Sam Nakhla - Academia.edu (original) (raw)

Papers by Sam Nakhla

Research paper thumbnail of Solar Wind Ion Sputtering of Sodium from Silicates Using Molecular Dynamics Calculations of Surface Binding Energies

The Astrophysical Journal Letters

For nearly 40 yr, studies of exosphere formation on airless bodies have been hindered by uncertai... more For nearly 40 yr, studies of exosphere formation on airless bodies have been hindered by uncertainties in our understanding of the underlying ion collisional sputtering by the solar wind (SW). These ion impacts on airless bodies play an important role in altering their surface properties and surrounding environment. Much of the collisional sputtering data needed for exosphere studies come from binary collision approximation (BCA) sputtering models. These depend on the surface binding energy (SBE) for the atoms sputtered from the impacted material. However, the SBE is not reliably known for many materials important for planetary science, such as plagioclase feldspars and sodium pyroxenes. BCA models typically approximate the SBE using the cohesive energy for a monoelemental solid. We use molecular dynamics (MD) to provide the first accurate SBE data we are aware of for Na sputtered from the above silicate minerals, which are expected to be important for exospheric formation at Mercur...

Research paper thumbnail of Sputtering of surfaces by ion irradiation: A comparison of molecular dynamics and binary collision approximation models to laboratory measurements

Journal of Applied Physics

We compare various sputtering simulation methods to experimental results in both the low energy (... more We compare various sputtering simulation methods to experimental results in both the low energy (<1 keV) and high energy (≥1 keV) impact regimes for argon ions impacting a pure copper substrate at normal incidence. Our results indicate that for high energy impacts, both binary collision approximation (BCA) and molecular dynamics methods can be used to generate reasonable predictions for the yield and energy distribution of the sputtered atoms. We also find reasonable agreement between the theoretical and experimental results down to impact energies of 600 eV. However, at 200 eV impact energies, significant discrepancies appear between the experimental and theoretical ejecta energy distributions in the peak position, the width of the energy distribution, and the magnitude of the high energy tail. These discrepancies appear to arise from the experimental results being only for atoms sputtered normal to the substrate surface, whereas the theoretical results are integrated over all 2π solid angles above the surface. Using the BCA code SDTrimSP and limiting the results to only atoms emitted within ±15°of the surface normal brings theory and experiment into reasonable agreement. These results suggest that for low energy impacts, the energy distribution of sputtered atoms is highly dependent on the emission angle of the ejecta.

Research paper thumbnail of Buckling And Post Buckling Behavior For Unsymmetrical Laminates Part I: Curing Cycle

Progress in Canadian Mechanical Engineering

This is the first of two companion papers that examine the elastic buckling and post-buckling beh... more This is the first of two companion papers that examine the elastic buckling and post-buckling behavior of thin unsymmetric cross-ply laminates. When cured in a flat mold these panels possess two cylindrical equilibrium configurations. From stability perspective this problem is identified as a bifurcation buckling problem. As explained in literature thermal mismatch between plies and existing geometric imperfections triggered this behavior. Therefore, and according to Koiter it is necessary to measure and account for these geometric imperfections to accurately predict the cured shapes. Since imperfection measurement is challenging and cannot be possible in the stage of design. This work applies a unified finite element methodology based on Koiter's theory to predict the cured shapes. This methodology is consistently applied in commonly used finite element computer codes, ABAQUS, ANSYS and LS-DYNA, and their predictions are compared.

Research paper thumbnail of Considerations when calculating the mechanical properties of single crystals and bulk polycrystals from molecular dynamics simulations

Molecular Simulation, 2020

The choice of a proper interatomic potential is critical to obtaining accurate and realistic mole... more The choice of a proper interatomic potential is critical to obtaining accurate and realistic molecular dynamics results. However, previous studies that have tested the suitability of a potential to predict mechanical properties often do so using elastic constants from a triaxial stress state that ignores Poisson’s effect. While this method is suitable it is not consistent with macroscale experimental methods and cannot provide the complete loading behaviour. Further, there is a lack of knowledge as to whether accuracy in predicting elastic constants from a fixed volume condition indicates accuracy for elastic moduli from uniaxial tensile simulations. Moreover, those studies that did account for Poisson’s effect studied only one crystal orientation and thus assumed potential accuracy is independent of crystal orientation. Results from the current study demonstrated that accuracy of a potential is dependent on the crystal direction. Further, the most accurate potentials for elastic co...

