基于延性材料RVE破断行为的结构完整性评价基础问题研究

Many defects can be inevitably found in the pressure vessels and pipelines which are commonly applied in the nuclear power, chemical industry and petroleum engineering. The integrity assessment of the structures containing defects is an important technique to ensure the safe operation of equipments, which shows remarkable benefit and economic value in the engineering application. Nowadays, some kinds of integrity assessment specifications have been issued for enforcement by different organizations. In the high-grading methods of integrity assessment relating to the efficiency maximization, some fundamental problems, such as uniaxial constitutive relationships of ductile materials, the prediction of quasistatic and fatigue fracture behaviors of cracked structures, still have not been solved. Therefore, this project plans to investigate the method for the full range uniaxial constitutive relationships up to failure of RVE (representative volume element) for ductile materials, reveal the critical failure mechanism of material under complicated tension and torsion loading, develop the theoretical forecasting methods for the quasistatic crack initiation and propagation of structural cracks, research the low cycle fatigue behaviors containing large deforming cyclic breakage of RVE for ductile materials, construct the theoretical method for predicting the fatigue crack propagation behaviors of cracked structures by introducing low cycle damage into the description of plastic zone near the crack tip. The expected achievements will provide the theoretical basis and technical supports on the high-grading integrity assessment methods for structures containing defects, which has widely engineering application prospects.

核电、化工、石油工程中的压力容器、管道等重大装备在服役过程中不可避免地存在某些缺陷，对含缺陷结构进行完整性评价是确保装备安全稳定运行的重要技术手段，其工程意义和经济价值突出。国际上已形成具有不同评价等级的各类完整性评价规范，在关乎效益最大化的高等级评价方法中，包括延性材料单轴本构关系、含裂纹结构的准静态与疲劳断裂行为预测等基础性问题尚未有效解决。为此，本项目拟研究获取延性材料RVE（Representative Volume Element）直至破坏的全程单轴本构关系的方法，并研究材料在拉扭复杂加载条件下的临界破坏机制，发展结构裂纹准静态启裂与扩展的理论预测方法；开展延性材料RVE含大变形循环破坏的低周疲劳行为研究，通过在结构裂纹尖端塑性区引入低循环损伤来建立描述含裂纹结构的疲劳裂纹扩展行为的理论方法。预计成果将为含缺陷结构的完整性高等级评价方法提供理论基础和技术支撑，具有重要的应用前景。

结构完整性评价是确保压力容器、管道等重大装备安全稳定运行的重要技术手段，其工程意义和经济价值突出。包括材料单轴本构关系获取、含裂纹结构的断裂行为预测等结构完整性评价中的基础性问题仍有待深入研究。本项目针对材料本构关系获取和含裂纹结构的断裂行为预测方法开展了研究，取得的研究成果包括：提出了获取延性材料直至破坏的全程等效应力-应变曲线的改进FAT方法。基于多种材料多种构形试样的破断试验和对比模拟分析，建立了Yao-Cai应力型临界破断准则；结合HRR裂尖场描述，提出了平面应变JR阻力曲线理论预测方法。提出了构建多种构元载荷-位移响应与材料本构行为的半解析关系的C-C能量等效原理。基于C-C能量等效原理，建立了预测材料等效应力-应变曲线、硬度和强度的压入试验方法，含裂纹构元的应力强度因子和J积分计算方法以及小冲杆测定材料等效应力-应变曲线的试验方法。通过低循环塑性损伤等效和应变能密度等效两种方式，分别提出了基于低周疲劳性能的纯I型贯穿裂纹、纯II型贯穿裂纹、纯III型贯穿裂纹、I型表面裂纹和I-II混合型裂纹的疲劳扩展速率理论预测方法。研究成果为结构完整性高等级评价方法提供了技术支撑。