Research on eddy current testing method for titanium alloy bolts based on phase difference

Abstract Aircraft profiled bolts, serving as critical components within the aircraft maneuvering system, are susceptible to fatigue cracks at the root of the threads during operational use, posing significant risks to both human and aircraft safety. In this study, a three-dimensional electromagnetic simulation model of bolts was established to address the challenge of distinguishing defective signals from thread signals during eddy current detection of profiled bolts. The simulation results demonstrated that utilizing a bridge differential probe sliding along the bolt axial direction with coil spacing equal to N times the thread pitch effectively suppressed thread signals, enabling accurate detection of bolt defects. Subsequent investigation revealed a strong resemblance between thread signals and lift-off signals, leading to the proposal of a novel thread noise suppression and defect identification method based on the ‘Thread-Lift-off’ signal equivalent. The method reveals the relationship between the thread signal and the defect signal so that the differential detection coil spacing does not have to be equal to N times the pitch. Subsequently, eddy current detection probes were designed and detection experiments and parameter optimization were carried out. The results show that defects at the root of the thread with a depth of 0.3 mm can be clearly detected by probes with different coil spacing, which verifies the effectiveness and reliability of the detection method.