51 research outputs found

    Modelling and experimental characterization of nanoindentation responses of various biocomposite materials

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    PhD ThesisIn the past decades, composite materials (which are usually classified into fibre-reinforced composites and particle-reinforced composites, depending on the geometry of the reinforcements) have been widely applied in tissue engineering as implant scaffolds. A lot of work has been done on the bulk mechanical properties of these composites. However, there is lack of nanomechanical characterization of such composites, which is crucial for understanding the cell-material interactions at small scale, and further optimizing the design of scaffold materials to promote the formation of new viable tissue. Nanoindentation has been used for nanomechanical characterization of a wide range of composite materials, but there is lack of comprehensive modelling of these composites. Therefore, this thesis begins with the modelling of the nanomechanics of inclusion-reinforced composite materials. In this part, finite element analysis (FEA) is adopted to study the spatial-dependent mechanical response of fibre/matrix and particle/matrix composites. The effects of various factors (such as inclusion geometry, indenter geometry, inclusion orientation and relative indentation location) on the nanomechanical response are studied. Various indentation-based empirical or semi-analytical models have been examined and novel analytical models are proposed to describe the nanomechanical behaviour of these inclusion-reinforced composites. Towards the end of this thesis, the nanoindentation characterization of typical biocomposite materials is presented, namely extracellular matrix. For these complex composites, the existing analytical models may not be directly applied. However, with the aid of a statistical model and FEA, it has been demonstrated that mechanical properties of each individual component can be determined

    Study of anelastic phenomena in materials of engineering interest

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    Mechanical spectroscopy (MS) studies absorption spectra of mechanical energy under the conditions of applied periodic external mechanical field, this technique is a powerful tool for the study of materials. In particular, it is shown that such a technique can help the mechanical characterisation of materials. The elastic and anelastic properties of the material have been investigated by using a vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations mode of cantilevered mounted reed samples for a wide range of materials of engineering interest

    Study of anelastic phenomena in materials of engineering interest

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    Mechanical spectroscopy (MS) studies absorption spectra of mechanical energy under the conditions of applied periodic external mechanical field, this technique is a powerful tool for the study of materials. In particular, it is shown that such a technique can help the mechanical characterisation of materials. The elastic and anelastic properties of the material have been investigated by using a vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations mode of cantilevered mounted reed samples for a wide range of materials of engineering interest

    Mechanical behaviour of human enamel and the relationship to its structural and compositional characteristics

