Electromagnetic testing q&a 004 by Charlie Chong

Electromagnetic Testing Questions & Answers -004 Part 1 ACFM,EC & FL,RF Testing-Book(E) 2009 My ASNT Level III Pre-Exam Preparatory My Self Study Notes 5th August 2015

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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Petrochemical Applications

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http://www.nbcnews.com/storyline/ukraine-crisis/chemical-plant-near-donetsk-eastern-ukraine-hit-explosion-n302821

Greek Alphabet

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http://greekhouseoffonts.com/

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Fion Zhang at Shanghai 5th August 2015

http://meilishouxihu.blog.163.com/

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IVONA TTS Capable.

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http://www.ivona.com/en/

My Mangoes

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My Mangoes

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Conductivity

Magnetic (Permeability & Dimensions)

A. Heat treatment give the metal

A. Length of the test sample

B. Cold working performed on the metal

B. Thickness of the test sample

C. Aging process used on the metal

C. Cross sectional area of the test sample

D. Hardness E. Crack & discontinuities

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ACFM-Alternating Current Measurement Testing

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ACFM Level I Q&A

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Answers to ACFM Level I 1.d 2.a 3.d 4c 5.b 6.c 7.a 8.d 9.c 10.d 11.b 12.a 13.c 14.d 15.a 16.b 17.c 18.c 19.b 20.c 21.d 22.a 23.b 24.d 25.a 26.a 27.b 28.b 29.d 30.b 31.c 32.a 33.b 34.b 35.a 36.c 37.d 38.d 39.a 40.c 41.b 42.a 43.b 44.d 45.d 46.b 47.a 48.c 49.d 50.c

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1. Which of the following frequencies has the least depth of penetration? a. 5 kHz b. 50 kHz c. 5 Hz d. 5 MHz B.2.23/D.318 2. Which of the following wires will have the greatest resistance? a. 0.158 cm (0.062 in.) diameter b. 0.318 cm (0.125 in.) diameter c. 0.635 cm (0.250 in.) diameter d. 1.27 cm (0.500 in.) diameter E.15

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3. Wbich of the following cable lengths will have the greatest resistance? a. 15.2 cm (6 in.) b. 45.7 cm (18 in.) c. 61.0 cm (24 in.) d. 142.2 cm (56 in.) E.15 4. Wbich of the following materials has the greatest depth of penetration a. aluminum b. copper c. stainless steel d. steel B.2.24/D.318

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5. When the permeability (μ) of a material is increased, the depth of penetration does which of the following? a. increases b. decreases δ=1/ √(πfμσ) c. remains constant d. increases the conductivity D.319 6. What is required in one circuit to change the current in an adjacent circuit? a. a continuous current must flow b. the resistance in both circuits must increase c. there must be a change in the current in the original circuit d. the magnetic field remains constant B.2.4/D.315

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7. What is the effect of increasing the number of turns of wire in a coil? a. the magnetic field produced will increase b. the magnetic field produced will decrease c. the magnetic field will remain constant Ф=N∙I/S, S= reluctance d. there will be a change in the electric current E.14/H.331·332 8. What is the symbol for conductivity? a. V b. Ω c. μ d. σ E.57

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Reluctance. Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance. The unit for magnetic reluctance is inverse henry, H-1. In a DC field, the reluctance is the ratio of the "magnetomotive force” (MMF) in a magnetic circuit to the magnetic flux in this circuit. In a pulsating DC or AC field, the reluctance is the ratio of the amplitude of the "magnetomotive force” (MMF) in a magnetic circuit to the amplitude of the magnetic flux in this circuit. (see phasors)

S = N∙I/Ф, Ф = N∙I/S, F =N∙I where S is the reluctance in ampere-turns per weber (a unit that is equivalent to turns per henry). F is the magnetomotive force (MMF) in ampere-turns Ф is the magnetic flux in webers. "Turns" refers to the winding number of an electrical conductor comprising an inductor 1/Henry = ampere/ weber, Henry= Weber/ampere?

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http://en.wikipedia.org/wiki/Magnetic_reluctance

9. Wbat is the symbol for permeability? a. H b. B c. Îź d. I E.47

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10. Which of the materials having the following relative permeability (Îźr) values will be the most easily magnetized? a. 60 b. 240 c. 800 d. 1000 E.47,62 11. When an electric current flows through a conductor, in which direction is the magnetic field set up? a. parallel to the conductor b. at 90 degrees to the current flow c. in the opposite direction to the current flow d. in the same direction as the current flow E.43

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12. If the current passing through a fixed number of turns is increased, what will be the effect on the magnetic field? a. the magnetic field will increase Đ¤ = NI/S b. the magnetic field will decrease c. the magnetic field will remain constant d. the magnetic field will change direction D.315 13. Which of the following has no effect on the inductance of a coil? a. increasing the number of turns b. increasing the coil diameter L = ÎźN2A/l https://en.wikipedia.org/wiki/Inductance c. increasing the current d. decreasing the number of turns E.13

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14. The depth of penetration in a material will be reduced by: a. increasing the permeability b. increasing the conductivity c. increasing the frequency of the test d. all of the above B.2.22-23/E.84 15. When a primary coil is brought close to a conductive material, what is the effect on the secondary magnetic field? a. it will be opposite to the primary magnetic field b. it will be opposite to the primary electric field c. it will be smaller but in the same direction d. it will be larger but in the same direction 8.2.2/0.319

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16. What is the coupling medium between an alternating current field measurement probe and a metal surface when operating in air? a. an electric current b. magnetic field c. none d. air A.250(Fig. 1) 17. "What generates a magnetic field in the Bz direction? a. uniform surface currents b. increasing surface currents c. curvature in the X and Y surface currents d. decreasing surface currents F.1

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18. What is the predominant electromagnetic property of a ferromagnetic material? a. resistance b. conductance c. high permeability d. a high frequency constant E.44

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19. If a current is flowing in a surface, then the magnetic flux density in the X direction is proportional to: a. the current in the X direction b. the current in the Y direction c. the curvature of the current in the X-Y plane d. the magnetic flux direction in the Z direction A.251 20. In what way does the magnetic field used for magnetic particle inspection differ from the one produced during alternating current field measurement inspection? a. it is equal and opposite b. it is in the same direction c. it is at 90 degrees to the alternating current field measurement magnetic field d. it is at 90 degrees to the surface of the material A.230,251

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20. In what way does the magnetic field used for magnetic particle inspection differ from the one produced during alternating current field measurement inspection? a. it is equal and opposite b. it is in the same direction c. it is at 90 degrees to the alternating current field measurement magnetic field d. it is at 90 degrees to the surface of the material The induced magnetic field run A.230,251 parallel to defect inducing B z

signals at crack tips â&#x20AC;&#x201C; indicating crack length, Bx indicating depth.

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The perturbated uniform electric field and the induced magnetic fields Bx, Bz

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The perturbated uniform electric field and the induced magnetic fields Bx, Bz

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Experiences in the use of ACFM for Offshore Platform Inspection in Brazil

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http://www.ndt.net/article/wcndt00/papers/idn233/idn233.htm

Simple Semi Elliptical Model

Bmax = a ? Charlie Chong/ Fion Zhang

http://www.shipstructure.org/pdf/91symp22.pdf

The Bx & Bz Display

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21. What is the symbol for resistance? a. H b. V c. I d. R E.15 22. Which of the following materials has the smallest depth of penetration for an applied AC magnetic field? a. mild steel b. stainless steel c. aluminum d. copper 8.2.22-24/0.318

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23. Which of the following best describes stainless steel? a. high permeability- low conductivity b. low conductivity- low permeability c. high conductivity - low permeability d. high permeability- high conductivity B.2.19,24

24. Which of the following statements is correct? a. the conductivity of a metal is proportional to the applied magnetic field b. the resistance in a circuit does not affect the current through the circuit c. a varying electric current cannot produce a current in an adjacent circuit d. the magnetic field produced by a current is proportional to the magnitude of the current Đ¤= NI/S E.13

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25. If it is required to generate a uniform electric field parallel to the weld, what direction will the magnetic field have to be to generate this electric field? a. at 90 degrees to the weld and parallel to the surface b. parallel to the surface and parallel to the weld c. a circular coil above the surface parallel to the weld d. a circular coil above the surface perpendicular to the weld A.251 26. If a current is flowing in a surface and the magnetic flux density is in the Y direction, what change would produce an effect? a. a change in the current in the X direction b. a change in the magnetic flux density in the X direction c. a nonmetallic coating d. a change in the current in the Z direction A.251

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27. Which of the following NDT methods or techniques can be used to estimate crack depth? a. MPI b. AC field measurement c. dye penetrant inspection d. ultrasonic creep wave inspection A.253

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28. During an alternating current field measurement weld inspection, the measurement of crack depth requires the following information: a. the frequency of the inspection b. the length of the crack c. the depth of penetration of the AC field d. all of the above F.8 Note: frequency f is not important!

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29. What is the indication produced in the magnetic field component representing crack depth that shows that a crack is present? a. the magnetic field is uniform b. the magnetic field is zero c. the magnetic field value is greater than the background field value d. the magnetic field value is lower than the background value F.6

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30. For accurate depth sizing when using electrical or electromagnetic inspection methods, what must the relationship between the crack length and depth be? a. they must be equal b. the length must be equal to or greater than twice the depth of the crack c. the crack depth must be more than 1.3 cm (0.5 in.) d. the crack length must be less than 5.1 cm (2.0 in.) A.258

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31. When a crack is detected in a ferrornagnetic material in an applied uniform magnetic field parallel to the weld and the crack is inclined to the toe of the weld, a magnetic field perpendicular to the surface (Bz) is produced at the crack ends. What form does the field take? a. a -ve peak at both ends b. a +ve peak at both ends c. a large +ve peak at one end and a small -ve peak (trough) at the other end d. large +ve peaks at each end and a â&#x20AC;&#x201C;ve peak in the center F.3 clock-wise current rotation gives peak (+ve) â&#x20AC;&#x201C; at weaker magnitude

Current flow

magnetic field parallel to the weld

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weld

8.9 Crack Geometry Effects: 8.9.1 A discontinuity at an angle to the scanâ&#x20AC;&#x201D;a discontinuity at an angle to the scan will reduce either the peak or the trough of the Bz as the sensor probe only passes through the edge of one end of the discontinuity. This produces an asymmetric X-Y plot. Additional scans may be made along the weld or parent plate to determine the position of the other end of the discontinuity.

clock-wise current rotation gives peak (+ve) â&#x20AC;&#x201C; at weaker magnitude

Current flow

magnetic field parallel to the weld

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weld

The Bx & Bz Display

magnetic field

current flow

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32. For accurate depth sizing, why must the length of a crack always be greater than twice the depth during alternating current field measurement inspection? a. to give sufficient current flow under the crack center b. the depth of the crack depends on the length c. no current will flow if the crack is too short d. to give sufficient current flow around the crack ends A.258 33. What effect would you expect if the coils in an AC field measurement probe were smaller and closer together? a. no effect b. improved detection of smaller discontinuities c. improved sizing on larger discontinuities d. smaller Bx signals A.255

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34. When using the alternating current field measurement inspection technique, what is the most important value that is measured? a. phase angle b. amplitude of the Bx magnetic field c. amplitude of the Bz magnetic field d. depth of the crack A.251

the most important value that is measured ?

