fatigue, frequnecy and cycles

[mfinke]

Dear all,

does anybody knows the depemdency between load frequency, load cycles and fatigue strenght.

I known that beyond approximately 10^2 load cycles we are talking about fatigue, but whats about the frequency. Is 1 cycle per hour or day enough to have fatigue effects in metals?

regards,

MAtthias

 

[tomirvine]

Load frequency must be considered together with a structure's natural frequency. The structure may amplify or attenuate the input energy depending on this frequency relationship. Damping is also a factor in this calculation.

The calculation also depends on whether the excitation is an applied force, a base excitation, or some other excitation type.

Otherwise, the load frequency should be irrelevant.

Tom Irvine

http://www.vibrationdata.com

 

[Drej]

LCF <= 1000 cycles (stress based)
HCF >= 50,000 cycles (stress based)

In between is no mans land, but can often be proved/argued based on size/number of striations on the material fracture face and the geometry of the failure. Fatigue strength (basic) or Fatigue Limit, based on infinite cycles, can be found in most strength of materials books for most of the engineering materials used. Usually quoted as endurance or fatigue limit based on 10^7 cycles of stress (strain is different). So, is 1 cycle per day enough? Possibly? If the amplitude is high enough it may be appropriate under LCF. If left long enough may even be HCF, although the component/system/etc will probably have fallen apart from corrosion many years earlier than this. Draw your own conclusions based on the facts above.

Cheers,

-- drej --

 

[ThomasH]

What is it that you want to analyze?

The reason for the question is this: The typical dynamic problem, vibration etc, is not a fatigue problem simply because the stresses are not that high or the number of cycles are so high that high stresses means faliure.

A typical, almost textbook, example is a car driving over a bridge. Say one car/hour for 5 years that gives 24*365*5=43800 cycles. That can be a problem, actually even one car/day can be a fatigue problem. Note that when I say "problem" I mean that fatigue applies, not that is is likely to be a design criteria.

Normally this type of fatigue is covered in codes or standards. The interresting part is stresspan, how often the full stresspan applies and the total number of cycles in the structures life.

Regards

Thomas

 

[EricZhao]

I think Matthias just touched a very good point. In most stress textbooks. The plot of S-N curve gives basic Fatigue life. The fatigue life didn’t consider how fast the stress changes (frequency). I bet there might be some influence from the stress alternating frequency.

 

[elogesh]

Hai MAtthias,

I think Tom already well responded about load frequency. The applied force gets amplified or attenuated depending upon the ratio of load frequency(excitation frequency) and natural frequency. Then proceed with normal calculations.


Ericzhao
"The fatigue life didn’t consider how fast the stress changes (frequency)" .
If the structure is rigid compared to the load frequency such that load frequency/natural frquency <<1,
then it is better to ignore load frequency.

Drej

LCF <= 1000 cycles (stress based).

I hope you meant Low cycle fatigue as strain based.....

Regards,
Loganathan.E