GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53
2nd Revised Pages9.12 Crack Initiation and Propagation • 321Figure 9.28 Fracture surface
of a rotating steel shaft that
experienced fatigue failure.
Beachmark ridges are visible
in the photograph.
(Reproduced with permission
from D. J. Wulpi,
Understanding How
Components Fail,American
Society for Metals, Materials
Park, OH, 1985.)On the other hand, fatigue striations are microscopic in size and subject to obser-
vation with the electron microscope (either TEM or SEM). Figure 9.29 is an electron
fractograph that shows this feature. Each striation is thought to represent the ad-
vance distance of a crack front during a single load cycle. Striation width depends
on, and increases with, increasing stress range.
At this point it should be emphasized that although both beachmarks and stria-
tions are fatigue fracture surface features having similar appearances, they are never-
theless different, both in origin and size. There may be literally thousands of striations
within a single beachmark.Figure 9.29 Transmission
electron fractograph showing
fatigue striations in
aluminum. Magnification
unknown. (From V. J.
Colangelo and F. A. Heiser,
Analysis of Metallurgical
Failures,2nd edition.
Copyright©c1987 by John
Wiley & Sons, New York.
Reprinted by permission of
John Wiley & Sons, Inc.)