Metal forming problems

Artificially increasing the speed of forming events is necessary to obtain an economical solution, but how large a speedup can we impose and still obtain an acceptable static solution? If the deformation of the sheet metal blank corresponds to the deformed shape of the lowest mode, the time period of the lowest structural mode can be used as a guideline for forming speed. However, in forming processes the rigid dies and punches can constrain the blank in such a way that its deformation may not relate to the structural modes. In such cases a general recommendation is to limit punch speeds to less than 1% of the sheet metal wave speed. For typical processes the punch speed is on the order of 1 m/s, while the wave speed of steel is approximately 5000 m/s. This recommendation, therefore, suggests a factor of 50 as an upper bound on the speedup of the punch velocity.

The suggested approach to determining an acceptable punch velocity involves running a series of analyses at various punch speeds in the range of 3 to 50 m/s. Perform the analyses in the order of fastest to slowest since the solution time is inversely proportional to the punch velocity. Examine the results of the analyses, and get a feel for how the deformed shapes, stresses, and strains vary with punch speed. Some indications of excessive punch speeds are unrealistic, localized stretching and thinning as well as the suppression of wrinkling. If you begin with a punch speed of, for example, 50 m/s, and decrease it from there, at some point the solutions will become similar from one punch speed to the next—an indication that the solutions are converging on a quasi-static solution. As inertial effects become less significant, differences in simulation results also become less significant.

As the loading rate is increased artificially, it becomes more and more important to apply the loads in a gradual and smooth manner. For example, the simplest way to load the punch is to impose a constant velocity throughout the forming step. Such a loading causes a sudden impact load onto the sheet metal blank at the start of the analysis, which propagates stress waves through the blank and may produce undesired results. The effect of any impact load on the results becomes more pronounced as the loading rate is increased. Ramping up the punch velocity from zero using a smooth step amplitude curve minimizes these adverse effects.

Springback

Springback is often an important part of a forming analysis because the springback analysis determines the shape of the final, unloaded part. While Abaqus/Explicit is well-suited for forming simulations, springback poses some special difficulties. The main problem with performing springback simulations within Abaqus/Explicit is the amount of time required to obtain a steady-state solution. Typically, the loads must be removed very carefully, and damping must be introduced to make the solution time reasonable. Fortunately, the close relationship between Abaqus/Explicit and Abaqus/Standard allows a much more efficient approach.

Since springback involves no contact and usually includes only mild nonlinearities, Abaqus/Standard can solve springback problems much faster than Abaqus/Explicit can. Therefore, the preferred approach to springback analyses is to import the completed forming model from Abaqus/Explicit into Abaqus/Standard.