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Large Area Maskless Photopolymerization (LAMP) is a disruptive additive manufacturing technology developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. Due to polymerization shrinkage during the layer-by-layer curing process, stresses are accumulated that can give rise to cracks and delaminations along the interfaces between adjacent layers. The objective of this doctoral dissertation is to investigate the mechanisms of stress evolution and cracking/delamination during the LAMP manufacturing process through theoretical modeling and experimental characterization methods. The evolving conversion degree in a layer was characterized through Fourier Transform Infrared Spectroscopy and this leads to a so-called print-through curve. The polymerization shrinkage strain in each exposed layer was calculated on the basis of the theoretical relationship between the volumetric shrinkage and the degree of conversion. Furthermore, the material’s elastic modulus, which also evolves with the degree of conversion, was characterized by three-point bending tests. With the degree of conversion, cure-dependent modulus and shrinkage strain as the three primary inputs, finite element modeling was conducted to dynamically simulate the layer-by-layer manufacturing process and to predict the process-induced stresses. To investigate the fracture process, Mode I and Mode II interlaminar fracture toughness of the LAMP-built laminates was characterized, using the double cantilever beam (DCB) test and the end notched flexure (ENF) test, respectively. In order to predict the crack initiation and propagation occurring in a LAMP-built part, a mixed-mode cohesive element model was developed. The Mode I and Mode II cohesive parameters, which are used to describe the bilinear constitutive behavior of the cohesive elements, were determined by matching the numerical load-deflection curves to the experimental ones obtained from the DCB tests and the ENF tests, respectively. Using this model, the fracture of a hollow-cylinder part was analyzed and the simulation results were compared with experiments. Finally, several possible strategies for mitigating the shrinkage related defects were investigated. Reducing the overall polymerization shrinkage, optimizing the print-through curve and delaying the gel point of resin composite were demonstrated to be effective in reducing stresses and cracks.

As a preliminary starting point for the present study, physical and mechanical properties of polypropylene nanocomposites (PPNCs) for samples received from Queen's University Belfast have been evaluated. Subsequently, polymer/clay nanocomposite material has been produced at Bradford. Mixing and processing routes have been explored, and mechanical properties for the different compounded samples have been studied. Clay intercalation structure has received particular attention to support the ultimate objective of optimising tensile and fracture behaviour of isotropic and die-drawn PPNCs. Solid-state molecular orientation has been introduced to PPNCs by the die-drawing process. Tensile stress-strain measurements with video-extensometry and tensile fracture of double edge-notched tensile specimens have been used to evaluate the Young's modulus at three different strain rates and the total work of fracture toughness at three different notch lengths. The polymer composite was analyzed by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, wide angle x-ray diffraction, and transmission electron microscopy. 3% and 5% clay systems at various compatibilizer (PPMA) loadings were prepared by three different mixing routes for the isotropic sheets, produced by compression moulding, and tensile bars, produced by injection moulding process. Die-drawn oriented tensile bars were drawn to draw ratio of 2, 3 and 4. The results from the Queen's University Belfast samples showed a decrement in tensile strength at yield. This might be explained by poor bonding, which refers to poor dispersion. Voids that can be supported by intercalated PP/clay phases might be responsible for improvement of elongation at break. The use of PPMA and an intensive mixing regime with a two-step master batch process overcame the compatibility issue and achieved around 40% and 50% increase in modulus for 3% and 5% clay systems respectively. This improvement of the two systems was reduced after drawing to around 15% and 25% compared with drawn PP. The work of fracture is increased either by adding nanoclay or by drawing to low draw ratio, or both. At moderate and high draw ratios, PPNCs may undergo either an increase in the size of microvoids at low clay loading or coalescence of microvoids at high clay loading, eventually leading to an earlier failure than with neat PP. The adoption of PPMA loading using an appropriate mixing route and clay loading can create a balance between the PPMA stiffness effect and the degree of bonding between clay particles and isotropic or oriented polymer molecules. Spherulites size, d-spacing of silicate layers, and nanoparticles distribution of intercalated microtactoids with possible semi-exfoliated particles have been suggested to optimize the final PPNCs property.

