Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

Leotoing, Lionel; Guines, Dominique

doi:10.1016/j.ijmecsci.2015.05.007

Cited by 61 publications

(32 citation statements)

References 25 publications

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“…For the Type 2‐A of nonlinear strain path, the FLCN shifts up with respect to the original FLCN under linear strain paths, and a small increase of formability at necking is observed in Figure a. This phenomenon is also reported by Leotoing et al . for an aluminium alloy sheet and Kuroda et al .…”

Section: Experimental Procedures and Resultssupporting

confidence: 79%

“…Using the in‐plane biaxial tensile test with the cruciform specimen to control the strain path can be an interesting alternative to overcome the drawbacks of conventional methods. The potentiality of the in‐plane biaxial tensile test with a cruciform specimen to study the effect of strain path change on the FLCN and to investigate the FLCF under linear strain paths was validated by the presenting authors . Firstly, the strain path during the test can be directly controlled by the motion of actuators along the two axes, which is sufficient to cover the whole forming limit diagram under linear and nonlinear strain paths.…”

Section: Introductionmentioning

confidence: 99%

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Effect of continuous strain path changes on forming limit strains of DP600

Song

Leotoing

Guines

et al. 2019

Strain

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The strain path may change in actual sheet metal-forming processes, so the determination of formability of sheet metal should consider the nonlinear strain path. For identifying the forming limit (FL) strains under nonlinear strain path, a conventional two-step procedure with unloading is classically used to produce the strain path change, which results in no continuous measure of strain. The in-plane biaxial tensile test with a cruciform specimen is an interesting alternative to overcome the drawbacks of conventional method.The strain path change can be made without unloading during a single test. In this work, the experimental FL strains of DP600 sheets under two types of nonlinear strain path are investigated and then compared with those under linear strain paths. The Oyane ductile fracture criterion is used in the finite element simulation to predict the experimental results. KEYWORDS FLCF, FLCN, in-plane biaxial tensile test, nonlinear strain path | INTRODUCTIONMost of the forming limit curves at necking (FLCNs) and the forming limit curves at fracture (FLCFs) have been experimentally determined using methods that produce proportional loading with insignificant changes in strain paths. However, in some innovative forming processes, such as single point incremental forming, complex strain paths with several changes have been identified by different authors. As an example, Figure 1 shows the evolution of the strain path during an incremental forming operation of a truncated aluminium cone with a wall angle of 60°. [1] As can be seen, drastic changes in the strain path are observed during the forming operation, without unloading, and a very high level of deformation can be achieved, well beyond the conventional forming limit strains at necking. In addition, it has been demonstrated that strain path change has an influence on the FLCN and more specifically on the high level of forming limits (FL). [2] Because the strain path change obviously affects the FL strains at necking, no unified FLCN exists in the strain space to represent the formability of sheet metal. [3] The strain paths in sheet metal forming are classically divided into two types: Linear strain path and nonlinear strain path. Traditionally, the identification of strain-based FLCN or FLCF is limited to the sheet metal undergoing linear strain paths. Three types of experimental FLCN (Figure 2) are concluded by Barata da Rocha et al. [2] to investigate the effect of strain path change on the FL strains at necking. For Type 1, the FLCN is determined under proportional loading. Every point of the FLCN is defined by major and minor strains at necking. As shown in Figure 2a, the whole FLCN (ABCD) is produced by different strain paths from biaxial stretching (OA) to uniaxial tension (OD) through plane-strain tension (OC). The red dash line shows the FLCF, which is higher than the FLCN. [4] For Type 2, the FLCN is identified under two steps of proportional loading. Different prestrain levels

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Section: Experimental Procedures and Resultssupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Effect of continuous strain path changes on forming limit strains of DP600

