Srikantha Phani


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Graduate Student Supervision

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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Growth, buckling, and rupture of abdominal aortic aneurysms (2023)

The objective of this work was to assess the influence of buckling on the growth and rupture of Abdominal Aortic Aneurysms (AAAs). AAAs are one of the most common under-diagnosed cardiovascular diseases in North America. They manifest as localized bulges in arteries, posing a risk of rupture with fatal consequences. Assessing the rupture risk of AAAs is complicated by complex mechano-biological factors; size-based criterion is often used to guide surgical interventions. It is often found that AAAs can exceed the size recommended (≥ 5.5 cm) for surgical intervention, and yet do not rupture, and vice versa. AAAs exceeding the clinically recommended size are often found in a tortuous state. Since in-vivo testing is impossible to assess the rupture risk of AAAs, inflation tests on hyperelastic tubes were performed to advance the mechanistic modelling, and to inform our clinical study. Mechanical instability in a pre-tensioned finite hyperelastic tube subjected to an internal pressure produces a spatially localized bulge. The fate of the bulge under continued inflation is governed by the end- conditions and the initial tension in the tube. Buckling was found to confer protection against rupture. Failure maps in the stretch parameter space and in stretch–tension space are constructed by extending existing membrane theory for bulge formation and buckling. Biologically relevant constitutive models for arteries are implemented in Finite element analyses (FEA). Buckled AAAs were found to grow without rupture under favourable arterial pre-tensions. Failure maps incorporating post-bifurcation (bulging and buckling) response are constructed to delineate the regimes of growth, buckling and rupture of an AAA. Finally, a retrospective review is performed on a database of patients who had undergone AAA repair to assess whether buckling (tortuosity) confers protection against rupture. Greater aortic buckling was found in the unruptured AAAs compared with the ruptured AAAs at the same matched diameter, pointing to the need to incorporate not only the size but also tortuosity in clinical interventions.

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Mechanics of polymer brush based soft active materials (2019)

A brush-like structure emerges from the stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. This structure is due to a competition between the conformational entropic elasticity of grafted polymer chains, and the intra and interchain excluded volume repulsions. Polymer brushes occur in biology: neurofilaments, articulate cartilage, extra cellular biopolymers etc. Recently, engineered soft active materials are developed to produce large controllable and reversible bending and stretching deformations. These materials are the focus of this work.New theoretical models, molecular simulations to assess them, and experimental studies are presented in this work. Mechanical stress within a brush and its dependence on the molecular parameters of the brush and external stimulus (temperature) is studied for the first time. A continuum beam model accounting for the Young-Laplace and the Steigman-Ogden curvature elasticity corrections is developed first to understand the large deformation of a flexible substrate due to a brush grafted on it. This model yields a generalized surface stress-curvature relation that enables one to determine stress from curvature measurements.Strong stretching theory (SST) from polymer physics is combined with continuum mechanics to obtain stress variation in a neutral brush with Gaussian chains. This theory predicts that the normal stress, parallel to the substrate, is a quartic function of the distance from the grafting surface with a maximum at the grafting surface. Idealizing the brush as a continuum elastic surface with residual stress, closed-form expressions for surface stress and surface elasticity as a function of molecular weight and graft density are derived. At a higher graft density, a more refined (semi) analytical SST with Langevin chain elasticity is advanced. Theoretical predictions are assessed by molecular dynamics simulation of a brush using bead-spring model.Experiments on a thermoresponsive brush grafted onto a soft beam showed the surface stress is ∼ −10 N/m and its magnitude decreases gradually, and reversibly, on increasing solvent temperature. Molecular scale parameters of the brush are estimated experimentally to enable qualitative comparison with SST theories.

