Advanced constitutive modeling for application to sheet forming

Continuum constitutive descriptions of plasticity suitable for finite element simulations of sheet forming processes are succinctly discussed. Although multi-scale approaches allow for a more explicit representation of the physical deformation mechanisms occurring at microscopic scales, they are usually not suitable for industrial applications because of the quick turnaround time needed for process design simulations. Therefore, advances in classical concepts such as plastic anisotropy and strain hardening are still very much in demand. This article describes possible ways to make use of multi-scale results for application to sheet metal forming simulations.11Ysciescopu

Evaluation of constitutive models for springback prediction in U-draw/bending of DP and TRIP steel sheets

U-draw/bending experiments and simulations were performed to investigate the characteristics of springback in sheet metals. The finite element method is frequently used for the simulation of springback, but the predictions are strongly influenced by the constitutive models such as yield criteria and hardening laws. In the present study, springback of DP and TRIP steel sheets after U-draw/bending was predicted with a finite element analysis. Various yield functions were considered, namely, the isotropic von Mises and anisotropic Hill models. For strain hardening, isotropic, non-linear kinematic and combined isotropic-kinematic hardening models were considered. In order to characterize the isotropic hardening behavior, both uniaxial and balanced biaxial (hydraulic bulge) tension tests were carried out. For the characterization of the combined isotropic-kinematic hardening model, forward-reverse simple shear tests were conducted. The springback predictions were greatly influenced by the choice of the hardening model but slightly affected by the choice of the yield criterion. The kinematic hardening and combined isotropic-kinematic hardening models provided satisfactory predictions for DP590 and TRIP590 sheets, respectively. Due to an extended measurable strain range, the use of the flow curves from the hydraulic bulge test led to more reliable results than those of uniaxial tension.ope

The formability of twinning: induced plasticity steels predicted on the base of Marciniak-Kuczynski theory

The purpose of this work is to predict and analyze the formability of twinning – induced plasticity steels through the Marciniak-Kuczynski (MK) theory with emphasis on the solutions for improving the prediction results. The selected constitutive equations involve the Yld00-2d of Barlat et al. (2003) plane stress yield function, the Swift strain–hardening power law and the dislocation density based constitutive model proposed by Kim et al. (2013), taking into account the dislocation glide, twinning and dynamic strain aging. Three types of high manganese content TWIP steels sheet were selected. To understand the formability of the TWIP steel and the factors influencing it, a sensitive study on the effect of the mechanical properties of the TWIP steel on the MK theory concept and the predicted forming limits is performed. Using the dislocation density based microstructural model, the deformation twinning effect and the contribution of dynamic strain aging to the FLDs of TWIP steel is analyzed. The relevant influence of the sharpness of the yield surface in the biaxial stretching region in the prediction of FLDs of TWIP steels is highlighted. The extended MK model can be adapted to predict the forming limits of the TWIP steels by using an unusual high initial geometrical defect imposed by their high strain hardening. In this way it was showed that the MK theory cannot be applied for predicting the forming limits of TWIP steels unless by applying imperfection factors that are not physically reasonable. Therefore, new failure models are required for TWIP steel.publishe

Observations on the Nonlinear Unloading Behavior of Advanced High Strength Steels

The unloading behavior was compared for three different steel grades: a dual-phase steel, a transformation-induced plasticity steel, and a twinning-induced plasticity steel. Steels that harden by phase transformation or deformation twinning exhibited a smaller component of microplastic strain during unloading and a smaller reduction in the chord modulus compared to the conventional hardening steel. As a result, unloading is closer to pure elastic unloading when the TRIP effect or TWIP effect is active.X111513sciescopu

Application of the Virtual Fields Method to determine dynamic properties at intermediate strain rates

Crash analysis simulation is now very important in automotive industry to assess automotive crashworthiness and safety. In order to acquire reliable crash simulation results, precise material behaviors at intermediate strain rates should be used as input data. To determine the stress-strain curves at various strain rates, the number of expensive and complicated experiments is large. The present study aims at determining the stress-strain curves of sheet metals at various strain rates from a single dynamic experiment. A new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were carried out. Full-field heterogeneous strain fields were measured by a digital image correlation technique using a highspeed camera. The load data was acquired from strain gauges attached to the elastic deformation region on the specimen. Then, an inverse identification scheme with a rate dependent hardening law was applied to retrieve dynamic parameters. The stress-strain curves of advanced high strength steel at intermediate strain rates (100 /s - 300 /s) were successfully obtained from a single experiment.11Ysciescopu

Measurement and analysis of the elastic-plastic deformation behavior of an ultra-thin austenitic stainless steel sheet subjected to in-plane reverse loading.

