Keywords: sandwich components, viscoelastic material, composite material, structural damping, loss factor, cork.

Passive damping has greater advantages in terms of energy efficiency and reliability of machines or structures than active damping, since this approach is based in simple solutions regarding the use of structural modifications, isolation techniques and, or damping materials [1]. This latter option typically requires viscoelastic materials with an intrinsic capacity of dissipating mechanical energy. Most of the time, these materials are combined with high strength or stiffness fibre reinforced polymers in the form of a hybrid composite [2,3].

The main objective of this paper is to study a passive damping system based on the addition of a single layer of a viscoelastic material to a primary structure. In this case, a CFRP laminate with a cork agglomerate layer placed in the middle plane was considered. The reason for considering cork as the viscoelastic material follows from the excellent energy absorption properties of this natural material. The viscoelastic layer was positioned in the middle-plane of a specimen considering two distinct configurations of the material: 1) a micro-sandwich typology with a 1mm cork agglomerate core; 2) a very thin cork dust layer embedded in the laminate through a manual ply-up staking operation.

A numerical analysis was undertaken to obtain the dynamic response of a rectangular flat plate representative of the sandwich with a cork agglomerate core. Results were compared with a plain laminate with the aim of determining the influence of the viscoelastic layer in the variation of the loss factor. Numerical results were complemented by an experimental determination of the loss factor based on the bandwidth method.

Results are conclusive about a significant damping effect caused by the viscoelastic layer within the laminate regardless the type of material, which is more evident for higher frequencies (around 1300Hz). This damping effect corresponds to an average 50% increase in the loss factor for sandwich type specimens with a cork core. Another conclusion is the decrease of the displacement field and the alteration of the natural frequencies as a consequence of the presence of the cork-based layer within the material.

As a general conclusion, this paper shows that the use of cork based composites is a viable and cost effective solution to improve the damping properties of high performance composites, which can be of paramount importance for some particular applications.

1

I. Finegan, R. Gibson, "Recent research on enhancement of damping in polymer composites", Composite Structures, 44, 89-98, 1999. doi:10.1016/S0263-8223(98)00073-7

2

S.H. Zhang, H.L. Chen, "A study on the damping characteristics of laminated composites with integral viscoelastic layers", Composite Structures, 74, 63-69, 2006; doi:10.1016/j.compstruct.2005.03.008

3

B.P. Dolgin, "Composite passive damping struts for large precision structures", United States Patent, Patent Number: 5,203,435, 1993.

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