Research paper thumbnail of Mechanical properties of nanocrystalline aluminium: a molecular dynamics investigation

Molecular Simulation

ABSTRACT Uniaxial deformation was performed using molecular dynamics to estimate the mechanical p... more ABSTRACT Uniaxial deformation was performed using molecular dynamics to estimate the mechanical properties of nanocrystalline aluminium. It was observed that the stacking faults and sliding of the grain boundaries affected the mechanical properties. In addition, accumulation of atoms near grain boundaries during deformation hardened the nanocrystalline material as the grain diameter increased (reverse Hall-Petch relation). Further, the effects of strain rate and temperature were investigated with various mean grain diameters. Investigation showed that mechanical properties were independent of tested strain rates (109–1010 s−1) and that the nanocrystalline material softened with increasing temperature. The elastic modulus was then compared to experimental results from literature at room temperature. The change in crystalline structure was observed with respect to percent strain and various mean grain diameters of nanocrystalline aluminium. It was observed that stacking faults increased with decreased mean grain diameter, which led to reduced mechanical properties.

Research paper thumbnail of Applied Finite Element Procedure for Morphing Wing Design

Applied Composite Materials

Bistable composite laminates provide an appealing platform for morphing applications. On the othe... more Bistable composite laminates provide an appealing platform for morphing applications. On the other hand they exhibit geometrically nonlinear behavior and they are sensitive to imperfections. Their curing behavior dictates bifurcation buckling analysis while their actuation requires snapthrough buckling analysis. This work proposes a generalized finite element analysis procedure applying Koiter’s asymptotic postbuckling theory to address their curing and actuation. Initially the postbuckling theory is discussed providing essential aspects required for its application into finite element analysis. A generalized scheme is established for the Koiter-based procedure to enable its incorporation into design optimization routines. To prove its generality, the procedure is implemented into three finite element commercial codes, namely, ABAQUS, ANSYS and LS-DYNA. Best practices for these implementations are provided, then their accuracies are assessed through multiple comparisons with published data. Moreover, Hyper-Elliptic Cambered Span (HECS) Wing design is developed utilizing bistable laminates. Stability characteristics of several design variations of the morphing HECS wing are assessed using the developed procedure. The Koiter-based finite element procedure is proven to be both general and suitable for implementation in different finite element codes to address designs with complex geometry. Therefore, this work provides a unique platform for novel designs employing bistable composites in various engineering applications. Furthermore, it presents a general framework to implement Koiter’s asymptotic postbuckling theory in finite element codes for bifurcation buckling and post-buckling studies of imperfection-sensitive structures.

Research paper thumbnail of Micro-Macro Finite Element Procedure To Predict Elastoplastic Behavior And Ultimate Failure Of Porous Metals

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Generalized Finite Element Procedure For Analysis Of Bistable Composites

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Bistable Composites: Design Platform For Hyper-Elliptic Cambered Span (Hecs) Morphing Wing

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Influence Of Isotropy On Mechanical Properties Of Nanocrystalline Iron

Progress in Canadian Mechanical Engineering. Volume 4

The influence of isotropy on the prediction of mechanical properties for nanocrystalline metals u... more The influence of isotropy on the prediction of mechanical properties for nanocrystalline metals using Molecular Dynamics (MD) is highly ignored in currently available studies found in literature. Many of these studies grossly ignore the isotropic nature of macroscopic metals when attempting to simulate a bulk nanocrystalline material. Moreover, said studies provide no substantial justification for the selection of the number of grains utilized. This is of utmost importance since there may not be a statistically significant number of grains to provide aggregate properties of a simulated bulk material due to the anisotropic nature of many metallic single crystals. As such, these studies unknowingly influence the prediction of mechanical properties such as elastic modulus, Poisson's ratio, and maximum stress. Therefore, the current investigation was performed to obtain reliable mechanical properties of nanocrystalline pure iron of a simulated bulk material through the use of MD. The current investigation provides a thorough guideline for obtaining an isotropic structure prior to obtaining mechanical properties while simultaneously highlighting the inaccuracies of currently available investigations found in literature.

