The effects of systematic variations in the size and volume fraction of reinforcing phase on the mechanical properties of and fracture processes in silicon carbide particlereinforced aluminium matrix composites have been studied. Tensile tests to failure have been performed to determine the mechanical properties of the composites. A simple model has been proposed for this behaviour. The micromechanisms of fracture have been investigated by a combination of fractographic and dynamic techniques. Matched fracture halves have been obtained from the composites and the fracture processes elucidated. Fracture proceeded by a ductile void nucleation, growth and coalescence mechanism. Void nucleation occurred at the reinforcing phase, with a change in nucleation mechanism on varying the micrstructural parameters. A simple critical stress criterion has been proposed for the nucleation process. Support for this proposal has been obtained by the study of sections through the failed tensile specimens. In situ scanning electron microscopy fracture studies have been performed. These revealed void nucleation before the onset of macroscopic cracking. Crack propagation has been shown to occur by the concurrent formation of microcracks ahead of the crack tip and failure of the joining matrix ligaments. The magnitude of matrix deformation has been shown to determine the extent of microcracking. Acoustic emissions have been monitored during tensile straining. Void nucleation events have been recorded from the onset of plastic deformation and continuing throughout the plastic régime until final failure. The suppression of void coalescence by the constaint imposed on matrix flow by rigidly-bonded interfaces has been proposed to account for the extended void growth in materials containing fractured particles. The importance of the local values of the microstructural parameters on the far-field strain at nucleation has been shown.

When solid particles of different types are mixed together, a random distribution of the components is rarely produced and deterioration of the mixture can occur on subsequent handling. Among the microscopic processes responsible, one important mechanism for free-flowing materials is thought to be interparticle percolation, the drainage of particles through the interstices between larger ones. If the larger particles are stationary this is called spontaneous percolation, whereas if it is produced by shear strain the term strain-induced percolation is used. Here a quantitative evaluation of both and some consequences are described. A practical application of spontaneous percolation has been the design and construction of a new static mixer or distributor, consisting of rows of angle bars mounted horizontally in a vertical channel. Material fed to the top of a unit bounces off the bars and is distributed across the channel. Two mixers were built; one dispersed material in one lateral direction only and could be used for feeding material onto a belt or distributing seed from a moving vehicle. The other produced a two-dimensional dispersion and would be useful in distributing material flowing into hoppers or whenever a good mixture were required. Optimisation of the design was investigated using a computer program which simulated the motion of a spherical particle as it fell through such a mixer. Design data was deduced from the record of the position of the particle. The mixers were not suitable for use with fine materials. Interpretation of experimental results from this equipment requires suitable statistical indices and two were developed here. One related the variance of sample compositions to the number of particles fed to the mixer by assuming that the distributions of material were ordered. The second, using the correlation coefficient between samples, related the variance to the sample size in those situations where two orthogonal processes are in operation. Both techniques are generally applicable to fields other than that of powder mixing. On the theoretical side, an existing model of spontaneous percolation for inelastic materials has been extended and improved. The original form did not account for the motion of a particle between collisions with bulk particles but this has now been included. An entirely new semi-empirical model for partly elastic materials has also been proposed. Both predict percolation velocities which agree with experimental data. In order to extend earlier experimental studies on strain-induced percolation, a simple shear cell was modified by installing a hydraulic drive which enabled the cell to be driven at a constant speed. Advantages of the use of such a cell include the possibility of detecting a percolating particle on entry to and exit from the bed and the constant strain throughout the material. Reliable and accurate readings of residence times of percolating particles were recorded and percolation velocities and both lateral and axial diffusion coefficients were calculated. These were functions of the relative particle size and density, the material properties of the percolating particle and bed conditions such as strain rate and normal stress Denser and softer particles percolated faster. Decreasing the diameter ratio between percolating and bulk particles from 0.67 to 0.27 caused a twenty-five fold increase in the percolation rate. The dependence of this rate on particle diameter was interpreted using statistical mechanics. The percolation rate has been shown to reach a constant value as the strain rate increases, in contrast to the deductions drawn in earlier work by Scott, whose procedure has been proved to be unsound.

A series of experiments are described which explore the relationship between biomechanical properties and the control of jaw movement in speech. This relationship is documented using kinematic analyses in conjunction with a mathematical model of jaw motion and direct measures of jaw stiffness. / In the first experiment, empirical and modeling studies were carried out to examine whether the nervous system compensates for naturally occurring forces acting on the jaw during speech. As subjects walk or run, loads to the jaw vary with the direction and magnitude of head acceleration. While these loads are large enough to produce a measurable effect on jaw kinematics, variation in jaw position during locomotion is shown to be substantially reduced when locomotion is combined with speech. This reduction in jaw motion is consistent with the idea that in speech, the control of jaw movement is adjusted to offset the effects of head acceleration. Results of simulation studies using a physiologically realistic model of the jaw provide further evidence that subjects compensate for the effects of self-generated loads by adjusting neural control signals. / A second experiment explores the idea that a principle mechanical property of the jaw---its spring-like behavior, or stiffness---might influence patterns of kinematic variation in speech movements. A robotic device was used to deliver mechanical perturbations to the jaw in order to quantify stiffness in the mid-sagittal plane. The observed stiffness patterns were non-uniform, with higher stiffness in the protrusion-retraction direction. Consistent with the idea that kinematic patterns reflect directional asymmetries in stiffness, a detailed relationship between jaw kinematic variability and stiffness was observed---kinematic variability was consistently higher under conditions in which jaw stiffness was low. Modeling studies suggested that the pattern of jaw stiffness is significantly determined by jaw geometrical properties and muscle force generating abilities. / A third experiment examines the extent to which subjects are able to alter the three-dimensional pattern of jaw stiffness in a task-dependent manner. Destabilizing loads were applied to the jaw in order to disrupt the ability of subjects to maintain a static jaw posture. Subjects adapted by increasing jaw stiffness in a manner that depended on the magnitude and, to a more limited extent, direction of the destabilizing load. The results support the idea that stiffness properties can be controlled in the jaw, and thus may play a role in regulating mechanical interactions in the orofacial system.

