DSpace Collection:
http://hdl.handle.net/2381/835
2016-12-04T22:27:00Z
2016-12-04T22:27:00Z
Dielectric response of filled high temperature vulcanization silicone rubber
Gao, Y.
Liang, X.
Dissado, L. A.
Dodd, S. J.
Chalashkanov, N. M.
http://hdl.handle.net/2381/38812
2016-12-03T04:04:33Z
2016-12-02T13:17:55Z
Title: Dielectric response of filled high temperature vulcanization silicone rubber
Authors: Gao, Y.; Liang, X.; Dissado, L. A.; Dodd, S. J.; Chalashkanov, N. M.
Abstract: The dielectric response of three kinds of filled high temperature vulcanization (HTV)
silicone rubber – SiO2 based nanocomposite with different alumina tri-hydrate (ATH)
micro-filler contents are investigated over a wide range of frequency and temperature.
The results are analyzed in terms of the many-body Dissado-Hill dielectric response
theory. In all three kinds of silicone rubber, a quasi-DC (QDC) process, a Dissado-Hill
loss peak process and a diffusion process are observed. Specifically, the ATH filler
contributes a QDC electrical transport/hopping process, which at higher temperatures
causes the formation of a diffusion layer at the electrodes. The nano-SiO2 particles
combined with rubber matrix give a Dissado-Hill loss peak dielectric relaxation. These
two dispersion processes are shown to be associated with the interior interfaces of filled
HTV silicone rubber. The effects of ATH filler content and temperature on the dielectric
response are also analyzed in depth.
2016-12-02T13:17:55Z
The Role of Bulk Charge Transport Processes in Electrical Tree Formation and Breakdown Mechanisms in Epoxy Resins
Chalashkanov, N. M.
Dodd, S. J.
Dissado, L. A.
Fothergill, J. C.
http://hdl.handle.net/2381/38810
2016-12-03T04:04:31Z
2016-12-02T13:07:12Z
Title: The Role of Bulk Charge Transport Processes in Electrical Tree Formation and Breakdown Mechanisms in Epoxy Resins
Authors: Chalashkanov, N. M.; Dodd, S. J.; Dissado, L. A.; Fothergill, J. C.
Abstract: Electrical treeing experiments have been conducted at different temperatures and levels
of absorbed moisture in Araldite CY1311 epoxy resin samples above their glass transition
temperature, i.e. when the resin was in a flexible state. The fractal dimension of the
electrical trees obtained and the rate of tree growth were found to depend on the
environmental factors: temperature and humidity. It has also been found that at certain
levels of temperature and moisture absorbed in the samples, a transition occurs from
electrical treeing degradation to breakdown by thermal runaway. Complementary
investigations of the dielectric properties of the same epoxy resin system have revealed
that a bulk quasi-dc (QDC) charge transport mechanism takes place above the glass
transition temperature, and we show that the characteristic features of the dielectric
response are related to the shape of the electrical treeing degradation and the transition
to thermal breakdown. This is explained qualitatively through the effect of the bulk QDC
charge transport process in modifying the local space charge electric field distribution.
2016-12-02T13:07:12Z
Study for performance characteristics of surface permanent magnet motor at various magnetization patterns
Petkovska, L.
Lefley, P. W.
Cvetkovski, G.
http://hdl.handle.net/2381/38801
2016-12-03T03:57:41Z
2016-12-02T11:54:46Z
Title: Study for performance characteristics of surface permanent magnet motor at various magnetization patterns
Authors: Petkovska, L.; Lefley, P. W.; Cvetkovski, G.
Abstract: Purpose – In the present paper the influence of magnetization patterns of the rotor magnets on the
performance characteristics for surface permanent magnet (SPM) motor is investigated. The objective is to
show how the electromagnetic and electromechanical characteristics of this type of motor can be
significantly changed by applying various magnetization patterns of permanent magnets placed on the rotor
surface.
Design/methodology/approach – At the beginning a survey of possible and most frequently used
magnetization patterns for permanent magnet motors is presented. The research is focussed on the
comparison of performance characteristics and is developed into three levels. It is started with a conventional
SPM motor having segmented permanent magnets, and two magnetization patterns are considered: parallel
and radial. Showing no significant difference of the motor performance at parallel and radial magnetization,
for further investigation only radial magnetization, being more conventional, is considered. In the second
step, the counterparts of SPM with two Halbach array configurations, under the constraint of fixed magnet
volume, are studied. Finally, detailed comparative analyses of SPM at radial, Halbach 1 and Halbach 2
magnet patterns are presented. Advantages and drawbacks for suggested magnet configurations are
discussed.
Findings – It is presented how the magnetization pattern of the rotor permanent magnets can have a
substantial impact on the SPM motor performance characteristics. From the analysis of magnetic field
properties at various types of magnetization, it is observed that both shape and rates of the characteristics, for
radial magnetization and Halbach 2 configuration, exhibit similar features. It is due to the fact that Halbach 2
array cancels magnetic flux above the permanent magnets, i.e. strengthens the magnetic field in the rotor,
and enhances the coupling between the rotor and stator magnetic field. It is worth to emphasise, that due to
less saturation of the magnetic core and lower iron loss at Halbach 1 and Halbach 2 magnetization, it is
possible to increase armature current and consequently to increase the electromagnetic torque. This fact
could be interesting for further research.
Originality/value – The paper presents an original comparative analysis for performance characteristics of a
surface permanent motor at various magnetization patterns. The novelty of the paper is seen in the
introduction of two Halbach magnetization array for permanent magnets and improvement of the
performance characteristics for the analysed motor.
Description: 12 month embargo from publication
2016-12-02T11:54:46Z
Reliability and performance enhancements for SSD RAID
McEwan, Alistair A.
Komsul, Muhammed Ziya
http://hdl.handle.net/2381/38672
2016-11-23T03:21:29Z
2016-11-22T15:11:08Z
Title: Reliability and performance enhancements for SSD RAID
Authors: McEwan, Alistair A.; Komsul, Muhammed Ziya
Abstract: NAND based solid state storage devices are almost ubiquitously used in safety-critical embedded devices, and recent advances have demonstrated RAID architectures specific to solid state storage devices resulting in increased data reliability, with architectural enhancements to solve the age convergence problem. However, these techniques require devices to be taken off-line while components are replaced—consequently these devices are of limited use in hard real time systems. There are further real time issues in that the conventional architectures ignore other characteristics of solid state devices such as garbage collection and meta data management. In this paper we investigate techniques that support the replacement of aged devices in the array in such a way that we provide continuous system reliability. We also improve the performance overhead of the reconstruction process using a novel data migration policy. The techniques are implemented and tested in a trace-driven simulator, and results demonstrate that average I/O response time is improved by up to 39% with improvement by up to 45% in its standard deviation, overheads in terms of device replacement time are negligible, and read performance is improved by an average of 8%.
2016-11-22T15:11:08Z
Power flow analysis and optimal locations of resistive type superconducting fault current limiters
Zhang, X.
Ruiz, Harold S.
Gang, J.
Shen, B.
Fu, L.
Zhang, H.
Coombs, T. A.
http://hdl.handle.net/2381/38668
2016-11-23T03:21:27Z
2016-11-22T11:48:55Z
Title: Power flow analysis and optimal locations of resistive type superconducting fault current limiters
Authors: Zhang, X.; Ruiz, Harold S.; Gang, J.; Shen, B.; Fu, L.; Zhang, H.; Coombs, T. A.
Abstract: Based on conventional approaches for the integration of resistive-type superconducting fault current limiters (SFCLs) on electric distribution networks, SFCL models largely rely on the insertion of a step or exponential resistance that is determined by a predefined quenching time. In this paper, we expand the scope of the aforementioned models by considering the actual behaviour of an SFCL in terms of the temperature dynamic power-law dependence between the electrical field and the current density, characteristic of high temperature superconductors. Our results are compared to the step-resistance models for the sake of discussion and clarity of the conclusions. Both SFCL models were integrated into a power system model built based on the UK power standard, to study the impact of these protection strategies on the performance of the overall electricity network. As a representative renewable energy source, a 90 MVA wind farm was considered for the simulations. Three fault conditions were simulated, and the figures for the fault current reduction predicted by both fault current limiting models have been compared in terms of multiple current measuring points and allocation strategies. Consequently, we have shown that the incorporation of the E–J characteristics and thermal properties of the superconductor at the simulation level of electric power systems, is crucial for estimations of reliability and determining the optimal locations of resistive type SFCLs in distributed power networks. Our results may help decision making by distribution network operators regarding investment and promotion of SFCL technologies, as it is possible to determine the maximum number of SFCLs necessary to protect against different fault conditions at multiple locations.