Research paper thumbnail of Influence Of Forcefield Selection On The Formation Of Viable Nanocrystalline Copper Structures Using The Melt Cool Method

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Characterization Of A Tissue-Like Hyperelastic Polymer

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Molecular Dynamics Simulations Of The Hydrogen Embrittlement Base Case: Atomic Hydrogen In A Defect Free Single Crystal

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Finite Element Modeling Of The Elastoplastic Behavior Of Bi-Layer Metalic Composites

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Validating Commonly Used Computer Models For Abdominal Aortic Aneurysms

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Simulating Ion Irradiation Of Surfaces

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Realistic Representative Volume Element Generation For Sintered Solids Part 1: Algorithm For Volume Computation And Geometry Creation

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Actual Microstructural Voids Generation In Finite Element Analysis Utilizing Computed Tomography Scan Of Highly Cross-Linked Epoxy

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Fracture Behavior of Heavily Cross-Linked Epoxy: Testing, Fractography, and Numerical Modeling

Journal of Engineering Materials and Technology

Heavily cross-linked epoxy was characterized under different types of loading. The scope of work ... more Heavily cross-linked epoxy was characterized under different types of loading. The scope of work involves detailed testing procedures utilizing high-precision digital image correlation (DIC) system for all strain measurements. Fractographic analyses using scanning electron microscopy (SEM) were also provided. Besides, computed tomography (CT) scans were employed to characterize existing manufacturing imperfections, i.e., voids. Numerical modeling using extended finite element method (XFEM) utilizing the actual microstructure is conducted. Testing results and fractographic analyses showed that microvoids led to failure initiation at micro lengths. An unstable fracture behavior dominated the final failure under different types of loading. Global plastic deformation was observed in the case of uniaxial tension, while local plasticity was observed in specimens under three-point loading. It can be concluded that epoxies failure under a combined state of stresses is sophisticated, and str...

Research paper thumbnail of A Finite Element Model to Study the Effect of Porosity Location on the Elastic Modulus of a Cantilever Beam

Progress in Canadian Mechanical Engineering, 2018

The effect of the location of porosity concentration on elastic modulus of a cantilever beam is i... more The effect of the location of porosity concentration on elastic modulus of a cantilever beam is investigated. First, two-dimensional investigation with beam theory, Euler-Bernoulli and Timoshenko, was performed to estimate the modulus based on load-deflection curve. Second, threedimensional finite element model (FEM) in Abaqus was developed to identify the effect of porosity concentration. The use of macro-models such as beam theory and threedimensional FEM enabled enhanced understanding of the effect of porosity on modulus.

Research paper thumbnail of Solar Wind Ion Sputtering of Sodium from Silicates Using Molecular Dynamics Calculations of Surface Binding Energies

The Astrophysical Journal Letters

For nearly 40 yr, studies of exosphere formation on airless bodies have been hindered by uncertai... more For nearly 40 yr, studies of exosphere formation on airless bodies have been hindered by uncertainties in our understanding of the underlying ion collisional sputtering by the solar wind (SW). These ion impacts on airless bodies play an important role in altering their surface properties and surrounding environment. Much of the collisional sputtering data needed for exosphere studies come from binary collision approximation (BCA) sputtering models. These depend on the surface binding energy (SBE) for the atoms sputtered from the impacted material. However, the SBE is not reliably known for many materials important for planetary science, such as plagioclase feldspars and sodium pyroxenes. BCA models typically approximate the SBE using the cohesive energy for a monoelemental solid. We use molecular dynamics (MD) to provide the first accurate SBE data we are aware of for Na sputtered from the above silicate minerals, which are expected to be important for exospheric formation at Mercur...

Research paper thumbnail of Sputtering of surfaces by ion irradiation: A comparison of molecular dynamics and binary collision approximation models to laboratory measurements

Journal of Applied Physics

We compare various sputtering simulation methods to experimental results in both the low energy (... more We compare various sputtering simulation methods to experimental results in both the low energy (<1 keV) and high energy (≥1 keV) impact regimes for argon ions impacting a pure copper substrate at normal incidence. Our results indicate that for high energy impacts, both binary collision approximation (BCA) and molecular dynamics methods can be used to generate reasonable predictions for the yield and energy distribution of the sputtered atoms. We also find reasonable agreement between the theoretical and experimental results down to impact energies of 600 eV. However, at 200 eV impact energies, significant discrepancies appear between the experimental and theoretical ejecta energy distributions in the peak position, the width of the energy distribution, and the magnitude of the high energy tail. These discrepancies appear to arise from the experimental results being only for atoms sputtered normal to the substrate surface, whereas the theoretical results are integrated over all 2π solid angles above the surface. Using the BCA code SDTrimSP and limiting the results to only atoms emitted within ±15°of the surface normal brings theory and experiment into reasonable agreement. These results suggest that for low energy impacts, the energy distribution of sputtered atoms is highly dependent on the emission angle of the ejecta.

