There are two basic answers that can always, or almost always, apply to anything that “failed.” These two basic answers are almost always true.

Why did it fail? Because somebody made a mistake!

This answer, I believe, is almost always true. I didn’t say these two answers would be immediately useful, and they are not. But it doesn’t take much to turn them into useful answers. This first one we can do by clarifying that a mistake was made by someone during the design, manufacturing, use or maintenance operations. Potentially more than one of these functions had problems, and multiple errors were made. Then the task is figuring out “where” and “when” the problem occurred and “what” it was (Fig. 1).

Of course, it’s also possible that no errors were made, but something really unusual and damaging happened. We might call it an Act of God, in legal terms, or we might blame Mother Nature. Many of us, in our hearts, blame Mother Nature even for our human failings.

Let’s look at an example. The recent earthquakes in Turkey were very strong. Maybe some older buildings would have collapsed because of the extreme stresses introduced into the structures by the sudden movement of the earth. However, many new buildings that collapsed were supposed to have been resistant to earthquakes of this magnitude (7.8 Richter scale). They were not, however, because politicians gave waivers that allowed contractors to build new structures without going to the trouble of meeting the codes. One building was even said to have had existing structural columns removed to make room for more parking. So even when there is an act of God, there are often, if not usually, mitigating circumstances. In fact, it is the extremely rare event that has a single cause.

Fig. 1. Four main areas where mistakes are made

Going back to Fig. 1, we see four main areas where mistakes are made. At the top, we have design. It’s amazing how many mistakes are still made today that were known to be mistakes for decades before they were made. How many components are made with corners sharper than they need to be? This can facilitate fatigue cracks, among other problems. How many marine components are specified to be made of 7000 series aluminum alloys that are susceptible to stress corrosion cracking with exposure to salt water?

The next category is manufacturing. Errors in manufacturing can result in discontinuities in the material, low strength levels, dimensions not as specified, grinding burn, coatings applied improperly and inadequate inspection methods.

Service or use errors happen all the time. The part gets loaded in a way that it was never designed to resist. Someone heats the assembly too hot and damages a component. Parts may be overheated to a less extreme degree, where they lose strength (perhaps over years) and eventually rupture. This happens in power plants. High-temperature components have a finite life. However, these types of fractures can be very dangerous. Therefore, management is supposed to inspect regularly and plan replacement before failure is imminent.

Finally, maintenance errors happen when the oil wasn’t changed properly, the timing chain was adjusted wrong, a control setting was unintentionally changed, a tool was left in the machine, and/or a lazy worker pretended to do a task and did not. I once investigated a $5 component that was inspected and deemed OK when it wasn’t. The poor boat owner later had thousands of dollars of damage when the degraded rubber seal leaked.

The second possible answer to the headline question is “because the stress exceeded the strength.” I first developed this hopefully attention-getting and pithy response for the question “Why did it break?” A little bit of metaphorical thinking can show us that stress can refer to extreme environmental conditions that facilitate corrosion or wear, as well as deformation or fracture. All physical components are subject to stresses over the course of their existence. For some parts, the greatest stress is during its manufacture. For others, it’s during one of the other lifecycle stages previously mentioned. If we are considering cradle-to-grave design, it might happen during the recycling process.

I would like to remind the reader of two things:

• Materials do not fail. They respond to their environments. Failure is always in the human realm. Components and assemblies made from materials experience damage, not failure.
• Failures rarely happen because the part did not meet the specification. If you want to argue with me about this, ask yourself if the part knows the spec. Most people are willing to acknowledge that the answer is no.

Next time, in September’s issue, we’ll delve more deeply into a methodology for turning “because the stress exceeded the strength” into a more widely useful answer.

Debbie Aliya is the owner and president of Aliya Analytical, Inc. in Grand Rapids, Mich., and specializes in failure analysis and prevention. She has a BS in Metallurgy and Materials Science from Carnegie Mellon University and an MS in Materials Science and Engineering from Northwestern University. She is also a member of ASM and SAE. Graphics provided by the author.