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Safety factor

The safety factor shows how much margin a structure has against yielding or fracture, accounting for uncertainty in load, material and model. The higher $n$ is, the larger the margin, but also more material and weight.

n = \dfrac{R_e}{\sigma_{actual}}

safety factor against yield

Symbols

$n$	safety factor
$R_e$	yield strength	Pa
$\sigma_{actual}$	actual (computed) stress	Pa

Example

Steel with $R_e = 355$ MPa, actual stress 142 MPa:

$n = 355/142 \approx 2.5$ .

n > 1

means margin against yield, not that the structure can take

n

times the load undamaged.

Practise stress and sizing for free →

← Normal stress · Strain and modulus of elasticity →

Part of Machine Design: Stress and sizing.