Primary goal of this thesis is to present proposed method for fatigue life evaluation applicable to light composite aircraft. The first chapter of this thesis is focused on certification specifications and requirements for fatigue life proof of structure. Significant part of this thesis is dedicated to fatigue life evaluation and guideline how to proceed in evaluation of fatigue data. Thesis also points out on possible methods of statistical fatigue data analysis and critical parts of structure in means of fatigue life. The last part of thesis is focused on design of experimental evaluation of structure including test rig and load sequence. This chapter also provides inside into possible means of introducing artificial defects into the structure.

As traffic demand on bridges increases, loading cycles on critical components will increase, reducing their service life. Modular bridge expansion joints, which are imperative to allowing the bridge superstructure to move, are susceptible to fatigue damage at their field splice. These splices are used to connect segments of the total joint, during staged construction. Current splice designs are either bolted or welded connections, which allow stress concentrations to induce pre-mature fatigue failure. This thesis examines the use of a hybrid FRP/steel design under fatigue loading for use as a splice detail. The splice detail consists of steel plates bolted to steel beam webs and CFRP pultruded plates adhesively bonded to the underside of the steel beam flanges. Two different moduli of CFRP were examined: Normal Modulus and Ultra High Modulus. Two beams of each modulus were tested under static conditions and six under constant amplitude fatigue loading. A testing rig was used to simulate similar bending moments experienced in bridge joints. In the static tests, slippage of the web plates caused considerable stiffness loss and the slippage load varied drastically between CFRP moduli. For the fatigue tests, the intention was to reach two million cycles at the different constant load ranges. Stiffness degradation was noticed during the fatigue process, and was likely due to bolt pre-tension loss and/or plastic deformation of the adhesive. Specimens that reached two million cycles were monotonically loaded to failure. Once the CFRP had failed, a secondary mechanism was observed for reserve load capacity. Simple beam mechanics were used to create prediction models for the initial spliced beam stiffness and peak CFRP load. Flexural and shear deformations of the spliced system were considered for beam stiffness. For the CFRP failure load prediction, a design peak strain in the CFRP was used to account for shear lag effects in the material and variability of the splice detail. While the model was inaccurate for beam stiffness, it provided a good approximate of the peak CFRP load. Based on the presented test data, the Normal Modulus CFRP hybrid splice detail showed better fatigue performance than conventional steel connection details. / Thesis (Master, Civil Engineering) -- Queen's University, 2013-07-24 11:28:19.728

All systems are unreliable in the sense that they degrade with age and usage and eventually fail. The degradation depends on the load (or stress) on the system and the maintenance actions. With higher load the productivity increases but this is achieved at the expense of faster degradation and more frequent failures. This in turn reduces the availability of the system. The yield (expected output per unit time), expected operational cost and the expected profit are functions of the load and the availability. The relationship between load and yield is a complicated function and depends on system reliability and maintenance actions. The thesis deals with determining the optimal load and maintenance actions to maximise the yield (or expected profit) or to minimise the expected cost. The thesis comprises of two parts (Parts I and II). Part I is theoretical in nature and looks at the modelling, analysis and optimisation issues. Part II deals with a real case study involving a dragline used in open cut mining. Part I starts with stress-life models which link the load on the system with the reliability of the system. It examines the empirical data in the form of S-N and P-S-N curves, and as WPP plots reported in the literature. The salient features are identified so as to assess the suitability of different models to model the stress-life data. Various Weibull models are assessed to determine their suitability to model stress-life data at the component level. This is followed by a discussion of the modelling at the system level. Optimal maintenance actions are derived based on component level and system level stress-life models. For the component level models, the optimal maintenance actions are derived for a variety of maintenance strategies (such as the age, block and periodic policies) for a specified load to minimise the asymptotic expected cost per unit time. Following this, the joint optimal choice of load and maintenance is examined. For the system level model, the optimal load is obtained to minimise the expected cost per unit time over the life of the system taking into account different actions such as minor and major preventive maintenance actions and corrective maintenance actions. Part II deals with a case study involving a BE1370W dragline. The dragline is used in open cut coal mining to remove the dirt (overburden) on top of the coal seam. This involves dragging the bucket, which is suspended from the boom, through the muckpile until full and then hoisting and swinging the bucket to the dumping point. The capacity of the bucket size determines the load on the dragline. The system level model studied in Part I is used to determine the optimal load to maximise the yield. The dragline was modelled as comprising of 25 components (or sub-systems). Each component was modelled using a system level model of Part I. The model parameters were estimated using real operating and maintenance data over two years. The optimal load to maximise the yield was derived and a sensitivity study carried out.

