7,178 research outputs found

    Hybrid time-dependent Ginzburg-Landau simulations of block copolymer nanocomposites: nanoparticle anisotropy

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    Block copolymer melts are perfect candidates to template the position of colloidal nanoparticles in the nanoscale, on top of their well-known suitability for lithography applications. This is due to their ability to self-assemble into periodic ordered structures, in which nanoparticles can segregate depending on the polymer-particle interactions, size and shape. The resulting coassembled structure can be highly ordered as a combination of both the polymeric and colloidal properties. The time-dependent Ginzburg-Landau model for the block copolymer was combined with Brownian dynamics for nanoparticles, resulting in an efficient mesoscopic model to study the complex behaviour of block copolymer nanocomposites. This review covers recent developments of the time-dependent Ginzburg-Landau/Brownian dynamics scheme. This includes efforts to parallelise the numerical scheme and applications of the model. The validity of the model is studied by comparing simulation and experimental results for isotropic nanoparticles. Extensions to simulate nonspherical and inhomogeneous nanoparticles are discussed and simulation results are discussed. The time-dependent Ginzburg-Landau/Brownian dynamics scheme is shown to be a flexible method which can account for the relatively large system sizes required to study block copolymer nanocomposite systems, while being easily extensible to simulate nonspherical nanoparticles

    DefGraspNets: Grasp Planning on 3D Fields with Graph Neural Nets

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    Robotic grasping of 3D deformable objects is critical for real-world applications such as food handling and robotic surgery. Unlike rigid and articulated objects, 3D deformable objects have infinite degrees of freedom. Fully defining their state requires 3D deformation and stress fields, which are exceptionally difficult to analytically compute or experimentally measure. Thus, evaluating grasp candidates for grasp planning typically requires accurate, but slow 3D finite element method (FEM) simulation. Sampling-based grasp planning is often impractical, as it requires evaluation of a large number of grasp candidates. Gradient-based grasp planning can be more efficient, but requires a differentiable model to synthesize optimal grasps from initial candidates. Differentiable FEM simulators may fill this role, but are typically no faster than standard FEM. In this work, we propose learning a predictive graph neural network (GNN), DefGraspNets, to act as our differentiable model. We train DefGraspNets to predict 3D stress and deformation fields based on FEM-based grasp simulations. DefGraspNets not only runs up to 1500 times faster than the FEM simulator, but also enables fast gradient-based grasp optimization over 3D stress and deformation metrics. We design DefGraspNets to align with real-world grasp planning practices and demonstrate generalization across multiple test sets, including real-world experiments.Comment: To be published in the IEEE Conference on Robotics and Automation (ICRA), 202

    Interview with Wolfgang Knauss

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    An oral history in four sessions (September 2019–January 2020) with Wolfgang Knauss, von Kármán Professor of Aeronautics and Applied Mechanics, Emeritus. Born in Germany in 1933, he speaks about his early life and experiences under the Nazi regime, his teenage years in Siegen and Heidelberg during the Allied occupation, and his move to Pasadena, California, in 1954 under the sponsorship of a local minister and his family. He enrolled in Caltech as an undergraduate in 1957, commencing a more than half-century affiliation with the Institute and GALCIT (today the Graduate Aerospace Laboratories of Caltech). He recalls the roots of his interest in aeronautics, his PhD solid mechanics studies with his advisor, M. Williams, and the GALCIT environment in the late 1950s and 1960s at the dawn of the Space Age, including the impact of Sputnik and classes with NASA astronauts. He discusses his experimental and theoretical work on materials deformation, dynamic fracture, and crack propagation, including his solid-propellant fuels research for NASA and the US Army, wide-ranging programs with the US Navy, and his pioneering micromechanics investigations and work on the time-dependent fracture of polymers in the 1990s. He offers his perspective on GALCIT’s academic culture, its solid mechanics and fluid mechanics programs, and its evolving administrative directions over the course of five decades, as well as its impact and reputation both within and beyond Caltech. He describes his work with Caltech’s undergraduate admissions committee and his scientific collaborations with numerous graduate students and postdocs and shares his recollections of GALCIT and other Caltech colleagues, including C. Babcock, D. Coles, R.P. Feynman, Y.C. Fung, G. Neugebauer, G. Housner, D. Hudson, H. Liepmann, A. Klein, G. Ravichandran, A. Rosakis, A. Roshko, and E. Sechler. Six appendices contributed by Dr. Knauss, offering further insight into his life and career, also form part of this oral history and are cross-referenced in the main text

