Fatigue behaviour of 3-d SiC/SiC Composites

A tension–tension fatigue damage analysis was performed using 3-d silicon carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/SiC) composites. Two groups of SiC/SiC specimens were tested. The first group consisted of samples without any oxidation protective top layer 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 coated and uncoated material had a fibre volume fraction of about 36% equally distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found to decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the fatigue test with the number of cycles to failure. In general, the presence of the coating layer decreases the static strength of the material. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxidation protective SiC coating. Microstructural examination has also been performed on the fractured specimens and it reveals some of the failure mechanisms of the composite that appeared under quasi-static and dynamic loading.