Page 27 - Mechanical Behavior of Materials

26 Chapter 1 Introduction

Figure 1.8 Development of a fatigue crack during rotating bending of a precipitation-
hardened aluminum alloy. Photographs at various numbers of cycles are shown for a test
requiring 400,000 cycles for failure. The sequence in the bottom row of photographs shows
more detail of the middle portion of the sequence in the top row. (Photos courtesy of
Prof. H. Nisitani, Kyushu Sangyo University, Fukuoka, Japan. Published in [Nisitani 81];
reprinted with permission from Engineering Fracture Mechanics, Pergamon Press, Oxford, UK.)

fatigue in the bridge, and sailboat rudders and bicycle pedals can fail in fatigue. Vehicles of all
types, including automobiles, tractors, helicopters, and airplanes, are subject to this problem and
must be extensively analyzed and tested to avoid it. For example, some of the parts of a helicopter
that require careful design to avoid fatigue problems are shown in Fig. 1.10.
If the number of repetitions (cycles) of the load is large, say, millions, then the situation is
termed high-cycle fatigue. Conversely, low-cycle fatigue is caused by a relatively small number of
cycles, say, tens, hundreds, or thousands. Low-cycle fatigue is generally accompanied by significant
amounts of plastic deformation, whereas high-cycle fatigue is associated with relatively small
deformations that are primarily elastic. Repeated heating and cooling can cause a cyclic stress due
to differential thermal expansion and contraction, resulting in thermal fatigue.