Research paper thumbnail of Buckling And Post Buckling Behavior For Unsymmetrical Laminates Part I: Curing Cycle

Progress in Canadian Mechanical Engineering

This is the first of two companion papers that examine the elastic buckling and post-buckling beh... more This is the first of two companion papers that examine the elastic buckling and post-buckling behavior of thin unsymmetric cross-ply laminates. When cured in a flat mold these panels possess two cylindrical equilibrium configurations. From stability perspective this problem is identified as a bifurcation buckling problem. As explained in literature thermal mismatch between plies and existing geometric imperfections triggered this behavior. Therefore, and according to Koiter it is necessary to measure and account for these geometric imperfections to accurately predict the cured shapes. Since imperfection measurement is challenging and cannot be possible in the stage of design. This work applies a unified finite element methodology based on Koiter's theory to predict the cured shapes. This methodology is consistently applied in commonly used finite element computer codes, ABAQUS, ANSYS and LS-DYNA, and their predictions are compared.

Research paper thumbnail of Considerations when calculating the mechanical properties of single crystals and bulk polycrystals from molecular dynamics simulations

Molecular Simulation, 2020

The choice of a proper interatomic potential is critical to obtaining accurate and realistic mole... more The choice of a proper interatomic potential is critical to obtaining accurate and realistic molecular dynamics results. However, previous studies that have tested the suitability of a potential to predict mechanical properties often do so using elastic constants from a triaxial stress state that ignores Poisson’s effect. While this method is suitable it is not consistent with macroscale experimental methods and cannot provide the complete loading behaviour. Further, there is a lack of knowledge as to whether accuracy in predicting elastic constants from a fixed volume condition indicates accuracy for elastic moduli from uniaxial tensile simulations. Moreover, those studies that did account for Poisson’s effect studied only one crystal orientation and thus assumed potential accuracy is independent of crystal orientation. Results from the current study demonstrated that accuracy of a potential is dependent on the crystal direction. Further, the most accurate potentials for elastic co...

Research paper thumbnail of Mechanical properties of nanocrystalline aluminium: a molecular dynamics investigation

Molecular Simulation

ABSTRACT Uniaxial deformation was performed using molecular dynamics to estimate the mechanical p... more ABSTRACT Uniaxial deformation was performed using molecular dynamics to estimate the mechanical properties of nanocrystalline aluminium. It was observed that the stacking faults and sliding of the grain boundaries affected the mechanical properties. In addition, accumulation of atoms near grain boundaries during deformation hardened the nanocrystalline material as the grain diameter increased (reverse Hall-Petch relation). Further, the effects of strain rate and temperature were investigated with various mean grain diameters. Investigation showed that mechanical properties were independent of tested strain rates (109–1010 s−1) and that the nanocrystalline material softened with increasing temperature. The elastic modulus was then compared to experimental results from literature at room temperature. The change in crystalline structure was observed with respect to percent strain and various mean grain diameters of nanocrystalline aluminium. It was observed that stacking faults increased with decreased mean grain diameter, which led to reduced mechanical properties.

Research paper thumbnail of Applied Finite Element Procedure for Morphing Wing Design

Applied Composite Materials

Bistable composite laminates provide an appealing platform for morphing applications. On the othe... more Bistable composite laminates provide an appealing platform for morphing applications. On the other hand they exhibit geometrically nonlinear behavior and they are sensitive to imperfections. Their curing behavior dictates bifurcation buckling analysis while their actuation requires snapthrough buckling analysis. This work proposes a generalized finite element analysis procedure applying Koiter’s asymptotic postbuckling theory to address their curing and actuation. Initially the postbuckling theory is discussed providing essential aspects required for its application into finite element analysis. A generalized scheme is established for the Koiter-based procedure to enable its incorporation into design optimization routines. To prove its generality, the procedure is implemented into three finite element commercial codes, namely, ABAQUS, ANSYS and LS-DYNA. Best practices for these implementations are provided, then their accuracies are assessed through multiple comparisons with published data. Moreover, Hyper-Elliptic Cambered Span (HECS) Wing design is developed utilizing bistable laminates. Stability characteristics of several design variations of the morphing HECS wing are assessed using the developed procedure. The Koiter-based finite element procedure is proven to be both general and suitable for implementation in different finite element codes to address designs with complex geometry. Therefore, this work provides a unique platform for novel designs employing bistable composites in various engineering applications. Furthermore, it presents a general framework to implement Koiter’s asymptotic postbuckling theory in finite element codes for bifurcation buckling and post-buckling studies of imperfection-sensitive structures.

