Research Of Numerical Simulation On Flexible Bending Technology For 3-D Surface

Flexible bending forming is a new technology to achieve the sheet flexible bending for 3-D surface, It's the organically combine with multi-point forming and the traditional bending forming technology. Its basic principle: Firstly, disperseing the forming parts of the bending machine into a number of small discrete cell rollers. Secondly, adjusting the relative position of the cell rollers by regulating device, then the small rollers composed of different curves with various curvature, so continuous partial forming for different 3-D surfaces of sheet metal can be realized. Products exploitation cycle can be shorter greatly by this surface forming means, and also reduce the production cost, improve the flexility of product renewal. As compared with traditional bending machine, flexible bending still have other excellences, such as: simpler equipment, higher efficiency, be prone to actualize flexibility, series and automatization of product lines. As a novel and applied forming artifice of sheet metal, flexible bending technology will be fulfil the requirements of aviation, spaceflight and shipbuilding industry more effective. Shaped quality of the sheet metal is easily affected by certain facters during the forming process, for instance: plate thickness, metal stuff, fprming mode and desired 3-D surface's curvature etc. Its machining orderliness can be predominate better by doing research on the equations metioned hereinbefore, and these tasks are important guidance for 3-D surface forming of sheet metal.To study the influence on the part owing to the above parameters, dynamic explicit finite element software is introduced to simulate the flexible bending forming process of twist surface, saddle surface, spherical surface. The main contents and results of the research are as follows:1,The mechanical analysis for sheet metal forming in 3-D surfaceIn this paper, the bending course is divided into two phases: elastic transmutation and elastic-plastic transmutation. Distribution of elastic and plastic anamorphic area, change of equivalent stress and strain are analysed in this part. Bending stress and angle of broad-plate are ulteriorly discussed; In this paper, 3-D surface forming process is analysed, using mechanics of materials which is based the basic bending theory and characteristic, and it is devide into transverse and longitudinal deformation, as for the transverse deformation which has been generated equivalent transform into non-deformation plate with equal bending section coefficient, then analysis on the disposed sheet in longitudinal forming force, based on its forming features ,two stages were devided for next dicussing: The first stage--continuous loading bending, elastic bending of sheet metal is beginning, later with the sheet closeing to the center of upper roller, bending moment becoming bigger and bigger, sheet metal entering into plastic bending gradually; The second stage—continuous unloading springback, the elastic deformation of sheet metal forming occurred rebound. Analysis of distribution of the biggest stress during the forming process, relationship of the center distance of under rollers and non-round segment, linear segment that were easily to appeared after the forming, the analytical formulate are developed in detail.2,Establishment of FEA model of 3-D surface flexible bending processThe Explicit Dynamics (LS-DYNA) has been applied for numerical simulation of sheet metal 3-D surface formation. Selecting different simulation units for the course that according to the courved surface features. Cell SHELL163 with warping rigidity BWC arithmetic is utilized to twisting formation, cell SHELL163 with BT arithmetic is applied for saddle and spherical surface forming; Flexible rollers and sheet metal are respectively defined as rigid materials model and nonlinear elastic materials model on the basis of their different conditions in the course of flexible bending; Constraint conditions, contact status as well as friction problems are determined separately which based on flexible bending principles of 3-D surface in view of various curved surfaces; Make certain the virtual rotating speed and local coordinate system for the flexible rollers in the simulation, establish finite element models respectively for distortion,saddle surface and spherical surface.3,Numerical simulation on bending forming process of twisted surfaceThe deformation mechanism and characteristics that belong to the twisting forming process are studied, in addition, set up the finite element model of twisted numerical simulation. The influence which may caused by the distribution of stress and strain in the shaping course, thickness of sheet metal, tilt angle of the working rollers and it's lower displacement etc, is studied. Through the above calculation and analysis, the following results can be concluded: Equivalent stress and plastic strain of the twist workpiece that had zonal features in region distribution and symmetric with the geometry central line of sheet metal; With the increasing of thickness of the thin plate, forming force of work rolls increases, the extent and the frequency of the force fluctuation augment, edge corrugation of the shaping part decrease, the forming effect is better and better; Twisted angle and degree of deformation of the distortion forming process that are sensitive to the inclined angle which working rolls were regulated, the small change of inclined angle may have great effect on forming results, so ,the inclined angle of the working roll is one of the key parameters during twist formation; Twisted entend and distortion angle were correspondingly increase with the increase of lower displacement given by upper rolls, but meanwhile, bending deformation are easier to take place along the direction of the length and the width.4,Numerical simulation on bending forming of saddle and spherical surface Nuemerical simulation on flexible bending forming process of saddle and spherical surface, the main research contents are as follows: Forming traits and stress , strain in distribution of saddle and spherical surface;The effect of sheet metal's thickness on the shaping process; The influence of rolling times on the prosess; Effect of material on the forming quality ; Analysis of effect to sheet metal forming process considering discretization degree of flexible rolls etc. Finally, the following conclusions are summarized: Under the same condition, shaping effect of the saddle workpiece is better than the spherical workpiece, folds are easily come out on the edge location of spherical piece; With the increasing of thickness of the plate, wrinking that emerging in the process are decreaseing, quality of sheet forming becoming better and better; In the identical condition, corrugation phenomenon and non-uniform distribution of stress and strain were gradually decreased, even disappeared with the increasing of rolling times, shaping results are improveing obviously; Forming effect of steel plate is better than aluminum plate's, its froming force of the unit rolls as well as greater than aluminum when using the sheet metal with different material quality; The higher the discrete degree of flexible roll is,the better the surface quality of the sheet metal, and the force of the units roll during the froming course is more stable, when sheet processing at the same condition, as the discrete extent of the roll increasing, the force that equipment has to imposed for sheet metal is decreasing.