2016-11-22T11:48:55Z
Adaptive Augmenting Control Design for Time-Varying Polytopic Systems
Mahdianfar, Hessam
Prempain, Emmanuel
http://hdl.handle.net/2381/38587
2016-11-17T03:21:23Z
2016-11-16T11:52:01Z
Title: Adaptive Augmenting Control Design for Time-Varying Polytopic Systems
Authors: Mahdianfar, Hessam; Prempain, Emmanuel
Abstract: To increase the performance of closed-loop controlled systems in off-nominal conditions and in the presence of inevitable faults and uncertainties, a systematic approach based on robust convex optimization for adaptive augmenting control design is discussed in this paper. More specifically, this paper addresses the problem of adaptive augmenting controller (AAC) design for systems with time-varying polytopic uncertainty. First, a robust state-feedback controller is designed via robust convex optimization as a baseline controller. The closed-loop polytopic system with the baseline controller is considered as the desired time-varying reference model for the design of a direct state-feedback adaptive controller. Next using Lyapunov arguments, global stability of combined robust baseline and adaptive augmenting controllers is established. Furthermore, it is proved that tracking error converges to zero asymptotically. A case study for a generic nonminimum phase nonlinear pitch-axis missile autopilot is conducted. Simulation tests are performed to evaluate stability and performance of nonlinear time-varying closed-loop system in the presence of uncertainties in pitching moment and normal force coefficients, and unmodeled time delays. In addition, results of the simulations indicate satisfactory robustness in case of severe loss of control effectiveness event.
Description: No permission to make PDF available.
2016-11-16T11:52:01Z
Optimised prefactored compact schemes for linear wave propagation phenomena
Rona, A.
Spisso, I.
Hall, E.
Bernanrdini, M.
Pirozzoli, S.
http://hdl.handle.net/2381/38487
2016-11-14T09:36:36Z
2016-11-14T09:35:45Z
Title: Optimised prefactored compact schemes for linear wave propagation phenomena
Authors: Rona, A.; Spisso, I.; Hall, E.; Bernanrdini, M.; Pirozzoli, S.
Editors: Moin, P.
Abstract: A family of space- and time-optimised prefactored compact schemes are developed that minimize the computational cost for given levels of numerical error in wave propagation phenomena, with special reference to aerodynamic sound. This work extends the approach of Pirozzoli (2007) to the MacCormack type prefactored compact high-order schemes developed by Hixon (2000), in which their shorter Padè stencil from the prefactorization leads to a simpler enforcement of numerical boundary conditions. An explicit low-storage multi-step Runge-Kutta integration advances the states in time. Theoretical predictions for spatial and temporal error bounds are derived for the cost-optimised schemes and compared against benchmark schemes of current use in computational aeroacoustic applications in terms of computational cost for a given relative numerical error value. One- and two-dimensional test cases are presented to examine the effectiveness of the cost-optimised schemes for practical flow computations. An effectiveness up to about 50 \% higher than the standard schemes is verified for the linear one-dimensional advection solver, which is a popular baseline solver kernel for computational physics problems. A substantial error reduction for a given cost is also obtained in the more complex case of a two-dimensional acoustic pulse propagation, provided the optimised schemes are made to operate close to their nominal design points.
2016-11-14T09:35:45Z
Improving the twilight model for polar cap absorption nowcasts
Rogers, N. C.
Kero, A.
Honary, F.
Verronen, P. T.
Warrington, E. M.
Danskin, D. W.
http://hdl.handle.net/2381/38479
2016-11-12T04:11:06Z
2016-11-11T16:58:46Z
Title: Improving the twilight model for polar cap absorption nowcasts
Authors: Rogers, N. C.; Kero, A.; Honary, F.; Verronen, P. T.; Warrington, E. M.; Danskin, D. W.
Abstract: During solar proton events (SPE), energetic protons ionize the polar mesosphere causing HF radio wave attenuation, more strongly on the dayside where the effective recombination coefficient, αeff, is low. Polar cap absorption models predict the 30 MHz cosmic noise absorption, A, measured by riometers, based on real-time measurements of the integrated proton flux-energy spectrum, J. However, empirical models in common use cannot account for regional and day-to-day variations in the daytime and nighttime profiles of αeff(z) or the related sensitivity parameter, m = A / sqrt(J). Large prediction errors occur during twilight when m changes rapidly, and due to errors locating the rigidity cutoff latitude. Modeling the twilight change in m as a linear or Gauss error-function transition over a range of solar-zenith angles (χl < χ < χu) provides a better fit to measurements than selecting day or night αeff profiles based on the Earth-shadow height. Optimal model parameters were determined for several polar cap riometers for large SPEs in 1998–2005. The optimal χl parameter was found to be most variable, with smaller values (as low as 60°) postsunrise compared with presunset and with positive correlation between riometers over a wide area. Day and night values of m exhibited higher correlation for closely spaced riometers. A nowcast simulation is presented in which rigidity boundary latitude and twilight model parameters are optimized by assimilating age-weighted measurements from 25 riometers. The technique reduces model bias, and root-mean-square errors are reduced by up to 30% compared with a model employing no riometer data assimilation.
Description: We acknowledge the efforts of Don Wallis who was principally responsible for the scientific operation of the CANOPUS riometer array and the NORSTAR team for providing the riometer data used in this study, which are available from ftp://aurora.phys.ucalgary.ca/data/riometer/. SGO riometer measurements are available from http://www.sgo.fi/Data/Riometer/rioData.php, and IRIS (Kilpisjärvi) riometer data are available from http://spears.lancs.ac.uk/data/request.html. NRCan riometer data are provided as a data set in the supporting information and are also available on request from Donald.Danskin@Canada.ca. We also acknowledge the U.S. National Geophysical Data Centre for providing GOES satellite data, available from http://satdat.ngdc.noaa.gov/sem/, and Kp geomagnetic and solar activity indices, available from ftp://ftp.ngdc.noaa.gov. Dst geomagnetic indices were provided by the World Data Centre for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/dstdir/).
2016-11-11T16:58:46Z
Footbridge system identification using wireless inertial measurement units for force and response measurements
Brownjohn, James Mark William
Bocian, Mateusz
Hester, David
Quattrone, Antonino
Hudson, William
Moore, Daniel
Goh, Sushma
Lim, Meng Sun
http://hdl.handle.net/2381/38425
2016-11-10T03:20:26Z
2016-11-09T15:36:28Z
Title: Footbridge system identification using wireless inertial measurement units for force and response measurements
Authors: Brownjohn, James Mark William; Bocian, Mateusz; Hester, David; Quattrone, Antonino; Hudson, William; Moore, Daniel; Goh, Sushma; Lim, Meng Sun
Abstract: With the main focus on safety, design of structures for vibration serviceability is often overlooked or mismanaged, resulting in some high profile structures failing publicly to perform adequately under human dynamic loading due to walking, running or jumping. A standard tool to inform better design, prove fitness for purpose before entering service and design retrofits is modal testing, a procedure that typically involves acceleration measurements using an array of wired sensors and force generation using a mechanical shaker. A critical but often overlooked aspect is using input (force) to output (response) relationships to enable estimation of modal mass, which is a key parameter directly controlling vibration levels in service.
This paper describes the use of wireless inertial measurement units (IMUs), designed for biomechanics motion capture applications, for the modal testing of a 109 m footbridge. IMUs were first used for an output-only vibration survey to identify mode frequencies, shapes and damping ratios, then for simultaneous measurement of body accelerations of a human subject jumping to excite specific vibrations modes and build up bridge deck accelerations at the jumping location. Using the mode shapes and the vertical acceleration data from a suitable body landmark scaled by body mass, thus providing jumping force data, it was possible to create frequency response functions and estimate modal masses.
The modal mass estimates for this bridge were checked against estimates obtained using an instrumented hammer and known mass distributions, showing consistency among the experimental estimates. Finally, the method was used in an applied research application on a short span footbridge where the benefits of logistical and operational simplicity afforded by the highly portable and easy to use IMUs proved extremely useful for an efficient evaluation of vibration serviceability, including estimation of modal masses.
2016-11-09T15:36:28Z
Sliding Mode Control of Constrained Nonlinear Systems
Incremona, Gian Paolo
Rubagotti, Matteo
Ferrara, Antonella
http://hdl.handle.net/2381/38424
2016-11-10T03:20:25Z
2016-11-09T15:35:45Z
Title: Sliding Mode Control of Constrained Nonlinear Systems
Authors: Incremona, Gian Paolo; Rubagotti, Matteo; Ferrara, Antonella
Abstract: This technical note introduces the design of sliding mode control algorithms for nonlinear systems in the presence of hard inequality constraints on both control and state variables. Relying on general results on minimum-time higher-order sliding mode for unconstrained systems, a general order control law is formulated to robustly steer the state to the origin, while satisfying all the imposed constraints. Results on minimum-time convergence to the sliding manifold, as well as on the maximization of the domain of attraction, are analytically proved for the first-order and second-order sliding mode cases. A general result is presented regarding the domain of attraction in the general order case, while numerical results on the estimation of the domain of attraction and on minimum-time convergence are discussed for the third-order case, following a procedure applicable to a sliding mode of any order.
