Coastal regions are very important because they provide a lot of resources and benefits for all the humankind. Coastal defense structures protect coastal regions from wave attacks. However, the cost of construction such coastal defense structures are very high and need big investments. Hence, to reach the optimum design and minimize the risk of failure has vital importance during the design stage of these structures. Model studies are the most effective tool in optimizing the design of these structures. Rubble mound coastal defense structures were constructed with assembly of different sizes of armor stones and front slopes. Rubble mound coastal defense structures were designed by Van der Meer

Les châssis automobiles sont composés en majorité de pièces mécano-Soudées soumises à deschargements multiaxiaux complexes. Par leur géométrie particulière et les transformations induites par leprocédé de fabrication, les joints soudés sont des sites privilégiés d’amorçage et de propagation de fissures enfatigue. Une méthode de calcul fiable et maniable représente un enjeu majeur pour l’industrie automobile, carelle permet de détecter les points critiques dès la phase d’avant projet, d’éviter le surdimensionnement et deréduire le nombre des prototypes physiques.Afin de découpler les incertitudes et permettre l’analyse isolée des effets induits par le chargementmultiaxial lui-Même et par les caractéristiques particulières du joint soudé, cette étude est divisée en deux parties.La première partie se concentre sur le chargement multiaxial complexe. Le comportement d’un « matériauécole », l’acier C35, est étudié sous spectre de chargement de type « automobile ». Différentes méthodes deprévision de la durée de vie sont ensuite comparées sous deux critères : la qualité de prédiction et la simplicitéd’utilisation. La deuxième partie est centrée sur l’étude du comportement des assemblages soudés et lasimulation de leur durée de vie. Pour ce faire, une éprouvette représentative des pièces de liaison au sol et duprocédé de fabrication a été conçue. L’étude micrographique de l’assemblage combinée à des essais instrumentésa permis de construire un modèle EF représentatif de la structure. Deux modes de sollicitation sont utilisés lorsdes essais de fatigue dans le but d’activer différents mécanismes de dommage en amplitude constante et enspectre « automobile ».Finalement, la méthodologie « Weld Stress Gradient » est proposée pour intégrer l’effet du gradient decontraintes en proximité des points critiques du joint soudé au calcul de fatigue. Cette méthodologie, associée aucritère de Vu (prise en compte du chargement complexe) et à la règle de cumul de dommage non linéaire« DCA » (prise en compte de l’interaction entre les blocs de chargement du spectre), permet d’obtenir desrésultats encourageants pour l’assemblage étudié. / Automotive chassis are composed mostly of welded parts subjected to complex multiaxial loadings.Welds are potential sites to initiation and propagation of fatigue cracks because of their particular geometry andtransformations induced by manufacturing process. A reliable and tractable fatigue design methodology is achallenge for automotive industry designers because it allows detecting the critical points from the upstreamphase, avoiding oversizing and reducing the number of physical prototypes.This study is divided into two parts allowing the distinction of effects induced by multiaxial loadingsand particular characteristics of the welded joints. The first part focuses on the complex multiaxial loading. Thebehavior of a “well known material”, the 1045 steel, is studied under "automobile" spectrum loading. Differentfatigue life prediction methods are compared under two criteria: quality of prediction and simplicity of use. Thesecond part focuses on the study of behavior and fatigue life simulation of welded structures. To achieve thisgoal, a representative specimen of chassis parts and manufacturing process has been designed. A representativeFE model is constructed, based on micrographic observations and instrumented tests. Two loading modes areused in fatigue tests in order to activate different damage mechanisms at constant amplitude and under"automobile" spectrum.Finally, the methodology "Weld Stress Gradient" is proposed to incorporate in fatigue analysis theeffect of the stress gradient at the vicinity of weld joints critical points. This methodology, combined with Vucriterion (taking into account complex loadings) and "DCA" nonlinear cumulative damage rule (taking intoaccount interaction between spectrum loading blocks), provides encouraging results for the studied specimen.

