Determination of plastic material properties on different length scales by indentation techniques and inverse analyses
PhD Thesis, Ruhr University of Bochum (2012)
Determination of plastic mechanical properties by conventional materials testing procedures is a time-consuming process of high costs. Impeded by the fact that the small volumes as well as local material properties are difficult or sometimes impossible to assess, indentation techniques have gained considerable interest in the last years. This work deals with the determination of plastic material properties with the help of indentation techniques and inverse analyses. As this plastic material behavior can be divided into isotropic and anisotropic behavior, the determination presented here differentiates between them numerically as well as experimentally. In both cases it will be shown that the residual imprint formed by an indentation process is the essential element in the determination of those properties. For the case of isotropic material behavior a method is presented for the identification of yield strength and work hardening behavior using the residual imprint geometry formed by a sphero-conical indentaion. It will be shown that this method yields unique and reliable results that are verified by independent uniaxial straining experiments on various alloys and a functionally graded material based on a steel alloying concept. The fundamental finding of this part of the work is that the residual imprint can be regarded as the fingerprint of a material as it contains sufficient information about the plastic behavior of a material to uniquely extract values for the yield strength and the work hardening rate. Whereas the first part of the work is valid under the assumption of an isotropic hardening and material model, the concept is transferred and specified to anisotropic material behavior of plastically deforming single crystals in the second part of this work. Methods to derive parameters for the description of single-crystalline material behavior using finite-element crystal plasticity are presented. Parameters are indirectly verified with the help of tension test and cross-checks of indentaion in differently oriented single crystals. Using the identified single-crystalline constitutive behavior the homogenized mechanical response and effective properties of a polycrystal created through Voronoi tessellation is analyzed. Eventually, the identified parameters lead to results describing the yielding and hardening behavior of individual slip planes of the investigated alloys.