A study into the interaction of intergranular cracking and cracking at a fastener hole

This paper describes the results of a study into a new class of multi-site damage problems, viz: the interaction between multi-layer intergranular cracks and cracks that grow from small naturally occurring material discontinuities at a hole. The nature of the intergranular cracks, which are perhaps better thought of as delaminations rather than cracks, are chosen so as to approximate those found at fastener holes in RAAF AP3C (Orion) aircraft which arise due to ingress of the environment down the holes in the wing structure. The particular form of the multi-site damage studied involves two intergranular cracks (delaminations) that (in the vicinity of the hole) divide the plate thickness into thirds) interacting with cracks that emanate from the bore of the hole in a 7075-T6 aluminium plate. We first determine the effect of the intergranular cracks (delaminations) on the stress field at the hole. We then evaluate the effect of these intergranular cracks on the stress intensity factors associated with cracks that emanate from small naturally occurring material discontinuities at the hole and then on the reduction in the time for a crack at a hole, with multi-layer intergranular cracking, to grow to a size where it will be readily detectable.