In this thesis, vibration induced fatigue life analysis of axial leaded Tantalum &amp / Aluminum capacitors, PDIP and SM capacitors mounted on the printed circuit boards are performed. This approach requires the finite element model, material properties and dynamic characteristics of the PCB. The young modulus of the PCB material is obtained from 3 point bending tests, resonance frequencies are obtained from modal tests and transmissibility&amp / #8217 / s of the PCB are obtained from transmissibility tests which are used as fatigue analysis inputs. Step Stress Tests are performed to obtain failure times of the tested electronic components which are also used as the numerical fatigue analysis inputs. Consecutively, fatigue analysis of a sample PCB used in military systems is aimed since it is important to compare the calculated fatigue damage to estimated life limits in order to determine which component(s), if necessary, must be moved to positions of lower damage . For this purpose, power PCB of the power distribution unit used in Leopard 1 battle tank is examined. Numerical fatigue analysis coupled with accelerated life test whose profile is convenient to military platforms is performed. Furthermore, the effects of eccobond and silicone on the fatigue life of the components are also surveyed since these techniques are common in electronic packaging. In addition, mean-time-to-failure values are obtained for the tested components by using Weibull distribution. Finally, sensitivity analysis is performed to indicate the effect of certain parameters on the fatigue life of a sample axial leaded capacitor.

The aim of this diploma thesis is to describe the causes and mechanisms of degradation processes that affect the surface of tools made of tool steel and to design and verify a practical solution that would lead to higher resistance of tools to these processes. Chapters in the theoretical part deal with fracture behavior of the material and the possibilities of its elimination, the experimental part includes testing of the effect of coating on the durability of hot forming tools.

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Research in the field of reliability based design is mainly focused on two sub-areas: The computation of the probability of failure and its integration in the reliability based design optimization (RBDO) loop. Four papers are presented in this work, representing a contribution to both sub-areas. In the first paper, a new Second Order Reliability Method (SORM) is presented. As opposed to the most commonly used SORMs, the presented approach is not limited to hyper-parabolic approximation of the performance function at the Most Probable Point (MPP) of failure. Instead, a full quadratic fit is used leading to a better approximation of the real performance function and therefore more accurate values of the probability of failure. The second paper focuses on the integration of the expression for the probability of failure for general quadratic function, presented in the first paper, in RBDO. One important feature of the proposed approach is that it does not involve locating the MPP. In the third paper, the expressions for the probability of failure based on general quadratic limit-state functions presented in the first paper are applied for the special case of a hyper-parabola. The expression is reformulated and simplified so that the probability of failure is only a function of three statistical measures: the Cornell reliability index, the skewness and the kurtosis of the hyper-parabola. These statistical measures are functions of the First-Order Reliability Index and the curvatures at the MPP. In the last paper, an approximate and efficient reliability method is proposed. Focus is on computational efficiency as well as intuitiveness for practicing engineers, especially regarding probabilistic fatigue problems where volume methods are used. The number of function evaluations to compute the probability of failure of the design under different types of uncertainties is a priori known to be 3n+2 in the proposed method, where n is the number of stochastic design variables. / <p>QC 20160317</p>

This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&amp;D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&amp;D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses. / QC 20100706

