Numerical Analysis On The Local Stress And Local Deformation In Multi-point Stretch Forming

With the rapid development of industry, as well as the birth of the new type constructions, the demand for large-scale and three-dimensional surface parts of different shape and size is also increasing. The traditional method has been unable to meet the requirements of rapid development. when using traditional methods the different shapes parts need to make different dies, and the design, manufacture and commissioning of the dies is a very high cost, especially for the areas of updating products quickly and small batch processing, the development of new technologies are severely constrained by traditional processing methods. Therefore, the research and development of the new forming technology and equipment has become the problems calling for urgent solutions. The new technology and equipment with a high degree of automation and wide adaptability can adapt the requirements of parts'upgrade quickly.Multi-point stretch forming is a new sheet metal forming technology. Its main idea is dividing the curved surface of traditional dies into discrete elements, so it can meet the requirement of a rapid, low-cost, digital forming for sheet metal. According to the traditional die, there are also forming defects such as split and springback. However, due to the discontinuity of multi-point die there are stress concentration and local strain on the blank,which will lead to the defect of dimpling. In order to establish reasonable measures to suppress dimpling, we should clarify the influence of different processes on the stress and strain concentration and then select a reasonable process to make the distribution of stress and strain on the sheet metal uniform, also to improve the quality of multi-point stretch forming.In this paper, the finite element analysis software ANSYS/LS-DYNA was adopted to simulate multi-point stretch forming of workpieces of different shapes to analyze the distribution of stress and strain on the workpieces of different process measures. By comparing select a reasonable process measures which made the distribution of stress and strain on the sheet metal most uniform. The main contents and results are as follows:1) Establishing a reasonable finite element modelFinite element analysis is the research for discrete model, the accuracy of the results is closely related with the accuracy of the discrete model, so a reasonable finite element model is a prerequisite for accurate simulation results. By comparing the simulation results which the blank is dispersed by different element types and sizes it showed that the contours of stress with the application of solid164 can reflect the stress concentration of the areas which contact with the punch elements more accurately. When using solid164, the thickness direction of the blank is divided into four layers and the size of element on the plane is 3mm, the simulation results meet the requirement of accuracy and the computation time is reasonable. The influence of load mode and virtual speed on the simulation results is analyzed, and concluded that: in a virtual speed of less than 1m/s, the simulation results are almost identical with different load mode. When the virtual speed boost up to 5m/s, the distribution of stress and strain with the first load mode don't produce deviation and the stress concentration on the blank became more serious with the second load mode. When the virtual speed boost up to 10m/s, the distribution of stress produce large deviation with the two load modes. So, under the premise of ensuring the accuracy of simulation results,in order to save time and improve simulation efficiency, the simulations all use the first load mode and use the virtual speed of 5m/s.2) The influence on the distribution of stress and strain by the number of punch elementsIn the stretch forming process, the discretization of multi-point die will cause stress concentration and local deformation. Through the numerical simulation of multi-point stretch forming, the influence of the number of punch elements on equivalent stress, thickness stress, plastic strain, as well as thickness strain is analyzed. Through the numerical simulation of multi-point stretch forming, the law of influence on the simulation results by the number of punch elements is summed up. The law is that the more punch elements are used, the results of multi-point stretch forming will be more closed to the overall die's. Through the numerical simulation of multi-point stretch forming on spherical parts, the law is verified. So the laws of the influence on one-way and two-way curvature parts by the number of punch elements are identical. At the same time, with regard to a certain size part, the number of punch elements must meet certain limits. If the number of punch elements is small, in order to ensure effective forming area the larger size punch elements must be used. When the blank is thin and the size of punch elements is large, the stress will be complex, which results in unpredictable deformation and the quality and accuracy of multi-point stretch forming can not be assured.3) The restraining function of cushion for stress concentration and local deformation in multi-point stretch formingIn the process of multi-point stretch forming, increasing the number of punch elements can improve the situation of stress and strain, but the increase of elements' number will increase the cost of equipment. The use of elastic cushion can improve the forming quality without increasing the cost too much. The distribution of stress and strain is analyzed with different thickness of elastic cushion. The compressive stress of the stress concentration region can be reduced effectively with the use of elastic cushion. With the increase in the thickness of elastic cushion, the stress concentration and local deformation on the blank can be alleviated effectively, so the quality of multi-point stretch forming can be greatly improved. When the thickness of elastic cushion increase to a certain extent, the forming quality can't be improved obviously with the increase of cushion thickness, therefore, selecting the reasonable thickness of elastic cushion is a important issue. Through the comparative analysis of multi-point stretch forming, the appropriate thickness of elastic cushion is about 8mm.