Fracture mechanics of composites with residual stresses, imperfect interfaces, and traction-loaded cracks

By partitioning the total stresses in a damaged composite into either mechanical and residual stresses or into initial and perturbation stresses, it was possible to derive two exact results for the energy release rate due to crack growth. These general results automatically include the effects of residual stresses, traction-loaded cracks, and imperfect interfaces. These effects are normally not needed in the fracture mechanics of homogeneous materials, but they are commonly needed for the fracture mechanics of composites. The general results were used to consider mode I fracture in composites, fracture and thermal cracking for two-phase, isotropic composites, and interfacial fracture in the microbond and single-fiber, pull-out tests. The analysis of interfacial fracture illustrates the importance of including friction effects in the energy release rate and not as part of the toughness of the composite. Many composite damage modes consist of a series of events instead of stable crack propagation. A new analysis method, referred to as finite fracture mechanics, is proposed which predicts that the next event occurs when the total energy released by that event exceeds some critical value or toughness for that type of event. A finite fracture mechanics model for microcracking that can correlate the results from many laminates is described.