A modified failure envelope method for failure prediction of multi-bolt composite joints

This paper presents a modified failure envelope to predict final failure mode and strength of multi-bolt composite joints based on the conventional failure envelope method, which was presented by Hart-Smith from abundant strength tests of open hole laminates, double-lap single and multi-bolt joints with quasi-isotropic or near-quasi-isotropic lay-ups. In contrast to the bearing “cutoff” of conventional failure envelope method, the modified one takes into account the effect of bypass load on the bearing failure and a new polyline consisted of two oblique lines is proposed. It is able to be established by an additional compressive strength test of laminates and semi-analytical or numerical analysis of fastener-hole laminates and open-hole laminates, combined with the point stress criterion. A flowchart of integration structure failure analysis process with the modified failure envelope method is provided. Series of tests were carried out to provide basic parameters and finite element analysis was conducted for the modified failure envelope. The method proposed was applied to two-bolt and four-bolt carbon-to-carbon double-lap joints with near-quasi-isotropic lay-ups. The predictions of the method proposed and the conventional one were compared with the tests data. The results indicate that the method proposed can obtain effective prediction of failure modes, and more accurate ultimate failure loads in double-lap composite bolted joints.