Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer memb... more Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer membranes under hygrothermal and mechanical cyclic loading is of vital significance in designing more durable fuel cell stacks (1). Hence, we have developed a test procedure to shed light on the crack propagation characteristics of membranes as a function of loading and environmental conditions. Rectangular specimens with a width of 10 mm and cracks with an average length of 0.7 mm on both sides are used. The specimens are placed under cyclic mechanical loading in a controlled level of environmental conditions (temperature and humidity). The crack propagation rate in the membrane is measured as a function of temperature, humidity and the amplitude of mechanical loading (Figure 1). It is found that a small linear decrease in the magnitude of the force results in an exponential decrement in the crack propagation rate. Furthermore, for a given state of mechanical load, it is found that increasi...
This paper introduces a numerical model to investigate the vertical vibrations of a long cylindri... more This paper introduces a numerical model to investigate the vertical vibrations of a long cylindrical rigid bar that is partially embedded within a homogeneous, viscoelastic, transversely isotropic, three-dimensional half-space. In the present numerical scheme, the boundary-value problem corresponding to the case of time-harmonic distributed axisymmetric vertical and radial ring loads within a homogenous, transversely isotropic, 3D half-space is presented explicitly. Its solution results in the required influence functions for the modeling of the present problem. The rigid cylinder is modeled as multiple circular fields of distributed vertical and radial forces, which are evenly distributed in the vertical direction. The vertical and radial displacements in each of these circular surfaces depend on the vertical and radial tractions acting on all its neighboring circular surfaces. This strategy ensures that the present model properly takes into account the body forces acting on the column resulting from its dynamic excitation. The results obtained from the proposed numerical scheme are compared with previous results from the literature. The impedance of the system comprising the bar and its surrounding half-space are shown for different frequencies of excitation and for different material compositions. The present model contributes to the study of dynamic response of buried piles in homogeneous soils.
Recently, nano devices have been developed which use Carbon Nanotubes (CNTs) as structural elemen... more Recently, nano devices have been developed which use Carbon Nanotubes (CNTs) as structural elements. To define the range of applicability of CNTs in such devices, it is important to investigate failure modes such as the axial buckling limit. Classical continuum models are inaccurate as they are unable to account for the size-effects in such devices. In this work, a modified nonlocal continuum shell model for the axial buckling of CNTs is proposed and compared with a nonlocal model for torsional buckling. This is done through modifying classical continuum models by incorporating basic concepts from nonlocal elasticity. Furthermore, molecular dynamics (MD) simulations are performed on a range of nanotubes with different diameters. Compared to classical models, the modified nonlocal models provide a much better fit to MD simulation results. Using MD simulation results for axial buckling, values of the nonlocal constant and shell thickness are calculated.
The introduction of Graphene Oxide (GO), a nanomaterial, has shown considerable promise in improv... more The introduction of Graphene Oxide (GO), a nanomaterial, has shown considerable promise in improving the mechanical properties of cement composites. However, the reasons for this improvement are not yet fully understood and demand further research. This study aims to understand the effect of laboratory-produced GO, using Tour’s method, on the mechanical properties and morphology of cement mortar containing GO. The GO was characterized using Fourier-transform infrared spectroscopy, X-ray Photoelectron Spectroscopy (XRD), X-ray powder diffraction, and Raman spectroscopy alongside Scanning electron microscopy (SEM). This study adopted a cement mortar with GO percentages of 0.02, 0.025, 0.03, 0.035, and 0.04 with respect to the weight of the cement. The presence of GO in cement mortar increased the density and decreased the consistency and setting times. At the optimum of 0.03% GO viscous suspension, the mechanical properties such as the 28-day compressive strength, splitting tensile st...
