vortex induced vibrations in composite materials

Hi

I am working on vortex induced vibrations in composite materials. I am trying to solve the euler bernoulli equation for a riser (composite material) by using normal mode analysis. I realise the stiffness constant will vary with time. How is the stiffness constant of a composite determined and is there any way to model the stiffness constant in my problem.

Are there any good references available regarding this?

Thanks

Tuhin.