Off-Axis Compressive Strength of a Unidirectional CFRP Laminate at High Temperature and Its Modeling

The off-axis compressive strength and its rate dependence of a unidirectional carbon/epoxy laminate are examined for various fiber orientations at high temperature. Static compression tests are carried out at two different strain rates of 0.1 and 10%/min for each fiber orientation. The fiber-orientation and strain-rate dependence of off-axis compressive strengths and associated failure modes are first discussed. The off-axis compressive strengths are compared with the off-axis tensile strengths at the same strain rates and test temperature, and differences between the ultimate off-axis strengths in tension and compression are quantified. Then, the Hoffman failure criterion that distinguishes between the off-axis strengths in tension and compression is tested for the accuracy of prediction of the different off-axis tensile and compressive strengths and the associated failure envelope at different strain rates. It is demonstrated that while the fiber orientation dependence of the off-axis tensile and compressive strengths are predicted with reasonable accuracy, the failure envelope associated with off-axis compression is poorly described using the Hoffman failure criterion. This observation suggests that consideration of the influence of the transverse compressive stress on the in-plane shear strength along fibers is essential for accurate prediction of failure envelope. Finally, a modified form of the Hoffman failure criterion that meets the requirement is proposed, and it is demonstrated to have an improved predictive capability compared with the original form.