In this work, we sought to provide a deeper understanding of the energy-absorbing capabilities of cast lattice structures. These structures absorb large amounts of energy via plastic deformation, but their most attractive characteristic from a structural standpoint is the favorable energy absorption-to-weight ratio. Conventional machining techniques are not well suited for manufacturing such complex features; therefore, we combined additive manufacturing (AM) with a well-known understanding of the metalcasting process. We used AM to produce sand molds in different sizes and with additional features for various applications — these molds were then filled with molten metal. Current literature suggests that this when appropriately applied, this methodology results in complex geometries castings comparable properties to parts made with traditionally produced sand molds. We chose to study periodic lattice structures for their repeatability and subsequent ease of making predictions via computer simulations. We first produced lightweight cast metal-ceramic composite panels of 225 x 225 x 60 mm. Our AM molds included provisions to install ceramic or hard metal tiles before pouring the molten metal. The tiles were encapsulated in the final casting to yield a composite structure. The initial material selection consisted of an aluminum A356-T6 alloy matrix with silicon carbide tiles. The composite lattice structures were tested against high-velocity projectiles — 0.30 caliber armor-piercing (AP M2) and NATO 7.62 mm ball rounds. We anticipated that the metal matrix alone would not be able to defeat these threats. However, the panels did reduce the striking velocity by approximately 20%. The thickness of the ceramic tiles varied from 4 mm to 8 mm at 2 mm increments. As expected, the hard ceramic tiles proved effective at improving the penetration resistance of the composite lattice structures — the impacts on regions with 4 mm thick tiles resulted in the reduction of striking velocity up to 49%; moreover, as the thickness was increased to 8 mm, the panels defeated the projectiles. We used these results to produce and validate a finite element (FE) model capable of replicating the experimental data within 5%. This model was later used to study how the ceramic material interacts with the lattice to absorb large amounts of kinetic energy from incident projectiles. Following, we manufactured smaller versions of these panels—50 x 50 x 90 mm test specimens for uniaxial compression testing for this instance. Once again, we relied on the capabilities of the FE method to replicate the test results within 10% for peak load and maximum displacement. We utilized computer simulations to optimize the design of the lattice structure. Its energy-absorbing capabilities were improved significantly — in this case, a 30% increase in the specific internal energy (internal energy per unit mass) as the evaluating criteria. The FE model was also used to study the performance of several other truss topologies. Lastly, we used computer simulations to evaluate the feasibility of making these cast lattice structures with ferrous alloys. We chose to work with Fe30Mn4Al0.9C due to its lower density and higher toughness than other steel grades. The first challenge was the lack of thermophysical property data in the literature for this alloy system. Hence, we used the CALPHAD method to calculate all the datasets needed to perform the mold filling and solidification simulation. Several of these calculations were validated experimentally. The location and severity of porosity between the model and the casting were in good agreement. / Doctor of Philosophy / The advent of additive manufacturing (AM), commonly known as 3D printing is a group of different digital-era technologies that has facilitated the production of complex designs that are not feasible to manufacture using conventional techniques. In the realm of metallic components one such technique involves the use of a laser beam to consolidate metallic powders via a layer-by-layer deposition process. Despite its advantages, this process has unique challenges, such as limited material selection and relatively small part volume. In this work, we have employed a hybrid approach that combines the use of AM with expertise in metalcasting to produce lightweight components with complex geometries. We used 3D printed sand molds that are then filled with molten metal of different alloy systems such as aluminum and steel. At first, we incorporate hard ceramic materials to increase the performance of the final parts under ballistics testing. Our aim is to upscale the size of current designs such that these devices can be used in various applications that require high absorption of kinetic energy, and that are lightweight and easy to replace.

