Page 417 - Mechanical Behavior of Materials

418 Chapter 9 Fatigue of Materials: Introduction and Stress-Based Approach

Figure 9.1 Large horizontal-axis wind turbine in operation on the Hawaiian island of Oahu.
The blade has a tip-to-tip span of 98 m. (Photo courtesy of the NASA Lewis Research Center,
Cleveland, OH.)

9.2 DEFINITIONS AND CONCEPTS

A discussion of the stress-based approach begins with some necessary definitions and basic
concepts.

9.2.1 Description of Cyclic Loading

Some practical applications, and also many fatigue tests on materials, involve cycling between
maximum and minimum stress levels that are constant. This is called constant amplitude stressing
and is illustrated in Fig. 9.2.
The stress range, σ = σ max − σ min , is the difference between the maximum and the minimum
values. Averaging the maximum and minimum values gives the mean stress, σ m . The mean stress
may be zero, as in Fig. 9.2(a), but often it is not, as in (b). Half the range is called the stress