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Callister - An introduction - 8th edition

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260 • Chapter 8 / Failure<br />

Figure 8.21 Fracture<br />

surface of a rotating steel<br />

shaft that experienced<br />

fatigue failure. Beachmark<br />

ridges are visible in the<br />

photograph. (Reproduced<br />

with permission from D. J.<br />

Wulpi, Understanding How<br />

Components Fail, American<br />

Society for Metals, Materials<br />

Park, OH, 1985.)<br />

“clamshell marks”) are of macroscopic dimensions (Figure 8.21), and may be<br />

observed with the unaided eye. These markings are found for components that experienced<br />

interruptions during the crack propagation stage—for example, a machine<br />

that operated only during normal work-shift hours. Each beachmark band<br />

represents a period of time over which crack growth occurred.<br />

On the other hand, fatigue striations are microscopic in size and subject to observation<br />

with the electron microscope (either TEM or SEM). Figure 8.22 is an electron<br />

fractograph that shows this feature. Each striation is thought to represent the<br />

Figure 8.22 Transmission<br />

electron fractograph<br />

showing fatigue striations in<br />

aluminum. 9000. (From<br />

V. J. Colangelo and F. A.<br />

Heiser, <strong>An</strong>alysis of<br />

Metallurgical Failures, 2nd<br />

<strong>edition</strong>. Copyright © 1987<br />

by John Wiley & Sons, New<br />

York. Reprinted by<br />

permission of John Wiley &<br />

Sons, Inc.)<br />

1 m

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