Lower-cost manufacturing methods have increased the anticipation for economical mass production of vehicles manufactured from composite materials. One of the potential applications of composite materials in vehicles is in energy-absorbing components such as hollow shells and struts (these components may be in the form of circular cylindrical shells, square and rectangular tubes, conical shells, and frusta). However, constructions which result in brittle fracture of the composite tubes in the form of circumferential or longitudinal corner crack propagation may lead to unstable collapse failure mode and concomitant very low energy absorption. As a result, metal-composite hollow tubes have been developed that combine the benefits of stable ductile collapse of the metal (which can absorb crushing energy in a controlled manner) and the high strength-to-weight ratio of the composites. The relative and absolute thicknesses of metal or FRP section has a substantial effect on energy absorption of the hybrid tubes. In particular, likelihood of delamination occurrence raises with increase in FRP thickness. This can reduce the energy absorption capability of the metal-FRP hybrid tubes. Additionally, adding a very thick FRP section may result in a global buckling failure mode (rather than local folding). Until now, there are no studies specifically addressing the effect of FRP thickness on energy absorption of hybrid tubes. In this study, the effects of fiber orientation and FRP thickness (the number of layers) on the energy absorption of S2-glass/epoxy-304 stainless steel square tubes were experimentally investigated. In addition, a new geometrical trigger was demonstrated which has positive effects on the collapse modes, delamination in the FRP, and the crush load efficiency of the hybrid tube. To complete this study, a new methodology including the combination of experimental results, numerical modeling, and a multi-objective optimization process was introduced to obtain the best combination of design variables for hybrid metal-composite tubes for crashworthiness applications. The experimental results for the S2 glass/epoxy-304 stainless steel square tubes with different configurations tested under quasi-static compression loading were used to validate numerical models implemented in LS-DYNA software. The models were able to capture progressive failure mechanisms of the hybrid tubes. In addition, the effects of the design variables on the energy absorption and failure modes of the hybrid tubes were explained. Subsequently, the results from the numerical models were used to obtain optimum crashworthiness functions. The load efficiency factor (the ratio of mean crushing load to maximum load) and ratio between the difference of mean crushing load of hybrid and metal tube and thickness of the FRP section were introduced as objective functions. To connect the variables and the functions, back-propagation artificial neural networks (ANN) were used. The Non-dominated Sorting Genetic Algorithm–II (NSGAII) was applied to the constructed ANNs to obtain optimal results. The results were presented in the form of Pareto frontiers to help designers choose optimized configurations based on their manufacturing limitations. Such restrictions may include, but are not limited to, cost (related to the number of layers), laminate architecture (fiber orientation and stacking sequence) which can be constrained by the manufacturing techniques (i.e. filament winding) and thickness (as an example of physical constraints). / Ph. D.

This thesis documents the findings of a three year experimental investigation into the impact damage resistance and damage tolerance of composite honeycomb sandwich panels. The primary area of work focuses on the performance of sandwich panels under quasi-static and low-velocity impact loading with hemispherical and flat-ended indenters. The damage resistance is characterised in terms of damage mechanisms and energy absorption. The effects of varying the skin and core materials, skin thickness, core density, panel boundary conditions and indenter shape on the transverse strength and energy absorption of a sandwich panel have been examined. Damage mechanisms are found to include delamination of the impacted skin, core crushing, limited skin-core de bonding and top skin fibre fracture at high loads. In terms of panel construction the skin thickness is found to dominate the panel strength and energy absorption with core density having a lesser influence. Of the external factors considered the indenter noseshape has the largest effect on both failure load and associated damage area. An overview of existing analytical prediction methods is also included and the most significant theories applied and compared with the experimental results from this study. The secondary area of work expands the understanding obtained from the damage resistance study and assesses the ability of a sandwich panel to withstand in-plane compressive loading after sustaining low-velocity impact damage. The importance of the core material is investigated by comparing the compression-after-impact strength of both monolithic carbon-fibre laminates and sandwich panels with either an aluminium or nomex honeycomb core. The in-plane compressive strength of an 8 ply skinned honeycomb sandwich panel is found to be double that of a 16 ply monolithic laminate, with the type of honeycomb also influencing the compressive failure mechanisms and residual compressive strength. It is concluded that under in-plane loading the stabilising effect of the core opposes the de-stabilising effect of any impact damage.

This thesis describes improvements on newly-discovered high-strength pozzolan-based materials fabricated via a low-cost chemical reaction that takes place between 90 and 115 ℃ for 3 to 24 hours. The reported results focus on pozzolan constituents acquired from Coal Combustion Products (CCPs) such as cenospheres, fly ash C and F, as well as bottom ash. The thesis reports on various types of these materials with specific gravity ranging from 0.5 to 1.6; compressive strength ranging from 300 to 3600 psi, and compressive modulus ranging from 50 to 240 ksi. In addition to their good mechanical properties under compression that are attractive for the building and construction industries, the materials further exhibit great potential for applications as energy absorption cores in sandwich construction that could extend their value in other industries including the automotive and aerospace industries. For example, the load-displacement curve exhibits a short elastic zone followed by a long load-plateau; while the materials crush through a controlled vertical cracking process. Additionally, an attempt was made to further decrease the manufacturing cost of the material by investigating incorporation of chemicals that accelerates dehydration of the mixture. One such successful chemical reported in this thesis is aluminum phosphate; while it is not conclusive how the chemical improves any major property.