There is a need to monitor the existence and effects of damage in structural materials. Aircraft components provide a much publicized example, but the need exists in a variety of other structures, such as layered materials used in food packaging industries. While several techniques and models have been proposed for material characterization and condition monitoring of bulk materials, less attention has been devoted to thin sheets having no flexural rigidity. This study is therefore devoted to the development of a new method for acoustic Non-Destructive Testing (NDT) and material characterization of thin sheets used in food packaging materials or similar structures. A method for assessing the strength in the presence of crack of thin sheets used in food packaging is first presented using a modified Strip Yield Model (SYM). Resonance frequency measurement is then introduced and it is shown, at low frequency range (less than 2kHz), that a change in the physical properties such as a reduction in stiffness resulting from the onset of cracks or loosening of a connection causes detectable changes in the modal properties, specifically the resonance frequency. This observation leads to the implementation of a simple method for damage severity assessment on sheet materials, supported by a new theory illustrating the feasibility of the detection of inhomogeneity in form of added mass, as well as damage severity assessment, using a measurement of the frequency shift. A relationship is then established between the resonance frequency and the material’s elastic property, which yields a new modality for sheet materials remote characterization. The result of this study is the groundwork of a low-frequency vibration-based method with remote acoustic excitation and laser detection, for nondestructive testing and material characterization of sheet materials. The work also enhances the feasibility of the testing and condition monitoring of real structures in their operating environment, rather than laboratory tests of representative structures. The sensitivity of the new experimental approach used is liable to improvement while being high because the frequency measurement is one of the most accurate measurements in physics and metrology.

Carbon Capture and Storage (CCS) is recognised as having a significant role to play in tackling climate change and reducing carbon dioxide (CO2) emissions. In CCS schemes, CO2 is captured from anthropogenic sources, and transported to suitable sites either for EOR (Enhanced Oil Recovery) or storage. The transport of such huge amount of CO2 causes new challenges. The main concern is in the difference between natural gas and CO2 transportation pipelines. CO2 phase behaviour during decompression, existence of different impurities and very high operating pressure are some of the new challenges for pipeline designer and operators. This PhD study has taken a systematic approach to understand the mechanics involved in the fracture of pipes containing high pressure flue-gas CO2. The work involved the development of a novel weight function stress intensity factor solution that can be used with complex stress fields induced by residual and/or thermal stresses in addition to applied pressure. In addition, the thesis reports a substantial experimented test programme which involved low temperature fracture toughness tests linked to a detailed finite element based stress analysis. Overall, the thesis presents an integrated engineering criticality means to assess the suitability or otherwise of a pipeline system to transport high pressure flue-gas CO2.

Total joint arthroplasty surgeries are known to have a high success rate but the longevity of the implants still acts as a limiting factor. Ultrahigh molecular weight polyethylene is the material of choice for the implant bearing surfaces due to its excellent clinical and tribological performance. A common problem associated with the polymer is however the loosening of the implant from its surrounding bone tissue. This phenomenon is caused by a biological reaction to released wear particles. Reducing the release of wear particles will increase the lifespan of implants and can be accomplished by increasing the wear resistance of the material. Crosslinking of the polymer by means of gamma irradiation is a well known approach to achieve an increased wear resistance but eventually leads to oxidation of the polymer. The addition of vitamin E as antioxidant is known to reduce this without significant loss of mechanical properties. A second approach is based on adding reinforcements to the polymer in order to enhance its tribological performance. This work focuses on the thermal and mechanical characterisation of newly developed UHMWPE nano-composites with a focus on the addition of vitamin E and crosslinking by gamma irradiation. Based on previously published results indicating an increased fracture toughness for different composites, Nanodiamonds, multiwalled carbon nanotubes and graphene oxide nanoparticles were dispersed throughout the matrix and consolidated. The thermal characterisation was performed using differential scanning calorimetry, making it possible to identify the different thermal transitions and degree of crystallinity of the polymer. The fracture toughness, an important property in wear due to fatigue, was furthermore characterised by performing three point bending experiments.Finally, by means of a multiaxial pin-on-plate set-up the wear resistance of the materials was analysed.