2016-11-09T15:35:45Z
Nonlinear Analysis and Control of Interleaved Boost Converter using Real Time Cycle to Cycle Variable Slope Compensation
Haimeng, Wu
Pickert, Volker
Giaouris, Damian
Ji, Bing
http://hdl.handle.net/2381/38370
2016-11-08T03:23:57Z
2016-11-07T17:06:35Z
Title: Nonlinear Analysis and Control of Interleaved Boost Converter using Real Time Cycle to Cycle Variable Slope Compensation
Authors: Haimeng, Wu; Pickert, Volker; Giaouris, Damian; Ji, Bing
Abstract: Switched-mode power converters are inherently
nonlinear and piecewise smooth systems which may exhibit a
series of undesirable operations that can greatly reduce the
converter’s efficiency and lifetime. This paper presents a
nonlinear analysis technique to investigate the influence of system
parameters on the stability of interleaved boost converters. In this
approach, Monodromy matrix which contains all the
comprehensive information of converter parameters and control
loop can be employed to fully reveal and understand the inherent
nonlinear dynamics of interleaved boost converters, including the
interaction effect of switching operation. Thereby not only the
boundary conditions but also the relationship between stability
margin and the parameters given can be intuitively studied by the
eigenvalues of this matrix. Furthermore, employing the
knowledge gained from this analysis a real time cycle to cycle
variable slope compensation method is proposed to guarantee a
satisfactory performance of the converter with extended range of
stable operation. Outcomes show that systems can regain stability
by applying the proposed method within a few time periods of
switching cycles. The numerical and analytical results validate the
theoretical analysis, and experimental results verify the
effectiveness of the proposed approach
2016-11-07T17:06:35Z
Survey and Future Directions of Fault-Tolerant Distributed Computing on Board Spacecraft
Fayyaz, Muhammad
Vladimirova, Tanya
http://hdl.handle.net/2381/38257
2016-10-27T02:40:55Z
2016-10-26T15:27:47Z
Title: Survey and Future Directions of Fault-Tolerant Distributed Computing on Board Spacecraft
Authors: Fayyaz, Muhammad; Vladimirova, Tanya
Abstract: Current and future space missions demand highly reliable on-board computing systems, which are capable of carrying out high-performance data processing. At present, no single computing scheme satisfies both, the highly reliable operation requirement and the high-performance computing requirement. The aim of this paper is to review existing systems and offer a new approach to addressing the problem. In the first part of the paper, a detailed survey of fault-tolerant distributed computing systems for space applications is presented. Fault types and assessment criteria for fault-tolerant systems are introduced. Redundancy schemes for distributed systems are analyzed. A review of the state-of-the-art on fault-tolerant distributed systems is presented and limitations of current approaches are discussed. In the second part of the paper, a new fault-tolerant distributed computing platform with wireless links among the computing nodes is proposed. Novel algorithms, enabling important aspects of the architecture, such as time slot priority adaptive fault-tolerant channel access and fault-tolerant distributed computing using task migration are introduced.
Description: Following the embargo period the following license applies.
2016-10-26T15:27:47Z
A Mechanistic Model for Acidic Drug Release Using Microspheres Made of PLGA 50:50
Sevim, Kevser
Pan, Jingzhe
http://hdl.handle.net/2381/38157
2016-10-08T03:19:32Z
2016-10-07T10:48:11Z
Title: A Mechanistic Model for Acidic Drug Release Using Microspheres Made of PLGA 50:50
Authors: Sevim, Kevser; Pan, Jingzhe
Abstract: Polyester microspheres are extensively studied for controlled release drug delivery devices, and many models have been developed to describe drug release from the bulk polymer. However, the interaction between drugs and polymers is ignored in most of the existing mathematical models. This paper presents a mechanistic model which captures the interplay between acidic drugs and bioresorbable polyesters. The model considers the autocatalytic effect on polymer degradation arising from carboxylic acid end groups of oligomers and drug molecules. Hence, the enhancing effect of acidic drug on the rate of degradation was fully considered. On the other hand the drug release from polyester microspheres is controlled by drug diffusion from polymer matrix. The drug diffusion coefficient depends strongly on the level of degradation of the polymer. This effect is also included in the model. It is shown that the model can effectively predict experimental data in the literature for both polymer degradation and drug release. Furthermore, the model is used to design different systems of microspheres which release drugs with either a zero order profile or burst followed by zero order release profile.
Description: Following the embargo period the above license applies.
2016-10-07T10:48:11Z
The Beginnings of Wisdom: Challenges in Engineering Education
Atkinson, Helen V.
http://hdl.handle.net/2381/38071
2016-09-14T02:17:08Z
2016-09-13T14:49:05Z
Title: The Beginnings of Wisdom: Challenges in Engineering Education
Authors: Atkinson, Helen V.
2016-09-13T14:49:05Z
Near real-time input to a propagation model for nowcasting of HF communications with aircraft on polar routes
Warrington, E. Michael
Stocker, A. J.
Siddle, D. R.
Hallam, J. M.
Al-Behadili, H. A. H.
Zaalov, N. Y.
Honary, F.
Rogers, N. C.
Boteler, D. H.
Danskin, D. W.
http://hdl.handle.net/2381/37900
2016-08-04T02:17:04Z
2016-08-03T09:50:31Z
Title: Near real-time input to a propagation model for nowcasting of HF communications with aircraft on polar routes
Authors: Warrington, E. Michael; Stocker, A. J.; Siddle, D. R.; Hallam, J. M.; Al-Behadili, H. A. H.; Zaalov, N. Y.; Honary, F.; Rogers, N. C.; Boteler, D. H.; Danskin, D. W.
Abstract: There is a need for improved techniques for nowcasting and forecasting (over several hours) HF propagation at northerly latitudes to support airlines operating over the increasingly popular trans-polar routes. In this paper the assimilation of real-time measurements into a propagation model developed by the authors is described, including ionosonde measurements and total electron content (TEC) measurements to define the main parameters of the ionosphere. The effects of D region absorption in the polar cap and auroral regions are integrated with the model through satellite measurements of the flux of energetic solar protons (>1 MeV) and the X-ray flux in the 0.1–0.8 nm band, and ground-based magnetometer measurements which form the Kp and Dst indices of geomagnetic activity. The model incorporates various features (e.g., convecting patches of enhanced plasma density) of the polar ionosphere that are, in particular, responsible for off-great circle propagation and lead to propagation at times and frequencies not expected from on-great circle propagation alone. The model development is supported by the collection of HF propagation measurements over several paths within the polar cap, crossing the auroral oval, and along the midlatitude trough.
2016-08-03T09:50:31Z
Detection of degradation in polyester implants by analysing mode shapes of structure vibration
Samami, Hassan
Pan, Jingzhe
http://hdl.handle.net/2381/37846
2016-07-16T03:24:03Z
2016-07-15T11:06:10Z
Title: Detection of degradation in polyester implants by analysing mode shapes of structure vibration
Authors: Samami, Hassan; Pan, Jingzhe
Abstract: This paper presents a numerical study on using vibration analysis to detect degradation in degrading polyesters. A numerical model of a degrading plate sample is considered. The plate is assumed to degrade following the typical behaviour of amorphous copolymers of polylactide and polyglycolide. Due to the well-known autocatalytic effect in the degradation of these polyesters, the inner core of the plate degrades faster than outer surface region, forming layers of materials with varying Young׳s modulus. Firstly the change in molecular weight and corresponding change in Young׳s modulus at different times are calculated using the mathematical models developed in our previous work. Secondly the first four mode shapes of transverse vibration of the plate are calculated using the finite element method. Finally the curvature of the mode shapes are calculated and related to the spatial distribution of the polymer degradation. It is shown that the curvature of the mode shapes can be used to detect the onset and distribution of polymer degradation. The level of measurement accuracy required in an experiment is presented to guide practical applications of the method. At the end of this paper a demonstration case of coronary stent is presented showing how the method can be used to detect degradation in an implant of sophisticated structure.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-07-15T11:06:10Z
Similarity Solutions of Compressible Flow over a Rotating Cone with Surface Suction
Towers, P. D.
Garrett, Stephen J.
http://hdl.handle.net/2381/37825
2016-07-14T01:45:09Z
2016-07-11T12:33:12Z
Title: Similarity Solutions of Compressible Flow over a Rotating Cone with Surface Suction
Authors: Towers, P. D.; Garrett, Stephen J.
Abstract: We present solutions of the laminar compressible boundary-layer flows over the family of rotating cones subject to surface mass flux. The work is a generalization of previous studies of the compressible rotating-disk flow and incompressible rotating-cone flow without surface mass flux. Transformations are used which lead to a system of generalized von Kármán equations with boundary conditions parameterized by half-angle and a mass-flux parameter. Results are discussed in terms of wall temperature and local Mach number in the particular case of air, although the formulation is readily extended to other fluids. It is suggested that suction acts a stabilizing mechanism, whereas increased wall temperature and local Mach number have destabilizing influences.
2016-07-11T12:33:12Z
A framework for experimental determination of localised vertical pedestrian forces on full-scale structures using wireless attitude and heading reference systems
Bocian, Mateusz
Brownjohn, J. M. W.
Racic, V.
Hester, D.
Quattrone, A.
Monnickendam, R.
http://hdl.handle.net/2381/37689
2016-06-08T02:13:20Z
2016-06-07T10:40:43Z
Title: A framework for experimental determination of localised vertical pedestrian forces on full-scale structures using wireless attitude and heading reference systems
Authors: Bocian, Mateusz; Brownjohn, J. M. W.; Racic, V.; Hester, D.; Quattrone, A.; Monnickendam, R.
