In recent years, use of composites is increasing in most fields of engineering such as aerospace, automotive, civil construction, marine, prosthetics, etc., because of its light weight, very high specific strength and stiffness, corrosion resistance, high thermal resistance etc. It can be seen that the specific strength of fibers are many orders more compared to metals. Thus, laminated fiber reinforced plastics have emerged to be attractive materials for many engineering applications. Though the uses of composites are enormous, there is always an element of fuzziness in the design of composites. Composite structures are required to be designed to resist high stresses. For this, one requires a reliable failure criterion. The anisotropic behaviour of composites makes it very difficult to formulate failure criteria and experimentally verify it, which require one to perform necessary bi-axial tests and plot the failure envelopes. Failure criteria are usually based on certain assumption, which are some times questionable. This is because, the failure process in composites is quite complex. The failure in a composite is normally based on initiating failure mechanisms such as fiber breaks, fiber compressive failure, matrix cracks, matrix crushing, delamination, disbonds or a combination of these. The initiating failure mechanism is the one, which is/are responsible for initiating failure in a laminated composites. Initiating failure mechanisms are generally dependant on the type of loading, geometry, material properties, condition of manufacture, boundary conditions, weather conditions etc. Since, composite materials exhibit directional properties, their applications and failure conditions should be properly examined and in addition to this, robust computational tools have to be exploited for the design of structural components for efficient utilisation of these materials. Design of structural components requires reliable failure criteria for the safe design of the components. Several failure criteria are available for the design of composite laminates. None of the available anisotropic strength criteria represents observed results sufficiently accurate to be employed confidently by itself in design. Most of the failure criteria are validated based on the available uniaxial test data, whereas, in practical situations, laminates are subjected to at least biaxial states of stresses. Since, the generation of biaxial test data are very difficult and time consuming to obtain, it is indeed a necessity to develop computational tools for modelling the biaxial behavior of the composite laminates. Understanding of the initiating failure mechanisms and the development of reliable failure criteria is an essential prerequisite for effective utilization of composite materials. Most of the failure criteria, considers the uniaxial test data with constant shear stress to develop failure envelopes, but in reality, structures are subjected to biaxial normal stresses as well as shear stresses. Hence, one can develop different failure envelopes depending upon the percentage of the shear stress content. As mentioned earlier, safe design of the composite structural components require reliable failure criterion. Currently two broad approaches, namely, (1) Damage Tolerance Based Design and (2)Failure Criteria Based Design are in use for the design of laminated structures in aerospace industry. Both approaches have some limitations. The damage tolerance based design suffers from a lack of proper definition of damage and the inability of analytical tools to handle realistic damage. The failure criteria based design, although relatively, more attractive in view of the simplicity, it forces the designer to use unverified design points in stress space, resulting in unpredictable failure conditions. Generally, failure envelopes are constructed using 4 or 5 experimental constants. In this type of approach, small experimental errors in these constants lead to large shift in the failure boundaries raising doubts about the reliability of the boundary in some segments. Further, they contain segments which have no experimental support and so can lead to either conservative or nonconservative designs. Conservative design leads to extra weight, a situation not acceptable in aerospace industry. Whereas, a nonconservative design, is obviously prohibitive, as it implies failure. Hence, both the damage tolerance based design and failure criteria based design have limitations. A new method, which combines the advantages of both the approaches is desirable. This issue is also thoroughly debated in many international conference on composites. Several pioneers in the composite industry indicated the need for further research work in the development of reliable failure criteria. Hence, this is motivated to carry out research work for the development of new failure criterion for the design of composite structures. Several experts meetings held world wide towards the assessment of existing failure theories and computer codes for the design of composite structures. One such meeting is the experts meeting held at United Kingdom in 1991.This meeting held at St. Albans(UK) on ’Failure of Polymeric Composites and Structures: Mechanisms and Criteria for the Prediction of Performance’, in 1991 by UK Science & Engineering Council and UK Institute of Mechanical Engineers. After thorough deliberations it was concluded that 1. There is no universal definition of failure of composites. 2. There is little or lack of faith in the failure criteria that are in current use and 3. There is a need to carry out World Wide Failure Exercise(WWFE) Based on the experts suggestions, Hinton and Soden initiated the WWFE in consultation with Prof.Bryan Harris (Editor, Journal of Composite Science and Tech-nology)to have a program to get comparative assessment of existing failure criteria and codes with following aims 1. Establish the current level of maturity of theories for predicting the failure response of fiber reinforced plastic(FRP)laminates. 2. Closing the knowledge gap between theoreticians and design practitioners in this field. 