When a material is placed in service there is always some element of uncertainty as to the stresses that the material will be subject to. In addition to this, the specifications of the mechanical properties of a material are generally subject to variability.
It therefore follows that certain design allowances must be enforced to ensure the avoidance of catastrophic failures that might result in personal harm or economic loss.
These allowances may follow one of two principals – design stress or safe stress.
The design stress, σd, is calculated as the applied stress level, σc , multiplied by a design factor, N', where N' > 1.
The safe stress, σw, is based on the yield strength of the material and is calculated as the yield strength, σy, divided by a safety factor, N.
Preference is usually given to the use of design stress since it is based on the applied stress which more often has greater variability. N must also be chosen wisely. If N is too large then the component will be over designed.