Safety factors for ductile materials based on yield strength
Safety factor	N1	N2	N3
	Material properties (from tests)	Stress/load model accuracy	Service environment
1,3	Well known/characterized	Confirmed by testing	Same as material test conditions
2	Good approximation	Good approximation	Controlled, room-temperature
3	Fair approximation	Fair approximation	Moderately challenging
5+	Crude approximation	Crude approximation	Extremely challenging