Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53

2nd Revised Pages

292 • Chapter 9 / Failure

(a) (b)

Figure 9.3 (a) Cup-and-cone fracture in aluminum. (b) Brittle fracture in a mild steel.

for fractographic examinations since 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 9.4a); this structure is characteristic of frac-

ture 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 9.4b. This parabolic shape may be indicative of

shear failure. Furthermore, other microscopic fracture surface features are also pos-

sible. Fractographs such as those shown in Figures 9.4aand 9.4bprovide valuable

Figure 9.4 (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©c1989 by John Wiley & Sons, New York.

Reprinted by permission of John Wiley & Sons, Inc.)