Factor of Safety: Factor of Safety of ductile and brittle materials

Factor of Safety

It is defined, in general, as the ratio of the maximum stress to the working stress.

Mathematically,

Factor of safety = Maximum stress/ Working or design stress

In case of ductile materials e.g. mild steel, where the yield point is clearly defined, the factor of safety is based upon the yield point stress. In such cases,

Factor of safety = Yield point stress/ Working or design stress

In case of brittle materials e.g. cast iron, the yield point is not well defined as for ductile materials. Therefore, the factor of safety for brittle materials is based on ultimate stress.

Factor of safety = Ultimate stress/ Working or design stress 

This relation may also be used for ductile materials.

The above relations for factor of safety are for static loading.

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