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The aim of this study was to evaluate the effect of different concentrations of hydrofluoric acid on the fatigue failure load of premolars restored with feldspathic ceramic inlays. Sixty upper premolars were embedded in PVC cylinders with acrylic resin up to 2 mm below the cement- enamel junction and prepared using an adapted device for that purpose. Teeth were randomly divided into three groups (n=20): HF1, HF5, and HF10, (conditioning with hydrofluoric acid for 60 s at concentrations of 1%, 5%, and 10%, respectively). Preparations were scanned using the CAD/CAM system with prefabricated blocks of feldspathic ceramic. The internal surfaces of the inlays were treated according to the groups, followed by application of silane coupling agent, and RelyX ARC resin cement was applied for cementation. The fatigue failure load was determined using the staircase method (10 Hz and 105 cycles in each step). The initial load (585.5 N) was applied on the slopes of the cusps through a cylinder attached to the test machine (InstronElectroPuls E3000). The fractured teeth were analyzed under a stereomicroscope for failure analysis. Statistical analysis was performed using one-way ANOVA. There was no statistical difference among the fatigue failure load of the tested groups: HF1 (448.5 N ±79.1), HF5 (360.7 N ±55.4), and HF10 (409.5 N ±121.1). Regarding the fracture mode, there was a predominance of interface fracture (50%), followed by cusp fracture (34.6%). It can be concluded that hydrofluoric acid concentrations used (1%, 5%, and 10%) did not influence the fatigue failure load of feldspathic ceramic inlays cemented on premolars. / Este estudo avaliou o efeito do condicionamento de inlays de cerâmica feldspátic com diferentes concentrações de ácido fluorídrico na carga de fratura sob fadiga de pré-molares. Sessenta pré-molares superiores foram embutidos em cilindros de PVC com resina acrílica, 2 mm abaixo da junção cemento-esmalte e preparados com caixas oclusais (2 mm) e proximais (4mm) padronizadas através de uma máquina adaptada para preparo. Os dentes foram divididos de forma randomizada em três grupos (n = 20): HF1; HF5 e HF10 (condicionamento com ácido fluorídrico durante 60 s nas concentrações de: 1%, 5% e 10% respectivamente). Os dentes foram moldados, os troquéis obtidos escaneados e as restaurações fresadas através do sistema CAD/CAM a partir de blocos pré-fabricados de cerâmica feldspática (VitaBlocks Mark II). A superfície dentária foi tratada com ácido fosfórico (38%) seguida da aplicação do sistema adesivo (Single Bond). A superfície interna das inlays foi condicionada de acordo com os respectivos grupos, seguido da aplicação do agente de união silano. A cimentação das inlays foi realizada com cimento resinoso (RelyX ARC). A carga de falha sob fadiga foi determinada utilizando o método da escada (staircase) a uma freqüência de 10 Hz e 105 ciclos, cuja carga inicial foi de 585,5 N, aplicada somente sobre as vertentes das cúspides, através de um cilindro de 8mm de diâmetro acoplado à máquina de teste (Instron Electropuls E3000). Os dentes fraturados foram analisados sob estereomicroscoscópio para classificação da falha. Análise estatística dos dados foi feita com ANOVA 1-fator. Não houve diferença estatística dos valores médios na carga de fratura sob fadiga entre os grupos testados: HF1-448,5N (±79,09); HF5-360,75 N (±55,4); HF10-409,5 N (±121,1). Considerando o modo de fratura, houve uma predominância de fraturas de interface (50%), seguida de fratura de cúspide (34,6%). Conclui-se que as concentrações de ácido fluorídrico utilizadas (1, 5 e 10%) não influenciam na carga de fratura sob fadiga de inlays de cerâmica feldspática cimentadas em pré molares.

Nickel superalloys are used for high-temperature application in energetic and aerospace industry. They are exposed to aggressive environment at high temperatures with the interactions between fatigue and creep processes, high-temperature oxidation, corrosion and erosion. Lifetime extension of such strained parts while increasing the performance of particular machine is possible by applying protective surface coatings. The subject of this work is to investigate the fatigue failure mechanisms of superalloy Inconel 713LC at 800 °C and to compare these mechanisms between material with a protective coating based on Al-Si and material without coating. The location of initiation fatigue cracks, their propagation and the fatigue crack propagation rate in some areas were analyzed by optical microscopy, scanning electron microscopy and confocal laser scanning microscopy. Based on previous research it was found that the application of the coating AlSi has a positive effect on lifetime of alloy Inconel 713LC. These results were confirmed and estimated in the diploma thesis.

