Cyclic-fatigue behavior of SiC/SiC composites at room and high temperatures

Tension-tension cyclic-fatigue tests of a two-dimensional-woven-SiC-fiber-SiC-matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000 °C. The cyclic-fatigue limit (107 cycles) at room temperature was approximately 160 MPa, which was approximately 80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000 °C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic-fatigue life at room temperature and at 1000 °C at stresses >180 MPa; however, cyclic-fatigue life decreased at 1000 °C at stresses <180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber-pullout length at 1000 °C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000 °C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interface between the matrix and the fibers.