• It is nothing but the reserve strength in the material.
  • While designing the component, it is necessary to ensure sufficient reserve strength in case of an accident. so it is ensured by taking suitable factor of safety

• It is defined as, $F.o.S = \frac{\text{maximum stress}}{\text{working or design stress}} = \frac{\text{failure stress or load}}{\text{allowable stress or load}}$

• For Ductile materials,

  F.o.S = $\frac{s_{yt}}{\sigma}$

  where, $s_{yt}$ = yield tensile strength $\sigma$ = allowable stress

• For brittle materials

  F.o.S =$\frac{s_{ut}}{\sigma}$ where $s_{ut}$ = ultimate tensile strength

The factor of safety ensures against

1. Uncertainty in the magnitude of external forces acting on the component
2. Variations in the properties like yield strength or ultimate strength.
3. Variations in the dimensions of the component due to imperfect workmanship.

The selection of proper factor of safety depends on the following factors:

1. Kind of material: For ductile material lower factor of safety is taken while for brittle material higher factor of safety is taken. The more the uncertainty about the composition and reliability of the material, the higher should be the factor of safety.

2. Kind of load: The load on the machine part may be static or varying in magnitude or suddenly applied in the the form of shock. So the factor of safety has to be higher for live loads as compared to static loads

3. Danger to life or property : The value of the factor of safety in designing a member of machine, by whose failure there is a danger of loss of life or valuable property, must be kept higher.

4. Cost: Generally cheaper machines have lower factor of safety in order to reduce cost of materials. For the machines having longer life, the factor of safety should be high as compared to those having shorter life.

5. Quality of production: Good quality components have less defects and hence chances of failures are reduced. So, factor of safety is kept low. Poor quality products have high factor of safety to deal against uncertainty of failures.

6. Stress concentration: Due to abrupt changes in the shape of the components stress concentration occurs leading to high factor of safety. Each of the above factors must be carefully considered and evaluated. The high factor of safety results in heavy sections and wastage of material, whereas a lower of factor of safety is unnecessary risk of failure.