Hamilton's principle for dynamical elasticity

There exists NO classical variational principle for continuum dynamics so far. The well-known Hamilton's principle is valid only for the conditions prescribed at the beginning and at the end of the motion and is therefore useless to deal with the usual initial-boundary condition problems. In this paper an exact classical variational theory is established, all initial conditions are converted into natural ones, furthermore, all natural final conditions automatically meet the physical requirements, leading to a vital innovation of Hamilton's principle.