Eventually, people figured out that although the nominal stress (i.e., average stress) was low, a sharp corner, a corrosion pit, a scratch or a dent, damage from weld heat or many of a long list of other factors could result in a local area with the actual stress approaching the tensile-stress value. Thus, with repeated load cycles creating a stress above the yield strength – even within an area as small as one or two grains – a crack may initiate. If you are unfamiliar with the idea of grains, you may wish to look up ASTM grain-size numbers.
The bottom line about fatigue cracks is that they are progressive. They happen over time. They do not happen all at once due to a single load cycle. Fatigue cracks in most common structural steels will look very different from cracks that happen all at once. Figure 1 shows a shaft from a stamping press. The crack initiated from an area roughened due to contact with the inner bearing race, which was supporting the shaft. Fatigue cracks commonly have beach marks, or alternating stripes of lighter and darker colors. The different colors result from different loading levels, different atmospheric conditions or both. Therefore, cracks that grow in uniform loading and environmental conditions do not have beach marks.