Detection of Thickness Variation in Steel using Pulse Eddy Current Technique in Time Domain

In contrast to single frequency conventional eddy current technique (ECT), in pulse ECT the coil is excited by repetitive broadband pulse. The pulsed excitation contains wide spectrum of frequencies. Each frequency has different penetrability and hence using pulsed eddy current (PEC) technique, large areas of structure with multiple variations in thickness can be scanned without the need for parameter or setup changes. This paper describes a pulsed eddy current based non-destructive testing system used for detection of thickness variation in the stainless steel sample. For this purpose, a PEC probe having ferrite core driving coil with giant magneto-resistive (GMR) sensor as pick up sensor has been developed. The PEC instrumentation consisting of coil driving circuit, instrumentation amplifier circuit, data acquisition module and signal processing software has been developed. The PEC response signals obtained from 1 mm to 6 mm thick stainless steel sample were investigated. Two time domain features, namely peak value and time to peak, of PEC response are used for extracting information about thickness variation in stainless steel plate. The variation of peak value and time to peak with thickness was compared. A customized signal processing software was developed to display the thickness variation of the tested sample. The technique can be used to detect the wall thinning in the pipelines of plants without removing the thermal insulation, where conventional NDT methods such as ultrasonic testing (UT) are difficult to apply.