Frédéric Barlat

Unconditionally Convex Yield Functions for Sheet Metal Forming Based on Linear Stress Deviator Transformation

Two non-quadratic orthotropic yield functions called Yld2011-18p (containing 18 param-eters) and Yld2011-27p (containing 27 parameters) are proposed. The formulations are based on theestablished concept of linear transformations operating on the stress deviator. Application examplesreveal the capabilities of both yield functions to accurately describe complex plastic anisotropy ofsheet metals.

A Theoretical Study of the Effect of the Double Strain Path Change on the Forming Limits of Metal Sheet

The present paper aims at a theoretical study of the forming limits of a sheet metal subjected to double strain path changes by using as reference material the AA6016-T4 aluminum alloy sheet. The simulation of plastic instability is carried out through the Marciniak-Kuczynski analysis. The initial shape of the yield locus is given by the Yld2000-2d plane stress yield function. The strain hardening of the material is described by the Voce type saturation law. Linear and several complex strain paths involving single and double strain path changes are taken into account. The validity of the model is assessed by comparing the predicted and experimental forming limits under linear and selected one strain path change. A good accuracy of the developed software on predicting the forming limits is found. A sensitive analysis of the influence of the type and value of the double prestain in the occurrence of the plastic flow localization is performed. A remarkable effect of the double strain path change on the sheet metal forming limits is observed.

Stress Relaxation for Formability Improvement

Improved formability has been reported due to stress relaxation when the continuous forming cycle is interrupted with steps by adjusting the punch motion. The contribution of stress relaxation and its parameters on the ductility of materials has not been established so far. In the present work, the stress relaxation behavior of three materials, low carbon steel, DP and TRIP steels are studied. The influence of strain rate and strain on the ductility enhancement due to stress relaxation is analyzed. It is observed that stress relaxation improved the ductility of materials in all the cases and therefore can be used as a potential method to improve formability in sheet metal forming.

Identification of Dynamic Flow Stress Curves Using the Virtual Fields Methods: Theoretical Feasibility Analysis

An inverse approach based on the virtual fields method (VFM) is presented to identify the material hardening parameters under dynamic deformation. This dynamic-VFM (D-VFM) method does not require load information for the parameter identification. Instead, it utilizes acceleration fields in a specimen’s gage region. To investigate the feasibility of the proposed inverse approach for dynamic deformation, the virtual experiments using dynamic finite element simulations were conducted. The simulation could provide all the necessary data for the identification such as displacement, strain, and acceleration fields. The accuracy of the identification results was evaluated by changing several parameters such as specimen geometry, velocity, and traction boundary conditions. The analysis clearly shows that the D-VFM which utilizes acceleration fields can be a good alternative to the conventional identification procedure that uses load information. Also, it was found that proper deformation conditions are required for generating sufficient acceleration fields during dynamic deformation to enhance the identification accuracy with the D-VFM.

Validation of homogeneous anisotropic hardening approach based on crystal plasticity

The current study investigates constitutive models at two different scales: 1) the micromechanical crystal plasticity framework using a dislocation density-based hardening model [1, 2]; 2) macroscale constitutive model based on a yield function that evolves according to the homogeneous anisotropic hardening (HAH) model [3, 4]. The polycrystalline aggregate, tuned for a low-carbon steel, is used to calculate the evolution of the yield surface during monotonic uniaxial tension. The results of the crystal plasticity model are used to train the anisotropic yield function and HAH parameters to demonstrate the flexibility of the macroscale constitutive approach. Through comparison between the two models, an improved rule for the HAH model is suggested.

Transient Negative Strain Hardening during Severe Plastic Deformation of Al-30wt%Zn Alloys

In this work we propose a new model (SOFTMAR) to describe the transient softening observed during severe plastic deformation of Al-30wt%Zn alloys. The model is divided in two main parts. The first one describes the softening process based on the evolution law of the mean free path of dislocations (L) with plastic strain . In the second part of the model emphazis is given to the relationship between the grain size and strain rate that in turn depends on diffusion-driven grain-boundary sliding.

Microstructural and Crystallographic Aspects of Yield Surface Evolution

The microstructural and crystallographic aspects, reflected at the macroscopic scale on yield surface and its subsequent evolution, are reappraised by application of crystal plasticity simulations. Strain hardening rule in the slip system is coupled to cope with latent hardening and Bauschinger effect. Uniaxial tension simulation on an isotropic polycrystalline aggregate leads to anisotropic strain hardening. Typical elements of phenomenological plastic anisotropy and hardening rules such as expansion, kinematic shift and distortion of the yield surface, are shown to be featured in crystal plasticity by tuning the slip system hardening rules appropriately.

Effect of Stress State and Temperature on the Kinetics of Martensitic Phase Transformation in TRIP-Assisted Steel

The effects of the stress state and temperature on the martensitic phase transformation behavior in TRIP 780 steel were investigated using multi-axial experimental techniques. Various mechanical experiments are performed to differentiate the stress state and temperature effects. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equibiaxial tension, plane strain tension and simple shear. A temperatures both 25 and 60 °C for each stress state condition except the simple shear test were investigated. In-situ magnetic measurements were performed to mesure the evolution of the martensite content throughout each experiment. Finally, a new martensitic transformation kinetics model for the TRIP 780 steel is proposed to take the effect of stress state and temperature into account.

Influence of Contact Friction on the Experimental Determination of the Balanced Biaxial Strain-Ratio Using the Disc Compression Test

In the present work the disc compression test used to determine the balanced biaxial strain-ratio

Prediction of Ridging by 3-Dimensional Texture in Ferritic Stainless Steels

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