“…Moreover, the variation of the total work done by the external force q(r, ϕ, t) is written as (9) Next, direct substitution of the strain-displacement relations (2) and (3) in (6) yields where the moment resultants in the above equation are defined as [34] (11) Now, employing (9), (10), and (12) into Hamilton's principle (Eq. 5), and making use of the so-called gradient theorem (see Appendix A), while taking advantage of the fundamental lemma of calculus of variation [32], results into the general form of equations of motion for forced vibrations of the ERF-based sandwich annular plate: (13) where i = 1, 3; δ 1 = 1 and δ 3 = −1, are given in Eqs. (7b), and the relevant shear resultants in the top and bottom skin layers are defined as [34]: (14) where i = 1, 3.…”