A safety factor was originally a number by which the ultimate tensile strength of a
material was divided in order to obtain a value of “working stress” or “design stress.”
These design stresses, in turn, were often used in highly simplified calculations that
made no allowance for such factors as stress concentration, impact, fatigue, differences
between properties of the material in the standard test specimen and in the manufactured
part, and so on. As a result, one can still find in handbooks safety factor recommendations
as high as 20 to 30. Modern engineering design gives a rational