This thesis deals with the reliability and maintenance of structures that are damaged by shocks arriving randomly in time. The degradation is modeled as a cumulative stochastic point process. Previous studies mostly adopted expected cost rate criterion for optimizing the maintenance policies, which ignores practical implications of discounting of maintenance cost over the life cycle of the system.Therefore, detailed analysis of expected discounted cost criterion has been done, which provides a more realistic basis for optimizing the maintenance. Examples of maintenance policies combining preventive maintenance with age- based replacement are analyzed. Derivation for general cases involving preventive maintenance damage level have been discussed. Special cases are also considered.

This thesis deals with the reliability and maintenance of structures that are damaged by shocks arriving randomly in time. The degradation is modeled as a cumulative stochastic point process. Previous studies mostly adopted expected cost rate criterion for optimizing the maintenance policies, which ignores practical implications of discounting of maintenance cost over the life cycle of the system.Therefore, detailed analysis of expected discounted cost criterion has been done, which provides a more realistic basis for optimizing the maintenance. Examples of maintenance policies combining preventive maintenance with age- based replacement are analyzed. Derivation for general cases involving preventive maintenance damage level have been discussed. Special cases are also considered.

Solid propellant rocket motor is the primary propulsion technology used for short and medium range missiles. It is also commonly used as boost motor in many di_erent applications. Its wide spread usage gives rise to diversity of environments in which it is handled and stored. Ability to predict the storage life of solid propellants plays an important role in the design and selection of correct protective environments. In this study a methodology for the prediction of solid propellant storage life using cumulative damage concepts is introduced. Finite element mesh of the solid propellant grain is created with the developed parametric grain geometry generator. Finite element analyses are carried out to obtain the temperature and stress response of the propellant to the environmental thermal loads. Daily thermal cycles are assumed to be sinusoidal cycles represented by their means and amplitudes. With the cumulative damage analyses, daily damage accumulated in the critical locations of the solid propellant grain are investigated. Meta-models relating the daily damage amount with the daily temperature cycles are constructed in order to compute probability of failure. The results obtained in this study imply that it is possible to make numerical predictions for the storage life of solid propellants even in the early design phases. The methodology presented in this study provides a basis for storage life predictions.

Microelectronic systems are subjected to thermal cycling, power cycling, and vibration environments in various applications. These environments, whether applied sequentially or simultaneously, affect the solder joint reliability. Literature is scarce on predicting solder joint fatigue failure under such multiple loading environments. This thesis aims to develop a unified modeling methodology to study the reliability of electronic packages subjected to thermal cycling, power cycling, and vibration loading conditions. Such a modeling methodology is comprised of an enriched material model to accommodate time-, temperature-, and direction-dependent behavior of various materials in the assembly, and at the same time, will have a geometry model that can accommodate thermal- and power-cycling induced low-cycle fatigue damage mechanism as well as vibration-induced high-cycle fatigue damage mechanism. The developed modeling methodology is applied to study the reliability characteristics of ceramic area array electronic packages with lead-based solder interconnections. In particular, this thesis aims to study the reliability of such solder interconnections under thermal, power, and vibration conditions individually, and validate the model against these conditions using appropriate experimental data either from in-house experiments or existing literature. Once validated, this thesis also aims to perform a design of simulations study to understand the effect of various materials, geometry, and thermal parameters on solder joint reliability of ceramic ball grid array and ceramic column grid array packages, and use such a study to develop universal polynomial predictive equations for solder joint reliability. The thesis also aims to employ the unified modeling methodology to develop new understanding of the acceleration factor relationship between power cycling and thermal cycling. Finally, this thesis plans to use the unified modeling methodology to study solder joint reliability under the sequential application of thermal cycling and vibration loading conditions, and to validate the modeling results with first-of-its-kind experimental data. A nonlinear cumulative damage law is developed to account for the nonlinearity and effect of sequence loading under thermal cycling, power cycling, and vibration loading.

