OBJECTIVES: To test and compare the effects of static and cyclic loading on fracture toughness (K1C) and microhardness of dental restorative CAD/CAM materials. MATERIAL AND METHODS: Five commercially available CAD/CAM restorative materials were included in this study: Lava™ Ultimate Restorative (3M ESPE), IPS Empress® CAD (Ivoclar Vivadent), Enamic® (VITA), IPS e.max® CAD (Ivoclar Vivadent), and CERASMART™ (GC Dental). Polished rectangular bars 4×2×14 mm (n=30) were prepared from mill blocks for each material. Single notch of 0.5-1 mm in depth was made on the center of one length edge. Ten specimens per group for each material were randomly selected for 1) static mode, 2) after 100k cyclic loads, and 3) after 200k cyclic loads. The survival bars after the fatigue test were then subjected to a three-point flexural test. K1C values were determined on ‘single-edge-pre-crack-beams’ (SEPB) method. In addition, random specimens after the flexural test were selected for Vickers microhardness test from each group. Additionally indentation fracture method (IF) was used to determine surface fracture toughness for e.max CAD and Empress CAD. All the results were analyzed via ANOVA with Tukey’s HSD test or least square regression model using JMP Pro 12.0. RESULTS: The mean fracture toughness (K1C) of the material tested in static mode (3.2 MPa.m1/2 for e.max CAD, 2 MPa.m1/2 for Lava Ult, 1.95 MPa.m1/2 for Empress CAD, 1.92 MPa.m1/2 for Enamic, and 1.65 MPa.m1/2 for Cerasmart). The 100k fatigue group (4.02 MPa.m1/2 for e.max CAD, 3.06 MPa.m1/2 for Cerasmart, 2.55 MPa.m1/2 for Lava Ult, 2.01 MPa.m1/2 for Enamic, 1.94 MPa.m1/2 for Empress CAD) The 200k fatigue group (3.14 MPa.m1/2 for Cerasmart, 2.83 MPa.m1/2 for Lava Ult, 2.68 MPa.m1/2 for e.max CAD, 2.01 MPa.m1/2 for Enamic, 1.72 MPa.m1/2 for Empress CAD). While there was a significant difference in the mean fracture toughness (K1C) and (VHN) after fatigue of material tested (p<0.05). CONCLUSION: The CAD/CAM materials tested exhibited a higher K1C values after cyclic loading, along with lower K1C compared to the static group. In addition, K1C values by IF method exhibit lower K1C values after fatigue that was not a good way to test the fracture toughness value. / 2018-09-28T00:00:00Z

Orientador: Vicente Afonso Ventrella / Resumo: Dentre os diversos aços ferramenta disponíveis atualmente, encontra-se o aço VP 100, aço ferramenta de estrutura bainítica empregado no setor de fabricação de moldes e matrizes. A literatura traz diversos estudos acerca desse aço frente à processos de usinagem. Entretanto, poucos estudos tratam do comportamento desse aço frente a utilização, direta ou indireta, de processos de soldagem. Desse modo, esse trabalho analisou a influência na estrutura e, nas propriedades mecânicas (dureza e taxa de desgaste) superficiais, do aço VP 100 quando submetido a um processo de tratamento térmico superficial. O processo de tratamento térmico superficial foi realizado através da execução de sucessivos cordões de solda, autógenos, na superfície do material, por meio de um sistema de laser de estado sólido Nd:YAG na condição pulsado. O aço VP 100 foi inicialmente investigado quanto a presença de isotropia em sua estrutura, visando excluir a direção adotada para tratamento térmico como fator influenciador nos resultados. Com uma estrutura visualmente semelhante em todas as direções e, com valores de dureza médios muito próximos entre direções, a direção adotada foi excluída como fator de influência no processo. Em seguida, realizou-se cordões de solda, autógenos, em seis condições distintas (SG, AR, N, N1, N2 e N3), analisados posteriormente através de ensaios de microdureza Vickers, para determinação da melhor condição para execução do tratamento térmico. As condições N1, N2 e N3 (todas com u... