Extract natural frequencies in abaqus

Hi all,

 I am new in the blog, so I would like to appologies if this post is already published.

 I am doing a research in vibracion-based health monitoring based in CFRP beams. As a part of my research, I am modeling a composite beam in abaqus and apply transient analysis on it. Before I would like to validate my model with a very simple a easy model for this reason I have elaborated this procedure:

 1. I have simulated a composite beam with the laminate feature that Abaqus has and I obtain the natural frequencies by linear peturbation/Frequency. Until here ano problem because using laminate section beam you cannot consider the interaction between layers (10 layers in this case). Then my first natural frequency was 25.77 Hz (just a reference data).

 2. I did the same but in this case instead of consider a 10 layers section, I just consider just one layer with the total thickness. Then the fist natural frequancy was 24.75 Hz (in the same order that before).

 3. As my analysis is going to introduce delamiantion, I have decided model each layer as a different part (instance) and conectect them to generate the total whole section. As I understand, interaction contact by cohesive elements cannot be used by Linear peturbation/frequency (I mean to extract the natural frequencies) because it is a nonlinear behabiour and it is impossible to obtain eigenvlaues of that. However if I will use tie constraits it should works becuase it cnsidere the union of two parts as a singel one with a contact between them which does not recognize any realtive displacement between layers (I mean it will work as unic section). In this case I have built my section by two layers (half thickness each) contact by Nodes to surface. the results that I obtained are wrong because the natural frequency is only for one single layers and it seems that it does not consider the connection between layers. 

My questions are:

 - is it possible to do linear pertubation with tie constraits between two layers?

 - if it is not could you please give some tips about? In which should I go?

Many thanks in advance. I am new in abaqus and might I have missed some point in the model.

All the suggestion will be more than appreciated.

David