• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1998년도 춘계학술대회논문집
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• pp.21-27
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• 1998

An analysis for the prediction of wrinkling formation in curved sheets during metal froming is presented. We construct "Wrinkling Limit diagram"(WLD) which represent the combinations of the critical principal stresses for wrinkling formation in curved sheet elements subjected to biaxial plane stress. Here the scheme of plastic bifurcation theory for thin shells based on the Donnell-Mushtari-Vlasov shell theory is used. In this study, the effects of the material variables (yield stress, plastic hardening coefficient, plastic anisotropic parameter, and so on) and sheet geometry on the critical conditions for wrinkling is carried out numerically.merically.

• 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.

• 소성∙가공
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• 제14권6호
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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.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 춘계학술대회 논문집
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• pp.92-96
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• 2005

Among the failure modes which can be occurred 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 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 path-dependent limitation of FLD makes the application to hydroforming process, where strain path is no longer linear throughout forming process, more careful. 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 Ihe state of stresses during hydroforming operation, and then FLSD is utilized as forming limit criterion. In addition, the approach is verified with 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 farming severity in hydroforming processes.

• 대한기계학회논문집A
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• 제36권2호
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• pp.179-186
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• 2012

자동차 산업에서 대기오염을 줄이고 연비를 향상시키기 위해 경량화가 중요한 과제로 여겨지고 있다. 이를 위해 알루미늄 소재의 적용이 증가하고 있다. 판재를 차체에 적용하기 위해서는 주로 프레스 가공 공정을 거치게 된다. 이때, 재료, 제품설계 및 프레스 공정의 부적절한 가공 변수의 사용으로 인하여 파단, 주름, 및 스프링 백 등에 의한 다양한 형태의 가공 불량이 발생한다. 따라서 이들 변수들의 적절한 조화 뿐 만 아니라 엄격한 공정 관리가 요구된다. 이에 본 연구에서는 자동차 판재에 주로 사용되는 Al5454 재료에 대한 이론적으로 유도한 소성 불안정 조건을 구하고, MATLAB을 이용하여 성형 한계도를 도출하였다. 또한, 장출 인장 실험을 통해 얻어진 실험값과 이론적으로 도출한 성형 한계도와의 비교를 수행하였다.