Mode-II Fracture at the Interface between a Base and a Repair Graphite/Epoxy Laminates

Experimental investigation of the characteristics of the interface between a base laminate [0(8)](T) and a repair laminate [0(8)](T) was carried out through Mode II fracture tests, compliance calibration tests, and microscopic observations. The material used for manufacturing the test samples is unidirectional carbon fiber prepreg (Newport Composites NCT-301 graphite/epoxy). The laminate lay-up used in all experiments was [0(16)](T). The specimens were subjected to variations in the autoclave pressure in the repair curing cycle. A standard sample (reference) was subjected to the manufacture recommended cure cycle. The repair samples were subjected to the manufacturer recommended curing cycle (for temperature) with different values for pressure. Results show a significant effect of pressure. However, the effect of the magnitude of the pressure is not significant after some threshold values. More than 28% drop in the fracture toughness is experienced by the lack of bonding between the older previously cured matrix and the newly added composite laminate. There is some difference between the Mode-II test and compliance calibration test, but no preference, in terms of accuracy, can be given to what method to be used to obtain Mode II fracture toughness.