Fracture mode separation for delamination and interlaminar fracture for composites

A computationally efficient approach combining a generalized crack-tip element model and a fracture mode separation technique was first developed to calculate mode mixities for delamination cracks in composite laminates under pseudo three-dimensional condition. A generalized plate crack-tip element was derived based on classical laminated plate theory. Fracture mode separation was achieved by comparing the total strain energy release rate from plate theory with that from the three-dimensional near-tip solution, along with two supplementary finite element analyses for special loadings. Four different types of specimens were then developed to investigate the interlaminar strength and interlaminar fracture energy in some composites. Interlaminar stresses, total strain energy release rate, stress intensity factors and fracture mode mixities were computed for various ply stacking sequences and fiber orientations. Attempts were made to select the best stacking sequence and bimaterial interface based on minimizing the aforementioned parameters. In the third part, interlaminar fracture behavior and fracture toughness of multidirectional composite laminates were investigated experimentally with relatively wide end-notched flexure specimens. The interlaminar fracture toughness was obtained from the experimental records of load-displacement histories, which was then calibrated by classical laminated plate theory and finite element analysis based on the assumption of pseudo three-dimensional deformation. Tests of both unstable and stable crack extensions were performed. Influence of the friction between crack surfaces was evaluated by classical laminated plate theory and finite element analysis based on the assumption of pseudo three-dimensional deformation. Tests of both unstable and stable crack extensions were performed. Influence of the friction between crack surfaces was evaluated by reducing the contact area. In addition, effects of the overhang and span during ENF testing and of the specimen width on the interlaminar fracture toughness were also examined. Finally, the size of K-dominance region was determined by comparing the K-based stress field and the full stress field, which was obtained by finite element analysis. Both end-notched flexure and edge-delaminated specimens were considered for analysis. Effects of the specimen configuration, material properties, loading condition, stacking sequence, and off-axis fiber orientation on the size of K-dominance region were investigated. Limitations of the stress intensity approach in LEFM for composite laminates were discussed.