Abstract: A major weakness among loading models for pedestrians walking on flexible structures proposed in recent years is the various uncorroborated assumptions made in their development. This applies to spatio-temporal characteristics of pedestrian loading and the nature of multi-object interactions. To alleviate this problem, a framework for the determination of localised pedestrian forces on full-scale structures is presented using a wireless attitude and heading reference systems (AHRS). An AHRS comprises a triad of tri-axial accelerometers, gyroscopes and magnetometers managed by a dedicated data processing unit, allowing motion in three-dimensional space to be reconstructed. A pedestrian loading model based on a single point inertial measurement from an AHRS is derived and shown to perform well against benchmark data collected on an instrumented treadmill. Unlike other models, the current model does not take any predefined form nor does it require any extrapolations as to the timing and amplitude of pedestrian loading. In order to assess correctly the influence of the moving pedestrian on behaviour of a structure, an algorithm for tracking the point of application of pedestrian force is developed based on data from a single AHRS attached to a foot. A set of controlled walking tests with a single pedestrian is conducted on a real footbridge for validation purposes. A remarkably good match between the measured and simulated bridge response is found, indeed confirming applicability of the proposed framework.
2016-06-07T10:40:43Z
Empirical Studies for the Assessment of the Effectiveness of Design Patterns in Migration between Software Architectures of Embedded Applications
Lakhani, Farah
Pont, Michael J.
http://hdl.handle.net/2381/37669
2016-06-01T02:16:43Z
2016-05-31T12:14:38Z
Title: Empirical Studies for the Assessment of the Effectiveness of Design Patterns in Migration between Software Architectures of Embedded Applications
Authors: Lakhani, Farah; Pont, Michael J.
Abstract: Two main architectures used to develop software for modern embedded applications are “event triggered” (ET) and “time triggered” (TT). ET designs involve creating systems which handle multiple interrupts; by contrast, only one interrupt is ever enabled in a TT design, and this interrupt is usually linked to a timer “Tick.” Although TT architectures are widely used in safety-related designs, they are less familiar to developers of mainstream embedded systems. The work on this research began from the premise that—for a broad class of systems—the use of a TT architecture would improve reliability. The overall goal of the work presented here was to identify ways in which the effort involved in migrating between existing ET architectures and “equivalent” TT architectures could be reduced. The specific goal of the research was to explore whether the use of an appropriate set of design patterns could assist developers who wished to migrate between ET and TT designs. An empirical evaluation of the efficacy of a newly proposed pattern collection is described in this paper. The results of these trials demonstrate that the proposed collection of patterns has the potential to support developers by helping them to take appropriate decisions during the migration process.
2016-05-31T12:14:38Z
A study of the temperature dependence of bienzyme systems and enzymatic chains
Kotov, N. V.
Baker, R. E.
Dawidov, D. A.
Platov, K. V.
Valeyev, Najil V.
Skorinkin, A. I.
Maini, P. K.
http://hdl.handle.net/2381/37660
2016-05-28T03:09:58Z
2016-05-27T14:50:23Z
Title: A study of the temperature dependence of bienzyme systems and enzymatic chains
Authors: Kotov, N. V.; Baker, R. E.; Dawidov, D. A.; Platov, K. V.; Valeyev, Najil V.; Skorinkin, A. I.; Maini, P. K.
Abstract: It is known that most enzyme-facilitated reactions are highly temperature dependent processes. In general, the temperature coefficient, Q10, of a simple reaction reaches 2.0-3.0. Nevertheless, some enzyme-controlled processes have much lower Q10 (about 1.0), which implies that the process is almost temperature independent, even if individual reactions involved in the process are themselves highly temperature dependent. In this work, we investigate a possible mechanism for this apparent temperature compensation: simple mathematical models are used to study how varying types of enzyme reactions are affected by temperature. We show that some bienzyme-controlled processes may be almost temperature independent if the modules involved in the reaction have similar temperature dependencies, even if individually, these modules are strongly temperature dependent. Further, we show that in non-reversible enzyme chains the stationary concentrations of metabolites are dependent only on the relationship between the temperature dependencies of the first and last modules, whilst in reversible reactions, there is a dependence on every module. Our findings suggest a mechanism by which the metabolic processes taking place within living organisms may be regulated, despite strong variation in temperature.
2016-05-27T14:50:23Z
Effects of Surface Diffusion and Heating Rate on First-Stage Sintering That Densifies by Grain-Boundary Diffusion
Pan, Jingzhe
Luo, Wendong
http://hdl.handle.net/2381/37485
2016-07-14T01:45:07Z
2016-05-05T11:51:25Z
Title: Effects of Surface Diffusion and Heating Rate on First-Stage Sintering That Densifies by Grain-Boundary Diffusion
Authors: Pan, Jingzhe; Luo, Wendong
Abstract: This paper presents a computational study of the role played by surface diffusion on first-stage sintering of powders that densify by grain-boundary diffusion. Coupled grain-boundary and surface diffusion is considered as the mechanism for matter redistribution. By using several novel approaches of presentation of the numerical data, it is shown that the assumption of fast surface diffusion is invalid for typical sintering conditions and materials in first stage of sintering. The study reveals a simple explanation for the role played by surface diffusion in matter redistribution of combined grain-boundary and surface diffusion—surface diffusion changes direction from moving atoms away from a contact neck to depositing atoms onto it as the rate of surface diffusion increases. The reverse surface diffusion blunts the neck and retards densification. It is shown that this mechanism is significant not only for free sintering but also for pressure assisted sintering. It is further confirmed that the widely observed beneficial effect of spark plasma sintering on densification can be, at least partially, attributed to its fast heating rate, which quickly passes through sintering at low temperatures where surface diffusion dominates.
Description: The file associated with this record is under a 12-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-05-05T11:51:25Z
An atomic finite element model for biodegradable polymers. Part 2. A model for change in Young's modulus due to polymer chain scission
Gleadall, Andrew
Pan, Jingzhe
Kruft, Marc-Anton
http://hdl.handle.net/2381/37484
2016-05-06T02:07:09Z
2016-05-05T11:28:59Z
Title: An atomic finite element model for biodegradable polymers. Part 2. A model for change in Young's modulus due to polymer chain scission
Authors: Gleadall, Andrew; Pan, Jingzhe; Kruft, Marc-Anton
Abstract: Atomic simulations were undertaken to analyse the effect of polymer chain scission on amorphous poly(lactide) during degradation. Many experimental studies have analysed mechanical properties degradation but relatively few computation studies have been conducted. Such studies are valuable for supporting the design of bioresorbable medical devices. Hence in this paper, an Effective Cavity Theory for the degradation of Young's modulus was developed. Atomic simulations indicated that a volume of reduced-stiffness polymer may exist around chain scissions. In the Effective Cavity Theory, each chain scission is considered to instantiate an effective cavity. Finite Element Analysis simulations were conducted to model the effect of the cavities on Young's modulus. Since polymer crystallinity affects mechanical properties, the effect of increases in crystallinity during degradation on Young's modulus is also considered. To demonstrate the ability of the Effective Cavity Theory, it was fitted to several sets of experimental data for Young's modulus in the literature.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-05-05T11:28:59Z
An atomic finite element model for biodegradable polymers. Part 1. Formulation of the finite elements
Gleadall, Andrew
Pan, Jingzhe
Ding, Lifeng
Kruft, Marc-Anton
Curcó, David
http://hdl.handle.net/2381/37483
2016-05-06T02:07:07Z
2016-05-05T11:15:09Z
Title: An atomic finite element model for biodegradable polymers. Part 1. Formulation of the finite elements
Authors: Gleadall, Andrew; Pan, Jingzhe; Ding, Lifeng; Kruft, Marc-Anton; Curcó, David
Abstract: Molecular dynamics (MD) simulations are widely used to analyse materials at the atomic scale. However, MD has high computational demands, which may inhibit its use for simulations of structures involving large numbers of atoms such as amorphous polymer structures. An atomic-scale finite element method (AFEM) is presented in this study with significantly lower computational demands than MD. Due to the reduced computational demands, AFEM is suitable for the analysis of Young's modulus of amorphous polymer structures. This is of particular interest when studying the degradation of bioresorbable polymers, which is the topic of an accompanying paper. AFEM is derived from the inter-atomic potential energy functions of an MD force field. The nonlinear MD functions were adapted to enable static linear analysis. Finite element formulations were derived to represent interatomic potential energy functions between two, three and four atoms. Validation of the AFEM was conducted through its application to atomic structures for crystalline and amorphous poly(lactide).
