Sheet Metal Forming Simulation in Industry

JournalofELSEVIER Journal of MaterialsProcessingTechnology60 (1996) 19-26 Materials Processing Technology Sheet metal forming simulation in industry A. Makinouchi The Institute o f Physical and Chemical Research - RIKEN, 2-1 Hirosawa, Wako, 351-01 Japan Abstract In order to see actual present situation of finite element simulation system introduced to industry to be utilized at the stamping tool design section, four different examples are introduced. It is clearly shown from the example that each industry has its own purpose to the simulation; prediction of wrinkle, prediction of surface deflection, study of tearing limit condition, determination of blank geometry, prediction of springback, evaluation of sheet thickness and residual stress, and so on. Present state of simulation systems integrated into CAD is also investigated. Keywords: Finite Element Simulation System, Sheet Metal Forming, Die Face Design L Introduction World wide efforts have been made to develop finite element codes to simulate sheet metal forming processes. These codes are expected to be a powerful tools to industrial users who are working in the tooling design section. Recent advances of this technological field can be seen ,for example, in the proceedings papers of international conferences specified to the sheet forming simulation, such as Symposium on Computer Modeling of Sheet Metal Forming Process (1985) [1], FE Simulation of 3-D Sheet Metal Forming Processes in Automotive Industry (1991) [2] and N U M I S H E E ~ 9 3 (1993) [3]. The progress is also clearly seen in the benchmark tests organized under the specific purpose to clarify the prediction capability of sheet forming defects; OSU test [4], VDI test [2] and NUMISHEET'93 test [3]. However, in m a n y cases the performance and capability of individual code demonstrated in a paper and/or a benchmark test are the so called ~champion data" and thus they are not easily achieved in the industrial production design phase. There always exists a rather large gap between research phase and industrial phase. The purpose of the p a p e r is to look at present situation of finite element simulation system introduced to industry and to reconsider real requirements from industrial engineers, which m a y give an important information to a researcher for further development of a finite element system. 9.. Expected role of finite element simulation In order to illustrate the expected role of simulation, let us consider a case of automobile industry. The design and manufacturing procedure of stamping tools used for 0924-0136/96/$15.00 © 1996 ElsevierScienceS.A. All rightsreserved Pl10924-0 136 (96) 02303-5 production of car panels is figured out in Fig. 1, starting from the concept and style design of a new car, and ending with the commencement ofpreduction. Absolute time measure is not indicated in the figure, since it depends very much on the policy and technological attitude of individual automobile industry. However, most of industries m a k e very keen efforts to reduce time for manufacturing new stamping tool sets and consequently save cost and resources. Expected role of the finite element simulation is to meet this requirement. Simulation may be effectively performed at five different stages in Fig. 1 for the purpose of helping decision making in design and modification of parts and tools. The first simulation is at the production process design stage <1>. Purpose of the simulation at the stage is to m a k e a rough estimation whether panels of new designed car can be formed or not. I f answer is "no', the ear design m u s t be modified. However, at this stage geometry of body parts are not fully described in CAD system and no tool data exists, thus it is not possible to make full simulation. The precise simulation is achieved at stage <3> to determine number of stamping steps, such as first drawing, second drawing, trimming, edge bending, and to design die face geometry used at each stamping step. At this stage die face geometry is modeled by CAD surface description and thus modification of die face data is made rather easily according to the simulation results on same CAD system. The performance of integrated CAD and simulation system is crucial for obtaining well optimized stamping steps and die face shape in very limited time allocated to the production tool design. The simulation is also required at the tryout stage <4>, to find a solution to avoid the forming defects appeared during tryout. In order to study the mechanism of origination and 20 Starting of design A. Makinouchi /Journal of Materials Processing Technology 60 (1996) 19-26 Starting of production Starting of TIME design Starting of TIME production Production process design Car [Production I design[process design[ A Prot tool design X ~ol desp b Protot~rpe~or\~ t y W ¥ A oductiootoo, , ~a ~ ~ ~ W manufacturing m, modification I t~asti~[Machining [ l Trialin r ~]& assembl[y [production ~ ] Trisl Simulation <1> <2> <3> <4> <5> Simulation<i> <2> Fig. 1 Design and manufacturing process of stamping tools in automobile industry. - PRESENT propagation of defects, the systematic series of simulation can be t a k e n place after production stated <5 >, and obtained information is efficiently made use of in the next new model. Assume that the finite element simulation is powerful enough to predict all the forming defects and provide optim u m stamping tools and conditions, we may completely eliminate the prototype tools from the design and manufacturing procedure, and we also reduce number of trial and modification operations. Thus the process might be shorten dramatically as is illustrated in Fig. 2. This is the most ideal attitude of the finite element simulation system in sheet metal forming industry. Fig. 2 Design and manufacturing process of stamping tools. - Future tial sheet deformation on the die face due to weight and the spring back afterforming. Elasto-plastic finite element method - static implicit approach Since sheet stamping process is not really impact process, it may be appropriate to assume the quasistatic equi. librium through the process. The static implicit time integration scheme satisfy this requirement, since the equilibrium condition is hold at each time integration step. However, difficulty exists to overcome convergence problem, which is mainly due to continuous change of contact and friction state between tool and sheet during iteration. 3. Existing simulation code A powerful finite element code is primary important to construct a simulation system. Finite element codes participated to the benchmark test of NUMISHEET°93 [3] are listed in Table 1. Three different benchmark problems are proposed by the organizer; square cup deep drawing (intending to prediction of wrinkling and splitting for rather simple geometry), front fender s t a m p i n g (prediction of wrivkllr~g and splitting for very complex tool geometry), and 2-D draw bending (prediction of springback). The problems participated by each code are marked in the table. Finite element codes developed specifically for sheet metal forming simulation may be classified by its formulation and time integration algorithm as classified in Table 1. There are m a n y investigations for comparison between classified methods, and advantage and drawback of each method are pointed out [5-7]. I f we m a k e a very rough conceptual evaluation of each method, following remarks may be stated. Static explicit approach In order to avoid the convergence problem, tangent stiffness matrix equation is solved without iteration at each time integration step with limiting the step size to very small value by the r-minimum procedure. Large number of incremental steps are necessary to complete the entire forming process without having accumulation of error due to neglecting higher order terms in time integration. Dynamic explicit approach Dynamic equilibrium equation is the bases of this formulation. It is big advantage that the stiffness matrix is not necessary to be constructed and solved, so that the solution of one time step can be obtained much faster t h a n the static approach. Most important point is the dynamic nature of this approach. In order to obtain stable solution in this time integration scheme, incremental time must be limited to such value that a dilatation wave does not cross through any element, and thus 10 .6 secouds is the most standard time step size. In order to reduce calculation time, simulation are often performed under accelerated tools peed (in m a n y cases 100 times faster punch speed compared to actual speed is applied), sometimes leading to unrealistic results. Rigid-plastic or rigid-viscoplastic finite element method Formulation and implementation is rather simple, and fast calculation is achieved. Due to lack of the elastic region in material model, it is not possible to calculate ini- .4. Makinouchi /Journal of Materials Processing Technology 60 (1996) 19-26 21 RJ ; i d - ( v i a c o ) - p l a s t i c Benchmark Problems Program name SHEET-3 MFP2D MFP3D CASHE Organization Ohio State Univ. Univ. of Catalunya Univ. of Catalunya KAIST Country 'U.S.A. Spain !Spain Korea Korea O O O