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Numerische und experimentelle Untersuchungen zu den Spannungsumlagerungen von ermüdungsbeanspruchten Betonbauteilen im Very-High-Cycle-Fatigue BereichBirkner, Dennis, Marx, Steffen 03 January 2024 (has links)
Ein zentraler Baustein zur Reduktion von CO2-Emissionen ist der Ausbau der erneuerbaren Energien, insbesondere der Windenergie. Forschungsbedarf besteht dabei bei der ressourceneffizienten Herstellung der Turmstrukturen. Bei Nabenhöhen von über 100 Metern sind Hybridtürme aus vorgespannten Stahlbetonsegmenten die geeignetste Konstruktion. Hierfür ist jedoch eine genaue Kenntnis des Ermüdungsverhaltens von Beton erforderlich. In der Literatur existieren überwiegend Untersuchungen an kleinformatigen zylindrischen Probekörpern, deren Ergebnisse nur bedingt auf die großmaßstäblichen Bauteile übertragen werden können. Im Rahmen dieses Vorhabens wurden daher zum einen Großversuche mit zyklisch biegebeanspruchten, vorgespannten Betonbalken sowie Begleitversuche an zylindrischen Probekörpern und zum anderen numerische Simulationen der Balkenversuche durchgeführt. Das numerische Materialmodell wurde aufbauend auf einem additiven Dehnungsmodell im Finite-Elemente-Programm ANSYS Mechanical in einem iterativen Berechnungsablauf implementiert. Die Betondehnungen setzen sich hierbei aus vier Anteilen zusammen, einem elastischen, einem plastischen, einem viskosen und einem Temperaturdehnungsanteil. Somit konnte der kombinierte Einfluss der Anteile auf das Ermüdungsverhalten von Beton dargestellt werden.
In den Großversuchen konnte bei den Balkenprobekörpern ein Ermüdungsversagen der Betondruckzone erzeugt werden, das sich an dieser Stelle durch Risse parallel zur Drucknormalspannung sowie teilweises Abplatzen der Betondruckzone, die der größten Spannungsschwingbreite ausgesetzt war, einstellte. Es zeigte sich, dass dies erst nach deutlich mehr Lastwechseln eintrat als bei den axial beanspruchten Betonzylindern in den zyklischen Begleitversuchen mit derselben Spannungsschwingbreite. Dies ist auf die Spannungsumlagerung zurückzuführen, die im Querschnitt aufgrund der ermüdungsbedingten Materialdegradation und Steifigkeitsverringerung der stark beanspruchten Randbereiche stattfand. In den Begleitversuchen wurden die Materialparameter für das numerische Modell ermittelt, mit dem im Anschluss die Balkenversuche nachgerechnet wurden. Es konnten die in den Versuchen beobachteten Effekte der Steifigkeitsdegradation und Spannungsumlagerung und die daraus resultierende Lebensdauerverlängerung nachgebildet werden. Das Modell kann somit für weitergehende Lebensdaueruntersuchungen von ermüdungsbeanspruchten Betonbauteilen verwendet werden.:ABSCHLUSSBERICHT
1 Allgemeine Angaben
2 Zusammenfassung / Summary
3 Wissenschaftlicher Arbeits- und Ergebnisbericht
4 Veröffentlichte Projektergebnisse
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Very high cycle fatigue of duplex stainless steels and stress intensity calculationsTofique, Muhammad Waqas January 2014 (has links)
Very high cycle fatigue (VHCF) is generally considered as the domain of fatigue lifetime beyond 10 million (107) load cycles. Few examples of structural components which are subjected to 107-109 load cycles during their service life are engine parts, turbine disks, railway axles and load-carrying parts of automobiles. Therefore, the safe and reliable operation of these components depends on the knowledge of their fatigue strength and the prevalent damage/failure mechanisms. Moreover, the fatigue life of materials in the VHCF regime is controlled by the fatigue crack initiation and early growth stage of short cracks. This study was focussed on the evaluation of fatigue properties of duplex stainless steels in the VHCF regime using the ultrasonic fatigue testing equipment. The ultrasonic fatigue tests were conducted on the cold rolled duplex stainless strip steel and hot rolled duplex stainless steel grades. Two different geometries of ultrasonic fatigue test specimens were tested. Considerable attention was devoted to the evaluation of fatigue crack initiation and growth mechanisms using the high resolution scanning electron microscopy. The fatigue crack initiation was found to be surface initiated phenomena in all the tested grades, albeit different in each case. The second part of this thesis work was the development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane. Cracks with dimensions much smaller than the overall size of the domain were considered. The main goal of the development of this technique was the evaluation of stress intensity factor at each crack tip. The comparison of results from the stress intensity factor evaluation by the developed procedure and the well-established Finite Element Method software ABAQUS showed difference of less than 1% for Jacobi polynomial expansion of sixth order in the dipole density representation. / <p>Article III was still in manuscript form at the time of the defense.</p> / Very high cycle fatigue of stainless steels
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Behaviour of CFRP-Prestressed Concrete Beams under Sustained Loading and High-Cycle Fatigue at Low TemperatureSaiedi, Mohammad Reza 22 December 2009 (has links)
Fibre-reinforced polymers (FRPs) are becoming increasingly accepted in structural engineering applications. In particular, Carbon-FRP (CFRP) tendons are proving to be promising as prestressing reinforcement for concrete structures. While several studies have been conducted on CFRP-prestressed concrete beams, very little attention has been given to their long-term behaviour at low temperatures. This thesis investigates the behaviour of CFRP prestressed concrete beams in two studies: (a) under sustained loading at low temperature, and (b) under high-cycle fatigue at low temperature. Seven 13 year old, 4.4 m long precast concrete T beams were tested, of which five were prestressed to various levels with CFRP tendons and two with conventional steel strands.
