Fatigue failure of a material is always a concern to design engineers because it is difficult to calculate for a given part and therefore presents an unknown risk in the final product. Factors that affect fatigue strength include the material, shape of the part, any defects, corrosion, heat treatment and loading.Fatigue is defined as "the progressive and localised structural damage of a material subjected to cyclic loading", but within this definition there are two types of fatigue to consider.High cycle fatigue.This is the repeated cyclic loading of a part inside the material's elastic limit. Typical cycles to failure would be 100,000 or more. A good example is a bicycle spoke which undergoes tensile and compressive loads for each wheel revolution.Low cycle fatigue.This describes events which stress a part beyond its elastic limit, causing plastic deformation. The bending of a bicycle fork after an impact could be an example of this and after such an event the remaining strength of the part is reduced.In the bicycle world components are almost entirely operating in the high cycle fatigue arena under normal riding / loading conditions.SN CurvesFatigue performance of a metal sample can be tested in the lab to produce an S-N curve.S = stress. N=number of cycles. A graph for an aluminium sample is shown below.