    The determinants of value addition: a crtitical analysis of global software engineering industry in Sri Lanka

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    It was evident through the literature that the perceived value delivery of the global software engineering industry is low due to various facts. Therefore, this research concerns global software product companies in Sri Lanka to explore the software engineering methods and practices in increasing the value addition. The overall aim of the study is to identify the key determinants for value addition in the global software engineering industry and critically evaluate the impact of them for the software product companies to help maximise the value addition to ultimately assure the sustainability of the industry. An exploratory research approach was used initially since findings would emerge while the study unfolds. Mixed method was employed as the literature itself was inadequate to investigate the problem effectively to formulate the research framework. Twenty-three face-to-face online interviews were conducted with the subject matter experts covering all the disciplines from the targeted organisations which was combined with the literature findings as well as the outcomes of the market research outcomes conducted by both government and nongovernment institutes. Data from the interviews were analysed using NVivo 12. The findings of the existing literature were verified through the exploratory study and the outcomes were used to formulate the questionnaire for the public survey. 371 responses were considered after cleansing the total responses received for the data analysis through SPSS 21 with alpha level 0.05. Internal consistency test was done before the descriptive analysis. After assuring the reliability of the dataset, the correlation test, multiple regression test and analysis of variance (ANOVA) test were carried out to fulfil the requirements of meeting the research objectives. Five determinants for value addition were identified along with the key themes for each area. They are staffing, delivery process, use of tools, governance, and technology infrastructure. The cross-functional and self-organised teams built around the value streams, employing a properly interconnected software delivery process with the right governance in the delivery pipelines, selection of tools and providing the right infrastructure increases the value delivery. Moreover, the constraints for value addition are poor interconnection in the internal processes, rigid functional hierarchies, inaccurate selections and uses of tools, inflexible team arrangements and inadequate focus for the technology infrastructure. The findings add to the existing body of knowledge on increasing the value addition by employing effective processes, practices and tools and the impacts of inaccurate applications the same in the global software engineering industry

    Investigating and mitigating the role of neutralisation techniques on information security policies violation in healthcare organisations

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    Healthcare organisations today rely heavily on Electronic Medical Records systems (EMRs), which have become highly crucial IT assets that require significant security efforts to safeguard patients’ information. Individuals who have legitimate access to an organisation’s assets to perform their day-to-day duties but intentionally or unintentionally violate information security policies can jeopardise their organisation’s information security efforts and cause significant legal and financial losses. In the information security (InfoSec) literature, several studies emphasised the necessity to understand why employees behave in ways that contradict information security requirements but have offered widely different solutions. In an effort to respond to this situation, this thesis addressed the gap in the information security academic research by providing a deep understanding of the problem of medical practitioners’ behavioural justifications to violate information security policies and then determining proper solutions to reduce this undesirable behaviour. Neutralisation theory was used as the theoretical basis for the research. This thesis adopted a mixed-method research approach that comprises four consecutive phases, and each phase represents a research study that was conducted in light of the results from the preceding phase. The first phase of the thesis started by investigating the relationship between medical practitioners’ neutralisation techniques and their intention to violate information security policies that protect a patient’s privacy. A quantitative study was conducted to extend the work of Siponen and Vance [1] through a study of the Saudi Arabia healthcare industry. The data was collected via an online questionnaire from 66 Medical Interns (MIs) working in four academic hospitals. The study found that six neutralisation techniques—(1) appeal to higher loyalties, (2) defence of necessity, (3) the metaphor of ledger, (4) denial of responsibility, (5) denial of injury, and (6) condemnation of condemners—significantly contribute to the justifications of the MIs in hypothetically violating information security policies. The second phase of this research used a series of semi-structured interviews with IT security professionals in one of the largest academic hospitals in Saudi Arabia to explore the environmental factors that motivated the medical practitioners to evoke various neutralisation techniques. The results revealed that social, organisational, and emotional factors all stimulated the behavioural justifications to breach information security policies. During these interviews, it became clear that the IT department needed to ensure that security policies fit the daily tasks of the medical practitioners by providing alternative solutions to ensure the effectiveness of those policies. Based on these interviews, the objective of the following two phases was to improve the effectiveness of InfoSec policies against the use of behavioural justification by engaging the end users in the modification of existing policies via a collaborative writing process. Those two phases were conducted in the UK and Saudi Arabia to determine whether the collaborative writing process could produce a more effective security policy that balanced the security requirements with daily business needs, thus leading to a reduction in the use of neutralisation techniques to violate security policies. The overall result confirmed that the involvement of the end users via a collaborative writing process positively improved the effectiveness of the security policy to mitigate the individual behavioural justifications, showing that the process is a promising one to enhance security compliance