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Among the steels currently available, is steel VP 100, a tool steel used in the mold and matrix manufacturing sector. There are many research on the machining process forward in this steel. However, few studies deal with work from this steel versus a direct or indirect use of welding processes. In this way, this work analyzed the influence on the structure and, under the mechanical conditions (hardness and wear rate), of the VP 100 steel when subjected to a superficial thermal treatment process. The surface heat treatment process was performed through the execution of successive weld beads, autogenous, on the surface of the material by means of a Nd: YAG solid state laser system in pulsed condition. The VP 100 steel was first investigated for the presence of isotropy in its structure, aiming the analysis if the direction took adopted for the heat treatment as an influencing factor in the results. With a visually similar structure in all directions and, with the average hardness values very close between the directions, the adopted direction was excluded as a factor of influence in the process. In the next step, weld beads, autogenous, were produced in six different condition (SG, AR, N1, N2 and N3), posterior analyzed by means of Vickers microhardness tests to determine the best condition for heat treatment. The N1, N2 and N3 conditions (all using nitrogen gas) presented the best hardness results (superficially and in the depth range of 0.30 mm), being chosen for the executio... (Complete abstract click electronic access below) / Doutor

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No. of bitstreams: 1 crespo_gs_dr_ilha.pdf: 4967768 bytes, checksum: c8e5785db6eaf0a8ac2b750452764960 (MD5) Previous issue date: 2018-08-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Dentre os diversos aços ferramenta disponíveis atualmente, encontra-se o aço VP 100, aço ferramenta de estrutura bainítica empregado no setor de fabricação de moldes e matrizes. A literatura traz diversos estudos acerca desse aço frente à processos de usinagem. Entretanto, poucos estudos tratam do comportamento desse aço frente a utilização, direta ou indireta, de processos de soldagem. Desse modo, esse trabalho analisou a influência na estrutura e, nas propriedades mecânicas (dureza e taxa de desgaste) superficiais, do aço VP 100 quando submetido a um processo de tratamento térmico superficial. O processo de tratamento térmico superficial foi realizado através da execução de sucessivos cordões de solda, autógenos, na superfície do material, por meio de um sistema de laser de estado sólido Nd:YAG na condição pulsado. O aço VP 100 foi inicialmente investigado quanto a presença de isotropia em sua estrutura, visando excluir a direção adotada para tratamento térmico como fator influenciador nos resultados. Com uma estrutura visualmente semelhante em todas as direções e, com valores de dureza médios muito próximos entre direções, a direção adotada foi excluída como fator de influência no processo. Em seguida, realizou-se cordões de solda, autógenos, em seis condições distintas (SG, AR, N, N1, N2 e N3), analisados posteriormente através de ensaios de microdureza Vickers, para determinação da melhor condição para execução do tratamento térmico. As condições N1, N2 e N3 (todas com utilização de gás nitrogênio), apresentaram os melhores resultados de dureza (superficialmente e na faixa de profundidade de 0,30 mm), sendo escolhidas para a execução do tratamento térmico. Os corpos de prova tratados termicamente foram analisados através de difratogramas de raios-x e réplica por extração de carbono. Em nenhuma das duas análises houve a detecção da formação de nitretos no metal pós-tratamento. Ainda acerca dos difratogramas de raios-x, os dados obtidos mostraram a mudança da estrutura do material, de bainítica para martensítica, e a redução do tamanho dos cristalitos nas três condições. Os ensaios de taxa de desgaste mostraram melhores resultados para as condições de maior dureza, com melhores resultados na condição N3. Por fim, a condição N3 foi estipulada como a melhor condição, dentre as analisadas, para realização de tratamento térmico superficial no aço VP 100, apresentando além de uma elevação na dureza, uma baixa taxa de desgaste, fatores atribuídos à formação de uma estrutura martensítica, redução do tamanho dos cristalitos e intersticialização de nitrogênio no metal pós-tratamento térmico. / Among the steels currently available, is steel VP 100, a tool steel used in the mold and matrix manufacturing sector. There are many research on the machining process forward in this steel. However, few studies deal with work from this steel versus a direct or indirect use of welding processes. In this way, this work analyzed the influence on the structure and, under the mechanical conditions (hardness and wear rate), of the VP 100 steel