Bx L

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35. If a hysteresis loop were produced for a conductive material and the loop was wide, it could be said that the material has: a. low permeability and would be difficult to magnetize b. high permeability and would be easy to magnetize c. high permeability and would be difficult to magnetize d. none of the above E.47

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http://www.ndt.net/article/v08n09/wong/wong.htm

36. The AC field measurement technique differs from other crack detection technologies in that it measures: a. the through thickness dimension b. the length only of a crack ? c. the length of the crack beneath the surface d. detection only A.253 37. Electromagnetic test methods can be used for which of the following applications? a. cracks in nonmagnetic materials b. cracks in magnetic materials c. cracks in coated materials d. all of the above A.13,424/D.314

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37. Electromagnetic test methods can be used for which of the following applications? a. cracks in nonmagnetic materials b. cracks in magnetic materials c. cracks in coated materials d. all of the above A.13,424/D.314

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38. In which of the following environments can AC field measurement inspections be performed? a. water b. air c. elevated temperatures d. all of the above A.248,385/G.214 39. If a transverse discontinuity is detected in a longitudinal weld, the Bx indication: a. produces an increase in the magnetic field b. produces a reduction in the magnetic field c. has no effect on the magnetic field d. reduces the magnetic field to zero F.3

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40. When an electric current flows down a wire, what is associated with it? a. an electric current flowing in the opposite direction b. a cooling effect c. a magnetic field at right angles to the current d. a magnetic field parallel to the electric current B.2.2 41. Which of the following definitions most closely defines Ohm's law? a. the potential differences in a circuit =1 b. the current in a circuit is proportional to the potential difference across it c. the magnetic field produced by a current is proportional to the magnitude of the current d. the frequency of an AC system is determined by its inductance E.15 V=IR

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42. What is the symbol for electric current? a. I b. C c. L d. F E.14 43. The resistance of a direct current circuit is equivalent to which quantity in an alternating current circuit? a. inductive reactance b. impedance c. reluctance d. conductivity E.15,18

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44. Which NDT method below cannot be used to estimate crack depth? a. alternating current potential drop (ACPD) b. time-of-flight ultrasonics c. alternating current field measurement d. dye penetrant inspection A.S-9,253 45. Variations in scanning speed: a. could influence the detection of small discontinuities b. will change the amplitude of small discontinuities c. will have no direct effect on results d. both a and b F.7

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46. For which of the following inspection requirements could you not normally use alternating current field measurement for discontinuity detection? a. fatigue cracks in welded joints of high strength steel b. subsurface cracks in welded joints of mild steel c. inspection for cracks in a weld of high strength steel under an epoxy paint coating d. surface fatigue cracks in mild steel welds A.248,252-258 47. IACS is the recognized abbreviation for: a. International Annealed Copper Standard b. Induced Alternating Current System c. Inductively Activated Comparison System d. Internal Applied Current System B.2.15/E.57

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48. When alternating current field measurement test coils are wound to form small diameter coils, what would they normally be used to detect? a. gradual changes in crack length b. gradual changes in crack depth c. small surface discontinuities d. changes in permeability A.255 49. What term is used to describe the variation in the Bx display as the distance from the surface being tested to the coil varies? a. phase change b. fill factor c. edge effect d. lift-off A.252

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50. Alternating current field measurement cannot be used to inspect: a. steel b. aluminum c. wood d. copper A.248-249

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Recalling the mistakes

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ACFM Level II Q&A

O TW

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Answers to ACFM Level II 1.d 2.a 3.b 4.d 5.d 6.b 7.a 8.d 9.b 10.c 11.b 12.c 13.a 14.c 15.d 16.a 17.d 18.b 19.c 20.a 21.a 22.a 23.d 24.c 25.a 26.b 27.c 28.a 29.d 30.d 31.a 32.d 33.a 34.c

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1. The Bz, magnetic field can best be described as: a. magnetism perpendicular to the plate surface b. perpendicular to the current flow c. produced by curvature of the current d. all of the above A.251

Bz Bz

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2. The Bx, magnetic field is best described as: a. magnetism parallel to the plate surface b. 90째 to the longitudinal weld c. parallel to the electrical current d. zero A.251 3. If the Bx chart recorder trace is low on the graph: a. there is a high current density b. there is a low current density c. the electric current is high d. the Bx flux density is high A.251

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4. If a current is flowing in a surface (called the x-y plane), then: a. the magnetic flux density in the x direction is proportional to the current in the y direction b. the magnetic flux density in the y direction is proportional to the current in the x direction c. the magnetic flux density in the z direction (out of the x-y plane) is proportional to the curvature of the current in the x-y plane d. all of the above A.251

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5. When measuring the length of a crack with alternating current field measurement, why can you not use the length between the Bz positive and the Bz negative positions? a. this is not the true length of the crack b. the Bz locations are produced by circular current flow and will produce Bz locations just inside the length of the crack c. this size will be smaller than that given by the AC field measurement program d. all of the above F.8

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6. For accurate sizing, why must the length to depth ratio of a crack being measured be at least 2:1? a. below this ratio more current would flow to the bottom of the crack than around the ends b. below this ratio more current would flow around the ends of the crack than to the bottom c. electricity will follow the path of greatest resistance d. no current will flow A.257-258 7. What electrical property has the greatest effect on the depth of penetration? a. permeability b. resistance c. voltage d. Conductivity δ = (πfμσ) -½ D.319

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8. "What are the disadvantages when using ACPD? a. it cannot be used on paint b. it requires a very clean surface for the injection prods to make contact c. it requires a very clean surface for the potential difference prods to make contact d. all of the above A.248 9. The axes of the coils used to measure the magnetic fields at the surface of a plate are mounted: a. perpendicular to the Bx and Bz fields b. parallel to the Bx and Bz fields c. in the same direction d. transverse to the direction of the crack A.251

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10. If alternating current field measurement is to be used to inspect through a nonconductive coating, what is the recommended maximum thickness of the coating to still ensure detection of a 2 mm (0.08 in.) deep discontinuity? a. 0.1 mm (0.004 in.) b. 1 mm (0.04 in.) c. 5 mm (0.196 in.) d. 25 mm (0.98 in.) A.254 11. If corrosion is present on the surface to be inspected, producing heavy pitting, what effect could it have on the inspection results? a. none b. there will be reduced sensitivity to small cracks and accuracy of sizing c. a small amount of surface noise could be produced d. the depth of the corrosion pits could be measured A.266

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FIGURE 7. Coating thickness at which magnetic flux density Bx amplitude drops to 1 percent for solenoid probes of three sizes.

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A.254

12. What is the effect of lift-off on a differential probe? a. one coil is affected and the other is not b. none of the coils is affected c. one coil changes positive and the other changes negative, thus cancelling each other (wound oppositely) d. a large signal is produced 0.316 13. What will happen if an absolute coil is brought close to the edge of a plate a. there will be a significant effect on the display b. there will be no effect c. the current will change direction d. the magnetic field will change direction 0.319

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14. What effect will produce a reduced apparent crack depth when using alternating current field measurement? a. a paint coating b. a corroded surface c. a crack, which has line contacts across it d. a material of varying thickness A.265-266 15. If an indication has been produced by excessive grinding, how can it be differentiated from a crack? a. carry out a transverse scan b. increase the speed of the scan c. increase the gain d. carry out parallel scans away from the Weld F.3

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16. Which of the following has no effect on crack sizing when using the alternating current field measurement technique? a. size of the slots in the test block b. multiple cracking c. an inclined crack d. line contacts F.5

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17. A large transverse crack in a weld may be indicated during longitudinal scanning by: a. a positive and negative Bz signal with no change in Bx b. a drop in the value of the Bx signal with no change in the Bz signal c. a longitudinal scan will not detect a transverse discontinuity d. a rise in the Bx with a peak and a trough in the Bz F.3

clock-wise current rotation gives peak (+ve) â&#x20AC;&#x201C; at weaker magnitude

Current flow

magnetic field parallel to the weld

Charlie Chong/ Fion Zhang

weld

8.9 Crack Geometry Effects: 8.9.4 Transverse discontinuitiesâ&#x20AC;&#x201D;if a transverse discontinuity occurs during the scan for longitudinal discontinuities then the Bx may rise instead of falling and the Bz signal will remain the same as for a short longitudinal discontinuity. The X-Y plot will then go upwards instead of down in the representative plot of Fig. 2. This flux leakage effect is, however, related to the opening of the discontinuity, so it may not be seen for tightly closed discontinuities. To confirm the presence of transverse discontinuities, further scans should be made with the probe orientated to give an induced field perpendicular to the weld, or through use of an array probe with twin fields. Bx Bz

FIG. 2 Example X-Y Plot Produced by Plotting the Bx (vertical) and Bz (horizontal) Together (The orientation of the plot may differ depending upon the instrumentation.) Charlie Chong/ Fion Zhang

18. The method of crack depth sizing used with alternating current field measurement corresponds most closely to which of tbe following? a. calibration against test blocks containing slots b. comparison with predetermined mathematical models c. calibration against lift -off d. multiplication of crack length values A.250 19. A signal is suspected to have come from a weld repair in the toe of the weld. What procedure(s) should be carried out? a. scan at right angles to the suspect area and the scan will be different from that of a crack b. scan over the repair in order to produce smaller Bz signals c. scan over the repair in order to produce signals that will cover a wider area d. all of the above F.3

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20. What is the value of the Bz, magnetic field when a uniform field is applied to a flat surface of ferritic steel? a. zero b. less than the uniform field c. greater than the uniform field d. equal to the uniform field A.251 21. Which of the following is a limitation of crack sizing by AC field measurement? a. crack length is less than twice the depth b. crack length is less than 10 mm (0.39 in.) c. crack depth is greater than 10 mm (0.39 in.) d. crack depth is greater than tbe depth of penetration A.253,258

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22. When tbe permeability of a material is increased, the depth of penetration does which of the following? a. decreases b. remains constant c. becomes less inductive d. increases D.318-319 23. "What is the dominant electromagnetic property of a ferromagnetic material? a. low conductivity b. low electrical resistance c. low frequency constant d. high permeability D.319

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24. High and low current density results in a different distribution in the lines of flux. The number of lines in a unit area is defined as: a. magnetic coupling b. magnetic density c. flux density d. hysteresis density E.43 25. Which of the following frequencies will provide the greatest alternating current field measurement penetration in aluminum? a. 300Hz b. 10kHz c. 1kHz d. 3kHz D.318

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26. The coercive force of a permanent magnet would be_____ the coercive force of a soft iron electromagnetic core. a. less than b. greater than c. twice d. equal to H.175 27. It is believed that an AC field measurement instrument is operating defectively. What is the proper course of action? a. all of the rejected material should be retested b. all of the accepted material should be re tested c. all material should be retested from the time of the last function's test-block test d. none of the above F.6

