Fatigue of 2D and 3D Carbon-Fiber-Reinforced Polymer Matrix Composites and of a Unitized Polymer/Ceramic Matrix Composite at Elevated Temperature

Abstract

Tension–tension fatigue behavior of two polymer matrix composites (PMCs) and that of a unitized polymer matrix/ceramic matrix composite was studied at elevated temperature. The two PMCs consist of the NRPE polyimide matrix reinforced with carbon fibers but have different fiber architectures: one is a single-ply non-crimp 3D orthogonal weave composite and another, a laminated composite, reinforced with 15 plies of an eight-harness satin weave (8HSW) fabric. The unitized composite consists of a PMC co-cured with a ceramic matrix composite (CMC). The PMC part consists of the NRPE polyimide matrix reinforced with 12 plies of an 8HSW carbon fiber fabric. The CMC part consists of a zirconia-based ceramic matrix reinforced with three plies of an 8HSW quartz fiber fabric. In order to assess the performance and suitability of the three composites for the use in aerospace components designed to contain high-temperature environments, mechanical tests were performed under temperature conditions simulating the actual operating conditions. In all elevated temperature tests performed in this work, one side of the test specimen was at 329 °C, while the other side was open to ambient laboratory air. The tensile stress–strain behavior of the three composites was investigated, and the tensile properties measured for both on-axis (0/90) and off-axis (±45) fiber orientations. Elevated temperature had little effect on the on-axis tensile properties of the three composites. The off-axis tensile strength of the two PMCs decreased slightly at elevated temperature, while the off-axis strength of the unitized PMC/CMC remained unchanged. Tension–tension fatigue tests were conducted at elevated temperature at a frequency of 1.0 Hz with a ratio of minimum stress to maximum stress of R = 0.05. Fatigue runout was defined as 2 × 105 cycles. Both strain accumulation and modulus evolution during cycling were analyzed for each fatigue test. The laminated PMC exhibited better fatigue resistance than the other two composites. Specimens that achieved fatigue runout were subjected to tensile tests to failure to characterize the retained tensile properties. Posttest examination under optical microscope revealed severe delamination in the laminated PMC and the unitized PMC/CMC. The non-crimp 3D orthogonal weave composite offered improved delamination resistance.