O Square Cup O Front Fender O O 2D Draw FORMSYS-SHEET KAIST Elasto-plastic Static-implicit Benchmark Problems Program name MTLFRM Dieka LAGAMINE CALEMBOUR ABAQUS FLECHE NIKE3D AUTOFORM INDEED PROFIL MARC Static-explicit Benchmark Problems Program name R B S O UT Organization Ford Motor Univ. of Twente Univ. of Liege Ecole Central Paris H. K. S. Inc. Univ. Tech. Compiegne Livermore Software ETH Zwrich Ford Motor INPRO Inst. National Sci. Marc Analysis Country U.S.A. The Netherlands Belgium France U.S.A. France U.S.A. Switzerland U.S.A. Germany France U.S.A. Square Cup O O O O O O O Front Fender O 2D Draw 0 O 0 0 0 0 Organization Osaka Univ. RIKEN Inst. RIKEN Inst. Country Japan Japan JaPan Square Cup O O Front Fender O O 2D Draw O O ITAS-3D ITAS-2D D y n a m i c explicit Benchmark Problems Program name LS-DYNA3D PAM-Stamp RADIOSS ABAQUS/Explicit CES-3D Organization Livermore Software E. S. I. MECALOG H. K. S. Inc. Univ. Tech. Compiegne Univ. of Catalunya Country U.S.A. France France U.S.A. France Spain Square Cup O O O O O O Front Fender O 2D Draw O O O O O Table. 1 List of finite element codes participated to NUMISHEET'93 benchmark test. 22 .4. Makmouchi / Journal of Materials ProcessingTechnology60 (1996) 19-26 Front fender 4. Industrial example To see the present situation of finite element simulation system in industries four different examples are introduced. All the information of following examples are provided from industrial engineers who h a v e been in change of constructing the system in their section and are leading their simulation group. 4.1 Automobile panel forming in N I S S A N Motor System structure The die design section of NISSAN Motor Co. has an inhouse CAD system, which is intensively used to design die face geometry based on the geometrical data of parts described by curved surfaces. ITAS-3D finite element code is integrated to this CAD system. The generation of tool meshes, is in general, one of key issue which exerts large influence upon the quality of simulation and it takes very long time to accomplish a sufficiently good mesh for die, punch and blankholder. One advantage of ITAS-3D is the tool data description. ITAS-3D uses "point data" approach, in which three dimensional tool surface is defined by a collection of points distributed regularly in the x-y coordinate plane and is generated easily without help of operator. An in-house m e s h generator integrated into this CAD system can efficiently produce a set of tool data within one day. By using input p a r a m e t e r s t a k e n from the material data base and the drawbead force data base, modeling process is rather automated. The first example of application of this system is optimization of die face geometry for the blank-holding operation of the front fender panel at the production tool design stage <3> in Fig. 1. Main aim is to find optimum die face geometry which avoid the wrinkle formation while keeping size of the blank sheet as small as possible. Fig. 3 shows shape of simulated front fender, in which the effect of height H at A-A section is examined. The increase of H leads to increase of the blank size, on the contrary the decrease of H gives an uneven height of the die face leading to the wrinkle formation. A result obtained using opthnized die face is illustrated in Fig. 4a. The wrinkle formed at the actual prototype die tryout (Fig. 4b) is diminished in the simulated model. J f Fig. 4 Sheet geometry after blank holding operation. (a) Actual tryout result by prototype tool. (b) Simulation result by optimized tool. Trunk lid S E C A-A Fig. 3 Die face shape of front fender. The trunk lid outer panel is often subjected to the surface deflection which spoils appearance of a car. The surface defection of this panel is mainly formed near the corner as is illustrated in Fig. 5. This defect is supposed to be originated from the non uniform contact of the sheet to the A. Makinouchi /Journal of Materials ProcessingTechnology60 (1996) 19-26 23 Surface deflection Fig. 5 Trunk lid outer panel. Fig. 6 Distance between punch and sheet. Model 1 Model 2 Model 3 Fig. 7 Distribution of distance between punch and sheet, obtained by simulation under different tool geometry. have different combination of height B and C in Fig. 5. It is clearly shown that the even distribution of distance at the corner is achieved in the model 3. This e~Amlnation is performed at tool design stage <3> and the production tool employing model 3 geometry is produced. Satisfactory panel quality is obtained at the tryout stage as is shown in Fig. 8. 