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# Random Fatigue from Acceleration PSD

Creator

Hi all!

I have a cantilevered beam  with the fixed end subjected to an enforced acceleration with a determined PSD.

I would like to run a random fatigue analysis in Virtual.Lab.  How should I constraint the beam model in the modal analysis that I must submit to Virtual.Lab for the analysis?

In my analysis, I set  one end of beam fixed for the computation of normal modes. However the fatigue damage computed by Virtual.Lab is really far from the analytical solution that I took from the Steinberg book for reference.

Is Virtual.Lab capable of vibration fatigue analysis with an enforced acceleration psd?

4 REPLIES

# Re: Random Fatigue from Acceleration PSD

Siemens Genius

Dear Hen,

it is possible to use Random Fatigue with acceleration loads, but there are numerical constraints when it comes to rigid body movements (i.e. the frequency of the loading  being below the first eigen mode of the structure). In this case using acceletartion PSDs the numerical errors for low frequencies behave like 1/f^4.

Therefore we recommend to use in these cases a large mass approach. I.e. Add a large mass at the load application point and apply forces (using F=m a). Here the errors behave only like 1/f and you get good results for the local stresses and therefore the local damages.

Hope this helps

Michael

Dr.Michael Hack
Simulation and Testing Solutions

# Re: Random Fatigue from Acceleration PSD

Creator

Thank you for  your enlightening comment Sir!

The problem is that the beam excitation is a costant PSD acceleration of 3 g^2/Hz (from zero to 2000 Hz) and I have no idea how to convert  this PSD acceleration  to a force PSD for the large Mass approach.

Beside, the point of excitation for the enforced acceleration is the fixed end of the cantileverd beam and if I apply a PSD force  to this fixed end I get zero MPFs and zero stresses.

The modal  analysis  submitted to Virtual.Lab is performed with one beam end fixed and the other one free.

# Re: Random Fatigue from Acceleration PSD

Siemens Genius

Dear Hen,

seems your application is indeed having the problems at low frequencies.

For the question on converting acceleration PSD to Force PSD: Multiply by the square of the large mass (As F=m times a) and the load is quadratic in the PSD, i.e. 3 g^2 m^2 [N^2/Hz]. The large mass model is a free free model.

BR

Michael

Dr.Michael Hack