Page 414 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 16 Sheet-Metal Forming Processes and Equipment

(alignment of impurities, inclusions, and voids throughout the thickness of the sheet).
The relevance of this subject is discussed further in Section 16.4.

Grain Size. As described in Section 1.4, grain size affects mechanical properties
and influences the surface appearance of the formed part (orange peel). The smaller
the grain size, the stronger is the metal; the coarser the grain, the rougher is the sur-
face appearance. An ASTM grain size of 7 or finer (Table 1.1) is preferred for gener-
al sheet-forming operations.

Dent Resistance of Sheet Metals. Dents commonly are found on cars, appliances,
and office furniture. Dents usually are caused by dynamic forces from moving ob-
jects that hit the sheet metal. In typical automotive panels, for example, velocities at
impact range up to 45 mfs. Thus, it is the dynamic yield stress (yield stress under
high rates of deformation), rather than the static yield stress, that is the significant
strength parameter.
Dynamic forces tend to cause localized dents, whereas static forces tend to
diffuse the dented area. This phenomenon may be demonstrated by trying to dent a
piece of flat sheet metal, first by pushing a ball-peen hammer against it and then by
striking it with the hammer. Note how localized the dent will be in the latter case.
Dent resistance of sheet-metal parts has been found to (a) increase as the sheet thick-
ness and its yield stress increase and (b) decrease as its elastic modulus and its over-
all panel stiffness increase. Consequently, panels rigidly held at their edges have
lower dent resistance because of their higher stiffness.

I6.4 Formability Tests for Sheet Metals

Sheet-metal formability is of great technological and economic interest, and it gener-
ally is defined as the ability of the sheet metal to undergo the desired shape change
without failure, such as by necking, cracking, or tearing. As we shall see throughout
the rest of this chapter, sheet metals (depending in part on geometry) may undergo
two basic modes of deformation: ( 1) stretching and (2) drawing. There are impor-
tant distinctions between these two modes, and different parameters are involved in
determining formability under these different conditions. This section describes the
methods that generally are used to predict formability.

Cupping Tests. The earliest tests developed to predict sheet-metal formability were
cupping tests (Fig. 16.13a). In the Erichsen test, the sheet specimen is clamped be-
tween two circular, flat dies and a steel ball or round punch is forced into the sheet
until a crack begins to appear on the stretched specimen. The punch depth, d, at which
failure occurs is a measure of the formability of the sheet. Although this and similar
tests are easy to perform, they do not simulate the exact conditions of actual forming
operations and hence are not particularly reliable, especially for complex parts.

Forming-limit Diagrams. An important advance in testing the formability of sheet
metals is the development of forming-limit diagrams, as shown in Fig. 16.14. A
forming-limit diagram (FLD) for a particular metal is constructed by first marking
the flat sheet with a grid pattern of circles (see Fig. 16.15), using chemical or photo-
printing techniques. The blank then is stretched over a punch (Fig. 16.13a), and the
deformation of the circles is observed and measured in regions where failure
(necleing and tearing) has occurred. Although the circles typically are 2.5 to 5 mm
in diameter, for improved accuracy of measurement, they should be made as small
as is practical.