Dynamic stiffness of a structure

[owner21]

Hi everyone!

I want to calculate the dynamic stiffness of a structure using Abaqus. Structure is made of some metals and rubber, it has one fixed support (A) and one point of load application (B). I have a series of pre-loads and frequencies with excitation displacements applied to B point. Let’s say for one pre-load force 1000 N I need to get values of structure stiffness on frequencies 100 Hz, 200 Hz and 300 Hz with displacement ±0.1 mm. Which analysis procedure type should I use and how to interpret the results?

For now I’m thinking on acting the following scenario:
1) create static general step for calculating structure’s static response for pre-load force (A is fixed, load applied to B)
2) create linear perturbation step - Frequency, for extracting frequencies between given 100 and 300 Hz
3) create steady state dynamic step: modal in which I would like to apply displacement 0.1 mm to B point

I’m struggled with the third step. In boundary conditions I see only base motion, so I guess I should create base boundary condition in the 2nd step (frequency) applied to B point and then create displacement base motion in the 3rd step (mode-based steady state dynamics) applied to the same B point.

[ul]
[li]Can I be sure that this displacement will be oscillating from +0.1mm to -0.1mm on 100Hz, then 200Hz and so on?[/li]
[li]From the results I can see the magnitude and the phase angle of output variables. How should I interpret phase angles from physical point of view?[/li]
[li]Which force I should use for stiffness calculation?[/li]
[li]Maybe I should use another procedure type?[/li]
[/ul]
Thanks a lot!

 

[FEA way]

Your choice of step sequence is correct - static for preload, then frequency extraction and mode-based SSD using mode shapes obtained in the second step. And you are also right that you have to use base motion if you want to prescribe displacement in this type of analysis. Such an approach is often used for virtual shaker testing.

The excitation will be applied in a sinusoidal form, for a given range of frequencies. You can use envelope results to see maximum values if you are not sure which complex form to choose. Request nodal forces (NFORC) output if you want to get forces for the whole model and use them in further stiffness computations.