In the first study, three beams were exposed to −27 °C while being subjected to a sustained load of 25% of their flexural capacity for 163 days. The sustained load produced cracking in two beams with lower prestress levels. Results were compared to those obtained from three similar beams subjected to the same sustained load at room temperature. Deflection increase under sustained load at low temperature was generally small and similar to that at room temperature. Prestressing strain had a direct relationship with temperature in the CFRP prestressed beams.
After being subjected to sustained loading, all seven beams were tested in the second study. Only three of the five CFRP prestressed beams were subjected to cyclic loading, one at −28 °C and two at room temperature, while only one of the two steel prestressed beams was subjected to cyclic loading, at −28 °C. Cyclic loading consisted of 3 million cycles at a frequency of 0.85 Hz. The load range represented 21 to 42% of the flexural capacity of the CFRP prestressed beams and 30 to 60% of that of the steel-prestressed beam. Monotonic tests were run every 1 million cycles. Finally, all seven beams were monotonically loaded to failure. All CFRP prestressed beams survived the 3 million cycles but the steel prestressed beam failed after 185,000 cycles. However, the CFRP concrete bond was weakened by high prestress levels, cyclic loading, and low temperature during sustained loading and loading to failure. This resulted in bond failure at loads ranging from 69 to 91% of the full flexural capacity. Stiffness and camber gradually decreased during cyclic loading. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-12-21 15:16:33.381
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Vliv doby izotermické transformace na strukturu a mechanické vlastnosti ADI / The Influence of the Isothermal Transformation Dwell on the Structure and Mechanical Properties of ADIFalta, Petr January 2008 (has links)
The thesis ,,The Influence of Transformation dwell on the Structure and Mechanical Properties of ADI´´ deals with problems of optimal properties of the unalloyed nodular cast iron, which are obtained by the heat treatment. The heat treatment consists of austenitization and then rapidly quenching to the transformation temperature, isothermal transformation and water cooling in the end. The aim of the thesis is focused on estimating the influence of isothermal transformation on the content of stabilized austenite, static mechanical properties and on the fatigue properties in high-cycle fatigue region for selected samples. The theoretical part of this work analyses the production of the nodular cast iron and its possibilities of the heat treatment, final technological and mechanical properties. Furthermore, it examines the conditions of austempering (temperature and time of isothermal dwell), which has a fundamental influence on mechanical properties of ADI. Problems of fatigue properties are described at the end of this part. The experimental part contains a chemical composition of studied samples and their metallographic analysis. Among others the basic mechanical static properties were determined as well as the Wöhler curves including values of fatigue limits for ten on seventh cycles. Wöhler curves were obtained by a mathematical processing using data reached by fatigue tests.
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An investigation of CFD simulation for estimation of turbine RULMaré, Charl Francois January 2018 (has links)
Turbines encounter blade failures due to fatigue and creep. It has been shown in the literature that the primary cause of steam turbine blade failures worldwide can be ascribed to fatigue in low pressure (LP) turbine blades. The failure and damage to these blades can lead to catastrophic consequences. Some utilities use empirical methods to determine the forces experienced by turbine blades but desire more accurate methods. The inaccurate
prediction of high-cycle fatigue (HCF), thermal durability and stage performance
is introduced when one does not consider blade row interaction. Blade row interactions can, however, be accounted for by means of computational fluid dynamics (CFD). Furthermore, modern high- fidelity CFD tools would be able to contribute greatly in predicting the forces experienced by turbine blades.
Numerical tools such as CFD and nite element analysis (FEA) can greatly contribute to the estimation of the remaining useful life (RUL) of turbine blades. However, in this estimation process, there are various uncertainties and aspects that affect the estimated RUL. Understanding the sensitivity of the estimated RUL to these various uncertainties and aspects is of great importance if RUL is to be estimated as accurately as possible.