Research paper thumbnail of Micro-Macro Finite Element Procedure To Predict Elastoplastic Behavior And Ultimate Failure Of Porous Metals

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Generalized Finite Element Procedure For Analysis Of Bistable Composites

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Bistable Composites: Design Platform For Hyper-Elliptic Cambered Span (Hecs) Morphing Wing

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Influence Of Isotropy On Mechanical Properties Of Nanocrystalline Iron

Progress in Canadian Mechanical Engineering. Volume 4

The influence of isotropy on the prediction of mechanical properties for nanocrystalline metals u... more The influence of isotropy on the prediction of mechanical properties for nanocrystalline metals using Molecular Dynamics (MD) is highly ignored in currently available studies found in literature. Many of these studies grossly ignore the isotropic nature of macroscopic metals when attempting to simulate a bulk nanocrystalline material. Moreover, said studies provide no substantial justification for the selection of the number of grains utilized. This is of utmost importance since there may not be a statistically significant number of grains to provide aggregate properties of a simulated bulk material due to the anisotropic nature of many metallic single crystals. As such, these studies unknowingly influence the prediction of mechanical properties such as elastic modulus, Poisson's ratio, and maximum stress. Therefore, the current investigation was performed to obtain reliable mechanical properties of nanocrystalline pure iron of a simulated bulk material through the use of MD. The current investigation provides a thorough guideline for obtaining an isotropic structure prior to obtaining mechanical properties while simultaneously highlighting the inaccuracies of currently available investigations found in literature.

Research paper thumbnail of Influence Of Forcefield Selection On The Formation Of Viable Nanocrystalline Copper Structures Using The Melt Cool Method

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Characterization Of A Tissue-Like Hyperelastic Polymer

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Molecular Dynamics Simulations Of The Hydrogen Embrittlement Base Case: Atomic Hydrogen In A Defect Free Single Crystal

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Finite Element Modeling Of The Elastoplastic Behavior Of Bi-Layer Metalic Composites

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Validating Commonly Used Computer Models For Abdominal Aortic Aneurysms

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Simulating Ion Irradiation Of Surfaces

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Realistic Representative Volume Element Generation For Sintered Solids Part 1: Algorithm For Volume Computation And Geometry Creation

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Actual Microstructural Voids Generation In Finite Element Analysis Utilizing Computed Tomography Scan Of Highly Cross-Linked Epoxy

Progress in Canadian Mechanical Engineering. Volume 4

Research paper thumbnail of Fracture Behavior of Heavily Cross-Linked Epoxy: Testing, Fractography, and Numerical Modeling

Journal of Engineering Materials and Technology

Heavily cross-linked epoxy was characterized under different types of loading. The scope of work ... more Heavily cross-linked epoxy was characterized under different types of loading. The scope of work involves detailed testing procedures utilizing high-precision digital image correlation (DIC) system for all strain measurements. Fractographic analyses using scanning electron microscopy (SEM) were also provided. Besides, computed tomography (CT) scans were employed to characterize existing manufacturing imperfections, i.e., voids. Numerical modeling using extended finite element method (XFEM) utilizing the actual microstructure is conducted. Testing results and fractographic analyses showed that microvoids led to failure initiation at micro lengths. An unstable fracture behavior dominated the final failure under different types of loading. Global plastic deformation was observed in the case of uniaxial tension, while local plasticity was observed in specimens under three-point loading. It can be concluded that epoxies failure under a combined state of stresses is sophisticated, and str...

Research paper thumbnail of A Finite Element Model to Study the Effect of Porosity Location on the Elastic Modulus of a Cantilever Beam

Progress in Canadian Mechanical Engineering, 2018

The effect of the location of porosity concentration on elastic modulus of a cantilever beam is i... more The effect of the location of porosity concentration on elastic modulus of a cantilever beam is investigated. First, two-dimensional investigation with beam theory, Euler-Bernoulli and Timoshenko, was performed to estimate the modulus based on load-deflection curve. Second, threedimensional finite element model (FEM) in Abaqus was developed to identify the effect of porosity concentration. The use of macro-models such as beam theory and threedimensional FEM enabled enhanced understanding of the effect of porosity on modulus.