The motion of ocean waves can be captured and converted into usable electricity. This indicates that wave power has the potential to supply electricity to grids like wind or solar power. A point absorbing wave energy converter (WEC) system has been developed for power production at Uppsala University. This system contains a semi-submerged buoy on the water surface driving a linear synchronous generator placed on the seabed. The concept is to connect many small units together, to form a wave farm for large-scale electricity generation. A lot of effort has gone into researching how to enhance the power absorption from each WEC unit. These improvements are normally done separately for the buoy, the generator or the electrical system, due to the fact that modelling the dynamic behavior of the entire WEC system is complicated and time consuming. Therefore, a quick, yet simple, assessment tool is needed.  This thesis focuses on studying the use of the equivalent circuit as a WEC system modelling tool. Based on the force analysis, the physical elements in an actual WEC system can be converted into electrical components. The interactions between the regular waves, the buoy, and the Power Take-off mechanism can be simulated together in one circuit network. WEC performance indicators like the velocity, the force, and the power can be simulated directly from the circuit model. Furthermore, the annual absorbed electric energy can be estimated if the wave data statistics are known. The linear and non-linear equivalent circuit models developed in this thesis have been validated with full scale offshore experimental results. Comparisons indicate that the simplest linear circuit can predict the absorbed power reasonably well, while it is not so accurate in estimating the peak force in the connection line. The non-linear circuit model generates better estimations in both cases. To encourage researchers from different backgrounds to adapt and apply the circuit model, an instruction on how to establish a non-linear equivalent circuit model is supplied, as well as on how to apply the model to accelerate the decision making process when planning a WEC system.

Le fuselage des avions de nouvelle génération fera de plus en plus appel aux matériaux composites à matrice organique qui présentent des propriétés spécifiques particulièrement intéressantes. Or les structures aéronautiques sont exposées à de nombreuses exigences et en particulier à celle induite par l'impact de "petits fragments" à haute énergie. Le non percement du fuselage nécessite donc d'adjoindre une fonction de blindage à sa fonction habituelle de tenue mécanique. Par rapport à cette problématique, une approche avec intégration de fonction a été adoptée et a mené au développement d'un matériau composite multifonctionnel dans le cadre de ce travail de recherche. L'étude du comportement sous impact à basse vitesse (essais Charpy) et à haute vitesse (essais au canon à gaz) de composites à matrice organique aéronautiques courants et plus spécifiques au blindage a d'abord été réalisée. Cette étude a permis d'établir le lien entre les constituants des matériaux et les différents modes d'absorption de l'énergie d'impact. Plusieurs paramètres ont été discriminés : nature de la matrice (thermodurcissable - thermoplastique), nature de la fibre (minérale - organique), architecture du renfort (UD - tissée - tricotée), taux de porosité intra-mèche, apport d'éléments spécifiques aux inter-plis. Plusieurs concepts de multimatériaux sont ensuite proposés. Ils ont été définis à partir de combinaisons des différents comportements observés sur matériaux élémentaires et sont basés sur différents scénarii d'endommagement. Ils ont été testés à haute vitesse d'impact. Les observations révèlent une aptitude particulière de la fibre de polypara-phénylène-2,6-benzobisoxazole (PBO) à absorber une grande quantité d'énergie par déformation inélastique, notamment lorsqu'elle est faiblement imprégnée. / Next generation aircraft fuselage will increasingly use polymer matrix composites that exhibit interesting specific properties. Aeronautical structures are exposed to many requirements and amongst them to that induced by the impact of high energy "small fragments". In order to avoid fuselage break through, an armour function has to be added to its usual mechanical function. With respect to this issue, an approach aiming the integration of such function was adopted and led to the development of a multifunctional composite material within this research work. The study of the behaviour under low speed (Charpy tests) and high speed (gas gun tests) impact of common and more specific organic matrix composites dedicated to armour was first performed. This study allowed establishing the link between the material components and the various modes of the impact energy absorption. Several parameters were discriminated: matrix type (thermosetting - thermoplastic), fibre type (mineral - organic), reinforcement architecture (UD - woven - knitted), intra-mesh porosity level, addition of specific inter-ply elements. Several concepts of multimaterials were then proposed. They were defined from combinations of various behaviours observed in the basic materials and are based on different damaging scenarios. They were tested under high speed impact. The observations show a particular aptitude of the polyparaphenylene-2,6-benzobisoxazole (PBO) fibre to absorb a large amount of energy by inelastic deformation, especially when it is poorly impregnated.

