Electromagnetic testing asnt level iii study guide ect by Charlie Chong

Electromagnetic Testing

Study Guide Eddy Current Testing Revisited My ASNT Level III Pre-Exam Preparatory Self Study Notes 26th April 2015

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Aerospace Applications

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Aerospace Applications

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Aerospace & Defence Applications

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Power & Nuclear Applications

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Heavy Industry & Mining Applications

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Fion Zhang at Shanghai 26th April 2015

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CHAPTER 1 PRINCIPLES OF EDDY CURRENT TESTING

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HISTORICAL BACKGROUND Belore discussing the principles of eddy current testing, it seems appropriate to discuss brielly facets of magnetism and electromagnetism that serve as the foundation of our study of eddy current testing. In the period from 1775 to 1900, scientific experimenters Coulomb, A Ampere, Faraday, Oersted, Arago, Maxwell, and Kelvin investigated and cataloged most of what is known about magnetism and electromagnetism Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet (Figure 1.1). He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field to the disk causing eddy currents to allow in the disk producing a magnetic field by the disk that attracted the magnet. Arago's simple model is a basis lor many automobile speedometers used today.

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Figure 1.1- Aragoâ&#x20AC;&#x2DC;s Magnetic Experimentation, 1821.

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https://www.nde-ed.org/GeneralResources/Formula/ECFormula/ECFormula.htm

Oersted discovered the presence of a magnetic field around a currentcarrying conductor, and he observed a magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents Ilowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to manufacture noninductive, precision resistors. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Fig. 1.2)

Figure 1.2ä¸&#x20AC;Induced Current with Coil and Magnet Charlie Chong/ Fion Zhang

Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown路 in Figure 1.3. Coil A is connected to a battery through a switch S. A second coil a connected to a galvanometer G is nearby. When switch S is closed producing a current in coil A in the direction shown, a momentary current is induced in coil a in a direction(- a) opposite to that in A. If S is now opened, a momeritary current will appear in coil a having the direction of (- b). In each case, current flows in coil a only while the current in coil A is changing.

Figure 1.3-lnduced Current, Electromagnetic Technique

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FARADAY LAW The electromotive force (voltage) induced in coil a of Figure 1.3 can be expressed as follows: E = K ∙ N ∙ ∆Ф/∆t E = Average induced voltage N = Number of turns of wire in coil B ∆Ф/∆t = Rate of change of magnetic lines of force affecting coil B K = 10-8 constant

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Maxwell produced a two-volume work "A Treatise on Electricity and Magnetism" first published in 1873, Maxwell not only chronicled most of the work done in electricity and magnetism at that time, but he also developed and published a group of relations known as Maxwell's equations for the elec tromagnetic field. These equations form the base that mathematically describes most of what is known about electromagnetism today. In 1849 Lord Kelvin applied Bessel.'s equation to solve the elements of an electromagnetic field. The principles of eddy current testing depend on the process of electromagnetic induction. This process includes a test coil through which a varying or alternating current is passed. A varying current flowing in a test coil produces a varying electromagnetic field about the coil. This field is known as the primary field.

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Faraday Law Increasing current in a coil of wire will generate a counter emf which opposes the current. Applying the voltage law allows us to see the effect of this emf on the circuit equation. The fact that the emf always opposes the change in current is an example of Lenz's law. The relation of this counter emf to the current is the origin of the concept of inductance. The inductance of a coil follows from Faraday's law.

E ∝ ∆ Ф/ ∆t (Faraday Law)

Since the magnetic field of a solenoid is:

E = - N ∆Ф/ ∆t

B = μNI ∙ (l -1)

Ф = BA

Thus:

B = flux density A = Area under the influence of B

E = - NA ∙∆B/ ∆t, becomes;

For a fixed area and changing current, Faraday's law becomes:

E = - N A ∙∆ [μNI (l -1)] / ∆t E = - NAμN ∙(l -1) ∙ ∆I/∆t

E = - N ∆Ф/ ∆t = -N ∆BA/ ∆t

for L = N2Aμ (l -1)

for Ф = BA

E = -L ∆I/∆t #

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http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indcur.html

Faraday Law

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GENERATION OF EDDY CURRENTS When an electrically conducting test object is placed in the primary field, an electrical current will be induced in the test object. This c urrent is known as the eddy current. Figure 1.4 is a simple model that illust rates the relations hips of primary and induced (eddy) curre nts. Conductor A represents a portion of a test coil. Conductor B represents a portion of a test object.

Figure 1.4-1 Induced Current Relationships Charlie Chong/ Fion Zhang

Following Lenz's law and indicating the instantaneous direction of primary current Фp, a primary field Фp is developed about Conductor A. When Conductor B is brought into the influence of Фp, an eddy current lE is induced in Conductor B. This electrical current lE produces an electromagnetic field ФE that opposes the primary electromagnetic field Фp. The magnitude of ФE is directly proportional to the magnitude of lE. Characteristic changes in Conductor B such as conductivity, permeability, or geometry will cause lE to change. When lE varies, ФE also varies. Variations of ФE are reflected to Conductor A by changes in Фp. These changes are detected and displayed on some type of readout mechanism that relates these variations to the characteristic that is of interest. Ip = Primary current IE = Eddy current Фp = Primary magnetic flux ФE = Secondary eddy current flux

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FIELD INTENSITY Đ¤ Figure 1.5 presents a schematic view of an excited test coil. The electromagnetic field produced about the unloaded test coil in Figure 1.5 can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The electromagnetic field is most intense near the coil's surface. Ip Đ¤p

Figure 1.5-Eiectromagnetic Field Produced by Alternating Current Charlie Chong/ Fion Zhang

The field produced about this coil is directly proportional to the magnitude of applied current, rate of change of current or frequency, and the coil parameters. Coil parameters include inductance, diameter, length, thickness, number of turns of wire, and core material. To better understand the principles under discussion, we must again look at the instantaneous relationships of current and magnetic flux. The exciting current is supplied to the coil by an alternating current generator or oscillator. With a primary current lr flowing through the coil, a primary electromagnetic field Đ¤p is produced about the coil. When this excited test coil is placed on a conducting test object, eddy currents lE will be generated in that test object. Figure 1.6 illustrates this concept.

Figure 1.6-Generation of Eddy Current in a Test Object Charlie Chong/ Fion Zhang

Note the direction of lp, Đ¤p, and the resultant eddy current lE. Although Figure 1.6 shows lE by directional arrows on the surface of the test object, lE extends into the test object some distance. Another important observation is that lE is generated in the same plane in which the coil is wound. Figure 1.7 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Figure 1.7- Induced Current Flow in a Cylindrical l Part

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A more precise method of describing the relationships of magnetic flux, voltage, and current is the phase vector diagram or phasor diagrams (4).

Figure 1.8-a. Phasor Diagram of Coil Voltage without Test Object b. Phasor Diagram of Coil Voltage with Test Object Charlie Chong/ Fion Zhang

Figure 1.8-a. Phasor Diagram of Coil Voltage without Test Object

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage = 0 I = Excitation Current Фp = Primary Magnetic Flux Фs = Secondary Magnetic Flux = 0

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Figure 1.8 shows the effects of a non-ferromagnetic test object on a test coil. Figure 1.8a shows an encircling coil and the resultant phasor diagram for the unloaded coil . The components of phasor diagram 1.8a are as follows:

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage = 0 I = Excitation Current Đ¤p = Primary Magnetic Flux Đ¤s = Secondary Magnetic Flux = 0

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object

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The current (I) and primary magnetic flux (ФP) are plotted in phase, and the primary voltage (EP) is shown separated by 90 electrical degrees. Secondary magnetic flux Фs is plotted at zero because without a test object no secondary flux exists. Figure 1.8b represents the action of placing a nonerromagnetic test object into the test coil. The components of phasor diagram 1.8b for a loaded coil are as follows:

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage ET = Total Voltage I = Excitation Current Фp = Primary Magnetic Flux Фs = Secondary Magnetic Flux ФT = Total Magnetic Flux I = Excitation Current

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET ET∠ ≠90º

ФT Excitation current I

Фp

ФS ФT∠ ≠90º

Primary magnetic flux

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Ep + Es = ET

Secondary magnetic flux

Observing Figure 1.8b we can see by vectorial addition of Ep and Es we arrive at a new coil voltage (ET) for the loaded condition. The primary magnetic flux cflp and secondary magnetic flux ells are also combined by vectorial addition to arrive at a new magnetic flux (Đ¤T) for the loaded coil. Notice that for the condition of the test object In the test coil, Đ¤T is not in phase with the excitation current I. Also observe that the included angle between the excitation current and the new coil voltage Ep is no longer 90 electrical degrees. These interactions will be discussed in detail later in this study guide.

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET Ep + Es = ET ФT

Excitation current I

Фp Primary magnetic flux

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ФS

Secondary magnetic flux

CURRENT DENSITY The distribution of eddy currents in a test object varies exponentially. The current density in the test object is most dense near the test coil. This exponential current density follows the mathematical rules for a natural expo.nential decay curve (1/e) Usually a natural exponential curve is illustrated by a graph with the ordinate (Y axis) representing magnitude and the abscissa (X axis) representing time or distance. A common point described on such a graph is the "knee" of the curve. The knee occurs at the 37 percent value on the ordinate axis. This 37 percent point, or knee, is chosen because changes in X axis values produce significant changes in Y axis values from 100 percent to 37 percent, and below 37 percent changes in X axis values produce less significant changes in Y axis values. Applying this logic to eddy current testing, a term is developed to describe the relationship of current density in the test object. Consider the eddy current generated at the surface of the test object nearest the test coil to be 100 percent of the available current, the point in the test object thickness where this current is diminished to 37 percent is known as the standard depth of penetration (4). Figure 1.9 is a relative eddy current density curve for a plane wave of infinite extent with magnetic field parallel to the conducting test object surface.

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Figure 1.9 - Relative Eddy Current Density

δ = 1.98√(ρ/fμr)

0.37

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Current Density at Depth “x” The current density at any depth can be calculated as follows:

Jx = Jo e –x√(πfμσ) Where: Jx = Current density at depth x , amperes per square meter Jo = Current density at surface, amperes per square meter π = 3.1416 f = Frequency in hertz μ = Magnetic permeability, henries per meter (H∙m-1) x = Depth from surface, meters σ = Electric conductivity, mhos per meter (Siemens∙m-1?) The siemens (SI unit symbol: S) is the unit of electric conductance, electric susceptance and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is equal to the reciprocal of one ohm, and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.In English, the same form siemens is used both for the singular and plural

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MAGNETIC PERMEABILITY

Magnetic permeability μ is a combination of terms. For nonmagnetic materials: μ= μo = 4π∙ 10-7 H/m For magnetic materials: μ = μr∙μo Where: μr = Relative permeability, henries per meter (H∙m-1) μo = Magnetic permeability of air or nonmagnetic material, (H∙m-1)

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THE STANDARD DEPTH OF PENETRATION δ The standard depth of penetration can be calculated as follows:

δ = (πfμσ) -½ where: δ = Standard depth of penetration, meters π = 3.1416 f = Frequency in. hertz μ = Magnetic permeability, H/m σ = Electric conductivity, mhos· per meter

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Exercise: lt should be observed at this point that as frequency, conductivity, or permeability is increased, the penetration of current into the test object will be decreased. We can use the graph in Figure 1.9 (p. 6) to demonstrate many eddy current characteristics. Using an example of a very thick block of stainless steel being interrogated with a surface or probe coil operating at a test frequency of 100 kilohertz (kHz), we can determine the standard depth of penetration and observe current densities at other depths. Stainless steel {300 Series) is non-ferromagnetic. Magnetic permeability μ is 4π∙ 10-7 H/m and the conductivity is 0.14∙107 mhos per meter for 300 Series stainless steel. δ = (πfμσ) -½ δ = (100 x 103 x π x 4 π x 10-7 x 0.14 x 107) -½ m δ = 1.35 x 10-3m = 1.35mm

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Exercise: Using 1.35 mm as depth “x” from surface a ratio of depth/depth of penetration would be 1. Referring to Figure 1.9, a depth/depth of penetration of 1 indicates a relative eddy current density of 0.37 or 37 percent. What is the relative eddy current density at 3 mm? Depth “x” equals 3 mm and depth of penetration is 1.35 mm, therefore: 3/1.35 = 2.22δ Current density = (1/e) 2.22 = 0.11 or 11% This ratio indicates a relative eddy current density of about 0.1 or 10 percent. With only 10 percent of the available current flowing at a depth of 3 mm, detectability of variables such as conductivity, permeability, and discontinuities would be very difficult to detect. The obvious solution for greater detectability at the 3 mm depth is to lower the test frequency. Frequency selection will be covered in detail later in this text.

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PHASE/AMPLITUDE AND CURRENT/TIME RELATIONSHIPS Figure 1.10 reveals another facet of the eddy current. Eddy currents are not generated at the same in stant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous; i.e., phase is an electrical term used to describe timing relationships of electrical waveforms. Phase angle lagging

Figure 1.10 - Eddy Current Phase Angle Radians Lagging Î˛ = x/Î´ radian

Depth of penetration Charlie Chong/ Fion Zhang

Phase is usually expressed in either degrees or radians. There are 2π radians per 360 degrees. Each radian therefore is approximately 57 degrees. Using the surface current phase angle near the test coil as a reference, phase angle current deeper in the test object lags the surface current. The amount of phase lag is determined by: β = x/δ = x(πfμσ) -½ in radian where β equals the phase angle lag in radians. Figure 1.10 should be used as a relative indicator of phase lag. The exact phase relationship for a particular system may be different due to other variables, such as coil parameters and excitation methods. The amount of phase lag for a given part thickness is an important factor when considering resolution. Resolution is the ability to separate variables occurring in the test object; for example, distinguishing two discontinuities occurring at different depths in the same test object. As an example, let us establish a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.10 and observe the phase lag of the current at one standard depth of penetration. Where depth of interest (X) is 1 mm and depth of penetration (δ) is 1 mm, the X/ δ ratio is 1 and the current at depth X lags the surface current by 1 radian.

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Projecting this examination, let us observe the phase lag for the entire part thickness. The standard depth of penetration is 1 mm, the part thickness is 5 mm; therefore, the ratio X/Î´ equals 5. This produces a phase lag of 5 radians or approximately 287 degrees for the part thickness. Having a measurement capability of 1 degree increments, the part thickness could be divided into 287 parts, each part representing 0.017 mm. That would .be considered excellent resolution. There is an obvious limitation. Refer to Figure 1.9 and observe the resultant relative current density with an X/Î´ ratio of 5. The relative current density is near 0. lt should become apparent that the frequency can be adjusted to achieve optimum results for a particular variable. These and other variables will be discussed in Section 5 of this study guide.

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CHAPTER 1 REVIEW QUESTIONS

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Answer to Questions

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0.1-1 Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. magneto-restrictive forces. D. all of the above. 0.1-2 A secondary field is generated by the test object and is: A. equal and opposite to the primary field. B. opposite to the primary field, but much smaller. C. in the same plane as the coil is wound. D. in phase with the primary field. 0.1-3 When a non-ferromagnetic part is placed in the test coil, the coil's voltage: A. increases. B. remains constant because this is essential. C. decreases. D. shifts 90 degrees in phase.