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-05-05T11:15:09Z
A constitutive law for degrading bioresorbable polymers
Samami, Hassan
Pan, Jingzhe
http://hdl.handle.net/2381/37482
2016-05-06T02:05:19Z
2016-05-05T10:58:44Z
Title: A constitutive law for degrading bioresorbable polymers
Authors: Samami, Hassan; Pan, Jingzhe
Abstract: This paper presents a constitutive law that predicts the changes in elastic moduli, Poisson's ratio and ultimate tensile strength of bioresorbable polymers due to biodegradation. During biodegradation, long polymer chains are cleaved by hydrolysis reaction. For semi-crystalline polymers, the chain scissions also lead to crystallisation. Treating each scission as a cavity and each new crystal as a solid inclusion, a degrading semi-crystalline polymer can be modelled as a continuum solid containing randomly distributed cavities and crystal inclusions. The effective elastic properties of a degrading polymer are calculated using existing theories for such solid and the tensile strength of the degrading polymer is predicted using scaling relations that were developed for porous materials. The theoretical model for elastic properties and the scaling law for strength form a complete constitutive relation for the degrading polymers. It is shown that the constitutive law can capture the trend of the experimental data in the literature for a range of biodegradable polymers fairly well.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-05-05T10:58:44Z
Degradation mechanisms of bioresorbable polyesters. Part 2. Effects of initial molecular weight and residual monomer
Gleadall, Andrew
Pan, Jingzhe
Kruft, Marc-Anton
Kellomäki, Minna
http://hdl.handle.net/2381/37481
2016-05-06T02:05:20Z
2016-05-05T10:46:44Z
Title: Degradation mechanisms of bioresorbable polyesters. Part 2. Effects of initial molecular weight and residual monomer
Authors: Gleadall, Andrew; Pan, Jingzhe; Kruft, Marc-Anton; Kellomäki, Minna
Abstract: This paper presents an understanding of how initial molecular weight and initial monomer fraction affect the degradation of bioresorbable polymers in terms of the underlying hydrolysis mechanisms. A mathematical model was used to analyse the effects of initial molecular weight for various hydrolysis mechanisms including noncatalytic random scission, autocatalytic random scission, noncatalytic end scission or autocatalytic end scission. Different behaviours were identified to relate initial molecular weight to the molecular weight half-life and to the time until the onset of mass loss. The behaviours were validated by fitting the model to experimental data for molecular weight reduction and mass loss of samples with different initial molecular weights. Several publications that consider initial molecular weight were reviewed. The effect of residual monomer on degradation was also analysed, and shown to accelerate the reduction of molecular weight and mass loss. An inverse square root law relationship was found between molecular weight half-life and initial monomer fraction for autocatalytic hydrolysis. The relationship was tested by fitting the model to experimental data with various residual monomer contents.
2016-05-05T10:46:44Z
Degradation mechanisms of bioresorbable polyesters. Part 1. Effects of random scission, end scission and autocatalysis
Gleadall, Andrew
Pan, Jingzhe
Kruft, Marc-Anton
Kellomäki, Minna
http://hdl.handle.net/2381/37480
2016-05-06T02:05:10Z
2016-05-05T10:35:21Z
Title: Degradation mechanisms of bioresorbable polyesters. Part 1. Effects of random scission, end scission and autocatalysis
Authors: Gleadall, Andrew; Pan, Jingzhe; Kruft, Marc-Anton; Kellomäki, Minna
Abstract: A mathematical model was developed to relate the degradation trend of bioresorbable polymers to different underlying hydrolysis mechanisms, including noncatalytic random scission, autocatalytic random scission, noncatalytic end scission or autocatalytic end scission. The effect of each mechanism on molecular weight degradation and potential mass loss was analysed. A simple scheme was developed to identify the most likely hydrolysis mechanism based on experimental data. The scheme was first demonstrated using case studies, then used to evaluate data collected from 31 publications in the literature to identify the dominant hydrolysis mechanisms for typical biodegradable polymers. The analysis showed that most of the experimental data indicates autocatalytic hydrolysis, as expected. However, the study shows that the existing understanding on whether random or end scission controls degradation is inappropriate. It was revealed that pure end scission cannot explain the observed trend in molecular weight reduction because end scission would be too slow to reduce the average molecular weight. On the other hand, pure random scission cannot explain the observed trend in mass loss because too few oligomers would be available to diffuse out of a device. It is concluded that the chain ends are more susceptible to cleavage, which produces most of the oligomers leading to mass loss. However, it is random scission that dominates the reduction in molecular weight.
2016-05-05T10:35:21Z
Impact of dynamic voltage scaling and thermal factors on SRAM reliability
Rosa, F. R.
Brum, R. M.
Wirth, G.
Kastensmidt, F.
Ost, Luciano
Reis, R.
http://hdl.handle.net/2381/37452
2016-04-27T02:05:19Z
2016-04-26T14:20:53Z
Title: Impact of dynamic voltage scaling and thermal factors on SRAM reliability
Authors: Rosa, F. R.; Brum, R. M.; Wirth, G.; Kastensmidt, F.; Ost, Luciano; Reis, R.
Abstract: This work investigates the effects of temperature and voltage scaling in neutron-induced bit-flip in SRAM memory cells. Proposed approach allows determining the critical charge according to the dynamic behavior of the temperature as a function of the voltage scaling. Experimental results show that both temperature and voltage scaling can increase in at least two times the susceptibility of SRAM cells to soft error rate (SER). In addition, a model for electrical simulation for soft error and different voltages was described to investigate the effects observed in the practical neutron irradiation experiments. Results can guide designers to predict soft error effects during the lifetime of SRAM-based devices considering different power supply modes.
2016-04-26T14:20:53Z
Vacuum assisted flow initiation in arching powders
Sinka, Iosif Csaba
Baserinia, R.
Rajniak, P.
http://hdl.handle.net/2381/37437
2016-04-26T02:05:13Z
2016-04-25T09:41:24Z
Title: Vacuum assisted flow initiation in arching powders
Authors: Sinka, Iosif Csaba; Baserinia, R.; Rajniak, P.
Abstract: The discharge of powders from hoppers usually takes place in open atmosphere. However, in powder pressing industries (e.g. manufacturing of pharmaceutical tablets, detergents, ceramics, powder metallurgy etc.) there are handling operations where powders are filled into closed cavities such as dies. During this process the air pressure is increased as powder is delivered into the die. At the same time typical tablet production equipment creates a suction effect. A critical orifice measurement apparatus was developed to study powder flow initiation from an arching state into an enclosure where the air pressure is reduced. It was shown that a very small reduction of pressure changed the critical orifice diameter significantly. Dimensional analysis was carried out to relate powder properties (particle size and density) and processing parameters (geometry of the system and differential pressure necessary to break the arch). A relationship was developed to calculate the pressure difference necessary to initiate powder flow. The relationship has two empirical parameters which are calibrated by performing simple experiments using the testing rig developed.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-25T09:41:24Z
Hierarchical energy monitoring for task mapping in many-core systems
Castilhos, G.
Mandelli, M.
Ost, Luciano
Moraes, F.
http://hdl.handle.net/2381/37427
2016-04-22T02:05:16Z
2016-04-21T10:39:31Z
Title: Hierarchical energy monitoring for task mapping in many-core systems
Authors: Castilhos, G.; Mandelli, M.; Ost, Luciano; Moraes, F.
Abstract: This work addresses a research subject with a rich literature: task mapping in NoC-based systems. Task mapping is the process of selecting a processing element to execute a given task. The number of cores in many-core systems increases the complexity of the task mapping. The main concerns in task mapping in large systems include (i) scalability; (ii) dynamic workload; and (iii) reliability. It is necessary to distribute the mapping decision across the system to ensure scalability. The workload of emerging many-core systems may be dynamic, i.e., new applications may start at any moment, leading to different mapping scenarios. Therefore, it is necessary to execute the mapping process at runtime to support a dynamic workload assignment. The workload assignment plays an important role in the many-core system reliability. Load imbalance may generate hotspots zones and consequently thermal implications, which may generate hotspots zones and consequently thermal implications. More recently, task mapping techniques aiming at improving system reliability have been proposed in the literature. However, such approaches rely on centralized mapping decisions, which are not scalable. To address these challenges, the main goal of this work is to propose a hierarchical runtime mapping heuristic, which provides scalability and a fair workload distribution. Distributing the workload inside the system increases the system reliability in long-term, due to the reduction of hotspot regions. The proposed mapping heuristic considers the application workload as a function of the consumed energy in the processors and NoC routers. The proposal adopts a hierarchical energy monitoring scheme, able to estimate at runtime the consumption at each processing element. The mapping uses the energy estimated by the monitoring scheme to guide the mapping decision. Results compare the proposal against a mapping heuristic whose main cost function minimizes the communication energy. Results obtained in large systems, up to 256 cores, show improvements in the workload distribution (average value 59.2%) and a reduction in the maximum energy values spent by the processors (average value 32.2%). Such results demonstrate the effectiveness of the proposal.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-21T10:39:31Z
Crossed-magnetic-field experiments on stacked second generation superconducting tapes: Reduction of the demagnetization effects
Baghdadi, M.
Ruiz, Harold Steven
Coombs, T. A.
http://hdl.handle.net/2381/37410
2016-04-22T02:20:28Z
2016-04-21T08:54:02Z
Title: Crossed-magnetic-field experiments on stacked second generation superconducting tapes: Reduction of the demagnetization effects
Authors: Baghdadi, M.; Ruiz, Harold Steven; Coombs, T. A.
Abstract: The crossed-magnetic-field effect on the demagnetization factor of stacked second generation (2G) high temperature superconducting tapes is presented. The superconducting sample was initially magnetized along the c-axis by the field cooling magnetization method and after achieving the magnetic relaxation of the sample, an extensive set of experimental measurements for different amplitudes of an applied ac magnetic field parallel to the ab-plane was performed. On the one hand, a striking reduction of the demagnetization factor compared with the reported values for superconducting bulks is reported. On the other hand, the demagnetization factor increases linearly with the amplitude of the ac transverse magnetic field confirming the universal linear behavior for the magnetic susceptibility predicted by Brandt [Phys. Rev. B 54, 4246 (1996)]. The study has been also pursued at different frequencies of the ac transverse magnetic field in order to determine the influence of this parameter on the demagnetization factor measurements. We report an even lower demagnetization factor as long as the frequency of the transverse magnetic field increases. Thus, the significant reduction on the demagnetization factor that we have found by using stacked 2G-superconducting tapes, with higher mechanical strength compared with the one of superconducting bulks, makes to this configuration a highly attractive candidate for the future development of more efficient high-power density rotating machines and strong magnet applications.
