When a part breaks unexpectedly, it usually sets off a flurry of activity. Often, a team is formed and charged with finding the root cause of the failure. In my career, I have led or been a member of many failure analysis teams. Based on my experience, there are five mistakes that engineers often make when investigating failures. (I know because I've made them all myself.) Recognizing these mistakes can help you avoid them.
1. Looking at a part in isolation. As a materials engineer, people often send me a broken part in a cardboard box and ask, "Why did it break?" Of course, if the part had been in a cardboard box the whole time, it's unlikely that it would have broken.
Although there are many things that can be learned by inspecting a failed part, the part itself rarely tells the whole story. It's important to understand the mechanical system to which the part belongs and the part's role in that system. What loads does the part feel? Where do the loads come from? What might cause the loads to be higher or lower? It's also important to understand the environment in which the mechanical system operates. In what ways might the operating environment differ from what was anticipated in design?
Remember, no part ever fails by itself. It fails as a part of a given mechanical system under a given set of environmental conditions.
2. Focusing on conformance to specifications rather than root cause. When a part breaks, one of the first questions asked is, "Did it meet the print?" Of course, it's important to establish whether or not the part conformed with the design requirements.
But finding a non-conformance is not the same as finding the root cause of the failure. If a part is found to be defective, it's important to understand what role (if any) the defect played in the failure. In some cases, a defect may be a red herring, which leads you away from the root cause. In other cases, early failure of a defective part may reveal an underlying problem, which would also cause non-defective parts to fail over a longer period of time.
There's another reason not to focus excessively on the issue of conformance or non-conformance: It can result in a confrontational relationship with parts suppliers. This can undermine cooperation, which may be necessary for the success of the investigation. Failure analysis should always be about solving problems, not assigning blame. Of course, this is not to say that suppliers shouldn't correct non-conformances, or that there shouldn't be consequences for suppliers who consistently provide non-conforming parts. But this should be separate from the work of the failure analysis team.