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0.1-4 Refer to Figure 1.8b (p. 5): If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs. D. None of the above. 0.1-5 Eddy currents generated in a test object flow: A. in the same plane as magnetic flux. B. in the same plane as the coil is wound. C. 90 degrees to the coil winding plane. D. Eddy currents have no predictable direction. 0.1-6 The discovery of electromagnetic induction is credited to: A. Arago. B. Oersted. C. Maxwell. D. Faraday.

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET ET∠ ≠90º

ФT Excitation current I

Фp

ФS ФT∠ ≠90º

Primary magnetic flux

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Ep + Es = ET

Secondary magnetic flux

Discussion Subject: Reason out on the following: 0.1-4 Refer to Figure 1.8b (p. 5): If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs. D. None of the above.

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0.1-7 A standard depth of penetration is defined as the point in a test object where the relative eddy current density is reduced to: A. 25 percent. B. 37 percent. C. 50 percent. D. 100 percent. 0.1·8 Refer to Figure 1.9 (p. 6). If one standard depth of penetration was established at 1 mm in an object 3 mm thick, what is the relative current density on the far surface? A. 3 B. <0.1 c. 1/3 D. Indeterminate 0:1-9 Refer to Figure 1.10 (p. 8). Using the example in question 1.8, what is the phase difference between the near and far surfaces? A. Far surface leads near surface by 57 º B. Far surface leads near surface by 171 º C. Far surface lags near surface by 171 º D. Far surface lags near surface by 57 º Charlie Chong/ Fion Zhang

0.1-10 Calculate the standard depth of penetration at 10 kHz in copper; Ď&#x192; = 5.7â&#x2C6;&#x2122;107 mhos per meter. A. 0.1 mm B. 0.02 mm C. 0.66 mm D. 66 mm

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CHAPTER 2 TEST COIL ARRANGEMENTS

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Eddy Current Eddy Current (EC) testing is based on electromagnetic induction. The technology can be used to detect flaws in conducting materials or to measure the distance between a sensor and a conducting material. The measurement does not require the tested object to be in direct contact The principle The basic principle behind standard EC testing involves placing a cylindrical coil, which is carrying an alternating current, close to the test piece. The current in the coil generates a changing magnetic field, which produces eddy currents in the test piece. Variations in the phase and magnitude of these eddy currents are monitored using a second coil (search coil) or by measuring changes to the current flowing in the primary coil (excitation coil). Image Variations in the electrical conductivity or magnetic permeability of the test object or the presence of flaws will change the flow patterns of the eddy currents and there will be a corresponding change in the phase and amplitude of the measured current. Applications EC testing can be used to inspect physically complex shapes and to detect small cracks on or near the surface of a test piece. The inspected surfaces need only minor preparation and need to be perfectly even. The technique is also used for measuring electrical conductivity and the thickness of coatings.

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http://www.rosen-group.com/global/company/explore/we-can/technologies/measurement/eddy-current.html

TEST COIL ARRANGEMENTS Test coils can be categorized into three main mechanical groups: probe coils, bobbin coils, and encircling coils. PROBE COILS Surface coil, probe coil, flat coil, or pancake coil are all common terms used to describe the same test coil type. Probe coils provide a convenient method of examining the surface of a test object. Figure 2.1 illustrates a typical probe coil used for surface scanning.

Figure 2.1 -Probe Coil Charlie Chong/ Fion Zhang

Probe coils and probe coil forms can be shaped to fit particular geometries to solve complex inspection problems. As an example, probe coils fabricated in a pencil shape (pencil probe) are used to inspect threaded areas of mounting studs and nuts or serrated areas of turbine wheels and turbine blade assemblies. Probe coils may be used where high resolution is required by adding coil shielding. When using a high-resolution probe coil, the test object surface must be carefully scanned to assure complete inspection coverage. This careful scanning is very time consuming. For this reason, probe coil inspections of large test objects are usually limited to critical areas. Probe coils are used extensively in aircraft inspection for crack detection near fasteners and fastener holes. In the case of fastener holes (bolt holes, rivet holes), the probe coil is spinning while being withdrawn at a uniform rate. This provides a helical scan of the hole using a "spinning probe" technique.

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Spinning Encircling Probe Coil

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Spinning Thread Probe Coil

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Spinning Thread Probe Coil

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Shielded Probe Coils

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Thread Probe Coils Eddy Current Inspections on RPV bolts and in RPV flat bottom holes Because of the size of the inspection objects and the inaccessibility of the inside thread a mechanised inspection is necessary. The especially developed bolt inspection tables for the inspection of the thread and shaft regions enable a secured inspection. By the outline guidance of the thread an optimum sensor position is guaranteed, whereas the inspection of the shaft region provides an automatic feed that ensures the complete inspection of the total shaft surface. In case of the flat bottom hole thread inspection an optimum sensor guidance is obtained by a motorised compulsory guidance in the thread. Path sensors allow a detailed eddy current and path record and a resulting well analysable presentation of the C-scan.

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http://deltatest.de/en/dienstleistungen/gewinde_bolzen.php

Shielded Probe Coils

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http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

ENCIRCLING COILS Encircling coil, OD coil, and feed-through coil are terms commonty used to describe a coil that surrounds the test object. Figure 2.2 illustrates a typical encircling coil.

Figure 2.2-Encircling Coil Charlie Chong/ Fion Zhang

Encircling coils are primarily used to inspect tubular and bar-shaped products. The tube or bar is fed through the coil (feed-through) at relatively high speed. The cross section of the test object within the test coil is simultaneously interrogated. For this reason, circumferential orientation of discontinuities cannot be determined with an encircling coil. The volume of material examined at one time is greater using an encircling coil than a probe coil; therefore, the relative sensitivity is lower for an encircling coil. When using an encircling coil, it is important to keep the test object centered in the coil. If the test object is not centered, a uniform continuity response is difficult to obtained. It is common practice to run the calibration standard several times, each time indexing the artificial discontinuities to a new circumferential location in the coil. This procedure is used to insure proper response and proper centering.

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ENCIRCLING COILS

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ENCIRCLING COILS

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ENCIRCLING COILS

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ENCIRCLING COILS

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ENCIRCLING COILS

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http://www.mdpi.com/1424-8220/11/3/2525/htm

ENCIRCLING COILS

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BOBBIN COILS Bobbin coil, ID coil, and inside probe are terms that describe coils used to inspect from the inside diameter (ID) or bore of a tubular test object. Bobbin coils are inserted and withdrawn from the tube ID by long, semiflexible shafts or simply blown in with air and retrieved with an attached pull cable. These mechanisms will be described later in the text. Bobbin coil information follows the same basic rules stated for encircling coils. Figure 2.3 illustrates a typical bobbin coil.

Figure 2.3 illustrates a typical bobbin coil. Charlie Chong/ Fion Zhang

Probe coils, encircling coils, and bobbin coils can be additionally classified (5). These additional classifications are determined by how the coils are electrically connected. The three coil categories are absolute, differential, and hybrid. Figure 2.4 shows various types of absolute and differential coil arrangements.

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ABSOLUTE COILS An absolute coil makes its measurement without direct reference or comparison to a standard as the measurement is being made (6). Some applications for absolute coil systems would be measurements of conductivity, permeability, dimensions, and hardness.

DIFFERENTIAL COILS Differential coils consist of two or more coils electrically connected to oppose each other. Differential coils can be categorized into two types. One is the self-comparison differential, and the other is external reference differential. The self-comparison differential coil compares one area of a test object to another area on the same test object. A common. use is two coils, connected opposing, so that if both coils are affected by identical test object conditions, the net output is "0â&#x20AC;&#x153; or no signal. The selfcomparison arrangement is insensitive to test object variables that occur gradually. Variables such as slowly changing wall thickness, diameter, or conductivity are effectively discriminated against with the self-comparison differential coil. Only when a different condition affects one or the other test coils will an output signal be generated. The coils usually being mechanically and electrically similar allows the arrangement to be very stable during temperature changes. Short discontinuities such as cracks, pits, or other localized discontinuities with abrupt boundaries can be detected readily using the self-comparison differential coil. Charlie Chong/ Fion Zhang

The external reference differential coil, as the name implies, is when an external reference is used to affect one coil while the other coil is affected by the test object. Figure 2.5 illustrates this concept. This system is used to detect differences between a standard object and test objects. lt is particularly useful for comparative conductivity, permeability, and dimensional measurements. Obviously in Figure 2.5 it is imperative to normalize the system with one coil affected by the standard object and the other coil affected by an acceptable test object. The external reference differential coil system is sensitive to all measurable differences between the standard object and test object. For this reason it is often necessary to provide additional discrimination to separate and define variables present in the test object.

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Figure 2.5 - Ex terna l Reference Different ial System

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HYBRID COILS Hybrid coils may or may not be the same size and are not necessarily adjacent to each other. Common types of the hybrid coil are Driver/Pickup, Through Transmission, or Primary/Secondary coil assemblies. Figure 2.6 shows a typical hybrid arrangement. Pickup

Figure 2.6-Hybrid Coil Charlie Chong/ Fion Zhang

Driver

A simple hybrid coil consists of an excitation coil and a sensing coil. In the through transmission coil, the excitation coil is on one side of the test object and the sensing coil is on the other. The voltage developed in the sensing coil is a function of the current magnitude and frequency applied to the excitation coil, coil parameters of the exciting and sensing coils, and test object characteristics. In Figure 2.6 an encircling coil induces circumferential currents in a cylindrical test object, and the disturbances of these currents are detected by a small probe coil. ADDITIONAL COIL CHARACTERISTICS Coil configuration is but one of many factors to consider when setting up test conditions. Other coil characteristics of importance are mechanical, thermal, and electrical stability; sensitivity; resolution; and dimensions. The geometry of the coil is usually dictated by the geometry of the test object, and often sensitivity and resolution are compromised. The relative importance of test coil characteristics depends upon the nature of the test. A blend of theory and experience usually succeeds in selection of proper coil parameters. Coil design and interactions with test objects will be discussed later in this study guide. Charlie Chong/ Fion Zhang

More Reading â&#x20AC;&#x201C; Phase Array Technology

Charlie Chong/ Fion Zhang

Eddyfi Tangential Eddy Current Phase Array Technology

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Detecting Corrosion in Aluminum with Eddy Current Array Technology Corrosion is everywhere and aluminum is no exception. Whether used in the petrochemical, the power generation, or the aerospace industry, aluminum is subject to degradation. Without a doubt, there is a real need for a reliable and high-precision non-destructive testing (NDT) method. In many situations, one must detect and assess the extent of corrosion damage without having direct access to the region of interest. Indeed, assessing wall loss and pitting on the far side of an aluminum layer is key in a number of situations. The present document highlights the capabilities of eddy current array (ECA) technology using a particularly interesting application: corrosion detection in the storage tanks of nuclear power plants. The following describes how the technology is used to examine this important asset, which plays a critical role in the safe operation of nuclear plants.

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

The Challenge Storage tanks vary in size, shape and material. In the current situation, an aluminum tank with a slightly concave floor (approximately 5.2 m (17 ft) in diameter) was in need of inspection. As in most cases during in-service inspections, the far side of the aluminum plate was not accessible. This called for a solution capable of scanning through the floor plates in an effort to detect and characterize corrosion-related defects such as pitting and thinning. An enhanced technique was needed in place of conventional NDT methods such as ultrasonic testing (UT) or single-channel eddy current testing (ECT). In the application herein, the examination was originally performed with UT, which required couplant, a crew of four to five technicians and a significant amount of time because of the small active surface of the transducer (6.35 mm or 0.25 in.). Furthermore, a wide ECA probe would need to:  Adapt to the tank floor’s curvature and other geometric features  Offer sufficient penetration to scan through thick aluminum (6.35–7.94 mm or 0.250–0.313 in.)  Be robust enough to withstand extensive use

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

The Solution ECA technology uses several individual coils, grouped together in one probe. The coils are excited in sequence to eliminate interference from mutual inductance (something referred to as multiplexing). So doing, the coils work together to scan a wider inspection area than conventional ECT probes, which drastically cuts down on the time required to inspect an entire tank floor. The absence of couplant, inherent to eddy current testing, is also a natural advantage of the solution over UT. Ectane front The solution developed to answer this challenge consists of three elements — Eddyfi’s EctaneTM, a compact, rugged, battery-operated ECA data acquisition unit; Magnifi®, acquisition and analysis software for graphical display (C-scan), record keeping, and reporting; and, finally, because of the non-linear geometry found in this application, a semi-flexible probe whose active surface could match the tank floor’s geometry. The ECA probe developed for the application has a flexible active surface 128  mm (5.04  in.) wide adapting to slightly convex or concave geometries. The array features 33 coils, distributed in two rows, and uses multiplexing for enhanced performance. The coils, 6 mm (0.236 in.) in diameter, are perfectly matched to cover a low-frequency range of 0.6-20 kHz with a central frequency of 5 kHz. This design ensures excellent penetration, reaching the far side of the aluminum tank floor. Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Eddyfi Ectane

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Eddyfi Magnifi速

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Eddyfi Semi-flexible ECA probe

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Eddyfi Semi-flexible ECA probe

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

A calibration plate was used to validate the probe’s performance. To simulate both localized pitting and plain corrosion, it has a series of flat-bottom holes (FBH) ranging from 1.59 mm (0.063 in.) to 12.7 mm (0.5 in.) in diameter and 10% to 80% of the plate’s thickness.

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Thanks to Magnifi, it’s easy to use the phase angle to assess the extent of corrosion, discriminating between near-surface and more distant defects. In addition to the traditional impedance plane, ECA technology offers advanced imaging capabilities. Indeed, Magnifi can generate 2D and 3D C-scans, which proves extremely useful when interpreting signals. Scanning the calibration plate with the ECA probe yielded the following results:

2D C-scan, aluminum, thickness 6.35 mm (0.25 in.) Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

3D C-scan, aluminum, thickness 6.35 mm (0.25 in.)

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

The ECA probe can clearly detect pitting-like indications (down to the 1.59 mm (0.063 in.) FBH at 40% thickness) or thinning-like indications (down to the 12.7 mm (0.5 in.) FBH at 10% thickness). These results proved to be superior to those of the previous examination method. The solution was deployed on-site and led to the discovery of very degraded tank floor plates. The entire ECA inspection of a typical tank floor was performed in about a tenth of the time taken with the original UT inspection procedure, and was carried out by a single technician.

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

The Benefits The solution developed by Eddyfi to meet the challenge of corrosion detection in nuclear power plant storage tanks has several benefits, useful to other industries and applications as well:  Rapid scanning of large regions of interest  Improved versatility, adapting to curved or irregular surfaces  High-precision assessment of localized indications (e.g. pitting) and general degradation (e.g. thinning)  Easier interpretation with C-scan imaging  Full data recording and archiving capabilities Eddyfi develops a variety of products, of which the ones presented here are only a few. We have the expertise and flexibility to engineer solutions for the most challenging applications.

