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Why the Goodman Diagram Is Not So Good

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Dave Palmer
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Platinum
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
User Rank
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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