High-Temperature Off-Axis Fatigue Behavior of Unidirectional Carbon Fiber-Reinforced Composites with Different Resin Matrices.

Off-axis fatigue behavior of unidirectional continuous carbon fiber-reinforced composites has been studied for T800H/epoxy. AS4/PEEK, and T800H/Polyimide systems, with a particular emphasis on the effect of matrix and matrix/interface properties at elevated temperature. Tensiontension fatigue tests were performed at 100°C for various kinds of plain coupon specimens with different fiber orientation angles. It was observed for these composite systems that the fatigue strengths decreased with increasing off-axis angles. Regarding the epoxy- and PEEK-based laminae, almost linear off-axis S-N relationships were obtained over the whole range of fatigue loading up to 106 cycles. The off-axis fatigue strength of the polyimide-based laminae significantly dropped near the fatigue cycle of 104 which was ascribed to the initial defect of non-uniform fiber distribution. Normalizing the fatigue stress with the static strength at test temperature, the off-axis fatigue data eventually fell on a single S-N relationship for these composites. This characteristic was described well by a non-dimensional effective stress defined on the basis of the classical static failure theories. It was also demonstrated that the off-axis fatigue behavior of these unidirectional fiber composites was described by the classical composite fatigue failure models.