Charlie Chong/ Fion Zhang

http://www.ndt.net/search/docs.php3?id=14617&content=1

Reading Two â&#x20AC;&#x201C; More on Eddyfi Phase Array ECT Technology

Charlie Chong/ Fion Zhang

Rising to the Ferromagnetic Electromagnetic Testing Challenge We all rely on carbon steel (CS) welds in our daily lives, whether they are on the structures we use to commute, on the pipelines that carry the fuel we use in our cars, or on the wind turbines that generate the electricity we use to prepare meals. I think we can agree that we like our CS welds strong and secure. Hence the need to inspect them for defects thoroughly and effectively. Carbon Steel Welds are Everywhere Why is that? CS is easy to weld, doesn’t cost too much, and it’s extremely reliable. But. There’s always a but. CS welds are prone to cracking and are sometimes well hidden under layers of paint and coatings used in an effort to preserve assets. The crack defects in CS welds often break their surface and are usually too small for the naked eye to see. Furthermore, carbon steel is ferromagnetic. This means a high magnetic permeability and little to no penetration of eddy current. We’ve never shied away from a challenge

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

CS Welds

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

The Problems with Existing CS Weld Inspection Methods Conventional methods used to detect cracks in industries relying heavily on carbon steel welds include: • Penetrant testing (PT) • Magnetic particle testing (MT) • ECT pencil probes including ACFM These methods require extensive and time consuming surface preparation, the remains of which often end up released in the environment. Which adds to their high dependence on operator skills, somewhat unreliable results, inability to archive inspection data, and inherently low inspection speeds. Another method enjoying a degree of success - electromagnetic, this time - is alternating current field measurement ACFM . This method relies on mathematical models to assess cracks and estimate their depth. However, while it can do what the other techniques can’t, it’s also a slow one that needs, like ECT pencil probes, several scans to cover the entire geometry of the weld while only offering partial data.

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

ECT pencil probes including ACFM

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Pushing the Limits of Electromagnetic Inspection Technologies Being so widespread, but not supported properly, CS weld inspection deserved a better inspection method. To come up with it, we were faced with very interesting technological challenges: How do we scan the entire geometry of the weld in a single pass to speed up the inspection process? How do we do so without surface preparation? How do we achieve that with reliable positioning and depth information about crack defects? A typical eddy current array (ECA) solution would seem, at first glance, ideally suited to this type of application. It isn’t, however. That’s because typical ECA pancake coil configurations yield signals from which it is difficult to extract depth information. Furthermore, the presence of liftoff introduces a “drift” of the operation point along this hook, which produces significant phase changes, making depth sizing impossible from a practical standpoint.

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Impedance Phase Diagram

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Enters TECA – Tangential Eddy Current Array Through much R&D, we came to the conclusion that using “ tangential eddy current ” was the most promising avenue towards overcoming these challenges. As mentioned above, conventionally, the axes of pancake coils are positioned perpendicular to the surface under test. With tangential eddy current, coils are on their sides, with their axes parallel to the surface and the eddy current generated by the coil flowing parallel to the surface under test, “diving”, so to speak, under it. So how could we use tangential eddy current and leverage the power of an eddy current array? A multiplexed ECA would solve the single-pass problem, as arrays cover a wider area. We analyzed several parameters, including coil size/impedance/position/configuration, the operating frequency, and the multiplexing pattern (topology), among others, to create an optimal ECA solution. We tested and characterize more than 30 coil configurations over the course of a year of R&D, coming up with what we felt is the best coil configuration to leverage the power of ECA, striking a balance between coverage, penetration, and resolution. That’s how the tangential eddy current array (TECA™ ) was born. We were able to observe that TECA generated a relatively flat liftoff signal and defects approximately 90° from the liftoff signal, something that’s not possible using other inspection techniques. The multiplexed eddy current generated by TECA can dive under cracks down to 10 mm (0.4 in). But that doesn’t take care of the geometry issue.

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

A NEW EDDY CURRENT PROBE - Tangential Eddy Current Array Lift-off noise is unavoidable so long as the probe picks up the eddy current induced by exciting coil. There the authors have thought of two notions in order to design a new probe that suppresses lift-off noise and detects flaws: 1. One of the methods to eliminate lift-off noise in eddy current testing is to develop a probe picking up the component of eddy current that is generated only by flaws but not by the probe lift-off. 2. Each part of detecting coil windings picks up the parallel component of eddy current to itself. With the above two notions in mind, the authors have devised a new eddy current surface probe that is composed of a pancake exciting coil and a tangential detecting coil as shown in Figure 1. The circular exciting coil is adopted because it induces eddy current most efficiently. The exciting coil induces axi-symmetric circular eddy current in the test material with no eddy current circulating across the exciting coil circle when there is no flaw in the test material as shown in Figure 2(a). When there is a flaw crossing the circle, some eddy current circulates along the flaw crossing the circle. Since each part of the detecting coil winding picks up the parallel eddy current component to itself, the tangential detecting coil picks up only the eddy current circulating across the circle as shown in Figure 2(b)-(d). As the new probe scans over a flaw, the detecting coil generates a figure eight signal pattern. If the probe has two tangential detecting coils wound perpendicular to each other, it can detect all flaws in every orientation. The impedance of the exciting coil can also be used to monitor the probe lift-off in order to avoid the probe not detecting flaws in the material. The new probe is lift-off noise free because the lift-off of the probe from the material does not cause any eddy current to circulate crossing the exciting coil circle. Thus lift-off noise can be eliminated by detecting only the newly generated eddy current by flaws and by not detecting the eddy current induced by the exciting coil when there is no flaw in the test material. The probe is self-nulling because the detecting coil generates a signal only when a flaw causes some eddy current to circulate across the circle. Since the probe generates minimal lift-off noise, the authors have also thought that the probe lift-off does not influence much to the flaw signal and that the signal phase can be used for evaluating the depth of surface flaws.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn037/idn037.htm

Tangential Eddy Current Array

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn037/idn037.htm

Scanning an Entire Weld in One Pass This was also tricky. The TECA coil design had to be used in such a way as to cover the cap, toe, and heat-affected zone of CS welds, while dynamically adjusting to the weldâ&#x20AC;&#x2122;s uneven geometry. The challenge lay in bundling the coils in a mechanical package that struck a balance between resolution and sizing capabilities. After much testing, we designed an ingenious system of independent, springloaded fingers that adapt to weld geometries. The individual wedged fingers all incorporate an array of coils, which provides great resolution even at higher scan speeds, surfing over the uneven geometry of the weld and enabling the a single-pass scans of entire welds.

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

TECA coil design

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

What About Liftoff? And you would be right to ask. As I mentioned above, TECA generates a virtually flat liftoff signal, with crack-like indications approximately 90째 relative to this liftoff signal and all the indications featuring the same phase shift. The software processing Sharck probe data incorporates the equivalent of a three-dimensional depth-to-liftoff-to-vertical-amplitude depth plane that allows compensating for liftoff.

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

TECA Liftoff Response

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

The Final Touch Add to the design a removable high-resolution encoder and you have the final patent-pending Sharck probe capable of positioning cracks, measuring their length, and sizing them as deep as 10 mm (0.4 in), without surface preparation, at up to 200 mm/s (7.9 in/s).

Charlie Chong/ Fion Zhang

http://www.eddyfi.com/ndt/surface-inspection/rising-ferromagnetic-electromagnetic-testing-challenge/

Chapter 2 REVIEW QUESTIONS

Charlie Chong/ Fion Zhang

Answers:

Charlie Chong/ Fion Zhang

0.2-1 Differential coils are usually used in: A. bobbin coils. B. probe coils. C. OD coils. D. any of the above. 0.2路2 When using a probe coil to scan a test object, ____ _ A. the object must be dry and polished. B. the object must be scanned carefully to insure inspection coverage. C. the object must be scanned in circular motions at constant speeds. D. the probe must be moving at all times to get a reading. 0.2路3 A "spinning probe" would most likely be a (an): A. bobbin coil. B. ID coil. C. OD coil. D. probe coi I.

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0.2路4 A "feed-through" coil is: A. a coil with primary/secondary windings connected so that the signal is fed through the primary to the secondary. B. an encircling coil. C. an OD coil. D. both B and C. 0.2-5 When inspecting a tubular product with an encircling coil, which statement is not true? A. OD discontinuities can be found. B. Axial discontinuity locations can be noted. C. Circumferential discontinuity locations can be noted. D. ID discontinuities can be found. 0.2路6 An absolute coil measurement is made ____ _ A. by comparing one spot on the test object to another. B. without reference to or direct comparison with a standard. C. only with probe coils. D. by comparative measurement to a known standard. Charlie Chong/ Fion Zhang

Encircling Coil - Defect Detectability

For long defect the self comparison differential coilsâ&#x20AC;&#x2122; may cancelled each other leaving no indication.

Charlie Chong/ Fion Zhang

Encircling Coil - Defect Detectability

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0.2路7 When coils in a differential arrangement are affected simultaneously with the same test object variables, the output signal ____ _ A. is directly proportional to the number of variables. B. is "0" or near-"0." C. is indirectly proportional to the number of variables. D. is primarily a function of the exciting current. 0.2路8 Which coil type inherently has better thermal stability? A. Bobbin B. Absolute C. OD D. Differential

Charlie Chong/ Fion Zhang

Encircling Coil - Defect Detectability

When coils in a differential arrangement are affected simultaneously with the same test object variables

Charlie Chong/ Fion Zhang

0.2-9 A hybrid coil is composed of two or more coils. The coils ____ _ A. must be aligned coplanar to the driver axis: B. may be of widely different dimensions. C. must be impedance-matched as closely as possible. D. are very temperature sensitive. 0.2-10 Proper selection of test coil arrangement is determined by: A. shape of test object. B. resolution required. C. sensitivity required. D. stability. E. all of the above.

Charlie Chong/ Fion Zhang

Absolute and Differential Coils

Absolute Coil

Charlie Chong/ Fion Zhang

Differential Receiver Coils

3. TEST COIL DESIGN As discussed earlier, test coil design and selection is a blend of theory and experience. Many factors must be considered. These important factors are determined by the inspection requirement for resolution, sensitivity, impedance, size, stability, and environmental considerations. In order to better understand coil properties and electrical relationships, a short refresher in alternating current theory is necessary. First, we must examine electrical units-for example, current and its representative symbol I. Current not only suggests electron flow but also the amount. The amount of electrons flowing past a point in a circuit in one second is expressed in amperes; 2Ď&#x20AC;â&#x2C6;&#x2122;1018 electrons passing a point in one second is called 1 ampere. RESISTANCE Resistance is an opposition to the flow of electrons and is measured in ohms. Ohm's Jaw is stated by the equation: I = E/R Where: I = Current in amperes R = Resistance in ohms E = Electrical potential difference in volts Charlie Chong/ Fion Zhang

R=ρXl/A Resistance = Ohms Specific Resistance ρ = Ohm / circular mill foot Area = Circular mill Length = foot Thus, the resistance of a 10-foot length of 40 gauge copper wire with a specific resistance of 10.4 circular-mil-foot at 20ºC would be found as follows: R = 10.4 x 10/ 9.88 = 10.53Ω In an alternating current circuit containing only resistance, the current and voltage are in phase. In phase means the current and voltage reach their minimum and maximum values, respectively, at the same time. The power dissipated in a resistive circuit appears in the form of heat. For example, electric toasters are equipped with resistance wires that become hot when current flows through them, providing a heat source for toasting bread.

Charlie Chong/ Fion Zhang

R=ﾏ々l/A

Charlie Chong/ Fion Zhang

Inductance Heat generation is an undesirable trait for an eddy current coil. If the 10-foot length of wire used in the previous example was wound into the shape of a coil, it would exhibit characteristics of alternating current other than resistance. By forming the wire into the shape of a coil, the coil also would have the property of inductance. The role of inductance is analogous to inertia in mechanics, because inertia is the property of matter that causes a body to oppose any change in its velocity. The unit of inductance is the Henry (H). A coil is said to have the property of inductance when a change in current through the coil produces a voltage in the coil. More precisely, a circuit in which an electromotive force of one volt is induced when the current is changing at a rate of one ampere per second will have an inductance of one Henry. The inductance of a multilayer air core coil can be expressed by its physical properties, or coil parameters. Coil parameters such as length, diameter, thickness, and number of turns of wire affect the coil's inductance.

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Figure 3.1 illustrates typical coil dimensions required to calculate coil inductance. l r

Figure 3.1- Multilayer Coil

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An approximation of small, multilayer, air core coil inductance is as follows:

L = 0.8(rN)2 ∙ (6r + 9l + 10b)-1 Where: L = Self-inductance in microhenries (μH) N = Total number of turns r = Mean radius in inches l = Length of coil in inches b = Coil depth or thickness in inches For example, a coil whose dimensions are as follows: r = 0.1 inches l = 0.1 inches b = 0.1 inches N = 100 turns L = 32 μH

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As stated earlier, this inductance is analogous to inertia in mechanical systems in that inductance opposes a change in current as inertia opposes a change in velocity of a body. In alternating current circuits the current is always changing; therefore inductance is always opposing this change. As the current tries to change, the inductance reacts to oppose that change. This reaction is called inductive reactance in ohm.

Charlie Chong/ Fion Zhang

The unit of inductive reactance (XL) Is in ohms. Because the amount of reactance is a function of the rate of change of current and rate of change can be described as frequency, a formula relating frequency, inductance, and inductive reactance is:

XL = Ď&#x2030;L = 2Ď&#x20AC;fL where: XL = Inductive reactance in ohms f = Frequency in hertz L = Inductance in henries

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For example, using the 32 microhenry coil calculated earlier, operating at 100 kilohertz, its inductive reactance would be found as follows: XL = ωL = 2πfL XL = 2π ∙ 100 ∙ 103 ∙ 32 ∙ 10-6 XL = 20.106 ohms Therefore, this coil would present an opposition of 20 ohms to currents with a rate of change of 100 kilohertz due to its reactive component. Unlike a resistive circuit, the current and voltage of an inductive circuit do not reach their minimum and maximum values at the-same time. In a pure inductive circuit the voltage leads the current by 90 electrical degrees. This means that when the voltage reaches a maximum value, the current is at "0“

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Power is related to current and voltage as follows: P =El where: P = Power in watts E = Volts I = Current in amperes Notice that in a pure inductive circuit, when the voltage is maximum, the current is "0"; therefore, the product El = 0. Inductive reactances consume no alternating power where resistive elements consume power and dissipate power in the form of heat. The opposition to current flow due to the resistive element of the coil and the reactive element of the coil do not occur at the same time; therefore, they cannot be added as scalar quantities. A scalar quantity is one having only magnitude; i.e., it is a quantity fully described by a number, but which does not involve any concept of direction. Gallons in a tank, temperature in a room, miles per hour, for example, are all scalars.

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IMPEDANCE In order to explain the addition of reactance and resistance with a minimum of mathematical calculations we can again use the vector diagram or phasor diagram to explain this addition (19). A phasor diagram constructed with Imaginary units on the ordinate. or (Y) axis and real units on the abscissa or (X) axis is shown In Figure 3.2a.

Figure 3.2-lmpedance Diagram

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Substituting Inductive reactance (XL) and resistance (R) we can find the resultant of the vector addition of XL and R. This resultant vector Z is known as impedance. Impedance is the total opposition to current flow. Further observation of Figure 3.2b reveals XL, R, and Z appear to form the sides of a right triangle. The mathematical solution of right triangles states the square of the hypotenuse is equal to the sum of the squares of the other two sldes, or c2 = a2 + b2 , substituting the Z, R & XL, the equation becomes Z2 = R2 + XL2 Z = √(R2 + XL2) Let's try an example. What is the impedance of a coil having an inductance of 100 microhenries and a resistance of 5 ohms and being operated at 200 kilohertz? First we must convert inductance to inductive reactance and then, by vector addition, combine inductive reactance and resistance to obtain the impedance. Z = [ 52 + (2π∙200 ∙103 ∙100 ∙10-6)2]0.5 Z = 125.76 Ohm

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The term R + jXM is known as a rectangular notation. As an example, a resistance of 4 ohms in series with an inductive reactance of 3 ohms could be noted as Z = 4 + j3 ohms. The impedance calculation is then: Z = √(42+32) = 5 ohm In coil design it is often helpful to know also the included angle between the resistive component and impedance. A convenient method of notation is the polar form where Tan Ф = XL/R, where Ф is included angle between resistance and impedance. In the previous example our impedance magnitude is 5 ohms, but at what angle? Ф = tan-1 (3/4) = 36.9º Z = 5∠ 36.9º = |5|36.9º = 4+j3 Eddy current coils with included impedance angles of 60° to 90° usually make efficient test coils. As the angle between resistance and impedance approaches 0, the test coil becomes very inefficient with most of its energy being dissipated as heat..

