Mechanical conditioning has been shown to promote tissue formation in a wide variety of tissue engineering efforts. However the underlying mechanisms by which external mechanical stimuli regulate cells and tissues are not known. This is particularly relevant in the area of heart valve tissue engineering owing to the intense hemodynamic environments that surround native valves. Some studies suggest that oscillatory shear stress (OSS) caused by time-varying flow environments, play a critical role in engineered tissue formation derived from bone marrow derived stem cells (BMSCs). There is strong evidence to support this hypothesis in tissue engineering studies of bone. From observing native heart valve dynamics, OSS can be created by means of pulsatility or by cyclic specimen geometry changes. However, quantification of the individual or combined effects of these variables for the maximization of OSS environments in vitro is to date, not known. Accordingly, in this study we examined and quantified the role that i) physiologically relevant scales of pulsatility and ii) changes in geometry as a function of specimen flexure, have in creating OSS conditions for dynamic culture of tissue. A u-shaped custom made bioreactor capable of producing flow stretch and flexure was used. Computational Fluid Dynamic (CFD) simulations were performed through Ansys CFX (Ansys, Pittsburgh, PA) for both steady and pulsatile flow. We have shown that OSS can be maximized by inducing pulsatile flow over straight scaffolds. We believe that OSS promotes BMSCs tissue formation.

In recent years, repetitive traumatic brain injuries have been linked to the progressive neurodegenerative disorder termed chronic traumatic encephalopathy. However, the mechanical characteristics of brain tissue exposed to repetitive loading still lack understanding. This research evaluated the response of porcine brain tissue undergoing cyclic, compressive loading in reference to three impact parameters: cycle number (N25, N50, N100, N150, and N200), strain level (15, 30, and 40%), and strain rate (0.00625, 0.025, 0.10, and 1.0/s). Following mechanical testing, tissue samples were processed for hematoxylin and eosin (H&E) staining. Stress values, hysteresis energy, and decreases in hysteresis energy for all parameters were compared. The data suggest that microstructural brain tissue damage is highly dependent on strain level and cycle number, whereas strain rate did not appear to cause permanent damage in the quasi-static range applied. The onset of permanent microstructural tissue damage may relate to movement of fluid molecules within the tissue.

Continuous Fiber Reinforced Ceramic Composites (CFCC's) are being currently investigated as potential materials for high temperature applications such as combustor liners in stationary gas turbines. The creep behavior of woven Enhanced SiC/SiC composites was studied at temperatures from 600 to 1200 °C and at 140 to 220 MPa stress levels. Most researchers studying the creep behavior of ceramic matrix composites (CMCs) use the time hardening model and rate equations for expressing the dependence of creep strain on time, temperature and stress. Such laws, although simple and easy to use, are inadequate to represent the creep behavior over a range of stress levels and temperatures and cannot be used to quantify the pest phenomenon commonly observed in CMCs. Hence, these laws were modified to include the pest phenomenon and an empirical equation was developed that can be used to represent the creep behavior at various stresses and temperatures. The modified equation was used in the finite element analysis and the results were compared with the time and strain hardening models. Microscopic observations on the fractured surfaces revealed the pseudo-ductile behavior of the material at high temperatures. A quasi-static test was conducted at 1200 °C to determine the unloading response of the material. The stress-strain response of the composite demonstrates a hysterisis loop and a small amount of permanent strain, which are characteristic of the CMC's [3]. Finally, a test was conducted at 1200 oC to investigate the recovery behavior of the material. The material exhibits a tendency to recover the accumulated creep strain as well as the small permanent strain upon unloading, if sufficient time is allowed for recovery. The creep data were also modeled using the representations such as Monkmon-Grant and Larsen-Miller equations. A modified Monkman-Grant equation was used to model the stratification of the creep strain rate data with temperature. A finite element model based on the plasticity theory was developed to simulate the quasi-static cyclic behavior of the material. Though the loading behavior of CMCs can be modeled using the bilinear or multilinear kinematic hardening plasticity models, the unloading behavior as predicted by the models is entirely different from the experimentally observed behavior. Hence, these models were modified to correctly predict the stress-strain behavior. The model, which was input via a user defined subroutine into the ANSYS finite element program uses the concept of state or internal variables to define the unloading portion of the stress-strain curve. The results were compared with the test data and they show very good agreement. The model was then used to predict the stress-strain response of a plate with a notch. The results from the analysis were compared with the experimental data and they show good agreement if average values of strains are considered. / Master of Science

