PhD Thesis Defence Presentations - ELLI BELLOU

Graphene-based materials exhibit exceptional mechanical properties and therefore they are suitable candidates for high performance real-world applications. However, these properties degrade in the presence of defects, which act as stress concentration points, leading to the premature failure of materials. Hence, it is essential to gain insight into the parameters affecting fracture and the crack impeding mechanisms developed due to the presence of graphene. This dissertation delves into the fracture mechanics of graphene, both supported and embedded in polymers, utilizing either in-situ Raman spectroscopy or Atomic Force Microscopy (AFM), combined with micromechanical tensile testing. A thorough investigation over the influence of a defect on the fracture behavior and the morphology of graphene is conducted, as the introduction of an induced defect can serve as a model to simulate real-world conditions, where imperfections are inevitable. The presented findings reveal that its existence significantly affects the mechanical behavior and the fracture toughness of both graphene and graphene-based nanocomposites. Therefore, it is essential to monitor the strain/stress distribution close to defects, when external deformation is applied, as an increased stress field could potentially result to crack initiation and propagation. Herein, the fracture behaviour of a novel class of nanocomposites, CVD graphene-based nanolaminates, and the fracture behaviour of mechanically exfoliated graphene is examined. The use of monolayer CVD graphene both as a sensor in polymers and as an enhancing filler impeding the fracture of graphene-based nanolaminates is studied. The analysis emphasizes on the determination of the stress distribution profiles at the vicinity of an induced defect. In the case of graphene-based nanolaminates, valuable information is provided over the fracture procedure, such as the fracture toughness, the energy dissipation and the mechanisms that impede the crack propagation. The insights of this investigation offer a comprehensive understanding of fracture mechanics in graphene-based materials offering pathways over the harnessing of their exceptional properties. In the case of mechanically exfoliated graphene, a comprehensive analysis over the fracture behaviour of mono- and few- layered graphene, in which a defect has been induced, is conducted, unravelling unique crack and wrinkle patterns, due to strain imposition.