Modeling Deformations in Viscoelastic Materials undergoing Microstructural Changes due to Mechanical Stimulus

This study presents a thermodynamics framework for describing responses of viscoelastic materials undergoing microstructural changes when exposed to mechanical loadings. The constitutive models are derived based on multiple natural configuration theory, in order to accommodate the evolution of the material microstructures in a simplistic way. Within the multiple natural configuration theory, the microstructures of such materials are assumed to evolving between different stress-free natural configurations. In this study, two natural configurations are considered, i.e., the initial natural configuration which is associated with the microstructures at original state where no external stimuli are applied, and the final configuration which is associated with the microstructures where all possible microstructural changes are completed. Therefore, the net effect of any possible microstructural changes, can be incorporated into the constitutive model using an internal state variable, which quantifies percent amount of microstructural changes. Within the thermodynamics framework, a three-dimensional constitutive model is developed for polyoxymethylene (POM) polymer, as an example for viscoelastic homogeneous material. As an additional application, with plant tissue as an example, constitutive model is developed to predict the responses of general anisotropic and heterogeneous viscoelastic materials. Furthermore, the constitutive model is modified to describe responses of POM polymer and glass fiber reinforced polyamide (PA6GF40), under different mechanical loading histories and isothermal temperatures. Elevated temperatures accelerate the microstructural changes in the viscoelastic polymers, which are captured by the model. Mechanical responses of POM polymer, plant tissue and PA6GF40 composite, such as quasi-static, creep, cyclic, are simulated and validated by comparing the simulated responses with experimental data. With a relatively small number of material parameters, the thermodynamically consistent models are capable of predicting the mechanical responses of viscoelastic materials undergoing microstructural changes. The multiple natural configurations based constitutive models are also computationally efficient, which makes them suitable for performing large scale structural analyses. As an application of the multiple natural configuration approach, structural analyses within shell finite element method are performed for a bilayer polymer comprising of different polymer constituents, e.g., elastic, viscoelastic, electro-active polymer, and light activated shape memory polymer (LASMP)