2016-04-21T08:54:02Z
A non-square sector condition and its application in deferred-action anti-windup compensator design
Turner, Matthew C.
Herrmann, G.
http://hdl.handle.net/2381/37279
2016-04-14T02:19:58Z
2016-04-13T09:20:49Z
Title: A non-square sector condition and its application in deferred-action anti-windup compensator design
Authors: Turner, Matthew C.; Herrmann, G.
Abstract: A sector condition for two connected deadzone nonlinearities is provided. By introducing an additional non-square operator which exploits their connectivity, a more general set of sector-like matrix inequalities is obtained. This “non-square” matrix inequality condition is applied to an anti-windup (AW) problem in which the AW compensator is not activated until the unconstrained control signal reaches a well-defined level beyond that of the physical actuator limits. The non-square sector condition allows such “deferred-action” AW synthesis to be performed in a manner much closer to traditional (“immediate”) sector-based AW with either lowered conservatism or decreased computational effort in contrast to recent work. The non-square condition is applicable to other AW problems.
2016-04-13T09:20:49Z
Lyapunov functions and L-2 gain bounds for systems with slope restricted nonlinearities
Turner, Matthew C.
Kerr, M.
http://hdl.handle.net/2381/37278
2016-05-04T01:45:09Z
2016-04-13T09:16:27Z
Title: Lyapunov functions and L-2 gain bounds for systems with slope restricted nonlinearities
Authors: Turner, Matthew C.; Kerr, M.
Abstract: The stability and L2L2 performance analysis of systems consisting of an interconnection of a linear-time-invariant (LTI) system and a static nonlinear element which is Lipschitz, slope restricted and sector bounded is revisited. The main thrust of the paper is to improve and extend an existing result in the literature to enable (i) concise and correct conditions for asymptotic stability of the interconnection and (ii) reasonably tight bounds on the L2L2 gain between an exogenous input and a given output to be obtained. Numerical examples indicate that the proposed algorithm performs well compared to competing results in the literature.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-13T09:16:27Z
Zames-Falb multipliers for absolute stability : from O'Shea's contribution to convex searches
Turner, Matthew C.
Carrasco, J.
Heath, W. P.
http://hdl.handle.net/2381/37277
2016-04-14T02:20:01Z
2016-04-13T09:12:34Z
Title: Zames-Falb multipliers for absolute stability : from O'Shea's contribution to convex searches
Authors: Turner, Matthew C.; Carrasco, J.; Heath, W. P.
Abstract: Absolute stability attracted much attention in the 1960s. Several stability conditions for loops with slope-restricted nonlinearities were developed. Results such as the Circle Criterion and the Popov Criterion form part of the core curriculum for students of control. Moreover, the equivalence of results obtained by different techniques, specifically Lyapunov and Popov׳s stability theories, led to one of the most important results in control engineering: the KYP Lemma.
For Lurye1 systems this work culminated in the class of multipliers proposed by O׳Shea in 1966 and formalized by Zames and Falb in 1968. The superiority of this class was quickly and widely accepted. Nevertheless the result was ahead of its time as graphical techniques were preferred in the absence of readily available computer optimization. Its first systematic use as a stability criterion came 20 years after the initial proposal of the class. A further 20 years have been required to develop a proper understanding of the different techniques that can be used. In this long gestation some significant knowledge has been overlooked or forgotten. Most significantly, O׳Shea׳s contribution and insight is no longer acknowledged; his papers are barely cited despite his original parameterization of the class.
This tutorial paper aims to provide a clear and comprehensive introduction to the topic from a user׳s viewpoint. We review the main results: the stability theory, the properties of the multipliers (including their phase properties, phase-equivalence results and the issues associated with causality), and convex searches. For clarity of exposition we restrict our attention to continuous time multipliers for single-input single-output results. Nevertheless we include several recent significant developments by the authors and others. We illustrate all these topics using an example proposed by O׳Shea himself.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text is available in the publisher's site.
2016-04-13T09:12:34Z
Cubature H-∞ Information Filter and its Extension
Gu, Dawei
Chandra, K. P. B.
Postlethwaite, I.
http://hdl.handle.net/2381/37218
2016-04-12T02:26:39Z
2016-04-11T08:27:12Z
Title: Cubature H-∞ Information Filter and its Extension
Authors: Gu, Dawei; Chandra, K. P. B.; Postlethwaite, I.
Abstract: State estimation for nonlinear systems with Gaussian or non-Gaussian noises, and with single and multiple sensors, is presented. The key purpose is to propose a derivative free estimator using concepts from the information filter, the H∞H∞ filter, and the cubature Kalman filter (CKF). The proposed estimator is called the cubature H∞H∞ information filter (CH∞IFCH∞IF); it has the capability to deal with highly nonlinear systems like the CKF, like the H∞H∞ filter it can estimate states with stochastic or deterministic noises, and similar to the information filter it can be easily extended to handle measurements from multiple sensors. A numerically stable square-root CH∞IFCH∞IF is developed and extended to multiple sensors. The CH∞IFCH∞IF is implemented to estimate the states of a nonlinear permanent magnet synchronous motor model. Comparisons are made with an extended H∞
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2016-04-11T08:27:12Z
Thixoforming of A356/SiC and A356/TiB2 Nanocomposites Fabricated by a Combination of Green Compact Nanoparticle Incorporation and Ultrasonic Treatment of the Melted Compact
Kandemir, S.
Atkinson, Helen V.
Weston, David P.
Hainsworth, Sarah V.
http://hdl.handle.net/2381/37216
2016-04-09T03:37:00Z
2016-04-08T12:11:23Z
Title: Thixoforming of A356/SiC and A356/TiB2 Nanocomposites Fabricated by a Combination of Green Compact Nanoparticle Incorporation and Ultrasonic Treatment of the Melted Compact
Authors: Kandemir, S.; Atkinson, Helen V.; Weston, David P.; Hainsworth, Sarah V.
Abstract: Thixoforming is a type of semi-solid processing which is based on forming metals in the semi-solid state rather than fully liquid or solid state. There have been no reports of the thixoforming of nanocomposites in the literature. The incorporation of ceramic nanoparticles into liquid metals is a challenging task for the fabrication of metal matrix nanocomposites due to their large surface-to-volume ratio and poor wettability. Previous research work by a number of workers has highlighted the challenges with the incorporation of nanoparticles into liquid aluminum alloy. In the present study, SiC and TiB2 nanoparticles with an average diameter between 20 and 30 nm were firstly incorporated into green compacts by a powder forming route, and then the compacts were melted and treated ultrasonically. The microstructural studies reveal that the engulfment and relatively effective distribution of the nanoparticles into the melt were achieved. The hardness was considerably improved with only 0.8 wt pct addition of the nanoparticles. The nanocomposites were successfully thixoformed at a solid fraction between 0.65 and 0.70. The microstructures, hardness, and tensile mechanical properties of the thixoformed nanocomposites were investigated and compared with those of the as-received A356 and thixoformed A356 alloys. The tensile properties of the thixoformed nanocomposites were significantly enhanced compared to thixoformed A356 alloy without reinforcement, indicating the strengthening effects of the nanoparticles.
2016-04-08T12:11:23Z
New multi-stage DC–DC converters for grid-connected photovoltaic systems
Hu, Y.
Cao, W.
Ji, Bing
Si, J.
Chen, X.
http://hdl.handle.net/2381/37184
2016-08-24T01:45:09Z
2016-04-07T10:38:54Z
Title: New multi-stage DC–DC converters for grid-connected photovoltaic systems
Authors: Hu, Y.; Cao, W.; Ji, Bing; Si, J.; Chen, X.
Abstract: Renewable energy is high on international and national agendas. Currently, grid-connected photovoltaic (PV) systems are a popular technology to convert solar energy into electricity. Existing PV panels have a relatively low and varying output voltage so that the converter installed between the PVs and the grid should be equipped with high step-up and versatile control capabilities. In addition, the output current of PV systems is rich in harmonics which affect the power quality of the grid. In this paper, a new multi-stage hysteresis control of a step-up DC–DC converter is proposed for integrating PVs into a single-phase power grid. The proposed circuitry and control method is experimentally validated by testing on a 600 W prototype converter. The developed technology has significant economic implications and could be applied to many distributed generation (DG) systems, especially for the developing countries which have a large number of small PVs connected to their single-phase distribution network.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-07T10:38:54Z
Optimised phase disposition pulse-width modulation strategy for hybrid-clamped multilevel inverters using switching state sequences
Ma, M.
He, X.
Cao, W.
Song, X.
