The fatigue characteristics of concrete are studied based on laboratory tests and numerical simulations. A series of compressive cyclic loading tests have been carried out on concrete samples. The effects of maximum and minimum load level on the evolution strain rate, energy dissipation, acoustic emissions (AE) and P-wave speed are analysed. Based on particle based numerical simulations, damage models corresponding to single-level and multi-level cyclic loading tests are proposed. The damage variable in the numerical model is time- and stress-dependent and is characterized by the progressive reduction of the bond diameter. The mechanical behaviour of concrete during cyclic loading tests is well reproduced in the numerical simulation. A real time fatigue failure prediction method is proposed based on the hysteresis occurrence ratio and hysteresis energy ratio. The AE characteristics during the laboratory tests are reproduced by the numerical simulations. AE counts and energy are characterized by broken bonds and released bond strain energy, respectively.

In this paper, a study based on experimental and numerical simulations is performed to analyze fatigue cracks in clinched joints. An experimental investigation is conducted to determine the failure modes of clinched joints under cyclic loading at different load amplitudes with single-lap shear tests. In addition, numerical FEM simulations of clinching process and subsequent shear loading are performed to support the experimental investigations by analyzing the state of stresses at the location of failure. An attempt is made to explain the location of crack initiation in the experiments using evaluation variables such as contact shear stress and maximum principal stress.

Das Örtliche Konzept dient der Berechnung von Lebensdauern zyklisch belasteter Konstruktionen. Dabei wird die Ermüdungsfestigkeit des ungekerbten Werkstoffes zugrundegelegt, um die Bauteillebensdauer abzuschätzen. Mit dieser Annahme verbunden ist ein starke Vereinfachung des Ermüdungsvorganges. Die Ursache ist die unzureichende Berücksichtigung wesentlicher Einflußgrößen, die die Festigkeit des interessierenden Bauteiles bestimmen und die andere Ausprägung als an der Werkstoffprobe erfahren. Dadurch ist die Zuverlässigkeit der Rechnung reduziert. Die vorliegende Arbeit untersucht und klassifiziert die Ursachen für dieses Übertragbarkeitsproblem. Mit einem sehr flexiblen multiplikativen Ansatz, der technologischen, statistischen und spannungsemchanischen Effekt berücksichtigt, läßt sich der Unterschied zwischen Festigkeit des Bauteils und der Werkstoffprobe rechnerisch erfassen. Zur numerischen Betrachtung technologischer und statistischer Einflüsse kann auf bereits bestehendes Wissen zurückgegriffen werden. Dies wird konzeptbezogen und mit Rücksicht auf die Anwendung angepaßt. Die Berücksichtigung des spannungsmechanischen Einflusses hingegen kann nicht von Bestehendem abgeleitet werden. Dazu wird aus einer umfangreichen Sammlung experimenteller Ergebnisse dieser Einflufaktor abgeleitet. Aus der so gegebenen Stichprobe lassen sich durch eine statistische Auswertung eine Reihe von Kennwerten identifizieren, die die Stützwirkung maßgeblich beeinflussen. Darauf aufbauend wird eine Bestimmungsgleichung für den spannungsmechanischen Einflußfaktor abgeleitet. Somit ist ein Stützwirkungskonzept geschaffen, das es erlaubt, die drei wesentliche Aspekte technologischen, statistischen und spannungsmechanischen Einfluß qualitativ uind quantitativ ins Örtliche Konzept zu integrieren. Mit dieser Vorgehsweise und einer vorliegenden Werkstoffwöhlerlinie läßt sich die bauteilspezifische Festigkeit abschätzen. Die so abgeleitete Wöhlerlinie erlaubt eine wesentlich genauere Lebensdauerberechnung, wie die Kotrolle an umfangreichen, unabhängigen Daten beweist. / The local strain concept serves to estimate the service life of cyclically loaded structures. Here the fatigue strength of the mere material coupon is taken as the basis in the calculation for notched components. This represents a distinct simplification of the complex fatigue process because of disregarding the relevant influences that determine strength and durability. By that the reliability of the calculation is not yet satisfying. The present investigation first classifies the reasons for this problem of transferabillity between notched component and unnotched specimen. A simple but flexible approach is used to describe the technological, statistical and and gradient effect. To consider the technological influences and statistical size effect numerically, known relationships and procedures are taken and adapted to the methodology of the concept. To catch gradient effects a new stress-relief-concept was developed. From a comprehensive database of experimental results gradient effects were were separated. By a statistical analysis relevant variables that govern the stress-relief are identified. Using these, an equation gets derived which enables to compute the gradient effects on fatigue strength. The developed stress-relief-concept allows to estimate a component-related strain(parameter-)-life-curve. Lifetime predictioins based on this are by far more reliable than those based on materials data only. This is verified by a check on an extensive and independent database.

