Fatigue limit rules of thumb 4

[mathlete7]

Does anyone know of any rules of thumb for estimating fatigue limits for various metallics (steel/aluminum) as a function of Ftu or Fty? Just looking for something for quick and dirty checks.
Seems like I've heard of rules of thumb in the past (infinite fatigue life at Ftu/3 or something of this nature) but can't remember what the rules of thumb are.

Thanks much!

 

[40818]

Steels exhibit an endurance limit, under which they have a pretty much infinite life. General rule of thumb do exist, but tend to be more specific in their application, i.e type of loading etc

 

[CoryPad]

The common rule of thumb is an endurance limit of 0.5 [·] Ftu for steels.

I recommend looking at the information contained in MIL-HDBK-5 to further develop some simple checks.

 

[edbgtr]

mathlete7
If you have access to helicopter fatigue design data, you'll get some good rule-or-thumb values from them. High cycle vibes produce high cycle fatigue alternating stress levels to be avoided. They're usually quoted for Kt=1 and then factored down from there.
Ed.

 

[GregLocock]

A very clever man told me that the endurance limit for steel is an artefact caused by poor analysis and test technique.

Trust the trend.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TVP]

Greg is correct, the endurance limit is essentially a myth. Numerous studies performed in the gigacycle (10[sup]9[/sup]) regime confirm that steels have reduced fatigue strength when compared to conventional "run outs" at 10[sup]7[/sup] cycles. The following is a Google search using "very high cycle fatigue" and "gigacycle fatigue" as keywords:

http://www.google.com/search?hl=en&...=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images

This is another good paper on the subject:

"Recent Findings to the Fatigue Properties of Helical Springs"

http://wwwsoc.nii.ac.jp/jssr/english/Proceeding/paper14.pdf

 

[tbuelna]

mathlete7,

The rule of thumb for analysis that I would recommend to you is that if you're not willing (or able) to do a thorough job of design, analysis, manufacturing, quality assurance and testing on your product, then make sure you apply a big fat factor of safety to all of your calculations.

There are no simple rules of thumb for fatigue life, since fatigue life can be significantly affected by so many variables. Things like material quality, grain structure, surface roughness, stress concentrations (Kt), load reversals (R), surface corrosion, fretting, duty cycles, magnitude of combined loads, magnitude of dynamic loads versus static loads, application of knock-down factors for materials in fracture critical parts, reductions in allowable material properties due to lower grades of NDI techniques, etc.

And finally, the values shown in most S-N curves is probabilistic. What that means is that you should expect a certain percentage of you parts to not meet that load-life rating.

Regards,
Terry