13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 3. Simulation method for correlating micro-structural characteristics with mechanical properties of two-phase steel (Mesoscopic approach) Second effort is to develop the mesoscopic method for correlating micro-structural characteristics with mechanical properties of two-phase steel, for instances Ferrite-Pearlite, Ferrite-Bainite, Ferrite-Martensite steel and so on. The authors have proposed a simulation method for predicting the effect of micro-structural morphology on ductile cracking for two-phase structural steel on the basis of micro-mechanism for ductile cracking through the following developments [3-5]: one is a meso-scale 3D micro-structural FE-model that enables the analysis of stress/strain localization behaviors dependent on the morphology of two phases with strength heterogeneity in real steel, another is a damage evolution and subsequent micro-void nucleation model. This model is expected to be able to predict the two ductile properties, which was found in the macroscopic approach to be the mechanical properties controlling CTOD-R curve, as well as stress-strain curves. In this chapter, it is demonstrated that the proposed model could simulate ductile cracking behaviors associated with heterogeneous micro-structure of two-phase, Ferrite-Pearlite, steel. Then, the effect of morphology of micro-structure of two-phase steel on two ductile properties, which are the critical local strain and the stress triaxiality dependent ductility, is analyzed. 3.1. Ductile cracking behaviors of two-phase steel with heterogeneous micro-structure The structural steel JIS SM490YB with Ferrite-Pearlite two phases (F-P steel) was used. The hardness of Pearlite phase is about 1.4 times larger than Ferrite phase. Volume fraction of Pearlite phase is about 30 %, the average grain size is about 25 !m. Table 2. Mechanical properties of SM490YB steel used. Ferrite Pearlite 344 540 64 17.6 31 198 276 El. : Elongation(G.L. = 36 mm, Dia. = 6 mm), Hv : Average Vickers hardness (Load : 25 gf) !Y : Lower yield stress, !T : Tensile strength, YR : Yield to tensile ratio = !Y/!T, "T : Uniform elongation !Y (MPa) !T (MPa) YR (%) "T (%) El. (%) Hv Figure 6. Microstructures of SM490YB steel used. Figure 7. Configuration of notched micro-tensile specimen used. The specimen for observing ductile cracking behaviors is flat micro-tensile specimen with side notches, as shown in Fig. 7. Figure 8 presents load P and displacement u curves obtained by tensile testing for three specimens. The specimens were unloaded at levels 1 and 2, and observed damage evolution up to failure. The ductile cracks generated from not only notch-root surface but also middle of net section, that is inside of the specimen, were observed at loading level 2, as shown in Fig. 9. These micro-cracks seem to mainly nucleate at Ferrite side with lower hardness near Ferrite-Pearlite boundary. This implied ductile damage was localized by stress/strain
RkJQdWJsaXNoZXIy MjM0NDE=