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Understanding Fatigue Failures

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Dave Palmer
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Re: Mastering the art of designing for fatigue
Dave Palmer   4/9/2012 11:40:20 AM
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@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.

Beth Stackpole
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Re: Mastering the art of designing for fatigue
Beth Stackpole   4/9/2012 11:12:48 AM
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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?!!!

Dave Palmer
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Re: Mastering the art of designing for fatigue
Dave Palmer   4/9/2012 11:06:02 AM
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@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.

A lot of fatigue data is available from ASM International; so much, in fact, that it may sometimes be difficult to find what you want.  We have a copy of their Atlas of Fatigue Curves, but, frustratingly, it almost never seems to have the data I'm looking for.  Right now they have a bargain-basement sale on a fatigue data book for light structural alloys which I might order.

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.

Beth Stackpole
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Mastering the art of designing for fatigue
Beth Stackpole   4/9/2012 8:23:52 AM
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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?

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