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28. What is the coupling medium between a coated metal surface and an alternating current field measurement probe when operating under water? a. magnetic field b. sea water c. electric current d. none F.2 29. Which of the following statements is incorrect? a. the resistance in a circuit affects the current through the circuit b. the magnetic field produced by a current is proportional to the magnitude of the current c. a varying magnetic field can produce a current in a conductor d. the conductivity of a metal is proportional to the applied magnetic field B.2.16

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30. Which of the following material properties governs the total flux in a magnetic circuit? a. the hardness b. the resistance c. the conductivity d. the reluctance Đ¤=NI/S 0.319 31. An eddy current is a circulating electrical current induced in a conducting material by: a. an alternating magnetic field b. a piezoelectric force c. X-rays d. none of the above 0.315

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32. Alternating current field measurement would have the deepest depth of penetration in which of the following materials? a. 90-10 Cu/Ni b. mild steel c. aluminurn d. stainless steel D.318 33. What generates a magnetic field perpendicular to the surface? a. uniform surface currents b. increasing surface currents c. curvature in the surface currents (generate Bz; peak & trough) d. decreasing surface currents A.251

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34. When scanning into tight geometry: a. the Bx signal will decrease b. the Bz signal will decrease c. the Bx signal will increase d. the Bz signal will increase F.3

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Recalling the mistakes

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ACFM Level III Q&A

e e r Th

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Answers to ACFM Level III 1.a 2.d 3.b 4.d 5.a 6.b 7.c 8.a 9.b 10.c 11.d 12.a 13.b 14.d

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1. In an alternating current field measurement technique system, a calibration standard may be used to: a. ensure repeatability and reliability of the setup b. calibrate the precise depth of detectable discontinuities c. both a and b d. measure the test frequency F.5 2. Skin effect causes eddy currents to tend to flow near the surface of the test piece. Which of the following factors alter the skin effect? a. testing frequency b. test piece temperature c. test piece hardness d. all of the above A.211

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3. Which of the following is not a common undesirable effect on the test coil caused by the testing environment? a. temperature variation b. crack in the test sample c. test object making contact with the test coil d. foreign object in the test coil field e. test coil vibration C.21-25 4. In many cases, AC field measurement can be reliably applied: a. underwater b. via remotely operated vehicles c. on railroad car axles d. all of the above A.260-261

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5. For AC field measurement discontinuity sizing techniques using standard theoretical models to produce the sizing tables, what is the common assumption? a. the input current is unidirectional and of uniform density b. the input current is omni-directional with varying density c. all discontinuities are axial surface cracks d. all discontinuities are subsurface cracks A.257-258 6. The reliability, as well as the ability, of AC field measurement to detect discontinuities has been specifically determined by: a. railroad cars and tracks not failing for years after AC field measurement examinations b. extensive trials carried out on realistic discontinuities under controlled conditions c. the number of satisfied customers around the world d. repeated field use around the world A.260

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7. A large, uniform input field in AC field measurement means: a. an improved focal point for localized detection b. increased ease of discontinuity sizing c. that the current perturbation from a discontinuity extends some distance away from the line of a discontinuity d. an increased probability of detection A.262

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Current Perturbation

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http://www.ndt.net/article/v05n03/topp/topp.htm

8. It is reasonable to expect that a discontinuity will be detected by AC field measurement if the Bx signal amplitude is: a. 1 percent or more b. 10 percent or more c. 80 percent or more d. the Bx signal has no influence on detection A.263 9. For AC field measurement inspections, probe/solenoid size: a. has no effect on the examination results b. should be a consideration while developing a successful AC field measurement technique c. is important only for underwater examinations d. all AC field measurement probes/solenoids are built the same A.263

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10. The false call rate for AC field measurement is: a. normally high b. very dependent on the underwater depth c. generally low d. greater than all other NDT techniques A.264 11. While performing AC field measurement, one material where particular care has to be used to avoid false calls is: a. A-214 CS b. A-179 CS c. 304 ss d. duplex steel A.264

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12. AC field measurement was originally designed for the detection and sizing of: a. fatigue cracks b. weld bursts c. intergranular attack (IGA) d. conductivity variations A.265

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13. Stress corrosion cracking (SCC) can form a dose series of cracks acting as a "colony“ or clusters of cracks. In these cases, as the signal response from AC field measurement can “superimpose” individual discontinuity signals on close neighboring discontinuity signals, which of the following statements is most correct? a. there is no reliable method for detection and sizing of SCC using AC field measurement b. in general, AC field measurement is a reliable method for detection and depth sizing of SCC c. AC field measurement can detect SCC but sizing models· are extremely inaccurate d. special calibration standards must be used when SCC is expected or encountered A.265

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14. With AC field measurement, a corrosion pit can look like a shallow crack on the initial scan. What could be an AC field measurement method for differentiating a crack from a pit? a. there is no reliable way to differentiate a crack from a pit using AC field measurement b. apply a mag-bias AC field measurement probe c. review the previous examination report d. scan the signal with different orientations of the interrogating signal; a pit will produce the same response regardless of probe/current orientation A.266

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Recalling the mistakes

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Eddy Current Testing

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Eddy Current Testing Level I Q&A

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Answers to Eddy Current Testing Level I 1.d 2.b 3.c 4.b 5.a 6.a 7.b 8.d 9.b 10.d 11.d 12.b 13.a 14.c 15.c 16.b 17.b 18.d 19.a 20.b 21.a 22.d 23.b 24.c 25.d 26.a 27.d 28.b 29.d 30.d 31.b 32.a 33.d 34.a 35.b 36.b 37.c 38.d 39.b 40.a 41.b 42.b 43.c 44.b 45.d 46.b 47.d 48.d 49.a 50.b 51.a 52.c 53.b 54.c 55.b 56.a 57.d 58.a 59.d 60.c 61.b 62.a 63.d 64.c 65.d 66.a 67.b 68.c 69.a 70.a 71.b 72.c 73.c 74.b 75.d 76.b 77.a 78.a 79.c 80.c 81.d 82.d 83.d 84.b 85.d 86.d 87.c 88.c 89.a 90.c 91.b 92.c 93.d 94.c 95.c 96.b

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1. In an electromagnetic testing situation, which of the following can provide sources of noise? a. geometrical changes b. permeability variations within the test object c. electrical interference d. all of the above B.7.26 2. The ratio of the response or amplitude from signals of interest to the response or amplitude of indications that contain no useful information for the test being conducted is referred to as: a. Poisson's ratio b. Signal to noise ratio c. the conductivity to permeability ratio d. the reactance to resistance ratio E.24

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3. Which of the following is not a basic component of an eddy current test system? a. amplifier b. sine wave generator c. couplant d. filter B.4.6 4. When conducting an eddy current test using a differential comparison coil arrangement that compares an external reference standard with the test specimen, the system should be nulled or balanced: a. with only the reference standard in one coil b. with the reference standard in one coil and an acceptable test specimen in the other coil c. with the reference standard in one coil and an unacceptable test specimen in the other coil d. with only the test specimen in one coil E.35

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5. The impedance of a test coil will increase if the: (XL=2Ď&#x20AC;fL) a. test frequency increases b. inductive reactance of the coil decreases c. inductance of the coil decreases d. resistance of the coil decreases B.1.22 6. Some of the products commonly tested using encircling coils are: a. rods, tubes and wire b. plate when volumetrically inspected c. sheets and metalized foil d. all of the above B.3.9/D.9/H.40.1

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7. Which of the following would normally be considered the best ftill factor when testing straight tubing with an encircling or feedthrough coil? a. 1.75 (175%) b. 0.95 (95%) c. 0.50 (50%) d. 0.25 (25%) C.47 8. When a metal part is placed in an eddy current test coil, the impedance of the coil will be changed by which of the following properties of the part? a. conductivity (electric factor) b. dimensions (magnetic factor) c.permeability (magnetic factor) d. all of the above C.67

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9. An eddy current test makes use of electrical current induced in a conductive article by: a. gamma rays b. an alternating magnetic field c. a piezoelectric force d. all of the above C.11/G(Vol. 1).1.25 10. The conductivity of a material can be altered by changing the: a alloy of the specimen b. heat treatment of the specimen c. temperature of the specimen d. all of the above B.5.2/E.58

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11. The depth to which eddy currents are induced in the material will be determined by the: a. conductivity of the material b. permeability of the material c. geometrical shape of the material (FOM, figure of merit?) d. all of the above δ = (2/ωμσ)½ = (πfμσ) -½ 8.2.18/0.3

12. When testing plate with a probe coil, it is noted that the eddy current output indication varies as the distance from the coil to the surface of the test part varies. The term used to describe this action is: a. fill factor b. lift-off c. phase differentiation d. edge effect E.89 13. The main purpose for spring-loaded eddy current probe coils is to: a. minimize lift -off variations b. minimize wear on the probe c. reduce operator fatigue d. eliminate edge effect B.3.2

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14. Lift-off is utilized in: a. measuring permeability changes b. measuring conductivity changes c. measuring the thickness of non conductive coatings d. determining proper test frequency B.5.13 15. In eddy current test systems where encircling coils are used, coupling efficiency is referred to as: a. lift-off b. edge effect c. fill factor d. phase differentiation C.45

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16. When inspecting products with a uniform cross section, an eddy current signal is produced when the leading end or trailing end of the product approaches the test coil. This phenomenon is referred to as: a. lift -off b. end effect c. fill factor d. phase discrimination E.62

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17. Which of the following conditions would be the most difficult to detect when eddy current testing a rod with an encircling coil? a. a deep surface crack that has a depth of 30% of the rod diameter b. a small inclusion in the center of the rod c. a 5% change in diameter d. a 10% change in conductivity B.5.13-14 18. When testing tubing with a bobbin coil, most of the eddy currents: a. flow in a longitudinal direction down the tubing b. flow radially in the tube c. flow around only the outside diameter of the tubing d. flow around the inside diameter of the tubing E.32

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19. Which of the following test frequencies would produce eddy currents with the largest depth of penetration? a. 100Hz δ = (2/ωμσ)½ = (πfμσ) -½ b. 10kHz c. 1 MHz d. l0MHz C.43 20. A test coil's opposition to the flow of alternating current, causing the current to Jag behind the voltage, is called: a. resistance b. inductive reactance (current lag voltage by π/2 or 90º) c. impedance d. capacitive reactance B.1.14,17

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Inductive reactance (XL) in terms of frequency and inductance is given by: XL = ω∙L = 2πf∙L Similarly the Capacitance Reactance: XC = 1/(ω∙C) = 1/ (2πf ∙C) Inductive reactance is directly proportional to frequency, and its graph, plotted against frequency (ƒ) is a straight line. Capacitive reactance is inversely proportional to frequency, and its graph, plotted against ƒ is a curve. These two quantities are shown, together with R, plotted against ƒ in Fig 9.2.1 It can be seen from this diagram that where XC and XL intersect, they are equal and so a graph of (XL − XC ) must be zero at this point on the frequency axis.