In this dissertation, a sensitivity analysis is performed with the purpose of establishing the sensitivity of the estimated RUL of the last stage rotor of an LP steam turbine, to the number of harmonics used in a nonlinear harmonic (NLH) CFD simulation. The sensitivity of the estimated RUL is evaluated in the HCF regime, where the cyclic stresses occur below the yield strength of the turbine blade. A CFD model, FE model, and fatigue model were therefore developed in such a manner that would suffice, regarding the purpose of the sensitivity analysis. The CFD model is validated by comparing the predicted CFD power to that of actual generated power of a dual 100MW LP steam turbine. The sensitivity analysis is performed for 3 operation conditions, and for each operational condition the aerodynamic forces were computed using 1, 2, and 3 harmonics in an NLH simulation.
The estimation process considers a weak coupling between the CFD model and FE model. NLH simulations are firstly performed to calculate the unsteady static surface pressure distributions on the last stage rotor. This is followed by the mapping thereof to the FE model, for which a transient structural analysis is performed. Finally, the RUL is estimated by performing a fatigue analysis on the stress history obtained from the transient structural analysis.
Based on the results of the sensitivity analysis, the following recommendations were made, from a conservative point of view. Firstly, in general, if the RUL is to be estimated with reasonable accuracy, just using 1 harmonic in an NLH simulation will not be sufficient and 2 harmonics should be used. Secondly, if the RUL has to be estimated with high accuracy, 3 harmonics should be used. / Dissertation (MEng)--University of Pretoria, 2018. / National Research Foundation (NRF) / Mechanical and Aeronautical Engineering / MEng / Unrestricted
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Iniciace a rozvoj cyklického poškození v oceli pro energetický průmysl / Initiation and development of cyclic damage in steel for the energy industryHorník, Vít January 2014 (has links)
The main content of this master's thesis is a determination of the fatigue properties by using non-destructive testing methods – the acoustic emission technique and the X-ray diffraction. The reactor steel, that is named GOST 15Ch2NMFA, is used as a model material and is used to manufacture VVER-1000 reactor pressure vessels. The supplied material was made of two different steel melting, designated as H and S. For the basic measurement of the fatigue properties the stress-cycle asymmetry R = -1 was used. Some of the samples were used for approximate determination of the fatigue strength stress-cycle asymmetry R = 0. During the loading process the resonance frequency of the loading machine and acoustic emission signals were recorded. The changes in microstructure were estimated from the record of acoustic emissions during the whole fatigue life. The change of the microstructure was experimentally observed by X-ray diffraction analysis. The mechanism of the fatigue can be described by the observation of the microstructural processes as well as the relevant bifurcation with sufficient sensitivity. The results of the used methods indicate a decay and re-growth of mosaic blocks during the whole loading process. Therefore, it is important to find a correlation of these two non-destructive methods, or to use other non-destructive testing methods in the future.
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High Cycle Fatigue Simulation using Extended Space-Time Finite Element Method Coupled with Continuum Damage MechanicsBhamare, Sagar D. January 2012 (has links)
No description available.
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An Understanding of the Quasi-static Behavior, High Cycle Fatigue and Final Fracture Behavior of a Titanium (Ti- 4 Al-2.5 V-1.5 Fe-0.25 O2) AlloyKuruvilla, Mithun 12 September 2008 (has links)
No description available.
