Bearing Behavior of Mechanically Fastened Joints in Composite Laminates--Part II: Failure Mechanisms

The bearing response and failure mechanisms of mechanically fastened carbon/epoxy laminated composite joints were investigated. Some results of the experimental study on strength and local strains were presented in Part I. The investigation on failure mechanisms is summarized in this paper. The major focus of the second part of the experiments was to determine the effect of the stacking sequence and the influence of clamping pressure exerted by a bolt on the bearing failure mechanisms. External damage was examined on a macroscopic scale and internal damage was evaluated using optical microscopy. To evaluate the extent of internal damage, polished cross-sections of the specimens were taken from a bearing plane and from a shear plane tangent to the hole boundary. Fiber orientation has an influence on the location around the hole circumference where failure initiates. Micrographs of the cross-ply and quasi-isotropic laminate specimens showed the importance of the 0degrees ply behavior. Intensive shear delamination appeared between the 0degrees ply and the adjacent inner ply. The pattern of bearing damage under the washers is similar for different clamping methods, but the ability of the joint to sustain damage increases with the level of clamping pressure.