Microstructure based evaluation of real structures in cold formable steels II
This project shall continue the work performed in IEHK-05-01 and consists of four tasks; micromechanical damage modelling of multiphase steels, study effects of microstructure morphology on failure behaviour of multiphase steels, development of homogenisation approaches for modelling the effect of microscopic damage on macroscopic component behaviour, and experimental validation of the models by measuring global stress-strain data and local deformation patterns. Concerning micromechanical damage modelling, damage initiation by grain boundary or phase boundary delamination due to high local strains will be investigated. From this damage models with physically meaningful parameters will be derived and implemented into a macroscopic RVE model to study the competition between different failure modes (fracture of brittle phases, interfacial delamination, plastic failure of soft phases). Since these damage phenomena can be understood as void nucleation events for the surrounding ductile phases, macro-micro-relations shall be determined in terms of an effective constitutive material law which results from numerical parameter studies. Finally, the model will be evaluated using several experimental techniques. Sheet metal forming tests as well as tensile tests with plane-strain specimens, pure shear specimens and central-hole specimens will be performed using optical systems to determine the local deformation patterns. These experiments will be simulated to check the accuracy of the models and evaluate their potential for industrial application.
Experimental results obtained from nanoindentations on various iron alloys and topology measurements provide the necessary information to identify macroscopic power-law hardening material parameters of elastoplastic materials. The crystal orientation relative to the indentation axis is measured by means of EBSD analysis and used for identification of single crystalline constitutive behavior. Finite element modeling (FEM) and a dislocation-based crystal plasticity model are implemented via the user subroutine UMAT. An optimization algorithm based on an effective meta modelling approach minimizes the mean square error between simulation and experiment with the benefit of a small number of computationally expensive function calls, especially for the crystal plasticity FEM runs. In the figure above, final results of an optimization run, where experimental results of indents in a ferritic phase of a dual-phase steel are used, is shown.