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William K.
User Rank
Platinum
Understanding overload failures.
William K.   3/30/2012 9:50:56 PM
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Dave Palmer certainly got it right in the comments about nonlinear stress analysis. And I would point out that vibration is often an unanticipated mechanism for the excess stress that starts those cracks. In addition, the vibration often leads to the fatigue failure that starts the process. Of course, the amplitude of the vibration is much greater in cases where there is resonance. So now there is a whole list of things to beware of.

williamlweaver
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
williamlweaver   3/29/2012 7:41:08 PM
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Hey @Dave Palmer, thanks for an awesome post! It's been a while since I've left my industry position, but all of the terms came flooding back. My development team was tasked with creating measurement techniques to complement / verify the FEA. We developed Temperature-sensitive Paint to measure and track propagating fatigue cracks while the part was under test. We developed Pressure-sensitive paint to measure aerodynamics and surface stress distributions. One of the most interesting projects was developing Strain-sensitive Paint for Ford Visteon to visualize whole-body strain-distribution on truck axles (now commercially known as Strain-Sensitive Skin, S3). All of that development was done in the late 1990's. I'm not sure how popular the techniques are now...

Dave Palmer
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Dave Palmer   3/27/2012 10:01:32 AM
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@Chuck: Usually, in mechanical engineering, the term "working stress" simply means the stress which a component experiences under working loads.  Typically, engineers and designers try to keep the working stress below the yield strength, divided by an appropriate factor of safety. (The yield strength divided by the factor of safety is sometimes called the "allowable stress" or the "design stress").

Charles Murray
User Rank
Blogger
Re: Migrating stress cracks versus quick breaks
Charles Murray   3/26/2012 8:05:52 PM
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Thanks, Dave. I brought the topic of yield strength because I seem to remember something from my distant past called, "working stress" design. I believe working stress was not based on yield strength, but I'm really not sure.

Dave Palmer
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Dave Palmer   3/26/2012 12:48:01 PM
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@bentarrow: I assume you're using a linear FEA package.  Nonlinear FEA would give you a much more accurate picture of localized yielding and load redistribution.  Otherwise, you may just want to point out to your customers that the high stresses at the transition are a result of the assumptions of the FEA model.  This may or may not be a satisfying answer to them, but it's true.

If you can do actual physical testing, this might also help to convince them.  You could look into using a product like Stresscoat to measure the actual tensile strains, and compare this to the results of your FEA model.  If the actual part is too big to test in the lab, you might be able to use a scale model; FEA can help you understand how to appropriately scale the loads so that you get the correct stresses. 

Dave Palmer
User Rank
Platinum
Re: Things break.
Dave Palmer   3/26/2012 12:24:49 PM
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@TJ McDermott: The all-important question is, "Where is the load coming from?" As you point out, responding to failures by simply beefing up whichever part happens to break often just results in other parts breaking.  At the end of the day, it results in assemblies which are extremely robust, but which are also unnecessarily heavy and expensive.

There's no substitute for a solid engineering analysis of the mechanical system as a whole.  If you understand where the load is coming from, you can address the source of the problem, rather than constantly beefing up parts to compensate.

As I said before, the part which breaks is often an innocent bystander.  Don't blame the part for the inadequacies of the design!

bentarrow
User Rank
Iron
Re: Migrating stress cracks versus quick breaks
bentarrow   3/26/2012 11:26:03 AM
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Dave,

       The subject of "getting the red out" is my current challenge as my design department is just now using FEA. I have had other FEA users at other company locations run FEA on parts in the past & early-on found this issue on linear static analysis of steel forgings. The loading is static and the cross sections are "L shaped" so that there is a bending stress at the inside transion from vertical to horizontal. I've found no amount of thickening the cross section ever completely eliminates the red (below yield). I have seen a report from a P.E. on a similar part where the P.E. concluded that "slight yielding in this area relieves the high stress concentration and then distributes the load more evenly accross the cross section of the part" - or words to that effect.

      That's all well & good but my issue is how to justify the "remaining red" in a report that will be reviewed by other engineers who are not M.E.'s (Petroleum Engineers) - and who are the customer in this case. Any brief suggestions and or recommended literature on this subject would be most welcome.

Thanks,

Leslie

ChasChas
User Rank
Platinum
Re: Things break.
ChasChas   3/26/2012 11:10:13 AM
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Yes, very intersting TJ - read a classic engineering poem:

http://www.legallanguage.com/resources/poems/onehossshay/

TJ McDermott
User Rank
Blogger
Re: Things break.
TJ McDermott   3/26/2012 10:49:20 AM
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It's interesting to watch what happens when a part is redesigned, "beefed up" because it's been breaking in the field.  If the redesign works correctly, the part is no longer the "weakest link", and something else now is.

The usual progression is a series of parts end up being redesigned, one at a time, as each becomes the weakest link in turn.

ChasChas
User Rank
Platinum
Things break.
ChasChas   3/26/2012 10:31:31 AM
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Great primer on overload.

My experience has been that operators have the unique ability to find every unintented use of a piece of machinery - causing real eningeering challanges when it comes time to find out what REALLY went wrong.

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