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

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Presentation on theme: "5 Current Field Measurement 5.1Alternating Current Field Measurement 5.2Direct Current Potential Drop 5.3Alternating Current Potential Drop."— Presentation transcript:

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) Coating Thickness [mm] ΔB x [%] 50  5 mm 20  2 mm 20  1 mm slot size

6 Slot Depth [mm] ΔB x and ΔB z [%] B x at 5 kHz B z at 5 kHz B x at 50 kHz B z at 50 kHz Axial Flaw 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 B x [T] Scanning time [a. u.] transverse flaw (normal to B) axial flaw (normal to E) B z [T] 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 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 Normalized Crack Depth, c / t Normalized Potential Drop, ΔV c / Δ V 0 a / t = 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 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 Frequency [Hz] Resistance [µΩ]

22 Skin Effect in Thick Nonmagnetic Plates 304 austenitic stainless steel, σ = 2.5 %IACS, experimental 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 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 kHz driver current: mA resistance range: 1-10,000 µΩ common mode rejection: 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] weld penetration (w) Weld Penetration [mil] Resistance [µΩ] b = 120 mils 80 mils 100 mils electrode separation (b)

27 Application Example: Erosion Monitoring Time [day] Temperature [ºC] Resistance [µΩ] Time [day] Temperature [ºC] Resistance [µΩ] erosion before compensationafter compensation β  [1/ºC] Temperature [ºC] Resistivity [µΩ cm] internal erosion/corrosion pipe


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