Factor of Safety
The Factor of Safety FS is the strength of a system divided by its maximum working load:
 P_{f} is the Failure Load (or Ultimate Load), i.e., the strength. Failure may be defined as the system (1) breaking
into two parts, (2) permanently deforming (yielding), or (3) deflecting too much.
This load is determined from tests or from
calculations based on the system's properties and geometry.
 P_{D} is the Working Load (or Design Load
or Allowable Load), i.e., the maximum load expected in service. This load is generally a result of specifications or codes, or tests on prototype systems.
 The strength and load must be given in the same units  force, stress, etc.).
