Factor of safety
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Factor of safety (FoS), also known as Safety Factor, is a multiplier applied to the calculated maximum load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. Thus, by effectively "overengineering" the design by strengthening components or including redundant systems, a Factor of Safety accounts for imperfections in materials, flaws in assembly, material degradation, and uncertainty in load estimates. An alternative way to use the safety factor is to derate the strength of the material to get a "design" strength.
- Sdesign = Syield / FoS
- Sdesign = Sproof / FoS
An appropriate factor of safety is chosen 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 use a safety factor of four or higher (often ten). Non-critical components generally have a safety factor of two. An interesting exception is in the field of Aerospace engineering, where safety factors are kept low (about 1.15 - 1.25) because the costs associated with structural weight are so high. This low safety factor is why aerospace parts and materials are subject to more stringent testing and quality control.
A Factor of safety of 1 implies no "overengineering" at all. Hence some engineers prefer to use a related term, Margin of Safety (MoS) to describe the design parameters. The relation between MoS and FoS is MoS = FoS - 1.
Example
In construction engineering the tensional stress σ is defined as σ = F / A where F is the force acting on the element and A is the cross sectional area. From laboratory testing it is known what the actual failure tensile stress σmax of materials is. To find the minimum safe cross section of an element, the force acting on the element is multiplied with the safety factor γ, (its magnitude depending on building codes and regulations). The minimum cross section is then found using Amin = F · γ / σmax
What Factor of Safety is Not
The physicist Richard Feynman wrote about misconceptions concerning the "factor of safety" concept in The Pleasure of Finding Things Out. During his involvement with the Challenger disaster investigation, he came across the ridiculous assertion that the fatal rubber o-rings had a safety factor of 3 because, when exposed to thermal expansion and contraction, the resulting cracks only went a third of the way through them. If an engineer designs a bridge with a safety factor of 3, Feynman wrote, it means the bridge could theoretically support three times its expected load, not that the expected load would only result in it falling one-third apart.