The inelastic behaviour of thermoplastic polymers below the glass transition temperature can involve shear plasticity or crazing, depending on the strain rate and temperature. Shear plasticity is driven by local shear stresses and is essentially volume preserving. In contrast, crazing is a failure phenomenon occurring under tension and causes significant volume change. In some cases, crazing can also result in large inelastic deformations at macroscopic scale, referred to as craze yielding. The aim of this study is to propose a thermodynamically-consistent constitutive framework that accounts for both shear plasticity and craze yielding in glassy polymers. The main assumption is that shear plasticity and craze yielding occur exclusively from each other, depending on the local stress state. Both are modelled as thermally-activated processes with different activation stresses and flow rules. The theory is validated against experimental data for polylactic acid (PLA). Our model is capable of reproducing the stress–strain response including yielding, softening, and drawing under both shear plasticity and craze yielding, and also accurately predicts the volumetric deformation under tension. In particular, our simulations well capture the interesting phenomenon that craze yielding stabilises localised deformations and prevents necking. Our model can also simulate complex scenarios where both mechanisms occur simultaneously at different locations of the specimen.

中文翻译：

---

考虑剪切塑性和银纹屈服的玻璃态聚合物本构模型


低于玻璃化转变温度的热塑性聚合物的非弹性行为可能涉及剪切塑性或开裂，具体取决于应变速率和温度。剪切塑性由局部剪切应力驱动，本质上是体积保持的。相反，裂纹是在张力下发生的失效现象，并导致显着的体积变化。在某些情况下，银纹还会导致宏观尺度上的大的非弹性变形，称为银纹屈服。本研究的目的是提出一种热力学一致的本构框架，该框架可以解释玻璃态聚合物的剪切塑性和银纹屈服。主要假设是剪切塑性和银纹屈服完全独立发生，具体取决于局部应力状态。两者都被建模为具有不同激活应力和流动规则的热激活过程。该理论已根据聚乳酸 (PLA) 的实验数据得到验证。我们的模型能够再现应力应变响应，包括剪切塑性和裂纹屈服下的屈服、软化和拉伸，并且还可以准确预测拉伸下的体积变形。特别是，我们的模拟很好地捕捉到了一个有趣的现象，即银纹屈服稳定局部变形并防止颈缩。我们的模型还可以模拟复杂的场景，其中两种机制同时发生在样本的不同位置。