Using a factor of safety for engineering design is a classical method of design.
A factor of safety, sometimes referred to as a design factor, is defined as the strength of the material divided by the stress in the material.
N = strength / stress
In the examples, a 5000 lbf weight is hung on the end of an aluminum 2014 rod (the force of gravity is also included in this example).
Aluminum 2014 has a tensile yield strength of 42,000psi. The rod has a diameter of 0.5 inches, and the resulting maximum stress on the rod is 35890psi. This results in a safety factor of 1.2.
To increase the desired safety factor, the rod diameter has been increased to 1 inch. The resulting maximum stress on this rod is 10371psi, resulting in a safety factor of approximately 4.
Determining the value of the factor of safety is up to the engineer or designer. The following are some considerations when making this determination:
Codes and Standards – The part or assembly being designed may come under conditions outlined by various codes and standards which must be met in the design.
Material Strength – The ultimate strength or yield strength may be used in the factor of safety. The value used for the factor of safety is typically different based on the strength used.
Type of Material – A material may be ductile or brittle. A ductile material will give a warning of impending failure. A brittle material will fail suddenly. The value for the factor of safety should reflect that. Confidence in the published properties of the material will also come into play.
Type of Loading – Loads can be classified as static, fatigue, or impact loads. The factor of safety selected should reflect that.
Use of the Part – Accidental overload or misuse of the part should be considered.
Difficulty of Analysis – If the loading or geometry of the part or assembly is very complex, the confidence in the analysis results may decrease.
Environment – Materials may degrade in certain types of environments.
Quality Control – Poor quality control may lead to a larger factor of safety.
Result of Failure – Will the failure be catastrophic? Will people be endangered by the failure? These questions should contribute to the selection of a factor of safety.
Cost – Typically, the larger the factor of safety, the larger the manufacturing cost. There needs to be a balance between safety and cost.