Modeling of progressive damage in metal-composite bolt joints: Insights into failure mechanisms

Accurate identification of the damage initiation location, propagation behavior, and damage modes of metal-composite joint structures is crucial for the design of civil aircraft. Considering the differences in the mechanical properties of metals and composites, a progressive damage model is developed based on a bilinear elastic-plastic hardening model and a three-dimensional (3D) Hashin damage model, which realizes the simultaneous analysis of the damage state of heterogeneous material joint structures. Experiments on seven typical multi-bolt metal-composite joints were conducted to investigate structural failure mechanisms. The results show that the geometric configuration and composite layup design are the dominant factors influencing the load-bearing capacity and failure modes. Optimizing the joint layout from a single-row to a double-row configuration increased the ultimate load by 57.8%. For the same configuration, optimizing the composite layup design improved stress redistribution between adjacent holes, enhancing the ultimate load by 19.5%. The improved finite element model demonstrated strong agreement with scanning electron microscopy (SEM) analysis, indicating that the competitive damage behavior between metal and composite components is the fundamental cause of diverse failure modes in the joints. The error is controlled within 5% compared with the experimental results, which verifies the accuracy of the model.