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Doctor of Philosophy(PhD) / 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.

Aim: The aim of this research was to measure functional intraoral pressures using a newly developed method; specifically, three areas were examined. Firstly, this new approach to measurement allowed the equilibrium theory of tooth position to be re-addressed. Secondly, it allowed investigation the patterns of pressure change in the palatal midline during water swallowing. Lastly, this approach allowed a preliminary investigation of the affect of the viscosity of the food ingested on the pressures generated in the mouth. Methods: The participants were 6 healthy volunteers (4 males, 2 females) recruited from the post-graduate students at the University of Otago, School of Dentistry. The age range was 25 to 35 years. All had full permanent dentitions, Angle Class I occlusions (normal) with acceptable overbite and overjet relationship, and none of them had a history of previous orthodontic treatment. For each of the subjects a cast chrome-cobalt baseplate was constructed to house 8 miniature strain gauge pressure transducers (Precision Measurement Co. Michigan). The location of the sensors were standardised as follows: Three sensors were paired on the buccal and lingual surfaces of the central, canine and first molar. Two palatal vault sensors were placed in the midline of the palate, one at level of the distal of the first premolar, and the second slightly anterior to the junction of the hard and soft palate. Simultaneous recordings were taken during a set of tasks including water swallows, saliva swallows and food ingestion. Results: The results showed that swallowing was a highly complex wellcoordinated event, and that each individual had their own unique signature pattern of swallowing, characterised by pressure changes of high frequency, in excess of �1000 kPa/s. Conclusions: The analysis of the pressure acting on the teeth showed that although the pattern and magnitude of pressure generated varied among the group, the observation of waveforms would suggest a tendency for no inherent balance between the buccal and the palatal pressures on the teeth during swallowing for any of the individuals tested. This investigation of the pressure in the midline revealed an aspect previously not fully explored; these experiments showed that there were large and persistent negative pressures generated during swallowing, that preceded the positive pressures and which appear important in bolus propulsion The patterns of swallowing and the magnitudes of pressure generated, changed with the different consistency of the various substances ingested. There was a general trend for increased pressures during swallowing of substrates that are more viscous; water showed the lowest pressures, followed by saliva and finally jelly. Keywords: Tongue pressure, Intraoral pressures, Tongue dynamics, Swallowing.

Passive knee motion is guided by the interaction of the articular surfaces and the restraining role of the soft-tissue structures. It is defined by characteristic kinematics within an envelope of motion. The main goal of this thesis was to simulate this characteristic motion by developing a subject-specific anatomically based finite element model. CT and MR image stacks were used to develop the geometry model and experimental (mechanical) test data was used as model input. Passive knee flexion was simulated and translational and rotational motion described using the Joint Coordinate System (JCS). The model was validated using clinical flexion and AP drawer tests. An ACL reconstruction model was also developed. Highest AP laxity was found at 30?? of flexion when the graft was positioned in the original native ACL insertion point. ACL tunnel positions were simulated according to surgical techniques. For this case, the highest AP laxity was displayed at 0?? of flexion. Four different graft materials were examined, with the quadriceps tendon graft exhibiting highest laxity, followed by the patellar tendon, braided hamstring and finally unbraided hamstring graft. The effect of malpositioning the graft's femoral attachment point from its central location was also investigated. The proximal femoral attachment point most closely mimicked the central attachment point in terms of AP laxity in the native ACL insertion group. In the ACL tunnel group, the posterior femoral attachment point most closely mimicked the intact knee. In this thesis it was found that changing the femoral insertion point of the graft can highly influence the AP laxity behaviour. Also using the surgical technique to create ACL tunnels may not necessarily produce the same kinematic behaviour as the intact knee. Lastly, this thesis has shown the importance of explicitly defining the local reference coordinate system when describing knee kinematics. Changing the coordinate system markedly alters the calculated kinematics. Ideally, a standardisation of local coordinate systems, similar to the JCS, would be proposed within the biomechanics community.

The research work in this thesis involves an experimental and theoretical investigation for high speed machining of AISI 4140 medium carbon steels and AISI D2 tool steels which are classified as being difficult to machine materials. An experimental program was carried out to determine the cutting forces, chip formation, the secondary deformation zone thickness and surface roughness at different cutting speeds using a 0.4mm and 0.8mm nose radii ceramic tools and -7?? rake angle for annealed (virgin) AISI 4140 and heat treated AISI 4140 steel. Another series of experiments was carried out on the annealed (virgin) and heat treated AISI D2 with 0.4mm, 0.8mm and 1.2mm nose radii CBN (Cubic Boron Nitride) tools under various cutting conditions. A theoretical model is developed by taking into account the flow stress properties of the AISI 4140 (0.44% carbon content) to use with the Oxley Machining approach. To find the flow stress data for AISI D2 tool steel, the Johnson and Cook empirical constitutive equation is used as the constitutive model. In addition, the magnitude of tool radius should be also considered to determine the prediction of cutting performances. To account for the effect of nose radius edge in hard machining, a simplified geometrical method is used to model the parameters for application in the Oxley Model and works for the cutting conditions considered here. These extensions to the Oxley machining theory were verified by experimental results. These results show a good agreement between the Oxley machining theory and hard machining experiment at data. The research work described in this thesis provides useful data for hard machining conditions.

The molecular weight distributions of two high molecular weight (M[subscript w]>>1 million) polyethylene polymers, WSR-308 and WSR-301, were characterized with gel permeation chromatography (GPC) coupled with a multi-angle laser light scattering detector (MALLS). The M[subscript w] of the WSR-308 was found to be 5.10x10⁶ g/mol with a molecular weight range from about 1 million g/mol to as high as 10 million g/mol. The M[subscript w] of the WSR-301 was found to be 3.16x10⁶ g/mol with the lowest molecular weight about 400,000 g/mol while the highest molecular weight component may have been as high as 8 million g/mol. Attempts to measure the M[subscript w] of the two polymers using static light scattering (SLS) techniques proved to be difficult. In conjunction with these studies, drag reduction and shear degradation studies of the two polymers in water were also conducted. Solutions of the two polymers, ranging from 1 to 10 ppm including mixtures of the two, were tested in a pipe-flow apparatus to obtain friction factor and %DR data. In every case, the greater the concentration and/or the molecular weight of the polymer, the greater the drag reduction effects. Additionally, the higher molecular weight polymer and mixtures with a greater weight percentage of the higher molecular weight polymer were found to shear degrade less quickly than otherwise. A unique point along the maximum drag reduction asymptote (MDA) termed the "divergence point" was a focus of this study and an energy model based on frictional losses correlates well to the data. The correlation developed here relates the difference in frictional losses between the solvent by itself and the polymer solution directly to the mass concentration and molecular weight of the polymer. This frictional difference was proportional to the product of the mass concentration and molecular weight where both quantities were to approximately the first power. / Graduation date: 2002

Graduation date: 2000

TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mechanical properties can be attributed mainly to the continuous transformation of retained austenite into martensite during straining (TRansformation Induced Plasticity). The aim of this thesis was to clarify the interaction between the hot rolling conditions, the formation of microstructure, and the resulting mechanical properties. Various rolling simulation techniques were employed to determine how the composite microstructure is formed during the various steps of multi-stage thermomechanical processing. The interaction between deformation and phase transformation is highlighted, particularly from the viewpoint of the transformation texture.