3. Stimulating the composites’ community into providing design engineers with more robust and accurate failure prediction methods, and the confidence to use them. The organisers invited pioneers in the composite industry for the program of WWFE. Among the pioneer in the composite industry Professor Hashin declined to participate in the program and had written a letter to the organisers saying that, My only work in this subject relates to failure criteria of unidirectional fiber composites, not to laminates. I do not believe that even the most complete information about failure of single plies is sufficient to predict the failure of a laminate, consisting of such plies. A laminate is a structure which undergoes a complex damage process (mostly of cracking) until it finally fails. The analysis of such a process is a prerequisite for failure analysis. ”While significant advances have been made in this direction we have not yet arrived at the practical goal of failure prediction”. Another important conference held in France in 1999, Composites for the next Millennium (Proceedingof Symposium in honor of S.W.Tsaion his 70th Birth Day Torus, France, July 2-3, 1999, pp.19.) also concludedon similar line to the meeting held at UK in 1991. Paul A Lagace and S. Mark Spearing, have pointed out that, by referring to the article on ’Predicting Failure in Composite Laminates: the background to the exercise’, by M.J.Hinton & P.D.Soden, Composites Science and Technology, Vol.58, No.7(1998), pp.1005. ”After Over thirty years of work ’The’ composite failure criterion is still an elusive entity”. Numerous researchers have produced dozens of approaches. Hundreds of papers, manuscripts and reports were written and presentations made to address the latest thoughts, add data to accumulated knowledge bases and continue the scholarly debate. Thus, the out come of these experts meeting, is that, there is a need to develop new failure theories and due to complexities associated with experimentation, especially getting bi-axial data, computational methods are the only viable alternative. Currently, biaxial data on composites is very limited as the biaxial testing of laminates is very difficult and standardization of biaxial data is yet to be done. All these experts comments and suggestions motivated us to carry out research work towards the development of new failure criterion called ’Failure Mechanism Based Failure Criterion’ based on initiating failure mechanisms. The objectives of the thesis are 1. Identification of the failure mechanism based failure criteria for the specific initiating failure mechanism and to assign the specific failure criteria for specific initiating failure mechanism, 2. Use of the ’failure mechanism based design’ method for composite pressurant tanks and to evaluate it, by comparing it with some of the standard ’failure criteria’ based designs from the point of view of overall weight of the pressurant tank, 3. Development of new failure criterion called ’Failure Mechanism Based Failure Criterion’ without shear stress content and the corresponding failure envelope, 4. Development of different failure envelopes with the effect of shear stress depending upon the percentage of shear stress content and 5. Design of composite laminates with the Failure Mechanism Based Failure Criterion using optimization techniques such as Genetic Algorithms(GA) and Vector Evaluated Particle Swarm Optimization(VEPSO) and the comparison of design with other failure criteria such as Tsai-Wu and Maximum Stress failure criteria. The following paragraphs describe about the achievement of these objectives. In chapter 2, a rectangular panel subjected to boundary displacements is used as an example to illustrate the concept of failure mechanism based design. Composite Laminates are generally designed using a failure criteria, based on a set of standard experimental strength values. Failure of composite laminates involves different failure mechanisms depending upon the stress state and so different failure mechanisms become dominant at different points on the failure envelope. Use of a single failure criteria, as is normally done in designing laminates, is unlikely to be satisfactory for all combination of stresses. As an alternate use of a simple failure criteria to identify the dominant failure mechanism and the design of the laminate using appropriate failure mechanism based criteria is suggested in this thesis. A complete 3-D stress analysis has been carried out using a general purpose NISA Finite Element Software. Comparison of results using standard failure criteria such as Maximum Stress, Maximum Strain, Tsai-Wu, Yamada-Sun, Maximum Fiber Strain, Grumman, O’brien, & Lagace, indicate substantial differences in predicting the first ply failure. Results for Failure Load Factors, based on the failure mechanism based approach are included. Identification of the failure mechanism at highly stressed regions and the design of the component, to withstand an artificial defect, representative this failure mechanism, provides a realistic approach to achieve necessary strength without adding unnecessary weight to the structure. It is indicated that the failure mechanism based design approach offers a reliable way of assessing critically stressed regions to eliminate the uncertainties associated with the failure criteria. In chapter 3, the failure mechanism based design approach has been applied to a composite pressurant tanks of upper stages of launch vehicles and propulsion systems of space crafts. The problem is studied using the failure mechanism based design approach, by introducing an artificial matrix crack representative of the initiating failure mechanism in the highly stressed regions and the strain energy release rate (SERR) are calculated. The total SERR value is obtained as 3330.23 J/m2, which is very high compared to the Gc(135 J/m2) value, which means the crack will grow further. The failure load fraction at which the crack has a tendency to grow is estimated to be 0.04054.Results indicates that there are significant differences in the failure load fraction for different failure criteria.Comparison with Failure Mechanism Based Criterion (FMBC) clearly indicates matrix cracks occur at loads much below the design load yet fibers are able to carrythe design load. In chapter 4, a Failure Mechanism Based Failure Criterion(FMBFC)has been proposed for the development of failure envelope for unidirectional composite plies. A representative volume element of the laminate under local loading is micromechanically modelled to predict the experimentally determined strengths and this model is then used to predict points on the failure envelope in the neighborhood of the experimental points. The NISA finite element software has been used to