IUTAM Symposium on Surface Effects in the Mechanics of Nanomaterials and Heterostructures, 2012
Nanoscale structures are the key components of NEMS-based sensor and actuator technology. They ha... more Nanoscale structures are the key components of NEMS-based sensor and actuator technology. They have a high surface area to volume ratio. Consequently, surface energy has a significant effect on the response of nanoscale structures, and is attributed to their size-dependent behaviour. In this paper, uniaxial buckling and transverse vibration of nanoscale beams are studied using Rayleigh’s energy method (Washizu, Variational principles in continuum mechanics. Department of Aeronautical Engineering Report, University of Washington, Seattle, WA, 1962). Surface energy is incorporated using Gurtin–Murdoch surface elasticity theory (Gurtin ME, Murdoch, Arch Rat Mech Anal 57(4):291–323; Gurtin ME, Murdoch AI, Arch Rat Mech Anal 59:389–390, 1975). Closed–form analytical solutions for critical compressive force and natural frequencies of free vibration of nanoscale beams under various boundary conditions (simply-supported, fixed-free and fixed-fixed ends) are derived using Rayleigh’s energy method based on Rayleigh quotient. The solutions from the energy method are shown to be in good agreement with those obtained using exact theory developed by the authors, previously (Liu C, Rajapakse RKND, IEEE Trans Nanotechnol 9(4):422–431, 2010; Liu C, Rajapakse RKND, Srikantha Phani A, J Appl Mech 78(3), 031014, 2011. Selected numerical results are presented for aluminum and silicon beams with [100] surfaces to demonstrate their salient response features. It is shown that both surface elasticity and surface residual stress influence the critical loads and natural frequencies. Their effects will become more pronounced with decreasing thickness of the beam. The influence of surface energy is shown to depend upon the boundary conditions. The analytical solution for natural frequency is further employed to fit the experimentally measured natural frequencies of GaAs fixed-free beams reported by Lagowski and his coworkers (Lagowski J, Gatos HC, Sproles ES Jr, Appl Phys Lett 26:493, 1975). The present formulation offers an explanation for the dependence of the experimentally observed first natural frequency of a GaAs fixed-free beam specimen. A technique for the determination of surface elastic constants and surface residual stress by measuring the natural frequency of free vibration is then proposed. The energy method incorporating surface effects provides an efficient approximation in cases where an equilibrium solution would not be practical due to variable geometry and material properties, and it can be applied to investigate the buckling and vibration of nanoscale beams encountered in the NEMS device design.
Carbon nanotubes (CNTs) have attracted considerable attention due to their remarkable properties.... more Carbon nanotubes (CNTs) have attracted considerable attention due to their remarkable properties. Torsional oscillators based on CNTs have been fabricated. Fabrication of CNT-based nanomechanical devices has to be supported in parallel by theoretical/computational models. It is recognized that modified continuum models that account for size-dependent behaviour are attractive in the analysis of nanomaterial systems. Constitutive properties such as Young’s modulus, shear modulus and Poisson’s ratio are essential in continuum modeling. This paper presents a comprehensive study of the torsional response of a CNT using MD simulations based on the REBO and AIREBO potentials. Numerical simulations are presented for different chiralities and diameters to understand the influence of potential function on size-dependency of the shear modulus. The size and potential function dependence of ultimate strength in torsion is also investigated. In addition, the mechanical response expressed in terms...
We investigated the temperature and strain rate dependent fracture strength of defective graphene... more We investigated the temperature and strain rate dependent fracture strength of defective graphene using molecular dynamics and an atomistic model. This atomistic model was developed by introducing the influence of strain rate and vacancy defects into the kinetics of graphene. We also proposed a novel continuum based fracture mechanics framework to characterize the temperature and strain rate dependent strength of defective sheets. The strength of graphene highly depends on vacancy concentration, temperature, and strain rate. Molecular dynamics simulations, which are generally performed under high strain rates, exceedingly overpredict the strength of graphene at elevated temperatures. Graphene sheets with random vacancies demonstrate a singular stress field as in continuum fracture mechanics. Molecular dynamics simulations on the crack propagation reveal that the energy dissipation rate indicates proportionality with the strength. These findings provide a remarkable insight into the ...
