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High-temperature off-axis fatigue behavior of unidirectional T800H/Epoxy composite has been studied. Effects of different temperature on off-axis fatigue strength are also discussed. Tension-tension fatigue tests are performed at room temperature and 100°C, respectively, on six kinds of off-axis plain coupon specimen. The absolute off-axis fatigue strengths are significantly reduced as the test temperature becomes higher. Irrespective of test temperature, almost linear S-N relationships are obtained for all off-axis angles over the range of fatigue life up to 106 cycles. Normalizing the maximum fatigue stress with respect to the static tensile strength at test temperature, the fatigue data for all off-axis angles eventually fall on a single S-N relationship. It is shown that a theoretical fatigue strength ratio can be defined as a non-dimensional effective stress based on the classical static failure theory and it succeeds in coping with the directional nature of off-axis fatigue strength. A fatigue damage mechanics model is developed using the non-dimensional effective stress, and its capability to describe the off-axis S-N relationship for the unidirectional T800H/Epoxy is demonstrated. Comparisons with the classical fatigue failure models for composites are also presented.

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