when subjected to a superficial thermal treatment process. The surface heat treatment process was performed through the execution of successive weld beads, autogenous, on the surface of the material by means of a Nd: YAG solid state laser system in pulsed condition. The VP 100 steel was first investigated for the presence of isotropy in its structure, aiming the analysis if the direction took adopted for the heat treatment as an influencing factor in the results. With a visually similar structure in all directions and, with the average hardness values very close between the directions, the adopted direction was excluded as a factor of influence in the process. In the next step, weld beads, autogenous, were produced in six different condition (SG, AR, N1, N2 and N3), posterior analyzed by means of Vickers microhardness tests to determine the best condition for heat treatment. The N1, N2 and N3 conditions (all using nitrogen gas) presented the best hardness results (superficially and in the depth range of 0.30 mm), being chosen for the execution of the thermal treatment. The thermally treated specimens were analyzed by x-ray diffraction and replication by carbon extraction. In none of the two analyzes was the detection of nitride formation in the metal after treatment. Still about the x-ray diffractograms, the data obtained showed the change in the structure of the material, from bainitic to martensitic, and the reduction of the size of the crystallites in the three conditions. The wear rate tests showed better results for the best hardness conditions, with better results in the N3 condition. Finally, the condition N3 was stipulated as the best parameters, among the analyzed ones, for the accomplishment of superficial thermal treatment in the steel VP 100, presenting a rise in the hardness besides a low rate of wear, factors attributed to the formation of a martensitic structure, crystallite size reduction and nitrogen interstitialization in the post-heat treatment metal. / CAPES:DS 1517021

Resumo: Neste trabalho foi feita uma análise detalhada dos micromecanismos de iniciação da fratura em amostras entalhadas para cinco aços e duas ligas de alumínio. Com esse objetivo foram obtidas curvas de transição frágil-dúctil e de tenacidade à fratura em função do raio da raiz do entalhe, e realizados ensaios interrompidos antes da fratura da amostra. Análises fractográficas e micrográficas das regiões de iniciação das fraturas foram realizadas por microscopia eletrônica de varredura. A variação dos micromecanismos de iniciação em função da variação da capacidade plástica local na raiz dos entalhes, como induzida pela variação da temperatura de ensaio e pela variação do raio da raiz, foi analisada. De um modo geral, três tipos de micromecanismos de iniciação foram observados. Um frágil, em que a iniciação envolve a nucleação de uma microtrinca à frente do entalhe e sua subsequente propagação instável. Dois dúcteis, um pela ruptura por cisalhamento localizado ao longo de linhas de cisalhamento máximo formadas na raiz do entalhe, e outro pela formação de microcavidades cuja ligação entre si e a ponta do entalhe envolve um processo misto de cisalhamento localizado e de coalescência de microcavidades. Aspectos de modelos que relacionam tenacidade com a microestrutura foram discutidos com base nos micromecanismos observados. / Abstract: In this study a detailed analysis of the micromechanisms of the fracture initiation in notched specimens was made, for five steels and two aluminum alloys. With that purpose brittle/ductile transition and fracture toughness x r1/2 curves were obtained, and interrupted tests before the fracture of the sample were used. Fractographic and micrographic analysis of the fracture initiation areas were accomplished by scanning electron microscopy. The variation of the initiation micromechanisms in function of the variation of the local plastic capacity in the notch root, as induced by the variation of the test temperature and by the variation of the notch root radius, was analyzed. In a general way, three types of initiation micromechanisms were observed. A brittle one, where the initiation involves the microcrack nucleation ahead of the notch and its subsequent unstable propagation. Two ductile, one by localized shear rupture along the maximum shear lines formed in the notch root, and other by microvoids nucleation whose link to each other and the notch tip involves a mixed process of localized shear and microcavoid coalescence. Aspects of relating models of fracture toughness with microstructure were discussed, based in the observed micromechanisms. / Orientador: Fathi Aref Ibrahim Darwish / Coorientador: Luís Rogério de Oliveira Hein / Banca: Valdir Alves Guimarães / Banca: Marcelo dos Santos Pereira / Banca: Luiz Carlos Pereira / Banca: Marcos Venicius Soares Pereira / Doutor