Made available in DSpace on 2014-12-17T14:57:45Z (GMT). No. of bitstreams: 1 Andre Luiz dos Santos Silva.pdf: 2919586 bytes, checksum: 16be0b485378f56625d3954714b825d8 (MD5) Previous issue date: 2006-05-26 / Structures capable of absorbing large amounts of energy are of great interest, particularly for the automotive and aviation industries, to reduce tbe impact on passengers in the case of a collision. The energy absorption properties of composite materials structures can be tailored, thus making these structures an appealing option a substitute of more traditional structures in applications where energy absorption is crucial. ln this research, the influence of some parameters, which affect the energy absorption capacity of composite material tubes, was investigated. The tubes were fabricated by hand lay-up, using orthophthalic polyester resin and a plain weave E-glass fabric Test specimens were prepared and tested under compression load. The ?nfluence of the following parameters on the specific energy absorption capacity of the tubes was studied: fiber configuration (0/90? or ? 45?), tube cross-section (circular or square), and processing conditions (with or without vacuum). The results indicated that circular cross-section tubes with fibers oriented at 0/90? presented the highest level of specific energy absorbed. Further, specimens from tubes fabricated under vacuum displayed higher energy absorption capacity, when compared with specimens from tubes fabricated without vacuum. Thus, it can be concluded that the fabrication process with vacuum produce composite structures with better energy absorption capacity / Estruturas com alta capacidade de absor??o de energia s?o de grande interesse, tanto na ?rea automotiva quanto na aeron?utica, na busca de reduzir ao m?ximo a energia de impacto sobre os passageiros em caso de colis?o. As propriedades de estruturas de materiais comp?sitos podem ser projetadas para absorver energia, fazendo dessas estruturas uma op??o atraente para a substitui??o daquelas de materiais mais tradicionais, em aplica??es onde a capacidade de absor??o de energia ? fundamental. Neste estudo foi realizada uma an?lise da influ?ncia de alguns fatores que afetam a capacidade de absor??o de energia de tubos de materiais comp?sitos. Os tubos foram fabricados pelo processo de lamina??o manual com matriz de resina poli?ster insaturada ortoft?lica, tendo, como refor?o, tecido bidirecional de fibra de vidro E. Corpos-deprova foram produzidos dos tubos e submetidos a carregamento de compress?o. Foram investigadas as influ?ncias dos seguintes par?metros na capacidade de absor??o de energia espec?fica dos tubos: disposi??o de fibra (0/90? ou +/- 45?), forma da se??o transversal (circular ou quadrada), e processo de fabrica??o (com ou sem a aplica??o de v?cuo). Os resultados mostraram que os tubos de se??o transversal circular, com disposi??o de fibra a 0/90?, apresentaram os maiores valores de absor??o de energia espec?fica. Al?m disso, os corpos-de-prova de tubos submetidos ? aplica??o de v?cuo no processo de fabrica??o, apresentaram maiores valores de capacidade energia espec?fica, quando comparados com os corpos-de- prova de tubos fabricados sem aplica??o de v?cuo. Assim, pode-se concluir que o processo de fabrica??o com aplica??o de v?cuo proporciona aos comp?sitos confeccionados, melhores propriedades com rela??o ? absor??o de energia

ix Flexibility has traditionally been considered an important component of human physical fitness but this conjecture lacks supporting empirical evidence. While there is extensive published research examining the relative importance of flexibility and the impact of various methods of stretching on levels of flexibility, performance and injury risk, the quality of studies has varied considerably, reliability and validity of methodology has not always been proven, and rationale has at times been questionable. Additionally, much literature has focused on static flexibility which is not necessarily related to properties of the musculotendinous unit and thus dynamic flexibility. This thesis was designed to fill gaps in the existing literature by using accepted methods to establish relative and absolute reliability of hamstring flexibility tests, consider the comparability of static and dynamic components of the global concept of flexibility and explore how dynamic flexibility and performance are influenced by fatiguing exercise and subsequent static stretching. The first aim was realised by a repeated measures study designed to establish the intraday and interday, intrarater reliability and measurement error of static and dynamic measures of hamstring flexibility. Significant relative reliability for measures of static and dynamic hamstring flexibility was demonstrated via intraclass correlation coefficient (3,1) but limits of agreement analysis indicated there was a degree of absolute measurement error that must be interpreted in relation to analytical goals. The second aim required evaluation of relationships shared by static and dynamic measures of hamstring flexibility. Significant relationships between the different static flexibility tests were established but the extent of unexplained variance indicated that only measurements from the same tests should be directly compared to each other. Relationships between different measures of dynamic flexibility and static flexibility varied from non-significant to moderately strong, suggesting that measures of static and dynamic flexibility are not identical and results should not be interchanged between the two types of tests. Due to a lack of explanatory empirical evidence, the final chapter aimed via a prospective randomised repeated measures study to investigate the impact of fatigue and post-exercise static stretching on measures of dynamic flexibility and performance. Fatigue resulted in no significant changes to passive or active dynamic flexibility measures but a significant worsening of static flexibility levels and perceived stiffness. Post-exercise stretch resulted in significantly increased passive and active energy absorption immediately and 18 hours post-exercise and in significantly reduced joint position sense immediately post-exercise. Effect sizes were small so the clinical meaningfulness of performing post-exercise static stretching is questionable, particularly if performed in place of other, potentially more beneficial practices. / Thesis (DPhil)--University of Pretoria, 2013. / gm2014 / Biokinetics, Sport and Leisure Sciences / unrestricted

