Effect of the adhesive layer on mechanical properties and load distribution in multi-bolt composite joints

The mechanical properties and load distribution of single-lap composite hybrid bonded/bolted joints under tensile loading were investigated experimentally and numerically. The damage loads, failure modes, and load distribution of the multi-bolt mechanical joints and the hybrid bonded/bolted joints were tested. The results show that the failure modes of the laminates are a mixture of tensile and extrusion damage for both joints, and the failure mode of the adhesive layer is adhesive peeling in the hybrid bonded/bolted joints. The presence of the adhesive layer makes the load distribution of the hybrid joints more uneven, so the overall structural damage load of the mechanical joints is as slightly greater than that of the hybrid bonded/bolted joints. ABAQUS display solver was used to establish the progressive damage model for multi-bolt mechanical and hybrid bonded/bolted joints. The VUMAT subroutine was used to compile the damage criterion of the composite material and to simulate the failure of the adhesive layer using the cohesive element. The model can effectively predict the damage load, failure mode and load distribution of the structure. The hybrid joint's adhesive layer damage evolution mechanism was described, and the influence of the adhesive layer on the load distribution during the loading process was analyzed. The results of the load distribution indicate that the load distribution of the mechanical joints shows a "basin" distribution with high sides and low middle. The hybrid joints' adhesive layer delays the bolt loading and changes the ratio of load distribution, which makes the outermost bolt take more load and accelerates the damage to the structure.