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Q or FIGURE OF MERIT The term used to describe coil efficiency is Q or merit of the coil. The higher the Q or merit of a coil, the more efficiently the coil performs as an inductor. The merit of a coil is mathematically stated as:

Q = XL/R Where: XL = Inductive reactance R = Resistance For example, a coil having an induct ive reactance of 100 ohms and a resistance of 5 ohms would have a Q of 20.

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PERMEABILITY AND SHIELDING EFFECTS The addition of permeable core materials in certain coil designs dramatically improves the Q factor. Permeable cores are usually constructed of high permeability "powdered iron." Probe coils, for example, are wound on a form that allows a powdered iron rod or slug to be placed in the center of the coil. lt is. common to increase the coil impedance by a factor of 10 by the addition of core materials. This increase in impedance without additional winding greatly enhances the Q of the coil. Some core materials are cylinder- or cup-shaped. A common term is cup core. The coil is wound and placed in the cup core. In the case of a probe coil in a cup core, not only is the impedance increased, but the benefit of shielding is also gained. Shielding with a cup core prevents the electromagnetic field from spreading at the sides of the coil. This greatly reduces the signals produced by edge effect of adjacent members to the test area, such as fasteners on an aircraft wing. Shielding, while improving resolution, usually sacrifices some amount of penetration into the part. Another method of shielding uses high conductivity material, such as copper or aluminum, to suppress high frequency interference from other sources and also to shape the electromagnetic field of the test coil. A copper cup would restrict the electromagnetic field in much the same manner as the "powdered iron cup core" discussed previously. A disadvantage of high conductivity, low or no permeability shielding is that the coil's impedance is reduced when the shielding material is placed around the test coil. The net effect is, of course, that the coil's Q is less than it was when the coil was surrounded by air.

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By Using Core: Ferromagnetic core  Q factor increase (increase impedance)  Shielding effect  Less penetration Non-ferromagnetic core  Q factor decrease (decrease impedance)  Shielding effect  Less penetration

Charlie Chong/ Fion Zhang

Magnetic Shielding

Charlie Chong/ Fion Zhang

Probe Shielding One of the challenges of performing an eddy current inspection is getting sufficient eddy current field strength in the region of interest within the material. Another challenge is keeping the field away from non-relevant features of the test component. The impedance change caused by non-relevant features can complicate the interpretation of the signal. Probe shielding and loading are sometimes used to limit the spread and concentrate the magnetic field of the coil. Of course, if the magnetic field is concentrated near the coil, the eddy currents will also be concentrated in this area. Probe Shielding Probe shielding is used to prevent or reduce the interaction of the probe's magnetic field with non-relevant features in close proximity of the probe. Shielding could be used to reduce edge effects when testing near dimensional transitions such as a step or an edge. Shielding could also be used to reduce the effects of conductive or magnetic fasteners in the region of testing. 1) Magnetically shielded with ferromagnetic materials Eddy current probes are most often shielded using magnetic shielding or eddy current shielding. Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high permeability and low conductivity. The ferrite creates an area of low magnetic reluctance and the probe's magnetic field is concentrated in this area rather than spreading beyond the shielding. This concentrates the magnetic field into a tighter area around the coil. 2) Eddy current shielding with non-magnetic materials Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil. The portion of the coil's magnetic field that cuts across the shielding will generate eddy currents in the shielding material rather than in the non-relevant features outside of the shielded area. The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to the skin effect in the shielding material. 3) Probe Loading with Ferrite Cores vs. Air Cores Sometimes coils are wound around a ferrite core. Since ferrite is ferromagnetic, the magnetic flux produced by the coil prefers to travel through the ferrite as opposed to the air. Therefore, the ferrite core concentrates the magnetic field near the center of the probe. This, in turn, concentrates the eddy currents near the center of the probe. Probes with ferrite cores tend to be more sensitive than air core probes and less affected by probe wobble and lift-off.

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/ProbesShielding.htm

Probe Shielding Magnetically shielded with ferromagnetic materials or non ferromagnetic materials

Probe Loading with Ferrite Cores vs. Air Cores Sometimes coils are wound around a ferrite core. Since ferrite is ferromagnetic, the magnetic flux produced by the coil prefers to travel through the ferrite as opposed to the air. Therefore, the ferrite core concentrates the magnetic field near the center of the probe. This, in turn, concentrates the eddy currents near the center of the probe. Probes with ferrite cores tend to be more sensitive than air core probes and less affected by probe wobble and lift-off.

Charlie Chong/ Fion Zhang

Another coil design used for inspection of ferromagnetic materials uses a saturation approach. A predominant variable that prevents eddy current penetration in ferromagnetic material is called permeability. Permeability effects exhibited by the test object can be reduced by means of magnetic saturation. Saturation coils for steels are usually very large and surround the test object and test coil. A steady state current is applied to the saturation coil. When the steel test object is magnetically saturated it may be inspected in the same manner as a non-ferromagnetic material. In the case of mild steel many thousands of gauss are required to produce saturation. In such other materials as nickel alloys (monel and inconel), the saturation required is much less and can usually be accomplished by incorporating permanent magnets adjacent to the test coil.

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More Reading on Shielded and Unshielded Probes Probes are normally available in both shielded and unshielded versions; however, there is an increasing demand for the shielded variety. Shielding restricts the magnetic field produced by the coils to the physical size of the probe. A shield can be made of various materials, but the most common are: ferrite (like a ceramic made of iron oxides), Mumetal, and mild steel. Ferrite make the best shielding because they provide an easy path for the magnetic field but has poor conductivity. This means that there is little eddy current loss in the shield itself (?) . Mild steel has more losses but is widely used for spot probes and ring probes due to its ease of machining when ferrite is not available in certain sizes or shapes. Mumetal is sometimes for pencil probes as it is available in thin sheet; however, it is less effective than ferrite. Shielding has several advantages: first, it allows the probe to be used near geometry changes, such as edges, without giving false indications; next, it allows the probe to touch ferrous fastener heads with minimal interference; last, it allows the detection of smaller defects due to the stronger magnetic field concentrated in a smaller area. On the other hand, unshielded probes allow somewhat deeper penetration due to the larger magnetic field. They are also slightly more tolerant to lift-off. Unshielded probes are recommended for the inspection of ferrous materials (steel) for surface cracks, and in particular with meter instruments. The reason for this is that the meter response is too slow to allow the signal from a shielded probe to be displayed at normal scanning speeds due to the smaller sensitive area.

Charlie Chong/ Fion Zhang

http://www.olympus-ims.com/en/ec-probes/selection/

Discussion: Subject: Ferrite make the best shielding because they provide an easy path for the magnetic field but has poor conductivity. This means that there is little eddy current loss in the shield itself (?) . Mild steel has more losses but is widely used for spot probes and ring probes due to its ease of machining when ferrite is not available in certain sizes or shapes. Mumetal is sometimes for pencil probes as it is available in thin sheet; however, it is less effective than ferrite.

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COIL FIXTURES Coil fixtures or holders may be as varied as the imagination of the designers and users. After the size, shape, and style have been decided upon, the next consideration should be the test environment. Characteristics of wear, temperature, atmosphere, mechanical stress, and stability must be considered. Normally wear can be reduced by selection of wear-resistant plastic compounds, or where severe wear is expected, artificial or genuine jewels may be used. Less expensive and very effective wear materials,.such as aluminum oxide or ceramics, are more commonly used. Temperature stability may be accomplished by using coil holder material with poor heat transfer characteristics. Metals have high heat transfer characteristics, and often coils made with metal holders are sensitive to temperature variations caused by human touch. For high temperature applications, materials must be chosen carefully. Most common commercial copper coil wire may be used up to 150 ~ 200째C. For temperatures above 200째C, silver or aluminum wire with ceramic or high temperature silicone insulation must be used.

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The Jewels

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Materials must be chemically compatible with the test object. As extreme examples, a polystyrene coil form would not be used to inspect an acetone cooler, or a lead or graphite housing allowed to come in contact with an inconel jet engine tail cone producing service-related stress cracks. Mechanical and electrical stability of the test coil can be enhanced by an application of epoxy resin between each layer of coil winding. This accomplishes many objectives: (1) it seals the coil to exclude moisture; (2) it provides additional electrical insulation; and (3) it provides mechanical stability. Characteristics listed are not in order of importance. The importance of each characteristic is determined by specific test requirements.

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Eddy Current Phasol Diagram

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Chapter 3 REVIEW QUESTIONS

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Answers:

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0.3-1 A coil's resistance is determined by: A. wire material. B. wire length. C. wire cross-sectional area. D. all of the above. 0.3-2 Inductance is analogous to: A. force. B. volume. C. inertia. D. velocity. 0.3-3 The unit of inductance is the: A. henry. B. maxwell. C. ohm. D. farad.

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0.3·4 The inductance of a multilayer air core coil with the dimensions l =0.2, r =0.5, b=0.1, and N=20, is: given that A. 1.38 henries. L = 0.8(rN)2 ∙ (6r + 9l + 10b)-1 in μH B. 13.8 microhenries. C. 13.8 ohms. D. 1.38 ohms. 0.3-5 The inductive reactance of the coil in Q.3-4, operating at 400 kHz, would be: A. 1380 ohms. B. 5520 ohms. C. 34.66 ohms. D. 3466 ohms. 0.3-6 The impedance of a 100 microhenry coil with a resistance of 20 ohms operating at 100 kHz would be: A. 62.8 ohms. XL = 2πfL = 62.83Ω, R=20Ω B. 4343.8 ohms. Z= √(XL2 + R2) C. 628 ohms. D. 65.9 ohms. Charlie Chong/ Fion Zhang

0.3-7 The Q or merit of a coil is the ratio of: A. Z/XL B. XL/Z C. XL/R D. R/XL 0 .3-8 The incorporation of magnetic shielding: A. improves resolution. B. decreases field extension. C. increases impedance.(?) D. does all of the above. (for ferromagnetic material shield materials only) 0.3-9 The purpose of a steady-state 稳态的 winding incorporated in a test coil is to: A. reduce permeability effects. B. provide magnetic saturation. C. provide a balance source for the sensing coil. D. both A and B.

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0.3路10 The most important consideration when selecting a test coil is: A. sensitivity. B. resolution. C. stability. D. test requirement and compatibility.

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4. EFFECTS OF TEST OBJECT ON TEST COIL As we have seen, the eddy current technique depends on the generation of induced currents within the test object. Perturbations or disturbances in these small induced currents affect the test coil. The result is variance of test coil impedance due to test object variables. These are called operating variables (19). Some of the operating variables are coil impedance, electrical conductivity, magnetic permeability, skin effect, lift-off, fill factor, end effect, edge effect, and signal-to-noise ratio. Coil impedance was discussed at length in the previous Section. In this Section coil impedance changes will be represented graphically to more effectively explain the interaction of other operating variables.

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ELECTRICAL CONDUCTIVITY In electron theory the atom consists of a positive nucleus surrounded by orbiting negative electrons. Materials that allow these electrons to be easily moved out of orbit around the nucleus are classified as conductors. In conductors electrons are moved by applying an outside electrical force. The ease with which the electrons are made to move through the conductor is called conductafce. A unit of conductance is the mho: The mho is the reciprocal of the ohm, or conductance G = 1/R, where G is conductance in mhos and R is resistance in ohms. In eddy current testing, instead of describing conductance in absolute terms, an arbitrary unit has been assigned. Since the relative conductivity of metals and alloys varies over a wide range, the need for a conductivity benchmark is of prime importance. The international electrochemical Commission established in 1913 aconvenient method of comparing of material to another. The commission established that a specified grade of high purity copper and uniform section of 1 mm2 measuring 0.017241 ohms at 20°C would be arbitrarily considered 100 percent conductive. The symbol for conductivity is Ω (sigma) and the unit is % IACS or percent of the International Annealed Conductivity Standard.

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Table 4.1 lists materials by conductivity and resistivity. A statement can be made about a conductor in terms of conductance or resistance. Note that a good conductor is a poor resistor. Conductance and resistance are direct reciprocals as stated earlier. Conductivity and resistivity, however, have different origins and units; therefore, the conversion is not so direct. As previously discussed, conductivity is expressed on an arbitrary scale in % IACS. Resistivity is expressed in absolute terms of micro ohm-centimeter. To convert to either unit, simply follow the equation:

Conductivity %IACS = 172.41 / Ď micro-ohm-cm in

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As the test coil is influenced by different conductivities, its impedance varies inversely to conductivity. A higher conductivity causes the test coil to have a lower impedance value. Figure 4.1 illustrates this concept. The coil's inductive reactance is represented by the Y axis and coil resistance appears on the X axis. The 0 percent conductivity point, or air point, is when the coil's empty reactance (XLo) is maximum. Figure 4.1 represents a measured conductivity locus (4). Conductivity is influenced by many factors. Table 4.1 lists conductivities of materials with different chemical compositions.

Figure 4.1 - Measured Conductivity Locus Charlie Chong/ Fion Zhang

Table 4.1- Eiectrical Resistivity and Conductivity of Several Common Metals and Alloys (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol. 11, 8th Ed.

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Some other factors affecting conductivity are temperature, heat treatment, grain size, hardness, and residual stresses. A change in the temperature of the test object will change the electrical conductivity of that object. In metals, as the temperature is increased, the conductivity is decreased. Carbons and carbon compounds have negative temperature coefficients; therefore, their conductivity increases as temperature is increased. Heat treatment also affects electrical conductivity by redistributing elements in the material. Dependent upon materials and degree of heat treatment, conductivity can either increase or decrease as a result of heat treatment. Stresses in a material due to cold working produces lattice distortion or dislocation. This mechanical process changes the grain structure and hardness of the material, changing its electrical conductivity. Hardness in "age hardenable" aluminum alloys changes the electrical conductivity of the alloy. The electrical conductivity decreases as hardness increases. As an example, a Brinell hardness of 60 is represented by a conductivity of 23, and a Brinell hardness of 100 of the same alloy would have a conductivity of 19.

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Impedance Plane Response for Conductivity As the test coil is influenced by different conductivities, its impedance varies inversely to conductivity. A higher conductivity causes the test coil to have a lower impedance value.

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Impedance Plane Response for Conductivity

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http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

More Reading on the Impedance Plane Eddy current testing is used to find surface and near surface defects in conductive materials. It is used by the aviation industry for detection of defects such as cracks, corrosion damage, thickness verification, and for materials characterization such as metal sorting and heat treatment verification. Applications range from fuselage and structural inspection, engines, landing gear, and wheels. Eddy current inspection involves initial setup and calibration procedures with known reference standards of the same material as the part. Probes of appropriate design and frequency must be used. Eddy current inspection is based on the principle of electromagnetic induction. An electric coil in which an alternating current is flowing is placed adjacent to the part. Since the method is based on induction of electromagnetic fields, electrical contact is not required.

