Eddy current analysis of mid-bore and corner cracks in bolt holes

Fatigue cracks are prone to develop around fasteners found in multi-layer aluminum structures on aging aircraft. Probability of Detection (POD) studies using eddy current techniques within the bolt holes contribute to risk assessments used in evaluating the serviceability of these aircraft. The signal response to corner and mid-bore cracks by eddy current testing using standard split-D differential probes has been examined. The data indicate that split-D probes are primarily sensitive to cracks at the corners of the Ds and that this gives rise to a double maximum for small mid-bore cracks as the corners pass over the crack. The double signal is not seen for corner cracks because the excitation coil, which is almost completely out of the hole, excites almost no eddy currents when the second corner of the D is over the crack. However, the eddy current response is more sensitive to corner cracks. The a90/95 for corner cracks was measured to be 0.22 mm (length) compared to 0.34 mm (depth) and 0.62 mm (length) for mid-bore cracks. The enhanced detectability of corner cracks is attributed to the presence of the edge, which restricts passage of eddy currents around the crack.