Deformation and failure mechanisms in austenitic steel under coupled compressive and torsional loading

了解材料在多轴机械载荷下的性能对结构材料的应用具有重要意义，因为构件在实际应用中经常受到多轴应力的影响。此外，负载通常是时变的和反向的。在如此复杂的载荷条件下进行实验研究是非常费力的，因此我们对材料在多轴反向载荷下塑性变形和破坏的机制的理解非常不足。此外，这导致了在这种条件下常见的固体力学失效假设是否有效的不确定性。提出的研究项目旨在通过实验研究氮合金奥氏体钢在叠加压缩和循环扭转载荷下的变形和损伤机制来弥补目前研究中的这一空白。采用高分辨显微技术分析了特殊加载条件对应变和损伤积累的微观组织机制及其变化的影响。在实验结果的基础上，建立了晶体塑性框架下的本构模型，该模型在微观结构水平上可靠地描述了多轴载荷下的循环塑性和损伤。除了单纯的位错塑性外，该模型还考虑了机械孪晶和晶界滑动。关于损伤机制，特别是发生在材料表面的过程进行了表征和建模。在压缩和循环扭转复合载荷作用下，试样发生塑性轴向应变，而压缩载荷本身远低于材料的屈服强度。一旦超过循环扭转的临界角，这种压缩应变就会发生。提出的研究项目调查这种现象是否可以用常见的失效假设来描述，或者是否需要相应的推广。此外，实验结果和数值模拟的比较可以导致对复杂机械载荷下的变形和损伤机制的基本理解。这些机制可以进一步与材料的软化和硬化行为相关联。为了尽可能扩大多轴载荷下疲劳细观力学模型的适用范围，实验研究了预冷加工对材料在压缩-扭转叠加载荷作用下行为的影响，并将其用于模型验证。

The understanding of material behavior under multiaxial mechanical loads is of great importance for the application of structural materials, because components are usually exposed to multiaxial stresses in practice. In addition, the load is typically time-dependent and reversing. Experimental investigations under such complex loading conditions are very laborious, such that our understanding of the mechanisms leading to plastic deformation and failure of materials under multi-axial reversing loads is quite insufficient. This leads, moreover, to an uncertainty whether the common solid mechanics failure hypotheses are valid under such conditions. The proposed research project seeks to close this gap in the current state of research by experimentally investigating the deformation and damage mechanisms in a nitrogen-alloyed austenitic steel with superimposed compressive and cyclic torsional loading. Effects of the special loading condition on microstructural mechanisms of strain and damage accumulation and their changes will be analyzed by high resolution microscopy. Based on the experimental results, a constitutive model is formulated within the framework of crystal plasticity that reliably describes cyclic plasticity and damage under multiaxial loads on the microstructural level. In addition to pure dislocation plasticity, mechanical twinning and grain boundary sliding are also considered within the model. Concerning the damage mechanisms, in particular the processes taking place at the material surface are characterized and modeled.Under combined compression and cyclic torsional loading, the phenomenon is observed that samples undergo plastic axial strain, although the compressive load by itself is well below the yield strength of the material. This compressive strain occurs as soon as a critical angle for the cyclic torsion is exceeded. The proposed research project investigates whether this phenomenon can be described with the common failure hypotheses or whether they need to be generalized accordingly. Moreover, the comparison of experimental findings and numerical modeling can lead to a fundamental understanding of the deformation and damage mechanisms under complex mechanical loads. These mechanisms can further be correlated with softening and hardening behavior of the material. In order to ensure the widest possible scope of the micromechanical model of fatigue under multiaxial loads, the influence of prior cold working on the material behavior under superimposed compressive-torsional loads is experimentally investigated and used for model validation.