Previous results have shown that tows of SiC Nicalon fibers are sensitive to the phenomenon of delayed failure, at temperatures below 700°C. The present paper examines the static fatigue of Hi‐Nicalon and Hi‐Nicalon S when subjected to constant load, at temperatures between 500° and 800°C in air. Multifilament tows and single filaments were tested. Experimental data show that the rupture times of tows depend on the applied stress according to the conventional power law tσn=A. In contrast, the stress‐rupture time data obtained on single filaments exhibit significant scatter. A model based on slow crack growth in single filaments shows that the stress‐rupture of fiber tows follows the conventional time power law. The dependence on temperature was introduced. The model allowed sound calculations of tow lifetimes and characteristics of the slow crack growth phenomenon to be extracted from the tow stress‐rupture time data.

Relations between fracture toughness and fiber/matrix interphases were examined on various SiC/SiC composites made by chemical vapor infiltration (CVI) and reinforced with woven fiber bundles. Strong and weak fiber/matrix bondings were obtained using multilayered interphases consisting of various combinations of carbon and SiC layers of different thickness and using fibers which had been previously treated. Fracture toughness was estimated using the J‐ integral and using strain energy release rate computed with a model taking into account the presence of a process zone of matrix microcracks. Both approaches evidenced similar trends. It appeared that higher toughness was obtained with those composites possessing strong interphases and subject to dense matrix microcracking.