Page 98 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 2.10 Failure and Fracture of Materials in Manufacturing and in Service 77
In a tension-test specimen, fracture begins at the center of the
Typical examples are (a) food and beverage containers with tabs (or entire tops)
which are removed by tearing the sheet metal along a prescribed path; (b) shear pins
on shafts that prevent machinery damage in the case of overloads; (c) perforated
paper or metal, as in packaging; and (d) metal or plastic screw caps for bottles.

2. I 0.l Ductile Fracture
Ductile fracture is characterized by plastic deformation, which precedes failure (H) (D) (C) (fi)
(Fig. 2.20a). In a tension test, highly ductile materials such as gold and lead may
neck down to a point before failing (Fig. 2.21d); most metals and alloys, however, FIGURE 2.2l Schematic
neck down to a finite area and then fail. Ductile fracture generally takes place illustration of the types of
along planes on which the shear stress is a maximum. Thus in torsion, for exam- fracture in tension: (a) brittle
ple, a ductile metal fractures along a plane perpendicular to the axis of twist; that fracture in polycrystalline
is the plane on which the shear stress is a maximum. Fracture in simple shear, metals; (b) shear fracture in
ductile single crystals-see
by contrast, is a result of extensive slip along slip planes within the grains. (See
Pig. 1.6) also Fig. 1.5a; (c) ductile
cup-and-cone fracture in poly-
Close examination of the surface of ductile fracture (Fig. 2.22) shows a
crystalline metals; (d) com-
Hbrous pattern with dimples, as if a number of very small tension tests have been plete ductile fracture in
Because of its appearance, the fracture surface of a tension-test
.... Z* ~t~..
carried out over the fracture surface. Failure is initiated with the formation of tiny
polycrystalline metals, with
1/oids, usually around small inclusions or preexisting voids, which then grou/ and
100% reduction of area.
~
coalesce, developing into microcracks which grow in size and eventually lead to
fracture.
this crack then propagates to the periphery of the necked region.
f ‘ ”‘

necked region as a result of the growth and coalescence of cavities
H
_,
seen in the midsection of the tension-test specimen in Fig. 2.23d; t,
(Fig. 2.23). The central region becomes one large crack, ‘as can be .xg ,F

pi
~>._
‘p
__,
.p
'V
if;
it ‘ii
»g
specimen is called a cup-and-cone fracture. *Q 'Q " ~~ fy f"' l"
inclusions have an important influence on ductile fracture and, con- "ff‘*“#§
,

f
sequently, on the workability of materials. Inclusions may consist of
il
impurities of various kinds and of second-phase particles, such as
Effects of Inclusions.
Because they are nucleation sites for voids,

in

»
V"i`
oxides, carbides, and sulfides. The extent of their influence depends ‘fa ia ; “Y
on such factors as their shape, hardness, distribution, and fraction FIGURE 2.22 Surface of ductile fracture in
of total volume; the greater the volume fraction of inclusions, the low-carbon steel, showing dimples. Fracture
lower will be the ductility of the material. is usually initiated at impurities, inclusions, or
Voids and porosity can also develop during processing of preexisting voids (microporosity) in the metal.
metals, such as the voids resulting from casting (Section 10.6) and Source: Courtesy of K.-H. Habig and D.
Klaffke.

Shear Fibrous
(G) (D) (C) (d) (e)
FIGURE 2.23 Sequence of events in the necking and fracture of a tensile-test specimen:
(a) early stage of necking; (b) small voids begin to form within the necked region; (c) voids
coalesce, producing an internal crack; (d) the rest of the cross section begins to fail at the
periphery, by shearing; (e) the final fracture, known as a cup- (top fracture surface) and-cone-
(bottom surface) fracture, surfaces.