4.2 Automobile panel forming in MAZDA Motor Simulation system MAZDA Motor Co. uses an in-house CAD system, and PAM-STAMP finite element software is integrated into the system. They made large efforts to develop an efficient automatic mesh generator for creating tool mesh directly on CAD surface. The new mesh generator is powerful enough to generate large tool mesh data such as the front fender within two days. This system is used in design and manufacturing stages <3> and <5> of Fig. 1. Among plenty of simulation cases two examples are shown in the following section which is performed in stage <5> intending to obtain information utilized in the next model design. Fig. 8 Produced panel and simulated panel using optimized tool. punch face in the drawing process. The distance between punch surface and sheet, as is illustrated in Fig. 6, is exAmined at several foaming steps. Fig. 7 shows comparison of three simulation results obtained using three tools which Side frame outer panel The side frame outer panel illustrated in Fig. 9 is very large parts which is subjected to many different forming defects. MAZDA has used the simulation system to find proper solution for minimizing wrinkling and avoiding tear- 24 .4. Makinouchl / Journal o f Materials Processing Technology 60 (1996) 19-26 Fig. 9 Side frame outer panel. \ Fig. I i Simulation model for prediction of tearing. High " a LOW , , , , ! High Small* • Large b Fig. 10 Simulation result of side frame outer panel. (a) Result used actual production tool. (b) Result used optimize tool. Lug. Dynamic-explicit code PAM-STAMP has advantage that large number of element can be utilized in modeling a sheet blank. But also such disadvantage exists that appropriate punch speed and modeling conditions have to be found to obtain reasonable results within reasonable calculation time. The first example is wrinkle simulation. The wrinkle was formed rather early stage of stamping as is shown in the simulated panel in Fig. 10a. Several modified tool was tested by simulation and an optimized tool shape was found (Fig. 10b), which was utilized in the next new car model. The second example is tearing problem. The tearing at the bottom of center pillar in Fig. 9 is always annoying problem which occurs in most of new models and have to be removed during the tryout with large effort. In order to find a general scheme to avoid this defect, rather simple stretch flanging simulation model, Fig. 11, was introduced, In the simulation effect of the edge radius R of sheet blank was exAmlned with using a set of tools. Simulation results are plotted in Fig. 12a whre the limited height is determined by the forming limit diagram of the sheet material. Blank edge radius R Formability Severe b /Formabili~ Low °rEma~1 tY [ • Teared laanel at tryout I : ~ a a ~ n ~ a ~ l ~ ~ ou t i , , 1 Blank edge radius R Fig. 12 Limit line evaluating tearing. (a) Tearing limit determined by simulation. (b) Tearing limit of actual panel. The result says that the limited height is linearly increases with the blank edge radius R. Based on this information actual tryout data of several different old panel was plotted on the same graph, and a clear relationship was obtained as shown in Fig. 12b. This limit line is used as the design standard for a new car model. Small, , Large A. Makinouchi /Journal of Materials Processing Technology 60 (1996) 19-26 25 Realproduct(axleease) i ~ F E M simulation result Fig. 13 Modeling of axle case forming by two dimensional code AXLE2D. Simulation Real axlecase Fig. 16 Simulated geometry of frame side member in forming process. 4.3 Forming of frame and member in Press Kogyo 1.58ram ~ 1 . ,o System structure Comparisonon mostdecreasethickness section Fig. 14 Comparison of cress sectional geometry of formed axle case between simulation and experiment. 45o -L74- Press Kogyo Co. utilizes three finite element codes, AXLE2D, ITAS-2D and ITAS-3D, which are integrated into pre and post processor IDEAS. Each code is used at different design stage. AXLE2D is special purpose program developed by the company Airnlrlgto be used only for design of axle case. Axle case -1 Re~ \ \ ,I Three dimensional forming process of axle case is modeled by a very simplified manner, that is, the plane stress 2D finite element method, as is shown in Fig. 13. By using this code, AXLE2D, simulation model is generated quickly, and using this model the mA~tiraum strain at stretch flanged part is roughly evaluated and the sheet blank geometry is determined automatically before forming tools are designed. This is a big advantage to have a special purpose code, and simulation results are feedbacked to the axle case