When stress appears on any section, the maximum allowable stress determines the value of its ability to bear load without deforming. If an object is subjected to stress and it crosses the stage of elastic limit, then the original cross-sectional area of the object changes. The working stress of an object should be less than yield stress and ultimate tensile stress. It is always less than elastic limit.
Factor of Safety or FoS:
The ratio of Yield Stress to the Working Stress determines the value of factor of safety. Principally, FoS is used to check the strength of an object how much it could bear load. After detailed analysis of the project, the value of factor of safety is decided.