O novo paradigma de se preservar o meio ambiente e de se utilizar produtos naturais vem contribuindo para um maior interesse na utilização de materiais derivados da biomassa. Neste sentido, os compósitos poliméricos com fibras vegetais surgem como uma boa alternativa no campo dos materiais para aplicações de engenharia. Os principais objetivos deste trabalho foram o processamento e a caracterização do compósito formado por uma resina poliuretano derivada do óleo de mamona e fibras de sisal e coco. O processamento foi realizado utilizando-se a técnica de moldagem por compressão. As fibras foram utilizadas em diferentes formas como fibras curtas, fibras longas, tecido e fios contínuos. A caracterização foi realizada através dos seguintes ensaios: tração, flexão, impacto, tenacidade à fratura, absorção d’água e DMTA. Foi também avaliado o efeito do tratamento com hidróxido de sódio (10%), nas fibras de sisal e coco, nas análises realizadas. O resultados mostraram que o desempenho dos compósitos com fibras de coco foi inferior aos compósitos com fibras de sisal, e mesmo ao poliuretano. Nos ensaios de tração e flexão, as fibras longas de sisal apresentaram o melhor efeito de reforçamento, seguido dos fios contínuos, fibras curtas e tecido. Nos ensaios de impacto e tenacidade à fratura, o melhor desempenho foi dos compósitos com tecido de sisal. O efeito do tratamento alcalino variou em função do tipo de ensaio e da geometria do reforço. Nos ensaios de tração e flexão, o seu efeito foi positivo para os compósitos com fibras longas e curtas, e negativo para os compósitos com tecido e fios, devido à deterioração da estrutura dos fios. Nos ensaios de tenacidade e impacto foi prejudicial, pois ao melhorar a aderência na interface, reduziu os principais mecanismos de absorção de energia, que são, a extração de fibras e o descolamento na interface. Com relação às medidas de absorção d’água, foi observado aumento no nível de absorção dos compósitos com o aumento da fração volumétrica de fibras. O máximo percentual de absorção foi de 17%, para os compósitos com fibras curtas de coco não tratadas. Este percentual diminuiu com o tratamento alcalino das fibras. Na análise térmica dinâmico mecânica, de uma forma geral, os compósitos mostraram acréscimo do módulo de armazenamento e decréscimo do amortecimento e da temperatura de transição vítrea, Tg, em relação ao poliuretano. Este comportamento foi proporcional ao aumento da fração volumétrica de fibras. / The new paradigm in preserving the environment and the use of natural products has contributed to increase the interest in the development and use of derived biomass materials. In this sense, the polymeric composites with natural fibers appear to be a good alternative for engineering applications. The main targets of this work were the processing and characterization of composites obtained by a polyurethane resin derived from castor oil and sisal and coir fibers. The compression moulding technique was used to process the composite. The fibers were employed in different forms such as: short fibers, long fibers, biaxial weave and continuos yarns. Tension, bend, impact, fracture toughness, water absorption and DMTA tests were used to characterize the composites. The sodium hidroxide (10%) treatment effect on the sisal and coir fibers was also evaluated. The results showed, in general, that the coir fibers composites performance were inferior to the sisal fibers composites, and even to the polyurethane matrix. Under tension and bending conditions, the long sisal fibers presented the best reinforcement effect, followed by the continuous yarns, short fibers, and the biaxial weave. Under impact and fracture toughness tests, the best performance was enhanced by sisal weave composites. The alkaline treatment effect varied in accordance with test type and reinforcement geometry. In tension and bending tests, its effect was positive for composites with short and long fibers and negative for composites with weave and yarns. This late was due to deterioration of the yarn structure. In the impact and toughness tests, the alkaline treatment effect was harmful, because when adherence is improved at the interface, the main energy absorption mechanisms are reduced, that mean, the fibers are pulled out and interface is debonded. Water absorption measurements showed an increase in the absorption level for the composites with higher volumetric fraction of fibers. The maximum water absorption was 17% for composites with non-treated coir short fibers. This percentage decreased for composites with treated fibers. In the dynamic mechanical thermal analysis (DMTA) the composites showed an increased storage modulus and a decreased glass transition temperature, Tg, when compared to polyurethane matrix. This behaviour was proportional to the increase of the fibers volumetric fraction.

