During cyclic fatigue at room temperature, matrix damage is determined by the maximum stress. It is created during the first cycles. Fatigue resistance is governed by the damage of fibers and the fiber — matrix bonds. Two different fatigue behaviors have been observed: after 1000 cycles, either the elastic modulus remains constant and the specimen is run­ning out, or it decreases until the specimen ultimately fails.48 The modulus degradation reflects either wear at cracked interfaces48 or growth of interface cracks. Under stresses smaller than 100 MPa, ultimate fail­ures are generally not observed after 106 cycles under tension-tension fatigue.

At high temperatures, additional phenomena acti­vated by environment (oxidation, creep, or slow crack growth) may operate and cause the extension of initial stress-induced matrix damage and interface cracks as well as the weakening of fibers by degradation or reloading. Reloading of fibers involves changes in load sharing. If the strength of tows is exceeded by the applied stresses according to the mechanism described previously, ultimate failure occurs. The rup­ture of tows dictates the lifetime.

The matrix cracks created upon loading become the pathways for the ingress of oxygen into the mate­rial. The PyC interphase is consumed, which causes fiber reloading. Creep of the SiC matrix (at very high temperatures, above 1200 °C) makes the stresses on the fiber to increase, which enhances matrix creep and further fiber reloading. Creep of fibers (at tem­peratures above 1200 °C) causes matrix reloading and possible matrix and interface cracking or crack prop­agation, leading to fiber reloading by decrease ofload carried by the matrix.

Finally, the slow crack growth in fibers (at tem­peratures below 1100 °C) is activated by oxidation of carbon grain boundaries, leading to delayed failure.49,50 This phenomenon, which was observed at

intermediate temperatures between 500 and 900 ° C, was first referred to as the ‘pest phenomenon’ by a few authors. The SiC/SiC were claimed to be susceptible to degradation by oxidation embrittlement.

In order to protect the PyC interphase against oxidation, multiple coating concepts have been explored and multilayered interphases and matrices have been developed.21 Such multilayered matrices contain phases that produce sealants at high temperatures, causing healing of the cracks and preventing oxygen from reaching the cracks and the interphases.2,22,51 Lifetime is also improved with oxidation-resistant interphases such as BN or multilayers.52

2.12.6.3 Thermal Shock

CVI SiC/SiC composites have been tested under thermal shock with excellent results.2,53 CVI SiC/ SiC generally had good strength retention after ther­mal shock cycles involving heating up to the desired temperature and then cooling down in water at 20 °C.