Combining Multi Axis PSD Vibration Results

- October 27, 2019 at 5:44 pm
  
  MechEngineer2019
  Subscriber
  
  Hi,
  
  I'm currently trying to run a tri-axial random vibration analysis (PSD). I don't see a way of combining random vibration results from each independent axis, but I did run a single case with accelerations in all three axis. Comparing this to the results from the single axis analysis, the equivalent stress results from the multi-axis run seems to be the RSS of the three independent axis. Is this the case?
  
  More generally, where in the help would this information be located? The web based help on the topic seemed fairly limited...
  
  Thanks!
- October 27, 2019 at 6:07 pm
  
  peteroznewman
  Subscriber
  
  You can apply all three axis PSD accelerations into one analysis, as long as they are uncorrelated. Random vibration analysis is a statistical analysis so a special algorithm is used to compute the equivalent stress.
  
  From the ANSYS help page.
  
  Multiple PSD excitations (uncorrelated) can be applied. Typical usage is to apply 3 different PSDs in the X, Y, and Z directions. Correlation between PSD excitations is not supported.
  
  Meaningful equivalent stress is computed using a special algorithm by Segalman-Fulcher. Note that the probability distribution for this equivalent stress is neither Gaussian nor is the mean value zero. However, the “3 σ” rule (multiplying the RMS value by 3) yields a conservative estimate on the upper bound of the equivalent stress.
- October 27, 2019 at 10:31 pm
  
  MechEngineer2019
  Subscriber
  
  Thanks, Peter.