Research On Fast Sheet Metal Forming Simulation Approach

In the process of automobile development, the cycle of development of autobodydies is important in the total cycle of development. The simulation software for metalsheet forming is universally applied in the process of the development of dies, inorder to shorten the cycle of development of dies. The simulation method for metalsheet forming can be divided into two kinds according to theoretical basis: One kindis based on incremental theory; another is based on deformation theory. Thecalculation accuracy of the former is higher, but the computation time is too long；Thelatter is represented by one step forming inverse algorithm, computing speed quickly,but poor precision. Therefore, the improvement research is done on the solutionprocesses of one step forming inverse algorithm using the modified gradient methodbased on large deformation elastoplastic theory; to deepen the study, the idea of onestep forming positive algorithm based on deformation theory is developed to be asimulation algorithm which has certain engineering practicability, and finallymulti-step forming positive algorithm which gives considerations to both calculationaccuracy and computational efficiency is proposed. The research content is briefly asfollows:1、In the frame of one step forming inverse algorithm, the improved gradient methodis proposed for iterative computations, and replacing the previous iteration algorithmbased on solving the equations. This method commences from the elementunbalanced force which generated from variations of element shape in the finiteelement analysis. The norm of the unbalanced force is defined to measure theunbalanced force situation in the blank. On each iteration step, firstly the unbalancedforce gradients generated from small disturbances can be calculated. Secondly theintelligence modifier optimization method is used to calculate the node coordinatesmodifier. Finally along the unbalanced force gradient descent direction, thecoordinates of the node on the initial blank is modified. To avoid the repeated computation of the unbalanced force vector on irrelevant nodes, the idea of localmodification from perturbation theory is introduced into the iteration process.2、On the basis of one step forming inverse algorithm, one step forming positivealgorithm is accomplished to simulate the sheet metal forming process. The spaceinitial solution of one step forming positive algorithm is obtained preliminary usingmapping relationships and area coordinates. Then, based on deformation theory theiterative solutions are carried out in three-dimensional coordinate system by using theimproved gradient method. During iterative process the contact process is introducedto ensure that the nodes on the spatial initial solution are not separated from diesurface. Compared to the inverse approach, the simulation process of the positiveapproach is more coincident with the forming process. The thickness of the initialblank is known, and the thickness of the final shape can be derived. The stampedshape of regular sheet can be calculated effectively. As one step forming positivealgorithm can solve some fundamental flaws of one step forming inverse algorithm,the precision is improved.3、On the basis of one step forming positive algorithm which is based on deformationtheory, introducing idea of incremental theory, the multi step forming positivealgorithm is proposed based on quasi-deformation theory. The influence of theloading history is considered by adding several intermediate shapes between theinitial blank and the final shape, and the simulation accuracy is further improved. Thebenchmark surfaces of the intermediate shapes can be built, in the basis of which theproject for the simplification of modified calculation is proposed, and the computationtime is reduced.4、The multi-step experimental scheme of square box is designed. The circularelements are marked on the flat blank using electric corrosion method, Five squarebox experiment samples of different steps including10mm,20mm,30mm,40mm,50mm areachieved. The deformation of element is measured by the method of opticalmeasurement, and then the strain of element can be calculated. The thickness of blankin different steps is measured using ultrasonic thickness gauge. Through thecomparison among several different algorithms on calculation precision and calculation efficiency, the multi-step forming positive algorithm is proved valid.Finally, the multi-step forming positive algorithm is applied to simulate the actualautomobile stamping parts. From the comparison among the simulation results ofmulti-step forming positive algorithm, the Dynaform incremental method and theresults of experimental measurement, it is observed that the multi-step formingpositive algorithm can also predict the forming results. Meanwhile, it is proved themulti-step forming positive algorithm can be commonly used.