Characterizing and Modeling the Free Recovery and Constrained Recovery Behavior of a Polyurethane Shape Memory Polymer

In this work, tensile tests are performed on a polyurethane shape memory polymer for both free recovery (extension recovery at zero load) and constrained recovery (stress recovery at constant extension) conditions. The experimental characterization is conducted on an electromechanical screw driven test frame, and a laser extensometer is used in conjunction with the electromechanical frame to provide a non-contact technique for measuring the deformation of the material. The specimens are deformed, above the glass transition temperature, to 10% extension. The SMP is then cooled, at a constant value of extension, to below the glass transition temperature to ‘lock’ the temporary shape. The extension recovery at zero load as well as the stress recovery at a constant value of extension is measured during the first shape memory cycle as the SMP is heated to above its glass transition temperature. The material is observed to recover 93% of the applied deformation when heated at zero load. In addition, a stress recovery of 1.5 MPa is observed when heated while holding a constant value of deformation (10% extension). After performing the experiments, the Chen and Lagoudas model, implemented in 1-D by Volk, et al., is used to simulate and predict the experimental results. The material properties used in the model — namely the coefficients of thermal expansion, shear moduli, and frozen volume fraction — are calibrated from a single free recovery experiment. The calibrated model is then used to simulate the material response for the free recovery tests as well as predict the response for the constrained recovery condition. The model simulations agree well with the free recovery experimental data but predict a larger compressive stress than what is observed during the constrained recovery experiment.