TABLE OF CONTENTS 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. ? a (sometimes ?, as in the figure) is the alternating stress, ? m is the mean stress, ? max is the maximum stress in the load cycle, and ? mi n is the minimum stress in the load cycle. Mathematically a load cycle (or stress cycle) is expressed as ? m ? a. Compressive stresses are taken as negative. It follows that
The stress range is S = 2 ? a = ? max ? ? mi n, and the stress ratio, R = ? min / ? max. The term stress ratio and its symbol, R, are very well established, and both are often used in the metal fatigue literature without explanation.
Very large numbers of constant amplitude fatigue tests have been carried out on plain (unnotched) metallic specimens. Test results are sensitive to the surface finish. Compilations of results are available,...
