Page 281 - Materials Science and Engineering An Introduction

8.3 Ductile Fracture • 253

Fracture

8.2 FUNDAMENTALS OF FRACTURE
Simple fracture is the separation of a body into two or more pieces in response to an im-
posed stress that is static (i.e., constant or slowly changing with time) and at temperatures
that are low relative to the melting temperature of the material. Fracture can also occur
from fatigue (when cyclic stresses are imposed) and creep (time-dependent deformation,
normally at elevated temperatures); the topics of fatigue and creep are covered later
in this chapter (Sections 8.7 through 8.15). Although applied stresses may be tensile,
compressive, shear, or torsional (or combinations of these), the present discussion will
be confined to fractures that result from uniaxial tensile loads. For metals, two fracture
ductile fracture, modes are possible: ductile and brittle. Classification is based on the ability of a material
brittle fracture to experience plastic deformation. Ductile metals typically exhibit substantial plastic de-
formation with high energy absorption before fracture. However, there is normally little
or no plastic deformation with low energy absorption accompanying a brittle fracture.
The tensile stress–strain behaviors of both fracture types may be reviewed in Figure 6.13.
Ductile and brittle are relative terms; whether a particular fracture is one mode or the
other depends on the situation. Ductility may be quantified in terms of percent elongation
(Equation 6.11) and percent reduction in area (Equation 6.12). Furthermore, ductility is a
function of temperature of the material, the strain rate, and the stress state. The disposi-
tion of normally ductile materials to fail in a brittle manner is discussed in Section 8.6.
Any fracture process involves two steps—crack formation and propagation—in
response to an imposed stress. The mode of fracture is highly dependent on the
mechanism of crack propagation. Ductile fracture is characterized by extensive plastic
deformation in the vicinity of an advancing crack. Furthermore, the process proceeds
relatively slowly as the crack length is extended. Such a crack is often said to be stable—
that is, it resists any further extension unless there is an increase in the applied stress.
In addition, there typically is evidence of appreciable gross deformation at the fracture
surfaces (e.g., twisting and tearing). However, for brittle fracture, cracks may spread
extremely rapidly, with very little accompanying plastic deformation. Such cracks may
be said to be unstable, and crack propagation, once started, continues spontaneously
without an increase in magnitude of the applied stress.
Ductile fracture is almost always preferred to brittle fracture for two reasons:
First, brittle fracture occurs suddenly and catastrophically without any warning; this is
a consequence of the spontaneous and rapid crack propagation. However, for ductile
fracture, the presence of plastic deformation gives warning that failure is imminent,
allowing preventive measures to be taken. Second, more strain energy is required to
induce ductile fracture inasmuch as these materials are generally tougher. Under the
action of an applied tensile stress, many metal alloys are ductile, whereas ceramics are
typically brittle, and polymers may exhibit a range of behaviors.

8.3 DUCTILE FRACTURE
Ductile fracture surfaces have distinctive features on both macroscopic and microscopic
levels. Figure 8.1 shows schematic representations for two characteristic macroscopic
fracture profiles. The configuration shown in Figure 8.1a is found for extremely soft
metals, such as pure gold and lead at room temperature, and other metals, polymers,
and inorganic glasses at elevated temperatures. These highly ductile materials neck
down to a point fracture, showing virtually 100% reduction in area.
The most common type of tensile fracture profile for ductile metals is that repre-
sented in Figure 8.1b, where fracture is preceded by only a moderate amount of necking.
The fracture process normally occurs in several stages (Figure 8.2). First, after necking