考虑蠕变变形与断裂机理影响的宽范围应力区蠕变裂纹扩展寿命研究

Investigation on the creep crack growth (CCG) life under considering mechanism of creep deformation and fracture over a wide range stress is a scientific problem which involving multiple disciplines, such as mechanical, material and mechanics. The CCG rate displays three line segments with different slopes in a wide range stress and can be different to the evaluation of CCG life of high temperature components under the combined effects of the creep deformation mechanism, creep fracture mechanism and the constraint. Based on the commonly used materials of CrMoV in high temperature components, the following issues will be focused in the present project by using the combined approaches of the macro-fracture mechanic experiment, micro-observation, finite numerical simulation based on ductility exhaustion model and theoretical analyses: (1) Investigation on the stress dependent mechanism of creep deformation and fracture; (2) Establishment of related model and finite element numerical calculation; (3) Discussion on the evaluation method to establish constraint dependent CCG life. The aim of this project is to reveal the rule of influence of stress dependent creep deformation and fracture mechanism on CCG rate, ascertain the mechanical essence of CCG behaviour in a wide range stress and obtain the method to predict creep constraint and CCG life. The results are with great significance of improving the reliability of high temperature components, ensuring the safe operation of equipment, selecting material of high temperature structure and fabrication process, and so on.

考虑蠕变变形与断裂机理影响的宽范围应力区蠕变裂纹扩展寿命研究是一个涉及机械、材料、力学等多个学科的科学问题。在蠕变变形机理、断裂机理、拘束三者共同影响下，蠕变裂纹扩展速率在宽范围应力区呈现出三段不同斜率的线性特征，给高温部件蠕变裂纹扩展寿命评价带来困难。本项目以高温部件常用CrMoV钢为牵引，综合运用宏观断裂力学实验、微观观察、基于损伤的延性耗竭模型有限元数值计算和理论分析等方法，重点开展如下研究：（1）应力区相关的蠕变变形与断裂机理研究；（2）模型构建及有限元数值计算；（3）拘束相关的蠕变裂纹扩展寿命评价方法探讨。目标在于揭示与应力水平相关的蠕变变形机理及断裂机理对蠕变裂纹扩展速率的影响规律、探明宽范围应力区蠕变裂纹扩展行为的力学本质、获得准确预测裂尖拘束及蠕变裂纹扩展寿命的方法。研究成果对于提高高温部件的可靠性、保障高温结构安全运行乃至结构选材和制造工艺等方面具有重要的理论和实际意义。

在宽范围应力区材料的蠕变变形机理和断裂机理与应力水平相关，在高应力区的蠕变变形主要是幂律蠕变，断裂方式主要是沿晶、穿晶混合延性断裂；在低应力区的蠕变变形主要是扩散蠕变，断裂方式主要是沿晶脆性断裂；在过渡应力区的蠕变变形是幂律蠕变与扩展蠕变的混合，断裂方式为沿晶延性断裂。不同应力区蠕变变形机理及断裂机理的变化使得材料的蠕变变形和断裂在宽范围应力区表现出应力相关性。基于单机理的传统延性耗竭模型无法准确预测高温部件在宽范围应力区的CCG行为。因此，为了实现高温部件宽范围应力区CCG寿命的可靠预测，迫切需要研究考虑蠕变变形机理、断裂机理、裂尖拘束的共同影响下的适用于高温部件CCG寿命评定技术。本项目以高温部件常用CrMoV钢为牵引，综合运用宏观断裂力学实验、微观观察、基于损伤的延性耗竭模型有限元数值计算和理论分析等方法，重点开展如下研究：(1)应力区相关的蠕变变形及断裂机理研究；(2)模型构建及有限元模拟；(3)小冲杆蠕变实验及缺口试样敏感性分析。目标在于揭示与应力水平相关的蠕变变形机理及断裂机理对蠕变裂纹扩展速率的影响规律、探明宽范围应力区蠕变裂纹扩展行为的力学本质、获得准确预测裂尖拘束及蠕变裂纹扩展寿命的方法。研究结果表明：高应力水平作用下拉伸试样为韧性断裂，应力指数在5.7左右；基于蠕变变形机理的CDM本构模型在宽范围应力区能和实验数据进行很好的关联，得到更为准确的预测结果；不同缺口长度对试样的蠕变断裂蠕变寿命产生显著影响，而且试样蠕变断裂寿命与缺口长度并不成比例，不同缺口长度试样的断裂机理与裂纹起裂位置及扩展方向相关，因而使得试样蠕变断裂寿命不同。研究成果对于提高高温部件的可靠性、保障高温结构安全运行乃至结构选材和制造工艺等方面具有重要的理论和实际意义。