Thank you all friends. Your information was very useful.
Furthermore, I hope to derive the equations of motion of a Kirchhoff plate,

starting with the displacements

u = u₀ - z * diff ( w₀ , x ),     v = v₀ - z * diff ( w₀ , y )     w = w₀ 

and strains

e_xx = diff ( u₀ , x ) - z * diff ( w₀ , x $2),     e_yy = diff ( v₀ , y ) - z * diff ( w₀ , y $2),     e_xy = diff ( u₀ , y ) +diff ( v₀ , x ) - z * diff(diff ( w₀ , y ),  x)   , e_xz = e_yz = e_zz =0

and constitutive relations

sigma₁ = Q11 e1 +Q12 e2     sigma₂ = Q12 e1 +Q22 e2     sigma₆ = Q66 e6 ,     sigma₃ = sigma₄ = sigma₅ = 0

using the principle of virtual work

int (D U , t = 0 .. T) = 0 ,     D U = the total differentiation of U

where,   D U = int int int [sigma_xx * D( e_xx )+ sigma_yy * D ( e_yy ) + 2 sigma_xy * D ( e_xy ) ] dzdxdy

finally, equating the coefficients of Du₀ , Dv₀ and Dw₀ by 0