The use of metal strain gauges and ultrasonic transducers have long been studied in the field of Nondestructive Evaluation (NDE) as a part of structural health monitoring (SHM). Strain gauges use electrical resistance to monitor strains during the loading of a component. Ultrasonic transducers are piezo devices that use a crystal-like sensing element with very low excitation energy that can monitor small strains such as acoustic emissions (AEs). These types of devices have been used to locate the sources of AEs from artificial sources, such as Hsu-Nielsen pencil lead break (PLB) tests, or natural sources such as quasi-static fracture or crack propagation. This type of evaluation has significant advantages over other types of damage inspections such as liquid die penetrant, Blue Light, Eddy Current, or X-ray inspections where visual inspections, large defects, and high levels of user experience are required. The ultrathin silicon membrane (USM) sensor developed by NanoSonic Inc. is a piezoresistive sensor, incorporating the best aspects of a conventional strain gauge and ultrasonic transducer. The sensor can measure both the strain of a component, as well as any acoustic emission that is emitted on the component. To the author's knowledge this is the only sensor capable of simultaneous measurement of these two data types. This paper presents the sensor's ability to be used for quasi-static fracture monitoring. The sensor is first compared to commercial ultrasonic transducers in an unloaded pencil lead break (PLB) test for determining the ability in measuring lamb waves for source location estimation. The NanoSonic USM sensor is further compared to commercial strain gauges and ultrasonic transducers during a PLB test under a tensile load where it is demonstrated the USM sensor yields similar measurements to both commercial sensors. The final test was a quasi-static fracture test, where the NanoSonic USM sensor was able to detect substantially lower energy AEs than the previous test and record the strain history during fracture. This duality of the USM sensor demonstrates an inherent usefulness to NDE and SHM fields. The sensor offers sensing capabilities comparable to commercially available sensors in a smaller package, with less power consumption, at a lower cost. / Master of Science / Nondestructive evaluation (NDE) is a field within structural health monitoring (SHM) that refers to determining any defects within a component that would hinder its performance without modifying its existing condition. This has historically been done by visual inspection by which experienced personnel examine the part for defects. This is inherently flawed as cracking below the surface of a component is common and would not be detected and extensive experience is required to successfully complete this task. Components in hard-to-reach places, with coatings, or that are small also prove challenges to visual inspection. Engineers have developed several new testing methods to combat these flaws. The use of acoustic emission (AE) testing allows sensors, called ultrasonic transducers, to receive and emit sounds at high frequencies to conduct the inspection. This can be done by emitting a sound which is then propagated as a wave along the surface of the component, if the wave hits a defect, it is scattered. A receiving sensor would then receive an unexpected signal, indicating that there is a problem. Furthermore, an array of these sensors can be employed to 'listen' for these surfaces waves that may be emitted during the standard operation of the components. Things like high loads, cracking, and impacts will all be able to be detected. The use of an array of sensors will allow the location of these events. This paper will discuss a new type of sensor, an ultrathin silicon membrane (USM) sensor developed by NanoSonic Inc. This type of sensor can detect high frequencies similar to an ultrasonic transducer, as well as measure large loads that would deform the part, resulting in an event known as strain. The novelty of the NanoSonic USM sensor is its ability to monitor both pieces of information simultaneously, which is believed to be the first to do so in the field. The ability to obtain information on strain and locations of acoustic events within a component during standard operation would be a valuable prospect for the aerospace, civil, and automotive industries.

Yes / A new approach of modelling the electromagnetic wave propagation and the penetration of small objects, are investigated and analysed. The travelling electromagnetic wave from source is simulated by time-dependent Maxwell's solutions. Subgridding technique is imposed at the point of interest for observing the electromagnetic field in high resolution. The computational burden caused by a large number of time steps has been parried by implementing the state of art of quasi-static approach. The induced electromagnetic fields near a buried pipeline runs parallel to a 400 kV power transmission lines are presented, and discussed.

