Fracture characterization of C/C composites under various stress modes by monitoring both mechanical and acoustic responses

C/C composites with different porosities, produced by chemical vapor infiltration have been mechanically tested under quasi-static loading in bending modes using uniform and notched specimens. The acoustic emission (AE) method was used to monitor the damage accumulation profile during loading up to fracture, supported by optical and scanning electron microscope characterization. Three stages in the damage buildup up to fracture were observed: Stage I, with no AE activity, Stage II, gradual growth in AE counts up to an abrupt jump and Stage III, sharp increases in AE counts. Moreover, the similarity in the profile between the cumulative AE counts vs. strain data and the predicted crack density vs. strain by the micro mechanical model suggested for interlaminar cracking, indicates the importance of AE in monitoring the damage evolution in composites in terms of AE counts. Fast Fourier transform analysis of the AE waves revealed three characteristic frequencies in Stage III, which is a sign of three main micro-mechanisms of failure which control the failure progress: fiber fracture, debonding and matrix cracking seem to be the active mechanisms.