Hello everyone, I am trying to evaluate the spring constant of a coaxial polymer cylindrical fiber. This fiber has a stiff core and a more soft sheath. I know the geometry of the system and I know the Young's moduli of both components. I can find separate models for the core or for the sheath where using the area moment of inertia I can extract the spring constant. My problem is that I don't know how to extract the spring constant of the whole object. I need this to have a theoretical value to compare it with the results of three point bending experiments in which I directly measure the spring constant and extract the bending modulus of the whole structure. Any help would be great!