The topic of this PhD. thesis is the issue of determining fatigue characteristics of selected cement-based composites. First, the history of fatigue of metal is mentioned, and some basic notions related to fatigue are clarified. The thesis also summarizes current research in the field of cement-based composites fatigue, and failure mechanisms of plain concrete during static and dynamic tests are described. Furthermore, the thesis deals with the evaluation of static and dynamic experiments’ results with extensive series of C30/37 and C45/55 class concrete specimens. In order to achieve relevant fatigue test evaluation which takes into account the age of the specimens, a new correction procedure of the data measured based on the approximation of the basic fracture-mechanics parameters of tested materials over time was suggested and verified. The annexes of this thesis present complete data obtained during static and dynamic experiments.

Fatigue failure plays an important role in engineering applications, especially when structural components experience significant cyclic thermal loading and complex force loading simultaneously. During the last decades, several post-processing techniques have been developed based on empirical investigations of experimental evidence to predict the fatigue life of materials. The work at hand postulates a conventional continuum damage theory for thermomechanical fatigue failure modeling. In particular, an implicit gradient-enhanced approach is employed to address the ill-posedness of the partial differential equation system when the damage onsets. An internal fatigue variable is phenomenologically defined based on the accumulation of viscoplasticity. In the sequel, a regularized fatigue variable is obtained to further yield the damagesoftening function, which straightforwardly applies to the stress, material tangent, and viscoplastic dissipation. A multi-field problem, consisting of the strain field, the temperature, and the non-local variable, is taken into consideration, leading to a fully coupled system. This numerical methodology is consistently derived and implemented into the context of the finite element method. Several representative and demonstrative examples are performed, which yield good numerical stability and agreement with experimental data. Conclusive findings and further perspectives close this article.

The main focus of this research is the transformation induced fatigue behavior of shape memory alloy (SMA) actuators undergoing thermally induced martensitic phase transformation. The recent development of aerospace applications employing shape memory alloys (SMAs) has expanded the need for fatigue life characterization and modeling. Lightweight, compact and with a great work output, SMAs are ideal materials for actuated structural components. However, fatigue life becomes a key factor in applications such as commercial airplanes. Therefore, it is necessary to not only perform fatigue testing but also to investigate the causes of fatigue failure. As a new class of materials, SMAs have unique characteristics and require novel test methodologies to conduct repeatable and reliable fatigue testing. For this research, two materials are being investigated: TiNiCu and Ni-rich NiTi. The experiments performed on the first selected alloy, i.e. TiNiCu SMA, explore three major parameters: the applied stress level, the amount of actuation, and the corrosive nature of the environment. Experimental results show that SMAs undergoing transformation induced fatigue exhibit a low-cycle fatigue behavior and the measurement of the accumulated plastic strain at failure is associated to a Manson-Coffin type failure criterion. Investigations conducted on the post-mortem microstructure showed evidence of a multiphysical coupling between corrosion and cyclic phase transformation, from which a novel cyclic damage mechanism is proposed and explained using the micromechanical shear lag model accounting for actuation and accumulated plastic strains. Thereafter, based upon the identified failure mechanism and considering damage accumulation through crack formation, a stress renormalization procedure is proposed in combination with the Miner’s rule to predict the reduction of number of cycles to failure due to cyclic phase transformation and corrosion. A direct method is first presented and the predictions show good agreement with experimental results. However, both corrosion and corrosion-free fatigue data are required. Therefore, a new approach is proposed: the inverse Miner’s rule, which requires corrosion fatigue data only to predict corrosion-free life. The new and attractive properties of the selected second alloy, i.e. Ni-rich NiTi SMA, have revived the motivation of the aerospace industry to design SMA actuators. One particular property is cyclic stability generated by precipitation hardening mechanism using precipitates. However, are also precipitates due to high Nickel content (60 wt.% or 55 at.%). Parameters such as processing, heat treatments, size effects, surface quality and environment are investigated. Thermomechanical response and fatigue life are discussed and the greatest impact is found to come from specimen surface quality. Finally, a detailed fractography presents the different microstructural aspects of the fatigue damage and concludes to a precipitation driven fatigue failure mechanism cause by precipitates.