Particle reinforced composites are widely used in tires, heat exchangers, thermal barrier coatings and many other applications, as they have good strength to weight ratio, excellent thermal insulation, ease of manufacturing and flexibility in design. During their service life, these composites are often subjected to harsh environments, which can degrade the thermo-mechanical properties of the constituents in the composites, affecting performance and lifetime of the composites. This study investigates performance of particle reinforced composites subjected to coupled heat conduction and thermo-elastic deformation at the macro and micro levels. A micromechanical model is used to determine the effective thermal and mechanical properties of the homogenized composite by incorporating microscopic characteristics of the composites. The constituent?s thermal conductivities of the composite are assumed to be functions of temperature and the elastic moduli to be functions of temperature and stress fields. The effective properties obtained from the micromechanical model represent average (macroscopic) properties. The effective heat conduction and thermo-elastic responses in the homogenized composites are compared with the responses of the composite with particles randomly distributed in the matrix (heterogeneous materials) which represent microscopic responses. For this purpose, two sets of finite element (FE) models are generated for composites with particle volume contents 12.5, 25, and 50%. The first FE model represents a homogenized composite panel and the effective responses from the micromechanical model are used as input for the material properties. The second FE model mimics composite microstructure with discontinuous particles randomly dispersed in a homogeneous matrix. Parametric studies on effects of conductivity ratio between particle and matrix, degree of nonlinearity, and volume fraction on the temperature distribution and steady state times have been studied. For lower volume fractions the temperature profiles of homogenized and heterogeneous composite models are in good agreement with each other. But for higher volume fractions, the detailed model showed a wavy profile whereas the effective model showed no signs of it. When the nonlinearity in thermal conductivity of the particle and matrix constituents is increased, the steady state time significantly deviates from the ones with constant constituent properties. When the volume fraction of particles in the composite increases, the steady state is reached in less time, since the thermal conductivity of particles are taken larger than that of the matrix. Effects of coefficient of thermal expansion (CTE) ratio of particle and matrix, temperature change, and volume fraction on the discontinuity of stress and strain fields at the interphase of matrix and particle have been studied. The stresses developed were more for higher CTE ratios and the magnitude of discontinuity also follows the same trend. As the volume fraction increases, the stresses developed and the magnitude of discontinuity also increase. Finally, sequentially coupled heat conduction and deformation analyses are performed on thermal barrier coating (TBC) systems to demonstrate the applicability of the micromechanical model in predicting overall thermo-elastic responses of the TBC.

Particle reinforced composites are widely used in tires, heat exchangers, thermal barrier coatings and many other applications, as they have good strength to weight ratio, excellent thermal insulation, ease of manufacturing and flexibility in design. During their service life, these composites are often subjected to harsh environments, which can degrade the thermo-mechanical properties of the constituents in the composites, affecting performance and lifetime of the composites. This study investigates performance of particle reinforced composites subjected to coupled heat conduction and thermo-elastic deformation at the macro and micro levels. A micromechanical model is used to determine the effective thermal and mechanical properties of the homogenized composite by incorporating microscopic characteristics of the composites. The constituent?s thermal conductivities of the composite are assumed to be functions of temperature and the elastic moduli to be functions of temperature and stress fields. The effective properties obtained from the micromechanical model represent average (macroscopic) properties. The effective heat conduction and thermo-elastic responses in the homogenized composites are compared with the responses of the composite with particles randomly distributed in the matrix (heterogeneous materials) which represent microscopic responses. For this purpose, two sets of finite element (FE) models are generated for composites with particle volume contents 12.5, 25, and 50%. The first FE model represents a homogenized composite panel and the effective responses from the micromechanical model are used as input for the material properties. The second FE model mimics composite microstructure with discontinuous particles randomly dispersed in a homogeneous matrix. Parametric studies on effects of conductivity ratio between particle and matrix, degree of nonlinearity, and volume fraction on the temperature distribution and steady state times have been studied. For lower volume fractions the temperature profiles of homogenized and heterogeneous composite models are in good agreement with each other. But for higher volume fractions, the detailed model showed a wavy profile whereas the effective model showed no signs of it. When the nonlinearity in thermal conductivity of the particle and matrix constituents is increased, the steady state time significantly deviates from the ones with constant constituent properties. When the volume fraction of particles in the composite increases, the steady state is reached in less time, since the thermal conductivity of particles are taken larger than that of the matrix. Effects of coefficient of thermal expansion (CTE) ratio of particle and matrix, temperature change, and volume fraction on the discontinuity of stress and strain fields at the interphase of matrix and particle have been studied. The stresses developed were more for higher CTE ratios and the magnitude of discontinuity also follows the same trend. As the volume fraction increases, the stresses developed and the magnitude of discontinuity also increase. Finally, sequentially coupled heat conduction and deformation analyses are performed on thermal barrier coating (TBC) systems to demonstrate the applicability of the micromechanical model in predicting overall thermo-elastic responses of the TBC.

