It is seen from fig. 10, the punch force is higher in first step and decreased by going on subsequent steps so that it was expected from generally accepted formula for predicting maximum punch load.
Punch force is in straight relation with punch diameter according to (1) [10]:
According to (1), the higher diameter of punch, then higher punch force.
Therefore, FEM results for punch load are in good agreement with (1) by about %25 differences between them.
Fig. 11 provides a comparison of the punch force variations versus punch stroke in various steps.
These results have extracted from numerical simulation. It is seen from fig. 11, the punch force for stage-2 has increased at end of punch stroke.
This demonstrates the deformed part in stage-2 is to be experiencing Ironing process at the end of punch stroke due to unsuitable clearance between punch and die.
Residual stresses at deformed parts in any steps have extracted from simulation analysis and are shown in Fig. 12.
These results were expected, because the central area of blank was not critical in deep drawing analysis, therefore residual stress in central section of blank was less than other sections.
On the other hand, the residual stresses are rising with proceeding in next stages. It is because of work hardening has been increased with next stages in deep drawing.
Comparison of experimental and FEM simulation results on the multi-stage drawing process were performed in this study.
It was found through comparison of thickness distribution in produced parts with simulated deep drawing parts, the finite element model have proven to be in qualitative agreement with those of experiment in primary steps, but because of the changes in plastic behavior of initial sheet, errors were increased in last steps.
Maximum errors in this simulation were up to 10% on the punch corners in fourth step.
Therefore, it is necessary to reinvestigation on material properties after any steps and to applying these changes in next steps. It was found through FE simulation, the punch force is higher in first stage and it is decreasing with proceeding in next stages.
Predicted punch forces were in good agreement with calculated punch force from formula in die design literatures.
Residual stresses are lesser in central area of blank and those are rising with proceeding in next stages.
Finite element modeling (FEM) can accurately portray forming from a particular die design without the need for costly trial and error. With simulation via FEM, designers can estimate field variables such as strain distribution, stress distribution, material flow and forming defects.
This information enhances the design capability and knowhow of an experienced process designer and leads to a reduced number of die-tryout tests.