Page 100 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 2.10 Failure and Fracture of Materials in Manufacturing and in Service 79

decrease in ductility and toughness and an increase in the strength
of plain-carbon and of some alloy steels.

2. I 0.2 Brittle Fracture
Brittle fracture occurs with little or no gross plastic deformation; in
tension, fracture takes place along the crystallographic plane
(cleavage plane) on which the normal tensile stress is a maximum.
Face-centered cubic metals usually do not fail by brittle fracture,
whereas body-centered cubic and some hexagonal close-packed
metals fail by cleavage. In general, low temperature and a high rate
of deformation promote brittle fracture. In a polycrystalline metal
under tension, the fracture surface has a bright granular appear- FIGURE 2.26 Fracture surface of steel that
ance, because of the changes in the direction of the cleavage planes has failed in a brittle manner. The fracture
as the crack propagates from one grain to another (Fig. 2.26). path is transgranular (through the grains).
Brittle fracture of a specimen in compression is more complex, and Magnification: 200><. Source: Courtesy of B.].
fracture may even follow a path that is theoretically at an angle of Schulze and S.L. Meiley and Packer Engineering
45° to the direction of the applied force. Associates, Inc.
Examples of fracture along a cleavage plane are the splitting
of rock salt and the peeling of layers of mica. Tensile stresses nor-
mal to the cleavage plane, caused by pulling, initiate and control
the propagation of fracture. Another example is the behavior of
brittle materials, such as chalk, gray cast iron, and concrete; in
tension, they fail in the manner shown in Fig. 2.21a. In torsion,
they fail along a plane at an angle of 45° to the axis of twist
(Fig. 2.10)-that is, along a plane on which the tensile stress is a
maximum.

Defects. An important factor in fracture is the presence of
defects, such as scratches, flaws, and preexisting external or inter-
nal cracks. Under tension, the sharp tip of the crack is subjected to
high tensile stresses, which propagate the crack rapidly.
FIGURE 2.27 Intergranular fracture, at two
The presence of defects explains why brittle materials ex-
different magnifications. Grains and grain
hibit such weakness in tension compared with their strength in
boundaries are clearly visible in this micrograph.
compression; see Table 8.2. The ratio of compressive to tensile
The fracture path is along the grain boundaries.
strength is on the order of 10 for rocks and similar materials,
Magnification: left, 100><; right, 500><. Source:
about 5 for glass, and about 3 for gray cast iron. Under tensile Courtesy of B.]. Schulze and S.L. Meiley and
stresses, cracks propagate rapidly, causing what is known as Packer Engineering Associates, Inc.
catastrophic failure.
In polycrystalline metals, the fracture paths most commonly observed are
transgranular (transcrystalline or intragranular); that is, the crack propagates
through the grain. In intergranular fracture, the crack propagates along the grain
boundaries (Fig. 2.27); it generally occurs when the grain boundaries are soft, con-
tain a brittle phase, or have been weakened by liquid- or solid-metal embrittlement
(Section 1.5.2).

Fatigue Fracture. Fatigue fracture typically occurs in a brittle manner. Minute ex-
ternal or internal cracks develop at preexisting flaws or defects in the material; these
cracks then propagate over time and eventually lead to total and sudden failure of
the part. The fracture surface in fatigue is generally characterized by the term beach
marks, because of its appearance (Fig. 2.28). Under high magnification (typically