Para realizar ensaios de fadiga, é preciso que a máquina ou equipamento de testes tenha uma durabilidade muito maior que as estruturas ou componentes a serem ensaiados. O presente trabalho propõe-se a investigar a estrutura de uma máquina de ensaio de fadiga de cadeira de rodas, com o objetivo de determinar sua vida em fadiga. Para isto, uma análise da interação entre cadeira de rodas e máquina de ensaio é feita, do ponto de vista dinâmico, através de dados experimentais e modelagem em elementos finitos. A máquina de ensaio é constituída por uma estrutura com dois tambores rotativos, que possuem ressaltos, e suportam as rodas da cadeira de rodas. Estimada a força no tempo que a cadeira exerce sobre os tambores, calculam-se as tensões na estrutura, identificam-se as regiões de maiores tensões e estima-se a vida em fadiga da estrutura. / Fatigue analysis of full-scale structures and components is possible by using appropriated devices and machines which has to be much more durable than the structures or components being tested. The present work is proposed to investigate the structure of a wheelchair fatigue testing machine and predict its fatigue life. To reach this goal a dynamic analysis based on experimental data combined with the finite element method of the testing machine and wheelchair interaction is done. The machine is made of a structure and two turning drums that have slats and support the wheelchairs. With the timedependent force that wheelchair applies to the drums, the stresses are calculated to the whole structure, the sites with higher stresses are identified and the fatigue life of the structure is predicted.

Cette thèse traite d’une approche pragmatique de l'endommagement par fatigue des structures sous un chargement cyclique. Un modèle anisotrope de dommage par fatigue a été développé. L'évolution de la dégradation des propriétés du matériau dépend d’un critère de fatigue (choisi) et de ses gradients. La dégradation du matériau anisotrope guidera la propagation des dommages. La propagation des dommages dépend principalement des crêtes de la «surface» du critère (gradients zéro). L'approche proposée décrit l'amorçage, la propagation des dommages et la rupture structurale sous un chargement multiaxial en fatigue. Pour chaque élément fini, des distributions anisotropes non homogènes des propriétés du matériau sont associées. Schématiquement, il apparaît comme un «surfer» matériel sur la «surface» du critère et les dommages suivent la crête de la «surface» du critère (niveau et gradient). Un critère d'approche globale, basé sur des invariants du tenseur des contraintes, est adopté. La réduction des propriétés des matériaux est affectée à un certain nombre de cycles et à un niveau global de contraintes, en utilisant une courbe de Wöhler expérimentale. Deux formes simplifiées du modèle sont proposées et les résultats sont comparés avec un exemple de référence expérimental (plaque cruciforme) et un cas industriel (pale d’une éolienne d’EDF). Une cartographie avec le critère de Dang Van est également calculée pour analyser les résultats numériques. / A new practical engineering methodology for the analysis of structures under cyclic loading is proposed in this work . A new anisotropic fatigue damage model is developed. The evolution of material properties degradation depends on a failure criterion and its gradient. The anisotropic material degradation will guide the damage propagation. The propagation of damage is mainly depending on the ridges of the criterion’s « surface » (zero gradients). The proposed approach can describe the initiation and propagation of the damage until the structural failure under fatigue loading. For each finite element, non-homogeneous anisotropic distributions of material properties are associated. Schematically, it seems like a material « surfing » on the criterion’s « surface » and damages follow the crest of the criterion’s « surface » (level and gradient). A global approach criterion, based on invariants of the stress tensor, is adopted. The reduction of material properties is assigned to a number of cycles and a global level of stresses, using an experimental Wöhler curve. Two simplified forms of the model are proposed and results are compared with a cruciform experimental reference example and an industrial case. A mapping with Dang Van criterion is also computed to analyze the numerical results.

