## Operating Principle of Compressible Constrained Layer Damping and Polymeric Foam Characterisation  
For the adaptation of the dynamic behaviour, an original concept of Compressible Constrained Layer Damping (CCLD) was already proposed by the authors in previous publications [27; 28]. CCLD is an extension of the well-known CLD design, where the usually incompressible viscoelastic damping layer is replaced by a compressible one. Thus the adaptation principle of CCLD is based on the pressure-controlled thickness variation of the viscoelastic damping layer (Figure 1). While the storage and loss shear modulus of the viscoelastic layer (\(G^{\prime}\) and \(G^{\prime\prime}\), which significantly affect the damping performance) and the thickness of the viscoelastic layer are constant for CLD, in the case of the CCLD they depend on the actuating pressure \(p_{1}\). The through-thickness actuation of the viscoelastic damping layer by \(p_{1}\) and the altering of the shear deformation amplitude \(\hat{\gamma}_{1}\) due to variable damping layer thickness \(h_{1}\) results finally in a simple yet efficient adaptation of the dynamic behaviour of the overall system. The actual damping mechanism is based, as for CLD, on the shear induced within the constrained damping layer. The underlying control principle bases on slight deformations (\"evanescent morphing\") when actuated by, for example, hydraulic fluid, compressed air or vacuum. In the presented work, exclusively vacuum actuation is considered. As a result, in the following investigations, the viscoelastic layer can only be compressed with a (negative) pressure of up to 1 bar from its uncompressed initial state. The here used term \"evanescent morphing\" means such a small shape change of the relevant dimension (e.g., thickness) of the oscillating structure that it only influences its dynamic behaviour, whereas the influence on the primary load carrying function is negligible. While the CCLD principle can be realised with a variety of intermediate layer materials, following material characteristics were identified as vital:  
* high damping to influence the structural dynamic behaviour using the minimal deformation through actuation principle,
* strong compression dependency of shear material properties to achieve a broad adjustment range of structural dynamic properties,
* high permeability for the actuation fluid and a technically feasible relation between compressive stress and compression, for example, allowing a considerable compression under vacuum actuation,
* low density to suit the lightweight aspect. An extensive product research of commercial flexible polymer foams revealed the unavailability of sufficient quantitative specifications of material properties related to the above mentioned characteristics, hampering a direct selection of suitable materials. Especially the relation between the applied pressure and the resulting compression as well as the accompanying change of shear dynamic properties are of key importance for the assessment of the CCLD potential as an adaptive damping treatment. They, however, could not be found for any compressible material even in specialised scholar publications. Therefore, a preselection of materials was conducted taking into account  
Figure 1: The Compressible Constrained Layer Damping (CCLD) setup is an extension of the well-known Constrained Layer Damping (CLD) design, usually applied for attenuating of bending vibrations. The typical incompressible viscoelastic damping layer is replaced in the CCLD by a compressible one; the adaptable dynamic shear behaviour results from the compacting of the damping material (by an actuating pressure \(p_{1}\)) and from the altering of the shear deformation amplitude \(\hat{\gamma}_{1}\) due to variable damping layer thickness \(h_{1}\). merely the available information about basic material features and the final selection was made based on the self conducted experimental material characterisation.