Research Papers - Department of Civil Engineering
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Publication Embargo Indentation of a nanolayer on a substrate by a rigid cylinder in adhesive contact(Springer Vienna, 2020-08) Tirapat, T; Senjuntichai, T; Rungamornrat, J; Rajapakse, R. K. N. DNanoindentation is employed to characterize the mechanical properties at the nanoscale. This paper considers the mechanical response of a nanoscale elastic layer on an elastic substrate that is indented by an adhesively bonded flat-ended rigid cylindrical punch. The complete Gurtin–Murdoch continuum model is employed to capture the size effects. The contact problem is analyzed by relating displacements of the contact region to contact stresses by a flexibility equation system, which is developed by discretizing the contact region into annular elements. The flexibility equation involves displacement influence functions corresponding to axisymmetric normal and radial surface ring loads applied on the layer-substrate system. The displacement influence functions are derived by using the Hankel integral transforms. Convergence and accuracy of the proposed solution scheme are verified by comparing with limiting cases such as the classical elasticity solution. Selected numerical results indicate that the substrate becomes stiffer and the elastic field is size-dependent due to the surface energy effects.Publication Open Access Effect of electric boundary conditions on crack propagation in ferroelectric ceramics(The Chinese Society of Theoretical and Applied Mechanics; Institute of Mechanics, Chinese Academy of Sciences, 2014-04) Rajapakse, R. K. N. D; Li, F-X; Sun, YIn this paper, the effect of electric boundary conditions on Mode I crack propagation in ferroelectric ceramics is studied by using both linear and nonlinear piezoelectric fracture mechanics. In linear analysis, impermeable cracks under open circuit and short circuit are analyzed using the Stroh formalism and a rescaling method. It is shown that the energy release rate in short circuit is larger than that in open circuit. In nonlinear analysis, permeable crack conditions are used and the nonlinear effect of domain switching near a crack tip is considered using an energy-based switching criterion proposed by Hwang et al. (Acta Metal. Mater., 1995). In open circuit, a large depolarization field induced by domain switching makes switching much more difficult than that in short circuit. Analysis shows that the energy release rate in short circuit is still larger than that in open circuit, and is also larger than the linear result. Consequently, whether using linear or nonlinear fracture analysis, a crack is found easier to propagate in short circuit than in open circuit, which is consistent with the experimental observations of Kounga Njiwa et al. (Eng. Fract. Mech., 2006).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 Numerical modelling of piezoelectric actuators exposed to hydrogen(Springer Vienna, 2014-10) Sapsathiarn, Y; Singh, Y; Rajapakse, R. K. N. DModern fuel injectors have been developed based on piezoelectric stack actuators. Performance and durability of actuators in a hydrogen environment are important considerations in the development of hydrogen injectors. 2D plane stress and 3D models for analysis of coupled diffusion and thermo-electromechanical response of actuators are presented. Chemical potential, electric field and temperature gradients are taken as driving forces for hydrogen transport. The explicit Euler finite difference method is used to solve the nonlinear diffusion governing equation. The finite element method is used for time-dependent analysis of fully coupled mechanical, electric and thermal fields. The diffusion process and thermo-electromechanical deformations are coupled through the dependence of piezoelectric properties on hydrogen concentration. Experimental results for the piezoelectric coefficient d 33 of PZT ceramics exposed to different hydrogen concentrations are used. A comparison of a fully coupled 2D model with 2D and 3D models with reduced coupling is made to examine the significance of coupling and computational efficiency. Selected numerical results are presented for time histories of hydrogen concentration, temperature and stroke of an idealized actuator unit cell to obtain a preliminary understanding of the performance of actuators exposed to hydrogen.Publication Embargo Performance of piezoelectric actuators in a hydrogen environment: Experimental study and finite element modelling(Pergamon, 2015-03-02) Singh, Y; Rajapakse, R. K. N. D; Kjeang, E; Mumford, DSignificant improvements in fuel efficiency and emissions can be achieved in internal combustion engines (ICE) by