TESTING THE TENSILE PROPERTIES OF CERAMIC-MATRIX COMPOSITES

This article reports the development of a mechanical testing methodolo gy (including fixtures, extensometry, temperature control, and calibra tion) and procedure (including control mode options, and analysis) for ceramic-matrix composites (CMCs). Six different CMCs were tensile tes ted at room temperature (RT) and 1,000-degrees-C. An appreciable reduc tion in fracture resistance was measured for most of the systems at th e high temperature, but there was little effect of an order-of-magnitu de change in strain rate at either temperature. The reduced properties at 1,000-degrees-C could be essentially duplicated at RT for several of the composites by prior exposure in air at 1,000-degrees-C. The sam e exposure in vacuum was much less damaging; thus, oxygen would appear to be the culprit. Fractographic evidence indicated that fiber pullou t in any system was a function of test temperature rather than fractur e strength. Moreover, the incidence and magnitude of energy-release ev ents during testing varied among the systems and revealed no unique co nnection with the state of damage induced by the prior exposures. Thes e observations do not fit readily into any simple theoretical model. H owever, they do serve to identify potential limitations for this class of composite in terms of sensitivity to thermal transients in a non-i nert environment.