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http://www.learnabout-electronics.org/ac_theory/lcr_series_92.php

Capacitor circuit: Current lead voltage by 90o

Inductor circuit: Current lagging voltage by 90o

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21. An increase in the impedance (Z) of a test coil will: a. produce an increase in the voltage through the test coil b. cause a decrease in the measured voltage across the test coil c. not affect the voltage in the test coil d. decrease the voltage applied to the coil (V=IZ?) C.32

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22. Which of the following will affect the impedance of a test probe? a. conductivity of a test specimen in the coil b. permeability of a test specimen in the coil c. fill factor d. all of the above B.2.11,34

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23. When a test coil consists of a double winding arrangement and one winding is referred to as a primary winding, the other winding is referred to as the: a. absolute winding b. secondary winding c. phase winding d. none of the above B.3.16 24. Large diameter surface scanning coils would most likely be used for tbe detection of: a. pitting b. small inclusions c. changes in conductivity d. porosity A.136/G(Vol. 11).1.32

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25. Small diameter probe coils would typically be used for tbe detection of: a. slow changes in dimensions b. gradual changes in conductivity c. variation in heat treatment d. small surface discontinuities C.54/G(Vol. 11).1.32 26. Direct current saturation coils would most likely be used when testing: a. steel b. aluminum c. copper d. brass B.2.13-14

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27. An absolute coil arrangement can be used in: a. encircling coils only b. probe coils only c. probe and encircling coils only d. probe, encircling and inside diameter coils B.3.11 28. Which of the following expresses the primary purpose( s) of the secondary winding(s) in an eddy current coil ? a. induce eddy currents in the test specimen b. detect changes in the eddy current flow c. induce eddy currents in the test specimen and detect changes in the eddy current flow d. provide DC saturation B.3.16

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29. Which of the following eddy current test coil arrangements uses one area of the test specimen as a reference standard against which another area on the same specimen is simultaneously compared? a. single absolute coil b. double absolute coil c. DC saturation coil d. differential coil B.3.13

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30. Reference standards used for electromagnetic testing: a. must contain artificial discontinuities such as notches and drilled holes b. must contain natural discontinuities such as cracks and inclusions c. must be free of measurable discontinuities d. may contain artificial or natural discontinuities or be free of discontinuities depending on the test system and the type of test being conducted 8.6.7-9 31. A reference standard used to ensure that the amplitnde and phase characteristics of a test system do not drift during continuous testing is called a: a. DGS standard b. calibration standard c. reference block d. none of the above 8.7.26

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32. Which of the following could not be tested by the eddy current testing method? a. a 102 mm (4 in.) thick plate to be tested for small discontinuities throughout the plate b. tubing to be tested for surface cracks c. rod to be tested for laps and seams d. tubing to be tested for variations in outside diameter B.2.20 33. Which of the following is not true about an eddy current test? a. it can be used for high-speed testing b. it can accurately measure conductivity c. it can be set up to provide high sensitivity to small discontinuities d. it can penetrate up to 152 mm (6 in.) in a test part B.2.20

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34. The entire cross-sectional area of a tube can be evaluated at one time when using: a. an encircling coil b. a probe coil c. a secondary winding d. none of the above B.3.9 35. When attempting to determine the exact circumferential location of a discontinuity in a tube, the test system should include: a. a phase shifter b. a probe coil c. an attenuator d. a penetrameter C.54

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36. It is often possible to sort various non ferromagnetic alloys of a metal by means of an eddy current test when: a. there is a unique range of permeability values for each alloy b. there is a unique range of conductivity values for each alloy c. the direction of induced eddy currents varies for each alloy d. the magnetic domains for each alloy are different E.69 37. Which of the following products would be most applicable to a test using an inside coil? a. sheet b. rod c. bolt hole d. coating thicknesses 6.3.2

Charlie Chong/ Fion Zhang

38. When eddy current testing rod for discontinuities, which of the following conditions could produce extraneous indications? a. improper adjustment of the hold-down rollers used to center the rod in the coil b. feeding rod through the coil at an excessive speed c. changes in dimension d. all of the above e. none of the above E.24 39. One method of reducing non relevant indications when eddy current testing ferromagnetic materials is to: a. decrease the pressure on the hold-down rollers b. use a direct current saturation coil to magnetically saturate the test specimen c. readjust the gain control d. increase the rate of feed through in the coil E.62

Charlie Chong/ Fion Zhang

40. In order to generate measurable eddy currents in a test specimen, the specimen must be: a. a conductor b. an insulator c. either a conductor or an insulator d. a ferromagnetic material 6.2.4 41. When using an encircling coil with both primary and secondary windings, the excitation alternating current is applied to: a. the secondary winding b. the primary winding c. either the primary or secondary winding depending on the setting of an instrument control d. both the primary and secondary coils E.14

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42. Which of the following is not a readout system used to present eddy current test information? a. alarms b. litmus paper c. cathode ray tube d. digital data storage E.26 43. Which of the following statements best describes the selection of eddy current test frequency? a. the frequency must equal the f/fc ratio to give an accurate test b. the frequency must be within Âą3% of the f/fc ratio to give an accurate test c. there is a range of suitable frequencies centered around the optimum frequency d. the frequency should be within Âą25% of the characteristic frequency E.83

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44. ·Which of the following options would not have the potential to improve the displayed signal-to-noise ratio of a test? a. change to a test frequency that will decrease the noise b. increase the amplification of the test instrument c. improve the fill factor d. enable filter functions in the instrument E.24 45. When conducting an eddy current test on tubing, the magnitude of the indication caused by a discontinuity is dependent on: a. the depth of the discontinuity b. the width of the discontinuity c. the length of the discontinuity (for MFL Amplitude =k∙D/W where D=depth, W=width)? d. all of the above E.63

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46. A coil's magnetic field may be viewed as a distribution of lines of flux around the coil. The number of lines in a unit area is defined as: a. magnetic probability b. flux density c. magnetic coupling d. hysteresis density B.2.19/E.43 47. Demagnetization is generally needed when the residual field in a specimen: a. may affect the operation or accuracy of instruments when the specimen is placed in service b. may interfere with the proper functioning of the part c. might cause particles to be attracted and held to the surface of moving parts d. any of the above reasons could make demagnetization necessary J.67

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48. Eddy current test techniques can be used to: a. measure nonmetallic coating thicknesses on conductors b. measure cladding thicknesses (metal-to-metal) c. gage the thickness of conductive sheets d. all of the above B.5.13 49. Eddy currents cannot be induced in: a. latex paint b. aluminwn c. steel d. copper E.13

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50. Which of the following statements is not true of ferromagnetic materials? a. they have permeability values much greater than I b. they do not exhibit any magnetic hysteresis characteristics c. they will have a definite saturation point on a B/H curve d. they will exhibit some degree of residual magnetism C.18 51. Eddy currents always travel: a. in circular paths b. in nonconductive materials c. radially when testing rod with an encircling coil d. in a gas B.2.5

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52. For online eddy current inspections in a tubing mill, the effective DC saturation point of ferromagnetic materials is reached when the current applied to the saturation coil is set at a value where for any further increase in that current, the eddy current signal-to-noise ratio will: a. start to increase b. start to decrease c. show no significant change d. suddenly drop to zero B.2.13

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53. When sorting on the variation between two distinct alloys, it is desirable to hold all indications within the bounds of the readout device in order to: a. eliminate overloading b. detect the possible existence of a third alloy c. make proper balancing possible d phase the readings correctly A.275 54. A term used to define a standardized condition in a device or system which results in zero output is: a. high frequency standardization b. integration c. null or balance d. differentiation B.6.19/K.203

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55. A term used to describe holes, grooves, notches, etc., that are introduced into a reference standard to provide accurately reproducible quality levels is: a. a natural discontinuity b. an artificial discontinuity c. an ellipse d. none of the above B.6.9 56. A term used to define one or more turns or loops of a conductor wound in such a manner as to produce an axial magnetic field when current passes through the conductor is: a. a coil b. a resistor c. a capacitor d. an oscillator B.1.14/C.21

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57. Nondestructive tests are commonly conducted to: a. determine the quality of finished products b. locate defective material before excessive fabrication is performed on the material c. monitor material production techniques and processes d. all of the above A.2/1.7 58. An eddy current coil arrangement that does not make a comparison with either two sections of the test specimen or one section of the test part and a reference standard is called: a. an absolute coil arrangement b. a self-comparison differential coil arrangement c. an external reference differential coil arrangement d. none of the above B.3.12

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59. Which of the following are common applications for eddy current testing? a. measurement of conductivity or a combination of conductivity and permeability b. measurement of the thickness of thin metal sections cladding or coating c. detection of surface and subsurface discontinuities d. all of the above E.91

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60. Which of the following is not a primary purpose for using an eddy current standard? a. to determine if the eddy current test system is capable of performing the test properly b. to determine if a discontinuity is a cause for rejection c. to determine the instrument control settings needed to pass the highest percentage of test specimens run d. to determine if the sensitivity of a test system has drifted with time B.6.4

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61. Material must be reasonably centered in an encircling test coil of a discontinuity detection system because, even with an acceptable fill factor ratio: a. the coil would otherwise be out of balance b. parts of the material farthest away from the coil, radially, may receive a less sensitive inspection c: the flow of eddy currents around the product will be disturbed, resulting in improper penetration d. lack of proper centering may change the phase adjustment of the instrument B.3.10

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62. When you determine that a discontinuity detection instrument has been set up incorrectly, or is defective, all material: a. should be retested since the time that a correct setup or proper operation was last verified b. accepted should be retested c. rejected should be retested d. none of the above C.61 63. An eddy current system will measure variations caused by differences in: a. size or shape b. grade or chemistry of the material c. the way in which the material has been processed d. all of the above B.4.8

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64. A length of tubing containing a notch of uniform Width and depth running from end to end, when tested with an encircling self-reference differential coil system, will produce: a. erratic signals b. a continuous, sustained signal c. no signal change d. excessive signals B.3.13 65. In encircling coil eddy current testing, optimum sensitivity to a known discontinuity placed at random on the surface of a nonferrous length of tubing is obtained by: a. proper choice of frequency b. centering the tubing in the coil c. proper adjustment of phase d. all of the above D.10

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66. One principal advantage of spinning probe eddy current testing is: a. The ability to locate the exact position of discontinuities b. its relative insensitivity to permeability variations c. the variety of internal and surface discontinuities that may be detected d. all of the above C.54

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67. At any given instant, a spinning probe eddy current instrument should be inspecting: a. one complete circumference of the product b. an area defined by the size of the probe coil c. one longitudinal line the length of the product d. none of the above C.54 68. A spinning probe eddy current instrument would be most useful in: a. measuring the hardness of ferromagnetic steel products b. discontinuity detection in hexagonal and shaped material c. detecting and precisely locating surface discontinuities d. inspection of copper bus bars for electrical conductivity C. 54

