Reduction of Pulsed Eddy Current Probe Footprint Using Sequentially Excited Multiple Coils

Abstract

In the detection of corrosion under insulation (CUI) using pulsed eddy current testing (PECT) method, it is of great signi cance to reduce the footprint of the probe for improving the spatial resolution to local corrosion. This paper presents a novel method to reduce the probe footprint by modifying the excitation coil into multiple sub-coils and driving them with sequential pulses of different delay time. Finite element simulations are conducted to reveal the underlying mechanism. It is found that by using the sequential excitation scheme, the diffusion and decay of eddy currents in the test piece are regulated, and both the footprint reduction and signal enhancement can be achieved. Afterwards, the effects of the sequence and the delay amount of the applying pulses on the probe footprint are analyzed. Results show that the optimal excitation sequence is to apply pulses with increasing delay time to the sub-coils from outside to inside; the probe footprint decreases with the increase of the delay amount. Experimental work is nally performed to verify the simulation results. A graphical method for measuring the probe footprint is proposed by moving the probe on a step wedge plate and plotting the evaluated thickness against the probe position. Footprint measurement results of a conventional probe and the presented 4-subcoil probe are compared. The effectiveness of the proposed method are validated and the differences between experimental and simulation results are analyzed.

Inspection of Liner Wall Thinning and Interface Debonding in Bimetallic Lined Pipes Using Pulsed Eddy Current Testing

“…Unfortunately, low frequency also causes other practical problems including low signal amplitude [ 15 ]. Compared with traditional ECT, pulsed eddy current testing (PECT) with pulsed- or square-wave excitation is found to have better performance in the inspection of deeply embedded defects, and its relevant applications have been frequently reported, especially on layered conductive structures [ 16 , 17 , 18 , 19 , 20 ]. Lebrun et al [ 16 ] applied PECT to detect cracks in riveted aeronautical structures and found that small cracks (1 mm of radial extent) buried 5 mm (0.2 in.)…”

“…Yang et al [ 17 ] developed a PECT-based giant magnetoresistive system for the automatic detection of small cracks under fasteners in multilayered structures at a depth of up to 10 mm. Xu et al [ 18 , 19 ] employed PECT to detect corrosion in thermally insulated pipes without removing the wrapped weatherproofing jacket and insulation material.…”