Dan Sorin Comsa

Analytical and Experimental Evaluation of the Stress-Strain Curves of Sheet Metals by Hydraulic Bulge Tests

This paper presents a new methodology for the determination of the biaxial stress – strain curves by hydraulic bulging tests with circular die. In order to validate the methodology, the authors have performed both stepwise and continuous bulging experiments. The pressure, polar height and curvature radius have been measured in different stages of the deformation process or continuously recorded during the test.

Determination of Stress‐Strain Curves of Sheet Metals by Hydraulic Bulge Test

The paper is focused on the determination of the biaxial stress‐strain curves by hydraulic bulging tests with circular die using a new methodology. This methodology is based on a modified version of Kruglov’s formula for the calculation of the polar thickness that takes into account the non‐uniformity of the strain distribution on the dome surface. In order to validate the methodology, the authors have performed both stepwise and continuous bulging experiments using the optical measurement system ARAMIS. The comparison with experimental data shows an improved accuracy of the modified Kruglov’s formula.

Prediction of the forming limit band for steel sheets using a new formulation of hora's criterion (MMFC)

The paper analyzes the dispersion of the mechanical parameters and its influence on the forming limit curves of sheet metals. The tests have been made for the case of the DC01 steel sheets. The dispersion of the mechanical parameters has been observed during the experimental research. On the basis of this dispersion, a forming limit band has been calculated using an alternate formulation of Hora’s model (MMFC).

Numerical Simulation of Sheet Metal Forming Processes Using a New Yield Criterion

The paper is focused on the development of a new phenomenological yield criterion able to describe the inelastic response of sheet metals subjected to cold forming. The model consists in two components: the equivalent stress and the hardening law. The equivalent stress is a function incorporating 8 material parameters. Due to these parameters, the new formulation is able to describe four normalized yield stresses (y0, y45, y90, yb) and four coefficients of plastic anisotropy (r0, r45, r90, rb). The hardening law is defined as a linearly asymptotic function containing 4 material parameters. The numerical tests presented in the last section of the paper prove the capability of the elastoplastic constitutive models based on the new yield criterion to model the earing as well as the wrinkling phenomena accompanying the deep-drawing process.

Effect of the mechanical parameters used as input data in the yield criteria on the accuracy of the finite element simulation of sheet metal forming processes

The accuracy of the finite element simulation of sheet metal forming processes is mainly influenced by the shape of the yield surface used in the mechanical model and, in particular, by the number of input values used in the identification of the yield surface. This paper investigates the effect of the input values used for identifying the BBC 2005 yield criterion on the accuracy of the finite element predictions. The accuracy assessment of the simulation is based on the comparison of the numerical predictions obtained using the commercially available FE programme AUTOFORM and experimental measurements obtained from the hydraulic bulging of sheet metals. Thickness and strain distributions, as well as the geometry of the bulged specimen were taken as comparison parameters. The accuracy of the finite element predictions obtained using the Hill-48 and Barlat-89 yield criteria is also studied and discussed in comparison with the results provided by the BBC 2005 yield and the experimental data.

A Procedure for the Evaluation of Flow Stress of Sheet Metal by Hydraulic Bulge Test Using Elliptical Dies

The paper describes a new experimental procedure for the determination of the curves relating the equivalent stress and equivalent strain of sheet metals by means of the hydraulic bulge tests through elliptical dies. The procedure is based on an analytical model of the bulging process and involves the measurement of only two parameters (pressure acting on the surface of the specimen and polar deflection).

Effect of the Constitutive Law on the Accuracy of Prediction of the Forming Limit Curves

The accuracy of the forming limit curves predicted by the Marciniak-Kuczynski model depends on the type and flexibility of the constitutive equations used to describe the mechanical response of the sheet metal. From this point of view, the yield criterion has the most significant influence. The paper presents a comparative analysis referring to the quality of the forming limit curves predicted by the Marciniak-Kuczynski model for the case when the plastic anisotropy of a DC04 sheet metal is described by the BBC2005 yield criterion. The coefficients included in the expression of the BBC2005 equivalent stress are evaluated using different identification strategies (with 4, 6, 7, and 8 mechanical parameters). The forming limit curves predicted by the Marciniak-Kuczynski model in each of the cases previously mentioned are compared with experimental data.

A simplified description of the uniaxial tensile test used for calibrating constitutive models of orthotropic porous sheet metals

ABSTRACT: In the present work, a simplified model of the uniaxial tensile test is developed for orthotropic metallic sheets. This model is mainly established for tensile test analysis and calibration of material parameters. The constitutive equations included in the model are based on an anisotropic Gurson-Tvergaard-Needleman model combined with the Hill 1948 quadratic yield criterion. At first, a detailed description of the constitutive equations along with their computer implementation is presented. Then, by comparing the force and void evolution diagrams predicted by the model with numerical and experimental results the efficiency and accuracy of the model are assessed. Finally, the effect of different parameters on the traction force and evolution of voids during uniaxial tensile tests are studied. The material parameters used in the calibration procedure are as follows: initial void volume fraction, two adjusting parameters, nucleation of void volume fraction, standard deviation, mean value of void nucleation strain, and sample orientation with respect to the rolling direction. The tests performed by the authors prove the capability of the simplified model to describe accurately the mechanical response of orthotropic sheet metals. Review History:

Effect of the Blank-Holding Load on the Drawing Force in the Deep-Drawing Process of Cylindrical and Square Cups

In this study, the influence of the blank-holding force (BHF) on the drawing force (DF) in the deep-drawing process of cylindrical and square cups has been investigated experimentally. For this purpose, different constant and variable BHFs have been applied to AA6016-T4 aluminum alloy and DC04 steel sheets during the forming process. It has been observed that an increased constant BHF leads to an increase of DF. On the other hand, the variable BHF approach, in which the BHF decreases in six steps throughout the punch stroke, reduces the DF.

Influence of the Identification Procedure of the Yield Criterion on the Thickness Prediction of the Square Cup

The accuracy of the forming limit curves reported by the Marciniak-Kuczynski model as well as the quality of the predictions provided by the FE programs depend on the yield criterion that describes the anisotropic plastic behavior of the sheet metal. Two identification procedures of Barlat89 and BBC2005 are used in this paper. The first procedure is a conventional one well described in the literature [, while the second approach is developed by adding an experimental parameter determined in plane-strain state that allows establishing the exponent of the yield criteria. The mechanical response of the IF steel shows that the limit strains are strongly influenced by a small alteration of the yield surface. The influence of the yield criterion on the thickness predictions obtained in the simulation of a square cup deep drawing simulation is also studied.