考虑多裂纹干涉影响的高温焊接管道蠕变寿命评估方法研究

For the aim of promoting the harmonious development between energy and environment, clean and high efficient Ultra Super Critical (USC) power plants have been widely constructed in China. While with the temperature and pressure increasing, the service environment for high temperature components in USC units is becoming more server and as a result more critical solutions are required to assure their safety. Many high-temperature components contain more than one crack-like defects and flaws. The interactions of these cracks or flaws are therefore ineluctable during the evolution of crack and could cause a change in the stress filed and the crack growth rate, which greatly affect the reliability of these components. However, the combination criteria for multiple cracks in prevailing assessment procedures for the high temperature structures are established under the elastic fracture mechanics and the effects of the creep deformation, constraint level and welding residual stress level on the multiple cracks interaction are not involved. Thus, these approaches have limitations on the reliability assessment and often result in an underestimated or overestimated life prediction. Hence, based on the elastic-plastic creep damage mechanics and high temperature fracture mechanics, the interaction behavior of coplanar multiple cracks in high temperature welded pipes during creep crack growth is systematically investigated by incorporating the experiment research, numerical simulation and theoretical analysis. Then the combination criteria for the multiple cracks during creep crack will be established, where the effects of the configurations and the growth of multiple cracks, the constraint level ahead of crack tip and the present residual stress level are all considered. Furthermore, the characterization of the creep crack growth rate under the multiple cracks interaction is proposed. Finally, the creep life assessment approach for the high temperature welded pipes with multiple cracks will be developed to assure the integrity of high temperature components.

为了促进能源与环境的协调发展，高效洁净的超超临界机组成为了我国目前火力发电的主力机组，而蒸汽参数的提高致使高温结构的服役环境更加恶劣，对其使用安全性提出了更高要求。高温部件往往出现多个裂纹共存，其相互干涉作用将改变裂纹前沿应力场和裂纹扩展速率，严重影响了结构的可靠性。然而现行评估方法中参照线弹性断裂方法建立的多裂纹合并准则，未考虑高温蠕变、拘束和残余应力等影响，具有较大的局限性，导致对评估结果偏保守或过于危险。本项目将针对高温焊接管道，基于高温弹塑性蠕变损伤力学和高温断裂力学，采用试验、数值模拟和理论分析手段相结合，围绕高温下共面多裂纹的干涉效应、裂纹扩展和合并规律展开系统研究，建立高温蠕变条件下综合考虑裂纹扩展、裂纹几何特征、拘束程度和残余应力的多裂纹合并准则，探明多裂纹干涉下蠕变裂纹扩展速率表征方法，发展考虑多因素的含多裂纹高温焊接管道蠕变寿命评估方法，保证高温焊接结构的完整性。

为实现裂纹扩展行为的准确模拟，构建了反映材料微观组织变化、由随机晶粒和晶界构成的理想化组织网格模型，耦合蠕变/疲劳/氧化损伤演变模型，实现了高温蠕变、蠕变/疲劳、蠕变/疲劳/氧化、缺口试样等的裂纹扩展行为的表征，既获得材料的裂纹扩展性能，同时探明了裂纹扩展不同阶段的断裂机制和断裂机理的变化，在国际上首次实现了高温裂纹扩展断裂机制的数值表征。通过含多裂纹高温结构断裂参量的数值模拟分析，探明了多裂纹的间距、形状、深度、蠕变性能以及载荷形式变化对高温蠕变下表面和埋藏多裂纹干涉行为的影响，除多裂纹之间相互距离以外，多裂纹的裂纹深度、蠕变性能以及载荷形式变化也决定了多裂纹的干涉水平，而裂纹形状变化的影响较小；在此基础上，建立了高温蠕变下多裂纹干涉因子的工程计算方法，便于基于工程结构实际缺陷尺寸和载荷工况，计算高温含缺陷结构多裂纹干涉效应。为表征拘束效应对多裂纹干涉行为的影响，提出了基于等效蠕变变形面积的高温蠕变干涉因子，准确反映了裂纹距离、裂纹深度、裂纹形状以及蠕变性能对多裂纹干涉行为的影响，并消除拘束效应对多裂纹干涉行为的影响。基于蠕变裂纹扩展数值模拟方法和高温蠕变多裂纹扩展的试验验证，探明了多裂纹共存下几何特征、载荷形式、残余应力、蠕变性能等变化对多高温蠕变下多裂纹干涉下裂纹扩展速率影响，提出利用裂纹扩展速率加速因子表征多裂纹共存下裂纹扩展行为的变化，分析了裂纹扩展速率加速因子和蠕变多裂纹干涉因子的关系，建立了高温蠕变条件下基于蠕变干涉因子的多裂纹合并判定准则和合并方法，降低了现行标准评估方法的保守性，完善了现行高温结构完整性评估准则，对保证我国高温结构完整性具有重要的工程价值。