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69. In a feed through encircling coil eddy current system, a calibration standard may be used to: a. ensure repeatability and reliability of the setup b. determine the circumferential extent of a discontinuity c. reduce sensitivity to vibration d. measure the test frequency C.61 70. What causes phase shift in an eddy current test coil? a. a change in the ratio of inductive reactance (XL) to resistance (R) b. a change in the sensitivity setting of the instrument c. use of modulation analysis d. none of the above C.32

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71. In a feedthrough encircling coil discontinuity detection eddy current system, what would be the purpose of running a calibration discontinuity several times but in various positions (such as top, bottom, left and right)? a. to check the phase selectivity b. to ensure proper centering of the material in the test coil c. to select the modulation analysis setting d. to select the proper operating speed B.3.10

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72. In a feedthrough encircling coil eddy current system, DC saturation would probably be most helpful in testing: a. copper water tubing b. titanium tubing c. ferromagnetic steel tubing d. brass rivet wire B.2.13 73. In an electromagnetic test system, the test frequency is controlled by the: a. oscilloscope b. sensitivity setting c. oscillator circuit d. modulation analysis setting A.175

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7 4. Variations in the chemical composition of a nonferromagnetic alloy would most likely affect its: a. permeability b. conductivity c. diameter d. frequency B.7.5 75. Which of the following frequencies will provide the greatest eddy current penetration in aluminum? a. 1kHz b. 10kHz c. 3kHz d. 300Hz C.43

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76. A symbol commonly used to express conductivity is: a. μ b. σ c. XL d. R C.Vii/E.57 77. A symbol commonly used to express permeability is: a. μ b. σ c. XL d. R E.47

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78. The method used to generate eddy currents in a test specimen by means of a coil can most closely be compared with the action of a: a. transformer b. capacitor c. storage battery d generator C.36

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79. When eddy current testing a nonferrous specimen, a discontinuity will: a. increase the effective conductivity of the specimen b. increase the effective permeability of the specimen c. decrease the effective conductivity of the specimen d. none of the above D.25 Comments: The presence of crack, decrease the effective conductivity of the specimen by the presence of break (discontinuity), reducing the eddy current flow; generating less secondary magnetic field that interact with the exciting/pick-up coil. The resistance component will decrease and the inductive reactance will increase as less energy is taken away from the exciting coil. ■ Decrease effective conductivity of specimen ■ Decrease effective resistance of exciting coil ■ Increase inductive reactance of exciting coil

Charlie Chong/ Fion Zhang

If the eddy current circuit is balanced in air and then placed on a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil, which shows up as resistance) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). If a crack is present in the material, fewer eddy currents will be able to form and the resistance will go back down and the inductive reactance will go back up. Changes in conductivity will cause the eddy current signal to change in a different way. Charlie Chong/ Fion Zhang

80. In multifrequency tubing inspection applications, signal analysis is most often performed using: a. phase angle analysis only b. signal amplitude analysis only c. a careful correlation of both amplitude and phase information d. the ellipse method B.86 81. When testing tubes with an ID bobbin probe: a. low voltage signals indicate insignificant discontinuity depths b. high voltage signals are always from rejectable discontinuities c. signals from support plates always hide discontinuity signals d. none of the above is true A.403

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82. A multicoil array probe used in a surface scanning application: a. should improve detection of localized discontinuities b. can cover a large surface area in one pass c. might increase the complexity of signal detection and interpretation d. all of the above B.8.14 83. Data from the external-reference differential channels of ID bobbin probe data can be used to detect: a. long, gradually tapering discontinuities b. one-sided erosion c. temperature differences d. all of the above D.14

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84. Self-comparison differential channels would be best for the detection of: a. long tapered discontinuities b. short abrupt discontinuities c. temperature differences d. all of the above B.3.13 85. Which of the following affects the characteristics of the primary coil in an eddy current system? a. the presence of a crack b. the presence of an edge c. change in material properties d. all of the above F.320

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86. Which of the following is applicable to eddy current testing? a. it can be used to provide high sensitivity to small discontinuities b. it can be used for high-speed testing c. it can be used to measure coating thicknesses d. all of the above F.314 87. A secondary magnetic field produced by eddy currents induced in a nonmagnetic conductor close to the inducing coils: a. induces a magnetic field parallel to the surface b. aids the coil's magnetic field c. opposes the coil's magnetic field d. decreases the frequency of the current F.319

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88. A test specimen used as a basis for calibrating test equipment or as a comparison when evaluating test results is referred to as a: a. null-balancer b. phase shifter c. reference standard d. high pass filter B.6.4/H.40.5 31. A reference standard used to ensure that the amplitnde and phase characteristics of a test system do not drift during continuous testing is called a: a. DGS standard b. calibration standard c. reference block d. none of the above 8.7.26

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89. The process of comparing the reading or output of an instrument, device or dial with a standard to determine the instrument's accuracy, capacity or graduations is referred to as: a. calibration b. differentiation c. integration d. phase shifting G(Vol. 11).2.13 90. Figure 1 is an illustration of a material's: a. phase analysis loop b. sine wave c. hysteresis loop d. none of the above C.19/H.33.3

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91. Ferromagnetic materials that have been magnetically saturated to suppress permeability variations may retain a certain amount of the magnetization. This is called: a. coercive force b. residual magnetism c. hysteresis loop d. hysteresis loss C.17-18 92. An alternating current flowing in a conductor will set up: a. an alternating current field around the conductor b. a periodically changing voltage tangent to the conductor c. an alternating magnetic field around the conductor d. none of the above B.2.4

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93. The characteristics of the alternating magnetic field in the vicinity of the AC coil are affected by: a. the coil parameters b. the magnitude of the applied AC current c. the frequency of the applied AC current d. all of the above I.346

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94. The region around a magnet that attracts other pieces of iron or steel is called: a. amaxwell b. retentivity c. the magnetic field d. alternating current H.30.2 95. Testing that requires that the test article be loaded and/or sectioned to verify and/or establish mechanical or physical properties is known as: a. impedance testing b. phase analysis testing c. destructive testing d. nondestructive testing H.4.1

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96. The depth of penetration would ____ if the localized permeability of a material increased. a. increase b. decrease c. remain constant d. increase the conductivity 8.2.22

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Recalling the mistakes

Charlie Chong/ Fion Zhang

Eddy Current Testing Level II Q&A

O TW

Charlie Chong/ Fion Zhang

Answers to Eddy Current Testing Level II 1.a 2.d 3.a 4.b 5.d 6.a 7.d 8.a 9.c 10.c 11.b 12.a 13.c 14.d 15.d 16.b 17.b 18.c 19.d 20.b 21.b 22.a 23.c 24.d 25.b 26.c 27.b 28.c 29.c 30.a 31.d 32.d 33.b 34.c 35.a 36.b 37.a 38.d 39.c 40.a 41.d 42.b 43d 44.c 45.c 46.d 47.d 48.b 49.b 50.d 51.a 52.d 53.b 54.d 55.c 56.b 57.b 58.a 59.d 60.a 61.a 62.c 63.c 64.c 65.a 66.a 67.c 68.a 69.b 70.d 71.b 72.a 73.b 74.a 75.c 76.c 77.d 78.a 79.a 80.c 81.a 82.a 83.b 84.c 85.d 86.d 87.b 88.c

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1. A method of minimizing the effect of indications caused by gradual dimensional changes while retaining the indications caused by discontinuities is to: a. enable a high-pass filter in the eddy current test equipment b. enable a low-pass filter in the eddy current test equipment c. increase the band pass of the amplifier d. use the impedance method of testing A.29,186 2. Which of the following methods may be used to improve the signal-to-noise ratio of an electromagnetic testing system? a. filtering or differentiation b. phase discrimination c. integration d. all of the above A.173

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3. In order to decrease the effect on conductivity readings due to variations in test part thickness: a. the test frequency should be increased b. the test frequency should be decreased c. the fill factor should be decreased d. there is no practical method for decreasing this effect A.54 4. When testing by the eddy current method, discontinuities will be most easily detected when the eddy currents are: a. coplanar with the major plane of the discontinuity b. perpendicular to the major plane of the discontinuity c. parallel to the major plane of the discontinuity d. 90 degrees out of phase with the current in the coil D.25/E.32

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5. A term used to describe the effect observed due to a change in the electromagnetic coupling between a test specimen and a probe coil when the distance of separation between them is varied is: a. fill factor b. edge effect c. end effect d. lift-off A.130,324 6. A term used to define the phenomenon where, at very high frequencies, the eddy current flow is restricted to an extremely thin outer layer of the conductor is: a. skin effect b. high-frequency filtration c. low-frequency filtration d. any one of the above A.131jD.23/K.8

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7. The fill factor when a 13 mm (0.5 in.) diameter bar is inserted in a 25 mm (I in.) diameter coil is: a. 0.5 (50%) b. 0.75 (75%) c. 1.0 (100%) [(D1/D2)2 , (.5/1)2 = 0.25] d. 0.25 (25%) D.24/E.89 8. Which of the following could be used to suppress unwanted high-frequency harmonics? a. low-pass filter b. oscillator c. phase discriminator d. high-pass filter A.186

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9.The impedance change of an eddy current test coil due to a change in test part characteristics can be most easily analyzed as a combined change in: a. capacitive reactance and resistance b. harmonic frequencies and inductive reactance c. signal amplitude and phase d. retentivity and harmonic frequencies E.65

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10. The inductive reactance of a test coil can be calculated using the formula: a. XL= 2πL b. XL = 0.5 π L c. XL= 2πfL d. X1.= IR E.18/H.36.1 (XL=ωL = 2πfL)

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11. When conducting, an electromagnetic test, variations in the test material can be detected as variations in: a. test speed b. impedance of a pickup coil c. none of the above d. both a and b A. 51-52 12. A term used to define two or more coils electrically connected in opposition such that any electromagnetic condition which is not common to both a test specimen and a reference specimen will produce a signal change in the system is: a. differential b. Absolute (absolute, differential, reflection) c. laminar d. flying probes A.52,129

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13. In a manufacturing environment, tubing is generally inspected using: a. U -shaped coils b. gap coils c. encircling coils d. none of the above D.9/E.32 14. When performing a hardness test on small parts with an encircling coil arrangement, which of the following test parameters may be considered important? a. length of the test sample b. thickness of the test sample c. cross-sectional area of the test sample d. all of the above A.74,271 Hardness affects conductivity?

