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Why the Goodman Diagram Is Not So Good

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Dave Palmer
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Re: GOODMAN DIAGRAMS
Dave Palmer   6/18/2014 8:00:22 PM
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@bobjengr: Thanks for your comments.  There are multiple CAE programs for fatigue analysis; most of them give you a choice between several different mean stress correction equations (Goodman, Gerber, Soderberg, SWT, Walker, etc.).

The fact that there are so many different equations to choose from should give you a clue that none of them works perfectly all the time.  Based on my research, the Walker equation is the best overall... but this is partially because the γ exponent is a "fudge factor" that allows you to fit the equation to your data.  So it's kind of a cheat.

On the other hand, the fact that γ has different values for different materials (i.e. γ=0.45 for aluminum, γ=0.65 for steel) suggests that mean stress sensitivity is a material property.  And if we actually understood why different materials have different sensitivities to mean stress, maybe "which equation to use" wouldn't be such a mystery.

I haven't decided if I'm going to pursue a PhD or not -- but if I do, it will be about this.

bobjengr
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Platinum
GOODMAN DIAGRAMS
bobjengr   6/13/2014 5:51:22 PM
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Excellent post Dave.  Boy do I remember those days.  I majored in mechanical engineering with concentration on machine design.  I remember one professor (Dr. Bob Maxwell) giving us explanation on how to use and plot the Goodman Diagram.  Somewhat cryptic at that time.  I certainly did not know the results might be 3 to 4 times the actual value.  The best part of your write-up is providing alternatives to the Goodman.  That's worth the read alone.  I imagine right now there are computer programs that will allow "plug and play" to do the actual calculation.  

Dave Palmer
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Platinum
Re: Correction
Dave Palmer   5/30/2014 10:53:54 AM
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Thanks, Jenn!

By the way, if anyone is interested in Professor Dowling's paper, you can find it here.  Unfortunately, it's not a free download.

Also, if you're interested in my work on fatigue of cast aluminum, check out my slide presentation here.  (This was my masters' thesis, by the way).

Jennifer Campbell
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Gold
Re: Correction
Jennifer Campbell   5/30/2014 8:11:23 AM
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The equation on the second page has been corrected.

Dave Palmer
User Rank
Platinum
Correction
Dave Palmer   5/30/2014 6:24:08 AM
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There's a typo in the equation on the second page.  It should read:

 
σeq = σaγa + σm) 1 - γ

 

Hopefully, Design News staff will be able to go in and fix this later this morning.

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