Failure curves and P-F intervals linked and explained

Tying the two most important reliability engineering curves together to generate a better picture of failure.

By Shon Isenhour May 16, 2016

During the early development of what would become reliability-centered maintenance, Nowlan and Heap gave us six failure curves. When folks first see that 68% fall into the infant mortality curve, the doubt fairy tends to show up: "Sixty eight percent of the failures in my facility are not instant or early on start-up." With this thought, they then discount the incredibly important failure-mode data provided to us from these studies. What they are missing is the connection to the P-F curve.

If the failure curves show the probability of defects introduced over time based on an individual failure mode, then the P-F shows the resistance to failure over time once the failure defect has occurred. Nolan and Heap did not say that 68% of your assets will catastrophically fail on or near start-up; rather, they said that 68% will have a defect introduced that will then travel down the curve of the P-F, becoming more prone to functional and catastrophic failure. 

This trip down the curve may take five days, five weeks or five years depending on the failure mode and operating context.

This means they don’t fail instantly, but they do fail prematurely because of the defects introduced during or shortly after maintenance activities. So, think of the six failure curves as the probability of introduction of a defect and the P-F as the path of that defect to functional and catastrophic failure. I hope this helps you send the doubt fairy packing, so you can begin to better understand both curves and the additional knowledge they can provide.

– Shon Isenhour is a founding partner of Eruditio. This article originally appeared on Eruditio’s blog. Eruditio is a CFE Media content partner. Edited by Erin Dunne, production coordinator, CFE Media,

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