Very high cycle fatigue vs. endurance limit
Very high cycle fatigue vs. endurance limit
(OP)
We build a fair number of horizontal shafts that operate with an overhung load. Materials are 1018 C/S, 304 S/S, and 316 S/S with a well radiused ground finish in critical areas. Total shaft life is required to be 1,000,000,000 to 5 billion cycles.
Does endurance limit still apply here? Do you have real-life experience of shafts designed to endurance limit and operating in "corrosion-free" conditions lasting this long?
I've looked for academic information on these situations but so far I've not found much.
I have read that endurance limit can be interpreted as the limit where stress intensity at the crack tip is not sufficient to cause crack propagation. In that regard it's plausible that endurance limit can work for VHCF situations if the material has been shown to have an endurance limit.
Thanks,
Dave
Does endurance limit still apply here? Do you have real-life experience of shafts designed to endurance limit and operating in "corrosion-free" conditions lasting this long?
I've looked for academic information on these situations but so far I've not found much.
I have read that endurance limit can be interpreted as the limit where stress intensity at the crack tip is not sufficient to cause crack propagation. In that regard it's plausible that endurance limit can work for VHCF situations if the material has been shown to have an endurance limit.
Thanks,
Dave





RE: Very high cycle fatigue vs. endurance limit
what is your loading like ... you say an over-hanging load, but does the shaft rotate under this ? (if so easy to build up cycles, spinning the shaft very quickly). or does the over-hanging load (magnitude) cycle ?? (harder to cycle quickly).
you can get some mileage out of the damage tolerance route .. you want to show that a typical flaw (0.005" radius surface flaw) is below the threshold stress intensity ... possibly less well documented than S/N data !
not much help i'm afraid ... good luck !
RE: Very high cycle fatigue vs. endurance limit
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Very high cycle fatigue vs. endurance limit
https://efatigue.com/
GregLocock,
I've heard the same thing from a couple of sources. One was my boss in early 1950's who had worked at Wright Patterson and the other was a Metallurgist from Bethlehem Steel when he was discussing 4140 steel shafts.
RE: Very high cycle fatigue vs. endurance limit
RE: Very high cycle fatigue vs. endurance limit
1018 mild steel and 304/316 cres should theoretically have some fatigue endurance limits. But establishing what those limits are might be tough.
Values like endurance limit and fatigue life for a particular material and structure can vary widely based on many factors. In the end, fatigue life mostly boils down to a statistical value that a certain percentage of representative sample parts of a consistent material, heat treat, manufacturing process, and load life cycle have been tested to.
Establishing S-N values for structural materials requires lots of very time consuming and expensive testing. There is data available for high performance structural metals in texts like MIL-HDBK-5, but I doubt you will find data for 1018 or 304/316 in there.
To be honest, once you apply the appropriate knock-down factors for things like R,Kt and combined stresses, and then analyze for something like 10^9 cycles, you'll likely find that the allowable stress for your chosen materials is very low. Probably less than 5 to 10KSI at best.
Good luck.
Terry
RE: Very high cycle fatigue vs. endurance limit
just some points to ponder ...
RE: Very high cycle fatigue vs. endurance limit
I'm hoping simply to find the state of the art in published information to better apply to our practices. We're currently using a modified s-n approach that does not recognize an endurance limit but does apply a single data point for a very high cycle test that effectively lowers the allowable stress below the published endurance limits. We take into account the various form, surface, stress concentration, etc factors. I'm concerned that the one data point we have for low load / high cycle fatigue is not representative. Our shafts are designed and finished for maximum fatigue life and the specimen in the data point was not.
I had a look at eFatigue and saw nothing other than classic fatigue theory for < 10^6 cycles and endurance limit for above that. Perhaps there's more under the hood of their calculation engine but nothing on the site suggests that's the case.
There must be some other data out there whether it's published or proprietary. I will try to dig deeper into MIL and NASA publications.
David
RE: Very high cycle fatigue vs. endurance limit
you're doing analysis after manufacture ... a common enough occurrence, and now you're trying to "show it good".
but you say "published endurance limits", which sounds like you have the answer you were looking for ??
RE: Very high cycle fatigue vs. endurance limit
Finding data for 1018, 304, and 316 in the gigacycle fatigue region will be difficult. For some general background, take a look at this previous Eng-Tips thread and a presentation from Prof. Socie, one of the foremost experts on fatigue:
thread2-242798: Fatigue limit rules of thumb
http
For the most contemporary information on this subject, I suggest reviewing articles published in the International Journal of Fatigue. The following Google Scholar keyword searches should be useful, and perhaps you can obtain the actual articles from a nearby University library:
http://sch
http://s
RE: Very high cycle fatigue vs. endurance limit
rb1957: yes, we've been manufacturing shafts in this configuration for some years. There is enough conservatism in the calculations and years of experience to know we can stay where we are. We're constantly designing new designs and/or new materials, so a better method is valuable and I'm looking to improve upon it.
Currently we do not use endurance limit. At 10^6 cycles, the endurance limit is where the s-n curve goes horizontal. We have one unofficial data point of some relevance that's in the gigacycle range and we use this point to form a curve on the s-n plot between the 10^6 point and through this data point. Therefore as cycles increase above 10^6 cycles, our allowable stresses decrease. I'm interested in other very high cycle fatigue data or a better calculation method if such a thing exists.
RE: Very high cycle fatigue vs. endurance limit
so you've got something that works and you want to make it better. a simple literature search might give you some ideas (though it appears it doesn't) but i doubt it'd be directly applicable to your specific application. if you can't afford testing on your own, why not try to interest a nearby university ? ... this sounds somewhat esoteric, = interesting to academia ??
RE: Very high cycle fatigue vs. endurance limit
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Very high cycle fatigue vs. endurance limit
RE: Very high cycle fatigue vs. endurance limit
Fatigue analysis is based on the Soderberg, Goodman, or
modified Goodman criteria.
NASA Handbook on Shaft Design
http://nt
Information on Designing for Fatigue.
ht
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RE: Very high cycle fatigue vs. endurance limit
Your scenario matches one of the common test setups so I would have thought you could find data that correlates well to the application. My usual suggestion to talk to the expert at the steel supplier because they often know where the data is.
RE: Very high cycle fatigue vs. endurance limit
ISZ
RE: Very high cycle fatigue vs. endurance limit
I would check IOP Science site. There are links at the top of the page.
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