I am doing a research in vibration-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 perturbation/Frequency. Until here no 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 frequency was 24.75 Hz (in the same order that before).

 3. As my analysis is going to introduce delamination, I have decided model each layer as a different part (instance) and connected them to generate the total whole section. As I understand, interaction contact by cohesive elements cannot be used by Linear perturbation/frequency (I mean to extract the natural frequencies) because it is a nonlinear behavior and it is impossible to obtain eigenvalues of that. However if I will use tie constraints it should work because it considered the union of two parts as a single one with a contact between them which does not recognize any relative displacement between layers (I mean it will work as unique section). In this case I have built my section by two layers (half thickness each) contact by:

a) note to surface: it does not work, it give the wrong natural frequency. 

b) surface to surface: it give me the right value but you can see in the model that the layers are moving separately in higher frequencies. I think if i will be able to connect the  nodes it will be solve it.

My questions are:

 - is it possible to do linear perturbation with tie constraints (no iteration contact) between two layers?

 - if it is not could you please give some tips about? In which should I go? It works to me in surface to surface but no in nodes to surface.

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