Without a doubt, the most common failure mode I see in my lab is fatigue. Unlike overload failures, which occur more or less immediately when a load is applied, fatigue failures are progressive in nature. In other words, they develop over time as a result of repeated loads. If you've ever bent a paperclip back and forth until it snapped, you've seen fatigue at work.
The word fatigue implies that the material is somehow getting tired, or that its properties are becoming exhausted by repeated application of loads. Of course, this is not actually the case. If you want to convince yourself of this, you can cut a small sample from of a part that failed in fatigue, and perform mechanical tests on the sample. In almost all cases, you will find that the strength is the same as it was before the part was put into service. This demonstrates that fatigue is not a matter of the material losing its strength or degrading over time. So why does fatigue occur?
Fatigue failures start with the formation of a small crack, which grows with repeated loads. Once the crack gets big enough, the part breaks.
Crack growth can be understood as an energy balance. When you apply stress to a part, you increase its strain energy. You can think of this as the energy stored in a stretched spring. Now let's think about a part with a crack in it. The surface of the crack has a surface energy associated with it. Making the crack larger increases its surface energy. So a part that has a crack in it can lower its strain energy by increasing the size of the crack by converting strain energy into surface energy. This is why a crack can grow under repeated loads. (Incidentally, it also explains why you can cut glass by scribing a line in it with a glass cutter, then bending it.)
But what causes the initial small crack to form, especially if the overall stress is low? Most parts contain stress risers, where the local stress may be significantly higher than the overall stress. Cracks initiate at these stress concentrations.
Some stress risers are geometric in nature, and can be minimized by good design. These include sharp corners, notches, and holes, among others. Other stress risers may come from manufacturing processes, and can be minimized by good manufacturing practices. These include casting porosity and other kinds of defects. Still, other stress risers may be internal to the material itself. For example, most steels contain nonmetallic inclusions, which act as internal stress risers. Bearing quality and aircraft quality steels are produced using special steelmaking techniques to minimize their inclusion content.
Dave, thanks for that input. The attitude is one that is, unfortunately, prevalent in several industries. Glad to hear that there are movements to open up things more in materials engineering. Industry associations can play a big part in such efforts.
Thanks for highlighting the fatigue failure mechanism in an new way. Interestingly enough the same design approaches that reduce stress risers for fatigue failure generally also improve the performance of a part in heavily loaded, non-cyclical stress situations. Good design practice often yields multiple performance benefits.
Yes, we work with ultrasonically driven dental tool manufacturing. You can quickly reach a fatigue failure at 30KHz if you have a surface finish that can initiate crack propagation. Excellent article.
I used to get frustrated by friends in my car hobbies discussing bolt failures of rperformance cars. Classic examples were the failures of the Corvette suspension bolts at the ends of the leaf springs. In many cases these were fatigue failures and the friends reactions were to the effect that "these are grade 8 bolts with very high tensile strength, you cannot get better than that!" Often these high strength bolts failed from brittle crack propagation from fatigue not from gross overstress. Salt pitting corrosion was often the culprit. The strength was less importent than corrosion resistance and anodic zinc plating. Sometimes aircraft hardware with controlled geometry under the head, good thread root shape, and good hole preparation was also helpful.
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