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Fatigue Failure Resulting from Variable Loading 273
Stress Concentration and Notch Sensitivity (Sec. 6–10)
The actual part may have a geometric stress concentration by which the fatigue behav-
ior depends on the static stress-concentration factor and the component material’s
sensitivity to fatigue damage.

Fluctuating Stresses (Secs. 6–11 to 6–13)
These sections account for simple stress states from fluctuating load conditions that are
not purely sinusoidally reversing axial, bending, or torsional stresses.

Combinations of Loading Modes (Sec. 6–14)
Here a procedure based on the distortion-energy theory is presented for analyzing
combined fluctuating stress states, such as combined bending and torsion. Here it is
assumed that the levels of the fluctuating stresses are in phase and not time varying.
Varying, Fluctuating Stresses; Cumulative
Fatigue Damage (Sec. 6–15)
The fluctuating stress levels on a machine part may be time varying. Methods are pro-
vided to assess the fatigue damage on a cumulative basis.

Remaining Sections
The remaining three sections of the chapter pertain to the special topics of surface
fatigue strength, stochastic analysis, and road maps with important equations.

6–3 Fatigue-Life Methods
The three major fatigue life methods used in design and analysis are the stress-life
method, the strain-life method, and the linear-elastic fracture mechanics method. These
methods attempt to predict the life in number of cycles to failure, N, for a specific level
3
of loading. Life of 1 ≤ N ≤ 10 cycles is generally classified as low-cycle fatigue,
3
whereas high-cycle fatigue is considered to be N > 10 cycles.
The stress-life method, based on stress levels only, is the least accurate approach,
especially for low-cycle applications. However, it is the most traditional method, since
it is the easiest to implement for a wide range of design applications, has ample sup-
porting data, and represents high-cycle applications adequately.
The strain-life method involves more detailed analysis of the plastic deformation at
localized regions where the stresses and strains are considered for life estimates. This
method is especially good for low-cycle fatigue applications. In applying this method,
several idealizations must be compounded, and so some uncertainties will exist in the
results. For this reason, it will be discussed only because of its value in adding to the
understanding of the nature of fatigue.
The fracture mechanics method assumes a crack is already present and detected. It
is then employed to predict crack growth with respect to stress intensity. It is most prac-
tical when applied to large structures in conjunction with computer codes and a peri-
odic inspection program.

6–4 The Stress-Life Method

To determine the strength of materials under the action of fatigue loads, specimens are
subjected to repeated or varying forces of specified magnitudes while the cycles or
stress reversals are counted to destruction. The most widely used fatigue-testing device