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    Objectives As the outer cover of teeth structure, enamel is the hardest, stiffest and one of the most durable load-bearing tissues of the human body. Also, enamel is an elegantly designed natural biocomposite. From a material science point of view, scientists are interested in the structure and function of the nature material. How does nature design the material to meet its functional needs? From a dental clinic point of view, dental practitioners are keen to know the properties of enamel and compare it with different dental materials. What kind of dental materials can best simulate enamel as a restoration in the oral cavity? The research presented in this thesis on the mechanical behaviour of enamel in respect of its structural and compositional characteristics will attempt to provide answers or indications to the above questions. Theoretical analysis, as well as experimental investigations of both man-made and natural composites materials, has shown that hierarchical microstructure and organic matrix glues the inorganic particles together and plays an important role in regulating the mechanical properties of the composite. Bearing this finding in mind, in the current investigations, we assume the hierarchical microstructure and trace protein remnants in enamel regulate the mechanical behaviour of the natural biocomposite to meet its functional needs as a load bearing tissue with superb anti-fatigue and wear resistant properties. One of the important reasons that dental hard tissues haven’t been thoroughly investigated is due to the limited sample volume. Fortunately, with the development of nanoindentation technique and equipment, it is now possible to explore the mechanical properties of small volume samples. The application of nanoindentation on dental hard tissues has been documented. However, most investigations have concentrated on only reporting the basic mechanical properties such as elastic modulus and hardness. Very few of them have taken the role of microstructure and composition of these natural biocomposites into their considerations. The main aim of this investigation is to interpret how microstructural and compositional features of enamel regulate its mechanical behaviour. To achieve this goal, the analytical methods considering nanoindentation data need to be expanded so that more information not only elastic modulus and hardness but also stress-strain relationship, energy absorption ability, and creep behaviour may be evaluated with this technique. These new methods will also be of benefit to dental material evaluation and selection. Materials and methods Based on the Oliver-Pharr method1 for the analysis of nanoindentation data, Hertzian contact theory2 and Tabor’s theory3, a spherical nanoindentation method for measuring the stress-strain relationship was developed. Furthermore, nanoindentation energy absorption analysis method and nanoindentation creep test were developed to measure the inelastic property of enamel. With the above methods, sound enamel samples were investigated and compared with various dental materials, including dental ceramics and dental alloys. • Firstly, using a Berkovich indenter and three spherical indenters with 5, 10 and 20 µm nominal radius, the elastic modulus, hardness and stress-strain relationship of different samples were investigated and compared. • Secondly, mechanical properties of enamel in respect to its microstructure were investigated intensively using different indenters by sectioning teeth at different angles. • Thirdly, inelastic behaviour of enamel such as energy absorption and creep deformation were observed and compared with a fully sintered dense hydroxyapatite (HAP) disk to illustrate the roles of protein remnants in regulating the mechanical behaviour of enamel. • Fourthly, to confirm the functions of protein remnants in controlling mechanical behaviour of enamel, enamel samples were treated under different environments such as burning (300°C exposure for 5 min), alcohol dehydration and rehydration to change the properties of proteins before the nanoindentation tests. • Lastly, micro-Raman spectroscopy was employed to measure and compare the indentation residual stresses in enamel and HAP disk to evaluate the role of both hierarchical microstructure and protein remnants in redistributing the stresses and reinforcing the mechanical response of enamel to deformation. Results and significance Nanoindentation is an attractive method for measuring the mechanical behaviour of small specimen volumes. Using this technique, the mechanical properties of enamel were investigated at different orientations and compared with dental restorative materials. From the present study, the following results were found and conclusions were drawn. Although some newly developed dental ceramics have similar elastic modulus to enamel, the hardness of these ceramic products is still much higher than enamel; in contrast, despite the higher elastic modulus, dental metallic alloys have very similar hardness as enamel. Furthermore, enamel has similar stress-strain relationships and creep behaviour to that of dental metallic alloys. SEM also showed enamel has an inelastic deformation pattern around indentation impressions. All of these responses indicated that enamel behaves more like a metallic material rather than a ceramic. Elastic modulus of enamel is influenced by highly oriented rod units and HAP crystallites. As a result, it was found to be a function of contact area. This provides a basis to understand the different results reported in the literature from macro-scale and micro-scale tests. Anisotropic properties of enamel, which arise from the rod units, are well reflected in the stress-strain curves. The top surface (perpendicular to the rod axis) is stiffer and has higher stress-strain response than an adjacent cross section surface because of the greater influence of the prism sheaths in the latter behaviour. Enamel showed much higher energy absorption capacity and considerably more creep deformation behaviour than HAP, a ceramic material with similar mineral composition. This is argued to be due to the existence of minor protein remnants in enamel. Possible mechanisms include fluid flow within the sheath structure, protein “sacrificial bond” theory, and nano-scale friction within sheaths associated with the degustation of enamel rods. A simple model with respect of hierarchical microstructure of enamel was developed to illustrate the structural related contact deformation mechanisms of human enamel. Within the contact indentation area, thin protein layers between HAP crystallites bear most of the deformation in the form of shear strain, which is approximately 16 times bigger than contact strain in the case of a Vickers indenter. By replotting energy absorption against mean strain value of a protein layer, data from different indenters on enamel superimposed, validating the model. This model partially explained the non-linear indentation stress-strain relationship, inelastic contact response and large energy absorption ability of enamel and indicated the inelastic characteristics of enamel were related to the thin protein layers between crystallites. Following different treatments, mechanical properties of enamel changed significantly. By denaturing or destroying the protein remnants, mechanical behaviour, especially inelastic abilities of enamel decreased dramatically, which indicates matrix proteins endow enamel better performance as a load bearing calcified tissue. Comparison of Raman derived residual maps about indentations in enamel and a sintered homogeneous HAP showed the hierarchical structure influenced the residual stress distribution within enamel. Moreover, less residual stresses were found in enamel and were a consequence of the protein remnants. These are evidence as to how the microstructure meets the functional needs of the enamel tissue. In general, evidence from different approaches indicated that the hierarchical microstructure and small protein remnants regulated the mechanical behaviour of enamel significantly at various hierarchical levels utilising different mechanisms. This investigation has provided some basis for understanding natural biocomposites and assisting with dental clinic materials selection and treatment evaluation procedures. References 1. Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res. 1992;7(6):1564-83. 2. Hertz H. Miscellaneous Papers. London: Jones and Schott, Macmillan; 1863. 3. Tabor D. Hardness of Metals. Oxford: Clarendon Press; 1951

