Effects of stretch induced softening to the free recovery behavior of shape memory polymer composites

During a cyclic tension test, many elastomeric materials exhibit an appreciable softening in their mechanical properties resulting from the previous stretch, known as the Mullins effect. This paper explores the influence of the stretch induced softening effect to the free recovery behavior of an acrylate shape memory polymer (SMP) composite by incorporating carbon black (CB) as filler materials. The observed softening effect in this SMP composite is considered to be a consequence of stretch induced alternation of filler–polymer interactions inside the composite. Further experiments find that a larger prior stretch gives a larger increase in material softening, which in turn decreases the shape recovery speed. To capture the experimental observations, a multi-branch one dimensional (1D) model is applied, where the modulus in the equilibrium branch is modeled to decrease with stretching deformation following a damage-like softening function. It is found that the loss in modulus due to softening consequently reduces the driving force for recovery and thus results in a slow recovery. Parametric studies further demonstrate that the discounted shape recovery speed will finally reach a saturated level when gradually increasing the programmed strain level in a shape memory cycle.