stady state dynamic in LS-DYNA

[Behdad.S]

I’m trying to understand how LS-DYNA SSD Module works. For this purpose, I’m solving a simple mass-spring system from a textbook.

I tried to solve it with two approaches:
1. Time-domain: The result matches very well with the book.

2. Frequency-domain (SSD): unfortunately, I could not replicate the same result. I could not figure out the possible problem. I used enforced displacement by the large mass method. I put both models in my google drive:

https://drive.google.com/drive/folders/1h8Jv-a72UImI1TZqFojsGqBqEIUnyZwx?usp=sharing

Could you help me to understand what my mistake is?

 

[onatirec]

Could you paste your resulting plot here? And what you are expecting to see?

In the frequency domain, it can be plotted a number of ways - one of which is a displacement ratio (X/Xb) vs frequency ratio (w/wn). This would of course give you a normalized amplitude over a range of frequency...

You also have transients here in the beginning of your signal - so it's unlikely that only the steady-state frequency response would fall out.

 

[Behdad.S]

I have just one excitation frequency; therefore, I expect to see just one number: the displacement in steady-state condition on node 2. The result should match the textbook (0.632 mm)

When I use the base displacement method, everything is just right.
Hier is the result of the base displacement method.
Frequency----------Amplitude-translational-z-disp @ 2
7.96E+00 ----------6.32E-04


Your comment about the transient response is absolutely right. An FFT analysis is showing that very well.