Faculty of Engineering
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Publication Embargo Atomistic and continuum modelling of stress field at an inhomogeneity in graphene(Elsevier, 2018-12-15) Dewapriya, M. A. N; Rajapakse, R. K. N. DThe influence of an atomic inhomogeneity on the resulting stress field of a nanoscopic matrix material can be remarkably different from the corresponding continuum descriptions due to the significance of surface energy and the discrete nature of matter at the nanoscale. In this work, we conducted a comprehensive molecular dynamics study to investigate the stress field at an atomic inhomogeneity, in the form of an elliptical hole or a circular hexagonal boron-nitride inclusion, in graphene. The results show that stress concentration factor at an inhomogeneity is higher than the corresponding classical continuum solution. We estimated the surface elastic constants for a modified continuum framework using the molecular dynamics results. Comparison between the atomic simulations and the modified continuum model reveals the limitations of such continuum-based models for the two-dimensional materials. Molecular dynamics results imply that the underlying atomic structure softens the effect of inhomogeneity compared to a continuum description thus causing an amplification of the stress filed. The molecular dynamics and modified continuum solutions for stress concentration are presented in simplified forms and design charts to facilitate preliminary design of graphene-based hybrid materials.Publication Open Access Atomistic modelling of size-dependent mechanical properties and fracture of pristine and defective cove-edged graphene nanoribbons(Multidisciplinary Digital Publishing Institute, 2020-07) Damasceno, D. A; Rajapakse, R. K. N. D; Mesquita, ECove-edged graphene nanoribbons (CGNR) are a class of nanoribbons with asymmetric edges composed of alternating hexagons and have remarkable electronic properties. Although CGNRs have attractive size-dependent electronic properties their mechanical properties have not been well understood. In practical applications, the mechanical properties such as tensile strength, ductility and fracture toughness play an important role, especially during device fabrication and operation. This work aims to fill a gap in the understanding of the mechanical behaviour of CGNRs by studying the edge and size effects on the mechanical response by using molecular dynamic simulations. Pristine graphene structures are rarely found in applications. Therefore, this study also examines the effects of topological defects on the mechanical behaviour of CGNR. Ductility and fracture patterns of CGNR with divacancy and topological defects are studied. The results reveal that the CGNR become stronger and slightly more ductile as the width increases in contrast to normal zigzag GNR. Furthermore, the mechanical response of defective CGNRs show complex dependency on the defect configuration and distribution, while the direction of the fracture propagation has a complex dependency on the defect configuration and position. The results also confirm the possibility of topological design of graphene to tailor properties through the manipulation of defect types, orientation, and density and defect networks.Publication Open Access Atomistic simulation of tensile strength properties of graphene with complex vacancy and topological defects(Springer Vienna, 2020-08) Damasceno, D. A; Rajapakse, R. K. N. D; Mesquita, E; Pavanello, RDefects including topological and vacancy defects have been observed in graphene during fabrication. Defects are also introduced to break the lattice symmetry of graphene and thereby obtain enhanced optoelectronic and other properties. It is important that gains in certain properties due to the presence defects are not at the expense of mechanical strength which is important in handling graphene and device fabrication. This paper presents a comprehensive study of the tensile strength and fracture strain of monolayer graphene with commonly observed topological defects and nanopores. Both molecular dynamics and the atomic-scale finite element method (AFEM) are used in this study, and the accuracy of AFEM in simulating complex topological and vacancy defects including line defects is established. It is found that the tensile strength properties have a complex dependency on the defect shape, size, and chirality. Certain defect geometries are found to be mechanically superior to other defect geometries thereby supporting the concept of topological design of graphene to optimize properties. The study also establishes AFEM as an efficient and potential tool for topological optimization of the mechanical behaviour of graphene.Publication Embargo Atomistic modelling of crack-inclusion interaction in graphene(Pergamon, 2018-05-15) Dewapriya, M. A. N; Meguid, S. A; Rajapakse, R. K. N. DIn continuum fracture mechanics, it is well established that the presence of crack near an inclusion leads to a significant change in the crack-tip stress field. However, it is unclear how atomistic crack-inclusion interaction manifests itself at the nanoscale where the continuum description of matter breaks down. In this work, we conducted molecular dynamics simulations to investigate the interactions of an atomic-scale boron nitride inclusion with an edge crack in a graphene sheet. Numerical simulations of nanoscale tensile tests were obtained for graphene samples containing an edge crack and a circular inclusion. Stress analysis of the samples show the complex nature of the stress state at the crack-tip due to the crack-inclusion interaction. Results reveal that the inclusion results in an