【文章內(nèi)容簡介】 t the draw wall during the stamping process,also,the side length of the punch head and the die cavity openingare different owing to the die gap. The sheet metal stretched between the punch head and the die cavity shoulder bees unstable owing to the presence of pressive transverse stresses. The unconstrained stretching of the sheet metal under pression seems to be the main cause for the wrinkling at the draw wall. In order to pare the results for the three different die gaps, the ratio 5β of the two principal strains is introduced, β being εmin/εmax, where εmax and εmin are the major and the minor principal strains, respectively. Hosford and Caddell [5] have shown that if the absolute value of β is greater than a critical value, wrinkling is supposed to occur, and the larger the absolute value of β, the greater is the possibility of wrinkling.The β values along the crosssection M–N at the same drawing height for the three simulated shapes with different die gaps, as marked in Fig. 4, are plotted in Fig. 5. It is noted from Fig. 5 that severe wrinkles are located close to the corner and fewer wrinkles occur in the middle of the draw wall for all three different die gaps. It is also noted that the bigger the die gap, the larger is the absolute value of β. Consequently,increasing the die gap will increase the possibility of wrinkling occurring at the draw wall of the tapered square cup. Effect of the BlankHolder ForceIt is well known that increasing the blankholder force can help to eliminate wrinkling in the stamping process. In order to study the effectiveness of increased blankholder force, the stamping of a tapered square cup with die gap of 50 mm,which is associated with severe wrinkling as stated above, was simulated with different values of blankholder force. The blankholder force was increased from 100 kN to 600 kN,which yielded a blankholder pressure of MPa and MPa, respectively. The remaining simulation conditions are maintained the same as those specified in the previous section.（An intermediate blankholder force of 300 kN was also used in the simulation.）The simulation results show that an increase in the blankholder force does not help to eliminate the wrinkling that occurs at the draw β values along the crosssection pared with one another for the stamping processes with blankholder force of 100 kN and 600 kN. The simulation results indicate that the _ values along the crosssection M–N are almost identical in both cases. In order to examine the difference of the wrinkle shape for the two different blankholder forces, five crosssections of thedraw wall at different heights from the bottom to the line M–N, as marked in Fig. 4, are plotted in Fig. 6 for both is noted from Fig. 6 that the waviness of the crosssections for both cases is similar. This indicates that the blankholder force does not affect the occurrence of wrinkling in the stamping of a tapered square cup, because the formation of wrinkles is mainly due to the large unsupported area at the draw wall where large pressive transverse stresses exist. The blankholder force has no influence on the instability mode of the material between the punch head and the die cavity shoulder.Distance(mm)Fig. 5. βvalue along the crosssection M–N for different die gaps.Fig. 6. Crosssection lines at different heights of the draw wall fordifferent blankholder forces. (a) 100 kN. (b) 600 kN.4. Stepped Rectangular CupIn the stamping of a stepped rectangular cup, wrinkling occurs at the draw wall even though the die gaps are not so 1(b) shows a sketch of a punch shape used for stamping a stepped rectangular cup in which the draw wall C is followed by a step D–E. An actual production part that has this type of geometry was examined in the present study. The material used for this production part was mm thick, and the stress–strain relation obtained from tensile tests is shown in Fig. 3.The procedure in the press shop for the production of this stamping part consists of deep drawing followed by the deep drawing process, no draw bead is employed on the die surface to facilitate the metal flow. However, owing to the small punch corner radius and plex geometry, a split occurred at the top edge of the punch and wrinkles were found to occur at the draw wall of the actual production part,as shown in Fig. 7. It is seen from Fig. 7 that wrinkles are distributed on the draw wall, but are more severe at the corner edges of the step, as marked by A–D and B–E in Fig. 1(b).The metal is torn apart along the whole top edge of