Fig. 3. The stress–strain relationship for the sheet metal.
for the simulations, the punch speed is set to 10 m s—1 and a coefficient of Coulomb friction equal to 0.1 is assumed.
3. Wrinkling in a Tapered Square Cup
A sketch indicating some relevant dimensions of the tapered square cup is shown in Fig. 1(a). As seen in Fig. 1(a), the length of each side of the square punch head (2Wp), the die cavity opening (2Wd), and the drawing height (H) are con- sidered as the crucial dimensions that affect the wrinkling. Half of the difference between the dimensions of the die cavity opening and the punch head is termed the die gap (G) in the
present study, i.e. G = Wd — Wp. The extent of the relatively unsupported sheet metal at the draw wall is presumably due
to the die gap, and the wrinkles are supposed to be suppressed by increasing the blank-holder force. The effects of both the die gap and the blank-holder force in relation to the occurrence of wrinkling in the stamping of a tapered square cup are investigated in the following sections.
3.1 Effect of Die Gap
In order to examine the effect of die gap on the wrinkling, the stamping of a tapered square cup with three different die gaps of 20 mm, 30 mm, and 50 mm was simulated. In each simulation, the die cavity opening is fixed at 200 mm, and the cup is drawn to the same height of 100 mm. The sheet metal
used in all three simulations is a 380 mm × 380 mm square sheet with thickness of 0.7 mm, the stress–strain curve for the
material is shown in Fig. 3.
The simulation results show that wrinkling occurred in all three tapered square cups, and the simulated shape of the drawn cup for a die gap of 50 mm is shown in Fig. 4. It is seen in Fig. 4 that the wrinkling is distributed on the draw wall and is particularly obvious at the corner between adjacent walls. It is suggested that the wrinkling is due to the large unsupported area at the draw wall during the stamping process, also, the side length of the punch head and the die cavity
Fig. 4. Wrinkling in a tapered square cup (G = 50 mm).
opening are different owing to the die gap. The sheet metal stretched between the punch head and the die cavity shoulder becomes unstable owing to the presence of compressive trans- verse stresses. The unconstrained stretching of the sheet metal under compression seems to be the main cause for the wrink- ling at the draw wall. In order to compare the results for the
three different die gaps, the ratio þ of the two principal strains is introduced, þ being cmin/cmax, where cmax and cmin are the major and the minor principal strains, respectively. Hosford and Caddell [5] have shown that if the absolute value of þ is greater than a critical value, wrinkling is supposed to occur, and the larger the absolute value of þ, the greater is the possibility of wrinkling.
The þ values along the cross-section 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 þ. Consequently, increasing the die gap will increase the possibility of wrinkling 拉中拉深壁起皱的分深模设计析英文文献和中文翻译(3):http://www.youerw.com/fanyi/lunwen_80828.html