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Although numerous models have been proposed to describe the strength differential (SD) effect, their appropriateness and efficiency in the case of advanced high strength steel (AHSSs) have not been investigated. To this end, this study investigates the SD effect in AHSS using a hydrostatic-pressure-dependent plasticity theory. Although pressure model can effectively predict the SD effect in AHSSs with only one coefficient, the original formulation is experimentally inconsistent in the first quadrant of a yield surface. Hence, this study developed a new formulation to remove the inconsistency in a simple manner and validated the pressure model using new experimental data reported for two recently developed AHSSs: DP980 and MART1500Y. The SD effect is characterized based on monotonic compression and tension tests in three different directions with respect to the rolling direction to obtain the stress–strain curves and r-values. Next, the pressure model is used to predict the plastic behavior of both steel sheets. The parallel transition of flow stress and similarity in r-values between the tension and compression states supports the appropriateness of the proposed pressure-based modeling. Furthermore, these insights substantiate the fact that deformation mechanisms cannot explain the SD effect in both steel sheets. Therefore, complex yield functions that describe the SD effect in general materials are not necessary in the case of AHSS. Additionally, the pressure model is incorporated into a distortional plasticity model for applying it to springback in the bending process. The distortional plasticity model with and without the pressure model is calibrated using tension–compression curves, thereby revealing that the SD significantly affects the calibration. Finally, a U-draw bending experiment and the corresponding finite element simulation are conducted to compare the springback; the results show that considering the SD effect improves the springback simulation accuracy. • Strength differential effect of high strength steel was observed experimentally. • Strength differential effect was modeled by pressure based approach.. • Pressure based modeling was validated by comparison of strength and r-values of tension and compression. • Effectiveness of pressure based modeling was proved in calibration of distortional plasticity and springback prediction. [ABSTRACT FROM AUTHOR]

• Fracture mechanism in absence of necking was investigated for steel sheets. • Micro- and macro-scale characterizations showed presence of shear and decohesion. • Finite element study was performed to model the forming behavior of steel sheets. • A simple fracture model was applied in finite element code to predict the fracture. The advanced high strength steels (AHSS) are well recognized for their high-strength-to-weight ratio combined with good shock absorption properties. Moreover, these steels are also known for their complex forming and fracture characteristics. This study is aimed at investigating the fracture behavior of AHSS sheets under the influence of an equal-biaxial state of stress. The work was conducted on two important AHSS sheets – (i) DP980 and (ii) TRIP1180. A hydraulic bulge test was performed to cause the fracture under biaxial loading condition. The strains near the pole of the bulge specimens were measured using the digital image correlation technique. A detailed micro- and macro-scale characterization was performed to examine the fracture aspects of both sheets. The results clearly indicated the occurrence of shear localization during fracture. Moreover, the likely sources of micro-crack initiation were also revealed. Apart from fracture characterization, a comprehensive finite element study was done using appropriate constitutive equations. Finally, a simple fracture criterion was applied to predict the onset of fracture for both steels. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Fracture behavior of DP980 and TRIP1180 sheets during 3-point bending was explored. • Macro- and micro-scale investigations showed presence of shear and decohesion. • A finite element study was done to model the bending behavior of the sheets. • A suitable set of mechanical tests is found for the calibration of fracture model. The goal of this study was to examine the fracture behavior of advanced high strength steel (AHSS) sheets in bending. For this purpose, 3-point bending (3 PB) tests were conducted on DP980 and TRIP1180 sheet samples. The test specimens were prepared in different orientations with respect to the rolling direction of the sheets. The onset of fracture and fracture strains were determined through digital image correlation technique. The fracture regions of the deformed specimens were observed at different magnifications to understand the crack initiation mechanism. Apart from fracture characterization, a finite element study was performed for both steels using an Abaqus/Explicit solver. The parameters required for the constitutive equations were obtained based on mechanical characterizations carried out on these sheets in a previous work. Finally, an uncoupled fracture model was adopted to predict the fracture location and onset of cracking during the 3 PB test. The predicted results were found in reasonable agreement with the experimental data when the fracture model was calibrated using an appropriate set of mechanical tests. [ABSTRACT FROM AUTHOR]

• The strength-differential (SD) effect of AHSS with ultimate tensile strength exceeding 1 GPa is modeled by a hydrostatic pressure-dependent distortional plasticity theory. • The finite element implementation of the pressure-dependent plasticity theory is proposed in the manuscript. The implemented theory is validated with the comparison study with the associated stand-alone code and experimental measurements. • Both the SD- and strain patch change effects of TIRP1180 are reproduced in finite element simulation by embedding the distortional plasticity model, HAH 20 , in the pressure-dependent yield condition. The effectiveness and applicability of the FE-implemented pressure-dependent distortional plasticity model in sheet metal forming simulations is validated through the springback prediction studies. In this work, the hydrostatic pressure-dependency of the distortional plasticity model HAH 20 was implemented using a finite element (FE) code to account for the strength-differential (SD) effect that has been observed in advanced high-strength steel sheets. To this end, a fully-implicit stress update algorithm formulation was introduced for the pressure-dependent plasticity theory. The implementation was validated by comparing the FE prediction of the material behavior during a number of tests with those of a stand-alone code of the constitutive description as well as with experimental data. In order to investigate the SD effect on the springback simulation results, the U-draw bending test was analyzed within this FE framework. Furthermore, in order to assess the effectiveness and stability of the formulation for a large-scale example, forming simulations of an automotive structural part were conducted. In addition to the SD effect, strain path changes and geometrical aspects were also investigated in this example. It was shown that the distortional plasticity-based pressure-dependent yield criterion well describes the asymmetric SD behavior in sheet metal forming simulations. [ABSTRACT FROM AUTHOR]