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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 safety 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.

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 σ_max 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 regulationIn the context of government and public services regulation (as a process) is the control of something by rules, as opposed to its prohibition. In economics, it is part of the government relationship with markets, often seen as the opposite of deregulatios). The minimum cross section is then found using A_min = F · γ / σ_max

2 See also

• structural engineeringStructural engineering is the field of civil engineering particularly concerned with the design of load-bearing structures. In practice, it is largely the implementation of mechanics to the design of structures, such as buildings, bridges, walls (includin
• strength of materialsStrength of materials is the scientific area of applied mechanics for the study of the strength of engineering materials and their mechanical behaviour in general (such as stress, deformation, strain and stress-strain relations). Strength is considered in
• safety engineeringSafety engineering is used to assure that a life-critical system behaves as needed even when pieces fail. Safety engineering is strongly related to systems engineering. Safety engineers distinguish different extents of defective operation: A "fault" is sa