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Figure 1. Schematic of Eddy Current absolute probe

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

An alternating current flowing through the coil produces a primary magnetic field that induces eddy currents in the part. Energy is needed to generate the eddy currents, and this energy shows up as resistance losses in the coil. Typical NDE application are designed to measure these resistance losses. Eddy currents flow within closed loops in the part.

Figure 2. Diagram illustrating Eddy Currents created in a part.

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

As a result of eddy currents, a second magnetic field is generated in the material. The magnetic fields of the core interact with those in the part and changes in the material being inspected affect the interaction of the magnetic fields. The interaction, in turn, affects the electrical characteristics of the coil. Resistance and inductive reactance add up to the total impedance of the coil. Changes in the electrical impedance of the coil are measured by commercial eddy current instruments. So, what does all of this have to do with nondestructive testing? The main method used in eddy current inspection is one in which the response of the sensor depends on conductivity and permeability of the test material and the frequency selected.

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

How eddy currents are created and sensed:  An alternating current creates a magnetic field (Oersted's Law).  The magnetic field causes a resulting eddy current in a part, which creates an induced magnetic field (Faraday's Law).  The magnetic field from the coil is opposed to the induced magnetic field from the eddy current.  A defect (surface or near surface) modifies the eddy current and therefore the magnetic field as well.  This change in the magnetic field is detected by a sensor and is indicative of a flaw.

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

How far do the eddy currents penetrate into a test piece? The strength of the response from a flaw is greatest at the surface of the material being tested, and decreases with depth into the material. The "Standard depth of penetration" is mathematically defined as the point when the eddy current is 1/e or 37% of its surface value. The "effective depth of penetration" is defined as three times the standard depth of penetration, where the eddy current has fallen to about 3% of its surface value. At this depth there is no effective impact on the eddy current and a valid inspection is not feasible. Penetration depth will: - Decrease with an increase in conductivity - Decrease with an increase in permeability - Decrease with an increase in frequency Conductivity is sensitive to cracks and material in-homogeneities - Cracks - Defects - Voids - Scattering of electrons Magnetic permeability is much more sensitive to structural changes in magnetic materials - Dislocations - Residual stress - Second phases - Precipitates

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Frequency selection will greatly affect eddy current response. Selection of the proper frequency is the essential test factor under the control of the test operator. The frequency selected affects not only the strength of the response from flaws and the effective depth of penetration, but also the phase relationship. How do we measure eddy current response? Eddy current response is viewed on an oscilloscope display, showing the impedance response (Z) from the test material, which is affected by factors depending on the specimen and experimental conditions. Specimen conditions affecting response: - Electrical conductivity - Magnetic permeability (unmagnetized ferromagnetic materials can become magnetized, resulting in large changes in impedance) - Specimen thickness - thickness should be limited to less then three times the standard depth of penetration Experimental conditions affecting response - AC frequency - Electromagnetic coupling between the coil and the specimen (a small liftoff has a pronounced effect) - Inspection coil size - Number of turns within the coil itself - Coil type Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

On an impedance plane diagram the signal of the resistance (R) component is displayed on the X axis and the inductive reactance (XL) component is displayed on the Y axis.

Figure 3. Electrical Conductivity changes for typical materials. Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Thickness changes in a sample can change the impedance response on an oscilloscope. Defects such as corrosion are found in this fashion.

Figure 4. Changes in conductivity curve due to thinning of a part

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Figure 5. Changes in conductivity curve due to corrosion damage

Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

There are two basic types of coil probes used in eddy current inspection; the absolute probe and the differential probe. An absolute probe consists of a single pickup coil which can be fashioned in a variety of shapes. Absolute probes are very good for sorting metals and detection of cracks in many situations. Absolute coils can detect both sharp changes in impedance and gradual changes. They are however, sensitive to material variations, temperature changes, etc.

Figure 6. Typical response for samples of different conductivity Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

A differential probe consists of two coils sensing different areas of the material being tested, which are linked electrically in opposition. The circuit will become unbalanced when one of the coils encounters a change in impedance. The response to this change in impedance creates what is known as a Lissajous figure. In general, the closer the element spacing the wider the "loop" in the signal. Differential probes are relatively unaffected by lift-off as long as the elements are balanced, and are suited for detection of small defects. The differential probe's nature allows for greater resolution of sharp discontinuities, however it makes it less likely to distinguish gradual changes in material.

Figure 7. Diagram of response of a differential probe over a defect Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Lissajous figure

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http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

Lift Off Lift-off from paint, coatings, etc. can cause variations that may mask the defects of interest. Lift-off may also be useful in determining the thickness of nonconductive coatings on a conductive component

Figure 8. Response of a probe due to lift off Charlie Chong/ Fion Zhang

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html

PERMEABILITY Permeability of any material is a measure of the ease with which its atoms can be aligned, or the ease with which it can establish lines of force. Materials are rated on a comparative basis. Air is assigned a permeability of 1. A basic determination of permeability, Îź (pronounced "mu"), is: Îźr =

Number of Lines Produced with material as a core Number of Lines Produced with air as a core

Ferromagnetic metals and alloys including nickel, iron, and cobalt tend to concentrate magnetic flux lines. Ferromagnetic material or sintered ionic compounds are also useful in concentrating magnetic flux. Magnetic permeability is not constant tor a given material. The permeability depends more upon the magnetic field acting upon it. As an example, consider a magnetic steel bar placed in an encircling coil. As the coil current is increased, the magnetic field of the coil will increase. The magnetic flux within the steel will increase rapidly at first, and then will tend to level off as the steel approaches magnetic saturation. This phenomenon is called the Barkhausen effect (?). When increases in the magnetizing force produce little or no change on the flux within the steel bar, the bar is magnetically saturated. When ferromagnetic materials are saturated, permeability becomes constant. With magnetic permeability constant, ferromagnetic materials may be inspected using the eddy current method. Without magnetic saturation, ferromagnetic materials exhibit such a wide range of permeability variation that signals produced by discontinuities or conductivity variations are masked by the permeability signal. Permeability effects are most predominant at lower frequencies. Other magnetic effects include diamagnetic and paramagnetic. Charlie Chong/ Fion Zhang

SKIN EFFECT Electromagnetic tests in many applications are most sensitive to test object variables nearest the test coil due to skin effect. Skin effect is a result of mutual interaction of eddy currents, operating frequency, test object conductivity, and permeability. The skin effect, the concentration of eddy currents in the test object nearest the test coil, becomes more evident as test frequency, test object conductivity, and permeability are increased (4). For current density or eddy current distribution inthe test object, refer to Figure 1.9 in Section 1.

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EDGE EFFECT The electromagnetic field produced by an excited test coil extends in all directions from the coil. As test object geometrical boundaries are approached by the test coil, they are sensed by the coil prior to the coil's arrival at the boundary. The coil's field precedes the coil by some distance (2) determined by coil parameters, operating frequency, and test object characteristics. As the coil approaches the edge of a test object, eddy currents become distorted by the edge signal. This is known as edge effect. Response to the edges of test objects can be reduced by the incorporation of magnetic shields around the test coil or by reducing the test coil diameter. Edge effect is a term most applicable to the inspection of sheets or plates with a probe coil. END EFFECT End effect follows the same logic as edge effect. End effect is the signal observed when the end of a product approaches the test coil. Response to end effect can be reduced by coil shielding or reducing coil length in OD encircling or ID bobbin coils. End effect is a term most applicable to the inspection of bar or tubular products. Charlie Chong/ Fion Zhang

LIFT-OFF Electromagnetic coupling between test coil and test object is of prime importance when conducting an eddy current examination. The coupling between test coil and test object varies with spacing between the test coil and test object. This spacing is called lift-off. The effect on the coil impedance is called I ift-off effect. NOTE: Absolute coil – more sensitive to lift off. Differential coil – lift off is compensated, thus less sensitive to lift off. Differential external reference – sensitive to lift off as it is not compensated.

Charlie Chong/ Fion Zhang

http://www.ndt.net/apcndt2001/papers/224/224.htm

Figure 4.2 - Lift-off Conductivity Relationship

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Figure 4.2 shows the relationship between air, conductive materials, and liftoff. The electromagnetic field, as previously discussed, is strongest near the coil and dissipates with distance from the coil. This fact causes a pronounced lift-off effect for small variations in coil-to-object spacing. As an example, a spacing change from contact to 0.001 in. will produce a lift-off effect many times greater than a spacing change of 0.010 in. to 0.011 in.. Lift-off effect is generally an undesired effect causing increased noise and reduced coupling resulting in poor measuring ability. In some instances, equipment having phase discrimination capability can readily separate lift-off from conductivity or other variables. Lift-off can be used to advantage when measuring nonconductive coatings on conductive bases. A nonconductive coating such as paint or plastic causes a space between the coil and conducting base, allowing lift-off to represent the coating thickness. Lift-oft is also useful in profilometry and proximity applications. Lift-oft is a term most applicable to testing objects with a surface or probe coil.

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Discussion Topic: Figure 4.2 shows the relationship between air, conductive materials, and lift-off. The electromagnetic field, as previously discussed, is strongest near the coil and dissipates with distance from the coil. This fact causes a pronounced lift-off effect for small variations in coil-to-object spacing. As an example, a spacing change from contact to 0.001 in. will produce a lift-off effect many times greater than a spacing change of 0.010 in. to 0.011 in..

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Fill factor Fill factor is a term used to describe how well a test object will be electromagnetically coupled to a test coil that surrounds or is inserted into the test object. Fill factor then pertains to inspections using bobbin or encircling coils. Like lift-off, electromagnetic coupling between test coil and test object is most efficient when the coil is nearest the surface of the part. Fill factor canbe described as the ratio of test object diameter to coil diameter squared. The diameters squared is a simplified equation resulting in the division of effective coil and part areas. The area of a circle (A) is determined using the equation: A1 = πd2/4, A2= πD2/4 where A1, A2 are the sample’s and coil’s area. π/4 appears in both numerator and denominator of the fractional equation; therefore; π/4 cancels, leaving the ratio of diameters squared d2/D2 = η (eta) = fill factor

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Fil I factor will always be a number less than 1, and efficient fill factors approach 1. A fill factor of 0.99 is more desirable than a fill factor of 0.75. The effect of fill factor on the test system is that poor fill factors do not allow the coil to be sufficiently loaded by the test object. This is analogous to the effect of drawing a bow only slightly and releasing an arrow. The result is, with thebow slightly drawn and released, little effect is produced to propel the arrow. In electrical terms, we say the coil is loaded by the test object. How much the coil is loaded by thetest object due to fill factor can be calculated in relative terms. A test system with constant current capabilities being affected by a conductive nonmagnetic bar placed into an encircling coil can be used to demonstrate this effect.

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Discussion

Topic: Comments on the illustration. Îˇ>1?

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Electromagnetic Testing with Bobbin Coil Expert at Works

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http://www.concosystems.com/sites/default/files/userfiles/files/techical-papers/energy-tech-magazine-ndt-testing-article-jk.pdf

For this example, the system parameters are as follows: (a) Unloaded coil voltage equals 10 volts, (b) Test object effective permeability (5) equals 0.3. (c) Test coil inside diameter equals 1 inch. (d) Test object outside diameter equals 0.9 inches. Fill Factor η = 0.81 An equation demonstrating coil loading is given by:

E = E0(1- η + ημeff) When the nonmagnetic test object is inserted into the test coil with μeff=0.3, the coil's voltage will decrease. E = 10 (1-0.81 + 0.81 • 0.3) E = 10 (0.19 + 0.243) E = 1 0 (0.433) E = 4.3 volts where: E0 = Coil voltage with coil affected by air E = Coil voltage with coil affected by test object η = Fill factor μeff = Effective permeability This allows 10 - 4.3 or 5.7 volts available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object. it is suggested that the reader calculate the resultant loaded voltage developed by a 0.5 inch bar of the same material and observe the relativesensitivity difference.

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DISCONTINUITIES Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage, and corrosion, generally cause the effective conductivity of the test object to be reduced. Discontinuities open to the surface are more easily detected than subsurface discontinuities. Discontinuities open to the surface can be detected with a wide range of frequencies; subsurface investigations require a more careful frequency selection. Discontinuity detection at depths greater than 0.5 inch in stainless steel is very difficult. This is in part due to the sparse distribution of magnetic flux lines at the low frequency required for such deep.penetrations. Figure 1.9 (p. 6) is again useful to illustrate discontinuity response due to current distribution. As an example, consider testing a non-ferromagnetic tube at a frequency that establishes a standard depth of penetration at the midpoint of the tube wall. This condition would allow a relative current density of approximately 20 percent on the far surface of the tube. With this condition, identical near and far surface discontinuities would have greatly different responses. Due to current magnitude alone, the near surface discontinuity response would be nearly 5 times that of the far surface discontinuity. Discontinuity orientation has a dramatic effect on response. As seen earlier, discontinuity response is maximum when eddy currents and discontinuities are at 90ยบ, or perpendicular. Discontinuities parallel to the eddy current flow produce little or no response. The easiest method to insure detectability of discontinuitles is to use a reference standard or model that provides a consistent means of adjusting instrumentation.

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SIGNAL-TO-NOISE RATIO Signal-to-noise ratio Is the ratio of signals of interest to unwanted signals. Common noise sources are test object variations of surface roughness, geometry, and homogeneity. Other electrical noises can be due to such external sources as welding machines, electric motors and generators. Mechanical vibrations can increase test system noise by physical movement of test coil or test object. In other words, anything that interferes with a test system's ability to define a measurement is considered noise. Signal-to-noise ratios can be improved by several methods. If a part is dirty or scaly; signalo-noise ratio can be lmproverl tly cleaning the part. Electrical interference can be shielded or isolated. Phase discrimination and filtering can improve signalto-noise ratio. lt is common practice in nondestructive testing to require a minimum signal-to-noise ratio of 3: to 1. This means a signal of interest must have a response at least three times that of the noise at that point.

