Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Fluid flow through porous media plays an important role in a variety of contexts of which filtration is one. Filtration efficiency of fibrous filters depends on the micro-structural characterization of these porous materials and is reflected in the permeability there-of. Compression of fibrous porous media has a significant effect on the permeability. Experimental data indicate that the permeability varies generally with more than an order of magnitude over the narrow porosity range in which the compression takes place. Relative to the amount of experimental studies regarding this phenomenon, there is a scarcity of geometric models in the literature that can account for the effect of compression on the permeability of a fibrous porous medium. Within the context of existing geometric porescale models based on rectangular geometry, a new model is presented and an existing model improved to predict the effect of one-dimensional compression in the streamwise direction. In addition, without compromising on a commitment to mathematical simplicity, empirical data of a non-woven fibrous porous medium was used to highlight the effect of model geometry on its predictive capability. Different mathematical expressions for the relationship between compression and porosity were considered. The permeability is expressed explicitly in terms of the fibre diameter and the compression fraction and implicitly in terms of the porosity. The porosity is incorporated through the relationship between the linear dimensions of the geometric model. The general applicability of the model(s) was validated by making use of data on airflow through a soft fibrous porous material as well as through glass and nylon fibres. The permeability predictions fall within the same order of magnitude as the experimental data. Given the mathematical simplicity of the model(s), the prediction capability is satisfactory. Attention is drawn to assumptions made and model restrictions within the analytical modelling procedure. A general predictive equation is presented for the permeability prediction in which a solid distribution factor is introduced. The proposed models serve as basis for further adaptation and refinement towards prediction capability. / AFRIKAANSE OPSOMMING: Vloei van vloeistowwe deur poreuse media speel ’n belangrike rol in ’n verskeidenheid kontekste waarvan filtrasie een is. Die filtrasie doeltreffendheid van vesel filters hang af van die mikro-strukturele karakterisering van hierdie poreuse materiale en word gereflekteer in die permeabiliteit. Kompressie van veselagtige poreuse media het ’n beduidende effek op die permeabiliteit. Eksperimentele data dui aan dat die verandering in permeabiliteit gewoonlik oor meer as ’n orde grootte strek oor die klein porositeitsinterval waarin die kompressie plaasvind. Relatief tot die aantal eksperimentele studies rakende hierdie verskynsel, is daar ’n tekort aan geometriese modelle in die literatuur wat die effek van kompressie op die permeabiliteit van veselagtige poreuse media in ag kan neem. Binne die konteks van bestaande geometriese kanaal-skaal modelle gebasseer op reghoekige geometrie, is ’n nuwe model voorgestel en ’n bestaande model verbeter om die effek van een-dimensionele kompressie in die stroomsgewyse rigting te voorspel. Sonder om die verbintenis tot wiskundige eenvoud prys te gee, is empiriese data van ’n nie-geweefde veselagtige poreuse medium gebruik om die effek van die geometrie van ’n model op sy voorspellingsvermo¨e uit te lig. Verskillende wiskundige uitdrukkings is oorweeg vir die verband tussen kompressie en porositeit. Die permeabiliteit is eksplisiet uitgedruk in terme van die veseldiameter en die kompressie breukdeel en implisiet in terme van die porositeit. Die porositeit is ge-inkorporeer deur die verhouding tussen die lineêre dimensies van die geometriese model. Die algemene toepaslikheid van die model(le) is gestaaf deur gebruik te maak van data oor lugvloei deur ’n sagte veselagtige poreuse materiaal sowel as deur glas en nylon vesels. Die voorspellings van die permeabiliteit val binne dieselfde groote orde as die eksperimentele data. Gegee die wiskundige eenvoud van die model(le), is die voorspellingsvermoë bevredigend. Aandag is gevestig op aannames wat gemaak is en modelbeperkings binne die analitiese modellerings prosedure. ’n Algemene voorspellingsvergelyking is voorgestel vir die voorspelling van die permeabiliteit waarin ’n vaste stof distribusie faktor geinkorporeer is. Die voorgestelde modelle dien as basis vir verdere aanpassing en verfyning van voorspellingsvermoë.

