Random Response Analysis

Hello everyone,

 

I have been following the OS-T 1325 Random Response Analysis OptiStruct tutorial last times, and I have a few questions about it:

 

- Firstly, eventhough I have clearly understood what Frequency Response and Transient Dynamic Analysis do, I don't see the point of the Random Response.

• Frequency Response Analysis gives the mechanical steady-state response y (such as displacement, stress, etc) for oscillatory excitation x(f) with a frequency f between a range [f1,f2]
• Transient Dynamic Analysis gives the mechanical response y(t) for time-dependent loads x(t). So we get mechanical values (stress, strain, etc) throughout the time history.

I understood that it is based on a Frequency Response Analysis with a PSD (which is kind of a Fourier transform) so a frequency information, and I understood that is useful when the load is said as 'random' that is to say we cannot predict the shape, values, mean such as wind on an aircraft, imperfect road surface on a car, etc. In any case, we need data as input so what do we use?

 

As an example, let's say we focus on a car moving on a road with imperfect surface and we record acceleration of a part throughout time. Why do we use Random Response Analysis in such a case? Is it because we cannot be sure that the signal is perfectly representative of the duty cycle of the car so it is unsuitable to run a Transient analysis? Thus, we run a Random Response Analysis which is more 'statistical' and will give mechanical responses that are likely to occur for a random signal with frequency in the range of the input? But if that is right, why the input is suddenly likely representative of the duty cycle? By the way, why do we use a PSD as an input?

I read this article https://blog.altair.com/random-response-fatigue-analysis/    and many other pages but even with that I'm still not convinced of what is the prupose of Random Response Analysis and how it orks. Concerning the results of such analysis, what does the RMS stress represent? Should we compare it to the yield strength? Or is it the stress you directly refer to in an S-N curve to assess the number of cycles before the structure fails, or do you set-up a fatigue analysis from it?

Finally, it is remains after all a Frequency Response Analysis so how does it use the PSD input? Does it multiply the Frequency response analysis ouput with the 'power' y-values of the PSD?

 

-Secondly, I don't understand in the RANDPS load collector what are the J and K input. It is said in the User Guide that there is a excited load and an applied load? How can they be different, what does that mean?

-Thirdly, what is the difference between the PSD(t) stress and the RMS stress?

 

 

Thank you for your help.

Nathan