    Structure and adsorption properties of gas-ionic liquid interfaces

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    Supported ionic liquids are a diverse class of materials that have been considered as a promising approach to design new surface properties within solids for gas adsorption and separation applications. In these materials, the surface morphology and composition of a porous solid are modified by depositing ionic liquid. The resulting materials exhibit a unique combination of structural and gas adsorption properties arising from both components, the support, and the liquid. Naturally, theoretical and experimental studies devoted to understanding the underlying principles of exhibited interfacial properties have been an intense area of research. However, a complete understanding of the interplay between interfacial gas-liquid and liquid-solid interactions as well as molecular details of these processes remains elusive. The proposed problem is challenging and in this thesis, it is approached from two different perspectives applying computational and experimental techniques. In particular, molecular dynamics simulations are used to model gas adsorption in films of ionic liquids on a molecular level. A detailed description of the modeled systems is possible if the interfacial and bulk properties of ionic liquid films are separated. In this study, we use a unique method that recognizes the interfacial and bulk structures of ionic liquids and distinguishes gas adsorption from gas solubility. By combining classical nitrogen sorption experiments with a mean-field theory, we study how liquid-solid interactions influence the adsorption of ionic liquids on the surface of the porous support. The developed approach was applied to a range of ionic liquids that feature different interaction behavior with gas and porous support. Using molecular simulations with interfacial analysis, it was discovered that gas adsorption capacity can be directly related to gas solubility data, allowing the development of a predictive model for the gas adsorption performance of ionic liquid films. Furthermore, it was found that this CO2 adsorption on the surface of ionic liquid films is determined by the specific arrangement of cations and anions on the surface. A particularly important result is that, for the first time, a quantitative relation between these structural and adsorption properties of different ionic liquid films has been established. This link between two types of properties determines design principles for supported ionic liquids. However, the proposed predictive model and design principles rely on the assumption that the ionic liquid is uniformly distributed on the surface of the porous support. To test how ionic liquids behave under confinement, nitrogen physisorption experiments were conducted for micro‐ and mesopore analysis of supported ionic liquid materials. In conjunction with mean-field density functional theory applied to the lattice gas and pore models, we revealed different scenarios for the pore-filling mechanism depending on the strength of the liquid-solid interactions. In this thesis, a combination of computational and experimental studies provides a framework for the characterization of complex interfacial gas-liquid and liquid-solid processes. It is shown that interfacial analysis is a powerful tool for studying molecular-level interactions between different phases. Finally, nitrogen sorption experiments were effectively used to obtain information on the structure of supported ionic liquids