Ji, Bing
http://hdl.handle.net/2381/37183
2016-04-08T02:25:04Z
2016-04-07T10:33:51Z
Title: Optimised phase disposition pulse-width modulation strategy for hybrid-clamped multilevel inverters using switching state sequences
Authors: Ma, M.; He, X.; Cao, W.; Song, X.; Ji, Bing
Abstract: This study describes an optimised modulation strategy based on switching state sequences for the hybrid-clamped multilevel converter. Two key control variables defined as ‘phase shift angle’ and ‘switching state change’ for a five-level hybrid-clamped inverter are proposed to improve all switches’ operation, and by changing their values, different control methods can be obtained for modulation optimisation purposes. Two example methods can solve the voltage imbalance problem of the dc-link capacitors and furthermore avoid two switches’ simultaneous switching transitions and improve the inverter's performance as compared with the traditional phase disposition pulse-width modulation strategy. A 6 kW prototype inverter is developed and a range of simulation and experiments are carried out for validation. It is found that simulation and experimental results are in a good agreement and the proposed modulation strategy is verified in terms of low-order harmonic reduction.
2016-04-07T10:33:51Z
Tractable stability analysis for systems containing repeated scalar slope-restricted nonlinearities
Turner, Matthew Christopher
Kerr, M. L.
Sofrony, J.
http://hdl.handle.net/2381/37160
2016-04-07T02:26:59Z
2016-04-06T10:24:46Z
Title: Tractable stability analysis for systems containing repeated scalar slope-restricted nonlinearities
Authors: Turner, Matthew Christopher; Kerr, M. L.; Sofrony, J.
Abstract: This paper proposes an LMI-based approach for studying the stability of feedback interconnections of a finite dimensional LTI system and a nonlinear element that consists of several identical scalar nonlinearities that have restrictions on their sector and slope. The results are based on the integral quadratic constraint stability analysis framework and other recent results that give a sharp characterisation of stability multipliers for monotone, repeated scalar nonlinearities. Several examples show the effectiveness of the proposed approach; the lack of conservatism in the results is noteworthy. Copyright © 2013 John Wiley & Sons, Ltd.
2016-04-06T10:24:46Z
Antiwindup Design for Zero-Phase Repetitive Controllers
Flores, J. V.
Gomes da Silva, J. M.
Sbarbaro, D.
Turner, Matthew Christopher
Salton, A. T.
http://hdl.handle.net/2381/37159
2016-04-07T02:25:01Z
2016-04-06T10:20:25Z
Title: Antiwindup Design for Zero-Phase Repetitive Controllers
Authors: Flores, J. V.; Gomes da Silva, J. M.; Sbarbaro, D.; Turner, Matthew Christopher; Salton, A. T.
Abstract: This paper addresses the antiwindup problem for linear systems equipped with the zero-phase repetitive controller (ZPRC). The antiwindup compensator is designed using a coprime factorization technique and conditions to characterize the sets of admissible references and disturbances are proposed. A numerical example illustrates the application and potentialities of the proposed methodology.
2016-04-06T10:20:25Z
What is the Process Window for Semi-solid Processing?
Zhang, Duyao
Dong, Hongbiao
Atkinson, Helen V.
http://hdl.handle.net/2381/37157
2016-11-05T02:45:09Z
2016-04-06T09:45:38Z
Title: What is the Process Window for Semi-solid Processing?
Authors: Zhang, Duyao; Dong, Hongbiao; Atkinson, Helen V.
Abstract: In semi-solid processing, the liquid fraction vs temperature is commonly used to define the process window. Conventionally, it is assumed that a freezing range is required for semi-solid processing but recently both high-purity aluminum and binary Al-Si eutectic alloy have been rheo-processed. Here, the kinetics during melting and solidification of pure metal are analyzed, and a comparison between the liquid fraction vs temperature and the liquid fraction vs time is presented. It is found that liquid fraction vs time is a significant criterion for semi-solid processing.
Description: The file associated with this record is under a 12-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-06T09:45:38Z
Does Shear Thickening Occur in Semisolid Metals?
Atkinson, Helen Valerie
Favier, V.
http://hdl.handle.net/2381/37155
2016-04-07T02:25:11Z
2016-04-06T09:22:53Z
Title: Does Shear Thickening Occur in Semisolid Metals?
Authors: Atkinson, Helen Valerie; Favier, V.
Abstract: In the various forms of semisolid processing such as thixoforming and thixoforging, the entry into the die occurs in a fraction of a second so it is the transient rheological behavior which governs the initial stages of flow. In experiments in the literature, this rheological behavior is probed through applying rapid transitions in shear rate under isothermal conditions. There is contradictory evidence as to whether the behavior during these transitions is shear thinning or shear thickening, although it is clear that once in the die the material is thinning. Here the data in the literature are reanalyzed to obtain a rationalization of the contradictions which has not previously been available. It is argued that if a suspension is initially in a disagglomerated state (i.e., one which is initially sheared), the instantaneous behavior with a jump-up in shear rate is shear thickening (even if the long-term steady-state behavior is shear thinning) provided the fraction solid is greater than about 0.36 and the final shear rate at the end of the jump is greater than about 100 s−1. If the jump-up in shear rate is made from rest then yield masks the shear thickening.
2016-04-06T09:22:53Z
Force control of semi-active valve lag dampers for vibration reduction in helicopters
Morales, Rafael Mauricio
Turner, M. C.
Court, P.
Hilditch, R.
Postlethwaite, I.
http://hdl.handle.net/2381/37146
2016-04-06T02:23:28Z
2016-04-05T11:04:00Z
Title: Force control of semi-active valve lag dampers for vibration reduction in helicopters
Authors: Morales, Rafael Mauricio; Turner, M. C.; Court, P.; Hilditch, R.; Postlethwaite, I.
Abstract: This study considers the design of a closed-loop force-tracking system for a semi-active damper, designed to be used to reduce in-plane vibrations caused by helicopter rotor blades during steady-state forward flight conditions. The study describes the development of the control law and includes details of (i) how the initial mathematical model of the system is adapted for controller design; (ii) how a non-linear dynamic inversion (NDI) control law is modified into a form suitable for implementation; and (iii) how the free parameters in the NDI controller can be optimised for various different operational modes. The success of the approach is demonstrated through both force-tracking simulations and also more comprehensive tests in which the controller is incorporated into a large-scale vibration simulation of the AgustaWestland 101 helicopter. The results show that the NDI-based controller can provide a satisfactory level of performance and hence greatly assist in the reduction of unwanted vibrations.
2016-04-05T11:04:00Z
Anti-windup and the preservation of robustness against structured norm-bounded uncertainty
Morales, Rafael Mauricio
Li, G.
Heath, W. P.
http://hdl.handle.net/2381/37143
2016-04-06T02:23:26Z
2016-04-05T10:19:58Z
Title: Anti-windup and the preservation of robustness against structured norm-bounded uncertainty
Authors: Morales, Rafael Mauricio; Li, G.; Heath, W. P.
Abstract: Anti-windup systems are modified control structures, which are designed to compensate against the detrimental effects of saturations. This manuscript considers primarily the stability robustness of two well-known anti-windup structures. Sufficient conditions for the stability robustness of the anti-windup structures and optimal robustness against structured norm-bounded plant uncertainty are found. A saturated loop is said to be optimally robust if the constrained loop is as robust as its unconstrained counterpart. The robustness condition is shown to be less conservative than existing results on additive uncertainty. Although it is known that the conventional internal model control provides optimal robustness against additive unstructured uncertainty, this is not the case against the more general uncertainty structure.
2016-04-05T10:19:58Z
Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa
Bagdanavicius, Audrius
Bowen, P. J.
Bradley, D.
Lawes, M.
Mansour, M. S.
http://hdl.handle.net/2381/37141
2016-04-06T02:25:03Z
2016-04-05T09:52:15Z
Title: Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa
Authors: Bagdanavicius, Audrius; Bowen, P. J.; Bradley, D.; Lawes, M.; Mansour, M. S.
Abstract: Independent research at two centres using a burner and an explosion bomb has revealed important aspects of turbulent premixed flame structure. Measurements at pressures and temperatures up to 1.25 MPa and 673 K in the two rigs were aimed at quantifying the influences of flame stretch rate and strain rate Markstein number, Masr, on both turbulent burning velocity and flame surface density. That on burning velocity is expressed through the stretch rate factor, Io, or probability of burning, Pb0.5. These depend on Masr, but they grow in importance as the Karlovitz stretch factor, K, increases, and are evaluated from the associated burning velocity data. Planar laser tomography was employed to identify contours of reaction progress variable in both rigs. These enabled both an appropriate flame front for the measurement of the turbulent burning velocity to be identified, and flame surface densities, with the associated factors, to be evaluated. In the explosion measurements, these parameters were derived also from the flame surface area, the derived Pb0.5 factor and the measured turbulent burning velocities. In the burner measurement they were calculated directly from the flame surface density, which was derived from the flame contours.
A new overall correlation is derived for the Pb0.5 factor, in terms of Masr at different K and this is discussed in the light of previous theoretical studies. The wrinkled flame surface area normalised by the area associated with the turbulent burning velocity measurement, and the ratio of turbulent to laminar burning velocity, ut/ul , are also evaluated. The higher the value of View the MathML sourcePb0.5, the more effective is an increased flame wrinkling in increasing ut/ul. A correlation of the product of k and the laminar flame thickness with Karlovitz stretch factor and Markstein number is explored using the present data and those of other workers. Some generality is revealed, enabling the wave length associated with the spatial change in mean reaction progress variable to be expressed by the number of laminar flame thicknesses, and the flame volume to be found.