Aufbauend auf historischen Anwendungen von Holzwerkstoffen in der Technik werden Vor- und Nachteile aus der Sicht des Maschinenbaus dargestellt sowie eine aktuelle Eignung dahingehend diskutiert. Besondere Aufmerksamkeit erfährt der integrative Leichtbau mit Holzfurnierlagenverbundwerkstoffen (Wood Veneer Composite, WVC). Durch deren modifizierbares Eigenschaftsprofil, in Verbindung mit einer an den Anwendungsfall angepassten Bauweise, ist eine zusätzliche Funktionsintegration möglich. Das Potenzial der Holzwerkstoffe wird an verschiedenen Konstruktionsbeispielen erläutert. Ergänzend werden Biege-Ersatz Emodul und Bauteilverformung eines Strukturelementes sowie Ergebnisse aus Ermüdungsversuchen der Verbindungstechnik dargestellt. Abschließend werden Probleme beider Umsetzung verschiedener Prototypen beschrieben und deren Lösung aufgezeigt. / Based on several historical applications of Wood Veneer Composites (WVC) the advantages and disadvantages of wood materials in today`s mechanical and plant engineering are discussed. Integrated WVC lightweight structures are especially taken into account. The highly modifiable mechanical properties of WVC’s allow application-adapted integration of further technical functions. The capabilities of integrated lightweight structures based on wood materials are demonstrated on several technical applications. Furthermore results of flexural tests, precisely the retrieved substitutet bending modulus and the maximal deflection as well as results of fatigue testing of the connecting components are given. Finally problems occurring during realization of WVC lightweight prototypes and their potential solutions are shown.

Ein metastabiler austenitischer Stahl der Zusammensetzung 16Cr-7Mn-6Ni wurde einerseits über die Methode der Rückumwandlung in Werkstoffzustände verschiedener Korngrößen überführt und andererseits additiv über das Electron Beam Melting (EBM)-Verfahren gefertigt. Das statische und das zyklische Verformungsverhalten werden stark von der Korngröße beeinflusst. Insbesondere der ultrafeinkörnige Zustand verzeichnete einen erheblichen Anstieg der Festigkeit bei gleichzeitig hoher Duktilität. Die Lebensdauer übertraf bei niedrigen Dehnungsamplituden die Lebensdauer der Vergleichszustände und lag - für ultrafeinkörnige Gefüge außergewöhnlich - selbst bei hohen zyklischen Beanspruchungen auf einem vergleichbaren Niveau. Im Gegensatz zur Korngröße hatten die prozessinhärenten Defekte der mittels EBM hergestellten Varianten kaum einen Effekt auf das Verformungsverhalten des Stahls. Auch die Absenkung der Lebensdauer fiel vergleichsweise gering aus. Diese hervorragende Schadenstoleranz wird der hohen Duktilität in Kombination mit der enormen Verfestigungskapazität zugeschrieben.

