Before revision:  In the fully discrete scheme, a Crank-Nicolson scheme is adopted for the approximation of the temporal derivatives of displacement and stress . Error estimates of the two schemes, as well as an unconditional stability result for the fully discrete scheme, are derived.
After revision:  In the fully discrete scheme, a Crank-Nicolson scheme is adopted for the approximation of the temporal derivatives of stress and velocity variables . Error estimates of the two schemes, as well as an unconditional stability result for the fully discrete scheme, are derived.