Temperature dependence of off-axis tensile creep rupture behavior of a unidirectional carbon/epoxy laminate

Stress-time-temperature extrapolation of off-axis creep rupture data on a T800H/2500 unidirectional carbon/epoxy laminate is studied. Off-axis tensile creep rupture tests are performed on plain coupon specimens with five kinds of fiber orientations θ = 0, 10, 30, 45 and 90° at each of the test temperatures of 60, 80 and 130 °C, respectively, within the time range up to 10 h. Creep rupture of unidirectional specimens takes place predominantly along the fibers in a brittle manner, regardless of the fiber orientation and test temperature. Straight lines can be fitted well to the log–log plots of creep stress level against the time to rupture over the restricted range of time for all fiber orientations, regardless of the test temperature. Those fitted straight lines for different fiber orientations at each test temperature are almost parallel to each other, and they are approximately extrapolated to the stress levels nearly equal to the off-axis tensile strengths at the test temperature. The fiber orientation dependence of the off-axis creep rupture data obtained at each test temperature can approximately be removed by normalizing the creep stress levels with the help of the off-axis tensile strengths, which allows identification of a single fiber-orientation-independent master creep rupture curve for each test temperature. The effect of temperature on creep strength is reflected by the change in slope of the normalized master creep rupture curve. For predicting the off-axis creep rupture lives at different stress levels and temperatures, two kinds of new and efficient engineering methods are developed. One method is based on a formula derived from a modified damage mechanics model for creep rupture. The other one is formulated on the basis of a grand master creep rupture curve built by means of a non-dimensional effective stress and the Larson–Miller parameter. The proposed damage mechanics and grand master curve approaches are validated in respect of accuracy of prediction of the off-axis creep rupture lives of the unidirectional carbon/epoxy composite at different stress levels over a range of temperatures.