determine the stresses in the representative volume element. From these micro-stresses, the strength of the lamina is predicted. A correction factor is used to match the prediction of the present model with the experimentally determined strength so that, the model can be expected to provide accurate prediction of the strength in the neighborhood of the experimental points. A procedure for the construction of the failure envelope in the stress space has been outlined and the results are compared with the some of the standard and widely used failure criteria in the composite industry. Comparison of results with the Tsai-Wu failure criterion shows that there are significant differences, particularly in the third quadrant, when the ply is under bi-axial compressive loading. Comparison with maximum stress criterion indicates better correlation. The present failure mechanism based failure criterion approach opens a new possibility of constructing reliable failure envelopes for bi-axial loading applications, using the standard uniaxialtest data. In chapter 5, the new failure criterion for laminated composites developed based on initiating failure mechanism as mentioned in chapter 4 for without shear stress condition is extended to obtain the failure envelopes with the shear stress condition. The approach is based on Micromechanical analysis of composites, wherein a representative volume consists of a fiber surrounded by matrix in appropriate volume fraction and modeled using 3-D finite elements to predict the strengths.In this chapter, different failure envelopes are developed by varying shear stress say from 0% of shear strength to 100% of shear strength in steps of 25% of shear strength. Results obtained from this approach are compared with Tsai-Wu and Maximum stress failure criteria. The results show that the predicted strengths match more closely with maximum stress criterion. Hence, it can be concluded that influence of shear stress on the failure of the lamina is of little consequence as far as the prediction of strengths in laminates. In chapter 6, the failure mechanism based failure criterion, developed by the authors is used for the design optimization of the laminates and the percentage savings in total weight of the laminate is presented. The design optimization of composite laminates are performed using Genetic Algorithms. The genetic algorithm is one of the robust tools available for the optimum design of composite laminates. The genetic algorithms employ techniques originated from biology and dependon the application of Darwin’s principle of survival of the fittest. When a population of biological creatures is permitted to evolve over generations, individual characteristics that are beneficial for survival have a tendency to be passed on to future generations, since individuals carrying them get more chances to breed. In biological populations, these characteristics are stored in chromosomal strings. The mechanics of natural genetics is derived from operations that result in arranged yet randomized exchange of genetic information between the chromosomal strings of the reproducing parents and consists of reproduction, cross over, mutation, and inversion of the chromosomal strings. Here, optimization of the weight of the composite laminates for given loading and material properties is considered. The genetic algorithms have the capability of selecting choice of orientation, thickness of single ply, number of plies and stacking sequence of the layers. In this chapter, minimum weight design of composite laminates is presented using the Failure Mechanism Based(FMB), Maximum Stress and Tsai-Wu failure criteria. The objective is to demonstrate the effectiveness of the newly proposed FMB Failure Criterion(FMBFC) in composite design. The FMBFC considers different failure mechanisms such as fiber breaks, matrix cracks, fiber compressive failure, and matrix crushing which are relevant for different loadin gconditions. FMB and Maximum Stress failure criteria predicts byupto 43 percent savings in weight of the laminates compared to Tsai-Wu failure criterion in some quadrants of the failure envelope. The Tsai-Wu failure criterion over predicts the weight of the laminate by up to 86 percent in the third quadrant of the failure envelope compared to FMB and Maximum Stress failure criteria, when the laminate is subjected to biaxial compressive loading. It is found that the FMB and Maximum Stress failure criteria give comparable weight estimates. The FMBFC can be considered for use in the strength design of composite structures. In chapter 7, Particle swarm optimization is used for design optimization of composite laminates. Particle swarm optimization(PSO)is a novel meta-heuristic inspired by the flocking behaviour of birds. The application of PSO to composite design optimization problems has not yet been extensively explored. Composite laminate optimization typically consists in determining the number of layers, stacking sequence and thickness of ply that gives the desired properties. This chapter details the use of Vector Evaluated Particle Swarm Optimization(VEPSO) algorithm, a multi-objective variant of PSO for composite laminate design optimization. VEPSO is a modern coevolutionary algorithm which employs multiple swarms to handle the multiple objectives and the information migration between these swarms ensures that a global optimum solution is reached. The current problem has been formulated as a classical multi-objective optimization problem, with objectives of minimizing weight of the component for a required strength and minimizing the totalcost incurred, such that the component does not fail. In this chapter, an optimum configuration for a multi-layered unidirectional carbon/epoxy laminate is determined using VEPSO. The results are presented for various loading configurations of the composite structures. The VEPSO predicts the same minimum weight optimization and percentage savings in weight of the laminate when compared to GA for all loading conditions.There is small difference in results predicted by VEPSO and GA for some loading and stacking sequence configurations, which is mainly due to random selection of swarm particles and generation of populations re-spectively.The difference can be prevented by running the same programme repeatedly. The Thesis is concluded by highlighting the future scope of several potential applications based on the developments reported in the thesis.

