From LCF to HCF fatigue

[sftvbr]

Hi All

Is it meaningfull of possible that we have to use low cycle fatigue and high cycle fatigue on the same part and same application for different loading cases?

 

[rb1957]

yes, if one load case is very infrequent (LC) and the other very frequent (HC)

another day in paradise, or is paradise one day closer ?

 

[EdStainless]

This is not uncommon.
For example a pump shaft. Starting or the open/close of valves may induce LCF.
But while running vibration or vane pass may produce multiple pulses per revolution, very much HCF.

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P.E. Metallurgy, Plymouth Tube

 

[FEA way]

These are two different assumptions. In case of low cycle fatigue (from about 10^4 to 10^5 cycles) we assume that plastic deformation is significant and cannot be neglected. On the other hand, in case of high cycle fatigue (from about 10^5 to 10^7 cycles) we assume that plastic deformation becomes low enough to ignore it. If one part can be subjected to both high and low number of cycles during its operation then we may use these two approaches for the single part.

 

[rb1957]

that last post reminded me of something. Airplane structures (wings specifically) are subject to varying loads. Unlike fuselages, which see a pretty consistent cyclic load, wings react a highly variable load (due to gusts). Now whilst even the high gust loads don't induce a plastic stress they do have a significant effect on the response to the smaller gust loads (what we call retardation, as it relates to crack growth). We don't call this HCF or LCF, its just "fatigue" to us.

The point is that if your component is subject to frequent low loads and occasional high loads, then Miner's rule is very conservative.

another day in paradise, or is paradise one day closer ?