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Chapter 4 REVIEW QUESTIONS

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Answers:

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Q.4路1 Materials that hold their electrons loosely are classified as: A. resistors. B. conductors. C. semiconductors. D. insulators. Q.4路2 100% IACS is based on a specified copper bar having a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms. Q.4路3 A resistivity of 13 micro ohm-cm is equivalent to a conductivity in % lACS of A. 11.032 B. 0.0625 C. 16.52 D. 13.26

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Q.4路4 A prime factor affecting conductivity is: A. temperature. B. hardness. C. heat treatment. D. all of the above. Q.4路5 Materials that tend to concentrate magnetic flux lines are ____ _ A. conductive B. permeable C. resistive D. inductive Q.4路6 Diamagnetic materials have ____ _ A. a permeability greater than air B. a permeability less than air C. a permeability greater than ferromagnetic materials D. no permeability

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0.4路7 When an increase in field intensity produces little or no additional flux in a magnetic test object, the object is considered: A. stabilized. B. balanced. C. saturated. D. at magnetic threshold. 0.4路8 Edge effect can be reduced by: A. shielding. B. selecting a lower frequency. C. using a smaller coil. D. both A and C. 0.4路9 Lift-off signals produced by a 0-10 mil spacing change are approximately _____ times greater than a 80-90 mil spacing change. A. 10 B. 2 C. 5 D. 100 Charlie Chong/ Fion Zhang

0.4-10 Calculate the effect of fill factor when a conducting bar 0.5 inches in diameter with an effective permeability of 0.4 is placed into a 1-inch diameter coil with an unloaded voltage of 10 volts. The loaded voltage is ____ _ A. 2 volts E = E0(1- η + ημeff) B. 4.6 volts η = 0.25 E = 10(1-.25+.25x0.4) = 8.5V C. 8.5 volts D. 3.2 volts 0.4·11 Laminations are easily detected with the eddy current (probe coil) method. A. True B. False 0.4-12 Temperature changes, vibration, and environmental effects are test coil inputs that generate: A. unwanted signals. B. magnetic fields. C. eddy currents. D. drift. Charlie Chong/ Fion Zhang

5. SELECTION OF TEST PARAMETERS As NDT engineers and technicians, it is our responsibility to industry to provide and perform nondestructive examinations that in some way assure the quality or usefulness of industry products. In order to apply a nondestructive test, it is essential that we understand the parameters affecting the test. Usually, industry establishes a product or component and then seeks a method to inspect it.This practice establishes test object geometry, conductivity, and permeability prior to the application of the eddy current examination. Instrumentation, test coil, and test frequency selection become the tools used to solve the problem of inspection. Test coils were discussed previously, and instrumentation will be discussed later in this text. Test frequencies and their selection will be examined in detail in this Section.

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Frequency Selection In Section 1, we observed that eddy currents are exponentially reduced as they penetrate the test object. We also observed a time or phase difference in these currents. The currents near the test coil happen first, or lead the current that is deeper in the object. A high current density allows good delectability, and a wide phase difference between near and far surfaces allows good resolution.

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http://www.eng.morgan.edu/~hubert/IEGR470/eddycurrent.html

Standard Depth Î´

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http://www.suragus.com/en/company/eddy-current-testing-technology

Single Frequency System unfortunately, if a low frequency is selected to provide good penetration and detectability, the phase difference between near and far surface is reduced. Selection of frequency often becomes a compromise. lt is common practice in in-service inspection of thin wall, non-ferromagnetic tubing to establish a standard depth of penetration δ just past the mid-point of the tube wall. This permits about 25 percent of the available eddy current to flow at the outside surface of the tube wall. In addition, this establishes a phase difference of approximately 150 to 170 degrees between the inside and outside surface of the tube wall. The combination of 25 percent outside, or surface current, and 170 degrees included phase angle provides good detectability and resolution for thin wall tube inspection. The depth of penetration formula discussed in Section 1, although correct, has rather cumbersome units. Conductivity is usually expressed in percent of the "International Annealed Copper Standard“ (% IACS). Resistivity is usually expressed in terms of micro-ohm-centimeter (μΩcm). Depths of penetration are normally much less than 0.5 inch.

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A formula using these units may be more appropriate and easier to use. A depth of penetration formula using resistivity, frequency, and permeability can be expressed as follows: δ = √(2/ ωσμ) = √2 / √(ωσμ) = √2 / √(2πfσμ) = √1/(πfσμ) = (πfσμ) -½ For non-magnetic conductor μr ≈ 1 δ = K (ρ/f)½ (given that μ = μr x μ0 = 4π∙10-7Hm-1 and For magnetic conductor μr ≠ 1 δ = K (ρ/fμr)½ where: δ = Depth of penetration in inches K = Constant = 1.98 Q = Resistivity in μΩcm f = Frequency in hertz μrel or μr = 1 for non-ferromagnetic conductors

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ρ = 1/σ)

As technicians and engineers, our prime variable is frequency. By adjusting frequency we can be selectively responsive to test object variables. Solving the non-ferromagnetic depth of penetration formula for frequency requires a simple algebraic manipulation as follows: δ = K (ρ/f)½ δ2/K2 = (ρ/f), f = K2ρ / δ2

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for English system

f =1.982ρ / δ2 f in Hertz. for a given standard penetration δ ρ in micro-ohm-cm. δ in inches. As an example of how this may be used consider inspecting an aluminum plate 0.3 inch thick, fastened to a steel plate at the far surface. Effects of the steel part are undesirable and require discrimination or elimination. The aluminum plate has a resistivity of 5 micro-ohm-cm. By establishing a depth of penetration at 0.1 inch, the far surface current will be less than 10 percent of the available current, thus reducing response to the steel part. The frequency required for this can be calculated by applying: f = 1960Hz.

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If detection of the presence of the steel part was required, the depth of penetration could be reestablished at 0.3 inch in the aluminum plate, and a new frequency could be calculated: f = 218Hz Area of interest aluminum plate

Î´= 0.3 in.

steel part

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Another approach to frequency selection uses argument "A" of the Bessel function where argument "A" is equal to unity or 1. A = fσμrd2/ 5066 f = Frequency in hertz σ = Conductivity meter/ohm-mm2 d = Diameter of test object, cm μr = Relative permeability A frequency can always be selected to establish factor "A" equal to 1. This frequency is known as the limit frequency and is noted by the term fg By substituting 1 for factor "A" and fg for f, the equation becomes:

fg = 5066/σμrd2 Limit frequency (fg) is then established In terms of conductivity, permeability dimension, and a constant “5066”. · Since limit frequency is based on these parameters, a method of frequency determination using a test frequency to limit frequency ratio f/fg can be accomplished. High f/fg ratios are used for near surface tests, and lower f/fg ratios are used for subsurface tests. Charlie Chong/ Fion Zhang

Often results of such tests are represented graphically by diagrams. These diagrams are called impedance diagrams. Impedance illustrated by vector diagrams in Section 3 shows inductive reactance represented on the ordinate axis and resistance on the axis of abscissa. The vector sum of the reactive and resistive components is impedance. This impedance is a quantity with magnitude and direction that is directly proportional to frequency. In order to construct a universal Impedance diagram valid for all frequencies, the jmpedance must be normalized. Figure 5.1 illustrates a normalization process.

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Figure 5.1-Effect of Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary lmpedance with Secondary Circuit

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Figure 5.1 a shows the effect on primary impedance Zp with changes in frequency (ω = 2πf). Figure 5.1 a represents primary impedance without a secondary circuit or test object. Figure 5.1b Illustrates the effect of frequency on primary impedance with a secondary circuit or test object present. The primary resistance R1 in Figure 5.1 a has been subtracted in Figure 5.1 b since resistance is not affected by frequency. The term ωLsG in Figure 5.1 b represents a reference quantity for the secondary impedance. The units are secondary conductance G and ωLs secondary reactance. Further normalization is accomplished by dividing the reactive and resistive components by the term ωLo or the primary inductive reactance without a secondary circuit present. Figure 5.2 shows a typical normalized impedance diagram. The terms ωL/ωLo and R/ωLo represent the relative impedance of the test coil as affected by the test object.

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Fig. 16 Normalized impedance diagram for a long coil encircling a solid cylindrical nonferromagnetic bar showing also the locus for a thin-wall tube. k, electromagnetic wave propagation constant for a conducting material, or √(ωμσ) ; r, radius of conducting cylinder, meters; ω , 2πf;f, frequency; √(ωLoG) , equivalent of √(ωμσ) for simplified electric circuits; μ, magnetic permeability of bar, or = 4π × 10-7 H/m if bar is nonmagnetic; σ, electrical conductivity of bar, mho/m; 1.0, coil fill factor.

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Signals generated by changes in ωL or R caused by test object conditions such as surface and subsurface discontinuities may be noted by ∆ωL or ∆R. The ∆ωLo and ∆R notation indicates a change in the impedance. Figure 5.3 shows the impedance variation in a non-ferromagnetic cylinder caused by surface and subsurface discontinuities. Figure 5.3 also illustrates a sensitivity ratio for surface and subsurface discontinuities. Notice with an f/fg ratio of 50, a relatively high frequency, the response to subsurface discontinuities is not very pronounced.

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Figure 5.3-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/f 9 = 50.

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Figure 5.4 shows responses to the same discontinuities with an f/fg ratio of 15. This lower frequency allows better detection of subsurface discontinuities as shown in Figure 5.4.

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Figure 5.4-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/fg = 15

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Multifrequency Systems lt becomes obvious that the technician must have a good working knowledge of current density and phase relationships in order .to make intelligent frequency choices. The frequency choice discussed to date deals with coil systems driven by only one frequency. Test systems driven by more than one frequency are called multifrequency or multiparameter systems. lt is common for a test coil to be driven with three or more frequencies. Although several frequencies may be applied simultaneously or sequentially to a test coil, each of the individual frequencies follows rules established by single frequency methods. Signals generated at the various frequencies are often combined or mixed in electronic circuits that algebraically add or subtract signals to obtain a desired result. One multifrequency approach is to apply a broadband signal, with many frequency components, to the test coil. The information transmitted by this signal is proportional to its bandwidth, and the logarithm of 1 plus the signalto-noise power ratio. This relationship is stated by the equation:

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C = W Log2 (1 + S/N) C = Rate of information transmitted in bits per second W = Bandwidth of the signal S/N = Signal-to-noise power ratio This is known as the Shannon-Hartley Jaw. Another approach to multi parameter methods is to use a multiplexing process. The multiplexing process places one frequency at a time on the test coil. This results in zero cross-talk between frequencies and eliminates the need for band pass filters. The major advantages of a multiplex system are (1) lower cost, (2) greater flexibility in frequency selection, and (3) no cross-talk between frequency channels. If the multiplex switching rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of eddy current signals by their phase angle and amplitude is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data. Two frequencies are required to remove each unwanted variable. Charlie Chong/ Fion Zhang

Keywords:  Multiple frequency testing- Multifrequency systems  Multiple frequency testing- Multiparameter systems  Broad band technique for multiple frequency testing.  Multiplexing technique for multiple frequency testing.  Phase angle and amplitude for characterization of eddy current signals.  Two frequencies are required to remove each unwanted variable (prime & subtractor frequencies).

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Practical multiparameter frequency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin-wall non-ferromagnetic heat exchanger tubing. Tubing is structurally supported by ferromagnetic tube supports at several locations. lt is desired to remove the tube support response signal from tube wall data. Solution: Apply a multiparameter technique to suppress tube support signal response. First, a frequency is selected to give optimum phase and amplitude information about the tube wall. We shall call this the prime frequency. At the prime frequency, the response to the tube support and a calibrating through- all hole are equal in amplitude response. A second frequency called the subtractor frequency is selected on the basis of tube support response. Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtractor frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole. If the mixing unit amplitude adjustments are set so that both prime and subtractor tube support signal amplitudes are equal and phased in a manner to cause signal subtraction, the tube support signals cancel, leaving only slightly attenuated prime data information. For suppressions of inside or near surface signals, a higher subtractor frequency would be chosen. A combination of prime, low, and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. Optimization of frequency then depends on the desired measurement or parameter of interest Charlie Chong/ Fion Zhang

Typical Heat Exchanger Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtractor frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole.

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Chapter 5 REVIEW QUESTIONS

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Answers:

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Table of information

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0.5·1 What frequency is required to establish one standard depth of penetration of 0.1 inch in Zirconium? A. 19.6 kHz δ = (πfμσ) -½ B. 196 Hz f = 1.982ρ/δ2 = 1.982 x 50 / (0.1)2 C. 3.4 kHz D. 340Hz 0.5-2 In order to reduce effects of far surface indications, the test frequency ____ _ A. must be mixed B. must be raised C. must be lowered D. has no effect 0.5-3 The frequency required to establish the Bessel function Argument "A" equal to 1 is called A. optimum frequency B. resonant frequency C. limit frequency D. penetration frequency Charlie Chong/ Fion Zhang

0.5·4 Calculate the limit frequency for a copper bar (σ = 50.6 meter/ohm-mm2) 1 cm in diameter. The correct limit frequency is ____ _ A. 50kHz 2 fg = 5066/σμrd B. 50.6 Hz fg = 5066 /(50.6 x 1 x12) = 100Hz C. 100Hz D. 100kHz 0.5-5 Using the example in Question 5.4, what is the f/fg ratio if the test frequency is 60 kHz? A. 1.2 B. 120 C. 60 D. 600 0.5-6 In Figure 5.1b the value ωLsG equaling 1.4 would be indicative of ____ A. a high resistivity material B. a high conductivity material C. a low conductivity material D. a nonconductor Charlie Chong/ Fion Zhang

Figure 5.1- Effect of Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary lmpedance with Secondary Circuit

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0.5路7 Primary resistance is subtracted from Figure 5.1 b because ____ _ A. resistance is always constant B. resistance is not frequency dependent C. resistance does not add to the impedance D. none of the above . 0.5-8 The reference quantity is different for solid cylinder and thin-wall tube in Figure 5.2 because A, the frequency is different B. the conductivity is different C. the skin effect is no longer negligible D. the thin-wall tube has not been normalized 0.5-9 A 25 percent deep crack open to the near surface gives a response ___ times greater than the same crack 3.3 percent of diameter under the surface (ref. Figure 5.4). A. 10 B. 2.4 C. 1.25 D. 5 Charlie Chong/ Fion Zhang

ratio = 5

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ration = 3

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0.5-10 When using multifrequency systems, low subtractor frequencies are used to suppress A. conductivity changes B. far surface signals C. near surface signals D. permeability changes

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6. INSTRUMENT SYSTEMS Most eddy current instrumentation is categorized by its final output or display mode. There are basic requirements common to all types of eddy current instrumentation. Five different elements are usually required to produce a viable eddy current instrument. These functions are: ■ ■ ■ ■ ■

excitation, modulation, signal preparation, Demodulation, signal analysis, and signal display.

An optional sixth component would be test object handling equipment. Figure 6.1 illustrates how these components interrelate.

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Figure 6.1- internal Functions of the Electromagnetic Nondestructive Test

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1. The generator provides excitation signals to the test coil. 2. The signal modulation occurs in the electromagnetic field of the test coil assembly. 3. Next, the signal preparation section, usually a balancing network, prepares the signal for demodulation and analysis. In the signal preparation stage, balance networks are used to "null" out steady-value alternating current signals. Amplifiers and filters are also part of this section to improve signal-to-noise ratio and raise signal levels for the subsequent demodulation and analysis stage. 4. The demodulation and analysis section is made up of detectors, analyzers, discriminators, filters, and sampling circuits. Detectors can be a simple amplitude type or a more sophisticated phase/ amplitude or coherent type. 5. The signal display section is the key link between the test equipment and its intended purpose. The signal can be displayed many different ways. Common displays include cathode ray tube (CRT) oscilloscopes, meters, recorders, visual or audible alarms, computer terminals, and automatic signaling or reject equipment.

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series of simple eddy current instruments is shown in Figure 6.2 a, b, c, and d (19).

