I'm not a composites expert but, I'm interested in helping you with your problem. What does the P, H, and a represent in your equation? What is your equation based on? I'm sure the equation is dependent on the type of composite you are using; orthotropic, anisotropic, etc.?
There are some other...
I dont understand why people would use a Metal Matrix Composite. Why not get rid of the graphite and other fibers and make it an isotropic material? An isotropic material is easier to model and easier to predict when under loading. What advantage do you get with using the fibers?