Determination of recovery energy densities of shape memory polymers via closed-loop, force-controlled recovery cycling

Shape memory polymers offer semi-active properties, which can be used for actuation. Possible applications are deployment or release mechanisms in spacecraft, or the shape readjustment of high precision composite parts. For these kinds of applications, especially the recovered energy density, which is the product of recovered strain and recovered stress, is an important figure of merit. The present study shows the experimental procedure for the determination of recovery energy densities for tension and compression samples. To ensure simultaneous recording of loaded recovered strain and stress, closed-loop, force-controlled recovery cycles are performed using an Instron tensile testing machine and a thermal chamber. More-dimensional working fields of a polystyrene-based shape memory polymer are determined, using the introduced experimental technique. The resulting more-dimensional working fields show the nonlinear relationships between recovery ratio, maximum strain and recovery stress. It was discovered that the optimal working points of the tension and compression mode are complementary, which leads to an extended working range of SMP actuators if both modes are considered. As an application example for SMP energy recovery, a finite element simulation of the shape readjustment of a high precision composite part is shown.