Figure 6.2-Four Types of Simple Eddy Current Instruments In Figure 6.2a, the voltage across the inspection coil is monitored by an ac voltmeter. This type of instrument could be used to measure large lift-off variations where accuracy was not critical. Figure 6.2b shows an impedance bridge circuit. This instrument consists of an ac exciting source, dropping resistors, and a balancing impedance. Figure 6.2c is similar to Figure 6.2b. In Figure 6.2c a balance coil similar to the inspection coil is used to provide a balanced bridge. Figure 6.2d illustrates a balancing coil affected by a reference sample. This is commonly used in external reference differential coil tests. In all cases, since only the voltage change or magnitude is monitored, these systems can all be grouped as impedance magnitude types (5). Charlie Chong/ Fion Zhang

Eddy current testing can be divided into three broad groups. The groups are: 1. Impedance (magnitude) testing, 2. Phase analysis testing, and 3. Modulation analysis testing. ď Ž Impedance testing is based on gross changes in coil impedance when the coil is placed near the test object. ď Ž Phase analysis testing is based on phase changes occurring in the test coil and the test object's effect on those phase changes. ď Ž Modulation analysis testing depends on the test object passing through the test coil's magnetic field at a constant rate. The amount of frequency modulation observed as a discontinuity passes through the test coil's field and is a function of the transit time of the discontinuity through the coil's field. The faster the transit time, the greater the modulation.

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1. IMPEDANCE TESTING With impedance magnitude instrumentation it is often difficult to separate desired responses, such as changes in conductivity or permeability, from dimensional changes. A variation of the impedance magnitude technique is the reactance magnitude instrument. In reactance magnitude tests, the test coil is part of the fundamental frequency oscillator circuit. This operates like a tuned circuit where the oscillator frequency is determined by the test coil's inductive reactance. As the test coil is affected by the test object, its inductive reactance changes, which in turn changes the oscillator frequency. The relative frequency variation â&#x2C6;&#x2020;f/f is, therefore, an indication of test object condition. Reactance magnitude systems have many of the same limitations as impedance magnitude systems.

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2. PHASE ANALYSIS TESTING Phase analysis techniques are divided into many subgroups depending on the type of data display. Some of the various types are (1) vector point, (2) impedance plane, (3) ellipse, and (4) linear time base. The vector point circuit and display are illustrated in Figure 6.3. 2.1 Vector Point The vector point display is a point of light on a CRT. The point is the vector sum of theY and X axis voltages present in the test coil (2). By proper selection of frequency and phase adjustment, voltage V1 could represent dimensional changes and voltage V2 could represent changes in conductivity.

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Figure 6.3-Vector Point Method (2, p. 3-15) Reprinted with permission.

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Figure 6.3-Vector Point Method (2, p. 3-15) Reprinted with permission. (continued)

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2.2 Ellipse The ellipse method is shown in Figure 6.4. As with the vector point method, the test object and reference standard are used to provide a balanced output. A normal balanced output is a straight horizontal line. Figure 6.5 shows typical ellipse responses. With the ellipse method the vertical deflection plates of a CRT are energized by an amplified voltage from the secondary test coils. The horizontal deflection plates are energized by a voltage that corresponds to the primary magnetizing current. With this arrangement, an ellipse opening occurs when a discontinuity signal is perpendicular to a dimensional variation in the impedance plane. The ellipse method can be used to examine many test object variables such as conductivity, permeability, hardness, dimensions, and discontinuities. When testing ferromagnetic parts with the ellipse and vector point methods, the relative permeability of the part will vary due to the nonlinear magnetization of the magnetizing field. This nonlinear magnetization creates odd harmonic frequencies to appear in the output data.

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Figure 6.4-EIIipse Method (2, p. 3-16) Reprinted with permission.

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Figure 6.5-CRT Displays for Dimension and Conductivity (2, p. 3-17) Reprinted with permission.

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2.3 Linear Time Base A test instrument system that is well suited to determine harmonic distortions present in the fundamental frequency uses the linear time base method of analysis. The linear time base unit applies a sawtooth shaped voltage to the horizontal deflection plates of a CRT. This provides a linear trace of the CRT beam from left to right across the CRT screen. The linear trace is timed so that it is equal to one cycle of the magnetizing current. This allows one cycle of the magnetizing sine wave voltage to appear on the CRT. Figure 6.7 illustrates a linear time base display.

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Figure 6.6-Linear Time Base Instrument Diagram (5, p. 40-29)

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Figure 6.7- Screen Image of a Linear Time Base Instrument with Sinusoidal Signals (5, p. 40路31)

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A slit or narrow vertical scale is provided to measure the amplitude of signals present in the slit. The base voltage is normally adjusted to cross the slit at "0" volts, the 180°point on the sine wave. The slit value "M" is used to analyze results. The slit value "M" is described by the equation: M = A sine ϴ where: M = Slit value A = Amplitude of the measurement in the slit ϴ = Angle between base signal and measurement effect In Figure 6.7, the angle difference A to B is approximately 90 degrees.

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MODULATION ANALYSIS TESTING Test instruments may also be classified by mode of operation. The mode of operation is determined by two functional areas within the instrument type. The first consideration is the method of test coil excitation. The second area is the degree of compensation, or nulling, and the type of detector used. The types of excitation include single frequency or multifrequency sinusoidal, single or repetitive pulse, and swept frequency. Compensation and detection can be accomplished by three modes. The three main input-detector modes are: 1. null balance with amplitude detector, 2. null balance with amplitude-phase detectors, and 3. selected off-null balance with amplitude detector.

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Mode 1 responds to any signal irrespective of phase angle. Mode 2, using amplitude-phase detectors, can discriminate against signals having a particular phase angle. With this system, the total demodulated signal can be displayed on an X-Y oscilloscope to show amplitude and phase relationships. Figure 6.8a shows a commercial null balance instrument with amplitude phase detectors.

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Figure 6.8a-Null Balance Instrument with Amplitude-Phase Detectors (Zetec, Inc.)

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Figure 6.8b-Typical Response to a Thin Wall Non-ferromagnetic Tube Calibration Standard (Zetec, Inc.)

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Mode 3 is a phase-sensitive system although it has only amplitude detectors. lt achieves phase sensitivity by operating at a selected off-balance condition. This off-null signal is very large compared with test object variations. Under this condition, the amplitude detector output varies in accordance with the test object signal variation on the large off-null signal. Two off-null systems are required to present both components of the test coil output signal. Figure 6.9 shows a block diagram of a stepped single frequency phaseamplitude instrument.

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Figure 6.9-lnstrument Providing Any One of Four Operating Frequencies

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The circuit in Figure 6.9 is capable of operating at any of the four frequencies. If the four frequencies are over a wide range, several different test coils may be required to use the instrument over the entire range. Most modern single frequency instruments use this principle; however, the four individual generators are usually replaced by one variable frequency generator with a wide operating range. A typical frequency range for such an instrument is 100 Hz to several megahertz. Figure 6.10 shows a block diagram for a multifrequency instrument operating at three frequencies simultaneously.

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Figure 6.10-Multifrequency Instrument Operating at Three Frequencies Simultaneously

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In Figure 6.10, excitation currents at each frequency are impressed on the coil at the same time. Multiple circuits are used throughout the instrument. The test coil output carrier frequencies are separated by filters. Multiple dual phase amplitude detectors are used and their outputs summed to provide separation of several test object parameters. A system similar to this is described in "In-Service Inspection of Steam Generator Tubing UsingMultiple Frequency Eddy Current Techniquesâ&#x20AC;&#x153;. another approach to the multifrequency technique uses a sequential coil drive called multiplexing. The frequencies are changed by a step-by-step sequence with such rapidity that the test parameters remain unchanged. The multiplex technique has the advantages of lower cost, continuously variable frequencies, and little or no cross-talk between channels.

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Figure 6.11 illustrates a commercial multifrequency instrument capable of operating at four different frequencies sequentially. Each of the frequency modules may be adjusted over a wide range of frequencies. In addition, two mixing modules are used to combine output signals of the various channels for suppression of unwanted variables. Results of such suppression are described in "Multi-Frequency Eddy Current Method and the Separation of Test Specimen Variables" .

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Figure 6.11-Commercial Multifrequency Instrument (Zetec, Inc.)

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Instruments are being developed that are programmable, computer or microprocessor based. With microprocessor controlled instruments, test setups can be stored in a programmable memory system. This allows complicated, preprogrammed test setups to be recalled and used by semiskilled personnel. Systems are designed with preprograms having supervisory code interlocks that prevent reprogramming by other than authorized personnel. Microprocessor-based instruments can interface with larger computer systems for control and signal analysis purposes. Figure 6.12 shows a single frequency portable microprocessor-based instrument. The CRT display applies the phase analysis technique for signal interpretation.

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Figure 6.12- Commercial Microprocessor-Based Instrument (Nortec Corporation)

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Other instruments being developed will be microprocessor based with the ability to excite several coils .at several frequencies. This would allow automatic supp-ression of unwanted variables and a direct link to larger computers for computer enhancement of test signal information. A test system using pulsed路excitation is shown in Figure 6.13. A pulse is applied to the test coil, compensating networks, and analyzers simultaneously. Systems having analyzers with one or two sampling points perform similar to a single frequency tester using sinusoidal excitation. Pulsed eddy current systems having multiple sampling points perform more like the multifrequency tester shown in Figure 6.10.

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Figure 6.13-Pulsed Waveform Excitation

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TEST OBJECT HANDLING EQUIPMENT Test object handling equipment is often a necessary component of a test system. Bars and tubes can be fed through encircling coils by means of roller fed assemblies. The stock being fed through the coil is usually transported at a constant speed. The transport speed is selected with instrument response and reject system response being of prime importance to the test. Pen marking and automatic sorting devices are common in automated systems. Spinning probes are used where the probe is rotated and the tube or bar is translated. Probe rotational speeds must be compatible with instrument response and translation speeds in order to obtain the desired inspection coverage and results. Small parts are often gravity fed through coil assemblies.

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A major problem with small parts is loading, inspecting, and unloading. A speed effect occurs when a conducting object is passed through a coil. As the object moves through the coil's magnetic field, an additional induced voltage within the object occurs. This additional induced voltage has the same frequency as the exciting current, and it causes a current flow and associated magnetic fields that produce signals proportional to the speed of the object through the coil. For larger or stationary structures, test probes are scanned over the part surface by manual or remotely operated systems. Scanning considerations are the same as tor tube and bar stock instrument response, marking or reject system response, and desired coverage. In the case of large heat exchangers, a probe positioning device is used to position the test probe over each tube opening to be inspected.

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Keywords: Speed Effect A major problem with small parts is loading, inspecting, and unloading. A speed effect occurs when a conducting object is passed through a coil. As the object moves through the coil's magnetic field, an additional induced voltage within the object occurs. This additional induced voltage has the same frequency as the exciting current, and it causes a current flow and associated magnetic fields that produce signals proportional to the speed of the object through the coil.

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Tubes to be inspected are identified by manual templates, or their coordinates are pro路 grammed into computer memory. Positive feedback is supplied to computer positioning systems by encoder devices. In manual template systems the tube end is viewed by a video camera. Tube identification and control feedback are supplied to the operator via a video display system. In each system, as the probe guide is positioned correctly, the probe is inserted and withdrawn from the heat exchanger tube bore, and res ults of the scan are recorded on chart paper and magnetic tape.

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Chapter 6 REVIEW QUESTIONS

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0.6路1 Signal preparation is usually accomplished by: A. detectors. B. samplers. C. balance networks. D. discriminators. 0.6路2 Most eddy current instruments have _____ coil excitation. A. square wave B. triangular wave C. sine wave D. sawtooth wave 0.6路3 When only coil voltage is monitored, the system is considered a(an) _____ type system. A. impedance magnitude B. phase analysis C. reactance magnitude D. resistance magnitude

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0.6路4 lt is easy to distinguish dimensional variations from discontinuities in a reactance magnitude system. A. True B. False 0.6路5 Eddy current systems can be grouped by: A. output characteristics. B. excitation mode. C. phase analysis extent. D. both A and B. 0.6路6 In modulation testing the test object must be ____ _ A. stationary B. moving C. polarized D. saturated

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0.6路7 Using the vector point method, undesired responses appear _____ on the CRT. A. vertical B. horizontal C. at 45潞 to horizontal D. random and cannot be set 0.6路8 When ellipse testing a rod, the f/fg ratio is lowered from 50 to 5 percent. The response from a 5 percent surface flaw: A. will appear more elliptical. B. will appear less elliptical. C. is unchanged. D. rotates 90 D clockwise. 0.6路9 Using the linear time base, harmonics appear: A. as phase shifts of the fundamental waveform. B. as distortions of the fundamental waveform. C. to have no effect on the fundamental waveform. D. as modified sawtooth signals. Charlie Chong/ Fion Zhang

0.6路10 Calculate the slit value "M" for a signal phase shift of 45 degrees at 10 divisions vertical amplitude. A. 14 B. 7 C. 0.7 D. 1.4 0.6路11 A multifrequency instrument that excites the test coil with several frequencies simultaneously uses the concept. A. multiplex B. time share C. broadband D. synthesized 0.6路12 A multifrequency instrument that excites the test coil with several frequencies sequentially usesthe concept. A. multiplex B. time base C. broadband D. Cartesian Charlie Chong/ Fion Zhang

0.6路13 In a pulsed eddy current system using a short duration and a long duration pulse, the short duration pulse is used to reduce ____ A. edge effect B. skin effect C. lift-off effect D. conductivity variations 0.6路14 When selecting feed rates for automatic inspection of tube and bar stock, consideration is given to: A. instrument response. B. automatic sorting response. C. speed effect. D. all of the above.

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7. READOUT MECHANISMS Eddy current test data may be displayed or indicated in a variety of ways. The type of display or readout depends on the test requirements. Test records may require archive storage on large inservice components so that corrosion or discontinuity rates of change can bemonitored and projected. In some production tests, a simple GO/NO-GO indicator circuit is all that is required. Some common readout mechanisms are indicator tights, audio alarms, meters, digital displays, cathode ray tubes, recorders, and computer printout or displays. INDICATOR LIGHTS A simple use of the indicator light is to monitor the eddy current signal amplitude with an amplitude gate circuit. When the signal reaches a preset amplitude limit, the amplitude gate switches a relay that applies power to an indicator light or automatic sorting device. With the amplitude gate circuit, high-low limits could be preset to give GO/NO-GO indications.

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AUDIO ALARMS Audio alarms can be used in much the same manner. Usually the audio alarm indicates only the abnormal condition. Alarm lights and audio alarms are commonly incorporated in eddy current test equipment. The indicator light and audio alarm give only qualitative information about the item, whether a condition is present or not. The degree of condition cannot normally be determined with these devices. Indicator lights and audio alarms are relatively inexpensive and can be interpreted by semi skilled personnel. METERS Meters can present quantitative information about a test object. Meters operate on the D'Arsonval galvanometer principle. The principle is based on the action between two magnetic fields. A common meter uses a strong permanent magnet to produce one magnetic field; the other magnetic field is produced by a movable coil wound on a core. The coil and core are suspended on jewelled bearings and attached to a pointer or "needle." The instrument output current is passed through the coil and produces a magnetic field about the coil that reacts to the permanent magnetic field surrounding the assembly. The measuring coil is deflected, moving the meter pointer. The degree of pointer movement can be related to test object variables as presented by the tester output signals.

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DIGITAL DISPLAYS Numerical digital displays or indicators provide the same type of information as analog meter systems. Many eddy current instruments have analog output circuits. Data handling of analog information in digital form requires analog information to be processed through analog-to-digital (A-D) converters. The A-D converter transforms analog voltages to numerical values tor display.

