Creep behavior of unidirectional and angle-ply T800H/3631 laminates at high temperature and simulations using a phenomenological viscoplasticity model

High-temperature creep behavior of symmetric angle-ply laminates made of unidirectional T800H/3631 carbon/epoxy composite is examined at relatively high stress levels. Constant-stress creep tests in tension are performed at 100 °C for 5 h on plain coupon specimens of three types of angle-ply laminates [±30]3S, [±45]3S and [±60]3S under load control conditions. For each angle-ply laminate, creep tests are carried out at three different stress levels. Creep strain recovery following the 5-h creep is also observed for 5 h at the same temperature, after completely removing the creep stress. Creep responses are clearly observed in all kinds of angle-ply laminates. The creep strain rate in the angle-ply laminates tends to rapidly disappear as the creep strain increases. The transient creep is thus dominant in the angle-ply laminates, regardless of the fiber orientations. The prior creep strain does not completely recover with time after removing the creep stress, which indicates certain inelastic mechanisms have operated with creep. Similar features are also observed for the off-axis creep behavior of the unidirectional laminates of the same composite system. A whole history consisting of the prior instantaneous elastoviscoplastic behavior at a constant strain rate and the subsequent creep response at a constant stress level is simulated using the classical laminated plate theory and a phenomenological viscoplasticity model for individual plies. Material constants involved by the ply viscoplasticity model are identified on the basis of the off-axis creep behavior for unidirectional laminates. It is demonstrated that excellent agreements between the predicted and observed results are obtained by additionally taking into account the fiber rotation induced by deformation.