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.
Really great explanation on the basic principles that cause fatigue failures--one which even non-engineers like myself can understand. Beyond the key equations you provided at the end of your piece, are there other sources of information about materials properties or specific design tools (perhaps CAE software) that can help engineers with this design challenge?
@Beth: There are a number of CAE tools for fatigue analysis. I don't have direct experience with any of them, but some I've heard of include fe-safe, MSC Fatigue, and nCode. I'm sure there are others. Some companies have their own in-house fatigue codes.
As far as sources for fatigue data, the American Iron and Steel Institute has an excellent, free, on-line database of fatigue data for a wide variety of steels. (Registration is required). Of course, it's important to understand what you're looking at. Not only do you need to find the data for the correct steel; it also needs to be the correct condition. I've seen engineers use data for a given steel in the annealed (soft) condition when the actual part is hardened, and vice versa.
Often, though, it's worthwhile to do your own testing rather than relying on published data. The main drawback of this is that a solid fatigue testing program can take months to complete. For example, at 30 Hz -- which is pretty fast for a hydraulic load frame -- a single 20 million cycle test will take over a week, and you need to perform several such tests in order to have statistically valuable data. The advantage, though, is that you can test the alloys you actually use, in the conditions you actually use them.
That extra info will be extremely helpful to the community. Thanks so much for providing it, Dave. Your point about having to do your own testing rather than rely on third-party material stuck me, however. Given that so many companies across industries are dealing with time-to-market pressures and there are so many aspects of a product that need vertification and testing, how do you prioritize what's required for fatigue testing, vs. say what's necessary for structural testing or something else? Can development teams really afford to take the time to do this, but I guess the bigger question is can they afford to not?!!!
@Beth: We try to do the best we can using published data, while at the same time accumulating our own body of test data about the materials we use most often. This way we don't hold up the development cycle waiting for test results; we just constantly fill in the gaps in our knowledge. I suspect that this is what a lot of other companies do, too.
We also try to avoid component-level fatigue testing where possible. For example, if you understand how die-cast 380 aluminum behaves in fatigue (and the effect of casting porosity on the properties), you can apply that knowledge to any die-cast 380 component; you don't have to repeat the test for each part. But for some components, like crankshafts, component-level testing is a must.
Thanks, Dave, for such a complete intro to fatigue failures. I also find it especially interesting to read about all the CAE tools for fatigue analysis. Beth's second question and your response is also intriguing. It sounds like there's a need for more centralized fatigue databases of materials and/or parts made with them. Each company doing all this on its own and building up its own database seems awfully wasteful of time and energy. Is this info just too hard to centralize and keep updated?
Dave, loved the article, and this response. I think you should qualify the use of software further though. You pointed out you need to use the right material, in the right condition. Anyone who reaches for fatigue software is likely NOT to find the correct material for their analysis at one point or another. Using a similar material as "close enough" is also likely to lead to erroneous and dangerous results.
@Ann: I think TJ's response sums up why it's often advantageous for companies to build up their own internal databases of fatigue data: "close enough" often isn't.
Dave, I understood what you meant about wanting to have your own internal database and why. What I was trying to find out was, at a broader view, isn't it more or less redundant with everyone else's internal database, and why can't all of this data be maintained in (one or more) centralized repositories, which might be accessible to the software tools? Perhaps the answer is there's too much data, or perhaps the answer is it's too product-specific to a manufacturer's/service provider's own products/services. Is it one of these or something else?
@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.
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.
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.
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)?
@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.
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.
McDonough and Braungart explain this point of things getting overdesigned in their cradle to cradle design concept. The lack of data leading to overdesigning is really a point worth noting
Many systems are over designed. It is just difficult to predict what aspects will be critical in actual applications. It is amazing the products and systems that last well beyond their design life (while others don't make it).
Over-design is a good thing if cost and weight is not a deciding factor. Things that are impossible to inspect after being built must be "over-designed" or designed for no post inspection.
@vimalkumarp: Thanks for the reference to McDonough and Braungart's Cradle to Cradle. It looks like an interesting book. I will try to find it.
Just to illustrate the point about how the lack of information affects design, right now I'm working on a die cast aluminum part which has an electroless nickel coating and operates at high temperatures (400 - 500°F).
We know something about how the die cast aluminum alloy behaves in fatigue at room temperature, but we don't know much about how it behaves at elevated temperatures. We know the fatigue strength will be lower, but by how much? We also don't know much about how the electroless nickel coating affects the fatigue strength. Again, we expect that the coating will reduce the fatigue strength, but we're not sure by how much.
Because we don't have the capability to do high temperature fatigue testing in-house, we might not fully answer these questions in the course of this project. Instead, we will have to make educated, conservative assumptions which will probably result in the component being somewhat overdesigned. As resources (hopefully!) become available in the future, we will try to do further testing to fill in the gaps in our knowlege.
By the way, it's very important to consider the effect of coatings on the mechanical properties of a material. In general, coatings which are more brittle than the substrate tend to reduce ductility, impact strength, and fatigue life. This is particularly true if the coatings apply tensile residual stresses to the substrate. These principles apply not only to plating of metals, but also painting of plastics. (For plastics, solvent attack on the substrate is another concern; in metals, the parallel to this is hydrogen embrittlement). You should never assume that you can apply a coating to a material without affecting its mechanical behavior.
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.
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.
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