ABSTRACT This paper presents a comprehensive molecular dynamics study on the effects of nanocrack... more ABSTRACT This 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 ( $J_\\mathrm{IC}$ ) of armchair graphene at various crack lengths. Results show that $J_\\mathrm{IC}$ depends on the crack length. This length dependency of $J_\\mathrm{IC}$ 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
Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer memb... more Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer membranes under hygrothermal and mechanical cyclic loading is of vital significance in designing more durable fuel cell stacks (1). Hence, we have developed a test procedure to shed light on the crack propagation characteristics of membranes as a function of loading and environmental conditions. Rectangular specimens with a width of 10 mm and cracks with an average length of 0.7 mm on both sides are used. The specimens are placed under cyclic mechanical loading in a controlled level of environmental conditions (temperature and humidity). The crack propagation rate in the membrane is measured as a function of temperature, humidity and the amplitude of mechanical loading (Figure 1). It is found that a small linear decrease in the magnitude of the force results in an exponential decrement in the crack propagation rate. Furthermore, for a given state of mechanical load, it is found that increasi...
This paper introduces a numerical model to investigate the vertical vibrations of a long cylindri... more This paper introduces a numerical model to investigate the vertical vibrations of a long cylindrical rigid bar that is partially embedded within a homogeneous, viscoelastic, transversely isotropic, three-dimensional half-space. In the present numerical scheme, the boundary-value problem corresponding to the case of time-harmonic distributed axisymmetric vertical and radial ring loads within a homogenous, transversely isotropic, 3D half-space is presented explicitly. Its solution results in the required influence functions for the modeling of the present problem. The rigid cylinder is modeled as multiple circular fields of distributed vertical and radial forces, which are evenly distributed in the vertical direction. The vertical and radial displacements in each of these circular surfaces depend on the vertical and radial tractions acting on all its neighboring circular surfaces. This strategy ensures that the present model properly takes into account the body forces acting on the column resulting from its dynamic excitation. The results obtained from the proposed numerical scheme are compared with previous results from the literature. The impedance of the system comprising the bar and its surrounding half-space are shown for different frequencies of excitation and for different material compositions. The present model contributes to the study of dynamic response of buried piles in homogeneous soils.
Recently, nano devices have been developed which use Carbon Nanotubes (CNTs) as structural elemen... more Recently, nano devices have been developed which use Carbon Nanotubes (CNTs) as structural elements. To define the range of applicability of CNTs in such devices, it is important to investigate failure modes such as the axial buckling limit. Classical continuum models are inaccurate as they are unable to account for the size-effects in such devices. In this work, a modified nonlocal continuum shell model for the axial buckling of CNTs is proposed and compared with a nonlocal model for torsional buckling. This is done through modifying classical continuum models by incorporating basic concepts from nonlocal elasticity. Furthermore, molecular dynamics (MD) simulations are performed on a range of nanotubes with different diameters. Compared to classical models, the modified nonlocal models provide a much better fit to MD simulation results. Using MD simulation results for axial buckling, values of the nonlocal constant and shell thickness are calculated.
The introduction of Graphene Oxide (GO), a nanomaterial, has shown considerable promise in improv... more The introduction of Graphene Oxide (GO), a nanomaterial, has shown considerable promise in improving the mechanical properties of cement composites. However, the reasons for this improvement are not yet fully understood and demand further research. This study aims to understand the effect of laboratory-produced GO, using Tour’s method, on the mechanical properties and morphology of cement mortar containing GO. The GO was characterized using Fourier-transform infrared spectroscopy, X-ray Photoelectron Spectroscopy (XRD), X-ray powder diffraction, and Raman spectroscopy alongside Scanning electron microscopy (SEM). This study adopted a cement mortar with GO percentages of 0.02, 0.025, 0.03, 0.035, and 0.04 with respect to the weight of the cement. The presence of GO in cement mortar increased the density and decreased the consistency and setting times. At the optimum of 0.03% GO viscous suspension, the mechanical properties such as the 28-day compressive strength, splitting tensile st...
