Page 306 - Materials Science and Engineering An Introduction

278 • Chapter 8 / Failure

Figure 8.24 Fatigue failure surface. A crack formed Region of slow
at the top edge. The smooth region also near the top crack propagation
corresponds to the area over which the crack propagated
slowly. Rapid failure occurred over the area having a dull
and fibrous texture (the largest area). Approximately
0.5 .
[From Metals Handbook: Fractography and Atlas of Fracto-
graphs, Vol. 9, 8th edition, H. E. Boyer (Editor), 1974. Repro-
duced by permission of ASM International, Materials Park, OH.]

Region of rapid failure
Often the cause of failure may be deduced after examination of the failure surfaces.
The presence of beachmarks and/or striations on a fracture surface confirms that the
cause of failure was fatigue. Nevertheless, the absence of either or both does not ex-
clude fatigue failure. Striations are not observed for all metals that experience fatigue.
Furthermore, the likelihood of the appearance of striations may depend on stress state.
Striation detectability decreases with the passage of time because of the formation of
surface corrosion products and/or oxide films. Also, during stress cycling, striations
may be destroyed by abrasive action as crack mating surfaces rub against one another.
One final comment regarding fatigue failure surfaces: Beachmarks and striations do
not appear on the region over which the rapid failure occurs. Rather, the rapid failure
may be either ductile or brittle; evidence of plastic deformation will be present for ductile
failure and absent for brittle failure. This region of failure may be noted in Figure 8.24.

Concept Check 8.4 Surfaces for some steel specimens that have failed by fatigue have a
bright crystalline or grainy appearance. Laymen may explain the failure by saying that the metal
crystallized while in service. Offer a criticism for this explanation.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

8.10 FACTORS THAT AFFECT FATIGUE LIFE 10
As mentioned in Section 8.8, the fatigue behavior of engineering materials is highly
sensitive to a number of variables, including mean stress level, geometric design, surface
effects, and metallurgical variables, as well as the environment. This section is devoted

10 The case study on the automobile valve spring in Sections M.14 through M.16 of the Mechanical Engineering (ME)
Online Module relates to the discussion of this section.