Srikantha Phani

Associate Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (2008-2018)
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.

https://open.library.ubc.ca/collections/24/items/1.0340530

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.

https://open.library.ubc.ca/collections/24/items/1.0074067

Master's Student Supervision (2010-2017)
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.

https://open.library.ubc.ca/collections/24/items/1.0223865

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.

https://open.library.ubc.ca/collections/24/items/1.0167015

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.

https://open.library.ubc.ca/collections/24/items/1.0072742

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.

https://open.library.ubc.ca/collections/24/items/1.0073261

Effective mechanical properties 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.

https://open.library.ubc.ca/collections/24/items/1.0072404

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.

https://open.library.ubc.ca/collections/24/items/1.0072408

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.

https://open.library.ubc.ca/collections/24/items/1.0072708

 

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