Page 32 - Mechanical Behavior of Materials

Section 1.3 Design and Materials Selection 31

Figure 1.12 Steps in the design process related to avoiding structural failure. (Adapted from
[Dowling 87]; used with permission; c Society of Automotive Engineers.)

For example, if X 1 = 2.0, the stress necessary to cause failure is twice as high as the highest stress
expected in service. Safety factors provide a degree of assurance that unexpected events in service
do not cause failure. They also allow some latitude for the usual lack of complete input information
for the design process and for the approximations and assumptions that are often necessary. Safety
factors must be larger where there are greater uncertainties or where the consequences of failure are
severe.
Values for safety factors in the range X 1 = 1.5 to 3.0 are common. If the magnitude of the
loading is well known, and if there are few uncertainties from other sources, values near the
lower end of this range may be appropriate. For example, in the allowable stress design method