Factors Affecting Fatigue Behaviour

Fatigue strength is most important property that is governed by fatigue behaviour. Fatigue life at a given stress level may also be regarded as a fatigue property. Increased fatigue strength and increased life at a given stress mark improvement in fatigue behaviour. Stronger materials have higher fatigue strength. Half of ultimate strength in tension is equal to fatigue strength obtained from smooth, polished specimen in rotating bending machine for steel. If stress cycle is pulsating the fatigue strength is 40% higher, and if reversible axial stress is applied the fatigue strength is 15% lower than the fatigue strength found in rotating bending machine on smooth-polished specimen.

Increasing size above 12 mm diameter reduces the fatigue strength while for very large size 200 mm diameter the size effect ceases. Surface finish is perhaps the single most important factor that affects fatigue strength. Fatigue strength improves if surface finish changes from as forged or drawn to turned and fine turned to ground and polished. Except in a few cases where inside of material has a defect, the fatigue crack initiates from the surface. This surface crack initiation is because of following

A.    surface is inherently weaker,
B.    surface is exposed to corrosion and oxidation,
C.    this is surface at which cutting and forming forces act, and
D.    stress is inherently highest at surface whether axial, bending, torsional or combination thereof.

Practical applications place some of the stress concentration on the surface. Corrosion and high temperature damage surface.Good surface finish & surface mechanical and heat treatment. (like shot peening, surface rolling, carburizing, nitriding, cyaniding) help obtain improved fatigue properties. Vanadium is the only alloying element that develops fatigue strength of steel.