electronically controlling the fuel injector opening valves with piezoelectric actuators. Hydrogen is considered an attractive alternative fuel with near-zero emissions at the point of use; however, the current understanding of the performance of piezoelectric actuators in a hydrogen environment is very limited. Variation in the performance of piezoelectric actuators due to their continuous and cyclic exposure to hydrogen at 100 °C and 10 MPa is experimentally investigated in the present work. The actuator's stroke-voltage relationship is evaluated under quasi-static as well as dynamic electric loading conditions within the ambient temperature range of 5–80 °C. A 3-D finite element model is also developed to simulate the behaviour of a single stack of an actuator exposed to hydrogen by using experimentally determined piezoelectric coefficients. The importance of coating technology to protect the actuator material from hydrogen is confirmed by the experimental study and numerical modelling.Publication Embargo An improved theoretical process-zone model for delayed hydride cracking initiation at a blunt V-notch(Pergamon, 2018-04-01) Huang, Y; Rajapakse, R. K. N. DDelayed hydride cracking (DHC) is an important concern for pressure tubes used in nuclear reactors. In this paper, an improved analytical process-zone model is developed based on the deformation fracture criteria. A V-notch with rounded root, which is widely adopted in mechanical testing of DHC, is considered and the proposed model includes the effect of both notch angle and tip radius. Comparisons with experiments show that the proposed model has a prediction accuracy closer to the current engineering process-zone model but with slightly less conservatism. The model is extended to account for plasticity and constraint effects at the flaw tip by introducing an empirical factor that depends on key material and geometric parameters.Publication Embargo Mechanistic models for nanobeams with surface stress effects(American Society of Civil Engineers, 2018-11-01) Sapsathiarn, Y; Rajapakse, R. K. N. DIn this paper, a mechanistic model for nanobeams with surface energy effects is developed by using a variational formulation. Thiswork is motivated by the unusual response of nanocantilevers predicted by models based on the Young-Laplace equation for surface stress.The governing equation and boundary conditions derived from the variational methods are compared with the governing equations andboundary conditions used in the Young-Laplace models and other formulations. A key difference in the shear force boundary condition isnoted. Analytical solutions for simply supported, cantilevered, and fixed-fixed beams are reexamined. It is shown that the unusual behavior ofnanocantilevers predicted by the Young-Laplace models is due to the shear force boundary condition used. The current formulation leads toconsistent solutions for beams under different boundary conditions.DOI:10.1061/(ASCE)EM.1943-7889.0001520.© 2018 AmericanSociety of Civil EngineersPublication 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 Open Access Atomic-scale finite element modelling of mechanical behaviour of graphene nanoribbons(Springer Netherlands, 2019-03) Damasceno, D. A; Mesquita, E; Rajapakse, R. K. N. D; Pavanello, RExperimental characterization of Graphene NanoRibbons (GNRs) is still an expensive task and computational simulations are therefore seen as a practical option to study the properties and mechanical response of GNRs. Design of GNR elements in various nanotechnology devices can be approached through molecular dynamics simulations. This study demonstrates that the atomic-scale finite element method (AFEM) based on the second generation REBO potential is an efficient and accurate alternative to the molecular dynamics simulation of GNRs. Special atomic finite elements are proposed to model graphene edges. Extensive comparisons are presented with MD solutions to establish the accuracy of AFEM. It is also shown that the Tersoff potential is not accurate for GNR modeling. The study demonstrates the influence of chirality and size on design parameters such as tensile strength and stiffness. Graphene is stronger and stiffer in the zigzag direction compared to the armchair direction. Armchair GNRs shows a minor dependence of tensile strength and elastic modulus on size whereas in the case of zigzag GNRs both modulus and strength show a significant size dependency. The size-dependency trend noted in the present study is different from the previously reported MD solutions for GNRs but qualitatively agrees with experimental results. Based on the present study, AFEM can be considered a highly efficient computational tool for analysis and design of GNRs.