基于“虚构无损伤”有效应力测量的半结晶聚合物损伤表征与机理研究

It is an effective tool to study failure behavior and lifetime prediction of semi-crystalline polymer (SCPs). The current damage characterization methods is not applicable for SCPs due to its complex molecular structure and strong nonlinear mechanical behavior. The scarcity of information in literature regarding the experimental results of effective stress-strain relationship in the fictitious undamaged configuration, as well as the lack of theoretical interpretation on the interaction between the macroscopic damage parameter and microstructure evolution, provide the motivation for this research. In this study, effects of strain rate, temperature and stress triaxiality on the damage evolution and molecular microstructure change of polyethylene (PE) materials will be studied based on the mechanical tests and microstructure characterization technique. The relationship between macroscopic damage evolution and microstructural damage characteristics will also be determined. Secondly, the quasi-static stress under various strain rates, temperatures and stress triaxialities will be studied based on the theory of viscoelastic-plastic theory and stress relaxation model. The effectives stress in the fictitious undamaged configuration will be finally determined using extrapolation methods. Moreover, the damage model based on the degradation of stress will be developed. Furthermore, numerical methods for the computation of fracture toughness of PE will be developed, the influence of stress triaxiality near the crack tip on the computation accuracy will also be discussed. This research will lay important foundation for the development of macroscopic, microscopic and mesoscopic damage theory and provide insight into the study of damage behavior in polymer materials.

损伤本构模型是研究半结晶聚合物(SCPs)断裂失效及寿命预测的有效手段。因其复杂的三明治分子结构和强非线性力学行为，SCPs损伤本构模型所依赖的“虚构无损伤”有效应力-应变关系及完备的损伤演化缺少实验结果，理论上对材料宏观损伤和微结构演化之间的关联也存在诸多空白。本研究以聚乙烯(PE)为实验对象，借助力学实验和微观结构表征技术，揭示应变率、温度和应力三轴度对PE宏观损伤变量及微观结构演化的影响机制，阐明宏观损伤与微观损伤特征的关系；借助粘弹塑性理论及应力松弛模型研究PE在不同应变率、温度和应力三轴度下的准静态应力-应变特性，采用外延法定量测得PE材料的“虚构无损伤”有效应力并建立基于应力衰减的损伤演化模型；建立PE断裂韧性的数值计算方法，明确裂纹尖端应力三轴度对断裂韧性计算精度的影响。本研究将为宏观-细观-微观多层嵌套连接的损伤理论的发展提供重要依据，为研究高分子材料的损伤行为提供思路。

损伤本构模型对研究材料的断裂失效行为有重要意义，但聚合物材料损伤演化的定量表征实验研究相对匮乏。本项目针对半结晶聚合物材料的损伤失效机理和损伤表征方法开展研究，较好地完成了项目的预期计划。主要研究工作及成果包括：（1）通过新设计的两阶段实验法定量描述了聚乙烯材料在不同应变率、温度和应力三轴度下的损伤演化过程；（2）采用宏观力学实验、微观表征技术和分子动力学模拟相结合的方法阐述了聚合物材料的变形、损伤机理，揭示了聚乙烯材料的微观变形/损伤机理与宏观损伤变量之间的关系；（3）基于聚乙烯材料的应力松弛模型获得了应变率、温度和应力三轴度对准静态应力的影响规律，进一步采用外推法建立了聚乙烯材料的虚构无损伤有效应力；（4）构建了聚乙烯材料的损伤本构模型，可以较好的描述聚合物材料的大变形与损伤断裂行为。本研究体现了机械、力学和材料多学科的交叉，对促进聚合物损伤力学理论的发展及新型高分子材料的制备与表征提供科学支持。项目实施以来共发表学术论文16篇（其中SCI收录8篇，EI收录4篇），授权国家发明专利2项，卢森堡国际专利1项，实用新型专利3项。