    Learning from biology to design stimuli-responsive capsules

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    As well as being model cell membranes, lipid vesicles are widely used as an encapsulation technology due to their impermeable membrane. Hydrogels are similarly useful due to their biocompatibility, mechanical strength, and potential for stimulus-responsive behaviour. By combining these two structures, the benefits of both can be reaped, giving a structure with both mechanical strength and the possibility to encapsulate actives within an impermeable membrane. However, the interactions between the gel and membrane, and their implications, are not well understood. This thesis considers two different composite structures of lipid vesicles and hydrogels as potential systems for encapsulation and controlled release. These structures are hydrogel-embedded vesicles, and Gel-Filled Vesicles (GFVs). The hydrogel-embedded vesicles are subjected to different types of mechanical stresses. Osmotic shocks are used to apply a uniform pressure on the lipid bilayer, and compression of the hydrogel by a micromanipulator is used to cause a uni-directional force. Agarose-embedded vesicles are shown to experience an adhesive interaction between the membrane and the gel, causing vesicle behaviours to be altered in comparison to free-floating vesicles. Of particular note is the formation of a buckled morphology for embedded vesicles subjected to hyperosmotic shocks. Additionally, the formation of GFVs demonstrating the poration mechanism of controlled release is attempted. A suitable gel core of poly(acrylamide-co-acrylic acid) is synthesised and characterised for Upper Critical Solution Temperature behaviour. In summary, this thesis demonstrates that interactions between the lipid bilayer and a hydrogel can strongly affect membrane behaviours, and therefore their uses for either encapsulation systems or for biophysical models

    A Robin-Neumann Scheme with Quasi-Newton Acceleration for Partitioned Fluid-Structure Interaction

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    The Dirichlet-Neumann scheme is the most common partitioned algorithm for fluid-structure interaction (FSI) and offers high flexibility concerning the solvers employed for the two subproblems. Nevertheless, it is not without shortcomings: To begin with, the inherent added-mass effect often destabilizes the numerical solution severely. Moreover, the Dirichlet-Neumann scheme cannot be applied to FSI problems in which an incompressible fluid is fully enclosed by Dirichlet boundaries, as it is incapable of satisfying the volume constraint. In the last decade, interface quasi-Newton methods have proven to control the added-mass effect and substantially speed up convergence by adding a Newton-like update step to the Dirichlet-Neumann coupling. They are, however, without effect on the incompressibility dilemma. As an alternative, the Robin-Neumann scheme generalizes the fluid's boundary condition to a Robin condition by including the Cauchy stresses. While this modification in fact successfully tackles both drawbacks of the Dirichlet-Neumann approach, the price to be paid is a strong dependency on the Robin weighting parameter, with very limited a priori knowledge about good choices. This work proposes a strategy to merge these two ideas and benefit from their combined strengths. The effectiveness of this new quasi-Newton-accelerated Robin-Neumann scheme is demonstrated for different FSI simulations and compared to both Robin- and Dirichlet-Neumann variants.Comment: Keywords: Partitioned Fluid-Structure Interaction, Robin-Neumann Scheme,Interface Quasi-Newton Method

    Parameter estimation with gravitational waves

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    The new era of gravitational wave astronomy truly began on September 14, 2015 with the detection of GW150914, the sensational first direct observation of gravitational waves from the inspiral and merger of two black holes by the two Advanced LIGO detectors. In the subsequent first three observing runs of the LIGO/Virgo network, gravitational waves from ∌50\sim 50 compact binary mergers have been announced, with more results to come. The events have mostly been produced by binary black holes, but two binary neutron star mergers have so far been observed, as well as the mergers of two neutron star - black hole systems. Furthermore, gravitational waves emitted by core-collapse supernovae, pulsars and the stochastic gravitational wave background are within the LIGO/Virgo/KAGRA sensitivity band and are likely to be observed in future observation runs. Beyond signal detection, a major challenge has been the development of statistical and computational methodology for estimating the physical waveform parameters and quantifying their uncertainties in order to accurately characterise the emitting system. These methods depend on the sources of the gravitational waves and the gravitational waveform model that is used. This article reviews the main waveform models and parameter estimation methods used to extract physical parameters from gravitational wave signals detected to date by LIGO and Virgo and from those expected to be observed in the future, which will include KAGRA, and how these methods interface with various aspects of LIGO/Virgo/KAGRA science. Also presented are the statistical methods used by LIGO and Virgo to estimate detector noise, test general relativity, and draw conclusions about the rates of compact binary mergers in the universe. Furthermore, a summary of major publicly available gravitational wave parameter estimation software packages is given