L’objectif de cette thèse est de comprendre les micro-mécanismes physiques de propagation et d’arrêt de fissure de clivage dans l’acier de cuve 16MND5 et de proposer un modèle de prédiction robuste et physiquement fondé, en s’appuyant sur une campagne d’essais de rupture fragile sur éprouvettes de laboratoire finement instrumentées, associée à la modélisation numérique de ces essais. Dans un premier temps, des expériences ont été menées sur des éprouvettes CT25 de différentes épaisseurs à cinq températures (-150°C, -125°C, -100°C, -75°C, -50°C). Des trajets de fissures rectilignes et branchées (deux fissures se développant de manière quasi-symétrique) ont été observés. Pour estimer la vitesse de propagation, une caméra ultra-rapide a été utilisée, associée à la mise au point d’un protocole expérimental permettant d’observer la face de l’éprouvette dans l’enceinte thermique, sans givrage. Des observations à 500 000 images.s-1 ont permis de caractériser finement la vitesse instantanée de la fissure sur le ligament complet de la CT (~25 mm). En parallèle, pour pouvoir analyser les essais et l’impact de la viscosité sur la réponse mécanique autour de la fissure, le comportement élasto-viscoplastique du matériau a été étudié jusqu’à une vitesse de déformation de 104 s-1 pour les températures étudiées. La méthode des éléments finis étendus (X-FEM) a été utilisée dans le code de calcul CAST3M pour modéliser la propagation de fissure. Les simulations numériques associent l’approche locale de la rupture en dynamique non linéaire et un critère de propagation en contrainte critique de type RKR à une distance caractéristique. Les travaux réalisés ont permis de confirmer la forme du critère proposé par Prabel à -125°C, et d’identifier les dépendances de ce critère à la température et à la vitesse de déformation. A partir d’analyses numériques en 2D et 3D, un critère multi-température fonction croissante de la vitesse de déformation est proposé. Des modélisations prédictives ont permis de valider le critère sur deux géométries d’éprouvettes (CT et anneau) en mode I à différentes températures. Des observations MEB et des analyses 3D au microscope optique montrent que le mécanisme de rupture est le clivage associé à des zones de cisaillement ductile entre les différents plans de fissuration. L’étude de la fraction surfacique des marches de cisaillement et des contraintes de fermeture associées tend à justifier le critère mis en place. Un modèle analytique est proposé permettant de justifier le critère déduit des modélisations numériques. Ce modèle considère que les ligaments retiennent la lèvre de la fissure et induisent donc des contraintes de fermeture au niveau de la pointe de fissure qu’il faut compenser pour atteindre la contrainte de clivage effective en pointe de fissure. Cette résistance des ligaments est directement reliée à la loi de comportement du matériau et justifie la dépendance du critère de rupture identifié à la vitesse de déformation. Enfin, les branchements de fissure ont été analysés via le dépouillement des vidéos obtenues avec la caméra rapide qui mettent en évidence un amorçage initial rectiligne, puis un amorçage de fissures multiples de part et d’autre du plan de fissure qui conduisent à l’arrêt de la fissure initiale, l’une de ces fissures ‘secondaires’ conduisant ensuite à la rupture de l’éprouvette. Les rôles essentiels de l’épaisseur et du chargement dans ce mécanisme de branchement sont soulignés. L’augmentation de l’épaisseur réduit la fréquence d’apparition de ce mécanisme et finit même par l’annuler. Logiquement l’intensité du chargement doit être suffisamment importante pour créer cette zone plastique étendue : les essais qui présentent une propagation rectiligne sont les essais pour lesquels les chargements à l’amorçage sont les plus faibles. / The purpose of this PhD thesis is