Stresses around fasteners in composite aircraft structures and effects on fatigue life

There is considerable interest in the use of composite materials in aerospace structures. One of the problems encountered in aircraft is that of fasteners and the stress distributions around fasteners in composite structural components are therefore of significance, particularly in relation to fatigue life in the region of stress concentration. This thesis concerns a programme of research on stresses around fasteners in multilayered Carbon Fibre Reinforced Plastic (CFRP) material; both theoretical work and experimental studies have been carried out. A commercially available finite element software, ANSYS, has been used to carry out three dimensional 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 on 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, after a large number of flexural loading cycles where damage initiated in the test specimens and the type of damage induced. A critical part of the work was to compare theoretically predicted regions of high stresses and induced damage in order to establish, if possible, damage inducing mechanisms and the levels, types and combinations of internal stresses which would cause damage to occur. Correlations have been made between the analytical and practical test results and a relationshp has been demonstrated between high values of direct stress throughout the specimen thickness and delamination. Cracks were associated with high values of shear stress in the plane of reinforcement of the specimens. Comparisons have been made for CFRP with three different stacking sequences in order to determine the most fatigue resistant of commonly used types of lay-ups.(DX184257)