Micro-mechanical investigation of deformation and failure scenario in dual phase steel using experimental and numerical methods
Final Report Abstract
In this project, the mechanical behavior of two cold rolled commercial dual phase steel grades DP600 and DP980 were analyzed experimentally and numerically on micro and macro scale. In WP1, the anisotropic behavior of both steel grades was investigated by standard tensile tests in three directions. The results show that material properties strongly depend on the particular process conditions of the DP-steel grade. In order to assess the influence of the stress state on the mechanical behavior, four different loading conditions (uniaxial tension, plane strain, biaxial tension and shear) were analyzed by ARAMIS. The ARAMIS results are utilized in WP4 for calibration and verification on the simulations. In WP2, microstructure analyses were conducted to assess the effect of martensite phase fraction, microstructure morphology and four loading conditions on damage evolution from the initial state to final failure. At first, the void patterns of both DP grades (DP600 and DP980) and for all four loading conditions were evaluated. As a result, different void formations were observed for each type of loading condition. Furthermore, the damage evolution was assessed for tensile loading in both materials at three stages (i.e., void initiation, void growth and void coalescence) by in-situ SEM testing. At lower strains, three different types of damage initiation mechanisms were observed in both materials, (a) in the center of a large ferrite phase, (b) at the interface of ferrite and martensite phases, and (c) at the trapped ferrite phase surrounded by martensite phases. At high strains, two effective micro-crack initiation mechanisms have been observed during in-situ testing: (i) at a thin martensite phase region due to strain intensification or shear band growth, and (ii) at the boundary between ferrite and martensite phases. DP600 and DP980 steels depict similar behavior in the micro-crack initiation stage. With increasing strain, micro-cracks propagate through the strain bands and then lead to final fracture in both steels. In WP3, micro-indentation tests were performed on both steel grades. For the DP600 steel grade, a nearly bimodal distribution of the hardness values was found. In the case of steel DP980 the results were found more difficult to interpret. It is supposed that the higher martensite volume fraction leads to a less distinct separation of the hardness valued for both phases. In WP4, a Modified Mohr-Coulomb (MMC) damage model was utilized in macro- and micro-mechanical analyses to predict the fracture behavior of DP600 and DP980 steels under the various loading conditions. At first, a VUMAT subroutine was developed to include a Modified-Mohr-Coulomb (MMC) damage model in the 3D macro models, and good agreement between the macro-mechanical model and experimental results was observed. However, in the micromechanical model, the MMC damage model can only predict the macroscopic fracture strain at near zero Lode angle, which are shear and plane strain loading conditions. The results of the observed micro-mechanical damage mechanisms revealed that the 3D micromechanical model is able to reproduce in-situ test results, but some discrepancies due to the random distribution of martensite phases in the out-of-plane direction in the 3D micromechanical model. Finally, fracture forming limit diagrams (FFLD) were calculated for both DP steels including the results from the macro- and micro-mechanical models. It was found that the 3D micromechanical model solely is not capable to calculate the FFLD curve for larger lode angles, which generally possess the tensile test and the bulge test. However, under plane strain conditions a good match with the experimental results was observed. In WP5, an appropriate distribution of the martensite morphology was created using a statistical reconstruction scheme. For each ferrite grain, the orientation of the crystallographic lattice was assigned by considering a crystal plasticity material model to include the influence of the texture. In a next step, different parameter sets for the extended Mohr-Coulomb Damage model were applied in modelling the DP steel microstructure. Then, crystal plasticity (CP)-FEM results were compared with the deformation patterns obtained from high resolution SEM figures in WP2. The predicted damage pattern and the void distribution show similar features with respect to the locations of high damage values adjacent to martensite islands and the shape of the voids depending on the load case. Finally, a comparison of the damage scenario performed in the micromechanical FEM model and in the CP-FEM model is presented. In the surrounding of the martensite phase, the strain fields are found to be very similar. In case of the crystal plasticity modeling approach, additional regions with high plastic deformation can be found in the ferritic matrix without any martensite in the surrounding. This can be explained with the orientation mismatch of neighboring ferrite grains since the regions with high plastic deformations coincide with ferrite grain boundaries. In the framework of this research project, comprehensive experimental and numerical studies with respect to the microstructure-property relation of the damage behavior of the two DP steel grades were conducted, successfully. New insights of the influence of martensite phase fraction and the phase distribution on the damage evolution were obtained. The FFLD curves of both DP steel grades could be predicted using the implemented MMC damage model.
Publications
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(2020) A comparative study on mechanical behavior and damage scenario of DP600 and DP980 steels. Mechanics of Materials 143 103339
Cheloee Darabi, Ali; Guski, Vinzenz; Butz, Alexander; Kadkhodapour, Javad; Schmauder, Siegfried
