Abstract In elastic and plastic deformation of glassy polymers and with the increase of deformation, the material would exhibit elastic, viscoelastic, and viscoelastic-viscoplastic (VE-VP) responses in sequence. These deformation behaviors should be constitutively modeled in such a way to accurately and efficiently simulate many important physical behaviors and phenomena involved in polymer deformation and processing. The elastic and viscoelastic deformation behavior in the pre-yield region can be well represented by the existing constitutive models. However, the VE-VP response in the post-yield region, such as stress relaxation and strain recovery behaviors, cannot be well modeled yet. In this research, a series of uniaxial compression tests of the stress relaxation and the loading-unloading-recovery behaviors of glassy polymer were firstly carried out. The experimental phenomena show strain-rate-dependent characteristics that cannot be explained by the thermally-activated viscosity/viscoplasticity theory. Therefore, it is proposed that a glassy polymer can be treated as a glassy network linked by the secondary bonds and entanglements; while the glassy network is locally yielded and elastically distorted in the pre-yield region, giving rise to a glassy network resistance (GNR) which accounts for the strain-rate-dependent characteristics. A finite strain thermodynamically-based constitutive model was then proposed to incorporate the effects of the GNR. Finally, the newly proposed model was calibrated and its predictive capability was elaborated with the comparison of experiments as well as two widely reported models. The predicted results demonstrate that the GNR is critical in controlling the post-yield stress relaxation and strain recovery. In addition, the nonlinear evolution of the GNR plays a key role in controlling the nonlinear pre-peak hardening and the nonlinear unloading curve. This research thus not only provides a new constitutive theory for modeling of the VE-VP deformation behavior of glassy polymers, but also advances the understanding of the mechanical responses of glassy polymers in deformation process.

中文翻译：

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基于有限应变热力学的玻璃态聚合物粘弹-粘塑性变形行为本构建模与分析

摘要 在玻璃态聚合物的弹塑性变形过程中，随着变形量的增加，材料将依次表现出弹性、粘弹性和粘弹-粘塑性（VE-VP）响应。这些变形行为应该以这样的方式进行本构建模，以准确有效地模拟聚合物变形和加工中涉及的许多重要物理行为和现象。预屈服区域的弹性和粘弹性变形行为可以由现有的本构模型很好地表示。然而，屈服后区域的 VE-VP 响应，如应力松弛和应变恢复行为，还不能很好地建模。在这项研究中，首先对玻璃态聚合物的应力松弛和加载-卸载-恢复行为进行了一系列单轴压缩试验。实验现象显示了热激活粘度/粘塑性理论无法解释的应变率相关特性。因此，建议将玻璃状聚合物视为由二级键和缠结连接的玻璃状网络；而玻璃网络在预屈服区域局部屈服并弹性变形，产生玻璃网络电阻（GNR），这解释了应变率相关的特性。然后提出了一种基于有限应变热力学的本构模型，以结合 GNR 的影响。最后，对新提出的模型进行了校准，并通过比较实验和两个广泛报道的模型来阐述其预测能力。预测结果表明 GNR 在控制屈服后应力松弛和应变恢复方面至关重要。此外，GNR 的非线性演化在控制非线性峰前硬化和非线性卸载曲线方面起着关键作用。因此，这项研究不仅为玻璃态聚合物的 VE-VP 变形行为建模提供了新的本构理论，而且促进了对玻璃态聚合物在变形过程中的力学响应的理解。预测结果表明 GNR 在控制屈服后应力松弛和应变恢复方面至关重要。此外，GNR 的非线性演化在控制非线性峰前硬化和非线性卸载曲线方面起着关键作用。因此，这项研究不仅为玻璃态聚合物 VE-VP 变形行为的建模提供了新的本构理论，而且促进了对玻璃态聚合物在变形过程中的力学响应的理解。预测结果表明 GNR 在控制屈服后应力松弛和应变恢复方面至关重要。此外，GNR 的非线性演化在控制非线性峰前硬化和非线性卸载曲线方面起着关键作用。因此，这项研究不仅为玻璃态聚合物 VE-VP 变形行为的建模提供了新的本构理论，而且促进了对玻璃态聚合物在变形过程中的力学响应的理解。