FACTORS AFFECTING FATIGUE STRENGTH

The fatigue strength - whether reversed or not is a structure sensitive property. It is affected by many factors; grain size, chemical composition, amount of cold working etc. Fatigue strength increases when the temperature is lowered, particularly below the ambient temperature. In addition, fatigue strength is critically influenced by surface conditions. This can affect fatigue strength in three ways:

a)         Through the geometry of the surface.

b)          Through the physical nature of the surface layer

c)    Through corrosion environment.

Roughness, porosity and foreign inclusion (slag, oxides) are characteristic geometrical and physical defects of metallic surfaces. Their presence can lower the fatigue life by as much as 20% depending on the material and seriousness of the defect. The main effect of corrosion is to produce sharp pits from which numerous cracks form, frequently meeting to form a serrated fracture.

DESIGN FOR FATIGUE

To secure satisfactory life it is necessary to observe the following points in design procedure:

1.          Modification of the design to avoid stress concentration by eliminating sharp recess and severe stress raisers.

2.    Precise control of surface finish by avoiding damage to surface by rough machining, punching, stamping, shearing etc.

3.    Control of corrosion and erosion or chemical attack in service and prevention of surface decarbonisation during processing or heat treatment.

4.    Surface treatment of metal.

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