Page 28 - Mechanical Behavior of Materials

Section 1.2 Types of Material Failure 27

Figure 1.9 Fatigue failure of a garage door spring that occurred after 15 years of service.
(Photo by R. A. Simonds; sample contributed by R. S. Alvarez, Blacksburg, VA.)

Cracks may be initially present in a component from manufacture, or they may start early in
the service life. Emphasis must then be placed on the possible growth of these cracks by fatigue, as
this can lead to a brittle or ductile fracture once the cracks are sufficiently large. Such situations are
identified by the term fatigue crack growth and may also be analyzed by the previously mentioned
technology of fracture mechanics. For example, analysis of fatigue crack growth is used to schedule
inspection and repair of large aircraft, in which cracks are commonly present.
Such analysis is useful in preventing problems similar to the fuselage (main body) failure in
1988 of a passenger jet, as shown in Fig. 1.11. The problem in this case started with fatigue cracks
at rivet holes in the aluminum structure. These cracks gradually grew during use of the airplane,
joining together and forming a large crack that caused a major fracture, resulting in separation of a
large section of the structure. The failure could have been avoided by more frequent inspection and
repair of cracks before they grew to a dangerous extent.

1.2.5 Combined Effects
Two or more of the previously described types of failure may act together to cause effects greater
than would be expected by their separate action; that is, there is a synergistic effect. Creep and
fatigue may produce such an enhanced effect where there is cyclic loading at high temperature. This
may occur in steam turbines in electric power plants and in gas-turbine aircraft engines.
Wear due to small motions between fitted parts may combine with cyclic loading to produce
surface damage followed by cracking, which is called fretting fatigue. This may cause failure at
surprisingly low stress levels for certain combinations of materials. For example, fretting fatigue