flexural rigidities of orthotropic composite laminates

Dear all,

Iam working in composite orthotropic laminates consists of many plies having different material properties in each ply. How to determine or calculate the flexural rigidity (D) of this laminate by transforming these composite into an isotropic model. The idea is that flexural rigidity of the orthotropic laminate is being smeared into a single isotropic formulation.

For instance: if I have three plies:

Ply1 : D1x and D1y

Ply2: D2x and D2y

Ply3: D3x and D3y

I am looking a formulation to simplify the above D1x, D1y, D2x, D2y, D3x and D3y into a single D that behaves as an isotropic model.

Any help is very much appreciated.

Thank you.