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Modeling the creping process in tissue making (2019)

Creping is a key operation in the manufacturing of low-density tissue paper. In this process, a wetweb is pressed and adhered onto a drying cylinder (Yankee) rotating at a high speed, dried onYankee, and then scraped off by a doctor blade. This controlled and violent interaction betweenthe web moving at a high speed with the stationary blade creates a series of invisible micro-folds,and explodes the thickness of the web through inter-fiber debonding. Various parameters governcreping and finding their optimal combination is currently limited to experience or costly trials. Aone-dimensional particle dynamics model is developed to study nonlinear deformations in thecreping process, and to understand the underlying mechanisms. Specifically, the web is modeledas a single layer of discrete particles connected by visco-elasto-plastic elements. A mixed-modediscrete cohesive zone model is embedded to represent the adhesive layer. Self-contact of the webis incorporated by a penalty method. First, a systematic parametric study is reported to assess therelative impact of various process parameters on the crepe structure and hence the tissue quality.Then, the model is extended to a multi-layered web to investigate the “sheet explosion”. A phasediagram for the creping regimes is constructed. Next, the effects of inhomogeneities on the crepingprocess are investigated. Three common inhomogeneities are considered separately: the formingfabric pattern; the non-uniform basis weight; and the non-uniform adhesion. Finally, a series ofexperiments have been conducted on an existing lab-scale creping apparatus to validate theproposed model, and qualitative agreement is observed. The model can serve as a tool toinvestigate the process-structure-property correlation in tissue making, and the findings in thisthesis offer practical guidance to the industry in the choice of forming and creping processparameters.

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Wave transmission in finite dissipative nonlinear periodic structures (2017)

Spatially periodic structures exhibit intriguing dynamic characteristics, contributing to their growing applications as phononic crystals, acoustic metamaterials and lightweight lattice materials. A striking feature, employed in many engineering applications, is their filtering effect, whereby waves can propagate only in specific frequency intervals known as pass bands. Other frequency components (stop bands) are spatially attenuated as they propagate through the structure. This thesis studies nonlinear wave transmission in periodic structures of finite extent in the presence of dissipative forces and externally induced nonlinear forces. Perfectly periodic structures with identical units are considered, as well as nearly periodic structures with small deviations from periodicity extended throughout the structure. At high amplitudes of motion, nonlinear forces gain significance, generating qualitatively new dynamic phenomena such as supratransmission. Supratransmission is an instability-driven transmission mechanism that occurs when a periodic structure is driven harmonically at one end with a frequency within its stop band. The ensuing enhanced transmission contrasts the vibration isolation characteristic of the same structure operating in the linear regime. In the context of engineering applications, three factors play a significant role: dissipative forces, symmetry-breaking imperfections induced by manufacturing constraints (disorder) and the finite size of the structure. This thesis systematically investigates the influence of these parameters on supratransmission in a one-dimensional periodic structure, studying the competition between the effects of dispersion, dissipation, nonlinearity and disorder-borne wave localization (Anderson localization). We identify the mechanism underlying supratransmission using direct numerical simulations and numerical continuation. Based on this insight, we obtain analytical expressions for the onset of supratransmission for weakly coupled structures using asymptotic analysis. Particularly, we highlight the non-trivial effects of damping on supratransmission in finite structures. We demonstrate that, regardless of the type of nonlinearity, dissipative forces can delay the onset of supratransmission, and high levels of damping can eliminate it. Given that the spectral contents of transmitted energies fall within the pass band, we expect a competition between supratransmission and Anderson localization. Using direct numerical simulations and continuation techniques, we demonstrate that disorder reduces the transmitted wave energy in the ensemble-average sense. However, the average force threshold required to trigger supratransmission remains unchanged.

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Position-dependent dynamics and stability of machine tools (2013)

Machine tool’s productivity and ability to produce a component of the required quality is directly influenced by its dynamic stiffness at the tool center point. Lack of dynamic stiffness may lead to unstable regenerative chatter vibrations which are detrimental to the performance. The chatter vibrations are influenced by the changing structural dynamics of the machine as the tool moves along the tool path, resulting in position-varying machining stability of the system. Evaluation of these varying dynamics at the design stage is a complex process, often involving the use of large order finite element (FE) models. Complexity and computational costs associated with such FE models limit the analyses to one or two design concepts and at only a few discrete positions. To facilitate rapid exploration of several design alternatives and to evaluate and optimize each of their position-dependent dynamic behavior, a generalized bottom-up reduced model substructural synthesis approach is proposed in this thesis. An improved variant of the component mode synthesis method is developed and demonstrated to represent higher order dynamics of each of the machine tool components while reducing the computational cost. Reduced substructures with position-invariant response are synthesized at their contacting interfaces using novel adaptations of constraint formulations to yield position-dependent response. The generalized formulation is used to evaluate the position-dependent behavior of two separate machine tools: one with a serial kinematic configuration, and another with hybrid serial-parallel kinematics. The reduced machine model is verified against full order models and is also validated against measurements by including joint characteristics in the model. The effects of position and feed-direction-dependent compliances on machining stability are investigated by using a novel position and feed-direction-dependent-process-stability performance criterion that evaluates the productivity of machine tools in its entire work volume. Parameters limiting the target productivity levels are identified and modified; and, the complete dynamics are rapidly re-analyzed using the developed models. Optimal design modifications are shown to increase productivity by ~35%. The proposed methods in this thesis enable efficient simulation of structural dynamics, stability assessment as well as interactions of the CNC and cutting process with the machine tool structure in a virtual environment.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Experimental characterization of tissue structure and the impact of forming fabric on tissue properties (2022)

