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In order to reduce vehicle weight while maintaining crashworthiness, advanced high strength steels (AHSSs), such as DP980, are extensively used for manufacturing automotive body components. During trimming operations, the high tensile strength of DP980 sheets tends to cause damage of the trim edge of D2 die inserts, which result in deterioration of the edge quality. The objective of this work is to study the damage microstructures at the trimmed edge of DP980 steel sheets as a function of the number of trimming cycles. A mechanical press equipped with AISI D2 tool steel inserts was used to continuously trim 1.4 mm thick sheets of DP980 at a rate of 30 strokes/min. Cross-sectional SEM images of the trimmed edges revealed that the sheared edge quality of the DP980 sheets decreased, indicated by an increase in the burr width, with an increase in the number of trims from 40,000 to 70,000. Plastic strains were estimated using the displacements of the martensite plates within plastic flow fields of ferrite. Sit...

A material model for more effective analysis of plastic deformation of sheet materials is presented in this paper. The model is capable of considering the following aspects of plastic deformation behavior of sheet materials: the anisotropy in yielding stresses in different directions by using a quadratic yield function (based on Hill’s 1948 model and stress ratios), the anisotropy in work hardening by introducing non-constant flow stress hardening in different directions, the anisotropy in plastic strains in different directions by using a quadratic plastic potential function and non-associated flow rule (based on Hill’s 1948 model and plastic strain ratios, r-values), and finally some of the cyclic hardening phenomena such as Bauschinger’s effect and transient behavior for reverse loading by using a coupled nonlinear kinematic hardening (so-called Armstrong-Frederick-Chaboche model). Basic fundamentals of the plasticity of the model are presented in a general framework. Then, the model adjustment procedure is derived for the plasticity formulations. Also, a generic numerical stress integration procedure is developed based on backward-Euler method (so-called multi-stage return mapping algorithm). Different aspects of the model are verified for DP600 steel sheet. Results show that the new model is able to predict the sheet material behavior in both anisotropic hardening and cyclic hardening regimes more accurately. By featuring the above-mentioned facts in the presented constitutive model, it is expected that more accurate results can be obtained by implementing this model in computational simulations of sheet material forming processes. For instance, more precise results of springback prediction of the parts formed from highly anisotropic hardened materials or that of determining the forming limit diagrams is highly expected by using the developed material model.

Electrohydraulic Forming (EHF) is a high velocity forming process, in which the strain-rate in the sheet metal can reach very high values depending on the prescribed input energy, the chamber geometry, the die geometry, instrumentation efficiency and the mechanical properties of the sheet material. In EHF, a high voltage discharge between electrodes that are submerged in a water-filled chamber generates a plasma channel that leads to propagation of a shockwave through the water that forms the sheet, with or without a die, in less than a millisecond. EHF generates a complex pressure pulse history that is extremely challenging to simulate. In this work, three-dimensional finite element simulations of DP590 sheet were completed in free-forming (EHFF) and die-forming (EHDF) conditions using ABAQUS/Explicit and a combination of Eulerian and Lagrangian elements. The Johnson-Cook constitutive plasticity model was selected and the parameters were calibrated based on uniaxial tensile test data at different strain-rates. A comprehensive numerical study was carried out with a view to understanding the differences between EHFF and EHDF in terms of the history of the deformation profile of the specimen, the strain-rate history, the loading path and through-thickness stresses. Higher strain-rates and more complex strain-paths were predicted in EHDF compared to EHFF due to dynamic sheet/die interaction. Good correlation between the experimental and numerical results demonstrated the ability of numerical models to accurately predict the history of the deformation profile in both EHDF and EHFF conditions.

Another factor that has a very significant effect on sheet formability is the loading path. Indeed, non-linear strain paths cause the forming limit curve (FLC) to translate in principal strain space and therefore the actual FLC for a given location on a part may be quite different from the as-received FLC. Consequently, the virtual design of a forming process will only be reliable if the forming limits used to assess the forming severity of the part were determined with due consideration of the (non-linear) strain paths in each critical location of the part. The FLC of sheet metals have often been predicted using the MK method which was proposed by Marciniak and Kuczynski (1967) and which models the inhomogeneity of the sheet material by a very shallow groove. Predictions of theoretical FLC using the MK method generally assume plane stress conditions, but in this work the MK analysis was modified to consider both the nonlinearity of the loading path and the presence of a through-thickness stress, simultaneously. Different magnitudes of prestrain were applied to a virtual sheet so as to simulate bi-linear strain paths and different values of the normal stress were applied for the prediction of the FLC. Published experimental FLC data obtained for different bilinear loading conditions were used to validate the theoretical work in both plane stress and general three-dimensional stress conditions. It was found that this modified MK model predicts FLC that correlate well with the limited experimental data.

In most predictions of forming limits for the evaluation of sheet metal forming severity, it is generally assumed that plane stress conditions apply and consequently the effects of the normal stress component on sheet metal formability are not considered. However, many industrial metal forming processes, such as tubular hydroforming, involve significant through‐thickness stresses in the tube wall while the part is being formed.In this work, the well‐known MK analysis originally proposed by Marciniak and Kuczynski was modified to account for general three‐dimensional stress states with a view to predicting the forming limit curve (FLC) and the stress‐based forming limit curve (SFLC) of sheet materials in the presence of a non‐negligible through‐thickness stress. This analysis was formulated in a way that the FLC/SFLC can be predicted for given ratios of the normal stress component to the major in‐plane stress component (β = σ3/σ1). The FLC and SFLC were then predicted for both plane‐stress conditions (β