Off-axis fatigue behavior of plain weave carbon/epoxy fabric laminates at room and high temperatures and its mechanical modeling

Off-axis fatigue behavior of a plain-woven carbon/epoxy fabric laminate at room temperature and 100 °C has been studied. Tension–tension fatigue tests are first performed on plain coupon specimens with five kinds of fiber orientations θ=0, 15, 30, 45, 90° at a frequency of 10 Hz in respective temperatures. Test results show that the fatigue behavior of the carbon fabric laminate significantly depends on fiber orientation. The on-axis S–N relationships plotted on logarithmic scales are almost linear over the range of fatigue life up to 106 cycle. By contrast, the off-axis S–N relationships are characterized by their S-shape. The log–log plot of the off-axis fatigue data becomes steeper in an intermediate range of fatigue life than before, suggesting increased sensitivity to fatigue, and it subsequently reaches a plateau indicating an apparent fatigue limit. The fiber orientation dependence of the off-axis fatigue strength can substantially be removed by normalization with respect to a modified static strength. These features are common to the fatigue behaviors at room temperature and 100 °C. Additional fatigue tests with a frequency of 2 Hz demonstrate that the shape of the off-axis S–N curve at room temperature is significantly affected by load frequency, as a result of a different specimen heating. Comparison between the high-temperature fatigue data at 2 and 10 Hz reveals that the S-shape of the off-axis S–N curve is caused partly by an intrinsic rate dependence of the carbon fabric laminate. Applicability of a phenomenological fatigue model that considers the effect of fiber orientation and of multiaxial state of stress is also examined for description of the off-axis fatigue behavior at 10 Hz. The results illustrate that the fatigue model can be used for describing the off-axis fatigue behavior of the carbon fabric laminate at room temperature and 100 °C for a given frequency of fatigue loading.