Reliability or life testing involves estimating the expected durability over time of an item.
This may be an entire system, a product, or an individual component. We may also focus on an element of a component, such has a material property.
At the end of the testing, we want to say something meaningful about the expected performance over time.
What’s a ‘good’ life test?
A ‘good’ life test focuses on the failure mechanisms in question.
For example, if I know the polymer’s elasticity will degrade due to chain scissioning, then designing a test that includes stress that causes chain scissioning to occur in a similar fashion as expected in use, then the results should reflect the actual performance.
The trick is to understand the failure mechanisms first, then select the test stresses.
If the development time is long enough to encompass the expected operating time of the item, then we can simply use the item as it is expected to be used. The test, in this case, is really just data collection and analysis. However, we rarely have ample time and require some form of acceleration.
Accelerated life testing, ALT, significantly highlights the need to apply the appropriate stress in order to age the item in a known fashion.
We attempt to cheat time and when doing the process well provides us with a glimpse into the future. When done poorly, we witness something that will not come to pass.
Focus on the failure mechanism
Do this simple test of your life test design: Ask, “What is the expected failure mechanism and how does the test stress encourage that mechanism to occur?”
If the answer is a failure mode such as the product ceases to function, then the design needs more work. What causes the product to fail? What fundamental mechanism leads to the loss of the function?
For example, if test a new product with elevated temperature and humidity, because it is something we have always done, or a customer requested it, or it’s an industry standard, we may or may not learn anything about the product’s expected lifetime. If the product is a handheld portable product, a high temperature and humidity test will not evaluate the very likely failure mechanism of shock stress due to dropping.
While we ‘pass’ the temperature and humidity test we learn nothing about the expected drop stress failures.
Sort out what is likely to fail, understand the failure mechanisms, then apply the appropriate stresses to excite those specific failure mechanisms.
Make sure the science and understanding support starting any life test.
Replicate the failures as expected in the field
Run the test to failure.
Did the test sample fail exactly or similarly enough (i.e. same failure mechanism) or did the test reveal a different path to failure? Running to failure allows you to check the validity of test design assumptions and avoid unpleasant surprises when the customers begin using the item.
Once you have a well-characterized failure mechanism, testing to show a minimum reliability with a success test (a test designed to have no failures) is feasible. Not before.
The key for any life testing is to replicate the failure mechanisms that will occur in actual use.
Use or build an acceleration model
For specific failure mechanisms that have a time to failure versus applied stress relationship, you can use that information to create an acceleration model.
The model provides an acceleration factor that translates the elevated time to failure test results to use condition expected time to failure performance.
If the failure mechanism is related to a flex motion (for example, a hinge), that occurs once a day in normal use. And, assuming we can replicate that motion in the lab 24 times a day. And, the increase in flex motion rate does not introduce spurious failure mechanisms, we have a 24x acceleration factor.
This means in one day of lab testing we replicate 24 days of use. We have that glimpse into the future.
Some failure mechanisms have complicated relationships with stress.
If the mechanism is chemical in nature, then the Arrhenius rate reaction formula connect temperature to the reaction rate if we know that specific chemical reaction’s activation energy (do not guess or use a standard based value here!).
The acceleration factor equations are either based on empirical evidence or detailed characterization of the failure mechanism. The field of physics of failure has catalogs of detailed formulas for specific failure mechanisms, plus the methods used to develop the formulas.
These models may provide a means to directly estimate the reliability of your item based on the specific failure mechanism or a method to design your own life test.
The best way to design a life test is to let your customers use the product. Then based on the understanding of how they use the product and what fails and when, we can design an adequate life test to predict what actually happens.
This is not practical, thus we make assumptions, sometimes confirmed by experimentation and characterization work, and attempt to peer into the future.
When you contemplate life testing, what questions do yo ask? What assumptions do you challenge? And, do you track how well your estimates reflect actual performance?