In opening this discussion on fatigue it seems that my first duty should be to state what fatigue is – I wish I knew. Just as Alice knew that if you drank much from a bottle marked “Poison” it was pretty sure to disagree with you sooner or later, so do we know that if a component be subjected too often to too high a load it is pretty certain to develop fatigue cracks sooner or later.

It is important to remember that Lewis Carroll was by profession a mathematician, because in a mathematical sense even these heavily conditioned statements should in fact convey a great deal. “Too often to too high a load” infers range of loading, and furthermore it implies a connection between range of load S and cycles N, by which in general N may be expected to increase as S decreases. “Sooner or later” indicates that N is not a unique function of S, and “pretty certain” provides for those pieces which, like certain batteries, obstinately go on long after the others have stopped. Then, lastly “to develop fatigue cracks” is a careful avoidance of a direct reference to failure.