fatigue - hand calculation help needed

[lodhss]

I'm trying to determine the endurance limit of a cylindrical cantilever beam (e.g. cylinder attached to a wall). Question: if I apply a load to it over and over again, will it eventually fail? Is there a simply engineering hand calculation that can be done to solve this? Thanks

 

[structSU10]

AISC has an appendix in the steel construction manual on fatigue (13th edition Appendix 3).

 

[dcarr82775]

Fatigue is typically limited to a certain maximum change in stress due to the application of the load. Simply solve for the stress and compare it to the specified limit (AISC manual or AASHTO give the same limits). If you have welds or bolts those will control the fatigue rating over bending in s typical beam.

 

[lodhss]

Looking at 3 different diameters with a single load, the stress in the Cobalt-Chrome beam were 693, 520, and 400MPa, respectively.

I don't have either of the manuals. Is the table public information? Let me know if you can help further. Thanks.

 

[lodhss]

i'd also be looking to load it 10 million times. i'll be happy to provide additional info if needed.

 

[kingnero]

do you have s/n (wohler) curves available for the material?

 

[structSU10]

You can download AISC spec from AISC website here

http://www.aisc.org/content.aspx?id=2884

 

[MiketheEngineer]

10 million times at full load?? Think not. Get the graphs!!

 

[TXStructural]

To answer your original question: there is no simple hand calc for this, as it varies by stress level (both tension and compression), cycles, and material. Fatigue endurance also changes with temperature, surface condition, corrosion (and exposure to chlorides or similar corrosion promoters)...

To quote an unnamed online reference: "The number of cycles that a metal can endure before it breaks is a complex function of the static and cyclic stress values, the alloy, heat-treatment and surface condition of the material, the hardness profile of the material, impurities in the material, the type of load applied, the operating temperature, and several other factors."

As a general rule, designers usually have little concern for fatigue in carbon steels until the stress reaches about half of yield (or half of ultimate for some designers and materials). The best course of action may be to keep the stress level lower than half of yield, (or possibly as high as 45% of ultimate) to avoid fatigue issues.

 

[Venturini]

As I see the only way to correctly evaluate the fatigue behavior of a structure would be to do testing to either assess the fatigue endurance of such structure as close as the final form and material (and environment, surface condition and number of cycles) as possible or to apply fracture mechanics to the design.

And yes, if it is under such stresses, it will fail eventually.

Out of that, it would be just guessing. Wohler curves are hardly applied on real life projects as it only true to the components tested and not a material behavior. Neither of the prior alternatives are simple neither they are cheap.