No description available.

Time-series power flow simulations are consecutive power flow calculations on each time step of a set of load and generation profiles that represent the time horizon under which a network needs to be analyzed. These simulations are one of the fundamental tools to carry out cost-benefit analyses of grid planing and operation strategies in the presence of distributed energy resources, unfortunately, their execution time is quite substantial. In the specific case of cost-benefit analyses the execution time of time-series power flow simulations can easily become excessive, as typical time horizons are in the order of a year and different scenarios need to be compared, which results in time-series simulations that require a rather large number of individual power flow calculations. It is often the case that only a set of aggregated simulation outputs is required for assessing grid operation costs, examples of which are total network losses, power exchange through MV/LV substation transformers, and total power provision from PV generators. Exploring alternatives to running time-series power flow simulations with complete input data that can produce approximations of the required results with a level of accuracy that is suitable for cost-benefit analyses but that require less time to compute can thus be beneficial. This thesis explores and compares different methods for shortening time-series power flow simulations based on reducing the amount of input data and thus the required number of individual power flow calculations, and focuses its attention on two of them: one consists in reducing the time resolution of the input profiles through downsampling while the other consists in finding similar time steps in the input profiles through vector quantization and simulating them only once. The results show that considerable execution time reductions and sufficiently accurate results can be obtained with both methods, but vector quantization requires much less data to produce the same level of accuracy as downsampling. Vector quantization delivers a far superior trade-off between data reduction, time savings, and accuracy when the simulations consider voltage control or when more than one simulation with the same input data is required, as in such cases the data reduction process can be carried out only once. One disadvantage of this method is that it does not reproduce peak values in the result profiles with accuracy, which is due to the way downsampling disregards certain time steps in the input profiles and to the averaging effect vector quantization has on the them. This disadvantage makes the simulations shortened through these methods less precise, for example, for detecting voltage violations.

This thesis considered the design of new and improved foldcore sandwich panels suitable for high-performance energy absorption applications. This was achieved by utilising origami geometry design techniques to alter foldcore structures such that they possessed different mechanical behaviours and failure modes. The major findings of this thesis were in three areas as follows. First, a modified planar foldcore geometry was developed by introducing sub-folds into a standard foldcore pattern. The new geometry, deemed the indented foldcore, successfully triggered a high-order failure mode known as a travelling hinge line failure mode. This was found to have a much higher energy absorption than the plate buckling failure mode seen in an unmodified foldcore structure. A comprehensive numerical, theoretical, and experimental analysis was conducted on the indented core, which included the development of a new foldcore prototyping method that utilised 3D printed moulds. It was shown that compared to available commercial honeycomb cores, the indented foldcore had an improved uniformity of energy absorption, but weaker overall peak and crushing stresses. Second, rigid origami design principles were used to develop extended foldcore geometries. New parametrisations were presented for three patterns, to complete a set of Miura-derivative geometries termed first-level derivatives. The first-level derivative parametrisations were then combined to create complex, piecewise geometries, with compatible faceted sandwich face geometry also developed. Finally, a method to generate rigid-foldable, curved-crease geometry from Miura-derivative straight-crease geometry was presented. All geometry was validated with physical prototypes and was compiled into a MATLAB Toolbox. Third, the performance of these extended foldcore geometries under impact loadings was investigated. An investigation of curved-crease foldcores showed that they were stronger than straight-crease foldcores, and at certain configurations can potentially match the strength, energy-absorption under quasi-static impact loads, and out-of-plane stiffness of a honeycomb core. A brief investigation of foldcores under low-velocity impact loadings showed that curved-crease foldcores, unlike straight-crease foldcores, strengthened under dynamic loadings, however not to the same extent as honeycomb. Finally, an investigation of single-curved foldcore sandwich shells was conducted. It was seen that foldcore shells could not match the energy-absorption capability of an over-expanded honeycomb shell, but certain core types did exhibit other attributes that might be exploitable with future research, including superior initial strength and superior uniformity of response.