design. After tools are designed, more precise evaluation of quality of formed axle case, such as thickness distribution of the wall, amount of the side wall curl and springback, Fig. 14, are performed by ITAS-3D. Frame side member of tracks 0 1000 20O0 Distance from front [ram] Fig. 15 Distribution of twisting angle along longitudinal direction of frame side member. Since the frame side member is not symmetric along the longitudinal direction, entire structure is often twisted due to the springback after forming. So that evaluation of amount of twisting is one of most important issue to de- 26 .4. Makinouchi / Journal of Materials Processing Technology 60 (1996) 19-26 sign tools. ITAS-3D is used at stage <3> for this purpose, providing sufficiently accurate twisted shape as shown in Fig. 15. Other defects such as wrinkle and springback of side wall are also predicted by this simulation. Fig. 16 demonstrates process of wrinkle formation due to shrink flanging deformation, which gives significant effect to side wall shape of formed frame. 4.4 Forming of tire disk wheel in Central Motor Wheel System structure CADCEUS is used to design tire disk wheel and the geometrical data is transmit to pre and post processor KSWAD. ITAS-2D finite element is incorporated to KSWAD for the purpose of two dimensional axisymmetric section analysis of the tire disk wheel forming process. Disk wheel This special purpose system is used for design of the multiple forming stages and the tool shapes for automobile disk wheels. The main ahn of the simulation is to predict thickness changes of the sheet at each stage, and evaluate the springback geometry and the residual stress distribution at the finial forming stage. The cross-sectional shape of the sheet at three successive forming stages obtained by simulation is shownin Fig. 17. The state of stress and strain in the sheet is transformed from previous forming stage to the next stage, and thus entire forming history is completely simulated to evaluate final formed state very precisely Fig. 18. This system is intensively used at the design section <3> of the company. First stage Second stage Third stage Fig.18 S h e e t t h i c k n e s s a n d r e s i d u a l stress a t final forming stage asked from industrial engineers. However, it is also true that ability of finite element codes is still very limited, so that if a user wants to obtain really useful information within reasonable time and cost, he/she has to limit the purpose of simulation to one or two specified items and optimize the code and model for this purpose. This situation means that a code must be customized under guidance of industrial engineers to achieve real efficiency. Acknowledgment The author would like to thank to Mr. H.Sunaga of Nissan Motor Co., Mr. T.Ogawa of Mazda Motor Co., Mr. I~Kazama of Press Kogyo Co. and Mr. T.Kakita of Central Motor Wheel Co. for their kind cooperation for preparing this paper. Most of the information about the shnulation system are kindly provided from them. References [1] Computer modehng of sheet metal forming process; edited by N.-M.Wang and S.C.Tang, The Metallurgical Society inc., 1985. [2] FE-simulation of 3-D sheet metal forming process in automotive industry, VDI Verlag, 1991. [3] NUMISHEET93 - 2nd international conference on numerical simulation of 3-D sheet metal forming process, edited by &Makinouchi, E.Nakamachi, E.Onate, R.H.Wagoner, 1993. [4] J.I~Lee, R.H.Wagoner and E.Nakamachi; "A benchmark test for sheet forming analysis", Report ERC/ NSM-S-90-22 Ohio State University, 1990. [5] E.Onate et al, NUMISTAMP: A research project for assessment of finite element models for stamping processes, NUMISHEET'93, p19, 1993. [6] D.Y.Yang et al; Comparative investigation into imphcit, explicit, and iterative/exphcit schemes for simulation of sheet metal forming process, NUMISHEET'93, p35, 1993. [7] J.C.Gelin et al; Quasi-static implicit and transient explicit analyses of sheet metal forming using a CO three node shell element, NUMISHEET'93, p53, 1993. After springback After springback After springback i Fig. 17 S i m u l a t e d cross-sectional s h a p e of t i r e disk w h e e l in successive f o r m i n g stages 5. D i s c u s s i o n The examples shown here tell us that each industry has its own purpose to use simulation system; such as prediction of wrinkle, prediction of surface deflection, study of tearing limit condition, determination of blank geometry, prediction of springback, evaluation of sheet thickness and residual stress, and so on. It is rather amazing that simulation can meet such variety of requirements