Fatigue Behavior of an Advanced Sic/Sic Composite with an Oxidation Inhibited Matrix at 1200C in Air and in Steam

The fatigue behavior of an advanced Silicon Carbide/Silcon (SiC/SiC) ceramic matrix composite (CMC) with oxidation inhibited matrix was investigated at 1200-C in laboratory air and in steam environments. The composite consisted of an oxidation inhibited SiC matrix reinforced with Hi-Nicalon fibers coated with pyrolytic carbon (PyC) with a boron carbide overlay woven into eight-harness-satin (8HS) weave plies. Tensile stress-strain behavior and tensile properties were evaluated at 1200-C. Tension-tension fatigue tests were conducted in both laboratory air and in steam at 1200-C at frequencies of 0.1 Hz, 1.0 Hz, and 10 Hz. The tension-tension fatigue tests had a ratio of minimum stress to maximum stress of R = 0.05, with maximum stresses ranging from 100 to 140 MPa in air and in steam. Fatigue run-out was defined as 105 cycles for the 0.1 Hz tests and as 2 x 105 cycles for the 1.0 Hz and 10 Hz tests. Strain accumulation with cycles and modulus evolution with cycles were analyzed for each fatigue test. The presence of steam degraded the fatigue resistance of the material at 0.1 Hz and 10 Hz. At 1.0 Hz, the presence of steam appeared to have little influence on the fatigue resistance for the fatigue stress levels lt;140 MPa. The presence of steam degraded the fatigue performance of the CMC at 1.0 Hz for the fatigue stress level of 140 MPa. Fatigue limit was 100 MPa (32.6% UTS) in air and steam at 1.0 Hz and in steam at 0.1 Hz.