IUTAM Symposium on Surface Effects in the Mechanics of Nanomaterials and Heterostructures, 2012
Nanoscale structures are the key components of NEMS-based sensor and actuator technology. They ha... more Nanoscale structures are the key components of NEMS-based sensor and actuator technology. They have a high surface area to volume ratio. Consequently, surface energy has a significant effect on the response of nanoscale structures, and is attributed to their size-dependent behaviour. In this paper, uniaxial buckling and transverse vibration of nanoscale beams are studied using Rayleigh’s energy method (Washizu, Variational principles in continuum mechanics. Department of Aeronautical Engineering Report, University of Washington, Seattle, WA, 1962). Surface energy is incorporated using Gurtin–Murdoch surface elasticity theory (Gurtin ME, Murdoch, Arch Rat Mech Anal 57(4):291–323; Gurtin ME, Murdoch AI, Arch Rat Mech Anal 59:389–390, 1975). Closed–form analytical solutions for critical compressive force and natural frequencies of free vibration of nanoscale beams under various boundary conditions (simply-supported, fixed-free and fixed-fixed ends) are derived using Rayleigh’s energy method based on Rayleigh quotient. The solutions from the energy method are shown to be in good agreement with those obtained using exact theory developed by the authors, previously (Liu C, Rajapakse RKND, IEEE Trans Nanotechnol 9(4):422–431, 2010; Liu C, Rajapakse RKND, Srikantha Phani A, J Appl Mech 78(3), 031014, 2011. Selected numerical results are presented for aluminum and silicon beams with [100] surfaces to demonstrate their salient response features. It is shown that both surface elasticity and surface residual stress influence the critical loads and natural frequencies. Their effects will become more pronounced with decreasing thickness of the beam. The influence of surface energy is shown to depend upon the boundary conditions. The analytical solution for natural frequency is further employed to fit the experimentally measured natural frequencies of GaAs fixed-free beams reported by Lagowski and his coworkers (Lagowski J, Gatos HC, Sproles ES Jr, Appl Phys Lett 26:493, 1975). The present formulation offers an explanation for the dependence of the experimentally observed first natural frequency of a GaAs fixed-free beam specimen. A technique for the determination of surface elastic constants and surface residual stress by measuring the natural frequency of free vibration is then proposed. The energy method incorporating surface effects provides an efficient approximation in cases where an equilibrium solution would not be practical due to variable geometry and material properties, and it can be applied to investigate the buckling and vibration of nanoscale beams encountered in the NEMS device design.
Carbon nanotubes (CNTs) have attracted considerable attention due to their remarkable properties.... more Carbon nanotubes (CNTs) have attracted considerable attention due to their remarkable properties. Torsional oscillators based on CNTs have been fabricated. Fabrication of CNT-based nanomechanical devices has to be supported in parallel by theoretical/computational models. It is recognized that modified continuum models that account for size-dependent behaviour are attractive in the analysis of nanomaterial systems. Constitutive properties such as Young’s modulus, shear modulus and Poisson’s ratio are essential in continuum modeling. This paper presents a comprehensive study of the torsional response of a CNT using MD simulations based on the REBO and AIREBO potentials. Numerical simulations are presented for different chiralities and diameters to understand the influence of potential function on size-dependency of the shear modulus. The size and potential function dependence of ultimate strength in torsion is also investigated. In addition, the mechanical response expressed in terms...
We investigated the temperature and strain rate dependent fracture strength of defective graphene... more We investigated the temperature and strain rate dependent fracture strength of defective graphene using molecular dynamics and an atomistic model. This atomistic model was developed by introducing the influence of strain rate and vacancy defects into the kinetics of graphene. We also proposed a novel continuum based fracture mechanics framework to characterize the temperature and strain rate dependent strength of defective sheets. The strength of graphene highly depends on vacancy concentration, temperature, and strain rate. Molecular dynamics simulations, which are generally performed under high strain rates, exceedingly overpredict the strength of graphene at elevated temperatures. Graphene sheets with random vacancies demonstrate a singular stress field as in continuum fracture mechanics. Molecular dynamics simulations on the crack propagation reveal that the energy dissipation rate indicates proportionality with the strength. These findings provide a remarkable insight into the ...
ABSTRACT This paper presents a comprehensive molecular dynamics study on the effects of nanocrack... more ABSTRACT This 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 ( $J_\\mathrm{IC}$ ) of armchair graphene at various crack lengths. Results show that $J_\\mathrm{IC}$ depends on the crack length. This length dependency of $J_\\mathrm{IC}$ 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
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