Emir, I notice in the Orthotropic properties, the Poisson's Ratio YZ is greater than 0.5 which I have never seen before. Please check that. For isotropic materials, 0.5 is the maximum possible value, which represents incompressible materials.

What support (if any) is in the Modal analysis? ANSYS will perform a modal analysis without supports, which is appropriate for structures floating in space.Â When you analyze structures with no support, the first 6 modesÂ are zero, so to see the first two non-zero modes, you have to request 8 modes since the default is to compute 6 modes. Click on Analysis Settings and in the details pane, the first line is Max Modes to Find and type 9, then Solve.Â

I built a version of your model with no supports and used shell, solid-shell and solid elements, but used 20x50 elements along the edges instead of the lower element count of 8x20 you had.

Natural Frequency (Hz) for different element types using Orthotropic Material shown above.

Natural Frequency (Hz) for different element types using IsotropicÂ Material.

Young's Modulus same as X direction above, Poisson's Ratio = 0.44.

Wow! Looks like you're onto something with the difference between Orthotropic and Isotropic properties for the Modal solver. Maybe others can comment?

For the experimental result you mentioned, how is the panel supported, how were the modes excited and how was the frequency measured? Do you have any experimental data to verify the properties in your orthotropic material model? ASTM D3043-00 "Standard Test Methods for Structural Panels in Flexure" describes how toÂ measureÂ the stiffness ofÂ thin strips in bending. You should try to validate your material properties by experimental means. You can also easily weigh a panel and measure its dimensions to calculate a density.