Rotational GMR magnetic sensor based eddy current probes for detecting buried corner cracks at the edge of holes in metallic structures

In this study, eddy current rotational probe based on giant magneto-resistance sensor (GMR) and circular coil were used to detect subsurface corner flaws at the edge of holes in metallic structures. Aluminum samples contain holes of 0.25" in diameter aligned in a row used to demonstrate this technique. The distance between the centers of two adjacent holes is 0.75". First sample has two 0.125" thick layers. A 0.06" long corner crack is located in one of the layers at the edge of a hole. The height of the corner crack is 0.04". Three configurations were tested to detect the corner crack at three different depths within the structure: a) at the bottom of the first layer; b) at the top of the second layer: c) at the bottom of the second layer. The second sample is a single layer and has a 0.25" in thickness and 0.075" long corner crack is located at the bottom of the sample; the height of the crack is 0.04". A GMR sensor is located above the coil containing the defect, at a distance equal to the mean radius from the center of the coil. The sensitive axis of the magnetic sensor was oriented tangential to the hole. The probe was then rotated about the axis of the hole containing the defect. Two circular coils of different mean diameter were used for this probe. The frequency and phase were optimized to detect corner cracks at different depths of the structure.