Investigation of microscale damage processes near adhesive-composite interfaces

The increasing use of composite materials in the aerospace industry has necessitated significant advancements in the prediction of damage in composite structures. Adhesive joints have recently become more widespread, as such joints offer key advantages over bolted joints, such as causing no damage during hole drilling and providing weight savings. Adhesive joints however, fail catastrophically. To advance the understanding of failure of composite adhesive joints, the problem is approached at the microscale to investigate the fundamental damage processes. Testing ofminiature adhesive joints (Lapped area: 5mm× 7mm) has been carried out using a micro tensile testing apparatus in an SEM chamber. Video recordings of the tests allowexamination of the evolution of damage processes during joint failure. Samples are tested underMode I dominant andMode II conditions. SEMimages of the post failure appearance of the failure surface are presented and failure mechanisms under Mode I and Mode II conditions are compared in the context of the ASTM standard for adhesive joint assessment. In conjunction with the experimental tests, a micromechanical finite element model of the interface region between a composite adherend and adhesive layer has been developed. An existing two-dimensional microscale RVE damage model is extended into three-dimensions, where accurate stress-strain response in comparison to experimental data is shown. Parameter studies onMode I andMode II strength at the fibre-matrix interface found that the Mode II interfacial strength had a negligible effect on the response of the RVE under loading in the transverse plane. The ply model is extended to represent the first ply and half of the adhesive region of an adhesive composite joint. The ply model is separated from an elastic-plastic adhesive layer using damageable cohesive elements. Parameter studies under Mode I and Mode II conditions were undertaken to demonstrate the ability of the model to reproduce the failure appearance of the joints in the experimental tests. Under Mode I conditions, bonds failed in both the adherend and adhesive, while under Mode II conditions, failure occurred exclusively at the adhesive-adherend interface. The damage parameters of the adhesive layer RVE are incorporated into a twodimensional global scale model through cohesive zone modelling. The results are compared with the experimental data. It was found that the microscale RVE underestimated the fracture energy of the experimental damage processes.