» Factor of Safety
The FACTOR OF SAFETY is the ultimate strength of a system divided by its maximum working load. Mathematically, the Factor of Safety (F.S.) is:

• P_f is the Failure Load (or Ultimate Load) - the load at which failure occurs. Failure may be the system (1) breaking into parts, (2) permanently deforming, (3) deflecting too much, etc. This load is generally determined from tests on the system, or from calculations based on the system's material properties (known from other tests).
• P_D is the Working Load (or Design Load or Allowable Load) - the maximum load that the system is expected to carry in service. This load is generally a result of specifications - the loads the system must support.

» Margin of Safety
In the aerospace industry, a common measure is the MARGIN OF SAFETY. The Margin of Safety (m) is a measure of the excess of the failure load compared to the working load. The definition is :

The values of m must be greater than 0, since F.S.>1 to avoid failure. For example, if F.S. = 1.5, m = 0.5

» Proof Testing
For many structural systems, it is important to test them to prove that they work before they are put into service. PROOF TESTING is especially necessary when the system or construction methods are new, the structure is difficult to analyze, or when the failure of the system can cause significant damage, especially in human life. The load at which the system is tested - the Proof Load, P_p - is greater than the Working Load, P_D but less than the Failure Load, P_f.