Multi-site crack initiation in local details of composite adhesive joints

Understanding the fracture mechanisms of thick bondline composite adhesive joints is of great importance for designing reliable large utility composite structures, e.g. wind turbine rotor blades. However, the complex fracture processes in the intricate local adhesive composite details are not yet fully revealed. This study aims to gain deep insights into the crack initiation behaviour of thick bondline composite adhesive joints through double cantilever beam tests and phase-field modelling. The crack initiation at multiple sites is examined and the interaction between cracks during the fracture process is investigated. We find that the fracture mode of the adhesive joint would change from the interface to the adhesive when voids exist in the adhesive front under Mode-I loading. If the primary crack occurs in the interface, the crack initiates in the composite layers. The crack initiation load and the peak load of the specimens are most affected by the critical energy release rate of different composites but much less by that of the adhesive. A slight modification on the composite layup leads to significant strength enhancement of the composite adhesive joints.