Bridging stresses in notched 2-D carbon/carbon composites under cyclic loading using acoustic emission monitoring

摘要

Crack advance in Ceramic Matrix Composites (CMCs) is always assumed in presence of a damage process zone, where the cracked matrix is bridged by intact and/or failed fibres, which debond, slip and pull-out. In general. crack bridging enhances the crack resistance of the material by partially shielding the crack tip from the appiled load. In the present work using modified compact tension (CT) test geometry and cyclic loading pattern the R-curve behaviour of 10mm thick 2-D Carbon/Carbon Composite is calculated. Subsequently, based on the knowledge of a R-curve and adopting the assumption of Linear Elastic Fracture behaviour of the investigated C/C material, the crack-face bridging stress distribution is concluded. The Acoustic Emission (AE) activity was monitored during testing and a systematic correlation of AE parameters to fracture mechanics has been performed. It was shown that during the phase of stable crack propagation both the rate of generation of AE event and the felicity ratio remains almost constant. Furthermore, the bridging force concludes by the analysis again remains almost constant during the stable crack propagation region, while its transient region is an indicator of the size of the bridging zone.