<p>Föreliggande avhandling omfattar en fallstudie av utmattningsrisken för järnvägsbroarna mellan Stockholm Central och Söder Mälarstrand. Ett stort antal utmattningssprickor i broarna över Söderström och Söder Mälarstrand har sedan länge varit kända. I samband med en bärighetsutredning av sträckan har ett stort antal anslutningspunkter i samtliga broar identifierats som kritiska avseende utmattningsrisk. Sträckan är den mest trafikerade i Sverige och de förenklade beräkningsmetoder som anges i Banverkets föreskrift för bärighetsberäkningar är i vissa fall inte tillämpliga.</p><p>De formella bärighetsberäkningarna har visat på flerfaldiga överskridanden i utnyttjandegrad gällande utmattning. I de punkter som beräkningarna visar störst risk för utmattning har inga sprickor påträffats, trots omfattande förbandsbesiktningar. Beräkningarna baseras på typiserade spänningskollektiv och givet antal spännings­växlingar, oberoende av den aktuella trafikmängden. Enligt bärighetsnormen ges möjlighet att beakta den verkliga trafikmängden baserat på historiskt bruttotonnage och typiserade trafiklaster. Dessa beräkningar har utförts och visar på ännu större risk för utmattning, jämfört med den förenklade metoden. I syfte att på ett ännu mer nyanserat sätt uppskatta utmattningsrisken har en metod använts, baserat på uppskattning av den faktiska trafikmängden och dess fördelningar. Beräkningarna baseras på trafikdata tillgänglig från Banverket. Metoden är tillämplig på andra järnvägsbroar på andra sträckor.</p><p>Utöver teoretiska beräkningar har töjningsmätningar utförts. Under 2006 utförde dåvarande Carl Bro AB töjningsmätningar på bro över Söder Mälarstrand och viadukt söder om Söder Mälarstrand, avseende några enskilda tågöverfarter. Under 2008 utförde KTH Brobyggnad omfattande töjningsmätningar på bro över Söderström, avseende all trafik under en månads tid.</p> / <p>The present thesis comprises a case study of the risk of fatigue of the railway bridges chaining between Stockholm Central Station and the district Söder Mälarstrand. A large number of fatigue-related cracks in the bridges at Söderström and Söder Mälarstrand have been known for a long time. During a capacity assessment of the current bridges, a large number of connections have been identified as critical concerning fatigue resistance. The route is the most frequent in all of Sweden and the simplified methods of fatigue assessment defined by Banverket may not always be applicable.</p><p>A conventional capacity assessment has shown numerous exceeds in fatigue resistance, using the stated safety margins. No fatigue cracks have been identified at the locations showing the largest theoretical risk of fatigue, in spite of extensive investigations. The conventional calculations are based on a uniform stress collective and a fixed number of stress cycles, independent of the actual traffic volume. According to the regulations stated by Banverket, the assessment may optionally be performed using historical data of the gross tonnage and standardised traffic loading. Such calculations have been undertaken and show even greater risk of fatigue, compared to the conventional assessment. To estimate the risk of fatigue in more detail, a method has been used, based on estimations of the real traffic volume and its distributions. The analysis is based on available data of the traffic volume and may be applied to other railway bridges on other locations.</p><p>Besides theoretical analyses, field measurements have been performed. In 2006, former Carl Bro AB carried out strain gauge measurements on the bridge passing Söder Mälarstrand and the viaduct south of Söder Mälarstrand. The measurements comprised a small amount of individual train passages. During 2008, the division of Structural Design and Bridges at KTH performed an extensive field measurement programme on the bridge passing Söderström. Continuous measurements collecting data of all traffic during a period of one month was performed.</p>