    High elastic modulus nanopowder reinforced resin composites for dental applications

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    Dental restorations account for more than $3 billion dollars a year on the market. Among them, all-ceramic dental crowns draw more and more attention and their popularity has risen because of their superior aesthetics and biocompatibility. However, their relatively high failure rate and labor-intensive fabrication procedure still limit their application. In this thesis, a new family of high elastic modulus nanopowder reinforced resin composites and their mechanical properties are studied. Materials with higher elastic modulus, such as alumina and diamond, are used to replace the routine filler material, silica, in dental resin composites to achieve the desired properties. This class of composites is developed to serve (1) as a high stiffness support to all-ceramic crowns and (2) as a means of joining independently fabricated crown core and veneer layers. Most of the work focuses on nano-sized Al2O3 (average particle size 47 nm) reinforcement in a polymeric matrix with 50:50 Bisphenol A glycidyl methacrylate (Bis-GMA): triethylene glycol dimethacrylate (TEGDMA) monomers. Surfactants, silanizing agents and primers are examined to obtain higher filler levels and enhance the bonding between filler and matrix. Silane agents work best. The elastic modulus of a 57.5 vol% alumina/resin composite is 31.5 GPa compared to current commercial resin composites with elastic modulus alumina, diamond/resin composites are studied. An elastic modulus of about 45 GPa is obtained for a 57 vol% diamond/resin composite. Our results indicate that with a generally monodispersed nano-sized high modulus filler, relatively high elastic modulus resin-based composite cements are possible. Time-dependent behavior of our resin composites is also investigated. This is valuable for understanding the behavior of our material and possible fatigue testing in the future. Our results indicate that with effective coupling agents and higher filler loading, viscous flow can be greatly decreased due to the attenuation of mobility of polymer chains. Complementary studies indicate that our resin composites are promising for the proposed applications as a stiff support to all-ceramic crowns

