Just another WordPress site - Ruhr-Universität Bochum

Abstract

Crystal plasticity (CP) models have proven to accurately describe elasto-plastic behavior on micro- and nanometer length scales in numerous publications. However, the parameters calibration, involving series of experiments at these length scales, and an inverse analysis are required. In this regard, nanoindentation test promises to be a well-suited tool to realize an inverse method approach to determine the typically large number of model parameters. The objective of this work is to develope a parameterization technique for nonlocal CP models by means of an accessible and reproducible workflow. To show its practical relevance, the complex microstructure of tempered lath martensite is chosen as test material. Its complexity is the hierarchical structure built of sub-units of lath, blocks and packets which appear in a prior austenite grain (PAG) during quenching. To achieve this goal, we combined finite-element simulations with nanoindentation tests. Latter have been performed into single packets of tempered lath martensitic specimen of different carbon contents to record two characteristics of the individual imprints: the load-displacement curves as well as the residual imprint topology around the remaining imprint surfaces with the help of atomic force microscopy. On the numerical side, a nanoindentation model is created and coupled to a nonlocal CP formulation (Ma2014). A parametric study is conducted to better understand the influence of different model parameters on mechanical response of lath martensite during nanoindentation. In the next step, nonlocal CP parameters are systematically adapted to mimic nanoindentation tests. Hence, an appropriate optimization algorithm is introduced. As a final validation step it is successfully demonstrated that the nanoindentation-informed CP model is able to predict the macroscopic stress-strain response of polycrystals. Two stories can be deduced from this study: on the one hand, materials science clearly states that the material properties can locally deviate very strong, which is caused by a dependency of the size distribution on the microstructure level. On the other hand, a methodological approach is pursued in which it could be shown that the inverse analysis is also possible for highly complex microstructures such as those of tempered lath martensite. [Ma2014] A. Ma and A. Hartmaier, ”On the influence of isotropic and kinematic hardening caused by strain gradients on the deformation behaviour of polycrystals”, Philosophical Magazine, Vol94, 125-140 (2014). Keywords: nanoindentation, inverse analysis, crystal plasticity