Inspection of the Lower Half of Wing Lap Joints with EMATs

Detection of fatigue cracks at the fastener holes in the lower portion of the C5A wing lap joint is complicated by lack of a direct line of access, and by the presence of fasteners and sealant material. Furthermore, any successful detection procedure must take into account the wide variation in the geometrical features of the joint. In this work, periodic permanent magnet EMATs (electromagnetic-acoustic transducers) have been employed to excite the n=o horizontally polarized shear mode of the skin at 200 kHz and 250 kHz. These modes are partially transmitted into the overlap region joined by the fastener. Spectral analysis of suitably time gated and apodized portions of the reflected waveform have allowed simulated cracks growing out of fastener holes to be detected, and preliminary sizing algorithms have been developed.