As cerâmicas a base de carbeto de silício, SiC, tem muitas aplicações na engenharia devido às suas excelentes propriedade mecânicas, térmicas e químicas. Para a produção de cerâmicas de SiC com propriedades específicas, seu processamento deve ser escolhido de maneira a se produzirem microestruturas adequadas para cada aplicação. Para isso, na maioria das vezes, essas cerâmicas são sinterizadas via fase líquida, usando como aditivos formadores dessa fase misturas de óxidos de alumínio e algumas terras raras. Esta dissertação tem dois objetivos principais: implantar o método SEVNB no Departamento de Engenharia de Materiais (DEMAR), na Escola de Engenharia de Lorena (EEL), para medida de KIC, propriedades mecânicas que, muitas vezes, limita aplicações das cerâmicas, e introduzir mais dois óxidos de terras raras para formação da fase líquida na sinterização do SiC, os óxidos de disprósio e itérbio. Após a mistura e secagem das matérias primas, os pós foram prensados com dupla ação de pistões e isostaticamente, e em seguida foram sinterizados em atmosfera de argônio utilizando seis ciclos diferentes para cada sistema de aditivos. Após sinterização as amostras foram caracterizadas e entalhadas com lâmina de barbear utilizando pastas diamantadas de 6 e 0,25 ?m e flexionadas em 4 pontos. Os resultados mostraram uma maior densificação para o sistema de aditivos Al2O3/ Yb2O3, de aproximadamente 97% da densidade teórica, prensados a 400 MPa e sinterizados na temperatura de 1950ºC por 1 hora, com formação de ?-SiC, DyAG e YbAG em todas as temperaturas estudadas. A máquina fabricada para entalhamento das amostras se mostrou eficiente, produzindo raios de entalhes atendendo a norma ISSO 23146:2008. Quanto as propriedades mecânicas, o sistema com itérbia possui os maiores valores de módulo de elasticidade e dureza, porém menor tenacidade à fratura em relação a cada ciclo de sinterização, resultado que sugere a influência do raio catiônico das terras raras formadoras da fase secundária nas cerâmicas a base de SiC. / The silicon carbide, SiC, has many applications in engineering due to its excellent mechanical, thermal and chemical properties. For the production of SiC ceramics with specific properties, the processing should be chosen to produce microstructures suitable for each application. For this reason, in most cases, these ceramics are liquid phase sintered using additives such as formers of this phase mixed oxides of aluminum and certain rare-earths. This dissertation has two main objectives: to implement the method SEVNB at Department of Materials Engineering (DEMAR), Engineering School of Lorena (EEL), to measure KIC, mechanical propertie, which often limits the ceramic\'s applications, and introduce two more rare-earth oxides for the formation of liquid phase sintering SiC, dysprosium and ytterbium oxide . After mixing and drying, the powders were pressed with a double acting piston and isostatically, and then sintered in an argon atmosphere using six different cycles for each additive system. After sintered, samples were characterized and V-notched using a razor blade and diamond pastes of 6 and 0.25 ?m and four-point flexured. The results showed a higher densification for the system Al2O3 / Yb2O3, approximately 97% , pressed at 400 MPa and sintered at a temperature of 1950 ° C for 1 hour, with formation of ?-SiC, DyAG and YbAG at all temperatures studied. The machine used to notched the samples proved to be effective, producing notch root according to the standard ISO 23146:2008. As for the mechanical properties, the system with ytterbium oxide has higher values of elastic modulus and hardness, but lower fracture toughness for each sintering cycle studied, result that suggests the influence of the rare-earth\'s cationic radius that forming the secondary phase in the SiC.

