• Transactions of Materials Processing
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• v.23 no.4
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• pp.199-205
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• 2014

The current study examines the effect of the analytical parameter values on the theoretical forming limit diagram (FLD) based on the Marciniak-Kuczynski model (M-K model). Tensile tests were performed to obtain stress-strain curves and determine the anisotropic properties in the rolling, transverse and diagonal direction of SPCC sheet materials. The experimental forming limit curve for SPCC sheet material was obtained by limiting dome stretching tests. To predict the theoretical FLD based on the M-K model, the Hosford 79 yield function was employed. The effects of three analytical parameters - the exponent of the yield function, the initial imperfection parameter and the fracture criterion parameter - on the M-K model, were examined and the results of the theoretical FLD were compared to the experimentally measured FLD. It was found that the various analytical parameters should be carefully considered to reasonably predict the theoretical FLD. The comparison of the acceptable forming limit area between the theoretical and experimental FLD is used to compare the two diagrams.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.10a
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• pp.156-159
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• 2004

Most formulations for a forming limit diagram (FLD) have been based on yield stress potentials defined in the stress field. Nevertheless, there are formulations where potentials defined in the stain-rate field are especially convenient to formulate the rigid plastic material. Based on a strain-rate potential proposed for materials exhibiting planar anisotropic, the formulations for the forming limit diagram has been developed applying M-K theory. As verification example, the formulation is applied for anisotropic AA5182-O sheet. The good verification results show that the formulation for the forming limit diagram has been successfully developed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.556-559
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• 2008

In this study, the failure in circular cup deep drawing simulation at warm temperature is predicted using forming limit diagram (FLD). The FLD is used in sheet metal forming analysis to determine the criterion for fracture prediction. The simulation with heat transfer of circular cup deep drawing at warm temperature was conducted. To predict the failure, the simulation with heat transfer used FLD at temperature in the vicinity of maximum thinning. The result of the simulation with heat transfer shows that the drawn depth increases with increasing temperature and is in accord with the experimental results above $150^{\circ}C$. The FLD provides a good guide for the failure prediction of warm forming simulation with heat transfer.

• Transactions of Materials Processing
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• v.24 no.5
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• pp.361-367
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• 2015

The forming limit diagram (FLD) is the most commonly used tool for evaluating of sheet metal formability in the manufacturing field as well as the finite element analysis (FEA)-based design process. Determination of the forming limits is considerably influenced by testing/measuring machines, techniques and conditions. These influences may cause a large scatter in FLD from laboratory to laboratory. Scatter is especially true when the ‘position-dependent method’, as is specified in most national and international standards, is used. In the current study a new ‘time-dependent method’ is proposed, which is to determine the forming limit strains more accurately and reasonably when producing a FLD from experimental data. This method is based on continual strain measurement during the test. The results are compared to those from the existing standardized methods.

• Transactions of Materials Processing
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• v.18 no.8
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• pp.596-600
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• 2009

Finite element simulation is an alternative method to practically find the forming limit diagram(FLD). In this paper, the novel fracture criterion is utilized to predict the FLD in conjunction with finite element analysis for sheet forming. The principal scheme of the fracture criterion in this paper is that growth of the micro voids leads up to fracture in the viewpoint of micro-mechanics. The numerical FLD is verified by results of the out-of plane stretching test using hemispherical punch. The verification is also conducted about two types of material. These results are in good accord with the experimental results. Especially, the proposed scheme is appropriate to predict FLDs for a restricted material with low ductility after the instability point or ultimate tensile strength.

