Influence of Specimen Geometry and Surface Quality on the Bending Strength of Short Fiber-Reinforced C/SiC

For continuous fiber-reinforced ceramic matrix composites, there is an extensive knowledge of the influence of specimen geometry on the failure mode and the determined strength in bending tests. In contrast, there are almost no studies on this topic for short-fiber-reinforced CMC, and as a consequence no testing standard for bending tests for such materials exists. In the present work, the influence of the specimen cross-section and the span width on the calculated mean and variance of the bending strength of different short fiber CMC (C/SiC with 3 mm and 10 mm fiber length) were examined. Below a certain specimen cross-section a decrease of the determined bending strength was observed. The relationships between the orientation of fiber bundles in the loaded area of the bending sample, the region of failure initiation and measured failure stress was investigated by high resolution X-ray CT in order to obtain further insight into the causes of strength scatter in these materials. The effect of different surface qualities prepared by grinding and milling on the measured bending strength in short fiber C/SiC was found to be negligible.