Song

Leotoing

Guines

et al. 2019

Strain

Self Cite

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“…A carrier blank with a central hole is usually used to avoid frictional contact between the sheet metal specimen and the punch, but optimising the dimension and geometries of carrier blank and punch is required in order to induce strain localisation and cracking in the unsupported region of the specimen, which complicates the test procedure and increases the cost of testing. Li and Ghosh [25] carried out a formability test of aluminium alloys 5754, 5182 and 6111 by using the Marciniak approach at a rapid [28] improved a cruciform specimen for the use of formability tests and determined the FLD of AA5086 at room temperature [29]. A servo-hydraulic biaxial testing machine was used to control loading paths in two vertical directions [30].…”

Section: Introductionmentioning

confidence: 99%

Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model

Shao

Lin

et al. 2017

International Journal of Mechanical Sciences

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Published: Z. Shao, N. Li, J. Lin, T. Dean, 2017, Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model, Int. J. Mech. Sci., 120, pp149-158. AbstractHot stamping and cold die quenching has been developed in forming complex shaped structural components of metals. The aim of this study is the first attempt to develop unified viscoplastic damage constitutive equations to describe the thermo-mechanical response of the metal and to predict the formability of the metal for hot stamping applications. Effects of parameters in the damage evolution equation on the predicted forming limit curves were investigated. Test facilities and methods need to be established to obtain experimental formability data of metals in order to determine and verify constitutive equations. However, conventional experimental approaches used to determine forming limit diagrams (FLDs) of sheet metals under different linear strain paths are not applicable to hot stamping conditions due to the requirements of rapid heating and cooling processes prior to forming. A novel planar biaxial testing system was proposed before and was improved and used in this work for formability tests of aluminium alloy 6082 at various temperatures, strain rates and strain paths after heating, soaking and rapid cooling processes. The key dimensions and features of cruciform specimens adopted for the determination of forming limit under various strain paths were developed, optimised and verified based on the previous designs and the determined heating and cooling method [1]. The digital image correlation (DIC) system was adopted to record strain fields of a specimen throughout the deformation history. Material constants in constitutive equations were determined from the formability test results of AA6082 for the prediction of forming limit of alloys under hot stamping conditions. This research, for the first time, enabled forming limit data of an alloy to be generated at various temperatures, strain rates and strain paths and forming limits to be predicted under hot stamping conditions.

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“…Depending on the test, results can be affected by friction between tools and sample, and by sample bending. In the last few years, the in-plane biaxial tensile test with a dedicated cruciform specimen has been proposed to identify forming limits at necking under linear paths [10] and non-linear paths [11]. Recently, with the same specimen shape, this test has also been used to determine the forming limits at fracture [7].…”

Section: Introductionmentioning

confidence: 99%

Characterization of forming limits at fracture with an optimized cruciform specimen: Application to DP600 steel sheets

Song

Leotoing

Guines

et al. 2017

International Journal of Mechanical Sciences

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International audienceIn order to identify experimental limit strains at fracture for metallic sheets under in-plane biaxial loadings, a specific specimen shape has to be defined. Ideally, the proposed geometry must lead to fracture at the specimen center whatever the strain path imposed by the experimental device. If such condition is ensured, the whole strain paths ranging from equi-biaxial to uniaxial could experimentally investigated. Based on literature review, four in-plane cross specimen shapes have been selected owing to strong potential to reach large strain and fracture in their central point. From finite element simulations, the behavior of these different shapes is evaluated. Based on these numerical results, a new shape is proposed and optimized for determining the forming limits at fracture of DP600 sheet metal with a thickness of 2mm. The forming limit strains at fracture are identified by a time-dependent method and the forming limit strains at necking are determined by a position-dependent method. The experimental forming limit strains at fracture of DP600 sheet metal are fitted by a forming line. Three ductile fracture criteria are then calibrated with experimental points. The Oyane criterion with two parameters gives the best prediction

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Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

Cited by 61 publications

References 25 publications

Effect of continuous strain path changes on forming limit strains of DP600

Effect of continuous strain path changes on forming limit strains of DP600

Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model

Characterization of forming limits at fracture with an optimized cruciform specimen: Application to DP600 steel sheets

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