Abdelmoneim Fawzy | Fracture Mechanics | December 22, 2015

Factors that affect fatigue life

ENVIRONMENTAL EFFECTS

Factors that affect fatigue life Magnitude of stress Quality of the surface

Solutions: Polish surface Introduce compressive stresses

(compensate for applied tensile stresses) into surface layer.

Shot Peening -- fire small shot into surface High-tech - ion implantation, laser peening.

Case Hardening: Steel - create C- or N- rich outer layer by atomic diffusion from surface

Harder outer layer introduces compressive stresses

Optimize geometry

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Avoid internal corners, notches etc.

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Factors affecting fatigue lifeEnvironmental effects

Thermal Fatigue. Thermal cycling causes expansion and contraction, hence thermal stress.

Solutions: change design! use materials with low thermal expansion

coefficients Corrosion fatigue.

Chemical reactions induce pits which act as stress raisers. Corrosion also enhances crack propagation.

Solutions: decrease corrosiveness of medium add protective surface coating add residual compressive stresses

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The Macroscopic Character of Fatigue Failure

Because of the manner in which the fracture develops, the surfaces of a fatigue fracture are divided into two areas with distinctly different appearances.

In most cases, the surface will have a polished or burnished appearance in the region where the crack grew slowly.

In the last stage, the surfaces developed are rough and irregular.

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Fractograph of fatigue failure in SAE 1050 pin, induction hardened to a depth of 5 mm ( 3/16 in.) and surface hardness of 55 HRC. Core hardness: 21 HRC. Fatigue initiated inside the grease hole at the metallurgical notch created by the very sharp case-core hardness gradient.