THE DEVELOPMENT OF FATIGUE DAMAGE AROUND FASTENER HOLES IN THICK GRAPHITE-EPOXY COMPOSITE LAMINATES

This paper discusses the mechanisms by which damage develops and grows around countersunk fastener holes in composite laminates under fatigu e loading conditions. Experiments have shown that the erosion of mater ial between ply layers nucleates delaminations which then grow through the laminate under the action of the fatigue loads. The damage at the hole bore surface produces sites for the nucleation of delaminations. The morphology of the delaminations and ply cracking was mapped exten sively and from these maps it was found that the volume of material ar ound the fastener hole, damaged by the fatigue loading, adopted a char acteristic shape; the volume of damaged laminate increased towards the faying surface of the laminate and (metal) fixture. This characterist ic damage volume was generated by the fastener rocking under the fatig ue loads. Growth of the delaminations has been shown to be preceded by intraply cracking and, as fatigue loading proceeded, more delaminatio ns were generated at the hole bore surface. The interfacial region bet ween the composite laminate and the fixture also provided the nucleati on site for fatigue cracks in the fasteners. The effects of initial fa stener-hole clearances on damage nucleation and growth did not appear to follow any clear pattern. However, coupons with excessive initial h ole clearances did appear to exhibit greater than expected damage grow th after only one sequence block. Several techniques for the measureme nt of damage growth (development) were investigated. Stiffness measure ments of the test system were found to show only small changes with ho le wear and fastener rocking (using shadow moire techniques) also show ed only small changes with hole wear. Ultrasonic C-scanning methods we re used to map the extent of damage around the fastener holes with fat igue loading. The experimental work has shown that damage development around fastener holes is a complex process, usually producing several delaminations in the region of the fastener hole which grow and may ul timately lead to the failure of the coupon.