Lucian Lazarescu

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.

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).

Evaluation of Drawing Force and Thickness Distribution in the Deep-Drawing Process with Variable Blank-Holding

In the deep drawing process, the blank-holding force (BHF) is an important process parameter affecting the energy consumption and the successful production of parts. In the present work, both experiments and finite element simulations have been conducted to investigate the influence of constant and time variable BHF on drawing force (DF) and thickness distribution in the deep drawing process of cylindrical and square cups. A finite element model was developed in the AutoForm software and validated with experiments. The developed model has been used for the simulation of deep drawing process of AA6016-T4 aluminum alloy sheet. The experimental and numerical results show that, using a variable instead of a constant BHF, the DF can be decreased in the expense of wall thickening.

Comparison of two methods for detection of strain localization in sheet forming

This paper presents a comparison of two criteria of strain localization in experimental research and numerical simulation of sheet metal forming. The first criterion is based on the analysis of the through-thickness thinning (through-thickness strain) and its first time derivative in the most strained zone. The limit strain in the second method is determined by the maximum of the strain acceleration. Experimental and numerical investigation have been carried out for the Nakajima test performed for different specimens of the DC04 grade steel sheet. The strain localization has been identified by analysis of experimental and numerical curves showing the evolution of strains and their derivatives in failure zones. The numerical and experimental limit strains calculated from both criteria have been compared with the experimental FLC evaluated according to the ISO 12004-2 norm. It has been shown that the first method predicts formability limits closer to the experimental FLC. The second criterion predicts values of strains higher than FLC determined according to ISO norm. These values are closer to the strains corresponding to the fracture limit. The results show that analysis of strain evolution allows us to determine strain localization in numerical simulation and experimental studies.This paper presents a comparison of two criteria of strain localization in experimental research and numerical simulation of sheet metal forming. The first criterion is based on the analysis of the through-thickness thinning (through-thickness strain) and its first time derivative in the most strained zone. The limit strain in the second method is determined by the maximum of the strain acceleration. Experimental and numerical investigation have been carried out for the Nakajima test performed for different specimens of the DC04 grade steel sheet. The strain localization has been identified by analysis of experimental and numerical curves showing the evolution of strains and their derivatives in failure zones. The numerical and experimental limit strains calculated from both criteria have been compared with the experimental FLC evaluated according to the ISO 12004-2 norm. It has been shown that the first method predicts formability limits closer to the experimental FLC. The second criterion predicts value...

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.

An Innovative Procedure for the Experimental Determination of the Forming Limit Curves

A new procedure for the experimental determination of the FLCs has been proposed. The procedure is based on the hydraulic bulging of two specimens. The upper blank has a pair of holes pierced in symmetric positions with respect to the center, while the lower blank acts both as a carrier and a deformable punch. By modifying the dimensions and reciprocal position of the holes, it is possible to investigate the formability in different strain combinations. The most important advantages of the method are the capability of investigating the entire deformation domain specific to sheet metal forming processes, simplicity of the equipment, and reduction of the parasitic effects induced by the friction, as well as the occurrence of the necking in the polar region. The comparison between the FLCs determined using the new procedure and the Nakazima test shows minor differences.

Influence of material property variability on the thickness in sheet metal subjected to the hydraulic bulging

In the sheet metal forming processes, the inherent unavoidable noise in material property, which can vary inside and between batches as well as between suppliers, constitutes an important source of uncertainty which could result in rejection of some formed parts. In the process design phase, a successful tool for the sensitivity analysis proved to be the finite element codes, in which case the material parameters are used as input into the yield criterion. This paper investigates the thickness sensitivity due to the noise in material property using the finite element simulation of hydraulic bulge test. The BBC 2005 yield criterion with eight constitutive parameters, which is implemented in the commercially finite element code AutoForm has been used in this study.