Dans les générateurs de vapeur des centrales nucléaires à eau pressurisée, les tuyauteries sont soumises à des chargements thermique et mécanique, qui sont variables et divisés en deux régimes différents : la fatigue oligocyclique et la fatigue gigacyclique. Les aciers au carbone – manganèse, type A42, A48 et Tu48 (normes françaises) sont souvent utilisés dans de telles applications. Les propriétés du matériau manifestent certains caractères spéciaux en mécanique et métallurgie comme le vieillissement dynamique conduisant à une augmentation de la valeur de la contrainte maximale et une diminution de la ductilité à la température 200 ℃. Le comportement en fatigue oligocyclique et gigacyclique sont étudiés à température ambiante et 200 ℃. Des essais de fatigue cumulée ont été mis en oeuvre pour étudier l’effet du cumul de dommage combinant des cycles de fatigue oligocyclique suivis de cycles en fatigue gigacyclique. Tous les résultats sont analysés en utilisant la mécanique de l’endommagement des milieux continus et l’analyse microfractographique. Les cycles d’hystérésis en fatigue oligocyclique sont dus à la déformation plastique de durcissement cinématique, ils peuvent être décrits par sous le modèle d’Armstrong – Frederick ; le durcissement isotrope est utile pour prédire l’évolution de l’amplitude de contrainte. Mais avec l’augmentation de la déformation plastique accumulée, le dommage ne peut être négligé. Le modèle de Chaboche d’endommagement par fatigue est utilisé pour décrire l’évolution des dommages oligocyclique et il est étendu au régime gigacyclique. Un modèle de fatigue cumulée des dommages a été développé à partir du modèle de Chaboche et appliqué à l’estimation des dommages de fatigue pour décrire le comportement de l’évolution de la contrainte en fonction du nombre de cycles. En fatigue oligocyclique à la température de 200 ℃, l’acier A48 est sensible au phénomène de vieillissement dynamique et il apparaît un durcissement secondaire, qui peut être prédit par la théorie des dislocations et est simulé dans la thése. L’analyse des surfaces de rupture est effectuée par fractographie au Microscope Electronique à Balayage pour les essais en oligocyclique, gigacyclique et cumul. En fatigue oligocylique, la fissure est initiée en surface. En fatigue gigacyclique, certaines fissures sont initiées sur des inclusions situées à l’intérieur d’éprouvettes. / In steam generators of nuclear power plants, typical pipes components are subjected to thermal and mechanical loading which are variable and divided into two different regimes: low cycle fatigue and gigacycle fatigue. Carbon-manganese steels A42, A48 and Tu48 steels (French standards) are often used in such applications. The material properties manifest some special characters in mechanics and metallurgy such as Dynamic Strain Aging, increasing UTS values in 200℃ temperature domain. The LCF and VHCF behaviors are investigated respectively by test method at room temperature and 200℃. The cumulative fatigue tests are implemented through referencing the load as prior LCF following gigacycle fatigue from the steam generator pipes thermal loads in order to obtain the performance of material under accumulated fatigue damage. All the test results are analyzed by using plastic mechanics, continuums damage mechanics and microscopic analysis. Hysteresis loops are due to plastic deformation in LCF which is the effect of kinematic hardening and they can be described by Armstrong – Frederick form models; the isotropy hardening is used to predict the evolution of stress amplitude in LCF. But with rising of accumulated plastic deformation, the damage can not be neglected. The Chaboche fatigue damage model is applied to describe the damage evolution of LCF and extended to VHCF regime. The cumulative fatigue damage model is extended from Chaboche model and applies to the estimation cumulative fatigue damage. The constitutive relationship and isotropy rule are coupled with fatigue damage model that can describe the whole fatigue behavior. In 200℃ for LCF, A48 is sensitive to dynamic strain aging and its secondary hardening behavior is important which can be predicted by dislocation theory and is simulated in the paper. The fractographic analysis is performed by SEM for LCF, VHCF and cumulative fatigue tests. The LCF crack is initiated in surface. Some of cracks of VHCF are given birth from the inclusions located at interior of sample.

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