• 한국정밀공학회지
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• 제14권10호
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• pp.127-134
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• 1997

열가소성 복합재료는 고상 성형법에 의해 저렴한 가격으로 부피가 튼 제품의 제조에 널리 사용될 수 있어 아주 좋은 전망을 가지고 있다. 그러나, 이러한 재료의 성형성에 대해선 아직 잘 알려지지 않았다. 본 연구의 첫번때 주안점은 2축 인장성형시 성형성에 대한 연구에 두었다. 실험에 사용된 재료는 임의의 방향으로 위치한 유리 섬유를 중량비로 20, 35, 40% 함유한 폴리프로필렌이다. 성형시험은 75 .deg. C 에서 150 .deg. C 사이의 온도에서 행했으며, 펀치 속도는 0.01cm/sec 와 1cm/sec 에서 행했다. 2축 인장성형에서 측정된 한계 변형률(Limiting Strain)은 Marciniak 불완전성 (Imperfection) 이론에 근거한 예견치외 비교되었다. 이론치와 실험치가 잘 일치함을 보였으며, 성형한계선도(Forming Limit Diagram) 로써 결과들을 요약하였다. 성형한계 변형률은 성형온도와 성형속도에 의해 크게 영향을 받는다는 것을 보인다. 이러한 결과들은 적절한 성형조건이 선택된다면 열가소성 복합재료의 인장성형은 실제 상업적으로 이용하기에 충분한 성형성을 갖는다는 것을 보인다.

• 대한기계학회논문집
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• 제12권5호
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• pp.935-943
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• 1988

본 연구에서는 유압벌지시험기를 이용한 저속 벌지시험과 낙하해머(drop hammer)를 응용한 충격벌지시험기를 제작, 사용하여 충격벌지시험을 하여 저속변형과 충격변형에서 판재금속의 성형한계선도를 구하고 변형율속도가 판재의 불안정 및 성형 성에 미치는 영향을 조사하고 다이의 형상비(단축/장축)를 0.5, 0.75, 1.0 등으로 달 리하여 실험하므로써 변형경로에 따른 한계변형율의 변화를 관찰하고 아울러 기존의 성형한계이론과 비교, 검토하였다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 춘계학술대회 논문집
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• pp.387-390
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• 2008

Electromagnetic forming (EMF) technology, which is one of the high speed forming methods, has been used for the forming process in various industry fields. Numerical approach by finite element simulation of the EMF process is presented in this study. The implicit code is used to obtain the numerical model of the time-varying currents that are discharged through the coil in order to obtain the transient magnetic forces. In addition, the body forces generated in the workpiece are used as the loading condition to analyze deformation of thin sheet metal workpiece using explicit code. Numerical approach for a dimpled shape by EMF process is carried out and the simulated results of the dimpled shape by EMF are reviewed in view of the deformed shape and formability evaluation.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.642-647
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• 2007

Determination of loading conditions for tube hydroforming(THF) process that implies an amount of the increment in axial feeding and internal pressure for each step is one of the most important constituents at the process design level. On account of the fact that those design factors mentioned above are imposed simultaneously during the process, suitable loading conditions are required to obtain robust products without any failure such as buckling, necking, bursting and so on. In which, especially, bursting is well known as the most frequently occurred failure in general THF process. In this study, therefore, determination of the loading condition based on the adaptive method was carried out to obtain safe loading paths. In addition, forming limit curves are applied to evaluate the derived loading conditions by using the simulation results. Consequently, it is found that described method in this study for THF process design is useful and has a feasibility.

• 소성∙가공
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• 제33권2호
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• pp.87-95
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• 2024

The current study aims to prove that high-speed forming has better formability than conventional low-speed forming. Experimentally, the quasi-static forming limit diagram was obtained by Nakajima test, and the dynamic forming limit diagram was measured by electrohydraulic forming. For the experiments, the LS-DYNA was used to create the optimal specimen for electrohydraulic forming. The strain measurement was performed using the ARGUS, and comparison of the forming limit diagrams confirmed that EHF showed better formability than quasi-static forming. Theoretically, the Marciniak-Kuczynski model was used to calculate the theoretical forming limit. Swift hardening function and Cowper Symonds model were applied to predict the forming limits in quasi-static and dynamic status numerically.

• 소성∙가공
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• 제23권4호
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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.

• 소성∙가공
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• 제26권2호
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• pp.121-126
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• 2017

The hot stamping process is an innovative forming method that could prevent the cracking of high strength steel sheets. The formability test of boron steel sheet using forming limit diagrams at elevated temperature is very complicated and time consuming job. In this paper, an alternative test method to evaluate the formability of boron steel in hot stamping has proposed. It measured the FLD0 instead of whole strain combinations of FLD with the tensile test machine and specially designed test rig. Test results shows that the proposed test method can simulate the plain strain condition fracture and can make the FLD of boron steel sheet at elevated temperature with less effort.

• 한국안전학회지
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• 제5권2호
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• pp.58-65
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• 1990

The purpose of this study is to consider the effects of strain rate on the stress-strain behavior of sheet metal at instability. The results and conclusions obtained as follows ： 1. As the strain rate increases, the fracture pressure increases and the polar height at fracture decreases. 2. The effect of strain rate on forming limit diagram produces a general lowering of the diagram with increasing strain rate but changes according to materials and strain paths. 3. The forming limit diagram predicted by swift instability theory is comparatively inconsistent with the experimental result at high strain rates, because there is inevitable gap between them.

• 소성∙가공
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• 제26권3호
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• pp.144-149
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• 2017

In order to characterize the macroscopic mechanical response of ultra-thin (0.1 mm thick) ferritic stainless steel sheet at various loading paths, a crystal plasticity finite element method (CP-FEM) was introduced. The accuracy of the prediction results was validated by comparing with the experimental data. Based on the results, the forming limit diagram (FLD) was predicted using a modified Marchinicak-Kuczinski model coupled to a non-quadratic anisotropic yield function, namely, Yld2000-2d. The predicted FLD was found to be in good agreement with the experimental data.

• 대한기계학회논문집A
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• 제31권8호
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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.