to understand physical mechanisms of cleavage crack propagation and arrest in the 16MND5 PWR vessel steel and to propose a robust predicting model based on a brittle fracture experimental campaign of finely instrumented laboratory specimens associated with numerical computations. First, experiments were carried out on thin CT25 specimens at five temperatures (-150°C, -125°C, - 100°C, -75°C, -50°C). Two kinds of crack path, straight or branching path, have been observed. To characterize crack propagation and to measure crack speed, a high-speed framing camera system was used, combined with the development of an experimental protocol which allowed to observe CT surface without icing inside the thermal chamber and on the specimen. The framing camera (520 000 fps) has allowed to have a very accurate estimation of crack speed on the complete ligament of CT (~ 25 mm). Besides, to analyse experiments and to study the impact of viscosity on the mechanical response around the crack tip, the elastic-viscoplastic behavior of the ferritic steel has been studied up to a strain rate of 104 s-1 for the tested temperatures.The eXtended Finite Element Method (X-FEM) was used in CAST3M FE software to model crack propagation. Numerical computations combine a local non linear dynamic approach with a RKR type fracture stress criterion to a characteristic distance. The work carried out has confirmed the form of the criterion proposed by Prabel at -125°C, and has identified the dependencies of the criterion on temperature and strain rate. From numerical analyzes in 2D and 3D, a multi-temperature fracture stress criterion, increasing function of the strain rate, was proposed. Predictive modeling were used to confirm the identified criterion on two specimen geometries (CT and compressive ring) in mode I at different temperatures. SEM observations and 3D analyzes made with optical microscope showed that the fracture mechanism was the cleavage associated with ductile shear zones between the different planes of cracking. The study of the surface fraction of ductile shear zones and associated closing stress tends to justify the established criterion. An analytical model is proposed to justify the criterion deduced from numerical modeling. This model assumes that the ligaments hold the crack lips and therefore induce closing stress at the crack tip which has to be compensated to achieve the effective cleavage stress at the crack tip. This resistance of ligaments is directly related to the mechanical behavior of the material and justifies the dependence of fracture stress criterion with strain rate. Lastly, the crack branching was analyzed thanks to movies obtained with the high-speed camera. The cleavage crack propagates in a straight way over few millimeters. Then, new cracks appear on both sides of the initial crack lips which leads to the arrest of the initial crack. One of the new cracks leads to the failure of the CT specimen. The essential role of the thickness and loading on this branching mechanism is emphasized. The increased thickness reduces the frequency of occurrence of this mechanism and eventually even cancel. Low thicknesses lead to more extensive plasticity at the crack tip and generates the germs for the triggering of secondary cracks that appear. Logically, the intensity of loading must be large enough to create this extended plastic zone : the experiments with a straight path are the tests for which the initial loadings are the lowest.

In the last decade, carbon nanotubes (CNTs) and Graphene nanoplatelets (GNPs) have attracted a lot of attentions in various polymeric and ceramic composite systems, in an effort to improve their mechanical and functional properties. Al2O3 has attracted considerable interests in ceramics community, in particular as a matrix material for composite fabrications. The high stiffness, excellent thermal stability and chemical resistance of Al2O3 make it practically a very important engineering material, and if we can overcome its brittleness issue, its applications will be much wider. Adding CNTs as a reinforcement to the Al2O3 matrix to improve the toughness is one of the most promising methods. Similarly, GNPs have recently also been shown to be very promising for the same purpose. It has been demonstrated that by adding a mixture of the 2D-GNPs and 1D-CNTs into a polymer