Page 413 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 16.3 Sheet-metal Charactenstrcs and Formablllty

Elongation. Sheet-metal-forming processes rarely involve simple uniaxial stretch-
ing like that in a tension test. However, observations from tensile testing are useful
and necessary for understanding the behavior of metals in these operations. Recall
from Section 2.2 that a specimen subjected to tension first undergoes uniform elon-
gation and that when the load exceeds the ultimate tensile strength of the material,
the specimen begins to neck and thus elongation is no longer uniform.
Because the material usually is being stretched in sheet forming, high uniform
elongation is desirable for good formability. The true strain at which necking begins
is numerically equal to the strain-hardening exponent (n) shown in Eq. (2.8). Thus,
a high n value indicates large uniform elongation (see also Table 2.3). Necking may
be localized or it may be diffuse, depending on the strain-rate sensitivity (ni) of the
material; this relationship is given in Eq. (2.9). The higher the value of rn, the more
diffuse the neck becomes. A diffuse neck is desirable in sheet-forming operations. In
addition to uniform elongation and necking, the total elongation of the specimen (in
terms of that for a 50-mm gage length) is also a significant factor in the formability
of sheet metals.

Yield-point Elongation. Low-carbon steels and some aluminum-magnesium alloys
exhibit a behavior called yield-point elongation: having both upper and lower yield
points (Fig. 16.12a). This behavior results in Liider’s bands (also called stretcher-
strain mar/as or worms) on the sheet (Fig. 16.12b)-elongated depressions on the sur-
face of the sheet, such as can be found on the bottom of cans containing common
household products (Fig. 16.12c). These marks may be objectionable in the final
product, because coarseness on the surface degrades appearance and may cause diffi-
culties in subsequent coating and painting operations.
The usual method of avoiding Luder’s bands is to eliminate or reduce yield-
point elongation by reducing the thickness of the sheet 0.5 to 1.5% by cold rolling
(temper or skin rolling). Because of strain aging, however, the yield-point elongation
reappears after a few days at room temperature or after a few hours at higher tem-
peratures. To prevent this undesirable occurrence, the material should be formed
within a certain time limit (which depends on the type of the steel).
Anisotropy. An important factor that influences sheet-metal forming is anisotropy
(directionality) of the sheet. Recall that anisotropy is acquired during the thermome-
chanical processing of the sheet and that there are two types of anisotropy: crystal-
lographic anisotropy (preferred orientation of the grains) and mechanical #hering

Yield-point
Yuppe, _ elongation

MOWGY _
3 Yielded metal
CD
LUder's band
275
Unyielded metal
0 Strain
(H) (D) (C)

FIGURE l6.l2 (a) Yield-point elongation in a sheet-metal specimen. (b) Luder’s bands in a
low-carbon steel sheet. (c) Stretcher strains at the bottom of a steel can for household
products. Source: (b) Courtesy of Caterpillar, Inc.