Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite single-lap joints

The strength and fatigue life of hybrid (bonded/bolted) joints with carbon-fibre reinforced plastic adherends have been investigated. The effect of adhesive material properties and laminate stacking sequence on the joint structural behaviour and failure modes were determined experimentally. Hybrid joints were shown to have greater strength, stiffness and fatigue life in comparison to adhesive bonded joints. However, the benefits were only observed in joints with lower modulus adhesives which allowed for load sharing between the adhesive and the bolt. Hybrid joints with high modulus adhesives showed no significant improvement in strength although increased fatigue life was observed due to the presence of the bolt. Three distinct stages in the fatigue life of hybrid joints were observed where the adhesive, the bolt and their combination were all contributing to the load transfer. Fatigue crack initiation was found to occur later in the hybrid joints where the bolt transferred a significant portion of the load. The failure mode of the joints was shown to be dependent upon the relation between the hybrid joint strength and the bearing strength of the laminate. Hybrid joints in which the strength greatly exceeded the laminate bearing strength failed catastrophically in net-section mode. In contrast, laminates with higher bearing strength failed in a non-catastrophic bearing mode. Careful comparison of the material bearing strength and adhesive bond strength is necessary to ensure increased joint strength and non-catastrophic failure.