    Improved methods for characterising acoustoplasticity

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    The benefits of high-power ultrasonics to industrial metal forming processes have long been demonstrated in uniaxial mechanical tests. The astonishing reductions in flow stress observed have been linked to changes to surface friction and to an interaction of the excitation with the mechanisms of plastic deformation in metals. Many advanced techniques and material models have been brought to bear on the problem of the underlying physics of acoustoplasticity, and yet all rely fundamentally on accurate force and extension data. The effects of inertia and inhomogeneity in the loading distribution on the specimen have been largely ignored, and yet are incompatible with commonly used instrumentation. This thesis reports investigations which address the error introduced into force measurement in mechanical testing by ultrasonic excitation. After reviewing experimental mechanics techniques, it was found that the piezoelectric force transducer retained its central role in defining true flow stress reduction. An inertia-based barrier to vibration was introduced between the force transducer and test machine crosshead, to impose the rigid boundary condition desired to ensure the force transducer coincided with a displacement node. Lumped-parameter modelling indicated that the dynamic response of the piezoelectric force transducer’s structure could significantly distort the amplitude of an oscillatory force measurand. Either amplification or attenuation could result depending on the proximity of excitation frequency to natural frequency of the force transducer’s first longitudinal mode. Simple impulse experiments provided the natural frequency of the force transducer in the free-free condition, a parameter used in later finite element (FE) modelling of the ultrasonic tensile test structure. Experimental Modal Analysis (EMA) was used to investigate the dynamic response of the ultrasonic tensile test structure, and to map the mode shape of the first longitudinal mode, the mode utilised in ultrasonic tensile testing. A finite element model was constructed of the test apparatus, and subsequently solved in an eigenvalue analysis to extract the natural frequency and mode shape of the first longitudinal mode. When the numerically predicted waveform was compared with that found from EMA, a significant difference was discovered between the horn and specimen. The compliance of the joint was adjusted until the simulated mode shape converged on its experimental counterpart. Once experimentally calibrated, the FE model was used to predict the force experienced by the force transducer for increasing values of vibration amplitude. Comparison with experimental force measurements found good agreement. Of greatest importance to the investigation of flow stress, the FE model predicted the indicated value from the force transducer to be 1.91 times greater than the measurand at the specimen-force transducer interface. Strain gauges were attached to the gauge section of the specimen in the ultrasonic tensile test apparatus, and the vibration varied over a range of amplitudes. By converting the oscillatory strain measurement into force on the specimen cross-section, the loading experienced by the specimen at the strain gauge location was compared to force measurements made simultaneously by the piezoelectric force transducer. The ratio of force amplitude from the force transducer over the force amplitude calculated from the specimen strain measurement was found to vary from 3.13 to 3.50, with a mean of 3.32. Repeating the experiment within the FE model calculated an amplitude ratio of 3.33, constant over all vibration amplitudes. This value was used to develop a correction factor to extrapolate force on the specimen from piezoelectric force transducer measurement. The correction was applied to an ultrasonic tensile test on a soft aluminium. Though the mean stress was reduced during the periods of excitation, no real reduction in flow stress was observed, which is consistent with the theory of stress superposition. The evolution of plastic deformation was studied over the gauge section of an ultrasonically excited specimen, using an optical metrology system adapted for use on the ultrasonic tensile test. To eliminate oscillatory motion from images, a high-speed strobe lit the specimen in bursts of light synchronised with the ultrasonic excitation. Digital Image Correlation was used to process the image sequence to find strain and strain rate across the whole face of the specimen gauge length. It was observed that the application of ultrasonic excitation disrupted the usual distribution of plastic deformation along the specimen length, focussing deformation towards the location of peak stress amplitude. Again, observations were consistent with the theory of stress superposition. This thesis demonstrates how the dynamic response of the structure of the specimen and force transducer in an ultrasonic tensile test can significantly distort the force measurement, crucial for accurately identifying a real reduction in flow stress. This has implications for studies of acoustoplasticity aiming at determining underlying physical mechanisms. It is found that, when the effect of inertia is accounted for, the theory of stress superposition is sufficient to explain the stress-strain relationship observed
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