matrix, the toughest or strongest man-made ropes have been made. However, the homogenous dispersion of CNTs or GNPs is more of a challenge in a ceramic matrix than in polymeric matrices, owing to the tendency of CNT agglomerations and more steps are needed to completely transfer the useful properties of CNTs and GNPs into ceramics. In this thesis, nanocomposites of Al2O3 reinforced with a hybrid of GNTs (a blend of GNPs and CNTs) were first fabricated. The hybrid GNT reinforcements were mixed with the Al2O3 using a wet chemical technique under ultrasonic treatment. The effects of varied GNT contents on the microstructural features and mechanical properties of the nanocomposites were then investigated. It is found that the well-dispersed GNT fillers resulted in high sintered densities (>99%) in the composites, whilst the fracture mode alteration, grain refinement and improved flexural strength of the composites are all associated with the inclusion of CNTs and GNPs. The average fracture toughness of the nanocomposites reached up to 5.7 MPa·m1/2, against 3.5 MPa·m1/2 of the plain Al2O3, and the flexural strength improved from 360 MPa to 424 MPa respectively, at a hybrid addition of 0.5 wt% GNPs and 1 wt% CNTs. The toughening mechanisms attributed with the unique morphologies and structures of the GNT fillers were also discussed based on analyses on the morphology, grain sizes and fracture mode. The effects of hot-pressing (HP) and spark plasma sintering (SPS) methods on the grain size, microstructural features, and mechanical behaviour of GNT-reinforced Al2O3 nanocomposites were then comprehensively studied. Identical overall reinforcement contents at various GNP/CNT ratios were selected to prepare the composites using both HP and SPS. Highly densified samples (>98%) were obtained at 1650°C under 40 MPa in Ar atmosphere, with dwell times of 1 h and 10 min for HP and SPS respectively. Both types of sample showed a mixture of inter- and trans-granular fracture behaviour. A 50% grain size reduction was observed for samples prepared by HP, compared with the SPS samples. Both types of samples achieved a high flexural strength and fracture toughness of > 400 MPa and 5.5 MPa·m1/2, respectively, whilst the properties of the SPS samples peaked at relatively lower GNT contents than those of the HP samples. Based on analyses of the morphology, grain sizes and fracture mode, similar toughening mechanisms for both types of sample were observed, involving the complex characteristics of the combined GNT fillers. The tribological performance of the HPed pure Al2O3 and its composites containing various hybrid GNT contents was further evaluated under different loading conditions using a ball-on-disc method. Benchmarked against the pure Al2O3, the composite reinforced with a 0.5 wt% GNP exhibited a 23% reduction in the friction coefficient along with a promising 70% wear rate reduction, and a hybrid reinforcement consisting of 0.3 wt.% GNPs + 1 wt.% CNTs resulted in even better performance, with a 86% reduction in the wear rate. The extent of damage to the reinforcement phases caused during wear was studied using Raman spectroscopy. The wear mechanisms for the composites were analysed according to the mechanical properties, brittleness index and microstructural characterization. The combination between GNPs and CNTs contributed to the excellent wear resistance properties for the hybrid GNT-reinforced composites. The GNPs played an important role in the formation of a tribofilm on the worn surface by exfoliation; whereas the CNTs contributed to the improvement in fracture toughness and prevented the grains being pulled out during the tribology test. Finally, Graphene Oxide (GO) was used to replace the GNPs in the hybrid, to prepare Al2O3-GONT nanocomposites, by adopting a new sol-gel processing, in addition to powder mixing. It has been found that sol-gel process leads to an impressive grain size reduction of 62%, the fracture toughness and flexural reached 6.2 MPa·m1/2 and 420 MPa (i.e. 70% and 14% improvement), respectively, than those of pure Al2O3, which even marginally outperformed the previously optimised Al2O3-GNP nanocomposites by 8% in fracture toughness. The success of our new sol-gel strategy opens up new opportunities for choosing hybrid reinforcements for the fabrication of advanced ceramic nanocomposites.