Para realizar ensaios de fadiga, é preciso que a máquina ou equipamento de testes tenha uma durabilidade muito maior que as estruturas ou componentes a serem ensaiados. O presente trabalho propõe-se a investigar a estrutura de uma máquina de ensaio de fadiga de cadeira de rodas, com o objetivo de determinar sua vida em fadiga. Para isto, uma análise da interação entre cadeira de rodas e máquina de ensaio é feita, do ponto de vista dinâmico, através de dados experimentais e modelagem em elementos finitos. A máquina de ensaio é constituída por uma estrutura com dois tambores rotativos, que possuem ressaltos, e suportam as rodas da cadeira de rodas. Estimada a força no tempo que a cadeira exerce sobre os tambores, calculam-se as tensões na estrutura, identificam-se as regiões de maiores tensões e estima-se a vida em fadiga da estrutura. / Fatigue analysis of full-scale structures and components is possible by using appropriated devices and machines which has to be much more durable than the structures or components being tested. The present work is proposed to investigate the structure of a wheelchair fatigue testing machine and predict its fatigue life. To reach this goal a dynamic analysis based on experimental data combined with the finite element method of the testing machine and wheelchair interaction is done. The machine is made of a structure and two turning drums that have slats and support the wheelchairs. With the timedependent force that wheelchair applies to the drums, the stresses are calculated to the whole structure, the sites with higher stresses are identified and the fatigue life of the structure is predicted.

This thesis deals with fatigue behaviour of magnesium alloy AZ91E produced by extrusion. In the frame of this work, basic mechanical characteristics and data from performed fatigue tests were obtained. Further metallographic analysis of microstructure and fractographic analysis of broken specimens after fatigue tests were carried out.

In this study, ABS Publication 115, “Guidance on Fatigue Assessment of Offshore Structures” is briefly reviewed. Emphasis is on the S-N curves based fatigue assessment approach of non-tubular joints, and both size and environment effects are also considered. Further, fatigue tests are performed to study the fatigue strength of load-carrying and non-load-carrying steel cruciform joints that represent typical joint types in marine structures. The experimental results are then compared against ABS fatigue assessment methods, based on nominal stress approach, which demonstrates a need for better fatigue evaluation parameter. A good fatigue parameter by definition should be consistent and should correlate the S-N data well. The equivalent structural stress parameter is introduced to investigate the fatigue behavior of welded joints using the traction based structural stress approach on finite element models of specimens, and representing the data as a single Master S-N curve.

