Damage tolerance characteristics of carbon fibre composites: behaviour under impact loads and post-impact fatigue.

One of the critical issue restraining the fully application of carbon fibre reinforced polymer (CFRP) composites to aircraft structures is barely visible impact damage (BVID) caused by low-velocity impact (LVI). Consequent internal damage, such as delamination, which is difficult to find by regular inspection, raising a great concern for the damage tolerance performance of CFRP. The main objective of this research study was to investigate the damage tolerance behaviour of carbon fibre composites under impact loads and post-impact fatigue. The project aims to explore and identify delamination growth and failure processes in impact-damaged carbon fibre epoxy composites under compressive cyclic loading and to explain the behaviour in the fracture mechanics method. Low velocity drop weight impacts were used to create the initial damage. There were four levels from 12J-25J. Maximum compressive loads during fatigue were between 70% and 80% of the nominal residual strength in compression after impact (CAI) tests. Delamination propagation was monitored at intervals during the test using DIC and C-scan techniques. It was found that compressive fatigue failure can be categorised into three phases: (1) local bending at impact damage site (2) local buckling mode change and (3) buckling propagation. Monitoring of local normal displacement at the impact damage site provided earlier indications of fatigue induced changes in delamination buckling than observations of delamination area growth. A three-dimension analytical model was developed and revealed that deep delamination has higher strain energy release rates at crack tip than delaminate at the surface of laminate.