Detection of Surface and Sub-surface Defects in Aluminium Plate Using Pulsed Eddy Current Technique

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1 More info about this article: Detection of Surface and Sub-surface Defects in Aluminium Plate Using Pulsed Eddy Current Technique H. M. Bapat, Gurpartap Singh, B. P. Singh and Manojit Bandyopadhyay Division of Remote Handling and Robotics Bhabha Atomic Research Centre, Trombay, Mumbai 485 Maharashtra, India Abstract Conventional eddy current technique is fast and reliable technique for detection of near surface defects in conducting materials. However, detection of defects located deep in to the material is very difficult due to skin effect phenomena. Pulsed Eddy Current (PEC) technique is emerging as new technique for detection of deeper defects in conducting material. In PEC technique the probe is excited with pulse having high amplitude and short duration. Exciting probe with pulse instead of continuous wave offers an advantage that more flux for short duration can be generated without heating of coil. A pulse contains continuum of frequencies which ensures that all frequency components are present and applied simultaneously to test material. This paper presents technique for detection of deeper surface and subsurface defects in aluminium plate using PEC technique. Notches of different depths were fabricated on aluminium plate and were used as defect standard. Pulsed eddy current probe having coil as pickup sensor is used for picking up the signal from defect standard. Transient response signal obtained from probe is analyzed in time domain. Peak amplitude and time of zero crossing of response signal provide information about the size of defect. These two parameters have been used for detection and characterization of defects. The technique can be used effectively for detection of defects in aluminium alloy material structures. Keywords: Pulsed Eddy Current, Surface Cracks, Peak Amplitude, Time of Zero Crossing 1. Introduction Conventional Eddy Current Testing (ECT) is fast and reliable technique for detection of surface defects. For detection of deep defects coil has to be excited with low frequency signal because depth of penetration varies inversely with excitation frequency, but with the lowering of frequency the rate of change of flux decreases and hence very less eddy 325 Non-Destructive Evaluation 216

2 currents are induced in test material. This limitation imposes difficulty in detection of deep defects in materials using conventional ECT. Pulsed Eddy Current (PEC) technique [1] has emerged as new technique for detection of deeper defects in conducting materials. In PEC technique, the probe is excited with short duration pulse of high amplitude. Exciting probe with pulse instead of continuous wave offers an advantage that more flux for short duration can be generated without heating of coil. A pulse contains continuum of frequencies which ensures that all frequency components are applied simultaneously to test material. Power consumption is also less in PEC testing due to pulse excitation. This paper presents technique for detection of deeper surface and subsurface defects in aluminium plate using PEC technique. Notches of different depths were fabricated on aluminium plate and were used as defect standard. Pulsed eddy current probe having coil as pickup sensor is used for picking up the signal from defect standard. Transient response signal obtained from probe is analyzed in time domain. Peak amplitude and time of zero crossing of response signal provide information about the size of defect. These two parameters have been used for detection and characterization of defects. 2. Experimental Setup The PEC experimental set-up comprises of PEC probe, test sample, PEC module and computer with data acquisition and signal processing software. Figure-1 shows the block diagram of the experimental set-up used for detection of defects in aluminium plate. Fig 1: Pulsed Eddy Current Experimental Setup The probe used for defect detection is having two coils one excitation coil and other pickup coil. The pickup coil is placed inside the excitation coil which picks up the response signal. The test sample used for experiments is aluminium plate 6mm thick, 1mm wide and 5mm long. To simulate the cracks in aluminium plate, small notches were machined on one face of sample as shown in figure Non-Destructive Evaluation 216

3 Fig 2: Aluminium Test Sample During experiments, in order to simulate surface cracks, probe is placed on same side of surface in which notches were machined, whereas for simulating sub-surface defects, probe is placed on opposite side of machined surface. The PEC Module is having pulse generator, pulse amplifier, pre-amplifier for conditioning the response signal and data acquisition module. Data acquisition module communicates with data acquisition & analysis software through ethernet. The response signal is analyzed in time domain [2]. Peak value and time of zero crossing of response signal are used to detect and characterize the surface and sub-surface defects. 3. Results and Discussion During the experiment, the PEC probe is excited with pulse having amplitude of 3V and width of 1 µsec. The probe was first placed on defect free portion of the test sample to acquire a reference signal. Then it is placed on defect in the test sample to acquire defect signal. The defect signal is subtracted from reference signal to generate the difference signal. The difference signal will be free from common mode signals and will appear only in the defect portion of the sample. This signal is used as PEC response signal for analysis. The scanning is carried out by keeping the PEC probe in close contact with surface to avoid any liftoff. The PEC response signal has a fast rise to a peak followed by a slower decay. The eddy current decays in the test sample to zero due to the electrical resistance of the material under test. Reference signal, defect signal and PEC response signal are shown in figure 3. Here, the reference & defe t sig al a e s aled do a fa to of / to keep the ithi the sa e s ee as the PEC response signal. 327 Non-Destructive Evaluation 216

