Load-rate dependency of ultimate tensile strength in ceramic matrix composites at elevated temperatures

Ultimate tensile strength of three continuous fiber-reinforced ceramic matrix composites, including SiCf/CAS-II (1D), SiCf/MAS-5 (2D) and SiCf/SiC (2D), was determined as a function of load (test) rate in air at 1100-1200 °C. All three composites exhibited a significant dependency of ultimate tensile strength on test rate. The application of the preload technique as well as the prediction of life from one loading configuration (constant stress-rate) to another (constant stress) suggested that the overall phenomenological macroscopic failure mechanism of the composites would be a power-law type of slow crack growth or damage evolution/accumulation. It was further found that constant stress-rate testing could be used as an alternative to life prediction test methodology for ceramic matrix composites, at least for a short range of lifetimes and when ultimate tensile strength is used as a failure criterion.