13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Hierarchical Damage Simulation to Correlate Micro-structural Characteristics of Steel with Ductile Crack Growth Resistance of Component Mitsuru Ohata1,*, Hiroto Shoji1, Fumiyoshi Minami1 1 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, JAPAN * Corresponding author: ohata@mapse.eng.osaka-u.ac.jp Abstract The final goal of this study is to develop a method for estimating the effect of micro-structural characteristics of steel (especially two-phase steel) on ductile crack growth resistance of a structural component. For this purpose, hierarchical approach to link the micro-structural characteristics of steel and ductile crack growth resistance curve of a component is proposed. First attention is paid to reveal mechanical properties that control ductile crack growth resistance curve (CTOD-R curve), so that the R-curve could be numerically predicted only from those properties. It is shown from the observation of a mechanism for ductile crack growth that two types of “ductile properties” of steel associated with ductile damage can mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from the surface of notch root, and the other one is a stress triaxiality dependent ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve is proposed taking these two “ductile properties” into account, where ductile crack initiation from crack-tip is in accordance with local strain criterion, and subsequent crack growth triaxiality dependent damage criterion. This macroscopic simulation can correlate the mechanical properties of steel with CTOD-R curve of a component. The second approach is to develop a simulation method to predict the effect of micro-structural characteristics of two-phase steel on the two types of ductile properties that were found to be ductile crack growth controlling mechanical properties. To simulate meso-scale ductile damage behaviors, 3D micro-structural FE-model is developed for analyzing the stress/strain localization behaviors by micro-structural strength mismatch and ductile damage model for reproducing damage evolution up to micro-void/micro-crack formation. This meso-scopic simulation can correlate micro-structural characteristics with mechanical properties of two-phase steel. Through the proposed hierarchical approaches, micro-structural morphology of two-phase steel to improve ductile crack growth resistance of a component can be discussed. Keywords Damage model, 3D-simulation, Microstructure, Ductile crack growth resistance 1. Introduction It is necessary to estimate ductile crack growth property of a steel structure for rational assessment of unstable fracture associated with ductile crack initiation and subsequent crack growth. Furthermore, from steel developing side, it should be required to provide a guideline for improving ductile crack growth resistance from material properties point of view. On the other hand, it is expected to improve resistance of ductile crack initiation and extension by controlling heterogeneous micro-structure of steel. However, no effective guideline in terms of material properties for improving resistance of ductile crack growth resistance of a cracked structural component has been necessarily established. For this purpose, the target was focused on proposing a hierarchical approach to link the micro-structural characteristics of multi-phase steel and ductile crack growth resistance of a structural component, which consists of macroscopic approach and mesoscopic approach. Figure 1 describes this concept. The final goal of this study is to develop a numerical simulation method for estimating the effect of micro-structural characteristics of multi-phase steel on ductile crack growth resistance of a structural component.
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