Off-Axis Fatigue and Its Damage Mechanics Modeling for Unidirectional Carbon Fiber-Reinforced Composite at Room and High Temperatures.

Effects of the temperature-dependence of matrix and fiber-matrix interface properties on the offaxis fatigue behavior of unidirectional T 800 H/Epoxy composite have been studied. Tensiontension fatigue tests were performed for seven kinds of plain coupon specimens with different fiber orientation angles at room temperature (RT) and 100°C. Using a non-dimensional effective stress defind on the basis of the classical static failure theories together with a formulation based on the continuum damage mechanics, a fatigue failure criterion was developed. The off-axis fatigue strengths at RT and 100°C decreased as the off-axial angle increased, and they were reduced significantly as the test temperature became higher. Almost linear S-N relationships were obtained in the whole range of fatigue loading up to 106 cycles, irrespective of off-axis angles and test temperarures. Normalizing the maximum fatigue stress with the static strength at test temperature, the off-axis fatigue data eventually fell on a single S-N relationship which showed no apparent dependence on temperarure. This characteristic was described well by the non-dimensional effective stress which may be interpreted as a theoretical strength ratio. It was also verified that the fatigue damage mechanics model proposed can describe the observed off-axis S-N relationships and it has a potential capability in more practical fatigue failure analyses for multidirectional laminates.