Doctor of Philosophy / Department of Civil Engineering / Hayder Rasheed / Most state highway agencies do not perform routine fatigue inspections on highway signs, luminaires, and traffic signals, thereby increasing the potential for unnoticed fatigue cracking. The Kansas Highway System utilizes over 450 sign trusses, most of which have been in service for 30-45 years. In addition, to aging support structures, the structural designs these signs and signals sometimes result in significant cyclical loading due to wind gust. This study conducted fatigue evaluations using nominal axial member-specific stress ranges corresponding to a wind speed database for a 45-year period, as well as, hundreds of structural analysis simulations. Potential fatigue failure was assessed for each member of the support structure by evaluating the ratio of consumed fatigue cycles to ultimate fatigue cycles using Miner’s rule to estimate finite life. If the ratio was close to zero after 45 years or any number of actual service years, the member was expected to have a practically infinite life. If the ratio was close to 1 after the service years, the member was expected to be at the end of its life. This information can help inspectors identify for critical spots that may have developed fatigue cracks that otherwise would be difficult to detect. Two approaches were hypothesized to account for fatigue life deterministically and probabilistically. Fatigue Life Simulator Software (FLSS) was developed to manage hundreds of simulations and determine the fatigue life of all members in a structure in specific areas of Kansas. FLSS is compatible and works simultaneously with STAAD Pro Software and Sign Truss Interface provided by KDOT, to generate results. Users apply the results to study the behavior of overhead structures and identify critical spots that should be physically inspected and potentially replaced. Results in Kanas indicated a range of structural fatigue life varying by city. Modifications were made to the output files of Sign Truss Interface to incorporate American Association of State Highway and Transportation Officials (AASHTO) load cases 1 and 2 and simulate wind speed into wind pressure using the effect of the two load cases. The modification also automatically incorporated 45-years of wind speed data into the Sign Truss Interface to simulate and generate structural models to determine corresponding stresses to the wind effect.

Ce travail de recherche donc concerne l'étude du système ultrasonique de fatigue en torsion de type indirect pouvoir étudier le comportement des matériaux métalliques à haute résistance, essentiellement au domaine de fatigue gigacyclique. Il existe actuellement deux types de système ultrasonique de torsion : un système qualifié de "direct" dans lequel le convertisseur délivre une rotation permettant, de manière simple et directe, la mise en vibration de torsion du système ; un système qualifié de "indirect" qui utilise un convertisseur longitudinal et nécessite une transformation en rotation pour créer la vibration en résonance de torsion. Premièrement, la présente d'étude sur le système indirect permet de comprendre son comportement de fatigue et ses propriétés. Cette étude est présentée à partir des calculs analytiques et numériques afin de dimensionner et modéliser ce système. Ensuite, l'optimisation du système à l'aide de la méthode des éléments finis est réalisée et un système plus effectif et plus robuste est obtenu. Des comparaisons entre les résultats numériques et les résultats de calibration sont proposées et une interprétation des écarts constatés est proposée avec les nouvelles simulations numériques. Deuxièmement, une campagne expérimentale à 20 kHz au-delà de 10⁸ cycles est menée en utilisant le système optimisé sur les aciers 16MnCr5 et 50CrV4 du projet FreqTigue. La méthode de l'escalier est utilisé dans les essais afin de décrire la courbe S-N, d'estimer la résistance à la fatigue et d'obtenir la durée de vie en fatigue. Des comparaisons des courbes S-N à différentes fréquences et sollicitations sont présentées. Les surfaces de rupture des éprouvettes sont analysées pour tenter d'identifier les causes de la rupture. / This research work concerns the study of the torsional ultrasonic fatigue system of indirect type which is able to study the behavior of high-strength metallic materials, mainly in the domain of gigacyclic fatigue. With the developments of technique, there are currently two types of ultrasonic torsional system: the first one is described as "direct" and its converter delivers a rotation which allows the system vibrate in torsion; the second one is described as "indirect", which uses a longitudinal converter and requires the transformation of a translation into a rotation to create a desired torsional resonant vibration in the specimen. Firstly, the study of the indirect system could understand the fatigue behavior and the properties of this system. This study is presented with the analytical and numerical calculations in order to determine and model the indirect system. Then, the optimization of this system by using the finite element method is performed and a more efficient and robust system is developed. Some comparisons between the numerical results and calibration’s results are proposed and an interpretation of differences is proposed by using new numerical simulations. Secondly, the tests of the 16MnCr5 and 50CrV4 steels of the FreqTigue project are carried out at 20 kHz and beyond 10⁸ cycles by using this optimized system. The staircase method is used in the tests to describe the S-N curve, to estimate the fatigue strength and to obtain the fatigue life. Then, the comparisons of the S-N curves at different vibration frequencies and stresses are presented, and the ruptured surfaces of the specimen are also analyzed to identify the causes of the rupture.