Yes / Composites have clinical application for their ability to mimic the hierarchical structure of human tissues. In tissue engineering applications the use of degradable biopolymer matrices reinforced by bioactive ceramics is seen as a viable process to increase osteoconductivity and accelerate tissue regeneration, and technologies such as additive manufacturing provide the design freedom needed to create patient-specific implants with complex shapes and controlled porous structures. In this study a medical grade poly(l-lactide) (PLLA) was used as matrix while apatite-wollastonite (AW) was used as reinforcement (5 wt% loading). Premade rods of composite were pelletized and processed to create a filament with an average diameter of 1.6 mm, using a twin-screw extruder. The resultant filament was 3D printed into three types of porous woodpile samples: PLLA, PLLA reinforced with AW particles, and PLLA with short AW fibres. None of the samples degraded in phosphate buffered solution over a period of 8 weeks, and an average effective modulus of 0.8 GPa, 1 GPa and 1.5 GPa was obtained for the polymer, particle and fibre composites, respectively. Composite samples immersed in simulated body fluid exhibited bioactivity, producing a surface apatite layer. Furthermore, cell viability and differentiation were demonstrated for human mesenchymal stromal cells for all sample types, with mineralisation detected solely for biocomposites. It is concluded that both composites have potential for use in critical size bone defects, with the AW fibre composite showing greater levels of ion release, stimulating more rapid cell proliferation and greater levels of mineralisation. / The research was funded in part by the UK EPSRC Centre for Doctoral Training in Additive Manufacturing and 3D Printing (EP/L01534X/1), the UK EPSRC Centre for Innovative Manufacture in Medical Devices (EP/K029592/1), and Glass Technology Services Ltd., Sheffield, UK.

The properties of composite materials in which spherical inclusions are embedded in a matrix of some kind, have been studied for many decades and many analytical models have been developed which measure these properties. There has been a steady progression in the complexity of models over the years, providing greater insight into the nature of these materials and improving the accuracy in the measurement of their properties. Some of the properties with which this thesis is concerned are, the elastic, thermal and electrical properties of such composites. The size of the spherical inclusion which acts as the reinforcing phase, has a major effect on the overall properties of composite materials. Once an inclusion is embedded into a matrix, a third region of different properties between the inclusion and matrix is known to develop which is called the interphase. It is well known in the composite community that the smaller the inclusion is, the larger the interphase region which develops around it. Therefore, with the introduction of nanoparticles as the preferred reinforcing phase for some composites, the interphase has a major effect on its properties. It is the aim of this thesis to consider the role of the interphase on the properties of composites by modeling it as an inhomogeneous region. There is much scientific evidence to support the fact that the interphase has an inhomogeneous nature and many papers throughout the thesis are cited which highlight this. By modeling the inhomogeneous properties by arbitrary mathematical functions, results are obtained for the various properties in terms of these general functions. Some specific profiles for the inhomogeneous region are considered for each property in order to demonstrate and test the models against some established results.

Carbonization of biomass is a type of pyrolysis that allows for the formation of byproducts that have applications in many other industries [1]. In the field of materials science concerned with environmental impact intersecting with desirable material properties and performance, the process of carbonization in particular with commonplace biomass such as food waste is of great interest. In this thesis, pistachio shell was used as the organic biomass of choice for carbonization, and reinforcement was provided by titanium powder. These two materials were milled together at two different compositions and milling times. Experimental conditions consisted of replicates of three bulk samples made from uniaxially pressed powder mixtures heat treated from 800 °C up to 1200 °C in increments of 100 °C. Heat treatment occurred in a tube furnace with a heating rate of 5 °C/min up to the heat treatment temperature, holding the temperature for 1 hour, then ramping back down to room temperature, all in an inert atmosphere. XRD was performed on heat treated samples before polishing, while optical microscopy and SEM were performed after mounting and polishing. TGA was performed on the milled powders, while hardness was performed on the heat treated bulk samples after mounting and polishing. Results obtained suggested that increasing heat treatment temperature and milling time decreased carbon matrix porosity. In addition, greater amounts of titanium seemed to result in increased porosity. However, at increased temperature, more surface cracking was observed, leading to the conclusion that an excessively high temperature is detrimental to mechanical properties. Finally, rutile TiO2 was formed as a result of the heat treatment process. In considering environmental impact, cost, and mechanical properties, a balance must be maintained between higher temperature processing, duration, milling time, and porosity present due to these factors. Future work includes further investigations into processing parameters and characterization such as XPS and abrasion testing. / Master of Science / Carbonization of organic materials such as wood or nut shells can be explained in short as a decomposition that occurs when those materials are heated up without allowing them access to oxygen as in a normal combustion like a fire. Because of that, carbonization can produce useful products and materials of interest to many. Adding titanium to pistachio shell powder, performing compaction and carbonization, then further heating up those samples, resulted in composite materials consisting of mostly carbon char and particles inside that improve the properties. After testing multiple experimental conditions and analyzing them using equipment such as X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA), optical microscopes, Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS), and a hardness tester, some trends in properties and structure were observed. Generally, increasing heat treatment temperature and milling time will reduce porosity in the matrix. On the other hand, decreasing amount of Ti powder added seems to reduce porosity. However, too high of a heat treatment temperature seems to have a detrimental effect on the part manufactured (i.e. surface cracking). In addition, considering processing costs and time costs could discourage one from using a very high temperature to heat treat these samples. Therefore, it is important to balance amount of energy used to heat treat, time spent, and resulting porosity of the final product for its applications. Future work should be done to further determine the effects of processing parameters by making more samples to test the properties of. Other characterization techniques like X-Ray Photoelectron Spectroscopy (XPS) and abrasion testing could be good to determine the exact makeup of the particles in the composite as well as see the sample's performance in its intended application (i.e. brake pads).