Steels_ Metallurgy and Applications, Third Edition

24 Steels: Metallurgy and Applications

40

E

30

-1-

20

.... | , I |

0.2 0.4 0.6

Polar thickness strain

Figure 1.21 Relationship between bulge height and polar thickness strain in sheets with
different ~ values. One 1.35 ~ material had a lower uniform elongation than the remaining
materials (After Horta et al. 42)

e-

"ii3 == .3

==

r

i~.2 1.95

I/55

0.9t3

1.35

* 1.20 and 1.35

25 I , 50 I

Distance from pole (mm)

Figure 1.22 Radial distribution of thickness strain, measured directly, in sheets with
different "i values bulged to give a polar thickness strain of 0.4. One 1.35 -~ material had
a lower uniform elongation than the remaining materials (After Horta et al. 42)

which gives a plot of the polar thickness strain developed in circular bulge tests
of progressively increasing height using steels with different rm value. It is seen
that for a given bulge height, the polar thickness strain is lower for a high rm
value steel than for a low rm value steel. The reason is, as shown in Figure 1.22,
that the strain is more uniformly distributed across the surface of the bulge for
the high rm value steel than for the low rm value steel. This type of influence of
rm on strain distribution is common to all pressings.
The Ar value is important because its ratio with the rm value determines the
height of ears that would be obtained in cups prepared using a cylindrical punch
and a circular blank. The ear height is defined as the mean distance between the