STRESSES AROUND FASTENERS IN COMPOSITE STRUCTURES IN FLEXURE AND EFFECTS ON FATIGUE DAMAGE INITIATION .1. CHEESE-HEAD BOLTS

This study concerns a programme of research on stresses around cheese-head bolts in multilayered Carbon Fibre Reinforced Plastic (CFRP) material; both theoretical and experimental studies have been carried out. Commercially available finite element (FE) software was used to carry out three dimensional (3D) stress analyses of CFRP material specimens with holes, loaded in flexure in a cantilever-type of arrangement, the specimen being attached to a rigid support via bolts through the holes. The types of specimens studied were those used in experiments in a robust fatigue rig which was built for the project. The theoretical models showed regions of high direct and shear stresses in the specimens when bent over and away from the clamping support. The practical fatigue tests showed, via ultrasonic scanning and electron microscopy, where damage initiated in the test specimens after a large number of flexural loading cycles and the type of damage induced. Comparisons between theoretically predicted regions of high stresses and induced damage were made in order to establish damage inducing mechanisms and the levels, types and combinations of internal stresses which cause damage to occur.