Eddy-current scratch inspection with high probe lift-off

High lift-off inspection in eddy current testing (ECT) has wide and important applications. However the reliable ECT detection for shallow scratches is generally carried out only on the surface or with a low lift-off height from probe because of the large lift-off noise of sensor and decreasing signal level in the high lift-off condition. In this paper, the proposed ECT probe is consisted of vertical rectangle exciting coil and spin-valve type giant magnetoresistive (SV-GMR) sensor to detect the surface scratches of 0.3 mm depth in a stainless steel plate with a 5 mm lift-off height. This structure of probe can suppress lift-off noise and pick up magnetic field signals up to several μT. In addition, 3-D finite element method (FEM) is applied to simulate the length information as well as scratches information using a single GMR sensor probe scanning. Experimental results prove that the proposed ECT probe has good detection capabilities in the condition of high lift-off height.