Low basis weight tissue papers (≤ 25 gsm) are extensively used in our day-to-day life. Theyare softer, stretchy and have high water absorbency compared to writing or printing paper. Duringtissue paper manufacturing, a slurry of pulp and water is carried on a forming fabric whichis pressed onto a chemically coated “Yankee” dryer which rotates at high speed (1200-2200m/min) and is maintained at ≈ 100◦C. After drying on Yankee, the sheet is scraped off Yankeewith the help of a stationary creping blade. The process of removing the sheet from the Yankeeis called “creping”. Creping imparts a folded structure to the sheet that is responsible for thedesired properties of tissue paper like high bulk, high failure strain, high work to rupture, andincreased softness and absorbency. A scientific understanding of the process and its influenceon the properties can be useful for enhancing the quality and efficiency of the manufacturingprocess. The current research studies the influence of creping by performing measurementsthrough experiments on tissue paper samples. An experimental technique based on focus stackingis used to image the edge of a tissue paper. Measurements made using this new techniqueare quantitatively compared to measurements from scanning electron microscope images. Theedge images captured are used to calculate a parameter that quantifies the creped structure andcorrelates well with the stretch. It provides a measure of foreshortening of the sheet that occursduring creping. The forming fabric imparts an embedded structure on the sheet before pressingon the Yankee. These patterns affect the creping process and thus the tissue quality. Surfaceimages of the forming fabric and the tissue paper formed on it in conjunction with measurementsfrom uni-axial tensile tests are used to study the influence of the forming fabric on tissuepaper. A method is proposed through which a surface image of the tissue can be used to estimatestretch. Tissue samples over the life of forming fabric are studied to understand the impact offorming fabric wear on tensile properties.

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Influence of flexure and crowding on nanoscale thermal transport (2021)

Nanoscale thermal transport has been studied by scientists for decades. Low dimensional materials have shown two significant characteristics - (1) Thermal conductivity (κ) can be dependent on the size of the system, (2) A significant reduction in κ has been observed in an array-like arrangement. Thus, it is essential to understand the mechanism to tailor material properties for different applications. Fourier’s law is an empirical relation between average thermal flux and temperature gradient. It indicates κ is an intrinsic material property, but studies have shown that it breaks down in low-dimensional systems. The heat flux (J) depends on the size of the system (N) by the relation J ∝ N α−1. Traditionally, 1D studies have mostly focused on the effect of two-body interactions on κ. In this thesis, we study the effect of multibody interactions in the presence of two-body interactions on thermal transport. We use Nℓ (number of persistence lengths) to define system size and study the asymptotic limit of NJ. The transition from ballistic to superdiffusive behaviour was observed near 100 Nℓ in the ordered systems. In contrast, disordered systems showed only superdiffusive transport. Coherent wave patterns emerged as thermal carriers in superdiffusive regimes. Further, modelling crowding as transverse pinning, we observe a non-monotonous transition from superdiffusive to ballistic behaviour as we increased the crowding. While the single chain models have been extensively studied to understand the length dependence of κ, simulation studies on their bundles and forests are very few. One such example is the experimentally observed reduced heat conduction in carbon nanotube (CNT) forests compared to an isolated CNT. Here, the all-atom simulations require a significant computational expense. Therefore, we have used a coarse-grained model to study the heat flow in molecular forests by incorporating the concepts known from polymer physics and thermal transport to propose a generic picture of the reduction of κ. We show that a delicate balance between the bond orientations, the persistence length of an isolated Q1DM (Quasi-one dimensional material), and the non-bonded inter-chain interactions govern the reduction of κ.

