失配蠕变固体裂纹的拘束效应研究

The focus of this project is to study the characterization of constraint effect for creep crack in mismatched solids and to develop the assessment method for time-dependent failure assessment diagram (TDFAD) considering mismatch constraint effect. This topic is the research frontier and hot topic in the field of high temperature solid mechanics. This research plays an important role in the development of creep fracture theory and possesses great application values in engineering practices. In fact, the theoretical and application foundations of constraint effect for high temperature fracture are still not well established. The targets of the project are as follows. The stress fields and properties for two-dimensional (2D) interface creep crack will be analyzed. The constraint effect for interface creep crack under 2D conditions will be studied. The suitable parameters to characterize constraint effect for interface creep crack will be proposed based on the theoretical analyses and numerical validation. High order asymptotic analysis for crack tip stress field of sub-interface creep crack will be studied. The characterization method to represent the geometry constraint effect, material constraint effect and loading constraint effect will be presented. The influences of crack shape, crack location and material mismatch on different types of constraint level will be also investigated. The influences of creep range and loading level on constraint effect of crack will be studied. The evaluation method with TDFAD considering constraint effect will be developed and presented. The assessment method for creep crack growth by taking account of material mismatch constraint effect will be also developed. The in-depth understanding of mechanism for material mismatch constraint effect of stress field for creep crack will be revealed through this project. Moreover, the influences of material mismatch constraint effect on the integrity assessment of high temperature structures will be studied. This research will provide a theoretical foundation and scientific basis for Chinese national standards and codes of high temperature structural integrity.

本项目拟研究高温非均匀固体裂纹的拘束效应表征及考虑拘束效应的蠕变裂纹扩展评估技术和时间相关失效评估方法，这是高温固体力学研究领域的国际前沿和热点，具有重要的理论意义和工程实用价值，目前尚未建立完善的理论基础和安全评估方法。本项目拟发展二维界面蠕变裂纹的拘束理论，提出表征二维界面蠕变裂纹尖端的拘束效应参数，分析界面蠕变裂纹的尖端场特性；发展二维亚界面蠕变裂纹的拘束理论，提出二维亚界面蠕变裂纹几何、材料、载荷拘束的表征方法，揭示裂纹构型、裂纹位置以及材料失配属性对于不同拘束参数的影响规律；研究蠕变范围和载荷水平对裂纹拘束效应的影响；提出基于以上断裂模式的考虑拘束效应的蠕变裂纹扩展评估技术，发展考虑拘束效应的时间相关失效评估方法。通过本项目研究，深入揭示高温蠕变裂纹尖端场失配拘束效应的作用机制及其对高温结构完整性的影响，为我国高温结构安全评定国家标准的制定提供重要的理论基础和科学依据。

蠕变是高温结构呈现的典型力学特征。蠕变断裂力学已经发展成为固体力学领域的一个重要研究方向，也是结构完整性领域的重要分支。本项目针对高温结构中的界面蠕变断裂问题开展了大量理论和数值模拟研究，具体来说有以下几个方面。其一，提出了表征二维界面蠕变裂纹尖端的拘束效应参数，分析了界面蠕变裂纹的尖端场特性；发展考略材料失配的二维亚界面蠕变裂纹的拘束理论，提出二维亚界面蠕变裂纹几何、材料、载荷拘束的表征方法，揭示了裂纹构型、裂纹位置以及材料失配属性对于不同拘束参数的影响规律；研究了蠕变范围和载荷水平对裂纹拘束效应的影响等方面开展了系统研究工作；提出基于以上断裂模式的考虑拘束效应的蠕变裂纹扩展评估技术，发展考虑拘束效应的时间相关失效评估方法。其二，提出并发展了表征材料失配引起的材料约束效应C*-A2g*-A2m*三参数理论。使用单轴下的中心裂纹板试样基于所提出的C*-A2g*-A2m*理论研究了单轴条件下失配对裂尖应力场的影响、以及失配裂尖场的材料约束效应及总体约束效应；研究发现总体上材料约束效应随着失配因子的增加而降低，同时发现低匹配条件下的约束水平为高约束状态，而高匹配条件下的约束水平则为低约束状态；从整体上，材料失配引起的约束效应可以占到整体约束水平的60%以上，因此应当给以重视。其三，研究了双轴应力状态下，失配材料结构中约束效应的表征问题。随着裂纹深度的加大，除双轴度为1的情况外，总体的约束水平在提高。受到材料失配的影响，整体上高匹配会引起约束水平和均匀材料相比偏低的值，而低匹配则会引起相对均匀材料而言更高的约束效应。如果在工程实际中在双轴载荷条件下，如果不考虑材料约束的影响，而直接采用均匀材料的断裂韧度作为设计的标准，对于实际处于高匹配状态的裂尖而言，则会引起过于保守的结果。反之，则会引起过于危险的结果。同样，如果设计时不考虑双轴效应的影响，而直接将单轴的断裂韧度拿过来使用，对于双轴度较低的试件则会产生过于危险的结果；反之，对于双轴度较高的试件则会产生过于保守的结果。上述成果为高温焊接结构的蠕变断裂安全评估提供了理论基础与科学依据。