Toughness characterization and acoustic emission monitoring of a 2-D carbon/carbon composite

The fracture behavior of carbon/carbon composites is mainly governed by the activation of multiple matrix cracking, shear band formation, fiber debonding and fiber pull out, promoting the stress redistribution and offering a kind of “stress shielding effect” against crack propagation. In order to understand these effects and quantify the evolution of different damage mechanisms in time, experiments have been conducted on compact tension (CT) test coupons, accompanied by continuous acoustic emission (AE) monitoring. Then, fracture mechanics analysis was applied on the experimental results and a signal pattern recognition classification process was developed for the AE data, supported by extensive microscopical examination and systematic ultrasonic inspection. Correlation between clusters, resulting from the classification algorithm of AE data, and damage mechanisms activated by load increase during the CT experiment was accomplished, using classification algorithm parameters. A relation between the felicity ratio and the effective crack length was introduced and the multiple matrix cracking resulting by the shear failure of carbon matrix was found to act as the dominant stress redistribution mechanism.