Osteoporosis is a bone degenerative disease characterized by reduced bone mass, quality and strength, along with changes in microarchitecture and increased incidence of fracture. The significant economic and social costs associated with osteoporotic fractures of the hip and spine have resulted in extensive research efforts directed towards developing an improved understanding of this disease, along with the relationships between osteoporotic bone degradation and fracture risk assessment. The current study investigates the failure behaviour of rat vertebral bodies collected from normal and osteoporotic donors based on the ovariectomized rat model (SHAM and OVX study groups, respectively), along with an additional control group intended to simulate postmenopausal patients with estrogen repletion (OVX+E study group). Simultaneous uniaxial compression testing and X-ray Micro-Computed Tomography (XμCT) were used to visualize and describe the failure behaviour of prepared vertebral body samples during mechanical testing, while providing standard measures of mechanical properties and bone geometry. In tandem with the incremental loading protocol required for simultaneous mechanical testing and XμCT imaging, a continuous loading protocol was also included. Three distinct failure behaviours were observed without bias in the three different study groups and two loading protocols. The primary and secondary failure modes involved fracture initiation in the direct vicinity of vascular apertures present in the dorsal surface of the cortical shell, with failure progression through the sample towards the ventral surface of the cortical shell (primary) or the cranial endplate (secondary). The tertiary failure mode involved localized rupture of the cranial endplate, likely resulting from end-effect related stress-concentrations. Diminished trabecular bone geometry (reduced BV/TV and Tb.N, along with increased Tb.Sp) was accompanied by reduced mechanical properties (reduced ultimate force and ultimate strength) in the OVX study group when compared with the SHAM and OVX+E study groups. Linear regression analyses revealed a shift in the determinants of failure initiation between the SHAM/OVX+E study groups and the OVX study group. In the SHAM and OVX+E study groups, ultimate force was best predicted by indices describing the vascular apertures present in the dorsal cortical shell. In the OVX study group, ultimate force was best predicted by indices describing trabecular bone geometry. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-11-21 19:07:38.661

Laboratory experiments were conducted to observe, document and evaluate the mechanical behavior of Fiber Reinforced Concrete after being submitted to five different environments for 8 months. The specimens were molded and reinforced with synthetic fibers with a composition similar to that used for dry-cast concrete. Four different types of fibers with different composition were used. The fibers were mixed with the concrete to create the samples and the samples were exposed to different environmental conditions. Some of these environments were meant to increase degradation of the interface fiber-concrete to simulate longevity and imitate harsh environments or marine conditions. The environments consisted of: a high humidity locker (laboratory conditions), submerged in the Intracoastal Waterway in a barge (SeaTech), a wet/dry cycle in seawater immersion simulating a splash/tidal zone, low pH wet/dry seawater immersion cycle and samples submerged in calcium hydroxide solution. The latter three were in an elevated temperature tank (87-95°F) to increase degradation process. The specimens were monitored weekly and the environments were controlled. Then, specimens were evaluated using different mechanical testing as the Indirect Tensile (IDT) test method, compressive strength according to ASTM standards. Results of testing were documented and observed in this study for further understanding of mechanical properties of Fiber Reinforced concrete. Forensic observation of fiber distribution after the IDT tests were also performed. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

With the use of grand canonical molecular dynamics, we studied the slow ompression(0.01m/s) of very thin liquid films made of equimolar mixtures of short and long alkane chains (hexane and hexadecane), and branched and unbranched alkanes (phytane and hexadecane). Besides comparing how these mixtures behave under constant speed compression, we will compare their properties with the behavior and structure of the pure systems undergoing the same type of slow compression. To understand the arrangement of the molecules inside the confinement, we present segmental and molecular density profiles, average length and orientation of the molecules inside well layered gaps. To observe the effects of the compression on the fluids, we present the number of confined molecules, the inlayer orientation, the solvation force and the inlayer diffusion coefficient, versus the thickness of the gap. We observe that pure hexadecane, although liquid at this temperature, starts presenting strong solid-like behavior when it is compressed to thicknesses under 3nm, while pure hexane and pure phytane continue to behave liquid-like except at 1.3nm when they show some weak solid-like features. When hexadecane is mixed with the short straight hexane, it remains liquid down to 2.8nm at which point this mixture behaves solid-like with an enhanced alignment of the long molecules not seen in its pure form; but when hexadecane is mixed with the branched phytane the system does not present the solid-like features seen when hexadecane is compressed pure.