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[en] ANALYSIS OF MICROSTRUCTURAL DAMAGE IN AN INTERSTITIAL FREE STEEL AFTER VERY HIGH CYCLE FATIGUE / [pt] ANÁLISE DE DANOS MICROESTRUTURAIS EM UM AÇO INTERSTITIAL FREE APÓS FATIGA DE ALTÍSSIMO CICLOHELDER KEITARO ARCARI AMBO 07 July 2022 (has links)
[pt] Pesquisas atuais evidenciam que diversos materiais com aplicação nas
indústrias automotiva, ferroviária, aeronáutica e aeroespacial não apresentam um
limite de fadiga. Porém, em torno de 1860 Wohler propôs a curva S-N (tensão -
número de ciclos), admitindo um conceito que há um limite de fadiga, estipulado
na época como 1E5 ciclos e considerado como vida infinita em fadiga. Contudo,
com os avanços tecnológicos, inúmeros componentes falham na condição de vida
infinita, de maneira contrária ao proposto por Wöhler. Atualmente, se considera 3
regimes distintos de fadiga: fadiga de baixo ciclo, fadiga de alto ciclo e fadiga de
altíssimo ciclo. Nas indústrias automotivas há um extensivo uso de chapas de aço
para a fabricação de componentes de geometria complexa. Neste contexto, o aço
interstitial free (IF – aços com baixa quantidade de átomos intersticiais), é um dos
materiais de largo emprego, em função da sua excelente estampabilidade. O aço IF
apresenta uma microestrutura ferrítica com uma baixíssima quantidade de
elementos intersticiais. Interstícios com carbono e nitrogênio são estabilizados pelo
titânio (Ti) ou nióbio (Nb) formando frações de volume extremamente pequenas na
matriz ferrítica de carbetos e nitretos. O trabalho baseia-se na investigação
experimental dos danos microestruturais no aço IF após carregamentos cíclicos de
longa duração. Amostras do aço IF estabilizado pelo Ti foram submetidas a ensaios
ultrassônicos de fadiga com carregamentos superiores a 1E7 ciclos. Se observou o
aparecimento de danos microestruturais (persistent slip bands, PSB s) após
determinado número de ciclos. Por meio de análise no microscópio ótico foi
estimado que 97,76 por cento da vida em fadiga foi destinado para nucleação de trinca
enquanto 2,23 por cento para a propagação de trinca. Houve o projeto de um novo corpo de
prova para a máquina de fadiga de altíssimo, com o intuito de acompanha a
superfície do material durante os ensaios. É proposto a equação da curva S-N fadiga
de altíssimo ciclo com os dados extraídos dos testes. E por fim, é sugerido um fator
de correção na equação de deformação plástica cisalhante nos PSB s formados / [en] Current research shows that several materials with application in the
automotive, railway, aeronautical and aerospace industries do not have a fatigue
limit. However, around 1860 Wohler proposed the S-N curve (stress - number of
cycles), admitting a concept that there is a fatigue limit, stipulated at the time as
1E5 cycles and considered as infinite fatigue life. Although, with technological
advances, numerous components fail in the condition of infinite life, contrary to
what Wöhler proposed. Currently, 3 different fatigue regimes are considered: low
cycle fatigue, high cycle fatigue and very high cycle fatigue. In the automotive
industries there is an extensive use of sheet steel for the manufacture of components
with complex geometry. In this context, interstitial free steel (IF steel) is one of the
widely used materials, due to its excellent stampability. IF steel has a ferritic
microstructure with a very low amount of interstitial elements. Interstices with
carbon and nitrogen are stabilized by titanium (Ti) or niobium (Nb) forming
extremely small volume fractions in the ferritic matrix of carbides and nitrides. The
work is based on the experimental investigation of microstructural damage in IF
steel after long-term cyclic loading. Samples of Ti-stabilized IF steel were subjected
to ultrasonic fatigue tests with loads greater than 1E7 cycles. It was observed the
arising of microstructural damage (persistent slip bands, PSB s) after a certain
number of cycles. By means of image analysis in the optical microscope, it was
estimated that 97.76 percent of the fatigue life was destined for crack nucleation while
2.23 percent for crack propagation in very high cycle fatigue regime. There was the
project of a new specimen for the very high fatigue machine, in order to accompany
the surface of the material during the tests. It is proposed the equation of the curve
S-N fatigue of very high cycle os with data extracted from the tests. Finally, a
correction factor is suggested in the plastic shear deformation equation in PSB s
based on experimental data.
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On efficient and adaptive modelling of friction damping in bladed disksAfzal, Mohammad January 2017 (has links)
This work focuses on efficient modelling and adaptive control of friction damping in bladed disks. To efficiently simulate the friction contact, a full-3D time-discrete contact model is reformulated and an analytical expression for the Jacobian matrix is derived that reduces the computation time drastically with respect to the classical finite difference method. The developed numerical solver is applied on bladed disks with shroud contact and the advantage of full-3D contact model compared to a quasi-3D contact model is presented. The developed numerical solver is also applied on bladed disks with strip damper and multiple friction contacts and obtained results are discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different nodal diameters of the bladed disk are systematically presented. The main parameters that influence the effectiveness of friction damping in bladed disks are engine excitation order, contact stiffnesses, friction coefficient, relative motion at the friction interface and the normal contact load. Due to variation in these parameters during operation, the obtained friction damping in practice may differ from the optimum value. Therefore, to control the normal load adaptively that will lead to an optimum damping in the system despite these variations, use of magnetostrictive actuator is proposed. The magnetostrictive material that develops an internal strain under the influence of an external magnetic field is employed to increase and decrease the normal contact load. A linearized model of the magnetostrictive actuator is used to characterize the magnetoelastic behavior of the actuator. A nonlinear static contact analysis of the bladed disk reveals that a change of normal load more than 700 N can be achieved using a reasonable size of the actuator. This will give a very good control on friction damping once applied in practice. / <p>QC 20170310</p> / TurboPower
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