Safety Factor

Products that fail may create an unsafe situation.

For catastrophic failure mechanisms, the design team may consider establishing a safety factor or margin of safety policy. This provides the design team guidance as they size structures, select components, and evaluate performance and reliability.

A safety factor or margin are measures of the separation of the stress and strength for a specific failure mechanism. If something has a 2x safety factor it implies the element is twice as strong as the expected stress.

One way to define a safety factor is with the ration of the mean strength over the mean stress.
$$ \large\displaystyle \text{Safety Factor:}=\frac{{{\mu }_{x}}}{{{\mu }_{y}}}$$

Where μx is the average strength and μy is the average stress.

In some cases, the safety factor is defined as the minimum strength and the maximum stress.

The margin of safety is a similar definition

$$ \large\displaystyle \text{Margin of Safety:}=\frac{{{\mu }_{x}}-{{\mu }_{y}}}{{{\mu }_{y}}}$$

And provides a measure of the relative separation between stress and strength.

It is common practice for an organization to establish a guideline for various elements of a system. For an aircraft for example, the wing structural attachment may have a very high safety factor requirements, where the individual overhead light switch may have a relative small safety factor.

Setting safety factors impacts the cost, weight, and durability (for example) of a specific design. Setting an appropriate safety factor is a balance of business, customer, technology, uncertainty, variability, and liability. If the cost of failure (loss of a passenger plane, for example) is only one consideration then designs would be very conservative, robust, and expensive.

Of course, the cost of failure is balanced by the cost of construction, maintenance, and operation. The safety factor is a formal way to balance the chance of failure with the various costs. Part of the process may include setting a value on human life.

Your specific situation may not have governing safety factor guidelines, therefore work with your management team to establish appropriate guidelines.

Related:

Stress Strength Normal Assumption (article)

Discovery Testing (article)

Expectation and Moment Generating Functions (article)