    Characterising the deformation behaviour of human tooth enamel at the microscale

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    Enamel plays an important role in tooth function. Optimal combinations of composition and structure endow enamel with unique mechanical properties that remain largely unexplored. Specifically, more detailed understanding of the loadbearing ability of enamel is needed to mimic it synthetically and to design next generation biocomposite materials. This research investigates the variables that influence deformation behaviour of tooth enamel in relation to its hierarchical structure. Initially, a new method was developed for preparing flat, finely polished tooth samples that were maintained in their normal hydrated state for nanoindentation testing. In contrast to conventional methods, which commonly utilise either inappropriate or excessive drying and/or chemically based embedding media (i.e., resins, glues), a novel embedding process was developed using an aqueous putty compound. Additionally, a custom-designed holder was manufactured for mounting wet tooth specimens on the nanoindentation stage that eliminated the need for hot wax or glue during testing. Considering that enamel is a functionally graded material that has different values of Young’s modulus (E) and hardness (H) over the enamel thickness, a new approach of data analysis was developed for interpreting the mechanical properties of enamel at a range of fixed constant indentation depths. Resultant functions were used for predictive purposes. The values of E and H obtained from the nanoindentation instrument demonstrated a well-known decreasing gradient from the enamel occlusal surface towards the enamel-dentine junction (EDJ). In contrast to studies using conventional methods, this research showed that both properties also decreased with increasing depths at fixed locations. Furthermore, experimental results showed that resin embedding had detrimental effects on the E and H of enamel (i.e., both properties decreased with increasing depth), but had positive effects on both mild and severe wear resistance parameters (i.e., both parameters increased with increasing depth). When contrasted against the mechanical properties of enamel samples prepared using conventional protocols, this study postulates that the new hydrated method has, for the first time, revealed the genuine E and H properties of this tissue. The effects of sample preparation methods on tooth microstructure, especially along the EDJ, were investigated with optical microscopy and scanning electron microscopy (SEM). The new method of sample preparation combined with a careful dehydration process maintained the integrity of the EDJ interface even after applying multiple Berkovich indents up to maximum load of 400 mN. In contrast, the EDJ and the enamel surface were commonly separated and fractured in teeth that had been resin-embedded. Accordingly, the new method of sample preparation proved to be reliable for investigating the genuine microstructural characteristics of teeth. The behaviour of the elastic region in tooth enamel was investigated with analytical and finite element models. The models were fitted into experimental values of E obtained from nanoindentation tests with a Berkovich indenter to identify a relationship between the mechanical responses of enamel under different loading conditions and microstructure. The decrease in E for enamel with increasing indentation depth was related to its enhanced load-bearing ability. The change of E was directly linked to the microstructural evolution (i.e., the rotation of mineral crystals) of enamel. The effective crystal orientation angle was found to be between 44o and 48o for indentation depths from 0.8 and 2.4 μm below according to the analytical model. The range of angles facilitated the shear sliding of mineral crystals and reduced the stress level as well as the volume of material under higher loads. The behaviour of the plastic region in healthy enamel was investigated with finite element models fitted to nanoindentation data obtained with a Berkovich indenter to determine deformation mechanisms that result in excellent mechanical responses for tooth enamel during loading. When nanoindentation was conducted with increasingly applied loads but at a fixed location, the values of H decreased with increasing indentation depth. The decreasing trend in H was simulated by finite element models and showed a reduction in stress level and yield strength with increasing load. This key mechanism of the loading dependence of mechanical properties resulted in remarkable enamel resilience and was related to the change of effective crystal orientation angle within the enamel microstructure. The mechanical behaviour of enamel with respect to its microstructure was also investigated on teeth exposed to commercially available whitening treatments (tooth bleaching). Enamels exposed to a 6% bleaching treatment exhibited degraded mechanical properties (E and H) compared to unbleached controls. Furthermore, the creep and recovery responses of bleached enamel were also significantly reduced compared to controls. To determine the variables regulating tooth enamel deformation mechanisms during whitening treatments, analytical models were fitted to stress-strain curves. The effective crystal orientation angle of healthy enamel and the protein shear stress, τc, were identified as 50o and 2.5 % of the transverse stiffness of a staggered composite (E2), respectively. After the bleaching treatment, the effective crystal orientation angle of enamel increased to 54o for τc = 1.5 % of E2. Notably, bleaching reduced shear (τc) by 40 % compared to normal readings for unbleached controls. The changes in mechanical responses of bleached enamel were linked to the decrease of the shear bearing ability of protein components in the enamel microstructure. It is envisaged that these findings will provide new perspectives on applications of bleaching treatments and lead to the development of bleaching agents with less damaging effects to healthy enamel. This work should stimulate new interest in understanding the deformation behaviour of tooth enamel at small scales, and offer new methods for the collection and analysis of data from samples prepared close to their native state, upon which novel and biologically relevant high-performance biocomposite materials can be engineered

    Identification of a soft matrix-hard inclusion material by indentation

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    peer reviewedA new procedure for identifying the mechanical behavior of individual phases within a bi-material (matrix-particles) is presented. The case of AlSi10Mg (large globularized Si-rich particles surrounded by an α-Al phase) processed by additive manufacturing and post-treated is taken as a typical example. Grids of nano-indentation tests are performed at different locations on the nanocomposite using a Berkovich indenter and show an impact of the hard inclusions on the experimental curves. The elastoplastic properties of the matrix are identified based on the lowest load–indentation depth curves. Several representative finite element (FE) models demonstrate the influence of the particles on the nano-indentation response. The capacity of the FE model to predict the indentation curve of a cube corner indenter experiment and the Berkovich grid result scattering was checked. A representative volume element (RVE) based on a scanning electron microscope (SEM) image is defined. The identified material parameters of the α-Al phase and Si phase, it allows the prediction of the stress-strain curve of a macroscopic experimental tensile test.LongLifeA

    Investigation of the effect of relative humidity on additive manufactured polymers by depth-sensing indentation