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Experimental studies on creping and its influence on mechanical properties of tissue paper products (2019)

Tissue papers are softer, stretchier, and more water absorbent than regular writing or packaging paper products. Creping of an adhesively bonded low-density paper web from the surface of a rotating Yankee drum is the key manufacturing technique in tissue production. Creping dedensifies and weakens the paper by partially damaging the fibers and the inter-fiber bonds in the fiber network. The process also imparts a signature microstructure, called crepe structure, to the tissue paper. Tissues thus produced have a high specific volume (bulk to basis weight ratio), work to rupture, failure strain (stretch), softness and absorbency. Mechanical properties of tissues are governed by the creping process. Therefore, a scientific understanding of the creping process, and its impact on the structural and mechanical properties of the tissue paper is important.The present research approaches the highly complex problem from an experimental perspective, with a view to complement ongoing physics based numerical models to simulate creping. Experimental techniques are developed to visualize the high speed creping process, quantify the crepe structure, and finally understand the influence of the crepe structure on the uni-axial tensile response of the tissue. A novel surface imaging based structural quantification technique is developed and successfully demonstrated on a commercial tissue machine. The surface image based quantification technique is also validated by micrographic observations of the tissue cross section under a Scanning Electron Microscope (SEM). This work lays the foundational techniques and protocols for future studies in the laboratory and opens the opportunity to observe crepe structure in real time for quality and process control.The surface imaging techniques are then used to observe the evolution of the creping microstructure under a tensile load. Local two dimensional strain fields are quantified using Digital Image Correlation (DIC) to gain insight into failure mechanisms at the macroscopic network level. Micro tensile tests are conducted under SEM to gain further insight into the deformation and failure mechanisms operative at fiber length scales. The studies showed the impact of the creping structure, formation, and inter fiber bonds on the tensile response of the tissue paper, specifically along machine direction.

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An experimental study of nonlinear oscillations in railroad friction control systems (2018)

Friction control at the wheel-rail interface has been an outstanding challenge in front of the rail road engineers throughout the world. On-board solid stick friction modifier system, simply named stick-applicator assembly, has proved to be one of the simple and efficient ways to tackle the excessive wear and rail corrugation. Interlocking solid sticks are applied to the wheel flange and tread by means of a mechanical applicator mounted on a bracket, which is connected to the bogie. Relative sliding motion in the stick-wheel interface provokes gradual transfer of solid lubricant film to the wheel-rail interface through the wheel’s motion. Consequently, friction control at wheel-rail interface could be achieved. Instability and failure of stick-applicator assembly due to stick-wheel interaction destabilize its performance. The present study uses a lab-scale setup to produce consistent instability, which helps examine the behavior of the stick-applicator assembly during instability. The lab-scale setup incorporates a mock-wheel connected to the stick-applicator assembly. Mock-wheel is used to simulate up - down and transverse motion based on the concept of parametric excitation in the presence of internal resonance. Dynamics of each substructure is investigated to gain better understanding of the behavior of the coupled system. Having known the characteristics of each substructure, the dynamics of the coupled system is studied. It is found that period doubling bifurcation occurs consistently in certain ranges of excitation frequencies and voltages. Lateral stiffness is identified as one of the design parameters of the lab-scale setup that governs the vibration level. Clearances in the stick-applicator assembly and looseness between each interlocking sticks are found to be parameters which weaken the lateral stiffness of the coupled system. Some modifications in the design of the main contributing parts to eliminate instability and suppress the vibration of the coupled system in the lab-scale setup are also addressed in this thesis. Furthermore, full-wheel rig experiments are carried out to check the practicality of the design modifications.