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Microgels are a class of colloids that are mechanically soft, and while in many cases can behave similarly to their hard-sphere counterparts, their interaction potentials are quite different. The softness of the interaction between microgels makes them capable of deformation and compression into more concentrated assemblies. This concentrated regime is interesting because little, if any, experimental work has been done to see how the bulk properties of soft-sphere assemblies deviate from those of hard-spheres at the point where their interaction potentials begin to diverge. In this thesis the effects on assembly phase behavior and dynamics of both particle compression and softness of the interaction potential are addressed. Poly(N-isopropylacrylamide) (pNIPAm) microgels are an excellent model system in which to study these effects. The thermoresponsivity of the polymer provides the experimentalist with a dial to tune the volume fraction of an assembly, while maintaining a constant particle number density in the system. Optical microscopy, particle tracking analysis and rheology have been used to investigate the effects of packing and particle structure on equilibrium phase behavior and localized perturbations to the phase of the assembly of this soft-sphere system. It has been elucidated from these experiments and others involving deswelling of large microgel particles in the presence of high concentrations of smaller microgels, that the soft, repulsive interaction between microgels is caused by a longer-range repulsion than was previously believed. The particles are acting on each other from a distance through the osmotic pressure of the assembly, which causes each particle to deswell without coming into direct contact with a neighboring particle.

A Thesis Submitted Towards the Partial Fulfilment Degree of Master of Technology (M.Tech.) FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT Cape Peninsula University of Technology 2011 / The split Hopkinson pressure bar (SHPB) test is the most commonly used method for determining material properties at high rates of strain. The theory governing the specifics of Hopkinson bar testing has been around for decades; however, it has only been for the last decade or so that significant data processing advancements have been made. It is the intent of this thesis to offer the insight of application of SHPB to determine the compressive dynamic behaviour for welded low carbon steel (mild steel). It also focuses on the tensile behaviour for unheat-treated and heat-treated welded carbon steel. The split Hopkinson Pressure bar apparatus consists of two long slender bars that sandwich a short cylindrical specimen between them. By striking the end of a bar, a compressive stress wave is generated that immediately begins to traverse towards the specimen. Upon arrival at the specimen, the wave partially reflects back towards the impact end. The remainder of the wave transmits through the specimen and into the second bar, causing irreversible plastic deformation in the specimen. It is shown that the reflected and transmitted waves are proportional to the specimen's strain rate and stress, respectively. Specimen strain can be determined by integrating the strain rate. By monitoring the strains in the two bars and the specimen's material, stress-strain properties can be calculated. Several factors influence the accuracy of the results, including the size and type of the data logger, impedance mismatch of the bars with the specimens, the utilization of the appropriate strain gauges and the strain amplifier properties, among others. A particular area of advancement is a new technique to determine the wave's velocity in the specimen with respect to change in medium and mechanical properties, and hence increasing the range of application of SHPB. It is shown that by choosing specimen dimensions based on their impedance, the transmitted stress signal-to-noise ratio can be improved. An in depth discussion of realistic expectations of strain gages is presented, along with closed form solutions validating any claims. The thesis concludes with an analysis of experimental and predicted results. Several recommendations and conclusions are made with regard to the results obtained and areas of improvement are suggested in order to achieve accurate and more meaningful results.

While the study of compression after impact of laminated composites has been under consideration for many years. the complexity of the damage initiated by low velocity impact has not lent itself to simple predictive models for compression strength. The damage modes due to non-penetrating. low velocity impact by large diameter objects can be simulated using quasistatic three-point bending. The resulting damage modes are less coupled and more easily characterized than actual impact damage modes. This study includes the compression testing of specimens with well documented initial damage states obtained from three-point bend testing. Compression strengths and failure modes were obtained for quasi-isotropic stacking sequences from 0.24 to 1.1 inches 'thick with both grouped and interspersed ply stacking. Initial damage prior to compression testing was divided into four classifications based on the type. extent, and location of the damage. These classifications are multiple through-thickness delaminations, isolated delaminations. damage near the surface. and matrix cracks. Specimens from each classification were compared to specimens tested without initial damage in order to determine the effects of the initial damage on the final compression strength and failure modes. A finite element analysis was used to aid in the understanding and explanation of the experimental results. It was found that specimens with multiple through-thickness delaminations experienced the greatest reduction in compression strength, from 50 to 75% below the strength of undamaged specimens. All the sublaminates formed by the delaminations failed at the same time. Individual sublaminate buckling was observed for isolated delaminations near 'the surface of the laminate. Delaminations far from the specimen surface had little effect on the final compression strength. Damage occurring in the outside 00 plies caused a 10 to 200/0 strength reduction according to both analytical and experimental results. The effects of increased interlaminar stresses near the specimen edges caused a reduction in undamaged strength of [05/455/-455/905]55 specimens, while having little effect on the [Osl60sl-605]75 specimens. / Master of Science