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15. In a test coil consisting of both a primary and secondary winding, the voltage across the secondary is a function of: a. test material permeability b. test frequency c. geometry of the test object d. all of the above D.28 16. Eddy current testing relies on the principle of: a. magnetostriction b. electromagnetic induction c. piezoelectric energy conversion d. magnetomotive force (Đ¤= NI/S, F=NI) A.28,33

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17. When the H field strength around a ferromagnetic material is increased and then decreased to zero, the induced field B within the part does not return to zero. The term to define this B value when H has returned to zero is: a. coercive force b. residual magnetism c. saturation value d. hysteresis loss A.48 18. The three major specimen parameters that influence eddy current testing are: a. electrical conductivity, frequency and material geometry b. density, permeability and frequency c. electrical conductivity, permeability and material geometry d. thermal conductivity, electrical conductivity and permeability E. 57 Charlie Chong/ Fion Zhang

19. Doubling the number of turns on a coil will: a. double the inductance b. halve the inductance (L= ÎźoN2A/L) c. decrease the inductance by a factor of four d. increase the inductance by a factor of four D.15/E.13 20. The frequency ratio f/fg can be defined as: a. the argument of the mathematical function describing the electromagnetic field within the test specimen b. test frequency divided by limit frequency c. neither a nor b d. both a and b D.28/E.85

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21. When testing tubing using an encircling coil, what is the phase relationship of the output signals from identical ID and OD discontinuities? a. the signals are in phase b. the phase of the OD discontinuity leads the phase of the ID discontinuity c. the phase of the OD discontinuity lags the phase of the ID discontinuity d. indeterminate E.86 22. A term used to define an instrumentation technique that discriminates between variables in the test part by the phase angle response differences seen on the output device is: a. phase analysis b. phase distortion c. phase shifting d. phase discrimination A.472/E.65,142

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23. If the characteristic frequency (fg) of a material is 125 Hz, the test frequency required to give an f/fg ratio of 10 would be: a. 1.25 Hz b. 12.5 Hz c. 1.25 kHz d. 12.5 kHz E.85

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24. At a fixed test frequency, in which of the following materials will the eddy current penetration be greatest? a. aluminum (35% IACS) b. brass (15% IACS) c. copper (95% IACS) d. lead (7% IACS) E.84

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25. If the test frequency increases while the field strength is held constant, the surface eddy current density: a. decreases b. increases c. remains the same d. could do any of the above E.83-84 26. As the fill factor decreases, the impedance variation of a pickup coil produced by a given change in conductivity will: a. increase b. remain the same c. decrease d. could do any of the above A.130

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27. The abscissa values on the impedance plane shown in Figure 2 are given in terms of: a. absolute conductivity b. normalized resistance c. absolute inductance d. normalized inductance D.29 Ď&#x2030;Lo = Inductive reactance of free space

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28. In Figure 2 (an impedance diagram for solid nonmagnetic rod), the fg or characteristic frequency is calculated by the formula: a. fg = σμ /d2 b. fg = δμ/d2 c. fg = 5066/σμrd2 d. fg = R/L

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29. In Figure 2, a change in the f/fg ratio will result in: a. a change in only the magnitude of the voltage across the coil b. a change in only the phase of the voltage across the coil c. a change in both the phase and magnitude of the voltage across the coil d. no change in the phase or magnitude of the voltage across the coil E.86

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30. In Figure 3, the solid curves are plots for different values of: a. fill factor (lift-off) b. heat treatment c. conductivity d. permeability

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31. Which of the following would be easier to detect in tubing by the eddy current method using the self-comparison differential coil arrangement? a. gradual changes in diameter b. gradual changes in conductivity c. changes in temperature d. short discontinuities A.156

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32. Figure 4 illustrates the fact that eddy current responses: a. can be caused by several parameters b. differ in phase and magnitude with different parameter changes c. can be made selective with regard to the variable of interest d. all of the above E.57

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33. To eliminate any dependence on the number of test coil turns, the inductance values of an impedance diagram are: a. disregarded b. Normalized ( divided by 1/Ď&#x2030;Lo) c. corrected to a value of 1 d. none of the above A.137 34. A term used to define a system that indicates only the magnitude of variations in the total coil impedance regardless of the phase or direction in which it occurs on an impedance plane is: a. inductive reactance magnitude system b. feedback-controlled system c. impedance-magnitude system d. impedance vector analysis system E.65

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35. When eddy current testing tubing with a system that includes adjustable filter functions, which of the following variables would be classified as a high frequency variable? a. small discontinuities b. conductivity changes c. diameter changes d. wall thickness variations E.82 36. A major problem for an untrained or inexperienced operator using the eddy current test method is the: a. inability of eddy current testing to accurately measure conductivity b. large number of variables that appear in the output readout mechanism c. need to test at low speeds to prevent skipping d. inability of eddy current testing to detect small discontinuities E.11

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37. Figure 5 illustrates the fact that the limit frequency (fg) equations and the impedance diagrams for long solid rods and long thin-walled tubing are: a. different b. identical c. similar d. unrelated E.85 Solid rod fg = 5066*/Gd2 Thin wall tube = 5066/GdiW What is G? G = ÎźoĎ&#x192;? *Could be 5060, 5056 or in this case 5066!

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thin-walled tubing

Solid rod fg = 5066*/Gd2 Thin wall tube = 5066/GdiW What is G? G = ÎźoĎ&#x192;? *Could be 5060, 5056 or in this case 5066!

long solid rods

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3.1.2 Limiting Frequency fg of Encircling Coils Encircling coils are used more frequently than surface-mounted coils. With encircling coils, the degree of filling has a similar effect to clearance with surface-mounted coils. The degree of filling is the ratio of the test material cross-sectional area to the coil cross-sectional area. Figure 3.7 shows the effect of degree of filling on the impedance plane of the encircling coil. For tubes, the limiting frequency [point where ohmic losses of the material are the greatest (and no more?) ] can be calculated precisely from Eq. (3.2 & 3.3):

fg = 5056/(σ∙di∙w∙μr)

(3.2)

Where: fg = limiting frequency σ = conductivity di = inner diameter w = wall thickness μr (rel) = relative permeability For Solid Rod:

fg = 5060/(σ∙μr∙d2)

(3.3)

Where: d= solid rod diameter Charlie Chong/ Fion Zhang

Introduction to Nondestructive Testing: A Training Guide, Second Edition, by Paul E. Mix

38. When a direct current saturation technique is applied to a ferromagnetic material, the flux density in the material is: a. less than the flux density generated by the test coil because of heat losses b. less than the flux density generated by the test coil because of resistivity c. the same as the flux density generated by the test coil d. greater than the flux density generated by the test coil B.2.20/E.101

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39. In eddy current testing, the specimen is coupled to the test coil by: a. core coupling b. magnetic saturation c. the coil's magnetic field d. magnetic domains A.51/E.89,136 40. The thickness of non conductive coatings on a conductive base can be most simply measured by: a. observing the lift-off effect caused by the coating b. testing both sides of the specimen c. varying the current in the test coil d. varying the test frequency over a given range during the test A.461

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41. The use of magnetic shielding around the exciting coil will generally: a. increase the magnetic field extension but decrease the eddy current penetration b. increase the magnetic field extension and eddy current penetration c. decrease the magnetic field extension but increase the eddy current penetration d. decrease the magnetic field extension and eddy current penetration A.139,144 42. Which of the following is not a factor that affects the inductance of an eddy current test coil? a. diameter of the coils Ф= NI/S, F= NI, XL=2πfL, L=μoN2A/L b. test frequency c. overall shape of the coils d. distance from other coils E.17

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43. The formula used to calculate the impedance of an eddy current test coil is: a. Z= 2≠ fL b. Z= XL +R c. Z=√(XL +R) d. Z= √(X2+R2) E.19 44. An out-of-phase condition between current and voltage: a. can exist only in the primary winding of an eddy current coil b. can exist only in the secondary winding of an eddy current coil c. can exist in both the primary and secondary windings of an eddy current coil (in the pure inductance circuit current lag voltage by 90º) d. exists only in the test specimen A.160

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45. A surface coil mounted so that the coil can be rotated about the circumference of a test specimen is known as a(n): a. bobbin coil b. encircling coil c. rotating coil d. gap coil E.406

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46. An eddy current test coil which uses high permeability material to purposely shape the magnetic field is known as a(n): a. bobbin coil b. encircling coil c. rotating coil d. shielded coil (magnetic shielding versus eddy current shielding) A.144

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47. The conductivity (Ď&#x192;) value for a metal is a function of the: a. heat treatment given the metal b. cold working performed on the metal c. aging process used on the metal d. all of the above A.53 48. Which of the following materials would be more likely used to create a holding fixture for a probe coil? a. aluminurn b. plastic c. copper d. steel 0.19

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49. A term used to define the timing relationships involved in alternating current signals is: a. magnitude b. phase c. impedance d. reactance B.1.16/E.15 50. When eddy current testing a nonferrous material, the choice of mode of operation and test frequency will be affected by the: a. degree of phase discrimination required b. eddy current penetration needed c. discontinuity detection and sizing criteria d. all of the above e. a and b only A.403

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51. Which of the following coil arrangements would eliminate or decrease the effects of minor variations in diameter, chemical composition, hardness, etc., which occur gradually along the length of a wire? a. self-comparison differential b. external reference absolute c. single coil absolute d. double coil absolute A.156

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52. Which of the following conditions are not valid criteria when selecting specimens to be used as reference standards? a. the specimen should be of the same size and shape as the piece to be tested b. the specimen should have the same heat treatment as the piece to be tested c. the surface finish of the specimen should be the same as the piece to beTested d. if the material is aluminum, the surface should always be anodized A.270

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53. An eddy current test coil's magnetic field intensity in air: a. increases with distance outside the coil and varies across the diameter inside the coil b. decreases with distance outside the coil and varies across the diameter inside the coil c. decreases with distance outside the coil and is assumed to be constant across the diameter inside the coil d. increases with distance outside the coil and is assumed to be constant across the diameter inside the coil E.66

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54. The circuitry or electronic functions that make it possible to minimize the effect of signals caused by variables of no interest on the readout mechanism are based on differences in: a. amplitude b. phase c. frequency d. any one or a combination of the quantities listed above A.174 55. When a nonferromagnetic rod is placed inside an encircling coil: a. the magnetic field generated by the coil is increased in intensity b. the distribution of eddy currents is uniform through the rod's cross section c. the distribution of eddy currents is at a maximum at the rod's surface, or near the surface, and decreases to essentially zero at the rod's center d. the temperature of the rod decreases E.31

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Discussion Subject: 55. When a nonferromagnetic rod is placed inside an encircling coil: will the magnetic field generated by the coil is decrease and not increased in intensity b. the distribution of eddy currents is uniform through the rod's cross section c. the distribution of eddy currents is at a maximum at the rod's surface, or near the surface, and decreases to essentially zero at the rod's center d. the temperature of the rod decreases E.31

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56. A decrease in conductivity is equivalent to: a. an increase in permeability b. an increase in resistivity (σ=1/ρ), ρ= Rl /A c. a decrease in permeability d. a decrease in resistivity A.53/8.2.8 57. When the voltage applied to a circuit and the current through the circuit both reach their maximums and minimums at the same time, the voltage and current are: a. additive b. in phase c. regenerative d. out of phase E.15

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Conductivity and Resistivity Conductivity and resistivity is related as follows: σ =1/ ρ Where σ = Conductivity (sigma) and ρ =Resistivity (rho). Conductivity can be quantified in Siemens per m (S/m) or in Aerospace NDT in % lACS (International Annealed Copper Standard). One Siemen is the inverse of an ohm. Another common unit used for conductivity measurement is Siemen per cm (S/cm).

