I have a quick question about interpreting the stress results of a Dynamic Random Vibration study. The values are RMS Von Mises stresses, which (as far as my understanding goes) are equal to one standard deviation, assuming a normal (Gaussian) distribution. So, when subjected to PSD acceleration, the stresses in each part have a 32% chance of exceeding the stresses shown.

The problem I'm having is determining a pass/fail criterion for this study. Many of the studies I've seen claim to use the PSD curve (for a specific amount of time?), and then provide results in a damage or life plot. I looked into using stresses created by the PSD curve and using them as inputs for a fatigue study, but it doesn't appear that SolidWorks has that capability.

I also tried assuming that 3*RMS stresses would ensure that stresses in the part would only have a 0.25% chance of exceeding the displayed RMS values (i.e.: the pass/fail limit is set to 1/3 of the material yield strength), but this seemed unrealistic. The RMS stresses in many of the robust parts we studied were coming fairly close to the limit (1/3 Sy).

Can anyone give me some insight on interpreting these results? For reference, I'm using PSD curves defined in MIL-STD-810G, Method 514.6C.

Hello Bryson

The best way of addressing this is to approach the system at "Qualification" levels. If you are using Maximum Predicted Environment (MPE), take the existing PSD map and multiply everything by 2, or equivalently adding 6 dB to the profile. Then examine the results and see what the RMS stress values are (IIRC, the RMS stress is due to RMS forces, which are, in turn from RMS accelerations).

If the RMS stress values at Qualification are below yield stresses for the system, then I would consider it a safe system.

What you are basically doing is doing what has been put into practice for physical testing but now at the simulation level. Qualification levels are generally never met during the life of physical service, but are used to effectively 'shake the system down'. If it works at Qual it usually passes into production...

Kirby