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Ying Li, Associate Professor, Department of Mechanical Engineering, University of Wisconsin-Madison
Abstract: Elastomers consist of long polymer chains joined together by chemical bonds through cross-linkers. They are usually capable of recovering their original shapes after finite deformation due to covalent cross-linkages. During a loading-unloading process, energy dissipates due to friction between polymer chains, and the elastomer exhibits viscoelastic behavior. The viscoelasticity of elastomers makes them widely used as energy-absorbing materials, such as coatings and foam cushions. Nevertheless, understanding the viscoelasticity of elastomers remains a challenge for the mechanics community since we are limited by phenomenological models. In this talk, I will present our recent efforts on developing a physics-informed constitutive model for the viscoelasticity of elastomers. Mathematically, the viscoelasticity of elastomers has been decomposed into hyperelastic and viscous parts, which are attributed to the nonlinear deformation of the cross-linked polymer network and the diffusion of polymers chains, respectively. The hyperelastic deformation of a cross-linked polymer network is determined by the cross-linking density, the molecular weight of the polymer strands between cross-linkages, and the entanglements between different chains. The viscous stress is governed by the diffusion of polymer chains, which can be understood through their tube dynamics. Combing a non-affine network model for hyperelasticity and a modified tube model for viscosity, both understood by molecular simulations, we develop a mechanistic-based constitutive model for the viscoelasticity of elastomers. This physics-aware constitutive model has been demonstrated experimentally to capture different chemistry signatures of two stockpile silicones (LVM97 and SE54), supplied by the Lawrence Livermore National Laboratory. Therefore, parametric materials design concepts can be easily gleaned from the constitutive model. Such a mechanistic constitutive model will allow us to identify the specific internal material processes playing the most significant role during the mechanical response of elastomers.