In order to clarify the deformation behavior of an ultra-thin austenitic stainless steel sheet (SUS301) used for manufacturing electronic parts a new testing devise is designed and built. The test material is 0.2 mm thick and has a 0.2 % proof stress of 1800 MPa. The testing apparatus is equipped with comb-type die couples to measure the stress-strain curves of the sample under tension-compression cyclic loading without buckling for a strain amplitude of 0.017. It is found that the stresses are higher in tension than in compression in the rolling direction (RD) for a strain range of lel 0.002, while in the transverse direction (TD) the stresses are higher in compression than in tension, and that the test material showed significant difference in the cyclic loading behavior between the RD and TD. (C) 2017 The Authors. Published by Elsevier Ltd.110Ysciescopu

Experimental verification of the tension-compression asymmetry of the flow stresses of a high strength steel sheet

110Yscopu

Characterization of dynamic hardening behavior using acceleration information

Crash analysis simulation is very important in automotive industry to assess automotive crashworthiness and safety. In the FE simulation, accurate dynamic hardening behavior should be used as input data to provide reliable results. But, it is difficult to obtain precise hardening properties at intermediate or high strain rates due to inaccurate measurement of load caused by the inertial effect. In this study, a new methodology was applied to retrieve dynamic strain hardening properties of sheet metal specimens. The virtual fields method (VFM) was adopted as an inverse method to identify hardening parameters without load information. As an initial study, Swift model for a rate independent hardening law was selected for an elasto-plastic constitutive model. In order to validate the proposed methodology in the experiments, a new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were performed. Digital image correlation technique using a high-speed camera was utilized to measure strain and acceleration fields so that the identification is carried out from the measured quantities. The validation of the proposed VFM identification procedure using the acceleration will be performed by comparing with the conventional procedure using a load-cell. (C) 2017 The Authors. Published by Elsevier Ltd.110Ysciescopu

FE implementation of HAH model using FDM-based stress update algorithm for springback prediction of AHSS sheets

The homogeneous anisotropic hardening (HAH) model was implemented into a finite element (FE) code in order to predict springback for an advanced high strength steel (AHSS) sheet sample after double-stage U-draw bending. The finite difference method (FDM) was utilized as an alternative way to calculate the derivatives of this advanced distortional plasticity model allowing the update of the equivalent plastic strain and stress tensor at each time step in the user-material subroutines (UMAT and VUMAT). The FDM makes it easier to derive the stress gradient of complex yield surfaces. The proposed FDM-based stress update algorithm was verified by comparing the springback profiles after the single- and double-stage U-draw bending tests for a DP980 sheet sample predicted with analytical and numerical approaches. In addition, the springback measurement parameters and computational efficiencies depending on both approaches were also compared. The results indicate that the computational efficiency and accuracy of the FE simulations with the FDM-based stress update algorithm were similar to those of the analytical method. © 2018 Institute of Physics Publishing. All rights reserved.11Ysciescopu

Application of homogeneous potentials for the modeling of the Bauschinger effects in ultra low carbon steel

In this work, an approach is proposed for the description of the plastic behavior of materials subjected to multiple or continuous strain path changes. In particular, although it is not formulated with a kinematic hardening rule, it provides a reasonable description of the Bauschinger effect when loading is reversed. This description of anisotropic hardening is based on homogeneous yield functions/plastic potentials combining a stable, isotropic hardening-type, component and a fluctuating component. The capability of this constitutive description is illustrated with applications on an ultra low carbon steel sheet sample deformed in three-stage uniaxial loading with two load reversals [1].ope