I have been playing with LS-Dyna for the past couple of months and I have some questions (or confusions) about modeling a single degree-of-freedom (SDOF) system with the software.
Recently, I was trying to model a spring-mass system as an undamped SDOF system for applying base acceleration using the *LOAD_BODY_X keyword card. According to dynasupport, the *ELEMENT_DISCRETE has only one DOF and stiffness can be inserted directly, which acts from n1 to n2. My intention was to record the displacement response and understand if my applied base acceleration is matching with my analytical solution. I assumed the stiffness of the spring is 174E5 with a nodal mass of 0.793.
To verify the system's mass and stiffness, I wanted to do an eigenvalue analysis, instructing the *CONTROL_IMPLICIT_EIGENVALUE deck to extract the fundamental period. Interestingly, the fundamental period it provided was very low, mismatching my analytical solution.
Later, after requesting the eigensolver to extract 10 solutions, it provided 3 - from which only the last one matches my analytical frequency.
Thus, to circumvent any mode interference, I applied boundary conditions at the free-end of the spring. This time, applying the same keyword cards, I did get one frequency, but it was much more higher (2799) than the calculated original (745). Unfortunately, I got the same thing when I applied the same input deck replacing the discrete with a beam model created before.
I think the following decks are enough for replicating the problem. I would be grateful if someone can help me with this (and subsequent questions).
(Edit: Removed the input deck for better accessibility. If anyone want to model similar system, please refer to https://www.dynaexamples.com/introduction/Introduction/example-17)
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