| Multi-Point Stretch-Forming (MPSF) is a great technical break though based on Multi-point forming(MPF)method.Its main idea is dividing a curved surface of the traditional stamping dies into a matrices of many elements.And then the three-dimensional shape of sheet metal panel is formed by the curved surface of elements (called element group of forming surface).During the forming process, the sheet metal is also held by the gripping jaw for stretch forming .The sheet metal is formed by loading in dual-dirctions.And the main characteristic of MPSF is to eliminate disadwantages of difficulty no design the whole streching die much time on maching and test,and cost on maintenance dies. So far, MPSF is being applied widely,especially on the sheet panels with large surface, small batch and high processing precision.MPSF is often used on producting aircraft skin because flexible MPF processing method which can reduce the springback and suppress defect effectively.However, MPSF is a complicated plastic forming process. Many technical parameter and effecting complications must be considered before stretch forming. moreover, S-shape skin is a complex shape skin. It is important to study on the MPSF for S-shape skin. The research context contains study on resilience cushion thickness, pre-stretch ratio, skin curvature and blank thickness. The author in thesis made numerical study using dynamic explicit finite element software.The paper concludes the rule that is influenced by every technical parameter, and the main study contents and results are as follows:1. To calculate the thickness of resilience cushion, which is set between of the sheet and matrices of punch elements to suppress the dimple defect and enhance forming precision.The paper built the mesh models of the sheet blank, cushion and punch elements and analysis the numerical simulation results: The conclusions are while the thickness of cushion is 6mm, there is obviously impress dimple defect, on the sheet where is gripped by jaws there is wrikle defect; and while the thickness of cushion is 8mm, dimples can not be suppressed obviously, however, there is some stress concentration on the right side of the blank; while the thickness of cushion is 10mm, the dimples disappeared,but the stress concentration exists also. The results also show that the resilience cushion can suppress dimple defect and improve the accuracy of shape.; but the accuracy of the shape becoming lower with the increasing thickness of resilience cushion. Therefore, it is necessary to adjust the punch elements and consider the deformation of the resilience cushion.2. To attain suitable pre-stretch ratio, which is a key parameter in MPSF and can control springback thoroughly.The paper made numerical simulation analysis when the pre-stretch ratio is 1%,2% and 3%.The conclutions are that the more pre-stretch ratio is, the smaller springback of blank is, the better blank stretching deformation is. When pre-stretch ratio is 1%, dimples is a bit, springback is a lot. With pre-stretch ratio gets bigger, dimples get more, springback gets little. When pre-stretch ratio is 3%, springback is the least, the stress on jawing position is the biggest. So in the stretch-forming process, the stress concentration must be controlled to improve the precision of sheets. It is the important and difficult problem in numerical simulation process.3. To study the effect of panel curvarure on the stress distribution and accuracy of the part.During MPSF process, the radius of panels must be considered .If the radius is much smaller and the curvature of the blank is much bigger, the contact points are not contained in the desire of design and it is easier to occur stress concentration. this effects the accuracy of the part. In this paper, when radius is 300mm, there is stress concentration obviously.When radius is 400mm or 500mm, the stress concentration on the blank is much more uniformity and shape of the workpiece meets the qulity reqirment.4. To study the effect of thickness of the blank on the stress distribution and accuracy of the part.The characteristics of material of the blank ,2024 aluminium is easy to plastic deformation, but is belong to duralumin . Ii is significant to study MPSF with the engineering material. The paper conludes that the more the thickness of blank is, the more symmetrical the stress of stretch forming blank is. In other words, if the forming region of blank is slick, the accuracy is good.5. To carry out numerical simulation research on multi-point stretch-forming for actual S-shape panel partThe size of real panel part is much big, stretch-forming is hard. Moreover, contrasting to the above analysis, the paper shows S-shape panel part is more hard to be formed. The simulation results can guide to the practical product of aircraft panel, reduce many labour, much cost to manufacture and improve the working efficiency. |
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