Eddy current in brief

Electromagnetic Testing Eddy Current in Brief

2014-December My ASNT Level III Pre-Exam Preparatory Self Study Notes 外围学习中

Charlie Chong/ Fion Zhang

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

Figure 1. Schematic of Eddy Current absolute probe

Charlie Chong/ Fion Zhang

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 port

Charlie Chong/ Fion Zhang

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

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

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

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

Charlie Chong/ Fion Zhang

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 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.

Charlie Chong/ Fion Zhang

The frequency selected affects not only: (1) the strength of the response from flaws and (2) the effective depth of penetration, but also (3) the phase relationship.

Charlie Chong/ Fion Zhang

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 testing 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.

Testing 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

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

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

The thinner the part the greater is the impedance (>R & >XL) Figure 4. Changes in conductivity curve due to thinning of a part Charlie Chong/ Fion Zhang

Figure 5. Changes in conductivity curve due to corrosion damage

Charlie Chong/ Fion Zhang

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.

Charlie Chong/ Fion Zhang

Figure 6. Typical response for samples of different conductivity

Charlie Chong/ Fion Zhang

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.

Keywords: Differential probes are relatively unaffected by lift-off as long as the elements are balanced

Charlie Chong/ Fion Zhang

Figure 7. Diagram of response of a differential probe over a defect

Charlie Chong/ Fion Zhang

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

Charlie Chong/ Fion Zhang

Birring NDE Center, Eddy Current Testing # 1 Basic Concept

https://www.youtube.com/watch?v=dxzsPzCnpVc

t r e g p n ex rri e Bi h T r.

M

Eddy Current testing and impedance plane display. Birring NDE Center is a NDT school in Houston that provides NDT training as per SNT-TC-1A. For training info see http://www.nde.com/training/

Charlie Chong/ Fion Zhang

https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang

Good Luck!

Charlie Chong/ Fion Zhang

Good Luck!

Charlie Chong/ Fion Zhang