Fatigue Behavior of an Advanced SiC/SiC Composite at Elevated Temperature in Air and in Steam

摘要

The fatigue behavior of an advanced Silicon Carbide/Silicon Carbide (SiC/SiC) ceramic matrix composite (CMC) was investigated at 1200 deg C in laboratory air and in steam environments. The composite consisted of a SiC matrix reinforced with Boron Nitride (BN) coated Hi-Nicalon fibers woven into eight-harness-satin (8HS) weave plies. Tensile stress-strain behavior and tensile properties were also evaluated at 1200 deg C. Tension-tension fatigue tests were conducted in both laboratory air and in steam at 1200 deg 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 = 0.1, with maximum stresses ranging from 80-120 MPa in air and 60-110 MPa in steam. Fatigue run-out was defined as 105 cycles for 0.1 Hz tests and as 2 x 105 cycles for 1.0 Hz and 10 Hz tests. Strain accumulation with cycles and modulus evolution with cycles were analyzed for each fatigue test. The CMC fatigue performance degraded after moving from the air to the steam environment. Fatigue limit was 100 MPa (46%UTS) in air and 80 MPa (37%UTS) in steam. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. All specimens tested in air retained 100% of their tensile strength and only one specimen tested in steam showed strength degradation of approximately 5%. The average modulus loss in air and in steam was limited to 12%.