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

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

2nd Revised Pages

320 • Chapter 9 / Failure

Stress amplitude (MPa) Stress amplitude (ksi)

25

20

15

10

5

0 0

2

3

1

PS PET

PMMA

PP

PE

PTFE

Nylon

(dry)

Number of cycles to failure

103 104 105 106 107

Figure 9.27 Fatigue curves (stress amplitude versus the number of cycles to failure) for

poly(ethylene terephthalate) (PET), nylon, polystyrene (PS), poly(methyl methacrylate)

(PMMA), polypropylene (PP), polyethylene (PE), and polytetrafluoroethylene (PTFE). The

testing frequency was 30 Hz. (From M. N. Riddell, “A Guide to Better Testing of Plastics,”

Plast. Eng.,Vol. 30, No. 4, p. 78, 1974.)

9.12 CRACK INITIATION AND PROPAGATION^8

The process of fatigue failure is characterized by three distinct steps: (1) crack ini-

tiation, wherein a small crack forms at some point of high stress concentration;

(2) crack propagation, during which this crack advances incrementally with each

stress cycle; and (3) final failure, which occurs very rapidly once the advancing crack

has reached a critical size. Cracks associated with fatigue failure almost always ini-

tiate (or nucleate) on the surface of a component at some point of stress concentra-

tion. Crack nucleation sites include surface scratches, sharp fillets, keyways, threads,

dents, and the like. In addition, cyclic loading can produce microscopic surface dis-

continuities resulting from dislocation slip steps that may also act as stress raisers,

and therefore as crack initiation sites.

The region of a fracture surface that formed during the crack propagation step

may be characterized by two types of markings termedbeachmarksandstriations.

Both of these features indicate the position of the crack tip at some point in time

and appear as concentric ridges that expand away from the crack initiation site(s),

frequently in a circular or semicircular pattern. Beachmarks (sometimes also called

“clamshell marks”) are of macroscopic dimensions (Figure 9.28), and may be ob-

served with the unaided eye. These markings are found for components that expe-

rienced interruptions during the crack propagation stage—for example, a machine

that operated only during normal work-shift hours. Each beachmark band represents

a period of time over which crack growth occurred.

(^8) More detailed and additional discussions on the propagation of fatigue cracks are to be
found in Sections M.3 and M.4 of Web Module M.