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5 Current Field Measurement

5 Current Field Measurement. 5.1 Alternating Current Field Measurement 5.2 Direct Current Potential Drop 5.3 Alternating Current Potential Drop. 5.1 Alternating Current Field Measurement. ~. ~. primary ac flux. normal (z). magnetic. transverse ( y ). flux. density. axial ( x ).

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5 Current Field Measurement

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

  2. 5.1 Alternating Current Field Measurement

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

  4. cw current normal (z) Bz [a.u.] magnetic transverse (y) flux electric density Axial Position axial (x) current Bz [a.u.] Bz < 0 Bx [a.u.] Bx [a.u.] Bz > 0 Axial Position ccw current Field Perturbation axial scanning above flaw magnetometer Bx0 axial flaw

  5. slot size 20  2 mm 50  5 mm 20  1 mm 8 7 6 5 4 ΔBx [%] 3 2 1 0 0 5 10 15 20 Coating Thickness [mm] Uniform Field effect of coating thickness on axial magnetic flux density Bx (ferrous steel, 5 kHz, δ 0.25 mm, 30-mm-long solenoid) • advantages: • testing through coatings • depth information • limited boundary effects disadvantages: • reduced sensitivity • sensitivity to geometry • flaw orientation

  6. 30 Bx at 5 kHz Bz at 5 kHz 25 Bx at 50 kHz Bz at 50 kHz 20 15 8 7 10 6 5 4 5 3 2 1 0 0 0 0.5 1 1.5 2 2.5 0 10 20 30 40 (parallel to B, normal to E) Axial Flaw rate of increase of the minimum of Bx with slot depth at the center 2-mm-diameter coil, ferrous steel ΔBx and ΔBz[%] ΔBxmper 1 mm Slot Depth [%] 40-mm-long solenoid 12-mm-long solenoid Slot Depth [mm] Slot Depth [mm] changes normalized to Bx0

  7. 0.025 0.17 0.020 0.16 transverse flaw (normal to B) axial flaw (normal to E) 0.150 0.15 Bx[T] Bz[T] 0.100 0.14 axial flaw (normal to E) 0.05 0.13 transverse flaw (normal to B) 0 0.12 -0.05 0.11 0 0 1 1 2 2 3 3 4 4 5 5 Scanning time [a. u.] Scanning Time [a. u.] Flaw Orientation eddy current mode magnetic flux mode

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

  9. 5.2 Direct Current Potential Drop

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

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

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

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

  14. 2b 2a I(+) V(+) V(-) I(-) Finite Plate Thickness I(+) I(-) n = +2 n = +1 V(+) V(-) 2t n = 0 t n = -1 n = -2

  15. 10 finite thickness a = 3b thin-plate appr. thick-plate appr. 1 Normalized Resistance, Λ 0.1 0.01 0.1 1 10 100 Normalized Thickness, t / a Resistance versus Thickness

  16. intact specimen cracked specimen I(+) V(+) V(-) I(-) I(+) V(+) V(-) I(-) c t infinite slot a = 3b 3 Normalized Potential Drop, ΔVc / ΔV0 2 a / t = 0.44 1.2 1.8 1 Normalized Crack Depth, c / t 0 0.2 0.4 0.6 0.8 1 Crack Detection by DCPD

  17. + polarity switch power supply _ + Vs _ electrodes specimen • 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 Technical Implementation of DCPD

  18. 5.3 Alternating Current Potential Drop

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

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

  21. 103 103 a = 20 mm, b = 10 mm, t = 2 mm a = 20 mm, b = 10 mm, σ = 50 %IACS 0.05 mm 1 %IACS 0.1 mm 2 %IACS 0.2 mm 5 %IACS 102 102 10 %IACS 0.5 mm 20 %IACS 1 mm 50 %IACS 2 mm Resistance [µΩ] Resistance [µΩ] 100 %IACS 5 mm 101 101 ft ft Frequency [Hz] Frequency [Hz] 100 100 100 100 101 101 102 102 103 103 104 104 105 105 analytical prediction Skin Effect in Thin Nonmagnetic Plates

  22. 104 50 mm 20 mm 10 mm 103 6.25 mm 2.5 mm Resistance [µΩ] 2 mm 1 mm 102 0.5 mm 0.2 mm 0.1 mm 101 0.05 mm 100 101 102 103 104 105 Frequency [Hz] 304 austenitic stainless steel, σ = 2.5 %IACS, experimental Skin Effect in Thick Nonmagnetic Plates a = 10 mm, b = 7.5 mm

  23. FE predictions (Sposito et al., 2006) f = 0.1 Hz 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) Current Distribution in Ferritic Steel

  24. 2a 2a 2b 2b 2 c 1 Electrode Gain, G0 0 1 2 3 Electrode Shape Factor, a / b Thin-Skin Approximation

  25. Vr low-pass filter A/D converter oscillator Vq 90º phase shifter differential driver low-pass filter + Vs PC processor _ electrodes specimen Technical Implementation of ACPD 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. clamshell catalytic converter welding 2 d = 0.120” current injection edge weld weldment voltage sensing a = 0.160” d b = 0.080” w = 0.054” w electrode separation (b) weld penetration (w) 80 200 70 60 150 50 Resistance [µΩ] Fracture Surface [mils] 40 100 b = 30 120 mils 20 50 100 mils 10 80 mils 0 0 0 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 Weld Penetration [mil] NDE [mil] Application Example: Weld Penetration

  27. 130 120 110 internal erosion/corrosion 100 301 302 Resistivity [µΩ cm] 90 303 304 80 309 310 70 316 321 pipe 60 347 403 50 0 200 400 600 800 Temperature [ºC] 25 25 33.0 33.0 24 24 32.8 32.8 23 23 32.6 32.6 Resistance [µΩ] Resistance [µΩ] Temperature [ºC] Temperature [ºC] 22 22 32.4 32.4 21 21 32.2 32.2 erosion erosion 20 20 32.0 32.0 0 0 5 5 10 10 15 15 20 20 Time [day] Time [day] Application Example: Erosion Monitoring β  0.001 [1/ºC] before compensation after compensation

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