Stress-strain behavior of thermoplastic polyurethanes

The large strain nonlinear stress-strain behavior of thermoplastic polyurethanes (TPUs) exhibits strong hysteresis, rate dependence and softening. Thermoplastic polyurethanes are copolymers composed of hard and soft segments. The hard and soft segments phase separate to form a microstructure of hard and soft domains typically on a length scale of a few tens of nanometers. Studies have revealed this domain structure to evolve with deformation; this evolution is thought to be the primary source of hysteresis and cyclic softening. In this paper, experiments and a constitutive model capturing the major features of the stress-strain behavior of TPUs, including nonlinear hyperelastic behavior, time dependence, hysteresis, and softening, are presented. The model is based on the morphological observations of TPUs during deformation. A systematic method to estimate the material parameters for the model is presented. Excellent agreement between experimental results and model predictions of various uniaxial compression tests confirms the efficacy of the proposed constitutive model.