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

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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.