Fatigue Calculation - equivalent stress
Fatigue Calculation - equivalent stress
(OP)
I need to estimate the mean life of a cross section that has a very significant stress concentration (approx 10X average stress) The loading in the section cycles from 25 to 100% and I expect it to fail around 100,000 cycles so infinite life does not apply.
What is the correct approach to calculate the equivalent fully reversing stress for the load? Do I just use the average stress in the cross section to get mean and amplitude values for Goodman? But the stress concentration messes up the endurance limit in Goodmans equation since it needs to be reduced for short life (which is unknown). Or do you need to iterate the life until an answer is found?
I've looked through several textbooks but havent found one that clearly estimates finite life with variable loading, only fully reversed loading. If you can recommend a good book for explaining this, or can it explain this it would be appreciated.
Thanks
What is the correct approach to calculate the equivalent fully reversing stress for the load? Do I just use the average stress in the cross section to get mean and amplitude values for Goodman? But the stress concentration messes up the endurance limit in Goodmans equation since it needs to be reduced for short life (which is unknown). Or do you need to iterate the life until an answer is found?
I've looked through several textbooks but havent found one that clearly estimates finite life with variable loading, only fully reversed loading. If you can recommend a good book for explaining this, or can it explain this it would be appreciated.
Thanks





RE: Fatigue Calculation - equivalent stress
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it might help.
desertfox
RE: Fatigue Calculation - equivalent stress
"But the stress concentration messes up the endurance limit in Goodmans equation since it needs to be reduced for short life (which is unknown). Or do you need to iterate the life until an answer is found?"
I don't really understand what you are on about here... can you expand please
you need to perform a rainflow analysis and sum the damage using miners rule
Regards
www.priamengineering.co.uk
RE: Fatigue Calculation - equivalent stress
Sounds like the rainflow method is what I need to do.
I was using the Marin equation from Mark's to determine the endurance limit. It states the following about stress concentration reductions - this really confuses me for finite life prediction:
Note that the miscellaneous effects factor (ke) for stress concentration applies to the endurance limit (Se) at (N =10^6) and greater. However, below (N =10^3) cycles it has no effect, meaning (Kf = 1) or (ke = 1). Similar to the process for finite life, between(N =10^3) and (N =106) cycles define a modified stress concentration factor (Kf ) where K f = aN^b and the coefficients (a) and (b), both dimensionless, are given in Eq. (7.18) as
a = 1/Kf and b = −1/3log(1/Kf)
where the reduced stress concentration factor (Kf ) is found from Eq. (6.23).
Thanks
RE: Fatigue Calculation - equivalent stress
I could however find no reference to the application of the strength reduction being different in the finite and infinite stress ranges.
As you have performed a finite element simulation with the geometry of the concentration accurately captured (I assume) and your increased stress in this area can be used instead of modifying your endurance strength. I am of course assuming that the stress concentration is NOT due to a weld (which is a totally different kettle of fish).
If you nee more help can you point me to the chapter in marks that you took the marin formula from?
Regards
www.priamengineering.co.uk
RE: Fatigue Calculation - equivalent stress
i think you've got other complex matters to ponder ... presumably at 100% load there is plasticity around the stress concentration. what's the material ? (ductile ??)
it's pretty common (in my world) to work with spectra that aren't fully reversed, usually with Smin = 0. look into "stress ratio". what fatigue data are you using ??
RE: Fatigue Calculation - equivalent stress
We are going to cycle test a bunch of these and would like to have a calculated life going in. We have some initial results on life of the part, but not enough of a sample size to be sure.
I was using Mark's Calculations for Machine Design chapter 7.3 - Marin Equation to determine the endurance limit. I am going to get my hands on Shigley as it appears to be a more commonly used reference, hopefully more info there.
Any recommendations on the approach? Or is this something that is difficult to calculate with really poor correlation anyway? Any other recommend books would be appreciated.
Thanks for the inputs.
RE: Fatigue Calculation - equivalent stress
www.priamengineering.co.uk
RE: Fatigue Calculation - equivalent stress
RE: Fatigue Calculation - equivalent stress
ok, it's a forging, but what material .. steel ? high ductility ?? (or high strength ?)
if you're doing FEA (like the forum name suggests) then do a NL run to investigate the scope of plasticity.
i think your problem is more an ultimate strength issue than a fatigue issue ... if you've got that much yielding, you probably don't have much life.
RE: Fatigue Calculation - equivalent stress
All "fatigue" fractures in the real world do not emmanate from notches. In classical fatigue analysis, if a number is desired instead of the infinity symbol, you pick the life you want to arrive at, then back-track to the notch or stress concentration factor that will give you that number. Sounds bad, and it is.
In either classical fatigue analysis or fracture mechanics, if you test a "dogbone" coupon started with the largest crack (crack not notch) that can be missed in inspection, you can obtain empirical data that can be used to emmulate crack growth or fatigue analytically.
In other words, Goodman, Oswald, S-n chart types of analysis are for college courses, not professional engineering. It took me three years to begin to understand my specialty of fracture/fatigue.
If you really want to look at fast numbers, just look up "fracture toughness (KIc)," and at least you'll be able to easily estimate what the critical crack size (crack length at failure) for your highest stress location. You don't really want to see what a professional has to do to estimate the inspection interval should be for a part unless that will be your only job.
RE: Fatigue Calculation - equivalent stress
For the question I had with Marks adjusting the stress concentration - it should not be done as others stated here, because the equation takes it's effect to zero for you at the low/high cycle limit. I think Marks just put it in there to explain the approach they are taking, not as an adjustment you need to make...a little confusing I must say...Shigleys and Juvinall were much more clear.
Thanks!