Polyether ether ketone (PEEK) is a semi-crystalline thermoplastic polymer with excellent thermo-mechanical properties, bio-compatibility, corrosion resistance, and 3D printability. Due to these merits, it has wide applications in aeronautics and biomedical devices. However, PEEK's excellent thermo-mechanical properties come from its complicated crystalline domains, making it hard to predict and to design PEEK structures under complex service conditions. In this paper, we studied the thermomechanical behaviors of PEEK with stretch-induced anisotropy and developed a constitutive model to incorporate the influence of the complex loading history along different loading axes. From the experiments, it was found that when it is stretched, PEEK demonstrates viscoplastic behaviors with reduced transversal modulus and yield stress in the subsequent loading, due to the initiation and growth of voids during stretching. The tensile sample also shows a necking behavior at relatively low temperature. To capture these behaviors, the constitutive model consists of two main parts. The undamaged part has three branches, one hyperelastic branch for the nonlinear elastic behavior, one viscoelastic branch for glass transition and relaxation in the amorphous domains, and one plastic branch for yielding and hardening in the crystalline domains. The damaged loose-chain part with history-dependent reduced relaxation time is used to capture the microscopic interface debonding between the crystallites and the amorphous domains. Compared with the experimental results, this model captures the stretch-induced volume expansion and the anisotropic evolution of material properties. This developed model is also able to capture the temperature-dependent necking phenomenon and the corresponding nominal stress-strain behaviors in the uniaxial tensile tests at different strain rates and temperatures. The developed model can be used to facilitate the design of PEEK-based structures under complicated loading conditions.

聚醚醚酮（PEEK）是一种半结晶热塑性聚合物，具有出色的热机械性能，生物相容性，耐腐蚀性和3D可印刷性。由于这些优点，它在航空和生物医学设备中具有广泛的应用。但是，PEEK的出色热机械性能来自其复杂的晶域，因此很难在复杂的使用条件下预测和设计PEEK结构。在本文中，我们研究了具有拉伸诱导各向异性的PEEK的热力学行为，并建立了本构模型，以考虑复杂载荷历史沿不同载荷轴的影响。从实验中发现，当拉伸时，PEEK表现出在随后的加载中具有减小的横向模量和屈服应力的粘塑性行为，这是由于拉伸过程中空隙的产生和增长所致。拉伸样品在相对较低的温度下也显示出颈缩行为。为了捕获这些行为，本构模型包括两个主要部分。未损坏的部分具有三个分支，一个用于非线性弹性行为的超弹性分支，一个用于在非晶域中的玻璃化转变和弛豫的粘弹性分支，以及一个用于在结晶域中屈服和硬化的塑性分支。具有依赖于历史的减少的弛豫时间的受损的松散链部分用于捕获微晶和非晶域之间的微观界面脱键。与实验结果相比，该模型捕获了拉伸引起的体积膨胀和材料特性的各向异性演变。此开发的模型还能够捕获在不同应变率和温度下的单轴拉伸测试中与温度有关的颈缩现象和相应的名义应力-应变行为。所开发的模型可用于简化复杂载荷条件下基于PEEK的结构的设计。