SETTING THE MARGIN FOR SAFETY

Estimating the set of stress and stress curves is an interesting exercise that may have a greater purpose: safety. The connection is clear when considering the potential consequences of failure. For example, the loss of braking power when landing an aircraft may result in the aircraft rolling off the end of the runway.

This could be into a river or road and may have a rather poor outcome, not only for the aircraft.  One way an aircraft braking system could fail is the over stress of a specific flange, causing it to fracture. I’m just making this up as I’m not all that familiar with aircraft brakes, yet I have enjoyed their ability to actually stop a landing aircraft on occasion.

We can calculate, simulate, and measure the applied load on the flange during braking. The data become the basis for the stress curve. Given the design, materials, and assembly process we likewise can calculate, simulate, and measure the ability of the flange to withstand the braking loads. This becomes the basis for the strength curve.

Given the two curves we can calculate the probability of an applied load fracturing a flange. This is the chance that the specific load is greater than the ability of the specific flange to hold without fracture. Given that any failure may result in catastrophe, what chance of failure is sufficiently low to be considered safe?

SAFETY FACTOR POLICY

The stress–strength calculations provide a chance of failure, yet we need a value to judge the calculated results. If the desire is to have less than a one in a million chance of flange fracture, then we have a specification to judge the stress–strength calculations. If the calculated value shows that there is a one in a thousand chance of failure, the strength is not sufficient and may require redesign or material change or assembly improvement. However, if the calculated chance of failure is one in a billion, then we may consider cost or weight savings.

No design or system is perfect and always has a chance of failure. The cost and available technology to reduce the risk of failure limits our ability to shift the strength curve away from the stress curve (to reduce chance of failure). Finding that balance is where the safety factor policy plays a role.

Your engineering team may have a policy aligned for different types of failures. Like an FMEA severity scale, the policy may prioritize work to reduce risk of failures that lead to catastrophic outcomes. The policy may dictate a specific chance of failure, such as a one in a million chance, or it may provide a ratio of how much stronger the strength has to be over the stress, or it could be stated as a margin of safety.

Any approach to stating the policy translates to the separation distance of the stress and strength curves. For example, we may set a policy that applies to the flange example as a safety factor of 5´. This means that the strength of the flange should be at least 5 times as strong as the expected stress it will experience during braking. The same policy may include a 1.5´ safety factor for noncritical failures. For example, failure of the elements that support my in-flight entertainment system is not life threatening, so may warrant a lower margin of safety.

Do you have a policy that allows you and your team to evaluate your design against the various types of potential failure consequences? If not, it’s time to set it, isn’t it?

Bio:

Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs. If you enjoyed this articles consider subscribing to the ongoing series at Accendo Reliability.