Factors to consider when choosing a design safety factor

If a grade of steel has a yield strength of 40,000 psi, any stress above this limit results in some amount of permanent deformation. If the design is not supposed to deform permanently by going beyond yield (true for most cases), then the maximum allowable stress in this case is 40,000 psi. At an actual stress of 40,000 psi, the safety factor is 1.0. If you want a safety factor of 2.0, then the material strength either needs to be 80,000 psi, or you must change the design to reduce the maximum stress to 20,000 psi.

In common practice, design stresses are limited to magnitudes significantly lower than the material yield strength. In other words, the safety factor is significantly more than 1.0. How much greater than 1.0 a safety factor should be depends on a number of considerations:

• A safety factor accounts for inaccuracies resulting from assumptions, simplifications, or unknowns in the modeling, setup, analysis processes, and material properties. The greater the number of assumptions that you must make, or the less certain that you are about the assumed properties and conditions, the more conservative the design safety factor should be.
• A safety factor can provide an allowance for dynamic loads. A load that is suddenly applied or cyclic (that is, dynamic) produces more stress than a static stress analysis predicts. You can exaggerate the applied load to account for dynamic effects or increase the required safety factor.
• A material subjected to repetitive loading cycles can fail, due to fatigue, at a stress significantly lower than the yield strength. You should consider the reduced allowable stress under fatigue conditions (known as the endurance limit of the material) when choosing a design safety factor.
• Consider the reliability of the assumed material properties. A large casting may contain porosity or contaminates that reduce the strength of the material locally. On the other hand, rolled, hot-forged or cold-worked materials have an improved grain structure, and their properties are more reliability. The chemical mixture of a particular grade of material may also vary from batch to batch. You should increase your design safety factors when you have less confidence in the reliability of the material properties.
• Consider the surface finish of the parts. Imperfections along a rough surface can act as stress concentration areas, effectively increasing the surface stress to something greater than the calculated magnitude. A ground and polished or finely machined surface is superior to a flame-cut or coarsely machined surface. Increase the design safety factor to account for rough finishes.
• Make an allowance for expected loss of material and surface roughening due to corrosion. Structures and parts must remain safe as deterioration occurs over time, whether caused by normal environmental factors or corrosive chemical exposure.
• Is the weight of the design of critical importance, such as when designing aircrafts or the equipment onboard them? In such cases, you cannot be overly conservative, and a lesser design safety factor is typical. However, you also must ensure the best possible accuracy of the model setup and reliability of the material properties. This type of design work requires that you minimize errors due to simplifications and assumptions.
• Finally, the repercussions of a failure must be considered. If a part failure does not produce consequential damages and is easy to repair, it has a relatively low impact. However, if a part failure could result in catastrophic structural failure, serious injury, or death, obviously greater safety factors are warranted. For example, an elevator should be designed using higher safety factors than a bracket used to mount a camera.