Constitutive modeling of shape memory polymers

Shape memory polymer has emerged as one kind of functional and smart material. Constitutive modeling is necessary in order to better understand its thermomechanical behavior. Based on nonlinear elasticity and viscoelasticity, constitutive models of shape memory polymers are developed.;A three-dimensional nonlinear elastic constitutive model, which gives stress in terms of deformation gradient and temperature histories, is developed to study thermomechanical behavior of shape memory polymers. The model describes large deformation, shape memory effect, and phase transition. The concepts of the active and frozen phases of the polymer are adopted and the phase transitions during cooling and heating are formulated. Two general constitutive functions are used for the active and frozen phases, respectively. The constitutive equation for the entire material is obtained by integrating the constitutive equations for individual material points over the entire body. Comparison between model prediction and experimental data is given. The linearized constitutive equation of the model is compared with that of the Chen-Lagoudas model. It is shown that the linearized constitutive equation of the present model is more general. For a particular four-path experiment with constant strain during cooling, it reduces to the linearized constitutive equation of the Chen-Lagoudas model.;Moreover, viscoelastic constitutive models of shape memory polymers are developed. The viscoelastic models describe shape memory effect, phase transition, and strain-rate dependent behavior. The strain function formulations with the Maxwell model are developed by using two different methods separately with mutually verifiable results. Moreover, two stress function formulations for shape memory polymers are developed by using the Maxwell model and the Kelvin-Voigt model, respectively.