Influence of the cyclic thermal and mechanical loading history on crack closure, fatigue crack growth and damage interaction of cast nickel-base alloys

燃气轮机的部件因启动和关闭循环而暴露在高热和机械负荷下。因此，使用过的镍基合金的热机械疲劳(TMF)是评估部件强度的主要标准。由于热瞬变和结构力学，在使用过程中，部件的关键区域存在不同的载荷历史，其中热载荷和机械载荷之间的相角局部变化(异相、同相、相移)。然而，只有了解裂纹扩展的寿命限制机制，才能获得有意义的疲劳寿命预测。在这方面，裂纹闭合和不同损伤机制的相互作用具有重要的影响，但今天无法量化，因为材料的性质是温度相关的，材料的行为是时间相关的，因此，相角起着重要的作用。在本项目中，一方面对裂纹扩展、闭合和损伤交互作用进行了实验研究。另一方面，用有限元方法进行数值研究，以确定裂纹闭合、裂纹扩展与材料性能之间的关系。在实验和数值计算结果的基础上，建立了一个基于机理的模型，该模型可以再现力学性能与温度、相角和时间的关系对多晶铸造镍基合金In 100 TMF裂纹扩展的影响。

Components of gas turbines are exposed to high thermal and mechanical loads resulting from start-up and shut-down cycles. Thus, thermomechanical fatigue (TMF) of the used nickel-base alloys is a major criterion for the assessment of the strength of the components. Due to thermal transients and structural mechanics, varying loading histories are present in critical areas of the components during service, where the phase angle between thermal and mechanical load varies locally (ouf-of-phase, in-phase, phase-shift). Meaningful fatigue life predictions can, however, only be obtained if the lifetime limiting mechanisms of crack growth are understood. In this regard, crack closure and the interaction of different damage mechanisms have a significant, but today not quantifiable influence, since the material properties are temperature-dependent, the material behaves time-dependent and, hence, the phase angle plays an important role. In this project, on the one hand, crack growth, crack closure and the damage interaction are investigated experimentally. On the other hand, numerical investigations with the finite-element method are used to identify correlations between crack closure, crack growth and material properties. On the basis of the experimental and numerical results, a mechanism-based model is developed which can reproduce the influence of the temperature-dependent mechanical properties in combination with the phase angle and the time-dependency on the TMF crack growth of the polycrystalline cast nickel-base alloy IN100.