- The non-linear (i.e., large deflection) free vibrations of thick, orthotropic annular plates with varying thickness are calculated. The formulation is based on the more general Reissner plate equations as well as the von Krmn plate equations for variable thickness annular plates. Numerical results for the ratio of the non-linear period to the linear period of natural vibration are compared with those existing in the literature. New results are also included for future comparisons. 1981.