Esse trabalho teve como objetivo analisar a influência da temperatura de revenimento na tenacidade à fratura dinâmica e na tenacidade à fratura assistida pelo ambiente de um aço inoxidável martensítico AISI 420. Para realização do tratamento térmico de revenimento as temperaturas de 380 e 540ºC foram selecionadas, sendo que esta última situa-se dentro da faixa de fragilização ao revenido desse material. Todos os ensaios reproduzidos neste trabalho foram realizados à temperatura ambiente utilizando corpos de prova da mecânica da fratura do tipo SE(B). As análises dos resultados obtidos mostraram uma grande susceptibilidade desse aço à fragilização ao revenido, inclusive com a mudança dos micromecanismos de fratura envolvidos no processo de falha, quando exposto ao ambiente de água do mar sintética sob proteção catódica de - 1100mVECS e a condições dinâmicas de carregamento, onde observou-se, respectivamente, uma redução na ordem de 60% e 30% do valor de tenacidade à fratura do material. / This study aimed to analyze the influence of tempering temperature on dynamic toughness and environment toughness of a stainless steel AISI 420. To carry out this study the tempering temperatures of 380 and 540ºC were selected, with the last one being within the range of the embrittlement of this material. All tests reproduced in this work were performed at room temperature on specimens of the mechanical fracture SE (B). The analysis of the obtained results showed a great susceptibility of this steel to the tempering embrittlement, with the change of fracture micromechanisms involved in the failure process, when exposed to the environment of synthetic sea water under cathodic protection of -1100mVECS and to dynamic conditions of loading, where a reduction in the order of 60% and 30% of the fracture toughness value of the material was observed, respectively.

This thesis investigates the quantity Crack Tip Opening Displacement (CTOD) as a means to assess fracture toughness when measured in the Single Edge Notched Bend (SENB) specimen setup. A particular objective is to assess the effectiveness of the test when used for high strain-hardening materials (e.g. stainless steels). This has been an increasing concern as the current available methods were generally designed for lower strain hardening structural steel. Experimental work on CTOD tests included silicone casting of the crack, and constant displacement tests were also performed. The silicone castings enable physical measurement of the crack under an optical microscope. Results from a series of Finite Element (FE) models were validated from the experiments. δ5 surface measurements were obtained using Digital Image Correlation (DIC) as a courtesy of TWI, which were compared to surface CTOD measurements from the silicone castings. In addition to the experiments and Finite Element modelling, archived test data from TWI was processed, showing analytical differences between current Standard CTOD equations. CTOD calculations from BS 7448, ISO 12135, ASTM E1820 and WES 1108 were compared to the experimental and FE modelling results. For high strain hardening material, CTOD predicted by Standard equations (apart from those in BS 7448 and single point CTOD from ISO 12135) were lower than the values determined from silicone measurements and modelling. This potentially leads to over conservative values to be used in Engineering Critical Assessments (ECA) or material approval. Based on a series of different strain hardening property models, a relationship between strain hardening and the specimen rotational factor, rp was established. An improved equation for the calculation of CTOD is proposed, which gave good estimation of the experimental and Finite Element modelling results. The improved equation will be proposed for future amendments of the ISO 12135 standard. The results of this research enable the accurate fracture characterisation of a range of engineering alloys, with both low and high strain hardening behaviour in both the brittle and ductile fracture regime.