Datasheets for most materials list a fatigue strength. Depending on the material, this represents either the level of stress below which fatigue does not occur (sometimes called the endurance limit), or the level of stress at which fatigue failure occurs in a given, usually large, number of cycles.
Fatigue strengths listed on datasheets are usually determined by means of fully reversed loading. This means that, during the test, the stress varies from a certain value in tension to an equal and opposite value in compression. This is the how the stress varies in a rotating shaft under a bending load, for example. However, it may not be the way the stress varies in your application. For instance, in your application, the load may repeatedly go from zero to a given value. Or your part may have a certain static tensile load, plus a vibration, which causes the load to oscillate around this value.
In these cases, you should not simply compare the maximum stress in the part to the fatigue strength. Remember, fatigue is all about cyclic stresses -- stresses that vary over time. If the stress doesn't fluctuate, fatigue can't happen. So in addition to the maximum stress, you also need to know the minimum stress at the same location; that is, the range of stresses in a given location over time. Another way of approaching this is to consider an average or steady stress, and an alternating stress, which is the amount by which the stress varies around this value over time.
There are a number of different equations for converting maximum and minimum stress values (or average and alternating stress values) to an equivalent fully reversed stress. The fact that there are so many different equations suggests that none of the equations works perfectly all the time; after all, if one of the equations worked, everyone would simply use that one. You can find these equations in any mechanical engineering textbook. One of the simplest and best known is the Goodman equation. However, I've found that, in most cases, the Walker equation is the most accurate.
As you can see, designing for fatigue is not as simple as designing for overload. This probably helps to explain why fatigue failures are so much more common than overload failures. However, observing these basic principles can help to avoid fatigue failures.
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.
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.
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.
If the Made for Monkeys column is any indication, there are a ton of folks out there that are well willing to juryrig products or trouble shoot issues just to get a longer product life span. Interesting, often those older products have failure points that don't necessary have to do with fatigue of parts, but rather quirky design choices that lead to issues.
@kenish: You are absolutely right; any interruption to the surface is a potential site for initiation of a fatigue crack. This might mean nicks or dings which occur either during manufacturing or during use.
Fatigue testing is usually done on highly polished samples. Broadly speaking, the rougher the surface, the lower the effective fatigue strength. Surface finish is a very important parameter, which unfortunately is very difficult to incorporate into a FEA model.
@Ann: I think it would be great if there were more cooperation between companies with this type of data. There are already some industry efforts in this direction. The American Iron and Steel Institute's Bar Steel Fatigue Database, which I linked to before, is a good example. The American Foundry Society has a database of fatigue properties for cast irons. USCAR and the Department of Energy have developed, or are developing, a similar but much larger database for light metals (aluminum, magnesium, etc.) -- I don't know much about this database, but I'd like to. Ultimately, assembling these databases is just one part of an much more ambitious project called integrated computational materials engineering.
However, in order for this to work, companies have to shed their old mentality of holding this kind of information closely. Industry associations and government can help to facilitate this.
Ncode/Somat has a good fatigue calculator, designed by Daryll Socie from the University of Illinois, Champaign. He also offers seminars/short courses for engineers who need to know more about designing for fatigue.
Using the NCode/Somat software to perform the calculations works fine, but you still have to know all of the properties of the materials in each situation to plug into the software in order to get accurate results.
If "overdesign" is the term for a design that includes only just barely enough to last until the warranty expires, possibly, then it is a very good attribute. The reality is that many products are sometimes used beyond the "typical" levels, and so they do need to be stronger than only enough to handle "typical". Designing only to the lower boundry of typical is why such a large portion of consumer goods are trash at the very instant that they are made.
Fatigue failure is indeed a whole lot more subtle than the other kinds, such as yield and wear failure, but it is avery important consideration in a lot of places. The article was both useful and needed.
Dave, great article. I'm a EE and this helped me visualize fatigue in a different way. Even I understood it! You mention design, manufacturing, and materials as sources of fatigue cracks. Aren't they also important to prevent minor damage in-service from turning into a fatigue site (along with periodic inspection of critical areas...I'm thinking about aircraft skins)?
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