Response of 2D materials due to mechanical stimuli : exploring the fracture regime
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Date
2024-12-10
Authors
Μπέλλου, Έλλη
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Abstract
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.
The content of this dissertation can be conceptually divided into 6 Chapters. An introduction to graphene and its exceptional properties, which renders its incorporation into nanocomposites as an enticing alternative towards the reinforcement of materials is provided in Chapter 1. Chapter 2 delves into fracture mechanics and its important parameters, focusing on the fracture toughness of graphene-based materials, as well as the role of factors affecting the mechanical properties of graphene-based nanocomposites (i.e. filler content, size, dispersion etc.). In Chapter 3, a detailed description of all experimental techniques used in this dissertation is provided. In Chapter 4, the use of CVD graphene (produced via Chemical Vapor Deposition) as a strain sensor for the quantification of strain close to defects, and for the extraction of valuable parameters related to fracture, such as Stress Concentration Factor (SCF) is proposed. The results were validated both analytically and numerically via Finite Element Analysis, with the SCF values being remarkably similar, suggesting that CVD graphene can be successfully used as a strain sensor for the assessment of strain/stress distribution close to defects in polymers, and therefore to polymer-based composites. Chapter 5 deals with the study of fracture mechanisms in a novel class of graphene nanocomposites, the CVD graphene/polymer nanolaminates, when subjected to tension. A thorough investigation over their fracture behaviour reveals that a low graphene content leads to the enhancement of the nanolaminate’s fracture toughness, whereas a higher graphene content leads to its embrittlement, despite the enhancement providing to the nanocomposite. In Chapter 6, fracture mechanics and strain-induced morphological alterations of notched mechanically exfoliated graphene flakes are investigated, unveiling novel insights and contribute to the advancement of nanotechnology field, through the design and optimization of nanomaterials with tailored properties and functionalities for future applications. Lastly, in Chapter 7, the results obtained in this thesis are summarized and potential topics for future work in the study of fracture of 2D materials and graphene-based nanocomposites are proposed.
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Keywords
Graphene, Graphene-based nanolaminates, Micro-mechanical testing, Raman spectroscopy, Atomic force microscopy, Fracture, Crack impeding mechanisms, Graphene-based strain sensors