Fiber fracture and strength degradation in unidirectional graphite/epoxy composite materials

An experimental study was conducted to determine the influence of load factor on fiber fracture development and residual strength of fatigue loaded unidirectional graphite/epoxy composite laminated. 8-phy composite laminates with a layer of release cloth imbedded at the middle ply were fatigued at different load levels and were examined for fiber fracture and residual strength at several stages of life based on the average number of cycles to failure (according to S-N data). From the experimental results, it is evident that the number of fiber fractures is nearly constant after the first few percent of the life. It is also suggested that the load level is much more important than the number of cycles of loading in the determination of the state of fiber fracture. This behavior was interrupted at high load levels (S60% S_u where the final fracture was highly affected by the longitudinal matrix splittings. Residual strength is found to be independent of the global fiber fracture density, and to be controlled by local behavior such as matrix cracking, local clustering of fiber fractures, and other local stress concentrations.