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Electrical Conductivity and Resistivity Resistance can be defined as follows: R = l /(Aσ) or R = ρl/A Where: R = the resistance of a uniform cross section conductor in ohms (Ω), l = the length of the conductor in the same linear units as the conductivity or resistivity is quantified, A=Cross Sectional area, σ = conductivity in S/m and ρ = Resistivity in Ω m.

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From Eq. (4.3) it can be seen that if the temperature is increased, resistivity increases and conductivity decreases from their ambient temperature levels. Conversely, if temperature is decreased the resistivity decreases and conductivity increases. To convert resistivity values, such as those obtained from Eq. (4.3) to conductivity in terms of% IACS, the conversion formula is,

%IACS = 172.41/ρ

(4.4)

Where: IACS = international annealed copper standard ρ = resistivity in Ω∙m

ρ100%IACS = 1.724110-8 Ωm = 100% IACS

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http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

3.3.2

Electrical Conductivity and Resistivity

In eddy current testing, instead of describing conductivity in absolute terms, an arbitrary unit has been widely adopted. Because the relative conductivities of metals and alloys vary over a wide range, a conductivity benchmark has been widely used. In 1913, the International Electrochemical Commission established that a specified grade of high purity copper, fully annealed measuring 1 m long, having a uniform section of 1 mm2 and having a resistance of 1.7241x10-8 ohm-meter at 20°C (100% IACS = 1.7241x10-8 ohm-meter at 20°C) - would be arbitrarily considered 100 percent conductive. The symbol for conductivity is σ and the unit is Siemens per meter. Conductivity is also often expressed as a percentage of the International Annealed Copper Standard (IACS). Note: 100% IACS = 1.7241x10-8 ohm-meter at 20°C

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Keywords:  100% IACS – (high purity copper, fully annealed - measuring 1 m long, having a uniform section of 1 mm2 and having a resistance of 1.7241x10-8 ohm-meter at 20°C. Absolute terms:  Resistivity ρ : 1.7241x10-8 ohm-meter at 20°C.  Conductivity 1/ρ : (1.7241x10-8)-1 /ohm-meter or 5.8001 x 107 Siemens/m at 20°C  Siemens = mho? or (ohm)-1

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https://www.nde-ed.org/GeneralResources/IACS/IACS.htm

Electrical Conductivity σ Electrical conductivity is a measure of how well a material accommodates the movement of an electric charge. It is the ratio of the current density to the electric field strength. Its SI derived unit is the Siemens per meter (mho/m), but conductivity values are often reported as percent IACS. IACS is an acronym for International Annealed Copper Standard, which was established by the 1913 International Electrochemical Commission. The conductivity of the annealed copper (5.8001 x 107S/m) ( 1/σ = ρ= 1.724 x 10-8 Ωm) is defined to be 100% IACS at 20°C . Therefore, iron with a conductivity value of 1.04 x 107 S/m, has a conductivity of approximately 18% of that of annealed copper and this is reported as 18% IACS. An interesting side note is that commercially pure copper products now often have IACS conductivity values greater than 100% IACS because processing techniques have improved since the adoption of the standard in 1913 and more impurities can now be removed from the metal.

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https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Physical_Chemical/Electrical.htm

58. Which of the following is not a commonly used eddy current testing readout mechanism? a. signal generator b. meter c. X-Y display d. strip-chart recorder A.174 59. Which of the following materials would have the highest resistivity value? a. aluminum with a 42% IACS rating b. magnesium with a 37% IACS rating %IACS â&#x2C6;? 1/Ď c. cast steel with a 10.7% IACS rating d. zirconium with a 3.4% IACS rating A.53

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60. The magnetic field generated by eddy currents induced in a test specimen: a. opposes the magnetic field that induced the eddy currents b. reinforces the magnetic field that induced the eddy currents c. cancels the magnetic field that induced the eddy currents d. has no effect on the magnetic field that induced the eddy currents A. 51 61. The impedance of a test coil usually can be represented by the vector sum of: a. inductive reactance and resistance b. capacitive reactance and resistance c. inductive reactance and capacitive reactance d. inductance and capacitance A. 51

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62. For age hardenable aluminum and titanium alloys, changes in hardness are indicated by changes in: a. retentivity b. permeability c. conductivity d. magnetostriction E.59

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Conductivity

Magnetic (Permeability & Dimensions)

A. Heat treatment give the metal

A. Length of the test sample

B. Cold working performed on the metal

B. Thickness of the test sample

C. Aging process used on the metal

C. Cross sectional area of the test sample

D. Hardness E. Crack & discontinuities

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63. The penetration of eddy currents in a conductive material is decreased when: a. test frequency or conductivity of the specimen is decreased b. test frequency is decreased or conductivity of the specimen is increased c. test frequency, conductivity of the specimen or permeability of the specimen is increased d. permeability of the specimen is decreased A. 54 δ= 1/√(πfμσ) = (πfμσ) -½

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64. When the alternating current in an eddy current coil reverses direction: a. the direction of the eddy currents in a part remains the same b. the eddy currents in the test part will change phase by 45째 c. the direction of the eddy currents in the part also reverses d. the eddy currents in the part remain the same E.14 65. To be useful, limit frequency equations fg must be: a. used with their impedance diagrams b. used to determine their impedance diagrams c. both a and b d. neither a nor b E.85

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66. When a rod is placed in an encircling type of coil, the density of eddy currents will be the greatest: a. at the surface b. at the center c. midway between the surface and the center d. none of the above A.130,151 67. The term fill factor applies to: a. a surface coil b. coaxial cable c. an encircling coil d. all of the above A.130

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68. An eddy current system using handling equipment with automatic marking of the defective areas primarily makes it possible to: a. remove defective areas of the product, if desired b. eliminate test surface contamination c. allow an inexperienced operator to establish the discontinuity cause d. pass rejectable material E.98

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69. Two test coils are often used in a bridge circuit to: a. eliminate skin effect b. determine the differences between a known standard sample and a test piece c. increase the conductivity of the circuit d. decrease the system sensitivity A.178 70. The actual frequencies used in any specific eddy current inspection should be selected on the basis of the: a. thickness of the material b. desired depth of penetration c. degree of sensitivity or resolution required d. all of the above E.83

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71. The term used to define the difference between actual instrument output and expected output as defined by a straight line calibration curve is: a. phase shift b. nonlinearity c. lift -off d. skin effect A.324-325 72. A term used to define any change in a sinusoidal signal when the output is not a linear reproduction of the input is: a. distortion b. linearity c. phase shifting d. dynamic range H.40.41

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73. The magnetic domains in an unmagnetized ferromagnetic material: a. are arranged to facilitate the conduction of electrons b. are randomly oriented and neutralize each other c. are uniformly oriented d. create a major north and south pole in the material E.44 74. When a magnetic material is placed in a region of an applied magnetizing force, a magnetic field is developed in the material by means of: a. induction b. conduction c. heat transfer d. magnetic domain transfer H.30.6

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75. The atoms of a metal showing magnetic characteristics are grouped into regions that are the smallest known permanent magnets. These regions are called: a. lattice structures b. cells c. domains d. planetary spins E.44 76. The flux density in a magnetized material is usually designated by the: a. symbol μ (permeability μ=B/H) b. symbol σ (conductivity σ= 1/ρ) c. letter B d. letter H (magnetic field intensity) H.30.22

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77. The magnetizing force of a saturation coil is usually designated by the: a. symbol μ (permeability μ=B/H)(H·m-1 or N·A-2) b. symbol σ (conductivity σ= 1/ρ)(Siemens/meter) c. letter B (Tesla, Gauss) d. letter H (magnetic field intensity) (Ampere-meter?) H.30.22 78. The ratio of a material's flux density to the applied magnetizing force can be used to determine the material's: a. Permeability μ=B/H b. conductivity c. resistivity d. lift-off 8.2.14

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79. The two sine wave signals (V and I) shown in Figure 6 are: a. in phase b. 45 degrees out of phase c. 90 degrees out of phase d. 180 degrees out of phase 8.2.36

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80. The two sine wave signals (V and I) shown in Figure 7 are: a. in phase b. 45 degrees out of phase c. 90 degrees out of phase current lags voltage by 90ยบ d. 180 degrees out of phase possibly inductive circuit 8.2.37

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81. The symbol commonly used to signify impedance is: a. Z b. XL c. R d. XC 82. In Figure 8, H represents: a. the magnetizing force b. the material's flux density c. the material's permeability d. the material's conductivity B.2.18

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83. In Figure 8, B represents: a. the magnetizing force b. the material's flux density c. the material's permeability d. the material's conductivity B.2.18 84. In Figure 8, the distance represented by A is a measure of the material's: a. permeability b. conductivity c. retentivity d. coercive force B.2.18

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85. In Figure 8, the distance represented by C is a measure of the material's: a. permeability b. conductivity c. residual magnetism d. coercive force B.2.18

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86. A term used to define a property of a test system which enables the separation of signals due to discontinuities in the test specimen that are located in close proximity to each other is: a. dynamic range b. sensitivity c. linearity d. resolution H.43.49 87. The inductive reactance of a coil is measured in units of: a. mhos b. ohms c. henries d. gauss 8.1.22

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88. A term used to defme a surface or internal rupture that is attributed to processing at too low a temperature or excessive working or metal movement during the forging, rolling or extruding operation is: a. a cold shut b. microshrinkage c. burst d. an inclusion B.A-10

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Recalling the mistakes

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Eddy Current Testing Level III Q&A

e e r Th

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Answers to Eddy Current Testing Level III 1.d 2.d 3.b 4.c 5.d 6.a 7.a 8.b 9.b 10.c 11.d 12.a 13.d 14.b 15.a 16.b 17.c 18.d 19.b 20.a 21.a 22.d 23.c 24.d 25.d 26.a 27.e 28.a 29.c 30.e 31.b 32.b 33.d 34.e 35.d 36.a 37.a 38.b 39.b 40.d 41.a 42.a 43.c 44.d 45.c 46.b 47.c

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1. In a feed through encircling coil eddy current system, a calibration standard may be used to: a. ensure repeatability and reliability of the setup b. calibrate the approximate depth of detectable discontinuities c. measure the test frequency d. both a and b A.271 2. A calibration standard may be used with a rotating probe eddy current instrument to: a. produce an indication relative to the depth of the discontinuity b. check the instrument for repeatability and freedom from drift c. check probe coil for possible damage d. all of the above A.271

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3. Handheld probe-type eddy current tests are most useful in: a. detection of surface and subsurface inclusions b. detection of surface discontinuities such as overlaps and seams c. detection of internal piping or bursts d. all of the above A.358

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4. In eddy current testing of tubing with low permeability values, a DC saturation approach might be provided by: a. an encircling solenoid b. permanent magnets c. either a or b depending upon the test environment d. neither a nor b D.19

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5. In Figure 9, AC current flowing through a primary coil sets up a magnetic field and causes a flow of eddy currents in the rod. The voltage induced in the secondary coil is dependent upon: a. these eddy currents b. the primary coil c. the generator d. all of the above D.11

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6. Which of the following is not a method that might be used to improve the signal-to-noise ratio? a. increase the amplification of the test instrument b. change to a test frequency that will decrease the noise c. improve the fill factor d. enable filter modes in the instrument A.173/0.25

