Cosserat thermo-mechanics under finite deformationstheory and finite element implementation

Zusammenfassung

The objective of this thesis is to propose a theory that is able to capture specific phenomena during machining operations on metals. Recent investigations proved that dislocations-driven mechanisms at molecular or atomic scale strongly affect the response of the medium when the deformations are localized in small areas compared to grain size. The classical continuum mechanics fails at predicting size effects, so new theories are required. Furthermore, during machining operations, strong thermal-microstructure interactions occur, and such pivotal relationships must be properly captured. In the last century, the strain-gradient theories have been used to overcome these shortcomings of the classical continuum mechanics. A special form of strain gradient theory is the Cosserat theory, which includes the microstructure grain rotation and its gradient as additional deformation measures, therefore allowing the theory to predict size-effects.A finite-deformation thermodynamically compatible elasto-visco-plastic Cosserat theory is presented in this investigation, alongside its numerical implementation in a Finite Element software. The theory is firstverified against previously derived analytical solutions and then used to analyze the size effect in a Hat-Shaped shear tests and simplified machining simulation of a Titanium Alloy. The focus of the investigations was to predict the correct thickness of the adiabatic shear band that forms in the metal during machining operations. Similar tests have been used to analytically identify the thickness of the adiabatic shear band. Finally, the effect of the additional material parameter (directly defining the intrinsic characteristic length of the continuum) on the thickness of the shear band has been examined during machining operations.