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CRTs Cathode ray tubes (CATs) play an important role in the display of eddy current information. Most CRTs are the "electrostatic" type. Three main elements comprise a cathode ray tube: (1) electron gun, (2) deflection plates, and (3) a fluorescent screen. The electron gun generates, focuses, and directs the electron beam toward the face or screen of the CAT. The deflection plates are situated between the electron gun and the screen. They are arranged in two pairs, usually called horizontal and vertical, or X and Y. The plane of one pair is perpendicular to the other pair and therefore considered X and Y. The screen is the imaging portion of the CAT. The screen consists of a coating or coatings that produce photochemical reactions when struck by the electron beam. The photochemical action appears in two stages. Fluorescence occurs as the electron beam strikes the screen. Phosphorescence enables the screen to continue to give off light after the electron- beam has been removed or has passed over a section of the screen. All screen materials possess both fluorescence and phosphorescence. Screen materials are referred to as phosphors. The color of fluorescence and phosphorescence may differ as the case for zinc sulfide: the fluorescence is bluegreen, and the phosphorescence is yellow-green.

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Fluorescence may appear blue, white, red, yellow, green, orange, or a combination of colors, depending on the chemical makeup of the screen. The amount of light output from the fluorescent screen depends on the electron beam accelerating potential, screen chemical composition, thickness of screen material, and writing speed of the electron beam. The duration of the photochemical effect is called persistence. Persistence is grouped as to low, medium, or high persistence. To display repetitive signals, a low or medium persistence CAT may be used. To display non recurrent or single events, a high persistence CAT should be used. Many modern CRTs have the capability of both low or medium and highpersistence. Storage or memory CRTs have the ability to display non recurrent signals. The image from a single event may remain visible on the CRT for many hours, if desired.

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Figure 7.1 illustrates a typical eddy current signal response on a storage CAT.

The amplitude of the signal in Figure 7.1 is an indicator of the volume of the discontinuity. The phase angle with respect to the X axis represents discontinuity depth and origin, origin indicating whether the discontinuity originated on the inside or outside surface of the tube.

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RECORDERS Recorders are also used to display data and to provide a convenient method of data storage. Recording is accomplished on paper strip charts, facsimile paper, facsimile photosensitive, magnetic tape (AM, FM, or video), or digital memory disks. Strip chart recordings are common in testing tubing or nuclear fuel rods where the discontinuity's location down the length of rod or tube is critical. The strip chart length is indexed to time or distance and pen response indicates normal or abnormal conditions. Fascimile recording is a technique of displaying data signals as a raster of lines which have varying levels of blackness which correspond to data-signal voltage changes. Facsimile recording is commonly referred to as C-scan recording. If no data is transmitted to the facsimile recorder, a uniform light or dark (depending on preference) line or series of lines (raster) would be recorded. In the case of light rasters, the incoming data signal would produce areas of different darkness. The darkness would be dependent on the incoming data signal. Facsimile recorders are used in conjunction with scanning mechanisms and scan rates, and locations are synchronized with the facsimile recorder to present an image of the object variances. Figure 7.3 illustrates a typical facsimile recording.

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Figure 7.2-Commercial Strip Chart Recorder (Gould Instruments)

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Figure 7.3-Facsimile Recording of Saw-cut Specimen (Copyright, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA. 19103. Reprinted, with permission.)

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Another common type recorder is the X路Y recorder. X-Yrecorders are usually used to present scanning type data. In X-Y systems, only data signals are printed; no raster is produced in a conventional X-Y recorder system. Magnetic tape recorders, usually frequency-modulated multichannel types, are used to provide a permanent record of test results. In the case of eddy current equipment with X路Y outputs, quadrature information is recorded and played back into analyzers for post inspection analysis.

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COMPUTERS Computers may be used to control data acquisition and analysis processes. Data handling techniques take a wide variety of approaches. Dodd and Deeds describe a computer-controlled multifrequency system. Figure 7.4 shows a computer-controlled eddy current system. Figure 7.4- Computer-controlled Eddy Current System (Oak Rid.ge Nationa l Laboratory, No. 1747-49)

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Chapter 7 REVIEW QUESTIONS

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0.7-1 Display requirements are based on: A. test applications. B. records requirement. C. need for automatic control. D. all of the above. Q.7-2 Amplitude gates provide a method of controlling: A. reject or acceptance limits. B. instrument response. C. display amplitude. D. all of the above. Q.7-3 Alarms and lights offer only: A. qualitative information. B. quantitative information. C. reject information. D. accept information.

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Q.7-4 The galvanometer principle is the basis for ____ A. corrosion rates B. metallographic deterioration C. a voltmeter D. light source illumination Q.7-5 In order for analog information to be presented to a digital computer, it must be processed through _______ _ A. an A-D converter B. a microprocessor C. a phase detector D. an amplitude detector 0.7-6 In a cathode ray tube, the electron gun: A. directs the beam. B. focuses the beam. C. generates the beam. D. all of the above.

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0.7路7 Photochemical reactions produced by electrons striking a CAT screen cause: A. photosynthesis. B. phosphorescence. C. fluorescence. D. both B and C. 0.7路8 High persistance CRT screens are normally used for repetitive signal display. A. True B. False 0.7-9 Length of a strip chart can indicate: A. flaw severity. B. distance or time. C. orthogonality. D. all of the above.

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0.7-10 A series of lines produced in facsimile recording is/are called: A. grid lines. B. raster. C. crosshatch. D. sweep display.

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8. APPLICATIONS Electromagnetic induction and the eddy current principle can be affected in many different ways. These effects may be grouped by discontinuity detection, measurement of material properties, dimensional measurements, and other special applications. With discontinuity, or the flaw detection group, we are concerned with locating cracks, corrosion, erosion, and mechanical damage. The material properties group includes measurements of conductivity, permeability, hardness, alloy sorting or chemical composition, and degree of heat treatment. Dimensional measurements commonly made are thickness, profilometry, spacing or location, and coating or cladding thickness. Special applications include measurements of temperature, flow metering of liquid metals, sonic vibrations, and anisotropic conditions.

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FLAW DETECTION The theoretical response to discontinuities has been discussed in previous Sections of this guide. In this Section, some actual practice examples are given to enhance the understanding of applied theory. A problem common to the chemical and electric power industries is the corrosion of heat exchanger tubing. This tubing is installed in large vessels in a high density array. It is not uncommon for a 4 foot diameter heat exchanger to contain 3000 tubes· This high density and limited access to the inspection areas often preclude the use of other NDE methods. Heat exchanger inspection systems and results are described. In most of these cases, the severity of the discontinuity is determined by analyzing the eddy current signal phase and amplitude. ■ ■

The signal amplitude is an indicator of the discontinuity volume. The phase angle determines the depth of the discontinuity and also the originating surface (ID or OD) of that discontinuity. (See Figure 6.8, above)

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Phase angle and amplitude relationships are usually established by using a reference standard with artificial discontinuities of known and documented values. The geometry of real discontinuities may differ from reference standard discontinuities. This difference produces interpretation errors as discussed by Sagar. Placement of real discontinuities near tube support members causing a complex coil impedance change is also a source of error. This, of course, is dependent upon the size of the discontinuity and its resultant eddy current signal in relation to the tube support signal. This follows the basic principle of signal-to-noise ratio. The signal-to-noise ratio can be improved at tube to tube support intersections by the use of multi-frequency techniques.

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In multifrequency applications, an optimum frequency is chosen for response to the tube wall and a lower than optimum frequency is chosen for response to the tube support. The two signals are processed through comparator circuits called mixers where the tube support response is subtracted from the tube wall response signal, leaving only the response to the tube wall discontinuity. Another industry that uses eddy current testing extensively is the aircraft industry. Many eddy current examinations are conducted on gas turbine engines and airframe structures. A common problem with gas turbines is fatigue cracking of the compressor or exhaust turbine blade roots.

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Usually these inspections are performed with portable instruments with meter response capability. The meter response is compared to the response of known discontinuities in a reference specimen. A determination is then made of the part's acceptance. The reference specimen and its associated discontinuities are very critical to the success of the test. Often models are constructed with artificial discontinuities that are exact duplicates of the item being inspected. The low frequency eddy current inspection of aircraft structures is explained by D.J. Hagemaier. The low frequency (100 - 1000 Hz) technique is used to locate cracks in thick or multiple layer, bolted or riveted aircraft structures. Again, models are constructed with artificial cracks, and their responses are compared to responses in the actual test object. Pulsed eddy current systems also are used for crack detection in thick structures.

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DIMENSIONAL MEASUREMENTS Dimensional measurements, such as thickness, shape, and position, or proximity of one item to another, are important uses of the eddy current technique. Often materials are clad with other materials to present a resistance to chemical attack or to provide wear resistance. Cladding or plating thickness then becomes an important variable to the serviceability of the unit. For nonconductive coatings on conductive bases, the "probe-tospecimen spacing", or lift-off technique can be applied. The case of conductive plating or cladding on conductive bases requires more refinement. The thickness loci respond in a complex manner on the impedance plane. The loci for multilayered objects with each layer consisting of a material with a different conductivity follow a spiral pattern. In certain cases, two frequency or multifrequency systems are used to stabilize results or minimize lift-off variations on the thickness measurement

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The depth of case hardening can be determined by measuring the nitride case thickness in stainless steel. The nitride case thickness produces magnetic permeability variations. The thicker the nitride, the greater the permeability. The coil's inductive reactance increases with a permeability increase. This variable is carefully monitored and correlated to actual metallographic results. Eddy current profilometry is another common way to measure dimensions; for example, the measurement of inside diameters of tubes using a lift-off technique. For this measurement, several small probe coils are mounted radially in a coil form. The coil form is inserted into the tube and each coil's proximity to the tube wall is monitored. The resultant output of each coil can provide information about the concentricity of the tube. An obvious problem encountered with this method is cantering of the coil holder assembly. The center of the coil holder must be near the center of the tube. When inspecting for localized dimensional changes, a long coil holder is effective in maintaining proper centering. Another function of the long coil form is to keep the coils from becoming "cocked" or tilted in the tube.

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CONDUCTIVITY MEASUREMENTS Conductivity is an important measured variable. In the aircraft industry, aluminum is used extensively. Aluminum conductivity varies not only with alloy but also with hardness and tensile strength. Eddy current instruments scaled in % IACS are normally used to inspect for conductivity variations. Secondary conductivity standards are commonly used to check instrument calibration. Common secondary conductivity standards range from 8% IACS to approximately 100% IACS. The secondary standards are usually certified accurate within ± 0.35 percent or ± 1 percent of value, whichever is less. Temperature is an important variable when making conductivity measurements. Most instruments and standards are certified at 20°C. Primary conductivity standards are maintained at a constant temperature by oil bath systems. Primary standards are measured by precision Maxwell bridge type instruments. This circuit increases measurement accuracy and minimizes frequncy dependence of the measurement

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HARDNESS MEASUREMENTS Hardness of steel parts is often measured with low frequency comparator bridge instruments. The reference and test coil are balanced with sample parts of known hardness. As parts of unknown hardness affect the test coil, the instrument output varies. The amount of output variation depends upon the degree of imbalance created by the unknown test object hardness. Signal output is then correlated to test object hardness by comparing to known hardness samples of the same geometry. For example, if a cathode ray tube were used to display hardness information, the "balance" hardness could be adjusted to center screen, lower hardness values could appear below center, and higher hardness values could appear above center on the CRT.

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ALLOY SORTING Alloy sorting can be accomplished in the same comparator bridge manner as hardness. A major consideration in both cases is the selection of correct and accurate reference specimens. Since most eddy current instruments respond to a wide range of variables, the reference specimen parameters must be controlled carefully. Test object and reference specimens must be the same or very similar in the following characteristics: 1. geometry, 2. heat treatment, 3. surface finish, 4. residual stresses, and 5. metallurgical structure.

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In addition, it is advisable to have more than one reference specimen for backup in case of loss or damage. In the case of steel parts, they should be completely demagnetized to remove the effects of residual magnetism on instrument readings. As in most comparative tests, temperature of specimen and test object should be the same or compensated. Many other measurements can be made using eddy current techniques. The electromagnetic technique produces so much information about a material, its application is only limited by our ability to decipher this information.

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Chapter 8 REVIEW QUESTIONS

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0.8-1 Conductivity, hardness, and composition are part of the group. A. defect detection B. material properties C. dimensional D. special 0.8路2 Using an ID coil on tubing and applying the phase/amplitude method of inspection, a signal appearing at 90潞 on a CRT would be caused by: A. ID flaw. B. OD flaw. C. dent. D. bulge. 0.8路3 Discontlnuitles in heat exchangers at tube support locations are easier to detect because the support plate concentrates the electromagnetic field at that point. A. True B. False

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0.8路4 Using multifrequency techniques on installed heat exchanger tubing, a tube support plate signal can be suppressed by adding a ____frequency signal to the optimum frequency signal. A. low B. high C. A orB D. none of the above 0.8路5 In the aircraft industry, a common problem in gas turbine engines is: A. corrosion. B. fatigue cracking. C. vibration damage. D. erosion. 0.8-6 Thick or multilayered aircraft structures are normally inspected by: A. low frequency sinusoidal continuous wave instruments. B. high frequency sinusoidal continuous wave instruments. C. pulsed systems. D. A and C. Charlie Chong/ Fion Zhang

0.8路7 Response to multilayer varying conductivity structures follow _____ loci. A. orthogonal B. spiral c. linear D. stepped 0.8路8 Nitride case thickness can be monitored in stainless steel cylinders by measuring ____ _ A. conductivity B. dimensions C. permeability D. none of the above 0.8-9 Conductivity is not affected by temperature. A. True B. False 0.8-10 Residual stresses in the test part produce such a small effect that they are usually ignored when selecting reference specimens. A. True B. False Charlie Chong/ Fion Zhang

Chapter 9 REVIEW QUESTIONS

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0.9路1 A precise statement of a set of requirements to be satisfied by a material, product, system, or service is a----'--A. standard B. specification C. procedure D. practice 0.9路2 A statement that comprises one or more terms with explanation is a ____ _ A. practice B. classification C. definition D. proposal 0.9-3 A general statement of applicability and intent is usually presented in the _____ of a standard? A. summary B. scope C. significance D. procedure Charlie Chong/ Fion Zhang

0.9路4 Military Standards are designated by "MIL-C-(number}." A. True MIL-STD-1537A B. False 0.9路5 In the structure of ASME the subcommittee reports to the subgroup. A. True B. False 0.9路6 In example QA 3, personnel Interpreting results must be: A. Level I or higher. B. Level 11 or higher. C. Level IIA or higher. D. Level Ill.

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0.9-7 The prime artificial discontinuity used to calibrate the system described in QA 3 is: A. 20% ID B. 50% OD C. 100% D. 50% ID 0.9-8 In QA 3, equipment calibration must be verified at least ____ _ A. every hour B. each day C. every 4 hours D. every 8 hours 0.9路9 QA 3 specifies a maximum probe traverse rate of _______ _ A. 12"/sec B. 14"/sec C. 6"/sec D. not specified

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0.9路10 The system in QA 3 is calibrated with an approved standard that is traceable to ___ _ A. NBS B. ASME C. a master standard D. ASTM Q.9路11 In accordance with QA 3, tubes whose data are incomplete or uninterpretable must be A. reinspected B. reported C. reevaluated D. removed from service 0.9路12 Referring to QA 3, QA 4.1 is a ____ _ A. calibration form B. data interpretation table C. data report form D. certification form Charlie Chong/ Fion Zhang

Good Luck

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More Reading http://www.ndt.net/article/ecndt02/322/322.htm http://www.proprofs.com/quiz-school/story.php?title=eddy-current-practise

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