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The standard ISO 12004-2 has recently established a methodology for estimating limit strains in Marciniak and Nakajima tests, that is, situations in which the strain gradient through the sheet thickness does not play an important role. This method, known as a “position-dependent method,” is based on the analysis of the major strain profile just before fracture occurrence. The present paper proposes an alternative method for detecting the onset of necking and determining the forming limits, based on temporal analysis of the major strain. This time-dependent methodology is applicable to more general situations where strain gradients in the sheet cannot be neglected, such as severe stretch-bending tests, small radius punches, etc. A series of stretching and stretch-bending experiments, close to plane strain conditions, have been carried out to check the ability of a number of methods to predict the onset of necking. Finally, a comparison of these methodologies is presented and their advantages and limitations are discussed in detail. [ABSTRACT FROM AUTHOR]

Two analytical approaches are detailed for the determination of Forming Limit Diagrams (F.L.D.) and compared with experimental results. The first one is the “Enhanced Modified Maximum Force Criterion EMMFC” and the second one is the “Through-Thickness Shear Instability Criterion TTSIC”. The criteria are both written in an intrinsic analytical form and are applicable to linear and non-linear strain paths as it occurs in any FEM codes for sheet-metal forming simulation. Finally, the two methods are complementary depending on the nature of failure and the predicted curves are in reasonable agreement with the trend of experimental results for a wide range of materials. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In the following paper, a full mechanical characterization of the AA6016 T4 aluminum alloy car body sheet DR100 is presented. A comprehensive experimental program was performed to identify and model the orthotopic elasto-plastic deformation behavior of the material and its fracture characteristics including criteria for localized necking, ductile fracture and shear fracture. The commercial software package MF GenYld + CrachFEM in combination with the explicit finite element code Ls-Dyna is used to validate the quality of the material model with experiments, namely, prediction of the FLD, deep drawing with a cross-shaped punch and finally, analysis of a simplified hemming process using a solid discretization of the problem. The focus is on the correct prediction of the limits of the material in such processes. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The forming limits of sheets subjected to the Single Point Incremental Forming process (SPIF) is generally several times higher than those found in the Forming Limit Curve (FLC). In this paper it is shown that the non-monotonic, serrated strain paths to which the material is subjected to during the SPIF process, play a role in the high formability, compared to the monotonic loading in the traditional FLC. The deformation history of an aluminium alloy truncated cone formed with the SPIF process is retrieved through a finite element (FE) model, and discussed. Subsequently, the strain paths at three different depths in the sheet are used as input into a Marciniak-Kuczynski (MK) forming limit model. The usage of different constitutive models in this analysis shows that anisotropic hardening contributes to the delay of the onset of necking in the SPIF process. The large difference in the predicted forming limits that were obtained from the different layers indicates that an interaction between these layers should be taken into account for more accurate forming limit predictions of sheets subjected to the SPIF process. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

A correct prediction of a possible sheet metal failure is essential to sheet metal forming simulations. The use of the conventional forming limit curve (FLC) is the standard approach on industrial level for this problem. The FLC concept is limited to the case of linear strain paths, however. The initial FLC is no longer valid in the case of nonlinear strain paths. The algorithm Crach allows for a transient prediction of the forming limit for localized necking in the case of arbitrary strain paths. For high strength steels and aluminium sheets there is also a risk of fracture without prior localized necking. This paper presents a fracture model that accounts for ductile fracture (caused by void nucleation, void growth and void coalescence) and shear fracture (caused by shear band localization). For both types of fracture, stress state parameters are introduced which can be applied for the plane stress state and for the general 3D stress state. The fracture limits are defined by the equivalent plastic strain at fracture as a function of the stress state parameter based on different experiments with nearly constant stress state parameter. These fracture limit curves are a basis for an integral damage accumulation in the sheet metal forming simulation for arbitrary nonlinear strain paths. The model Crach for the prediction of localized necking and the two fracture models for ductile and shear fracture are combined in the comprehensive failure model CrachFEM. CrachFEM can be linked via a user material model MF GenYld to different explicit FEM codes. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Two analytical approaches are detailed for the determination of Forming Limit Diagrams (F.L.D.) and compared with experimental results. The first one is the “Enhanced Modified Maximum Force Criterion EMMFC” and the second one is the “Through-Thickness Shear Instability Criterion TTSIC”. The criteria are both written in an intrinsic analytical form and are applicable to linear and non-linear given strain paths as it occurs in any FEM codes for sheet-metal forming simulation. Finally, the two methods are complementary depending on the nature of failure and the predicted curves are in reasonable agreement with the trend of experimental results for a wide range of materials. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]