Delamination onset prediction in mechanically fastened joints in composite laminates

A three-dimensional finite element model is created to simulate a mechanically fastened joint in a composite laminate. Using the results from the finite element model, a spline approximation for the through-thickness stress is applied in order to determine the stress state at the interfaces between the layers. A delamination onset criterion is then applied at each interface. After validation, the model is used to assess the effects of stacking sequence and clamping pressure on the delamination onset lends and surfaces. It is concluded that "blocked" laminates lead to lower delamination onset loads and larger initial delaminated regions. Finger-tight and clamped joints have higher delamination onset loads and smaller initial cracked regions than pin-loaded joints. These results are in agreement with experimental observations, but no significant difference is detected between finger-tight and clamped joints. It is therefore concluded that the through-thickness pressure has an effect not only on damage initiation but also on damage growth, as well as in the load transferred by friction between the washer and the laminate.