1. 5 Current Field Measurement 5.1Alternating Current Field Measurement 5.2Direct Current Potential Drop 5.3Alternating Current Potential Drop

2. 5.1 Alternating Current Field Measurement

3. Principle of Operation electric field magnetic flux density axial (x) transverse (y) normal (z) galvanic current injection  magnetometer magnetic injection: primary ac flux ~ ~

4. Bx0Bx0 Field Perturbation magnetic flux density magnetometer axial (x) transverse (y) normal (z) axial flaw cw current B z < 0 electric current B z > 0 ccw current axial scanning above flaw Axial Position B z [a.u.] Axial Position B x [a.u.] B z [a.u.] B x [a.u.]

5. Uniform Field advantages:  testing through coatings  depth information  limited boundary effects disadvantages:  reduced sensitivity  sensitivity to geometry  flaw orientation effect of coating thickness on axial magnetic flux density B x (ferrous steel, 5 kHz, δ  0.25 mm, 30-mm-long solenoid) 8 7 6 5 4 3 2 1 0 05101520 Coating Thickness [mm] ΔB x [%] 50  5 mm 20  2 mm 20  1 mm slot size

6. 30 25 20 15 10 5 0 Slot Depth [mm] ΔB x and ΔB z [%] 00.511.522.5 B x at 5 kHz B z at 5 kHz B x at 50 kHz B z at 50 kHz Axial Flaw 8 7 6 5 4 3 2 1 0 010203040 Slot Depth [mm] ΔB xm per 1 mm Slot Depth [%] 40-mm-long solenoid 12-mm-long solenoid rate of increase of the minimum of B x with slot depth at the center 2-mm-diameter coil, ferrous steel changes normalized to B x0 (parallel to B, normal to E)

7. Flaw Orientation 0.17 0.16 0.15 0.14 0.13 0.12 0.11 B x [T] 012345 Scanning time [a. u.] transverse flaw (normal to B) axial flaw (normal to E) 0.025 0.020 0.150 0.100 0.05 0 -0.05 B z [T] 012345 Scanning Time [a. u.] transverse flaw (normal to B) axial flaw (normal to E) eddy current mode magnetic flux mode

8. Magnetic Flux Mode electromagnet crack N I magnetometer Lateral Position Tangential Magnetic Field Lateral Position Normal Magnetic Field

9. 5.2 Direct Current Potential Drop

10. Inductive versus Galvanic Coupling specimen eddy currents probe coil magnetic field electric current V II injection current potential drop specimen advantages of galvanic coupling dc and low-frequency operation constant coupling (four-point measurement) awkward shapes absolute measurements inherently directional

11. Thin-Plate Approximation combined electric current and potential field 2a2a 2b2b t << a I (+) I (-) V (+) V (-) I (+) I (-) V (+) V (-)

12. Lateral Spread of Current Distribution 2w2w I (+) I (-) V (+) V (-) x y 2a2a J(0,w) J(0,0)

13. Thick-Plate Approximation t >> a 2a2a 2b2b I (+) I (-) V (+) V (-)  combined electric current and potential field I (+) I (-) V (+) V (-)

14. Finite Plate Thickness 2a2a 2b2b t I (+) I (-) V (+) V (-) n = 0 n = -1 n = +1 n = -2 n = +2 2t2t I (+) I (-) V (+) V (-)

15. Resistance versus Thickness 0.1 1 10 0.010.1110100 Normalized Thickness, t / a Normalized Resistance, Λ finite thickness thin-plate appr. thick-plate appr. a = 3b

16. Crack Detection by DCPD intact specimen I (+) I (-) V (+) V (-) t I (+) I (-) V (+) V (-) cracked specimen c 1 2 3 00.20.40.60.81 Normalized Crack Depth, c / t Normalized Potential Drop, ΔV c / Δ V 0 a / t = 0.44 1.2 1.8 a = 3b infinite slot

17. Technical Implementation of DCPD low resistance, high current thermoelectric effect, pulsed, alternating polarity control of penetration via electrode separation low sensitivity to near-surface layer no sensitivity to permeability power supply polarity switch + _ specimen electrodes VsVs + _

18. 5.3 Alternating Current Potential Drop

19. Direct versus Alternating Current DCPD ACPD higher resistance, lower current no thermoelectric effect control of penetration via frequency higher sensitivity to near-surface layer sensitivity to permeability

20. Thin-Plate/Thin-Skin Approximation 2a2a 2b2b t << a I (+) I (-) V (+) V (-)

21. Skin Effect in Thin Nonmagnetic Plates analytical prediction a = 20 mm, b = 10 mm, t = 2 mm 10 0 10 1 10 2 10 3 10 0 10 1 10 2 10 3 10 4 10 5 Frequency [Hz] Resistance [µΩ] 1 %IACS 2 %IACS 5 %IACS 10 %IACS 20 %IACS 50 %IACS 100 %IACS ftft a = 20 mm, b = 10 mm, σ = 50 %IACS 0.05 mm 0.1 mm 0.2 mm 0.5 mm 1 mm 2 mm 5 mm ftft 10 0 10 1 10 2 10 3 10 0 10 1 10 2 10 3 10 4 10 5 Frequency [Hz] Resistance [µΩ]

22. Skin Effect in Thick Nonmagnetic Plates 304 austenitic stainless steel, σ = 2.5 %IACS, experimental 10 1 10 2 10 3 10 4 10 0 10 1 10 2 10 3 10 4 10 5 Frequency [Hz] Resistance [µΩ] 50 mm 20 mm 10 mm 6.25 mm 2.5 mm 2 mm 1 mm 0.5 mm 0.2 mm 0.1 mm 0.05 mm a = 10 mm, b = 7.5 mm

23. Current Distribution in Ferritic Steel f = 0.1 Hz FE predictions (Sposito et al., 2006) f = 50 Hz f = 1 kHz a = 10 mm, b = 5 mm, t = 38-mm, c = 10 mm (0.5-mm-wide notches, two separated by 5 mm)

24. Thin-Skin Approximation 0 1 2 123 Electrode Shape Factor, a / b Electrode Gain,  0 2b2b 2a2a 2b2b 2a2a c

25. Technical Implementation of ACPD low-pass filter low-pass filter oscillator differential driver + _ 90º phase shifter A/D converter specimen electrodes PC processor VrVr VsVs VqVq frequency range: 0.5 Hz - 100 kHz driver current: 10-200 mA resistance range: 1-10,000 µΩ common mode rejection: 100-160 dB.

26. a = 0.160” b = 0.080” w = 0.054” 2 d = 0.120” voltage sensing current injection welding weldment d w edge weld clamshell catalytic converter Application Example: Weld Penetration NDE [mil] Fracture Surface [mils] 0 10 20 30 40 50 60 70 80 01020304050607080 weld penetration (w) Weld Penetration [mil] Resistance [µΩ] 0 50 100 150 200 020406080100120 b = 120 mils 80 mils 100 mils electrode separation (b)

27. Application Example: Erosion Monitoring 05101520 Time [day] 20 21 22 23 24 25 Temperature [ºC] 32.0 32.2 32.4 32.6 32.8 33.0 Resistance [µΩ] 05101520 Time [day] 20 21 22 23 24 25 Temperature [ºC] 32.0 32.2 32.4 32.6 32.8 33.0 Resistance [µΩ] erosion before compensationafter compensation β  0.001 [1/ºC] 50 60 70 80 90 100 110 120 130 0200400600800 Temperature [ºC] Resistivity [µΩ cm] 301 302 303 304 309 310 316 321 347 403 internal erosion/corrosion pipe