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Sound Transmission Characteristics of Sandwich Panels with a Truss Lattice Core (2016)

Sandwich panels are extensively used in constructional, naval and aerospace structures due to their high stiffness and strength-to-weight ratios. In contrast, sound transmission properties of sandwich panels are adversely influenced by their low effective mass. Phase velocity matching of structural waves propagating within the panel and the incident pressure waves from the surrounding fluid medium lead to coincidence effects (often within the audible range) resulting in reduced impedance and high sound transmission. Truss-like lattice cores with porous microarchitecture and \emph{reduced} inter panel connectivity relative to honeycomb cores promise the potential to satisfy the conflicting structural and vibroacoustic response requirements. This study combines Bloch-wave analysis and the Finite Element Method (FEM) to understand wave propagation and hence sound transmission in sandwich panels with a truss lattice core. Three dimensional coupled fluid-structure finite element simulations are conducted to compare the performance of a representative set of lattice core topologies. Potential advantages of sandwich structures with a lattice core over the traditional shear wall panel designs are identified. The significance of partial band gaps is evident in the sound transmission loss characteristics of the panels studied. This work demonstrates that, even without optimization, significant enhancements in STL performance can be achieved in truss lattice core sandwich panels compared to a traditional sandwich panel employing a honeycomb core under constant mass constraint.

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Vibration mode localization in coupled microelectromechanical resonators (2014)

State-of-the-art resonant sensors rely on shift in resonant frequency due to a change in its mass or stiffness caused by a physical quantity to be measured. However, they require low damping operating environment. As a result, applications such as biomolecular detection in aqueous environment pose formidable challenges. A promising, alternative sensing paradigm, minimally affected by damping, is based on normal mode localization in a weakly coupled, symmetric resonator system due to parametric changes. The higher sensitivity of mode shape compared to resonant frequency in a weakly coupled, symmetric resonator system results from the phenomena of eigenvalue veering and associated mode localization induced by symmetry breaking parametric changes in the system. The method offers added benefit of common mode rejection.This thesis critically examines the mode localization based resonant sensing paradigm using a combination of energy based analytical theory, Simulink models, and experimental studies on planar MEMS devices. Built-in asymmetry in fabricated devices and its influence on achievable sensitivity are highlighted. Increasing the number of degrees of freedom (DOF) is shown to enhance sensitivity, but a trade-off exists with the size and complexity of the device. Similarly, decreasing coupling enhances sensitivity at the expense of measurable range of parametric changes. Two and three DOF coupled resonator MEMS devices with tuneable linear coupling were designed, fabricated and tested to verify the above conclusions.In summary, this thesis demonstrates that mode localization based sensing is orders of magnitude more sensitive compared to resonant frequency shifts. The sensitivity can be further increased by decreasing coupling between resonators, or increasing number of DOF in a resonant MEMS device, or both.

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Design of Stent Expansion Mechanisms (2012)

Stents are widely used in the treatment of vascular disease and they represent one of the most valuable medical device markets. It has been observed that the mechanical characteristics of a stent influences clinical outcomes. This thesis is concerned with the design of expansion mechanisms of balloon expandable stents based on the principles of lattice mechanics. Balloon expandable vascular stents are mesh-like, tubular structures used mainly to prop open narrowed arteries, and also to provide sealing and anchorage in a stent-graft for treatment of aneurysms or dissections. Presence of a spatially repeating geometric pattern of a `unit' or a cell is a striking feature of stents. Lattice mechanics deals with such spatially periodic materials and structures. The focus is on the plastic expansion phase of a stent from the initial crimped configuration. The elastic post-expansion phase is also considered. Eight unit cell-based stent designs are selected for this work. Their expansion characteristics are analyzed and measured. Analytical methods based on kinematics of stent expansion mechanisms are presented first which are then validated with more detailed Finite Element (FE) calculations. Analytical methods developed in this work aid rapid design calculations in selecting appropriate unit cell geometries. Three of the designs are manufactured through laser micromachining and tested for their expansion characteristics. The analytical methods were validated as they predicted similar expansion characteristics as finite element and experiment. Additionally, the study confirmed that stent designs with positive, negative, or zero axial strain over expansion is possible. Finally, the study suggest that unit cell design can be tailored to obtain desired length-diameter and pressure-diameter characteristics over the expansion phase of stenting.

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Dynamic response localization in one-dimensional periodic systems (2012)

This thesis contributes a novel receptance coupling technique to analyse dynamic response localization induced by bandgap mechanisms in advanced periodic light weight material and structural systems. One-dimensional structural systems are used to illustrate the technique with experiments. Localization induced by disorder and nonlinearity is investigated using numerical simulations. Insights on bandgap localization mechanisms offered by the receptance technique can be used to design periodic composite materials such as Phononic Crystals and metamaterials, and periodic structures with enhanced vibroacoustic performance characteristics.

