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

多轴机械载荷下的材料行为的理解是非常重要的结构材料的应用，因为组件通常暴露在多轴应力在实践中。此外，负载通常是时间相关的和反向的。在这种复杂的加载条件下的实验研究是非常费力的，以至于我们对导致材料在多轴反向载荷下的塑性变形和破坏的机制的理解是相当不足的。这导致，此外，不确定性是否常见的固体力学失效假设是有效的，在这种条件下。拟议的研究项目旨在通过实验研究氮合金奥氏体钢的变形和损伤机制，并叠加压缩和循环扭转载荷，以缩小这一差距。利用高分辨率显微镜分析了特殊加载条件对应变和损伤累积的微观组织机制及其变化的影响。基于实验结果，本构模型制定的晶体塑性的框架内，可靠地描述循环塑性和损伤下的多轴载荷的微观结构水平。除了纯位错塑性，机械孪生和晶界滑动也被认为是在模型内。关于损伤机制，特别是在材料表面发生的过程进行了表征和modeled.Under组合压缩和循环扭转加载，观察到的现象，样品经历塑性轴向应变，虽然压缩载荷本身是远低于材料的屈服强度。一旦超过循环扭转的临界角，就会发生这种压缩应变。拟议的研究项目调查这种现象是否可以用常见的故障假设来描述，或者是否需要相应地进行推广。此外，实验结果和数值模拟的比较可以导致复杂的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。为了确保尽可能广泛的多轴载荷下的疲劳微观力学模型的范围内，在叠加的压缩-扭转载荷下的材料行为的影响，预先冷加工的实验研究和用于模型验证。

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.