[pt] A produção de petróleo em alta vazão a partir de reservatórios formados por arenitos friáveis requer a instalação de sistemas de contenção de sólidos para preservar equipamentos de superfície e subsuperfície. Os projetos de explotação para campos constituídos por estes reservatórios têm na completação uma etapa fundamental na construção do poço. Dentre as diversas operações de completação, a instalação de sistemas de contenção de sólidos é uma das mais complexas e envolve uma ampla gama de recursos humanos e financeiros. A alteração no estado de tensões atuante sobre a formação é uma das principais fontes de carregamento dos sistemas de contenção mecânica de sólidos instalados em poços horizontais. O objetivo deste trabalho é desenvolver um modelo para avaliação do desempenho de sistemas de contenção de sólidos do tipo gravel pack quando submetidos aos esforços relacionados ao comportamento geomecânico das formações produtoras e a variação de pressões durante a vida produtiva de um poço de petróleo, permitindo a otimização de projetos destes sistemas sob a ótica da resistência ao colapso das telas. O carregamento imposto sobre estes sistemas é avaliado através da implementação do modelo de Mohr Coulomb solucionado numericamente através do método de elementos finitos (MEF). O programa comercial ABAQUS™ é utilizado em função da sua flexibilidade para solução de modelos não-lineares. Foram analisados sistemas de contenção de areia com os conjuntos de telas tipicamente utilizados na indústria de petróleo. Em nenhum cenário analisado foram verificados indícios de colapso dos tubos indicando a possibilidade de redução da sua resistência mecânica. / [en] Global increase in energy demand and the lack of opportunities on shore or in shallow waters are driving production of hydrocarbons towards deep and ultra deepwater basins, where reservoirs are usually formed by weak and unconsolidated sandstones that require sand control methods to prevent damage in surface and subsurface equipments. Guidelines to select sand control systems are primarily based on sand exclusion, seeking to optimize balance between oil rate and fines production. Another aspect, often overlooked, is collapse strength of the system formed by the sand control equipment and the formation itself, subjected to mechanical loadings that change during life of the well. This contribution presents a method to evaluate collapse strength of sand control systems taking into account mechanical interaction between the formation and sand control screens. Elastoplastic models are used to represent granular materials. Three sand control systems were studied: gravel pack with premium screens, stand alone premium screens and pre-packed screens. A model to describe contact between granular materials (gravel and formation) and soil-pipe interaction is proposed. Results demonstrate that perforated base pipes used in premium screens may be oversized for applications under regular operating conditions.

[pt] Este trabalho visa simular situações que levam uma rocha a ruptura considerando o fluxo de fluidos, em uma tentativa de correlacionar os mecanismos envolvidos na ruptura ao processo conhecido como produção de areia em rochas reservatório, ou seja, a produção de sólidos durante a extração de hidrocarbonetos. Há algum tempo este assunto tem sido tratado do ponto de vista da mecânica das rochas dada a similaridade do seu comportamento com outros fenômenos estudados nesta área. Como se trata de um processo de extração de fluidos, a sua influência deve ser considerada no comportamento mecânico da rocha. Assim, simulações numéricas, representativas de situações favoráveis a este fenômeno, foram analisadas, utilizando-se, para tal, o programa comercial de elementos finitos ABAQUS, que permite a simulação do processo de acoplamento fluido mecânico. Dada a flexibilidade demonstrada pelo pacote numérico, possibilitou-se a implementação de um modelo elastoplástico e a verificação de sua eficiência na representação do comportamento da rocha reservatório neste processo. A implementação numérica baseou-se no conhecido modelo de Lade-Kim. Este modelo é apropriado à descrição do comportamento de materiais geomecânicos granulares. Trabalhos experimentais em paralelo procuram determinar os parâmetros do modelo de Lade-Kim. Estes parâmetros são usados na análise numérica. / [en] This work aims to simulate situations that take a rock the rupture considering the fluid flow, in an attempt to correlate the involved mechanisms in the rupture to the know process as sand production in rocks reservoir, that is, solid production during hydrocarbon extraction. Numerical simulations using the commercial finite element program ABAQUS, reproduce fluid-mechanical coupling, and allowing the implementation of an elastoplastic rock material behavior. More precisely, the author implemented a numerical routine based on the well-known Lade-Kim elastoplastic model, which can suitably describe the behavior of geomaterials subjected to external stress-strain fields.

