A Question about the Bathtub Myth
Do all components follow the bathtub curve? Is it possible to generate a bathtub curve for a component?
Thanks for your thoughts and insights!
This is a good question.
No. No component or product I’ve ever worked with follows the bathtub curve, or looks like those in textbooks. An individual component or product does follow any curve; it simply fails at some point in time for some particular cause. The probability of failure for that single component we use a best estimate to predict the failure) is a combination of the many curves describing the various failure mechanisms, or is a combined model based on similar past products.
An entire population of components, like a capacitor, generally has some portion of units that are damaged in production and escape quality checks – they become early life failures or fail due to the latent damage. These failures tend to occur quickly relative to expected life times. Yet, they can also occur very late in the expected life cycle. There is no rule that they have to fail early, but in general latent damage failures occur early.
There is no such thing in my experience as a constant rate or flat part of the bathtub curve – I’ve never, ever seen one with real data. There are times when the change in failure rate is small enough over a short enough duration that it could be reasonably estimated as constant, but that is very rare. Hence my rather vehement objection to MTBF, which assumes constant failure rates.
Wear out is real. It occurs for many reasons and depending on the product, use environment and failure mechanism, it may have a well-defined model to describe the expected failure rate over time. There are many models that describe specific failure mechanisms. The rate of failure varies dramatically based on failure mechanisms. Metals rust or migrate, pn junctions degrade, polymers break down, gaskets become brittle, materials abrade, bearings and grease diminish, etc. For a specific product there are thousands of ways it can fail, and it is a grand race to cause the eventual failure.
Some products have a dominant failure mechanism. For the brake systems in cars, the brake pads abrade as they create friction and braking force. They wear and eventually will lose the ability to create braking force due to dimensional change, i.e. wear. They are replaced and the remaining equipment within the braking system lasts much longer. When the pads wear out and need replacing we don’t deem the system as failed; although it has, it was predicted and expected.