Structural failure can be avoided if the loads that a structure is capable of supporting are greater than the loads it will be subjected to when in service. Since strength is the ability of a structure to resist loads, the preceding criterion can be restated as follows: The actual strength of a structure must exceed the required strength.
The ratio of the actual strength to the required strength is called the factor of safety (n):
The factor of safety always must be higher than 1.0 if failure is to be avoided. Depending upon the circumstances, the factor of safety from slightly above 1.0 to as much as 10 is used.
The incorporation of factors of safety into the design of material is not a simple matter because both strength and failure have many different meanings. Strength may be measured by the load-carrying capacity of a structure, or the stress in the material may measure it. Failure may mean the fracture and complete collapse of a structure, or it may mean that the deformations have become so large that the structure can no longer perform its intended functions. The latter kind of failure may occur at loads much smaller than those that cause actual collapse.
Determination of a factor of safety must be taken into account, its determination depends on the following things:
- The probability of accidental overloading of the structure by loads that exceed the design loads;
- Types of loads (static or dynamic)
- Whether the loads are applied once or are repeated
- Accurately the how loads are known
- Possibilities for fatigue failure
- Inaccuracies in construction
- Variability in the quality of workmanship
- Variations in properties of materials
- Deterioration due to corrosion or other environmental effects
- The accuracy of the methods of analysis
Whether the failure of material is gradual or sudden (no warning); consequences of failure (minor damage or a disaster) and other such considerations.
If the factor of safety number is too low, the likelihood of failure will be high and the structure design will be unacceptable; if the factor of safety is too high, the structure will be wasteful of materials and perhaps unsuitable for its operations.
Because of these complexities and uncertainties, factors of safety must be determined on a probabilistic basis. They usually are established by groups of experienced engineers who write the codes and specifications used by other designers and sometimes they are even enacted into law.
In aircraft design, it is customary to speak of the margin of safety rather than the factor of safety (n). The margin of safety is defined as the factor of safety minus one:
The margin of safety is frequently expressed as a percent, in which the value given above is multiplied by 100. Thus, a structure having an actual strength that is 1.75 times the required strength has a factor of safety of 1.75 and a margin of safety of 0.75 (or 75%). When the margin of safety is reduced to zero or approaches to zero, there is a higher probability structure will fail.