Trees are exposed to many stresses over their lifetime and withstand them due to their woody skeleton which provides excellent mechanical support. Wood has therefore been one of the most used materials throughout the history of humanity. However, the mechanical properties of wood vary considerably depending on wood anatomy and also show significant differences between and within trees. Wood is a cellular solid, characterised by a high degree of anisotropy at all levels of organisation and is formed by cells which are oriented largely in the longitudinal and radial directions, making wood mechanics rather complicated. Therefore, there is a need for an understanding of the mechanical properties of wood in different species and in different parts of the tree and its relationship to wood anatomy. This study began with two investigations into the transverse toughness of green trunk wood in different tree species including both hardwood and conifers. Double-edge notched tensile tests were conducted on the specimens to quantify their specific fracture energies and evaluate their failure fashions. The influence of wood anatomy on the toughening mechanism of wood was observed using both electron microscopy and light microscopy. It was found that the fracture properties of woods were mainly affected by the wood density and anatomy. Hardwoods were found to have higher fracture energies than conifers due to their denser woods and higher volume fraction of rays. The results also found that the specific fracture energies of RL and RT systems were around 1.5-2 times greater than TL and TR systems. This difference was mainly explained by the presence of rays which provided toughness in the radial direction, at least in hardwoods, as breaking across rays resulted in spiral fractures of the cell walls. The mechanical properties of green branches and coppice shoots of three temperate tree species (chestnut, sycamore and ash), were then investigated at three distances from the tip. The study also investigated how bending failure was influenced by the morphology and anatomy of branches and coppice shoots. Coppice shoots were shown to be more likely to buckle in bending, whereas branches failed with a clean fracture. It was shown that ash and sycamore had greater properties in their coppice shoots than their branches, while chestnut showed better properties in their branches. It was suggested that this occurred because increasing the leaf node frequency resulted in a decrease in mechanical properties; ash and sycamore had more leaf nodes in their branches, thus lower properties in their branches, while chestnut had more leaf nodes in its coppices. The mechanical properties also decreased from base to tips of branches and coppice shoots because of falls in diameter of shoots and wood density. The results also suggested why coppice shoots can act as a useful structural material. Finally, this thesis investigated how and why the fracture properties vary around the structure of tree forks. The fracture properties of green hazel forks were examined using double-edge notched tensile tests in the RT and TR directions. The fracture surfaces were also observed using scanning electron microscopy in both fracture systems. The results showed that the central apex of forks were considerably tougher than other locations, suggesting they provide the load-bearing capacity of tree forks. It was shown that the increased toughness was related to both higher wood density and an interlocking wood grain pattern. Interestingly, the TR fracture system was found to be tougher than the RT fracture system at the central apex of forks, probably related to the orientation of the fibres. These results provide insight into the relationship between wood mechanics and anatomy, particularly showing the importance of rays. They can also help us understand how our ancestors shaped wood and designed tools and how we could design better structures.

Carbon capture and storage (CCTS) is a transitional technology offering a nearterm method of mitigating climate change. Pipelines are considered to be the most suitable systems for CCTS; however, structural integrity of pipeline has to be guaranteed in order for this technology to become a practical technical solution. The investigation detailed here is based on a systematic experimental approach to investigate the structural integrity of API X100, X60 and X70 steels exposed in simulated flue-gas under dynamic conditions. A core of the structured experiments through some methods such as aging test, tensile properties, fracture toughness, residual stress and engineering critical assessment was accomplished in parent material and exposed samples on flue-gas. The temperature range of evaluation for tensile test covers -70C to 21C while fracture toughness was over the range -196C to 21C. Tensile properties of virgin material show that steels meet standard specification while aging samples do not show significant scatter compared with parent steels. Ovalisation of the fracture surface and splitting phenomenon was observed which is related with steel anisotropy. Fracture toughness obtained from experiment was compared with that calculate by two existing correlations. However both correlations did not predict the level of fracture toughness expected indicating the methods used in this work has limited applicability under the test conditions used here. Residual stress (RS) induced in API X100 steel by cold rolling method was characterised using two complementary techniques known as Neutron Diffraction (ND) and Incremental Hole Drilling (IHD). The RS distribution shows good agreement for both techniques used but reproducibility of them depends on their own inaccuracies. An Engineering Criticality Assessment (ECA) was performed based in Failure Assessment Diagram (FAD) approach using all the experimental data obtained by a leak-before-break method under three operational pressures. The results showed the effect on the integrity of material under the presence of a flaw length assessed. Overall, the thesis presents a combined engineering critical assessment which involved the examination of materials used to transport flue-gas and established a methodology to determine fracture toughness alongside with the FAD to assess the integrity of pipelines.

