Callister - An introduction - 8th edition

238 • Chapter 8 / Failure
Figure 8.3 (a) Cupand-cone
fracture in
aluminum.
(b) Brittle fracture
in a mild steel.
(a)
(b)
Fractographic Studies
Much more detailed information regarding the mechanism of fracture is available
from microscopic examination, normally using scanning electron microscopy. Studies
of this type are termed fractographic. The scanning electron microscope is preferred
for fractographic examinations because it has a much better resolution and
depth of field than does the optical microscope; these characteristics are necessary
to reveal the topographical features of fracture surfaces.
When the fibrous central region of a cup-and-cone fracture surface is examined
with the electron microscope at a high magnification, it will be found to consist of numerous
spherical “dimples” (Figure 8.4a); this structure is characteristic of fracture resulting
from uniaxial tensile failure. Each dimple is one half of a microvoid that formed
and then separated during the fracture process. Dimples also form on the 45 shear lip
of the cup-and-cone fracture. However, these will be elongated or C-shaped, as shown
in Figure 8.4b. This parabolic shape may be indicative of shear failure. Furthermore,
other microscopic fracture surface features are also possible.Fractographs such as those
shown in Figures 8.4a and 8.4b provide valuable information in the analyses of fracture,
such as the fracture mode, the stress state, and the site of crack initiation. 1
(a)
5 m (b)
4 m
Figure 8.4 (a) Scanning electron fractograph showing spherical dimples characteristic of
ductile fracture resulting from uniaxial tensile loads. 3300. (b) Scanning electron
fractograph showing parabolic-shaped dimples characteristic of ductile fracture resulting
from shear loading. 5000. (From R. W. Hertzberg, Deformation and Fracture Mechanics
of Engineering Materials, 3rd edition. Copyright © 1989 by John Wiley & Sons, New York.
Reprinted by permission of John Wiley & Sons, Inc.)
1 Web Module M, “Investigation of Engineering Failures,” discusses how fractographic
studies are used in failure analyses to reveal causes of failure.