Cette thèse s’effectue au sein de l’université de Bordeaux. Ce projet concerne la construction en bois et en particulier la compréhension des phénomènes à l’origine des fissures dans les structures bois. L’un des objectifs étant de concevoir des méthodes efficaces de renforcement local pour les éléments de structure. Pour cela, l’étude se décompose en plusieurs étapes, la compréhension des phénomènes mis en jeu dans la création des fissures sur des bois de construction. Ce travail s’orientera sur la modélisation de fissure en mode mixte, la recherche de solution de renforcement avec compréhension fine de l’impact de types de renforts sur la propagation de la fissure. Cette étude sera accompagnée d’une campagne d’essais, afin de vérifier l’efficacité des renforcements choisis et d’identifier l’impact de l’effet d’échelle sur les modèle de prédiction. Des essais de grandes dimensions seront donc réalisés pour mieux appréhender les effets de groupes et les effets d’échelle sur du matériau d’emploi. On vise ensuite à définir des outils prédictifs de la résistance des systèmes renforcés et de moyens de contrôles pour les Plan d’Assurance Qualité. / The purpose of reinforcing assemblies and structural elements inwood is to overcome the resistance limits of the material, by transferring greaterefforts in areas which can lead to premature cracking in structures. The reinforcementsused can be made of steel, composite materials or wood. Their hook can bemechanical (screwed bodies) or by adhesion (structural bonding like glued-in rodsfor example). In both cases, the transfer of solicitations remains poorly known, andthe effect of the beginning and the deflection of crack are not well apprehended. Inengineering techniques, the wood resistance in the reinforced area is neglected, whichis in line with the precautionary principle. Currently, the scientific investigations areinterested in the resistance of those kind of techniques without considering the interactionsbetween the quasi-brittle behavior of the wood and the reinforcementswhich govern the gain in mechanical performance. However, these solutions can leadto a failure caused by the progressive splitting of the wood and the anchor loss ofthe reinforcement. So it seems accurate to propose predictions of the short-termstrength for splitting of reinforced and unreinforced beams, which can be used tofurther exploration of the long-term failure mechanism. That is why, in this study, aglobal prediction model of the ultimate strength of structural components subjectedto splitting, reinforced and unreinforced ones, was developed. It considers the quasibrittlebehavior of the wood and crack propagation in mixed mode, using a mixinglaw established on the R-curves. The relevance of this modeling was then comparedto the current dimensioning methods of the Eurocodes 5, for notched beams, withexperimental campaigns conducted at different scales.

Propagation of cracks in functionally graded materials (FGMs) under cyclic loading was investigated via experiments and finite element (FE) analysis. Alumina-epoxy composites with an interpenetrating-network structure and tailored spatial variation in composition were produced via a multi-step infiltration technique. Compressed polyurethane foam was infiltrated with alumina slip. After foam burn-out and sintering, epoxy was infiltrated into the porous alumina body. Non-graded specimens with a range of compositions were produced, and elastic properties and fatigue behaviour were characterised. An increase in crack propagation resistance under cyclic loading was quantified via a novel analytical approach. A simulation platform was developed with the commercial FE package ANSYS. Material gradient was applied via nodal temperature definitions. Stress intensity factors were calculated from nodal displacements near the crack-tip. Deflection criteria were compared and the local symmetry criterion provided the most accurate and efficient predictions. An automated mesh-redefinition algorithm enabled incremental simulation of crack propagation. Effects of gradient and crack-geometry parameters on crack-tip stresses were investigated, along with influences of crack-shape, crack-bridging, residual stresses and plasticity. The model provided predictions and data analysis for experimental specimens. Fatigue cracks in graded specimens deflected due to elastic property mismatch, concordant with FE predictions. In other FGMs, thermal or plastic properties may dominate deflection behaviour. Weaker step-interfaces influenced crack paths in some specimens; otherwise effects of toughness variation and gradient steps on crack path were negligible. Crack shape has an influence, but this is secondary to that of elastic gradient. Cracks in FGM specimens initially experienced increase in fatigue resistance with crack-extension followed by sudden decreases at step-interfaces. Bridging had a notable effect on crack propagation resistance but not on crack path. Similarly, crack paths did not differ between monotonic and cyclic loading, although crack-extension effects did. Recommendations for analysis and optimisation strategies for other FGM systems are given. Experimental characterization of FGMs is important, rather than relying on theoretical models. Opportunities for optimization of graded structures are limited by the properties of the constituent materials and resultant general crack deflection behaviour.