The examiner of cloud computing systems in the last few years observes that there is a trend of the emergence of new Big Data frameworks every single year. Since Hadoop was developed in 2007, new frameworks followed it such as Spark, Storm, Heron, Apex, Flink, Samza, Kafka ... etc. Each framework is developed in a certain way to target and achieve certain objectives better than other frameworks do. However, there are few common functionalities and aspects that are shared between these frameworks. One vital aspect all these frameworks strive to achieve is better reliability and faster recovery time in case of failures. Despite all the advances in making datacenters dependable, failures actually still happen. This is particularly onerous for long-running “big data” applications, where partial failures can lead to significant losses and lengthy recomputations. This is also crucial for streaming systems where events are processed and monitored online in real time, and any delay in data delivery will cause a major inconvenience to the users.<div>Another observation is that some reliability implementations are redundant between different frameworks. Big data processing frameworks like Hadoop MapReduce include fault tolerance mechanisms, but these are commonly targeted at specific system/failure models, and are often redundant between frameworks. Encapsulating these implementations into one layer and making it shared between different applications will benefit more than one frame-work without the burden of re-implementing the same reliability approach in each single framework.<br></div><div>These observations motivated us to solve the problem by presenting two systems: Guardian and Warden. Guardian is tailored towards batch processing big data systems while Warden is targeted towards stream processing systems. Both systems are robust, RMS based, generic, multi-framework, flexible, customizable, low overhead systems that allow their users to run their applications with individually configurable fault tolerance granularity and degree, with only minor changes to their implementation.<br></div><div>Most reliability approaches carry out one rigid fault tolerance technique targeted towards one system at a time. It is more challenging to provide a reliability approach that is pluggable in multiple Big Data frameworks at a time and can achieve low overheads comparable with single targeted framework approaches, yet is flexible and customizable by its users to make it tailored towards their objectives. The genericity is attained by providing an interface that can be used in different applications from different frameworks in any part of the application code. The low overhead is achieved by providing faster application finish times with and without failures. The customizability is fulfilled by providing the users the options to choose between two fault tolerance guarantees (Crash Failures / Byzantine Failures) and, in case of streaming systems; it is combined with two delivery semantics (Exactly Once / At Most Once).<br></div><div>In other words, this thesis proposes the paradigm of dependable resources: big data processing frameworks are typically built on top of resource management systems (RMSs),and proposing fault tolerance support at the level of such an RMS yields generic fault tolerance mechanisms, which can be provided with low overhead by leveraging constraints on resources.<br></div><div>To the best of our knowledge, such approach was never tried on multiple big data batch processing and streaming frameworks before.<br></div><div>We demonstrate the benefits of Guardian by evaluating some batch processing frame-works such as Hadoop, Tez, Spark and Pig on a prototype of Guardian running on Amazon-EC2, improving completion time by around 68% in the presence of failures, while maintaining around 6% overhead. We’ve also built a prototype of Warden on the Flink and Samza (with Kafka) streaming frameworks. Our evaluations on Warden highlight the effectiveness of our approach in the presence of failures and without failures compared to other fault tolerance techniques (such as checkpointing)<br></div>

<p dir="ltr">The process of metal additive manufacturing is characterized by the layer-by-layer construction of components, where each individual layer may be subjected to distinct thermal variations, resulting in differences in cooling rates and thermal gradients. These variations can impact the microstructure and, subsequently, mechanical properties of the final product, especially as the height of the build increases. In the present investigation, an evaluation was undertaken to ascertain the impact of build height on the structural integrity of Ti-6Al-4V samples produced using the laser powder bed fusion (LPBF) technique. The study encompassed a comprehensive examination of microstructural features, the microhardness measurement, as well as an evaluation of defect characteristics including size, location, and distribution, with respect to the build height. Tensile and fatigue tests were conducted to elucidate the potential dependence of fatigue and tensile failures on the build height. Two groups of specimens were fabricated: the first, underwent continuous fabrication, while the second involved a pause at the half height, with the process resuming after a 24-hour interval. The