The influence of viscoelastic film thickness on the dynamic characteristics of thin sandwich structures

Passive damping techniques by means of viscoelastic materials are widely used for structural vibrations control. Sandwich structures composed of viscoelastic adhesive films and metallic constraining layers result in thin composite structures with improved dynamic capabilities. The nature and the small thickness of these sandwich structures enable them to be processed in conventional metal sheet transformation techniques to obtain components of complex geometries. In this work the influence of the viscoelastic film thickness on the dynamic properties, stiffness and damping, of thin sandwich structures is analysed from experimental and numerical results. Sandwich structures composed of same viscoelastic material but three different core thicknesses are tested and the dynamic properties of the viscoelastic film are obtained. From the experimental results a material model with fractional derivatives is proposed for the shear complex modulus of the viscoelastic adhesive film. The results show the viscoelastic film thickness has a greater influence in the loss factor than in the storage modulus, being this effect more pronounced at high frequencies. The bending stiffness of the sandwich structure is increased with core thickness even if the storage modulus is decreased. Therefore, the viscoelastic film thickness determines the vibrational response of thin sandwich structures.