4 15 PEC Response Signal Amplitude (mv) Time (usec) Reference Signal/64 Defect Signal/64 PEC Response Signal/1 Fig 3: Reference Signal, Defect Signal and PEC Response Signal The PEC response signal obtained from different depth surface defects is shown in figure 4. The peak amplitude and time of zero crossing of response signal is analyzed for characterizing the defects. Peak amplitude is the highest amplitude of response signal. Time of zero crossing is the time at which the PEC response signal crosses the time axis for the first time while decaying. A comparative table showing peak amplitude and time of zero crossing obtained from different depth surface defects are shown in table 1. The PEC response signal curve broadens for higher depth surface defects. As the depth of surface defect increases, both the peak amplitude and time of zero crossing increases. For small depth defects both peak amplitude and time of zero crossing is less as compared to large depth defects. During experiments, the smallest depth surface defect which is detected successfully was.25mm deep where as highest depth was 4mm deep. 328 Non-Destructive Evaluation 216

5 Apmlitude (mv) Surface defects Time (usec) 4mm Depth 2mm Depth 1mm Depth.5mm Depth.25mm Depth Fig 4: The Pulsed Eddy Current response to surface defects having different depth Length (mm) Width (mm) Depth (mm) Peak Amplitude (mv) Time of Zero Crossing (µsec) Defect Defect Defect Defect Defect Table 1: Variation of Peak amplitude and time of zero crossing with surface defect depth 329 Non-Destructive Evaluation 216

6 The Plot of Peak amplitude and time of zero crossing of PEC response signal versus defect depth is shown in figure 5. It is observed that these values follow a second order polynomial fitting. The fitted curve can be used to estimate the defects of unknown size within this limit. Peak Amplitude (mv) Surface Defect Depth (mm) Time of Zero Crossing (usec) Surface Defect Depth (mm) Peak Amplitude (mv) Poly. (Peak Amplitude (mv)) Zero Crossing Poly. (Zero Crossing) Fig 5:Peak amplitude and time of zero crossing following a second order polynomial fitting PEC response signals were obtained for subsurface defects also. Sub surface defects were simulated by scanning the same aluminium plate from opposite side of surface where surface defects are located. Subsurface defects at depth of 2mm and 4mm were detected successfully. The response curves obtained from sub-surface defects were observed in positive direction compared to surface defects responses which were in negative direction. The response curves obtained by scanning sub-surface defects are shown in figure Non-Destructive Evaluation 216

7 12 SubSurface defects 1 8 Amplitude (mv) Time(us) 2mm Depth 4mm Depth Fig 6: The Pulsed Eddy Current response to Sub surface defects having different depth 4. Conclusion Pulsed eddy current technique to detect surface and sub-surface defects in aluminium plate is presented. PEC probe having coil as pickup sensor is used to detect the defects. Peak amplitude & time of zero crossing of response signal are used to characterize the defect. Using this technique surface defects having depth of.25mm to 4mm were detected successfully while subsurface defects at depth of 4mm and 2mm were detected successfully. The technique can be used for detection of defects in plate type aluminium structures. Acknowledgment The authors are thankful to Associate Director, DMAG and Head, Division of Remote Handling and Robotics for providing all support in carrying out these experiments. The authors acknowledge the help and support given by all supporting staffs of DRHR. References 1. Basi s of Pulsed Eddy Cu e t Testi g, H. M. Bapat, Gurpartap Singh, B. P. Singh, Manojit Bandyopadhyay and R.K. Puri, Journal of Non-Destructive Testing & Evaluation, Volume 11, Issue 4, March Dete tio of Thi k ess Va iatio i teel usi g Pulse Eddy Cu e t Te h i ue i Ti e Do ai H. M.Bapat, Gurpartap Singh, B. P. Singh, R. K. Puri and Manojit Bandyopadhyay, Asia Pacific Conference on Non-Destructive Testing APCNDT 213, November 18-22, 21,Mumbai. 331 Non-Destructive Evaluation 216