Page 433 - Mechanical Behavior of Materials

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

Figure 9.14 Various fatigue test specimens, all shown to the same scale. (Adapted from
[Hartmann 59]; used with permission.)

Figure 9.15 Axial fatigue testing machine based on a resonant vibration caused
by a rotating eccentric mass. (From [Collins 93] p. 179; reprinted by permission of
John Wiley & Sons, Inc. copyright c 1993 by John Wiley & Sons, Inc.)

connecting rod from the eccentric drive give different mean deflections and, hence, different mean
stresses. The test specimens are often flat, with the width tapered the proper amount to give a
constant bending stress, despite the linearly varying moment. One of the test specimens shown in
Fig. 9.14 is of this type. If yielding occurs in such a test, the stresses cannot be readily determined
from the deflections, and the force or specimen strain must be specifically measured. Axial stressing
with various mean levels can be achieved by a modification of this device.
A cyclic stress with zero mean level can be achieved by exciting a resonant vibration in an
elastic system, such as the axial testing machine based on a rotating eccentric mass shown in
Fig. 9.15. More complex resonant devices capable of providing mean stresses are also used, and
similar principles can be applied to bending or torsion. In addition, the vibration can be induced
by other means, such as electromagnetic, piezoelectric, or acoustic effects. Frequencies of cycling