In practice it is not possible to design a mechanical component or structural component permitting stressing up to ultimate stress for the following reasons.
- Reliability of material may not be 100 percent. There may be small spots of flaws.
- The resulting deformation may obstruct the functional performance of the component.
- The loads taken by designer are only estimated loads. Occasionally there can be overloading. Unexpected impact and temperature loading may act in the lifetime of the member.
- There are certain ideal conditions assumed in the analysis (like boundary conditions). Actually ideal conditions will not be available and, therefore, the calculated stresses will not be 100 percent real stresses.
Hence, the maximum stress to which any member is designed is much less than the ultimate stress, and this stress is called Working Stress. The ratio of ultimate stress to working stress is called factory of safety. Thus
Factor of Safety = Ultimate stress / Working stress
In case of elastic materials, since excessive deformation create problems in the performance of the member, working stress is taken as a factor of yield stress or that of a 0.2 proof stress (if yield point do not exist).
Factor of safety for various materials depends up on their reliability. The following values are commonly taken in practice.
- For steel – 1.85
- For concrete – 3
- For timber – 4 to 6
