Effect of skin-core debonding on the dynamic behaviour of GFRP composite beams

A new generation composite sandwich made up of glass fibre reinforced polymer (GFRP) skins and high strength phenolic core material has been developed in Australia. Sandwich structure relies on the adhesive bond between the face-sheet and core for its overall stability and consistency. Although perfect bond between the skin and the core is a common assumption, an important issue that needs to be considered in using a composite beam or slab is the development of debonding between the skin and the core, which is a predominant failure mode of these sandwiches. Debonding can happen during fabrication or under service conditions that leads to changes in free vibration behaviour which in turn can result in resonance. Therefore, it is important to make accurate predictions of changes in natural frequencies in such structural elements. This paper investigates the influence of debonding on dynamic behaviour of sandwich beams of different debonding sizes and end conditions. Numerical model is developed with Strand7 finite element software, and the developed model is used for 3D finite element simulation. Free vibration behaviour reported in the literature for composite beams will first be used to compare the analytical results with the fully bonded and debonded beams. Study is extended to depict the effect of debonding on free vibration behaviour of novel composite beams. It is observed that the decrease in natural frequency with the increase in the extent of debonding is more dependent on the width of debonding across the beam than the length along the beam. The end conditions of the beam are also a governing factor dictating which modes are more affected. It is also perceived that full width debonding leads to increased participation of twisting modes in comparison to half-width debonding in clamped-clamped end condition.