The effect of debonding on the natural frequencies of laminated fibre composite sandwich plates

A new generation composite sandwich made up of glass fibre reinforced polymer skins and lightweight but high strength core material has been developed in Australia. A structural composite plate or slab can be manufactured by gluing a number of these sandwiches to satisfy the design requirements. The presence of unbonded areas in the glue line due to manufacturing faults or deterioration in bond in some of the originally fully bonded areas due to aging is a possibility in a FRP structure consisting of glue-laminated fibre composite sandwich plates. The presence of no bonding in some areas of the glue line may cause changes to the vibration parameters, such as natural frequencies and mode shapes, of the plate structure. In particular, unbonded areas could reduce the natural frequency and if the reduced natural frequency is close to the working frequency of the plate structure, resonance could happen. Therefore, it is necessary to make accurate predictions of changes in natural frequencies in such plates. This paper presents a numerical investigation into changes in natural frequency due to various amounts of debonding along the glue line in a laminated fibre composite sandwich plate structure. The need for 3D finite element modelling is avoided by innovatively using the plate and link elements in Strand7 finite element software package to model the plate behaviour by giving due consideration to bonded and unbonded surfaces between plate layers. The accuracy of the numerical solution is verified by comparing present results with bench mark solutions reported in the literature. Variation of the natural vibration frequencies with different debonding areas and locations for a 4 layer sandwich plate structure is presente