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

Neste trabalho os resultados da metodologia de normalização de E. D. Reese e K. H. Schwalbe para a obtenção de curvas J-R, são comparadas com aquelas obtidas pela tradicional Técnica de Variação de Flexibilidade Elástica. Ambos os métodos foram aplicados à um aço de grau nuclear A508-CL3A, quando vários corpos-de-prova miniaturizados 0.394TC[S], foram testados em temperaturas típicas de operação de vasos de pressão nuclear. Corpos-de-prova miniaturizados formam tratados termicamente tentando simular o comportamento exibido pelos aços estruturais quando submetidos à diferentes níveis de exposição de nêutrons, sendo que outros foram ensaiados na condição de como-recebidos. Todos os procedimentos experimentais e analíticos empregados neste estudo foram totalmente computadorizados, incluindo aquisição e refinamento dos dados e o traçado da curva J-R. Esta última foi realizada conforme a mais recente versão das normas ASTM, denominada ASTM-E1820. As variáveis avaliadas influentes do comportamento das curvas de J-R, incluem o incremento de propagação de trinca e o número de pontos de dados de carga versus deslocamento. Também foi analisada a efetividade do método de normalização quando comparada com a técnica de variação de flexibilidade elástica. Os resultados desta pesquisa mostraram que a normalização é uma metodologia confiável e muito simples, tão logo a programação de software seja desenvolvida, para ser aplicada em uma vasta variação de comportamentos mecânicos como amostrados neste estudo. / In this research, results from Reese; Schwalbe (1993) linear normalization methodology for deriving J-R curves are compared to those obtained from the traditional unloading elastic compliance technique. Both methodologies were applied to a nuclear grade steel A508-CL3A. Several miniaturized 0.394TC[S] specimens, were tested at typical NPV operating temperatures. Small scale testpieces were heat treated aiming to simulate the embritlemment exhibited by the structural steel when submitted to different levels of neutron exposure, whereas other specimens were tested in the as-received condition. All experimental and analytical procedures employed in this study have been fully computerized, including data acquisition, data refinement and J-R curve plotting, when the guidelines supplied by most recent version of ASTM standard for J-R curves, namely ASTM-E1820, were followed. The assessed variables affecting J-R curves behavior comprised crack propagation increment, number of load-Ioad line displacement data points, and it was also analyzed the agreement of the normalization method when compared to the compliance technique. Research results have shown that linear normalization is a trustworthy and very simple methodology to be applied in a vast range of mechanical behavior as sampled in this study, as long as computer programming is employed.

Neste trabalho os resultados da metodologia de normalização de E. D. Reese e K. H. Schwalbe para a obtenção de curvas J-R, são comparadas com aquelas obtidas pela tradicional Técnica de Variação de Flexibilidade Elástica. Ambos os métodos foram aplicados à um aço de grau nuclear A508-CL3A, quando vários corpos-de-prova miniaturizados 0.394TC[S], foram testados em temperaturas típicas de operação de vasos de pressão nuclear. Corpos-de-prova miniaturizados formam tratados termicamente tentando simular o comportamento exibido pelos aços estruturais quando submetidos à diferentes níveis de exposição de nêutrons, sendo que outros foram ensaiados na condição de como-recebidos. Todos os procedimentos experimentais e analíticos empregados neste estudo foram totalmente computadorizados, incluindo aquisição e refinamento dos dados e o traçado da curva J-R. Esta última foi realizada conforme a mais recente versão das normas ASTM, denominada ASTM-E1820. As variáveis avaliadas influentes do comportamento das curvas de J-R, incluem o incremento de propagação de trinca e o número de pontos de dados de carga versus deslocamento. Também foi analisada a efetividade do método de normalização quando comparada com a técnica de variação de flexibilidade elástica. Os resultados desta pesquisa mostraram que a normalização é uma metodologia confiável e muito simples, tão logo a programação de software seja desenvolvida, para ser aplicada em uma vasta variação de comportamentos mecânicos como amostrados neste estudo. / In this research, results from Reese; Schwalbe (1993) linear normalization methodology for deriving J-R curves are compared to those obtained from the traditional unloading elastic compliance technique. Both methodologies were applied to a nuclear grade steel A508-CL3A. Several miniaturized 0.394TC[S] specimens, were tested at typical NPV operating temperatures. Small scale testpieces were heat treated aiming to simulate the embritlemment exhibited by the structural steel when submitted to different levels of neutron exposure, whereas other specimens were tested in the as-received condition. All experimental and analytical procedures employed in this study have been fully computerized, including data acquisition, data refinement and J-R curve plotting, when the guidelines supplied by most recent version of ASTM standard for J-R curves, namely ASTM-E1820, were followed. The assessed variables affecting J-R curves behavior comprised crack propagation increment, number of load-Ioad line displacement data points, and it was also analyzed the agreement of the normalization method when compared to the compliance technique. Research results have shown that linear normalization is a trustworthy and very simple methodology to be applied in a vast range of mechanical behavior as sampled in this study, as long as computer programming is employed.

The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.

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This master´s thesis deals with the evaluation of fracture behavior of ODS steel MA956 at high temperature range. This behavior was tested by using miniaturized CT specimens, on which were performed experiments to measure of ductile crack growth resistance curves (J-R curves). The value of the fracture toughness was determined from these J-R curves. Fracture properties were consequently evaluated by using fractographic analysis of the fracture surfaces. Structural properties of material was identified by hardness measurement and analyzed by metallographic methods. Results of the measurements show drop of the fracture toughness with respect to the increasing temperature.