A 3D finite deformation constitutive model for amorphous shape memory polymers: A multi-branch modeling approach for nonequilibrium relaxation processes |
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Authors: | Kristofer K. WestbrookPhilip H. Kao Francisco CastroYifu Ding H. Jerry Qi |
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Affiliation: | Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, United States |
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Abstract: | Shape memory polymers (SMPs) are materials that can recover a large pre-deformed shape in response to environmental stimuli. For a thermally activated amorphous SMP, the pre-deformation and recovery of the shape require the SMP to traverse its glass transition temperature (Tg) to complete the shape memory (SM) cycle. As a result, the recovery behavior of SMPs shows strong dependency on both the pre-deforming temperature and recovery temperature. Generally, to capture the multitude of relaxation processes, multi-branch models (similar to the 1D generalized viscoelastic model or Prony series) are used to model the time-dependent behaviors of polymers. This approach often requires an arbitrary (usually numerous) number of branches to capture the material behavior, which results in a substantial number of material parameters. In this paper, a multi-branch model is developed to capture the SM effect by considering the complex thermomechanical properties of amorphous SMPs as the temperature crosses Tg. The model utilizes two sets of nonequilibrium branches for fundamentally different modes of relaxation: the glassy mode and Rouse modes. This leads to a significant reduction in the number of material parameters. Model simulation comparisons with a range of thermomechanical experiments conducted on a tert-butyl acrylate-based SMP show very good agreement. The model is further utilized to explore the intrinsic recovery behavior of an SMP and the size effects on the free recovery characteristics of a magneto-sensitive SMP composite. |
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Keywords: | Shape memory polymers Constitutive models Thermomechanical behaviors Finite deformation |
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