Experimental and numerical study of process-induced defects and their effect on fatigue debonding in composite joints

Laboratory coupon joints for fatigue debonding tests usually have narrow width and a through-width initialdisbond. However, realistic structural joints are much wider and may contain process-induced defects and ac-cidental damage; both are much smaller than the joint width. Small and discrete damage may behave differentlyfrom the idealised through-width disbond crack. This has brought a question on whether the laboratory couponjoint can accurately represent the fatigue behaviour of wider structural joints. This paper presents an experi-mental and numerical study of fatigue behaviour of a wide bonded lap joint with a process-induced defect ofsemi-circular shape. Fatigue debonding propagation was monitored by ultrasound inspection. Fatigue life waspredicted using a normalised strain energy release rate parameter calculated byfinite element method, and theadhesive material fatigue crack growth rate data measured under single and mixed mode conditions. Simulationof process-induced defect and validation by experiments have brought a better understanding of fatigue de-bonding behaviour in wide joints containing realistic damage. Suggestions are given for fatigue fracture tests ofbonded joints.