" Also how well do you know the material properties (you certain of all the properties – often there is a variation) – sometimes we need to alter/change them a bit to get better agreement with test data."
Your question is valid , I am using FR4 properties given in Ansys Mechanical while this PCB board is made from Isola 370HR (FR4) dielectric material. For this i don't have all the 9 properties values required for an orthotropic material.
From datasheet of Isola 370HR , i got few properties which are as below:
E1 (along length direction) : 3744 ksi ( ~ 25814 MPa)
E2 (along cross direction ) : 3178 ksi (~ 21912 MPa)
Poisson's Ratio :
A. Length direction : 0.177
B. Cross direction : 0.171
The other properties details are missing like E3 , Vz , G12 , G13 & G23.
Since , these properties are not available , i have rely on FR4 properties given in Ansys Mechanical. Here, also i observed that for shear modulus calculation formula used is valid for isotropic material.
like Ex : 20.4 GPa
vxy : 0.11
if you see the Gxy value it is 9200 MPa which will come from G = E/ (2*(1+v)).
I don't have exact material properties , that may be one of the reason that modal frequency is not matching.
Please suggest if you came across this material or having mechanical properties.