Random Vibration Fatigue Cycles
Random Vibration Fatigue Cycles
(OP)
If I run a stress analysis with a random vibration input spectrum, the output stresses will be in Grms (1-sigma), and therefore for a fatigue analysis I can also determine the 2-sigma and 3-sigma stresses. However, the thing I'm uncertain about is how to determine the number of cycles at each stress level. It seems unrealistic to base the number of cycles on the peak input frequency. Alternatively, I can calculate the resonant frequencies, but since this is effectively a flat random profile, all resonances will be excited simultaneously.
Any suggestions gratefully received.
Any suggestions gratefully received.





RE: Random Vibration Fatigue Cycles
Cheers
Greg Locock
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RE: Random Vibration Fatigue Cycles
RE: Random Vibration Fatigue Cycles
Then you need an algorithm to count every time the signal passes through zero.
Divide the number, given by your counter, by 2 to get the number of cycles.
RE: Random Vibration Fatigue Cycles
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: Random Vibration Fatigue Cycles
Various methods of counting have been proposed in the literature, leading to different results and therefore (for some) to errors in the calculation of the fatigue lives.
They are peak count, range-restricted peak count, level-restricted peak count, mean-crossing peak count, range count, range-mean count, range-pair count, ordered overall range method, racetrack method, level crossing count, modified level crossing count, peak valley pair (PVP) counting, fatigue-meter count, rainflow count, national luchtvaart laboratorium (NRL) counting, and time spent at a given count level.
As you see, rainflow count is just one method among a lot of other methods...
RE: Random Vibration Fatigue Cycles
Cheers
Greg Locock
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RE: Random Vibration Fatigue Cycles
As a corollary to this, I strongly suspect I can synthesise a signal that is statistically equivalent to a given random signal, but that has a consistently greater number of zero crossings, and I know that when real time based fatigue loads are prepared for a rig test many zero crossings are eliminated.
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: Random Vibration Fatigue Cycles
When you write some lousy messages, I say to myself : " This guy is just an idiot" but I don't write it to the entire community.
RE: Random Vibration Fatigue Cycles
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?