Constant Amplitude Fatigue

Part of the Solid Mechanics and Its Applications book series (SMIA, volume 145)


A constant amplitude fatigue loading (or constant amplitude loading) is a fatigue loading in which all the load cycles are identical. A load cycle is usually called either just a cycle or a fatigue cycle. The notation used is shown in Figure 3.1. A cycle is the smallest unit of the stress history which repeats exactly. Cycles are often, but not always, sinusoidal. σa (sometimes σ, as in the figure) is the alternating stress, σm is the mean stress, σmax is the maximum stress in the load cycle, and σmin is the minimum stress in the load cycle. Mathematically a load cycle (or stress cycle) is expressed as σm ± σa. Compressive stresses are taken as negative. It follows that
$$ \sigma _{max} = \sigma _m + \sigma _a , $$
$$ \sigma _{min} = \sigma _m - \sigma _a , $$
$$ \sigma _m = \frac{{\sigma _{max} + \sigma _{min} }} {2}. $$
The stress range is S = 2σa = σmaxσmin, and the stress ratio, R = σmin/σmax. The term stress ratio and its symbol, R, are very well established, and both are often used in the metal fatigue literature without explanation.


Fatigue Crack Fatigue Life Fatigue Limit Fatigue Crack Propagation Load Cycle 
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© Springer 2007

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