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Effective mechanical properites of lattice materials (2012)

Lattice materials possess a spatially repeating porous microstructure or unit cell. Their usefulness lies in their multi-functionality in terms of providing high specific stiffness, thermal conductivity, energy absorption and vibration control by attenuating forcing frequencies falling within the band gap region. Analytical expressionshave been proposed in the past to predict cell geometry dependent effective material properties by considering a lattice as a network of beams in the high porosity limit. Applying these analytical techniques to complex cell geometries is cumbersome. This precludes the use of analytical methods in conducting a comparative study involving complex lattice topologies. A numerical method based on the method of long wavelengths and Bloch theory is developed here and applied to a chosen set of lattice geometries in order to compare effective material properties of infinite lattices. The proposed method requires implementation of Floquet-bloch transformation in conjunction with a Finite Element (FE) scheme. Elastic boundary layers emerge from surfaces and interfaces in a finite lattice, or an infinite lattice with defects such as cracks. Boundary layers can degrade effectivematerial properties. A semi-analytical formulation is developed and applied to a chosen set of topologies and the topologies with deep boundary layers are identified. The methods developed in this dissertation facilitate rapid design calculation and selection of appropriate core topologies in multifunctional design of sandwichstructures employing a lattice core.

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Friction-induced vibrations in railway transportation (2012)

Controlling friction at the wheel-rail interface is indispensable for extending track life, minimising wheel-flange wear, improving fuel efficiency, reducing noise and lateral forces. A particular implementation of friction modifier system consists of a stick-tube assembly, attached through a bracket which is suspended from the railway bogie frame. Inside the tube, a set of interlocking solid sticks resides with one end pressed against the tread or flange of the wheel, and the other end against a constant force tape spring. Rubbing action at the stick-wheel interface and the action of the spring results in a gradual transfer of friction modifier film to the wheel and thence to the rail through the wheel-rail contact. This results in effective friction management between the wheel and the rail. Friction modifier systems can experience unstable friction-induced vibrations due to a complex set of in situ contact conditions. Stability prediction is important for efficient functioning of friction control systems. This dissertation contributes a stability analysis procedure in frequency domain based on Frequency Response Functions (FRFs) of the wheel and the applicator-bracket subsystems. The stability analysis yields stability maps delineating stable and unstable regions of operation in the design parameter space defined by speed of train, angle of applicator, and friction coefficient. Stability characteristics of three bracket designs are compared using experiments and finite element models. Results are summarised in the form of stability diagrams indicating the operating conditions that will lead to unstable vibrations. This methodology can easily incorporate design changes to the bracket and/or applicator, thus facilitating a rapid comparison of different designs for their stability characteristics even before they are built.

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Molecular dynamics study of effects of geometric defects on the mechanical properties of graphene (2012)

Graphene is a flat monolayer of carbon atoms arranged in a two-dimensional hexagonal lattice. It is the strongest material ever measured with strength exceeding more than hundred times of steel. However, the strength of graphene is critically influenced by temperature, vacancy defects (missing carbon atoms) and free edges. A systematic Molecular Dynamics (MD) simulation study is performed in this thesis to understand the effects of temperature, free edges, and vacancy defects on the mechanical properties of graphene. Results indicate that graphene has a positive coefficient of thermal expansion. However, the amplitude of intrinsic ripples (out-of-plane movement of carbon atoms) increases with increasing temperature, which reduces the net effect of thermal expansion. This is probably the reason for negative values of thermal expansion coefficient observed in some experiments. The MD simulations provide significant insights. At higher temperatures the sheets are observed to fail at lower strains due to high kinetic energy of atoms. Excess edge energy of a narrow graphene sheet is found to induce an initial strain at equilibrium configuration. Free edges have a greater influence on the mechanical properties of zigzag sheets compared to those of armchair sheets. Simulation of sheets with vacancy defects indicates that a single missing atom could reduce the strength by nearly 20%. It is also found that the calculated strength based on Griffith's theory falls below the results from MD simulations. The results obtained in this study are useful to the design and fabrication of graphene based nano-devices.

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