Dual frequency fusion for defect signal enhancement in EC-GMR inspecton of riveted multilayer structures

Reliable detection of defects under fasteners at rivet sites is one of the most critical problems in inspection of multilayer structures. Rotating eddy current (EC) excitation with giant magnetoresistive (GMR) sensor array field pick up (EC-GMR) has proved to be a promising approach in this application. In this paper, fusion of dual frequency is applied to EC-GMR scan data acquired with a novel differential array probe, to suppress the strong fastener response and hence improve the detectability of defects. Performance of the data fusion algorithm is evaluated on a two-layer aluminum sample with steel fasteners and notches introduced in the bottom layer using the electric discharge machine (EDM). Experimental results demonstrate that defects of different sizes can be successfully detected by evaluating the residual field after fusion. Defect orientation angle has minimal impact on the sensitivity of the probe owing to rotating eddy current excitation.