• Transactions of Materials Processing
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• v.14 no.6 s.78
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• pp.527-532
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• 2005

Among the failure modes which can occur in tube hydroforming such as wrinkling, bursting or buckling, the bursting by local instability under excessive tensile stresses is irrecoverable phenomenon. Thus, the accurate prediction of bursting condition plays an important role in producing the successfully hydroformed part without any defects. As the classical forming limit criteria, strain-based forming limit diagram (FLD) has widely used to predict the failure in sheet metal forming. However, it is known that the FLD is extremely dependant on strain path throughout the forming process. Furthermore, The application of FLD to hydroforming process, where strain path is no longer linear throughout forming process, may lead to misunderstanding for fracture initiation. In this work, stress-based forming limit diagram (FLSD), which is strain path-independent and more general, was applied to prediction of forming limit in tube hydroforming. Combined with the analytical FLSD determined from plastic instability theory, finite element analyses were carried out to find out the state of stresses during hydroforming operation, and then FLSD is utilized as forming limit criterion. In addition, the approach is verified by a series of bulge tests in view of bursting pressure and shows a good agreement. Consequently, it is shown that the approach proposed in this paper will provide a feasible method to satisfy the increasing practical demands for judging the forming severity in hydroforming processes.

• Journal of Industrial Technology
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• v.30 no.A
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• pp.49-57
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• 2010

A forming limit diagram (FLD) defines the extent to which specific sheet material can be deformed by drawing, stretching or any combination of those two. To determine the forming limit curve (FLC) accurately, it is necessary to perform the tests under well-organized conditions. In this study, the influence of several geometric or process parameters such as the blank shape and dimensions, strain measuring equipments, test termination time, forming speed and lubricants on the FLC is investigated.

• Transactions of Materials Processing
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• v.9 no.1
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• pp.3-9
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• 2000

The new FLD of the laser welded blank, which includes FLCs of welded zone and base metals, is introduced. For the forming limits of welded zone, the hemispherical dome punch tests were performed with various widths of asymmetric specimen. The FLC0 as well as the dome height at fracture associated with various specimen widths in the same and different thickness combinations were found to see the formability depending on thickness combinations. In order to show the application of the new FLD, the measured strains of squared cup drawing and simulated strains of door inner panel stamping were compared with those of FLCs. The successful prediction of fracture in the applications reveals that the forming limits of welded zone and base metals should be separately found for more accurate evaluation of the formability and workability of the laser welded blank.

• Transactions of Materials Processing
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• v.17 no.7
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• pp.501-506
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• 2008

In this study, a forming magnesium alloy circular cup was simulated accounting for heat transfer at elevated temperatures. In order to predict the failure of magnesium alloy sheet during simulation, the forming limit diagram, which is used in sheet metal forming analysis to determine the criterion for failure, was investigated. For the failure prediction in the simulation accounting for heat transfer, the forming limit diagram for a temperature the same as the temperature of the blank element was used. The result of the simulation showed that the drawn depth increases with the increase of the die-holder temperature, and is in accord with the experimental results above the die-holder temperature of $150^{\circ}C$. The forming limit diagram provided a good guide for the failure prediction of warm forming simulation accounting for heat transfer. In addition, the effect of the tool shoulder radius on the drawn depth at various tool temperatures is verified using the simulation conditions which agreed with the experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.8
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• pp.833-838
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• 2007

In tube hydroforming process, several defective products could be obtained such as bursting, wrinkling, folding, buckling. Because, especially, bursting is most frequently occurred failure among the well known failures, it is mostly important to predict the onset of bursting failure on tube hydroforming process. For most sheet metal forming processes, strain based forming limit diagram(FLD) is used often as a criteria to estimate the possibility of onset of the failures proposed above. However, FLD has a shortcoming that it is dependent on strain path while stress based diagram is independent on strain history. Generally, tube hydroforming consists of three main processes such as pre-bending, pre-forming, and hydroforming and it means that the strain histories of final products are nonlinear. Therefore, forming limit stress diagram(FLSD) is more suitable to predict forming limit for hydroforming parts. In this study, FLSD is applied to estimate bursting failure for an engine cradle of an automobile part. Consequently, it is proved that application of FLSD to predict forming limit is available for tube hydroforming parts.