Working Stress and Factor of Safety

Working Stress:

Working stress is the safe stress taken within the elastic range of the material. For brittle materials, it is taken equal to the ultimate strength divided by suitable factor of safety. However, for materials possessing well defined yield point, it is equal to yield stress divided by a factor of safety. It is the stress which accounts all sorts of uncertainties.

It is also called allowable stress, permissible stress, actual stress and safe stress.  

Factor of Safety:  

Factor of safety is a number used to determine the working stress. It is fixed based on the experimental works on the material. It accounts all uncertainties such as, material defects, unforeseen loads, manufacturing defects, unskilled workmanship, temperature effects etc. Factor of safety is a dimensionless number. It is fixed based on experimental works on each material. 

It is defined as the ratio of ultimate stress to working stress for brittle materials or yield stress working stress for ductile materials.