Thickness Vibrations of Sandwich Plates and Beams and Delamination Detection

In multi-hull marine vehicles assembled by FRP sandwich composite materials, problems with delamination and skin/core debonding are reported. An early damage warning (EDW) system for delamination detection in marine vessels based on measurements of bending vibrations in sandwich structures are under development. Related to this EDW system development, this paper presents a theory for qualitative determination of thickness vibrations in sandwich plates and beams. The core is modelled as a two-parameter foundation where shearing interaction effects and normal stress effects in the core are included. The skins are modelled as ordinary plates or beams on a foundation. Expressions for both thickness shear and thickness stretch modes are given. In addition to the theoretical development, experiments with a simply supported sandwich beam, using a TV-holography technique, were performed and good agreement between theory and experiments was achieved. With the expressions for thickness shear and thickness stretch frequencies presented in this paper one is able to qualitatively determine thickness vibration frequencies. It is particularly important to determine whether thickness vibrations are present in the same frequency range as the bending frequencies of interest in the EDW system. The occurrence of a thickness shear frequency and an occasional non-existence of a higher bending frequency may be misinterpreted as a down shift of the bending frequency and thus lead to an incorrect delamination detection.