Page 429 - Mechanical Behavior of Materials

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

Figure 9.7 Sample record of stresses at the steering knuckle arm of a motor vehicle,
including the original stress–time history (a), and the separation of this into the vibratory load
due to roadway roughness (b) and the working load due to maneuvering the vehicle (c).
(From [Buxbaum 73]; used with permission; first published by AGARD/NATO.)

S-N curves from constant amplitude testing can be used to estimate fatigue lives for irregular
load-time histories. The methodology will be introduced near the end of this chapter.

9.4 FATIGUE TESTING

Materials testing to obtain S-N curves is a widespread practice. Several ASTM Standards address
stress-based fatigue testing for metals, especially Standard No. E466. The resulting data and curves
are widely available in the published literature, including various handbooks, as listed in a special
section of the References. An understanding of the basis of these tests is useful in effectively
employing their results for engineering purposes.

9.4.1 Test Apparatus

One of the machines employed by W¨ ohler tested a pair of rotating test specimens subjected to
cantilever bending, as shown in Fig. 9.11. Springs supplied a constant force through a bearing,
permitting rotation of the specimen, so that the bending moment varied linearly with distance from
the spring. In such a rotating bending test, any point on the specimen is subjected to a sinusoidally
varying stress as it rotates from the tension (top) side of the beam to the compression (bottom) side,
◦
completing one cycle each time the specimen rotates 360 .
Equipment for rotating bending tests operating on similar principles is still in use today. A
variation involving four-point bending has probably been more widely used than any other type of