Determination of the in situ fiber strength in ceramic-matrix composites from crack-resistance evaluation using single-edge notched-beam tests

A model for the single-edge notched-beam (SENB) test is presented which all ows the load-displacement curve of fiber-reinforced ceramic-matrix composit e (CMC) specimens to be calculated using the crack-opening function. This f unction considers the effects of the main parameters of the composite compo nents that govern the fracture of the composite (e.g., fiber strength and i nterfacial properties). Then, in an experimental study, the SENB curves of an oxide-oxide composite are measured in the as-processed state and after e xposure at elevated temperatures. Comparison between the measured and calcu lated SENB curves indicates that the characteristic loss of the maximum loa d in the SENB curves due to the annealing is caused by the corresponding re duction of the in situ fiber strength. This method then is proven to be an excellent tool to evaluate the actual fiber strength in CMCs, which is, oth erwise, barely measurable. It will be shown that the SENB test is very much qualified for a proper investigation of the relationship between in situ f iber strength and composite strength (the maximum SENB load), because of th e limited influence of other parameters (Weibull modulus of the fiber stren gth and the friction coefficient) on the maximum SENB load.