FATIGUE BEHAVIOR OF 3-D SIC SIC COMPOSITES

A tension-tension fatigue damage analysis was performed using 3-d sili con carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/ SiC) composites. Two groups of SiC/SiC specimens were tested. The firs t group consisted of samples without any oxidation protective top laye r coating, whilst the latter one contained samples covered with a well fitting, chemical vapour deposited (CVD) SiC system. This coating is necessary for the material to sustain high temperatures. Both the coat ed and uncoated material had a fibre volume fraction of about 36% equa lly distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found t o decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the f atigue test with the number of cycles to failure. In general, the pres ence of the coating layer decreases the static strength of the materia l. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxi dation protective SiC coating. Microstructural examination has also be en performed on the fractured specimens and it reveals some of the fai lure mechanisms of the composite that appeared under quasi-static and dynamic loading.