1 INTRODUCTIONThe sheet metal forming process has beenused in various industrial areas because of its highefficiency in mass production and its stability inproduction quality. The spring back is one of thekey factors influencing the quality of stamped sheetmetal parts. Many researchers made efforts toevaluate and/or decrease the spring back<1 5>.There are numerous factors that influence theamount of spring back of the metal sheet, such asthe shape of the die, the properties of the material,the boundary conditions and the interfacial loads.All of these factors should be taken into account ina spring back simulation. In order to obtain thefinal desired shape it is necessary to accuratelyassess the amount of spring back during the form ing process.Early in 1950s, a generalized mathematicalanalysis for spring back corrections in the purebending of metals, such as aluminum, nickel, tita nium and ferrous alloys was carried out. In laterwork, researches were done in order to give a theo retical analysis of the influence of the Bauschingereffect on spring back in sheet metal forming andspring back angles and curvatures for bending andbending unbending areas of the test specimen<6,7>.With the development of the computer technology,computer numerical calculation was adopted tosimulate the forming process of the sheet<8 13>. Alot of researchers investigated the numerical calcu lating method of the sheet metal forming process.Lee et al<14> evaluated the numerical factors influen cing spring back quantitatively by using the Tagu chi method. To clarify the effect of each factor,the U draw bending process was chosen as an eval uation problem because of its large spring back. Intheir analysis, the explicit time integration methodwas used in the simulation of the forming stage,and the implicit time integration method was ap plied to the spring back stage. Papeleux et al<15>used a benchmark method, ie U draw bending tosolve the spring back problem. Narasimhan etal<16> also predicted spring back in sheet metalforming by using an explicit to implicit sequentialsolution procedure. SONG<17> utilized the softwarepackage ANSYS/LS DYNA and DYNAFORM tosimulate the spring back of sheet metal forming forthe HT 7U vacuum. Samuel et al<18> made an as sessment of numerical parameters influencingspring back in explicit finite element analysis ofsheet metal forming process.In construction industry, the large concrete mixer truck is one of the most important machines,and its blades are the key components. The entireblade inside the drum consists of 13 sectors. Afterthe sectors are stamped separately, they are allfield welded together to form a helix during the fi nal assembly. The drum of the truck, in which theblades are installed, has an outermost radius of2.1 m and a height of 3.8 m. The thickness of eachblade is only 4 mm. Compared with its shell di mension, the thickness is much smaller.During the blade stamping process, high dis tortion and warping are generated. In order to ac quire the best mixing effects by controlling theshape of the blade, predicting the spring back isvery important in the blade processing.In this paper, by using the finite elementmethod the stamping process of a NURBS surface3 dimensional CAD model was simulated to predictthe spring back effect and the distribution of thick ness, stress and strain in the blade. Most impor tantly, numerical factors influencing spring backwere selected and their effects on spring back werecomprehensively and quantitatively assessed inorder to find the optimal shapes for the technicalsurface of the die and punch by applying finiteelement software MARC.2 ESTABLISHMENT OF MODELThe schematic diagram of the drum and theblade is shown in Fig.1. In this paper, one of the13 blades from
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