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Understanding Fatigue Failures

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Ann R. Thryft
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Re: Mastering the art of designing for fatigue
Ann R. Thryft   4/10/2012 1:20:32 PM
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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?


Dave Palmer
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Re: fatigue failure
Dave Palmer   4/10/2012 12:15:20 PM
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@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.

ChasChas
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Re: fatigue failure
ChasChas   4/10/2012 11:02:06 AM
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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.

Great article by Dave.

 

naperlou
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Re: fatigue failure
naperlou   4/10/2012 9:57:05 AM
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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).

vimalkumarp
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fatigue failure
vimalkumarp   4/10/2012 12:11:14 AM
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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

vimalkumarp
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Understanding Fatigue Failures
vimalkumarp   4/9/2012 11:55:48 PM
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your point is excellent that lack of data is leading to overdesigning.

TJ McDermott
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Re: Mastering the art of designing for fatigue
TJ McDermott   4/9/2012 4:25:06 PM
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Lack of good data also means things get overdesigned (hopefully in the right spots).

Dave Palmer
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Re: Mastering the art of designing for fatigue
Dave Palmer   4/9/2012 3:56:28 PM
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@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.

TJ McDermott
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Re: Mastering the art of designing for fatigue
TJ McDermott   4/9/2012 1:10:15 PM
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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 R. Thryft
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Re: Mastering the art of designing for fatigue
Ann R. Thryft   4/9/2012 1:01:23 PM
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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?


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