- FOS -
Design Factors of Safety are often published in technical standards but there is no dedicated standard to the subject.
Note that for statutory items such as cranes and pressure vessels FOS are specified in the design codes.
|Factor of Safety
- FOS -
|For use with highly reliable materials where loading and environmental conditions are not severe and where weight is an important consideration
|1.3 - 1.5
|For use with reliable materials where loading and environmental conditions are not severe
|1.5 - 2
|For use with ordinary materials where loading and environmental conditions are not severe
|2 - 2.5
|For use with less tried and for brittle materials where loading and environmental conditions are not severe
|2.5 - 3
|For use with materials where properties are not reliable and where loading and environmental conditions are not severe, or where reliable materials are used under difficult and environmental conditions
|3 - 4
In general there is a linear connection between load and stress and the factor of safety can within mechanical engineering for normal stress be modified to
FOS = σfail / σallow (2)
σfail = failure normal stress (N/m2, psi)
σallow = allowable normal stress (N/m2, psi)
FOS for shear stress can be expressed as
FOS = τfail / τallow (3)
τfail = failure shear stress (N/m2, psi)
τallow = allowable shear stress (N/m2, psi)
Be aware that in some cases there may not be a linearity between applied load and stress.
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