6.2.1 Classification of Loadings

Periodical stresses which exist in service are generally sinusoidal, and in accordance with the value of the superimposed static stress ? _s, we distinguish:

• pure or symmetric alternating stress ? _s = 0 ( ? _m = 0);

• dissymmetric alternating stress ? _s < ? _a (0 < ? _m < ? _a);

• repeated stress ? _s = ? _a ( ? _m = ? _a);

• fluctuating stress ? _a < ? _s ( ? _a < ? _m).

with ? _a as the variable stress amplitude and ? _m the mean stress (equal to the static stress, ? _s) (Figure 6.3).

Figure 6.3: Classification of loadings

The stress ratio R is also defined as the algebraic ratio of minimum stress to maximum stress:

6.2.2 Stress-Number of Cycles to Failure Curve (W hler Curve)

The simplest fatigue test consists of subjecting each specimen to load cycles of constant maximum amplitude and frequencies and noting the number of cycles N _r at which failure occurs. The number N _r is plotted generally on a logarithmic scale, according to the maximum stress of the cycles. Each specimen corresponds to a point on the plane ( ?, N _r) and, from a batch of specimens subjected to different stress amplitudes, a curve is obtained,...