Which resonant point is more dangerous?

[remlap]

Hi!

Sorry, another fundamental mech question from an elec eng, but you guys are just SO helpful!!

If I have a sinusoidal sweep results graph that shows a 10dB resonance at 35Hz and another 10dB resonance at 250Hz, at which frequency should I do my sinusoidal endurance test?

Thanks.
Ray (Japan)

 

[GregLocock]

I'd have to know much more about the likely excitation in real life, and the system under test, but usually it is the low frequency that kills things.

Oh, those amplitudes are accelerations aren't they?

Cheers

Greg Locock

 

[remlap]

Thanks Greg,

Yes, accelerations.

The application is a 40kg resistor assembly mounted on a locomotive.

Our customer's spec says to test using a constant G sweep, and then 8hrs at the resonant point.
My supplier just chose the low F point, but I thought (without knowing the mech theory of course!) that the energy would be higher in the higher frequency vibration. But perhaps the displacement is higher in the lower F point ...?

Ray

 

[bjpil]

Not knowing more about your assembly I would test it at both frequencies because the resonances may not occur at the some location, causing damage at two different spots. This of course if the assembly sees both frequncies under working conditions.

BJP

 

[electricpete]

Using a simple sdof model, we would conclude that low frequency is most likely the way to go:
- You have the same acceleration at two different frequencies a factor of 7 apart.
- Velocity will be a factor of 7 higher at the lower frequency.
- Displacement (indicator of strain) will be a factor of 49 higher at the lower frequency.
- If the material is within it’s linear range, fatigue depends upon displacement times frequency => fatigue rate is factor of 7x more at the lower frequency.
- If the material is not within it’s linear range, then we will have a better change of identifying this non-linear and plastic deformation problesm at the higher displacements ie lower frequencies.
All of the above suggests lower frequency is the best place to test.

Now one fly in the ointment. Each of those frequencies corresponds to a different mode shape. Even though the point you are monitoring shows the same acceleration at high frequency as low frequency, there may be other non-monitored points in space that show much higher accekeration at the frequency than the low frequency. If one of them is your weak link you may miss it by testing only at the lower frequency. How concerned you should be imho depends on how sure you are that you have selected the proper critical points for monitoring.