To fully exploit the strengths of La2Zr2O7 pyroclores and promote them as a next-generation thermal barrier coating (TBC), the improvements of their thermally insulating property and fracture toughness are studied in this thesis. A strong phonon scattering source, rattlers, is found in Y3+-doped La2Zr2O7 pyrochlores. Rattlers dramatically flatten k (thermal conductivity)-T curves, or even make k approach the amorphous limit. The presence of rattlers is strongly dependent on (1) oversized atomic cages that are formed in pyrochlores; and (2) the occupation of smaller guest ions in those oversized cages. To maximize the rattling effect, In3+/Sc3+ ions that are much smaller than Y3+ are introduced to the La2Zr2O7 lattice. As envisaged, the smaller ions in the oversized lattice voids make k glass-like at a much lower doping content. Nevertheless, they are still not effective in reducing the high temperature plateau kmin. Instead, oxygen vacancies are very effective in reducing kmin, because they generate an electrostatic repulsion force among cations surrounding them, resulting in stronger lattice anharmonicity and weaker bonds. The plateau kmin is reduced dramatically by the filling of the B-sites in La2Zr2O7 with a 21% larger (and 50% heavier) Ce4+ guest ion rather than a 96% heavier (but similar-sized) Hf4+ ion, suggesting that a large absolute size of substitutional atoms is more effective in reducing kmin than a heavy absolute mass. This is because: (1) kmin is proportional to (E/M)0.5 (where E is the elastic modulus and M is the average atomic mass); (2) a larger size of guest ions tends to produce a weaker ionic bond and consequently, a lower E; and (3) the changing extent of E by introducing larger guest ions is much greater than that of M induced by adding heavier ones. Lastly, the fracture toughness (KIc) has been increased by dispersing the tetragonal 3 mol% Y2O3-stabilized zirconia (t-3YSZ) particulates in the La2Zr2O7 (LZ) matrix. The tendency of the dispersive t-3YSZ second phases transforming to monoclinic (m) phases strongly depends on the volume fraction introduced. For samples made from equilibrium route, they are toughened by phase transformations within the dispersive t-3YSZ second phases and a crack shielding effect arising from the residual compressive stress within the LZ matrix. An anticipated increase of KIc from ferroelastic toughening together with the residual compressive stress toughening highlights a potential to improve coating durability by depositing t’-3YSZ/LZ composite TBCs by the non-equilibrium route.

A madeira apresenta diferentes comportamentos, dependendo das solicitações às quais está sujeita. Em geral, a madeira pode resistir a solicitações de maior magnitude se a ação ocorre num curtíssimo intervalo de tempo. A propriedade mecânica envolvida nestes casos é a tenacidade, cujos valores são determinados em ensaios de impacto na flexão. Diversas aplicações relacionadas à madeira dependem da mencionada propriedade, não muito estudada para as madeiras usualmente empregadas no Brasil. Com o intuito de preencher tais lacunas o presente trabalho foi realizado para avaliar a influência da umidade e da orientação dos anéis de crescimento da madeira no valor da sua tenacidade. Para isso foram utilizadas as espécies Angelim Saia (Vatairea sp), Eucalipto Citriodora (Eucalyptus citriodora), Eucalipto Grandis (Eucalyptus grandis), Pinus Elliottii (Pinus elliottii), com corpos-de-prova convenientemente amostrados e climatizados. Além disso foram quantificadas as relações entre: a tenacidade e a densidade da madeira a 12% de umidade; a tenacidade e a resistência da madeira à compressão paralela às fibras; a tenacidade e a resistência convencional da madeira no ensaio de flexão estática. Para isto foram considerados os resultados obtidos para trinta e seis espécies tropicais brasileiras, sendo no mínimo doze amostras por espécie. / Wood often exhibits different behavior, depending on the kind of acting forces. Usually, wood can resist to greater solicitations if short time intervals are considered. The mechanical property involved in these situations is toughness, which values are determined in impact bending tests. Several basic uses of timber depend on the toughness, property not widely studied to tropical species grown in Brazil. This work, which was conducted to improve this situation, aimed to evaluate the influence of moisture content and grain angle in values of toughness. In order to reach these objects, species like Angelim Saia (Vatairea sp), Eucalipto Citriodora (Eucalyptus citriodora), Eucalipto Grandis (Eucalyptus grandis), Pinus Elliottii (Pinus elliottii), were employed, with well-sampled and climatized specimens. Besides, relationship between toughness and wood density at 12% of moisture, toughness and wood strength at compression parallel to the grain, toughness and modulus of rupture in static bending test were determined. To reach these subjects, results obtained for thirty six tropical species grown in Brazil, no less than twelve especimens for specie, were considered.