results of this investigation unveiled a discernible influence of the height of the build on the structural integrity of components under cyclic loading. Most fatigue specimens were observed to exhibit failure in the upper portion of the gage section with respect to the build direction. Analyses of microstructure revealed a consistent grain morphology in alignment with the build direction, and a uniform distribution of hardness throughout the build height was noted. However, for the specimens in the first group, more process-induced defects were detected within the top half of the gage section in comparison to the bottom half, while there was no noticeable difference in the distribution of defects in the second group. The results suggest that in LPBF process, as the build height is increased, there is a higher likelihood of process-induced defect formation, ultimately resulting in a reduction in structural integrity at greater build heights.</p>

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24847号 / 工博第5164号 / 新制||工||1986(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 杉浦, 邦征, 教授 高橋, 良和, 教授 山本, 貴士, 教授 北根, 安雄 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Knots are usually regarded as defects when grading lumber. In order to evaluate a member under out-of-plane loading, shear strength is one of the major mechanical properties, specifically, rolling shear (RS) strength is one of the critical mechanical properties of Cross-Laminated Timber (CLT), which determines the flexural strength of CLT under short-span bending loads. Lower grade lumber with a higher percentage of knots is recommended to be utilized for the cross-layer laminations which are mainly responsible for resisting shear stresses. Firstly, shear tests were performed in order to evaluate the effect of knots on longitudinal shear strength using shear blocks. After that, the effect of knots on the RS strength of 3-ply southern yellow pine CLT were investigated by experimental tests and an analytical model. Center-point bending tests with a span-to-depth ratio of 6 and two-plate shear tests with a loading angle of 14° were conducted on six CLT configurations composed of different types of cross layer laminations: clear flatsawn lumber with/without pith, lumber with sound knots with/without pith, and lumber with decayed knots with/without pith. The shear analogy method was implemented to evaluate the RS strength values from the bending test results, which were also compared against the results from the two-plate shear tests. It was found that: (1) The shear blocks containing sound knots had higher shear strength than matched clear shear blocks, the shear blocks containing unsound knots had lower shear strength than the matched clear shear blocks. (2) CLT specimens with cross-layer laminations with either sound knots or decayed knots had higher RS strength. (3) In general, the shear analogy method underestimated the RS strength of CLT specimens containing knots and pith.

<p> The use of AA 5754 Al-Mg alloy for automotive applications is limited by its rapid shear failure process, due to shear banding. This failure mechanism is further complicated by the presence of inhomogeneous plastic deformation, so-called Portevin-Le Chatelier (PLC) effect, during deformation. Therefore, the purpose of this study was primarily to investigate the impact of Portevin-Le Chatelier (PLC) banding towards shear banding in this commercial alloy. The second objective was to study the PLC banding as a function of prior deformation under positive strain rate sensitivity condition.</p> <p> The experimental work involved pre-straining experiments coupled with a non-contact strain measurement technique. Pre-straining experiments were carried out by deforming the sample at 223 K, at which the PLC effect is significantly suppressed, up to a prescribed amount of true strain prior to room temperature testing. A non-contact strain measurement technique, based on digital image correlation (DIC), was utilized in order to observe PLC band behavior during tensile tests at room temperature and subsequently to measure the amount of plastic strain carried within the band.</p> <p> The results showed the appearance of random nucleation deformation bands, associated with type B PLC banding, with short distance propagation during constant strain rate tensile test at room temperature. A change in the nature of PLC banding, marked by distinct band propagation, was observed once a critical amount of pre-strain is given. However, there is no evidence of a relationship between two existing phenomena, PLC banding and shear banding, in this alloy.</p> / Thesis / Master of Applied Science (MASc)