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    Additive manufacturing methods have been developed from rapid prototyping techniques and are now being considered as alternatives to conventional techniques of manufacturing. Stereolithography is one of the main additive methods and is considered highly accurate and consistent. Polymers are used as stereolithography materials and exhibit features such as high strength-to-weight ratio, corrosion resistance, ease of manufacturing and good thermal and electrical resistance properties. However, they are sensitive to environmental factors such as temperature, moisture and UV light, with moisture being identified as one of the most important factors that affect their properties. Moisture generally has an adverse effect on the mechanical properties of polymers. Investigation of the effects of moisture on polymers can be carried out using a number of experimental techniques; however, the benefits of the depth sensing indentation method over bulk tests include its ability to characterise various mechanical properties in a single test from only a small volume of material and the investigation of spatial variation in mechanical properties near the surface. The aim of this research was to investigate the effects of varying relative humidity on the indentation behaviour of stereolithography polymers and to develop a modelling methodology that can predict this behaviour under various humidities. It was achieved by a combination of experimental and numerical methods. Depth sensing indentation experiments were carried out at 33.5 %, 53.8 %, 75.3 % and 84.5 % RH (relative humidity) and 22.5 °C temperature to investigate the effects of varying humidity on the micron scale properties of the stereolithography resin, Accura 60. In order to minimise the effects of creep on the calculated properties, appropriate loading and unloading rates with suitable dwell period were selected and indentation data was analysed using the Oliver and Pharr method (1992). A humidity control unit fitted to the machine was used to condition the samples and regulate humidity during testing. Samples were also preconditioned at 33.5 %, 53.8 %, 75.3 % and 84.5 % RH using saturated salt solutions and were tested at 33.5 % RH using humidity control unit. It was seen that properties such as indentation depth increased and contact iv hardness and contact modulus decreased with increasing RH. The samples conditioned and tested using the humidity control unit at high RH showed a greater effect of moisture than the preconditioned samples tested at 33.5 % RH. This was because the samples preconditioned at high RH exhibited surface desorption of moisture when tested at ambient RH, resulting in some recovery of the mechanical properties. In order to investigate these further, tests were performed periodically on saturated samples after drying. Ten days drying of samples conditioned for five days at 84.5 % RH provided significant, though not complete, recovery in the mechanical properties. These tests confirmed that Accura 60 is highly hygroscopic and its mechanical properties are a function of RH and removal of moisture leads to a significant recovery of the original mechanical properties

    Isolating the role of geometrical structure on the mechanical properties of nanoporous metals

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    Nanoporous (NP) metals are three-dimensional (3D) structures with characteristic length-scale of its constituents (ligaments, junctions, and pores) in the range from a few to hundreds of nanometers. Such materials are of great interest for many applications, including catalysis, biological material analogues, and the next generation interconnect materials in electronics packaging. Investigations targeting understanding of the mechanical properties of such materials have tried to separate effects of the geometrical arrangement of the 3D network from those due to the nanostructure (abundance of surfaces, presence of grains, and other defects). Traditionally, this has been achieved by assuming that the network is geometrically similar to that of a macroscopic, low-density metal foam. The goal of this work is to attack the problem using a comprehensive approach that involves isolating the prominent geometry and size scale effects and examining their specific contributions individually for a range of relative densities. Specifically, 3D printed models printed and tested in compression at the macroscale replicate the geometrical arrangement of the nanoporous 3D network independently of any size effects. The relative modulus and relative compressive yield strength for the 3D printed structures with same arrangement as the nanoporous solid exhibit different scalings with density compared to stochastic macroscale foams. The deformation mechanism in stochastic macroscale foams is isolated in the ligaments and switches from bending to compression dominated behavior as the relative density increases. In contrast, due to the presence of enlarged junctions, the deformation mode for the 3D printed nanoporous structures remains bending dominated even at high relative densities. Nanoscale experiments and molecular dynamics (MD) simulations provide a glimpse into the relative modulus and strength scalings and reveal a more nuanced dependence, with unexpected enhancement in both the relative modulus and relative strength. Obtaining a clear understanding of the contribution of geometrical structure on the properties of nanoporous metals will significantly advance our understanding of how to tailor NP metal microstructure such as grains, interfaces, and surfaces to enhance the physical properties of the material. Thus, the findings reported here could inform future studies to maximize the versatility and potential of nanoporous metal structures.Ph.D

    Ceramic Materials

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    This is the first book of a series of forthcoming publications on this field by this publisher. The reader can enjoy both a classical printed version on demand for a small charge, as well as the online version free for download. Your citation decides about the acceptance, distribution, and impact of this piece of knowledge. Please enjoy reading and may this book help promote the progress in ceramic development for better life on earth
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