Common Slope Assumption

Abstract

Chris and Fred discuss a ‘common slope assumption’ … which is something that only those of you with reliability engineering experience will have heard about. This is all about what we called ‘accelerated life testing’ where we increase stresses to accelerate failure. And software that is used to model accelerated life testing. Keen to learn more? Listen to this podcast!

Key Points

Join Chris and Fred as they discuss a common term used in ‘accelerated life testing’ or ALT. This is where we increase the stress our system is exposed to in order to accelerate failure. Why do we do this? Well … if we can increase stresses to our system and replicate 10 years worth of use in two weeks worth of testing … then we save lots of time and money. But as we know, failure is a random process. And one way of characterizing the way things fail (at least statistically) is with what we call the ‘slope’ of those times to failure. Tires that wear out will usually have a similar ‘slope.’ Steel that corrodes will typically have a similar … but different slope. So what does this mean?

Topics include:

Failure mechanisms tend to have the same slope. It doesn’t matter if it is wear, diffusion, corrosion, fatigue … they all have similar ‘slopes.’ Wear has a ‘typical slope.’ Diffusion has a different ‘typical slope.’ Fatigue has yet another, ‘typical slope.’ So even if you speed up failure by increasing stress (like temperature), the slope that describes the shorter times to failure will be the same.
Accelerated life testing must not change the mechanism. Otherwise, we can’t be sure that two weeks worth of testing equates to 10 years worth of use. If you (for example) increase the temperature, and your system melts, then there is nothing you learn about actual time to failure. But it can be a lot more subtle than this. Increasing the temperature can change the characteristics of materials. Some materials (like plastic) become softer, meaning that you may accelerate time to failure in an essentially uncontrolled way.
So why do we have the ‘common slope assumption?’ This should be more of a rule. And not an assumption. The slopes of the times to failure for systems exposed to different stresses should be the same. If they are not … then you almost certainly have different mechanisms being created by the increased stress. Unfortunately, many software packages and reliability engineers (neither of which are good at critical thinking) simply assume this to be the case. Which often leads to disastrous conclusions!
What can go wrong? You might drastically underestimate reliability, and go back to the drawing board and waste time and money in creating a more robust design for no reason. Alternately, you can drastically overestimate reliability and launch a product that will very quickly fail in the hands of your users and customers.
Takeaway? Always CHECK assumptions. Including common slopes for accelerated times to failure.

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