Perturbation analysis of frequency shifts in an electroelastic body under biasing fields

We analyze the eigenvalue problem associated with small-amplitude vibrations superposed on finite-biasing fields in an electroelastic body. The widely used first-order perturbation integral by Tiersten is generalized in two different ways: a second-order perturbation analysis is given when the biasing fields are not infinitesimal and their second order effects need to be considered; a first-order perturbation analysis is given when an eigenvalue is associated with more than one eigenvector (a degenerate eigenvalue)