Ultrasonic imaging of cracks around fastener holes

The principles of a non-destructive ultrasonic transmission techniques for imaging and sizing cracks around fastener holes is described. This method relies on the detection of the shadow region of the crack when illuminated with focused shear waves, and which, given a sealant layer with sufficient acoustic transparency, permits measurements on cracks in the rear layer of a two-layer system, with the fastener in place, and using single-sided access. The primary advantage of a transmission method over a pulse-echo to image the crack. This also means that so long as the crack casts a shadow; the orientation of position with respect to an interface or surface is not critical. It is anticipated that with further design of a suitable probe head and coupling arrangement, then the proposed method would make repair of structures where accessibility and working environment considerations are important, more cost-effective.