PhD Thesis Defence Presentations - Christos Katsiropoulos

The damping characteristics of advanced lightweight composites are of paramount importance for a variety of applications particularly those employing composite parts or components operating under dynamic external loading. Graphene has been shown to possess high stiffness and strength and is widely esteemed as an ideal additive for enhancing the mechanical properties of polymeric, ceramic and even metallic composite matrices. However, it has been postulated that if such behaviour can be combined with increased fracture toughness and damping characteristics then a whole new class of multifunctional composites could emerge. 

     In the present thesis a systematic investigation is conducted on the damping response of polymers and carbon fibre/polymer composites (CFRPs) whose matrix is doped with graphene nanoplatelets (GNPs) at various concentrations, with the aim of improving the damping capacity of the materials under examination. Indicatively, free oscillation tests are performed on a custom-designed and manufactured device. Furthermore, mechanical and thermomechanical tests (DMA), as well as a multitude of other tests (e.g., Atomic Force Microscopy-AFM, X-ray Photoelectron Spectroscopy-XPS, Micro-Computed Tomography-μCT, Scanning Electron Microscopy-SEM, etc.) have been employed to support and better understand the results.

     Furthermore, a computational model is proposed to predict the damping behaviour of these polymer nanocomposites and hybrid CFRPs. The adopted numerical approach utilizes the structural transient finite element method (FEM) applied to periodic representative volume elements (RVEs) under sinusoidally time-varying axial loads. The damping ratio is estimated based on the computed time lag between the input load and the resulting deformation. 

     The experimental results indicate that the addition of GNPs into both the neat polymer and the CFRP, affects significantly the damping behaviour. In particular, considering polymer nanocomposites the damping ratio ζ exhibits an increasing trend with nanofiller concentration up to approximately 10.0 wt.% and then it forms a plateau. For the case of hybrid CFRPs a significant increase of damping capacity is observed for low concentrations in the range of 0.15-1 wt.%, whereas for higher concentrations the corresponding enhancement does not follow a clear trend. Respective results are also obtained from the mechanical tests performed. The numerical results are compared to the experimental measurements from the free vibration tests, demonstrating good agreement regarding the overall damping behaviour versus the mass fraction of the GNPs. A plausible explanation for this behaviour is also presented, through a systematic investigation of the energy dissipation mechanisms occurred.