Amostras de um aço inoxidável martensítico AISI 410 temperado e revenido foram nitretadas a plasma em baixa temperatura usando o tratamento de nitretação plasma DC e a nitretação a plasma com tela ativa. Ambos os tratamentos foram realizados a 400 °C, utilizando mistura gasosa de 75 % de nitrogênio e 25 % de hidrogênio durante 20 horas e 400 Pa de pressão. As amostras de aço AISI 410 temperado e revenido foram caracterizadas antes e depois dos tratamentos termoquímicos, usando as técnicas de microscopia óptica, microscopia eletrônica de varredura, medidas de microdureza, difração de raios X e medidas de teor de nitrogênio em função da distância à superfície por espectrometria WDSX de raios X. A resistência à erosão por cavitação do aço AISI 410 nitretado DC e com tela ativa foi avaliada segundo a norma ASTM G32 (1998). Os ensaios de erosão, de erosão - corrosão e de esclerometria linear instrumentada segundo norma ASTM C1624 (2005) somente foram realizados no aço AISI 410 nitretado com tela ativa. Ensaios de nanoindentação instrumentada forma utilizados para medir a dureza (H) e o módulo de elasticidade reduzido (E*) e calcular as relações H/E* e H3/E*2 e a recuperação elástica (We), utilizando o método proposto por Oliver e Pharr. Ambos os tratamentos produziram camadas nitretadas de espessura homogênea constituídas por martensita expandida supersaturada em nitrogênio e nitretos de ferro com durezas superiores a 1200 HV, porém, a nitretação DC produziu maior quantidade de nitretos de ferro do que o tratamento de tela ativa. Os resultados de erosão por cavitação do aço nitretado DC mostraram que a precipitação de nitretos de ferro é prejudicial para a resistência à cavitação já que reduziu drasticamente o período de incubação e aumentou a taxa de perda de massa nos estágios iniciais do ensaio; entretanto, depois da remoção desses nitretos de ferro, a camada nitretada formada somente por martensita expandida resistiu bem ao dano por cavitação. Já no caso do aço nitretado com tela ativa, a resistência à erosão por cavitação aumentou 27 vezes quando comparada com o aço AISI 410 sem nitretar, fato atribuído à pequena fração volumétrica e ao menor tamanho dos nitretos de ferro presente na camada nitretada, às maiores relações H/E* e H3/E*2 e à alta recuperação elástica da martensita expandida. A remoção de massa ocorreu, principalmente, pela formação de crateras e de destacamento de material da superfície dos grãos por fratura frágil sem evidente deformação plástica. As perdas de massa acumulada mostradas pelo aço nitretado foram menores do que aquelas do aço AISI 410 nos ensaios de erosão e de erosão corrosão. O aço nitretado apresentou uma diminuição nas taxas de desgaste em ambos os ensaios de aproximadamente 50 % quando comparadas com o aço AISI 410. O mecanismo de remoção de material foi predominantemente dúctil, mesmo com o grande aumento na dureza. Os resultados de esclerometria linear instrumentada mostraram que a formação de martensita expandida possibilitou uma diminuição considerável do coeficiente de atrito em relação ao observado no caso do aço AISI 410 sem nitretar. O valor de carga crítica de falha foi de 14 N. O mecanismo de falha operante no aço nitretado foi trincamento por tensão. / Specimens of a quenched and tempered AISI 410 martensitic stainless were low temperature plasma nitrided using DC pulsed plasma treatment and the pulsed plasma active screen technic. Both treatments were carried out at 400 °C in a mixture of 75 % of nitrogen and 25 % of hydrogen during 20 hours and 400 Pa of pressure. Nitrided and non-nitrided AISI 410 specimens were characterized by optical and scanning electron microscopy, micro and nanohardness measurements, X ray diffraction and determination of the nitrogen content as a function of the depth using wavelength dispersive spectroscopy WDSX. Cavitation erosion tests were carried out according to ASTM G32 (1998) standard for both DC nitrided steel and active screen nitrided steel, whereas, the erosion, erosion - corrosion tests and scratch tests according to ASTM C1624 (2005) were conducted only for active screen nitrided steel. Nanoindentation tests were carried out in order to assess the hardness (H), the reduced elastic modulus (E*) the H/E* and H3/E*2 ratios and the elastic recovery (We) of the active screen nitrided steel according to the procedure proposed by Oliver and Pharr. Both nitrided treatments produced thick nitrided cases composed of nitrogen supersaturaded expanded martensite and iron nitrides, however, the DC treatment promoted the precipitation of large quantities of iron nitrides in comparison to the active screen technic. The cavitation erosion results of the DC nitrided steel showed that iron nitrides precipitation is harmful for the cavitation resistance as it drastically reduced the incubation period, despites this, after the removal of those iron nitrides, the nitrided case composed solely of expanded martensite resisted the cavitation damage. On the other hand, the active screen technic increased 27 times the cavitation erosion resistance of the AISI 410 steel. The increase in cavitation erosion resistance was attributed to minor quantities of smaller size iron nitrides, the higher H/E* and H3/E*2 ratios and to the higher elastic response of the expanded martensite. The material removal mainly comes from the formation of craters and from debris detachment from the grain surfaces due to brittle fracture, without plastic deformation. The active screen nitrided steel showed the lower cumulative mass losses in erosion and erosion - corrosion tests. The nitrogen addition decreased around 50 % the erosion rate in both tests. The active screen nitrided steel showed a ductile behavior despite the intense increase in hardness. The scratch tests showed that expanded martensite formation led to a significant decrease of the friction coefficient. The critical load was 14 N and the failure mechanism acting in the nitrided case was tensile cracking.

