To estimate the stress in your flywheel, have a look at equation 3.7.7, below. Note that in this equation, stress is not a function of the thickness of the disk This equation will not provide an exact stress but will provide a reasonable first estimate of the order-of-magnitude of the stress at various points on your disk. You will probably need to consider your flywheel to be 3 different disks with three different inside and outside diameters.