External loads, such as those due to the orientation of body segments relative to gravity, affect the extent to which control signals result in the achievement of desired goals. The degree to which subjects adjust control signals to compensate for loads provides a measure of what the nervous system knows about forces affecting motion and gives an indication of the complexity of control signals needed for voluntary movement. In the present study, we have explored the hypothesis that subjects take no account of the orientation of the head relative to gravity when making jaw movements during speech. We used a simulation model of the jaw to predict the kinematic effect of using a single set of motor commands (which take no account of the relative direction of the gravitational force) to produce speech-like movements while the body was in three different orientations: upright, prone and supine. The simulations predict a systematic change in jaw pitch angle and horizontal translation resulting from the change in body orientation. Empirical results for five subjects tested under the same conditions as those explored in the simulations are for the most part consistent with the pattern predicted by the model. This suggests that in the case of jaw movements during speech, control signals are not adjusted to account for changes in head and body orientation relative to gravity.

The mechanisms underlying the effect of alterations in Type I collagen on bone mechanical properties are not well defined. Clinical tools for evaluating fracture risk, such as dual energy x-ray absorptiometry (DXA) and quantitative ultrasound (QUS) focus on bone mineral and cannot detect changes in the collagen matrix. The mechanical response tissue analyzer (MRTA) is a potential tool for evaluating fracture risk. Thus, the focus of this work was to investigate the effects of collagen degradation on bone mechanical properties and examine whether clinical tools can detect these changes. Female and male emu tibiae were endocortically treated with 1 M potassium hydroxide (KOH) solution for 1-14 days and then either mechanically tested in three-point bending, fatigued to failure or fatigued to induce stiffness loss. Computed Tomography scans, DXA, QUS, MRTA and three-point bend testing in the elastic region were performed on emu tibiae before and after either KOH treatment or fatigue to induce stiffness loss. Fracture surfaces were examined to determine failure mechanisms. Bone mineral and bone collagen were characterized using appropriate techniques. Bone mineral-collagen interface was investigated using Raman spectroscopy and atomic force microscopy (AFM). Endocortical KOH treatment does not affect bone mineral however, it causes in situ collagen degradation, rather than removal and may be weakening the mineral-collagen interface. These changes result in significantly compromised mechanical properties. Emu tibiae show significant decreases in failure stress and increased failure strain and toughness, with increasing KOH treatment time. The significant increase in toughness of KOH treated bones is due to structural alterations that enhance the ability of the microstructure to dissipate energy during the failure process, thereby slowing crack propagation, as shown by fracture surface analysis. KOH treated samples exhibit a lower fatigue resistance compared to untreated samples at high stresses only for both sexes. Partial fatigue testing results in similar decreases in modulus for all groups and sexes. The MRTA detected these changes whereas DXA and QUS did not. MRTA detects changes in bone mechanical properties induced by changes in collagen quality and fatigue and could be a more effective tool for predicting fracture risk.

Carbon nanotubes (CNTs) and carbon nanofirbers (CNFs) have excellent properties (mechanical, electrical, magnetic, etc.), which can make them effective nanoreinforcements for improving the properties of materials. The incorporation of CNT/Fs in a wide variety of materials has been researched extensively in the past decade. However, the past study on the reinforcement of cementitious materials with these nanofilaments has been limited. The findings from those studies indicate that CNT/Fs did not significantly improve the mechanical properties of cementitious materials. Two major parameters influence the effectiveness of any discrete inclusion in composite material: The dispersion quality of the inclusions and the interfacial bond between the inclusions and matrix. The main focus of this dissertation is on the dispersion factor, and consists of three main tasks: First a novel thermodynamic-based method for dispersion quantification was developed. Second, a new method, incorporating the utilization of silica fume, was devised to improve and stabilize the dispersion of CNFs in cement paste. And third, the dispersion quantification method and mechanical testing were employed to measure, compare, and correlate the dispersion and mechanical properties of CNF-incorporated cement paste produced with the conventional and new methods. Finally, the main benefits, including the increase in strength and resistance to shrinkage cracking, obtained from the utilization of CNFs in cement paste will be presented. The investigations and the corresponding results show that the novel dispersion quantification method can be implemented easily to perform a wide variety of tasks ranging from measuring dispersion of nanofilaments in composites using their optical/SEM micrographs as input, to measuring the effect of cement particle/clump size on the dispersion of nano inclusions in cement paste. It was found that cement particles do not affect the dispersion of nano inclusions in cement paste significantly while the dispersion of nano inclusions can notably degenerates if the cement particles are agglomerated. The novel dispersion quantification method shows that, the dispersion of CNFs in cement paste significantly improves by utilizing silica fume. However, it was found that the dispersion of silica fume particles is an important parameter and poorly dispersed silica fume cannot enhance the overall dispersion of nano inclusions in cementitious materials. Finally, the mechanical testing and experimentations showed that CNFs, in absence of moist curing, even if poorly dispersed, can provide important benefits in terms of strength and crack resistance.

Thesis (M.S.)--University of Hawaii at Manoa, 2006. / Includes bibliographical references (leaves 37-41). / viii, 52 leaves, bound ill. 29 cm

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.

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.

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.