Failure mechanisms in bolted CFRP

An experimental investigation on the damage mechanisms of mechanically fastened joints in composite laminates is presented. This information concerning damage mechanisms is required before developing damage progression models. For double-lap joints with finger-tight washers, specimens that fail in the bearing, tension and shear-out modes are investigated. For fully failed specimens and for specimens loaded to several percentages of failure load. the damage mechanisms are investigated using X-radiography and sectioning. It is concluded that failure occurs by a process of damage accumulation, where the failure mechanisms present are fiber fracture, delamination at the laminate loaded hole, matrix cracks and related fiber microbuckling, and internal delamination. Modelling this interaction between failure mechanisms requires further analytical efforts. Due to the importance of the through-thickness stresses present at the hole boundary, delamination has a significant effect on the joint strength, so the use of a three-dimensional failure criterion is suggested.