Stress Relaxation Behavior of Unidirectional CFRP at Elevated Temperature and Analysis Using a Viscoplasticity Model.

Abstract

Off-axis stress relaxation behavior of a unidirectional carbon fiber-reinforced composite (T 800 H/3631) has been examined at high temperature. Stress relaxation tests are performed at 100°C on plain coupon specimens with various off-axis angles: θ=0, 10, 30, 45, and 90°. For each of these off-axis angles, the stress relaxation tests are carried out at three different strain levels. Just after the total strain is kept constant, the stress rapidly relaxes with time in a short period. Then, the relaxation rate of the composite stress tends to vanish. The extent of the transient stress relaxation is significantly dependent on the total strain kept constant and the fiber orientation angle. Simulation of the off-axis stress relaxation behavior is performed using a macromechanical viscoplasticity model based on an overstress. It is demonstrated that the model succeeds in moderately reproducing the time-dependent behavior of this system.