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7. Which of the following is a property of eddy currents induced in a conductor by an encircling coil? a. the eddy current flow is affected by permeability variations in the sample b. the magnitude of eddy current flow is large compared to the current flow in the coil c. the eddy current flow dissipates no power in the conductor d. none of the above D.3 8. Which of the following is a property of eddy currents induced in a homogeneous conductor by an encircling coil? a. they are weakest on the conductor surface b. the phase of the eddy currents varies throughout the conductor c. they travel in straight lines d. they are maximum along the coil axis D.4 Jx =Joe–x/δ , δ = (πfμσ) -½ , ϴ = x/δ rad. Charlie Chong/ Fion Zhang

9. Which factor does not affect the phase shift between the transmitted signal and the reflected signal for a reflection coil (assuming the part is nonferrornagnetic)? a. the conductivity of the sample b. the magnitude of the transmitted signal θ = x/δ = x∙√(πfμσ) c. the thickness of the sample d. the presence of discontinuities in the sample (?) D.11,56

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10. The tubular product parameter having the greatest influence on the primary flux distribution throughout a nonferromagnetic part is the: a. surface roughness of the product b. outside diameter of the product c. wall thickness of the product d. length of the product A.148 11. Any handling equipment used in an eddy current system must take into consideration: a. the operator's abilities b. the end use of the product being tested c. speed, frequency of the test, sorting speed and physical control of the product d. all of the above A.356

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12. An eddy current system can be calibrated to provide qualitative rating and material marking where the product being inspected: a. does not allow defective areas to be removed b. is not defective c. is of inferior quality d. has inconsistent quality A.356 13. When conducting an eddy current inspection using an automatic handling system, it is advisable to calibrate and adjust the sensitivity levels to: a. some electronic source b. another NDT method c. an NIST standard d. an actual test part being inspected A.274

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14. A distinct advantage of using handling equipment in an eddy current test system is to reduce the potential for errors caused by: a. instrument drift b. lift-off c. skin effect d. all of the above e. none of the above A.364

Charlie Chong/ Fion Zhang

15. Decreased coupling or fill factor will resultin decreased test sensitivity because: a. reduced coupling between the specimen and the pickup coil results in smaller voltages across the pickup coil b. reduced coupling between the driver coil and the specimen increases eddy current flow in the specimen c. reduced coupling between the driver coil and the specimen will increase the signal-to-noise ratio d. all of the above 0.24 16. Why is it desirable to hold the fill factor or lift-off constant? a. to avoid arcing between the coil and the specimen b. to minimize test output signal changes that are not relevant to conditions within the specimen to be tested c. because a fill factor or lift-off change will shift the operating frequency d. to minimize the load on the constant current AC excitation circuits A.356

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17. The reactance component is decreased by placing a conducting object in the coil's electromagnetic field. Why is this so? a. the secondary field is exactly in phase with the primary field b. the secondary field is at precisely 90° with the primary field c. the phase angle between the two field components is always greater than 90° which partially cancels the primary field d. the secondary field is 180° out of phase with the primary field which causes a large phase shift D.4 Comment: the reasoning for choosing “d” is the Lenz’s law. (more reading on the correct choice of “c”)

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a. changes in the test specimen, such as cracks, metallurgical or dimensional changes, will alter the secondary field phase and amplitude b. the primary alternating current must be 60 cycles to produce this effect c. a temperature rise in the specimen d. a mismatch of the Hp and Hs fields produces a change in the output D.5

Figure 10

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18. Test coils may be shielded with either high conductivity or high permeability materials to: a. shape the field b. increase sensitivity c. increase resolution d. all of the above e. none of the above A.144/0.22 19. When a ferromagnetic bar is placed in the coil's electromagnetic field, the coil's reactance is increased. What causes this phenomenon? a. the coil becomes magnetically saturated b. the ferromagnetic test sample's permeability raises the inductance of the test coil c. the ferromagnetic test sample's conductivity increases the reactance value of the coil d. none of the above; a ferromagnetic bar would cause the coil's reactance to decrease 0.22 Charlie Chong/ Fion Zhang

20. Eddy currents flowing in the test object at any depth produce magnetic fields at greater depths, which oppose the primary field, thus reducing its effect and causing what kind of change in current flow as the depth increases? a. a decrease b. an increase c. a frequency change d. none of the above 0.5

Charlie Chong/ Fion Zhang

21. An alternating current produces eddy currents in a test object. The vector Hs represents the secondary AC field in the test piece. What function occurs to produce a workable test situation? (See Figure 10.) a. changes in the test specimen, such as cracks, metallurgical or dimensional changes, will alter the secondary field phase and amplitude b. the primary alternating current must be 60 cycles to produce this effect c. a temperature rise in the specimen d. a mismatch of the Hp and Hs fields produces a change in the output D.5 Figure 10

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22. Skin effect causes eddy currents to tend to flow near the surface of the test piece. Which of the following factors alter the skin effect? a. testing frequency: b. test piece temperature c. test piece hardness d. all of the above D.23 23. When an excitation voltage is applied to a coil, the primary magnetic flux is in phase with the current. No secondary magnetic flux is present When a test object is inserted in this coil, what action takes place? a. the object gets hot and no information is available b. insertion of the object cancels all information c. a secondary magnetic flux is produced, thus creating a new total magnetic flux which can be used to supply test information d. by subtractiug the relative values of the primary flux and voltage vectors from the secondary components, it is possible to predict the net oscillator power consumption 0.5 Charlie Chong/ Fion Zhang

24. The electromagnetic field in a test specimen during eddy current testing is described by: a. Bessel functions b. exponential functions c. algebraic functions d. all of these A.33/D.2/E.85/H.36.13 25. There is one function that responds to variations in eddy current discontinuity and magnetic field conditions. This function actually produces the output signal from the coil. What is this function? a. phasing b. resistance c. reactance d. impedance D.21

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26. To separate cracks and diameter effects for steel cylinders, the optimum frequencies correspond to f/fg ratios of less than (see Figure 11): a. 10 b. 15 c. 50 d. 100 0.28,30

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27. The inductive reactance of a test coil, which is one of the most important impedance quantities, depends upon which of the following? a. frequency, coil inductance and coil resistance b. coil inductance only c. coil resistance and inductance only XL=2Ď&#x20AC;fL d. frequency and coil resistance only e. frequency and coil inductance only D.16

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28. Figure 12 indicates that the largest eddy current indications from subsurface cracks will occur when the frequency ratio (f/fg) is: a. 5 or less b. 15 c. 50 d. 150 or more D.28,30

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29. An operating frequency of 100kHz will have the deepest penetration in: a. titanium b. copper c. stainless steel d. aluminum D.22,27 30. Disadvantages of using a surface probe coil for the inspection of smalldiameter tubing include: a. inability to detect small discontinuities b. slow inspection speed c. inherent Mechanical problems d. both a and c e. both b and c D.9/E.29

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31. Figure 13 indicates that the magnitude of a signal from a subsurface crack will increase when the frequency ratio (f/fg): a. remains the same b. decreases c. increases d. none of the above D.28,30 Comment: Maximum at f/fg = 15 decreasing on both sides

Charlie Chong/ Fion Zhang

32. Figure 13 indicates that when inspecting for surface cracks in nonferromagnetic cylinders, the optimum frequency ratio (f/fg) is between: a. 5 and 10 b. 10 and 50 c. 50and100 d. 100 and 150 D.28,30 33. Of the following eddy current systems, which would be considered best for detection of small discontinuities in tubing? a. surface probe coils b. encircling coils c. inside probe (ID) bobbin coils d. both b and c 0.9/E.29

Charlie Chong/ Fion Zhang

34. In a send-receive system, when the sensing coil is placed near the excitation coil, which of the following statements is true? a. almost the same magnetic flux threads both coils b. signals from either coil can be used to provide information about the test Piece (?) c. the arrangement is never sensitive to small discontinuities d. all of the above e. both a and b D.11 35. A primary-secondary coil inspection system is being used. To suppress the primary coil signal induced in the secondary coil, one might: a. use a differential-type secondary coil b. add a 180ยบ out-of-phase input signal to the induced signal c. subtract an in-phase (input) signal from the induced signal d. any of the above E.33

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36. Which material should be selected to provide the most effective shielding of a test coil from an external RF field? a. Fiberglass b austenitic stainless steel c. copper d. brass D.18 37. Which type of readout mechanism would be best for the detection of a nonrepetitive discontinuity indication during high speed testing? a. a storage-type LCD screen (?) b. a meter c. a system using short â&#x20AC;&#x201C;persistence window logic d. a dual beam type D.40

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38. Impedance changes produced by small lift -off variations are greatest when: a. there is a nonconductive coating on the test material b. the probe is usually in contact with the test material c. lower test frequencies are used d. large diameter coils are used D.23 39. As the operating frequency is increased, the impedance of the empty coil: a. decreases b. increases c. remains the same XL= 2Ď&#x20AC;fL, Z= â&#x2C6;&#x161;(R2+XL2) d. none of the above D.17/J.81

Charlie Chong/ Fion Zhang

40. When testing for the unknown conductivity of a metal, the measure conductivity (Ď&#x192;) is strongly affected by: a. I2R b. material thickness variations beyond the three delta point in tbe test specimen c. coil diameter d. specimen temperature D.21 41. In selecting a coil for multifrequency tests: a. bandwidth is of major importance b. frequency response is of minor importance c. the Q of the coil should be less than 1 d. the Q of the coil should be less than the inductance D.37

Charlie Chong/ Fion Zhang

42. In developing a multifrequency test, the parameter separation limitations are greatest for those parameters producing: a. nearly similar signals b. 0째 to 90째 phase shifts c. signals having no electrical relation d. 90째 to 110째 phase shifts D.37 43. Mutual inductance describes: a. the effect two coils have on each other b. the coupling between the test coil and test part c. a and b d. neither a nor b E.17 Comment: No restricted to coil-coil interaction only! This include test piece-coil mutual inductance.

Charlie Chong/ Fion Zhang

44. Increasing the heat-treating temperature of nonferrous metals: a. increases electrical conductivity b. will have no effect on electrical conductivity c. will decrease electrical conductivity d. may either increase or decrease electrical conductivity, depending upon the alloy and the nature of the heat treatment D.22/H.42.19 45. Limit frequency fg is defined when the argument of the mathematical function describing the electromagnetic field within a test specimen is set equal to: a. 0 b. 0.37 c. 1 d. 6.25 D.28

Charlie Chong/ Fion Zhang

46. Figure 14 illustrates that the separation of diameter effects and conductivity effects: a. can be accomplished at any f/fg ratio b. can best be accomplished at f/fg ratios larger than 4 c. can best be accomplished at f/fg ratios less than 4 d. cannot he accomplished at any f/fg ratio D.28

Charlie Chong/ Fion Zhang

47. Figure 15 illustrates: a. variations in fill factor for a solid rod b. variations in fill factor for a thin-walled tube c. transition between a solid rod and thin-walled tube d. all of the above D.24,29

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