Amostras de um aço inoxidável martensítico AISI 410 temperado e revenido foram nitretadas a plasma em baixa temperatura usando o tratamento de nitretação plasma DC e a nitretação a plasma com tela ativa. Ambos os tratamentos foram realizados a 400 °C, utilizando mistura gasosa de 75 % de nitrogênio e 25 % de hidrogênio durante 20 horas e 400 Pa de pressão. As amostras de aço AISI 410 temperado e revenido foram caracterizadas antes e depois dos tratamentos termoquímicos, usando as técnicas de microscopia óptica, microscopia eletrônica de varredura, medidas de microdureza, difração de raios X e medidas de teor de nitrogênio em função da distância à superfície por espectrometria WDSX de raios X. A resistência à erosão por cavitação do aço AISI 410 nitretado DC e com tela ativa foi avaliada segundo a norma ASTM G32 (1998). Os ensaios de erosão, de erosão - corrosão e de esclerometria linear instrumentada segundo norma ASTM C1624 (2005) somente foram realizados no aço AISI 410 nitretado com tela ativa. Ensaios de nanoindentação instrumentada forma utilizados para medir a dureza (H) e o módulo de elasticidade reduzido (E*) e calcular as relações H/E* e H3/E*2 e a recuperação elástica (We), utilizando o método proposto por Oliver e Pharr. Ambos os tratamentos produziram camadas nitretadas de espessura homogênea constituídas por martensita expandida supersaturada em nitrogênio e nitretos de ferro com durezas superiores a 1200 HV, porém, a nitretação DC produziu maior quantidade de nitretos de ferro do que o tratamento de tela ativa. Os resultados de erosão por cavitação do aço nitretado DC mostraram que a precipitação de nitretos de ferro é prejudicial para a resistência à cavitação já que reduziu drasticamente o período de incubação e aumentou a taxa de perda de massa nos estágios iniciais do ensaio; entretanto, depois da remoção desses nitretos de ferro, a camada nitretada formada somente por martensita expandida resistiu bem ao dano por cavitação. Já no caso do aço nitretado com tela ativa, a resistência à erosão por cavitação aumentou 27 vezes quando comparada com o aço AISI 410 sem nitretar, fato atribuído à pequena fração volumétrica e ao menor tamanho dos nitretos de ferro presente na camada nitretada, às maiores relações H/E* e H3/E*2 e à alta recuperação elástica da martensita expandida. A remoção de massa ocorreu, principalmente, pela formação de crateras e de destacamento de material da superfície dos grãos por fratura frágil sem evidente deformação plástica. As perdas de massa acumulada mostradas pelo aço nitretado foram menores do que aquelas do aço AISI 410 nos ensaios de erosão e de erosão corrosão. O aço nitretado apresentou uma diminuição nas taxas de desgaste em ambos os ensaios de aproximadamente 50 % quando comparadas com o aço AISI 410. O mecanismo de remoção de material foi predominantemente dúctil, mesmo com o grande aumento na dureza. Os resultados de esclerometria linear instrumentada mostraram que a formação de martensita expandida possibilitou uma diminuição considerável do coeficiente de atrito em relação ao observado no caso do aço AISI 410 sem nitretar. O valor de carga crítica de falha foi de 14 N. O mecanismo de falha operante no aço nitretado foi trincamento por tensão. / Specimens of a quenched and tempered AISI 410 martensitic stainless were low temperature plasma nitrided using DC pulsed plasma treatment and the pulsed plasma active screen technic. Both treatments were carried out at 400 °C in a mixture of 75 % of nitrogen and 25 % of hydrogen during 20 hours and 400 Pa of pressure. Nitrided and non-nitrided AISI 410 specimens were characterized by optical and scanning electron microscopy, micro and nanohardness measurements, X ray diffraction and determination of the nitrogen content as a function of the depth using wavelength dispersive spectroscopy WDSX. Cavitation erosion tests were carried out according to ASTM G32 (1998) standard for both DC nitrided steel and active screen nitrided steel, whereas, the erosion, erosion - corrosion tests and scratch tests according to ASTM C1624 (2005) were conducted only for active screen nitrided steel. Nanoindentation tests were carried out in order to assess the hardness (H), the reduced elastic modulus (E*) the H/E* and H3/E*2 ratios and the elastic recovery (We) of the active screen nitrided steel according to the procedure proposed by Oliver and Pharr. Both nitrided treatments produced thick nitrided cases composed of nitrogen supersaturaded expanded martensite and iron nitrides, however, the DC treatment promoted the precipitation of large quantities of iron nitrides in comparison to the active screen technic. The cavitation erosion results of the DC nitrided steel showed that iron nitrides precipitation is harmful for the cavitation resistance as it drastically reduced the incubation period, despites this, after the removal of those iron nitrides, the nitrided case composed solely of expanded martensite resisted the cavitation damage. On the other hand, the active screen technic increased 27 times the cavitation erosion resistance of the AISI 410 steel. The increase in cavitation erosion resistance was attributed to minor quantities of smaller size iron nitrides, the higher H/E* and H3/E*2 ratios and to the higher elastic response of the expanded martensite. The material removal mainly comes from the formation of craters and from debris detachment from the grain surfaces due to brittle fracture, without plastic deformation. The active screen nitrided steel showed the lower cumulative mass losses in erosion and erosion - corrosion tests. The nitrogen addition decreased around 50 % the erosion rate in both tests. The active screen nitrided steel showed a ductile behavior despite the intense increase in hardness. The scratch tests showed that expanded martensite formation led to a significant decrease of the friction coefficient. The critical load was 14 N and the failure mechanism acting in the nitrided case was tensile cracking.