CRACK DEVELOPMENT IN CARBON POLYIMIDE CROSS-PLY LAMINATES UNDER UNIAXIAL TENSION

This study addresses matrix dominated failures in carbon fiber/polymer matrix composite laminates in a cross-ply layup. The events of interest were intralaminar fracture in the form of transverse cracks in the 90-degrees plies and longitudinal splitting in the 0-degrees plies, and interlaminar fracture in the form of 0/90 delamination. These mechanisms were observed using various non-destructive evaluation (NDE) techniques employed during static tensile tests. X-ray, acoustic emission, and edge view microscopy were the principal methods utilized in a real time environment. A comparison of the NDE results with an analytical model based on the classical linear fracture mechanics concept of strain energy release rate as a criterion for crack growth was performed. The virtual crack-closure theory was incorporated with a finite element model to generate strain energy release rate curves for the analytical case. Celion carbon fiber/polyimide matrix (G30-500/PMR-15) was the material tested with cross-ply lay-ups of (0(2)/90(6))s and (0(4)/90(4))s. The test specimens contained thermally induced cracks caused by the high temperature processing. The investigation of the damaged laminates gave insight into their capability of sustaining further loads, and the sequence of damage events was obtained. The analytical model was updated to compensate for the initial damage and to study further accumulation by taking into account the crack interactions. By correlating the experimental and analytical data, the critical energy release rates were found for the observable events of interest.