With the development of the design and manufacture technology, composite materials are widely used in the aeronautical industry. But, one of the main concerns which affects the application of composites is foreign object impact. The damages induced by the Low Velocity Impact (LVI), which can significantly reduce the strength of the structures, can’t be easily inspected routinely. The so-called Barely Visible Impact Damages (BVID) due to LVI typically includes interlaminar delamination, matrix cracks and fibre fracture at the back face. Previous researches have shown that the results of LVI test are similar to that of the Quasi-Static Load (QSL) test. The initiation and propagation of delamination can be detected more easily in the QSL test and the displacement and reaction force of the impactor can be controlled and measured much more accurately. Moreover, it is easier to model QSL tests than dynamic impacts. To investigate the impact damage induced by LVI, a Finite Element (FE) model employing cohesive elements was used. At the same time, the ply clustering effect, when several plies of the same orientation were stack together, was modelled in the FE model in terms of damage resistance and damage size. A bilinear traction-separation law was introduced in the cohesive elements employed to simulate the initiation and propagation of the impact damage and delamination. Firstly, a 2D FE model of the Double Cantilever Beam (DCB) and End Notched Flexure (ENF) specimens were built using the commercial FEM software ABAQUS. The results have shown that the cohesive elements can be used to simulate mode I and mode II delamination sufficiently and correctly. Secondly, an FE model of a composite plate under QSL but without simulating damage was built using the continuum shell elements. Agreement between the FEA results with published test results is good enough to validate the capability of continuum shell elements and cohesive elements in modelling the composite laminate under the transverse load condition (QSL). Thirdly, an FE model containing discrete interface delamination and matrix cracks at the back face of the composite plate was built by pre-setting the cohesive failure elements at potential damage locations according to the experimental observation. A cross-ply laminate was modelled first where fewer interfaces could be delaminated. Good agreement was found in terms of the delamination area and impactor’s displacement-force curve. Finally, the effect of ply clustering on impact damage resistance was studied using Quasi-Isotropic (QI) layup laminates. Because of the limited time available for calculation, the simulation was only partly completed for the quasi-isotropic laminates (L2 configuration) which have more delaminated interfaces. The results showed that cohesive elements obeying the bilinear traction-separation law were capable of predicting the reaction force in quasi-isotropic laminates. However, discrepancies with the test results in terms of delamination area were observed for quasi-isotropic laminates. These discrepancies are mainly attributed to the simplification of matrix cracks simulation and compressive load at the interface in the thickness direction which is not taken into account.

This thesis presents a comprehensive study of the compressive response of hexagonal honeycomb panels from the initial elastic regime to a fully crushed state. Expanded aluminum alloy honeycomb panels with a cell size of 0.375 in (9.53 mm), a relative density of 0.026, and a height of 0.625 in (15.9 mm) are laterally compressed quasi statically between rigid platens under displacement control. The cells buckle elastically and collapse at a higher stress due to inelastic action. Deformation then first localizes at mid-height and the cells crush by progressive formation of folds; associated with each fold family is a stress undulation. The response densifies when the whole panel height is consumed by folds. The buckling, collapse, and crushing events are simulated numerically using finite element models involving periodic domains of a single or several characteristic cells. The models idealize the microstructure as hexagonal, with double walls in one direction. The nonlinear behavior is initiated by elastic buckling while inelastic collapse that leads to the localization observed in the experiments occurs at a significantly higher load. The collapse stress is found to be mildly sensitive to various problem imperfections. For the particular honeycomb studied, the collapse stress is 67% higher than the buckling stress. It was also shown that all aspects of the compressive behavior can be reproduced numerically using periodic domains with a fine mesh capable of capturing the complexity of the folds. The calculated buckling stress is reduced when considering periodic square domains as the compatibility of the buckles between neighboring cells tends to make the structure more compliant. The mode consisting of three half waves is observed in every simulation but its amplitude is seen to be accented at the center of the domains. The calculated crushing response is shown to better resemble measured ones when a 4x4 cell domain is used, which is smoother and reproduces decays in the amplitude of load peaks. However, the average crushing stress can be captured with engineering accuracy even from a single cell domain. / text