Les zéolithes sont des solides microporeux cristallins largement utilisés en adsorption, catalyse, échange ionique et comme tamis moléculaires. Les zéolithes hydrophobes purement siliciques (zéosils) peuvent être utilisées pour le stockage et l’absorption de l’énergie mécanique par intrusion d’eau à haute pression. En fonction du système “zéosil-eau”, lorsque la pression est relâchée (extrusion), le système peut restituer, dissiper ou absorber l’énergie mécanique fournie pendant la compression (intrusion) et donc, il peut montrer un comportement de type ressort, amortisseur ou pare-chocs. Récemment, il a été remarqué que l’intrusion de solutions salines peut améliorer considérablement les performances énergétiques de ces systèmes par une augmentation de la pression d’intrusion. Pendant ce travail, l’intrusion d’eau et de solutions de LiCl a été étudiée pour différentes zéosils pour mieux comprendre la relation qui existe entre la structure des zéosils (dimension des pores, type et dimensionnalité du système poreux) et le comportement ou les performances énergétiques du système “zéosil-liquide intrusé”.Les expériences avec des zéosils qui présentent une structure à cage ont confirmé une pression d’intrusion plus faible par rapport à celles observées par les zéosils ayant une structure à canaux. La pression d’intrusion augmente fortement avec la concentration de LiCl pour les zéosils caractérisés par de petites ouvertures des pores, spécialement pour ceux qui ont des cages, tandis que cette augmentation est plus faible lorsque de grandes ouvertures de pores sont présentes. Il a été aussi montré une influence de la concentration du sel sur le comportement, probablement, due à la nature particulière des solutions très concentrées. / Zeolites are microporous crystalline solids widely used in adsorption, catalysis, ion exchange and molecular sieving. Hydrophobic pure-silica zeolites (zeosils) can be used for mechanical energy absorption and storage by high pressure intrusion-extrusion of water. Depending on the “zeosil-water” system, when the pressure is released (extrusion), the system is able to restore, dissipate or absorb the supplied mechanical energy during the compression step (intrusion) and therefore to display a spring, shock absorber or bumper behavior. Recently, it was found that the use of aqueous salt solutions could considerably improve the energetic performances of such systems by an increase of the intrusion pressure.In this work the intrusion of water and LiCl solutions was studied for different zeosils in order to understand the relationship between the structure of zeosils (pore size, pore system type and dimensionality) and the behavior or the energetic performances of “zeosil-liquid” systems. The experiments with cage-type zeosils confirmed a lower intrusion pressure in comparison with channel-type ones. The intrusion pressure strongly increases with the LiCl content for the zeosils with small pore openings, particularly, for the cage-type ones, while for larger pores this increase is less important. An influence of salt concentration on the behavior of “zeosils-liquid” systems probably due to the particular nature of highly concentrated solutions was also shown.

In the transportation sector, crash structures are often used to protect their inhabitants inthe event of a collision. These crash structures frequently utilize thin-walled tubes as energyabsorbers. The process of developing thin-walled tubes is iterative based and requires mul-tiple simulations, making it resource intensive. This thesis researches how thin-walled tubesare developed today, what kind of challenges exist in the development process and whattools and methods are used to shorten the development lead times. Later a new methodfor assessing TWBs crashworthiness before a simulation is investigated. In this method43 cross-section geometries from thin-walled tubes used in automobiles are parameterized.These tubes are later subjected to a dynamic crash simulation along their longitudinal axis.Results from these simulations are correlated to their respective parameters in order to findmeaningful relation between the parameters and results. It was found that the circumferenceof a cross-section correlates with its crashworthiness. With this finding, the developmentlead times of thin-walled tubes could be shortened by reducing the amount of required FEMsimulations.