## 6 Conclusions  
For the adaptation of the dynamic behaviour, an original concept of Compressible Constrained Layer Damping (CCLD) was proposed. The control principle bases on minimal deformation of the intermediate, viscoelastic layer when actuated by for example, hydraulic fluid, compressed air or vacuum. Such approach enables an adjustment of the damping and stiffness properties of the overall vibrating system.  
Since, the compression-dependent shear damping/stiffness as a parameter of key importance could not be found for polymeric foams extensive material testing was performed. Finally, three PU foams and a melamine foam (as a material with completely different morphology) with favourable viscoelastic properties and significantly different densities were used in the investigations. To estimate the potential of the CCLD, an analytical model was developed which describes the dynamic behaviour of a beam structure, taking into account the compression-driven properties of the intermediate layer. The model was used in a series of simulations to analyse the CCLD potential regarding the damping capacity and its adaptation as well as to understand the physical relationships of parameters describing the system configuration. The influence of the base beam slenderness on the CCLD performance is presented in this study. It can be observed that already the application of an unactuated CCLD on the bare beam increases the system's damping. This can be accounted to a purely passive CLD treatment and it ensures a favourable fail-safe system feature. An understandable trend showing decreasing vibration amplitudes for more slender beams can be observed directly. This can be accounted to the decreasing ratio of base structure size to the constant CCLD treatment size. The results of the simulations confirm the principal correctness of the initial theoretical assumptions about the adaptive dynamic behaviour of structures with the original CCLD treatment. A significant vibration damping as well as a remarkable adaptability were observed but both show--as the results of the material characterisation--a complex dependence on the system configuration.  
Alternative damping materials that exhibit higher specific dissipation energies combined with lower actuation strokes compared to polymeric foams were screened. Herein, polyester needle-punched nonwovens (PNPN) were identified as a promising candidate. First preliminary investigations revealed that the damping capacity is between 1.6 (high compression, large shear deformation) and 21.5 (low compression, small shear deformation) times better than that of the polymeric foams focused so far. These results are encouraging but further studies are necessary as non-linear material behaviour was observed under shear stress due to the now different type of dissipation mechanism.  
Conceptualization, T.E. and P.K.; Data curation, T.E.; Formal analysis, K.H. and P.K.; Funding acquisition, N.M. and P.K.; Investigation, T.E.; Project administration, P.K.; Resources, N.M.; Software, K.H.; Supervision, N.M. and P.K.; Validation, P.K.; Visualization, T.E.; Writing--original draft, T.E. and K.H.; Writing--review & editing, N.M. and P.K.  
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)--315011510. The presented work is part of the research within the context of the Priority Program (SPP 1897) \"Calm, Smooth and Smart--Novel Approaches for Influencing Vibrations by Means of Deliberately Introduced Dissipation\", subproject \"Lightweight Structures with Adaptive Dynamic Behaviour through Evanescent Morphing\".  
Open Access Funding by the Publication Funds of the TU Dresden.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
CT Computed tomography  
CCLD Compressible Constrained Layer Damping  
CLD Constrained Layer Damping  
PNPN Polyester needle-punched nonwovens  
PU Polyurethane  
TTS Time-temperature superposition