Engineering
Intermediate

Factor of Safety

Ratio of ultimate strength to allowable stress.

Formula

FoS=σultσallowFoS = \frac{\sigma_{ult}}{\sigma_{allow}}

Variables

\sigma_{ult}Ultimate strength
\sigma_{allow}Working stress

Example

A part failing at 800 MPa but used to 200 MPa has a safety factor of 4.

Did You Know?

Elevators are typically built with a safety factor of around 11 — their cables could hold far more than a full car.

safetystructuraldesign