Description: The file associated with this record is under a 24-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-05T09:52:15Z
Exergy and exergoeconomic analysis of a Compressed Air Energy Storage combined with a district energy system
Bagdanavicius, Audrius
Jenkins, N.
http://hdl.handle.net/2381/37140
2016-04-06T02:24:58Z
2016-04-05T09:49:24Z
Title: Exergy and exergoeconomic analysis of a Compressed Air Energy Storage combined with a district energy system
Authors: Bagdanavicius, Audrius; Jenkins, N.
Abstract: The potential for using heat generated during the compression stage of a Compressed Air Energy Storage system was investigated using exergy and exergoeconomic analysis. Two Compressed Air Energy Storage systems were analysed: Compressed Air Energy Storage (CAES) and Compressed Air Energy Storage combined with Thermal Storage (CAES-TS) connected to a district heating network. The maximum output of the CAES was 100 MWe and the output of the CAES-TS was 100 MWe and 105 MWth. The study shows that 308 GW h/year of electricity and 466 GW h/year of fuel are used to generate 375 GW h/year of electricity. During the compression of air 289 GW h/year of heat is generated, which is wasted in the CAES and used for district heating in the CAES-TS system. Energy efficiency of the CAES system was around 48% and the efficiency of CAES-TS was 86%. Exergoeconomic analysis shows that the exergy cost of electricity generated in the CAES was 13.89 ¢/kW h, and the exergy cost of electricity generated in the CAES-TS was 11.20 ¢/kW h. The exergy cost of heat was 22.24 ¢/kW h in the CAES-TS system. The study shows that CAES-TS has the potential to be used both as energy storage and heat source and could be a useful tool for balancing overall energy demand and supply. © 2013 Elsevier Ltd. All rights reserved.
2016-04-05T09:49:24Z
Combined analysis of electricity and heat networks
Liu, X.
Wu, J.
Jenkins, N.
Bagdanavicius, Audrius
http://hdl.handle.net/2381/37139
2016-04-06T02:23:31Z
2016-04-05T09:43:15Z
Title: Combined analysis of electricity and heat networks
Authors: Liu, X.; Wu, J.; Jenkins, N.; Bagdanavicius, Audrius
Abstract: Energy supply systems are usually considered as individual sub-systems with separate energy vectors. However, the use of Combined Heat and Power (CHP) units, heat pumps and electric boilers creates linkages between electricity and heat networks. Two combined analysis methods were developed to investigate the performance of electricity and heat networks as an integrated whole. These two methods were the decomposed and integrated electrical-hydraulic-thermal calculation techniques in the forms of power flow and simple optimal dispatch. Both methods were based on models of the electrical network, hydraulic and thermal circuits, and the coupling components, focusing on CHP units and circulation pumps. A case study of Barry Island electricity and district heating networks was conducted, showing how both electrical and heat demand in a self-sufficient system (no interconnection with external systems) were met using CHP units. The comparison showed that the integrated method requires less iteration than the decomposed method.
2016-04-05T09:43:15Z
Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K((R))
Pang, H. T.
D'Souza, N.
Dong, Hongbiao
Stone, H. J.
Rae, C. M. F.
http://hdl.handle.net/2381/37134
2016-04-06T02:23:33Z
2016-04-05T08:32:08Z
Title: Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K((R))
Authors: Pang, H. T.; D'Souza, N.; Dong, Hongbiao; Stone, H. J.; Rae, C. M. F.
Abstract: A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature T IM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the T IM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the T IM of the material approached the solidus temperature of the alloy.
2016-04-05T08:32:08Z
Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment
Welton, D.
D'Souza, N.
Kelleher, J.
Gardner, S.
Dong, Z. H.
West, G. D.
Dong, Hongbiao
http://hdl.handle.net/2381/37133
2016-04-06T02:23:37Z
2016-04-05T08:28:44Z
Title: Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment
Authors: Welton, D.; D'Souza, N.; Kelleher, J.; Gardner, S.; Dong, Z. H.; West, G. D.; Dong, Hongbiao
Abstract: Discontinuous precipitation in single-crystal Ni-base superalloys during solution heat treatment has been studied. It is found that discontinuous precipitation occurs at temperatures approaching the solvus, where volume diffusion is dominant. Diffusion of Al ahead of the boundary leads to gamma prime precipitation and is accompanied by a loss in the driving force available for advancement of the grain boundary. The rate of gamma prime precipitation was tracked using in situ neutron diffraction during isothermal hold. Gamma prime precipitation is accompanied by super-saturation of Cr and W within the channels ahead of the interface. The driving force calculated for the initial stages of DP was [10-5 to 10-4] N/[μm2 of the grain boundary]. The results provide an insight into discontinuous precipitation during solution heat treatment of Ni-base single-crystal alloys and are useful in optimizing the heat treatment process to avoid surface defect formation.
2016-04-05T08:28:44Z
Gamma Prime Precipitation, Dislocation Densities and TiN in Creep-Exposed Inconel 617 Alloy
Krishna, R.
Atkinson, H. V.
Hainsworth, Sarah V.
Gill, S. P. A.
http://hdl.handle.net/2381/37132
2016-10-29T01:45:08Z
2016-04-04T12:12:34Z
Title: Gamma Prime Precipitation, Dislocation Densities and TiN in Creep-Exposed Inconel 617 Alloy
Authors: Krishna, R.; Atkinson, H. V.; Hainsworth, Sarah V.; Gill, S. P. A.
Abstract: Inconel 617 is a solid-solution-strengthened Ni-based superalloy with a small amount of gamma prime (γ′) present. Here, samples are examined in the as-received condition and after creep exposure at 923 K (650 °C) for 574 hours and 45,000 hours and at 973 K (700 °C) for 4000 hours. The stress levels are intermediate (estimated, respectively, as of the order of 350, 275, and 200 MPa) and at levels of interest for the future operation of power plant. The hardness of the specimens has been measured in the gage length and the head. TEM thin foils have been obtained to quantify dislocation densities (3.5 × 10[Superscript: 13] for the as-received, 5.0 × 10[Superscript: 14], 5.9 × 10[Superscript: 14], and 3.5 × 10[Superscript: 14] lines/m[Superscript: 2] for the creep-exposed specimens, respectively). There are no previous data in the literature for dislocation densities in this alloy after creep exposure. There is some evidence from the dislocation densities that for the creep-exposed samples, the higher hardness in the gage length in comparison with the creep test specimen head is due to work hardening rather than any other effect. Carbon replicas have been used to extract gamma prime precipitates. The morphology of γ′ precipitates in the ‘as-received’ condition was spheroidal with an average diameter of 18 nm. The morphology of these particles does not change with creep exposure but the size increases to 30 nm after 574 hours at 923 K (650 °C) but with little coarsening in 45,000 hours. At 973 K (700 °C) 4000 hours, the average gamma prime size is 32 nm. In the TEM images of the replicas, the particles overlap, and therefore, a methodology has been developed to estimate the volume fraction of gamma prime in the alloy given the carbon replica film thickness. The results are 5.8 vol pct in the as-received and then 2.9, 3.2, and 3.4 vol pct, respectively, for the creep-exposed specimens. The results are compared with predictions from thermodynamic analysis given the alloy compositions. Thermodynamic prediction shows that nitrogen content is important in determining the gamma prime volume fraction. This has not previously been identified in the literature. The higher the nitrogen content, the lower the gamma prime volume fraction. This may explain inconsistencies between previous experimental estimates of gamma prime volume fraction in the literature and the results here. The observed decrease in the γ′ volume fraction with creep exposure would correspond to an increase in TiN. At present, there are insufficient experimental data to prove that this predicted relationship occurs in practice. However, it is observed that there is a higher volume fraction of TiN precipitates in the gage length of a creep sample than in the head. This suggests that secondary TiN particles are precipitating at the expense of existing γ′ due to the ingress of N from the atmosphere, possibly via creep cracks penetrating in from the surface of the gage length. This effect is not expected to be observed in real components which are much larger and operate in different atmospheres. However, this highlights the need to be conscious of this possibility when carrying out creep testing.
Description: The file associated with this record is under a 12-month embargo from publication in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
2016-04-04T12:12:34Z
Re-examination of the dielectric spectra of epoxy resins: Bulk charge transport and interfacial polarization peaks
Chalashkanov, N. M.
Dodd, Stephen John
Dissado, L. A.
Fothergill, J. C.
http://hdl.handle.net/2381/37130
2016-04-05T02:25:02Z
2016-04-04T11:34:35Z
Title: Re-examination of the dielectric spectra of epoxy resins: Bulk charge transport and interfacial polarization peaks
Authors: Chalashkanov, N. M.; Dodd, Stephen John; Dissado, L. A.; Fothergill, J. C.
Abstract: The dielectric properties of two amine cured bisphenol-A epoxy resin systems, Araldite CY1301 and Araldite CY1311 have been characterized using dielectric spectroscopy over the frequency range 1 mHz to 100 kHz. These two epoxy resin systems were chosen to allow the dielectric response to be studied from above and below the glass transition, as Araldite CY1311 is a modified version (with added plasticizer) of Araldite CY1301. The dielectric response was found to comprise both bulk and interfacial features. Above the glass transition temperature, two processes were identified, a low frequency process usually ascribed to interfacial polarization is shown to be a bulk process termed Quasi-DC (QDC) conduction and a dielectric dispersion usually taken to be a bulk process has been shown to be an interfacial effect.
2016-04-04T11:34:35Z