Fracture Mechanisms of SiC/SiC Composite by means of Acoustic Emission Analysis

A SiC/SiC composite formed from woven sheets of carbon coated SiC fibers has been examined during the tensile loading of both un-notched and double-edge-notched specimens. As the specimens were deformed at a slow crosshead speed of 0.02 mm/min, the load, strain and acoustic emission (AE) were all measured and characterized. Analysis of AE amplitude distributions, coupled with the stress strain curves and SEM observations indicates that the fracture mechanisms of the SiC/SiC composite consist of matrix cracking, debonding, fiber pull-out and fiber breakage. The fracture surfaces of the un-notched specimens were characterized by fiber pull-out. The fiber pull-out was a result of the stress concentration due to pores located in the woven yarns which produced heavy matrix cracking leading to fiber bundle breakage. Different behavior was found for the notched samples which demonstrated quasi-yield behavior during tensile loading. The notch was found to produce a localized stress concentration inducing debonding of the 0° direction fibers. Then fiber pull-out occurred after the breaking of fibers without causing substantial matrix damage. In the SiC/SiC coposite, pores located in the woven yarns act as an initial defect which causes matrix cracking. The resultant stress concentration from the matrix cracking causes numerous localized fiber breaks.