## Interface force recovery from Random Vibration using FEA

## Interface force recovery from Random Vibration using FEA

(OP)

Typically we use quasi-static loads on our system level structures. Because the system level models are so large we often recover interface forces from the system model (of different mating components) and then apply these forces at bolted joints of detailed stand alone 'fitting' models. Then we recover stresses in the joints. In this particular instance we have a random vibe load for our system model. We are using cbush elements to recover the forces and as you all know the RMS force values have no phase information and thus are all 'positive' in nature. I realize the correct manner to do the proper stress analysis would be to incorporate the detailed fitting model into the system model and recover RMS stresses. Does anyone have any experience applying random vibe interface forces to stand alone highly detailed fitting models? Applying all 'positive' values isn't really appropriate.

## RE: Interface force recovery from Random Vibration using FEA

Cheers

Greg Locock

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## RE: Interface force recovery from Random Vibration using FEA

Don't confuse the RMS magnitudes of the forces whether individually or summed, with the time evolution of the actual forces. If you do the phase information is lost. In otherwords, all modes word be excited in phase and produce grossly misleading results.

Basic maths...

## RE: Interface force recovery from Random Vibration using FEA

I'll check out the vibrationdata site.

## RE: Interface force recovery from Random Vibration using FEA

- for each cbush in your joint extract the FRF (based on 1g sine sweep analysis). Each FRF is complex
- at each frequency you can add all the Re and Im part of the all the FRFs
- once you have the SUM FRF at each freq. f, estimate the magnitude: SQRT(Re^2+Im^2)
- if you then take at each freq. f: PSDinterfarce(f)= mag^2*PDS(f),you might end up with the PSD "resultant" for the interface

Just a thought and might be totally wrong !!## RE: Interface force recovery from Random Vibration using FEA

I'll look into that. Thanks for your thoughts.

## RE: Interface force recovery from Random Vibration using FEA

## RE: Interface force recovery from Random Vibration using FEA

1. Create 'long' time domain signal that satisfies the RMS profile

2. From FEA, extract individual transfer functions of force_at_bolts/input_motion (complex numbers)

3. Apply transfer functions on input signal to calculate forces in time domain

The output will have phase content. Interactions of closely spaced modes could be accounted for. Statistical analysis needs to be performed on the input and output to verify the law of large numbers is satisfied.

In principle, evaluations can be done in the time domain peak loads.

Kind regards,

Jason

## RE: Interface force recovery from Random Vibration using FEA

## RE: Interface force recovery from Random Vibration using FEA

A simple Matlab code below plots the standard deviation of different data block size. At around 10^5, it appears to converge.

## CODE --> matlab

Kind regards,

Jason