Factor of safety
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Factor of safety (FoS) can mean either the fraction of structural capability over that required, or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. The two senses of the term are completely different in that the first is a measure of the reliability of a particular design, while the second is a requirement imposed by law, standard, contract or custom. Careful engineers refer to the first sense as a factor of safety, or, to be explicit, a realized factor of safety, and the second sense as a design factor, but usage is inconsistent and confusing, so engineers need to be aware of both.
The realized factor of safety is just a definition and needs no elaboration.
Appropriate design factors are based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually can use a safety factor of four or higher (often ten). Non-critical components generally have a design factor of two. An interesting exception is in the field of aerospace engineering, where design factors are 1.50 – 3.00) because the costs associated with structural weight are high. This low design factor is why aerospace parts and materials are subject to more stringent quality control.
A factor of safety of 1.0 implies no "overengineering" (not exceeding design requirements). Many government agencies and companies require the use of a Margin of Safety (M.S.) to describe the ratio of the strength of the structure to the requirements. The relationship between M.S. and FoS is M.S. = FoS − 1. Margin of Safety is sometimes, but infrequently, used as a percentage, i.e., a 0.50 M.S vs. a 50% M.S. The equivalent factor of safety would be 1.5.
