多轴载荷下热机疲劳失效机制及寿命预测方法研究

Equipment in nuclear power plants is often subjected coupling cyclic temperature and mechanical loading, such as pressure pipe subjected to thermal stratification, thermal shock and thermal transient. It results in thermo-mechanical fatigue damage under multiaxial loading, which has drawn great attention from domestic and international researchers. However, no design standard concerning this problem has been accomplished in the world. In this project, our researches focus on the thermo-mechanical cyclic deformation behaviors, thermo-mechanical multiaxial low cycle fatigue failure mechanism and fatigue life prediction theory for forging 316LN stainless steel of nuclear pressure pipe. The effects of loading rate, non-proportionality of loading, dynamic strain aging, cyclic creep, and temperature-mechanical loading phase on thermo-mechanical cyclic deformation behaviors and microscopic deformation mechanism will be studied during the temperature of 150~550℃. Visco-plastic constitutive model will be developed by considering the influencing factors to predict thermo-mechanical cyclic deformation. We will verify thermal-mechanical fatigue damage mechanism due to multiaxial loading, dynamic strain aging, creep and temperature-mechanical loading phase. The thermo-mechanical multiaxial low cycle fatigue life prediction method and theory will be developed. The achievements of this project will provide the theory fundamentals of the establishment of standard as well as the design of high temperature pressured equipment (such as pressure vessel and piping).

核电设备通常经受着温度和机械载荷的联动循环，如压力管道受到热分层、热冲击和热瞬态的作用，由此产生的多轴载荷下热机疲劳损伤倍受关注,国内外对此没有设计标准。本课题研究核电管道用材锻造316LN不锈钢多轴热-机循环变形性能、热-机多轴低周疲劳失效机理和寿命预测理论。试验研究的温度变化在150~550℃范围内，探究加载率、载荷非比例度、动态应变时效、循环蠕变和温度-机械载荷相位角等因素对热机循环变形性能的影响及微观变形机制；建立考虑这些因素的能有效预测热机循环变形的粘塑性本构模型；探明多轴载荷、动态应变时效、循环蠕变和温度-机械载荷相位角等因素对热-机疲劳损伤影响的机制，建立考虑这些因素的热机多轴低周疲劳寿命预测方法和理论，从而为高温承压设备（如压力容器和管道）结构的设计及规范的制定提供理论支撑。

核电结构在长期服役过程中会同时承受交变的机械载荷和因大量热瞬态导致的循环热应力，热机械疲劳失效已经成为威胁核电结构完整性的重要原因之一。本项目对国产核级高氮316LN不锈钢的单轴/多轴高温疲劳及热机械疲劳性能开展了全面系统的研究。掌握了复杂加载条件下材料的循环力学行为、微观结构演化行为、损伤失效机理及疲劳寿命。本项目获取的大量试验数据丰富了国产核结构材料数据库，建立了316LN材料的高温疲劳设计曲线，提出了新的寿命预测模型。研究成果为钠冷快堆管道结构的可靠性设计及寿命评估提供了重要的数据支撑和理论依据。