Tensile deformation behavior of TRIP-aided bainitic ferrite steel in the post-necking strain region
This study validated the methods for the evaluation of stress triaxiality using finite element (FE) simulations. The FE simulations tracing actual mechanical tests demonstrate derived local stress triaxiality at the fracture portion. Conventionally, such numerically derived stress triaxiality identifies ductile fracture loci for advanced high-strength steels (AHSSs). Although the method has been confirmed, the stress evaluation by FE simulations have not been validated by comparing with actual values. Thus, this study conducted the direct measurement of stress triaxiality in the small round-bar tensile tests using synchrotron X-ray diffraction. Two types of AHSSs—dual-phase (DP) and transformation-induced plasticity (TRIP) steels—were used for the study. As an additional advancement of this study, the modified Swift law was introduced to the FE simulations as a work-hardening law to facilitate an accurate prediction of the stress vs. strain curve in the large strain region. Consequently, the numerically simulated stress triaxiality with the modified Swift law agreed well with the measurement for the DP steel. However, the FE simulation did not agree with the measurements for the TRIP steel due to the stress partitioning of the α- and γ-phases, and transformation from the γ- to the α-phases.
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