increase (amplification) or a decrease (shielding) of the crack-tip stress field depending on the location of the inclusion relative to the crack-tip. Our numerical experiments unveil that inclusions of specific locations could lead to a reduction in the fracture resistance of graphene. Results of the crack-inclusion interaction study were compared with the corresponding results of crack-hole interaction problem. The study also provides an insight into the applicability of well-established continuum crack-microdefect interaction models for the corresponding atomic scale problems.Publication Embargo Effects of free edges and vacancy defects on the mechanical properties of graphene(IEEE, 2014-08-18) Dewapriya, M. A. N; Rajapakse, R. K. N. DDefects are unavoidable during synthesizing and fabrication of graphene based nanoelecromechanical systems. This paper presents a comprehensive molecular dynamics simulation study on the mechanical properties of finite graphene with vacancy defects. We characterize the strength and stiffness of graphene using the concept of surface stress in three-dimensional crystals. Temperature and strain rate dependent atomistic model is also presented to evaluate the strength of defective graphene. Free edges have a significant impact on the stiffness; the strength, however, is less affected. The vacancies exceedingly degrade the strength and the stiffness of graphene. These findings provide a remarkable insight into the strength and the stiffness of defective graphene, which is critical in designing experimental and instrumental applications.Publication Embargo Influence of hydrogen functionalization on the fracture strength of graphene and the interfacial properties of graphene–polymer nanocomposite(Pergamon, 2015-11-01) Dewapriya, M. A. N; Rajapakse, R. K. N. D; Nigam, NUsing molecular dynamics and classical continuum concepts, we investigated the effects of hydrogen functionalization on the fracture strength of graphene and also on the interfacial properties of graphene–polymer nanocomposite. Moreover, we developed an atomistic model to assess the temperature and strain rate dependent fracture strength of functionalized graphene along various chiral directions. Results indicate that hydrogen functionalization at elevated temperatures highly degrade the fracture strength of graphene. The functionalization also deteriorates the interfacial strength of graphene–polymer nanocomposite. Near-crack-tip stress distribution depicted by continuum mechanics can be successfully used to investigate the impact of hydrogen passivation of dangling carbon bonds on the strength of graphene. We further derived a continuum-based model to characterize the non-bonded interaction of graphene–polymer nanocomposite. These results indicate that classical continuum concepts are accurate even at a scale of several nanometers. Our work provides a remarkable insight into the fracture strength of graphene and graphene–polymer nanocomposites, which are critical in designing experimental and instrumental applications.Publication Embargo Influence of temperature and free edges on the mechanical properties of graphene(IOP Publishing, 2013-08-12) Dewapriya, M. A. N; Phani, A Srikantha; Rajapakse, R. K. N. DA systematic molecular dynamics simulation study is performed to assess the effects of temperature and free edges on the ultimate tensile strength and Young's modulus of a single-layer graphene sheet. It is observed that graphene sheets at higher temperatures fail at lower strains, due to the high kinetic energy of atoms. A numerical model, based on kinetic analysis, is used to predict the ultimate strength of the graphene under various temperatures and strain rates. As the width of a graphene reduces, the excess edge energy associated with free edge atoms induces an initial strain on the relaxed configuration of the sheets. This initial strain has a greater influence on the Young's modulus of the zigzag sheet compared with that of the armchair sheets. The simulations reveal that the carbon–carbon bond length and amplitude of intrinsic ripples of the graphene increases with temperature. The initial out-of-plane displacement of carbon atoms is necessary to simulate the physical behaviour of a graphene when the Nosé–Hoover or Berendsen thermostat is used.Publication Embargo Atomistic and continuum modelling of temperature-dependent fracture of graphene(springer, 2014-06-01) Dewapriya, M. A. N; Rajapakse, R. K. N. D; Phani, A.SThis paper presents a comprehensive molecular dynamics study on the effects of nanocracks (a row of vacancies) on the fracture strength of graphene sheets at various temperatures. Comparison of the strength given by molecular dynamics simulations with Griffith’s criterion and quantized fracture mechanics theory demonstrates that quantized fracture mechanics is more accurate compared to Griffith’s criterion. A numerical model based on kinetic analysis and quantized fracture mechanics theory is proposed. The model is computationally very efficient and it quite accurately predicts the fracture strength of graphene with defects at various temperatures. Critical stress intensity factors in mode I fracture reduce as temperature increases. Molecular dynamics simulations are used to calculate the critical values of J integral (JIC) of armchair graphene at various crack lengths. Results show that JIC depends on the crack length. This length dependency of JIC can be used to explain the deviation of the strength from Griffith’s criterion. The paper provides an in-depth understanding of fracture of graphene, and the findings are important in the design of graphene based nanomechanical systems and composite materials