Aufbauend auf historischen Anwendungen von Holzwerkstoffen in der Technik werden Vor- und Nachteile aus der Sicht des Maschinenbaus dargestellt sowie eine aktuelle Eignung dahingehend diskutiert. Besondere Aufmerksamkeit erfährt der integrative Leichtbau mit Holzfurnierlagenverbundwerkstoffen (Wood Veneer Composite, WVC). Durch deren modifizierbares Eigenschaftsprofil, in Verbindung mit einer an den Anwendungsfall angepassten Bauweise, ist eine zusätzliche Funktionsintegration möglich. Das Potenzial der Holzwerkstoffe wird an verschiedenen Konstruktionsbeispielen erläutert. Ergänzend werden Biege-Ersatz Emodul und Bauteilverformung eines Strukturelementes sowie Ergebnisse aus Ermüdungsversuchen der Verbindungstechnik dargestellt. Abschließend werden Probleme beider Umsetzung verschiedener Prototypen beschrieben und deren Lösung aufgezeigt. / Based on several historical applications of Wood Veneer Composites (WVC) the advantages and disadvantages of wood materials in today`s mechanical and plant engineering are discussed. Integrated WVC lightweight structures are especially taken into account. The highly modifiable mechanical properties of WVC’s allow application-adapted integration of further technical functions. The capabilities of integrated lightweight structures based on wood materials are demonstrated on several technical applications. Furthermore results of flexural tests, precisely the retrieved substitutet bending modulus and the maximal deflection as well as results of fatigue testing of the connecting components are given. Finally problems occurring during realization of WVC lightweight prototypes and their potential solutions are shown.

In the present thesis, fatigue behavior of nodular cast iron (NCI) is investigated using micromechanical simulations. An elastic-plastic porous material experiences an increase in a void volume fraction with each cycle of loading. This is called void ratchetting. The hypothesis of this thesis is to explain the fatigue failure of NCI using void ratchetting mechanism. The strain-life, stress-life, notch support effect, and fatigue crack growth are studied using the micromechanical simulations. In all these studies, matrix material is defined as an elastic-plastic with isotropic/kinematic hardening. No damage law is used to define material degradation. The axisymmetric cell model is developed to study strain-life and stress-life approaches for fatigue. The cell model is subjected to cyclic loading and cycle by cycle simulations are carried out until failure. The failure of the cell model is defined based on the drop in the macroscopic response of the cell model. The notch support effect is investigated using a 2D plane strain model within stress-life concept. From the simulation results, strain-life and stress-life curves are extracted, and they are in qualitative and quantitative agreement with experimental data collected from literature. The fatigue crack growth is studied using a micromechanical cell model under small scale yielding conditions. The graphite particles are considered as voids, and they are resolved discretely in fracture process zone. The region outside of the fracture process zone is considered as a homogenized medium. When positive alternating loads are applied, ligaments in the fracture process zone show ratchetting behavior, which is responsible for an effective fatigue crack growth. This mechanism is relevant for the fatigue crack growth in NCI. The 2D plane strain boundary layer model is able to predict the effect of load ratio on threshold for the fatigue crack growth and the fatigue crack growth rate. The fatigue crack growth rate curves obtained from the simulations are compared with experimental data. It is essential to note that the void ratchetting (plastic collapse of the intervoid ligaments) is a crucial mechanism in NCI and more focus should be given to this mechanism as it is simple to implement and gives satisfying simulation results.

The desire for energy and resource savings brings magnesium alloys as lightweight materials with high specific strength more and more into the focus. Most structural components are subjected to cyclic loading. In the course of computer aided product development, a numerical prediction of the fatigue life under these conditions must be provided. For this reason, the mechanical properties of the considered material must be examined in detail. Wrought magnesium semifinished products, e.g. magnesium sheet metals, typically reveal strong basal textures and thus, the mechanical behavior considerably differs from that of the well-established magnesium die castings. Magnesium sheet metals reveal a distinct difference in the tensile and compressive yield stress, leading to non-symmetric sigmoidal hysteresis loops within the elasto-plastic load range. These unusual hysteresis shapes are caused by cyclic twinning and detwinning. Furthermore, wrought magnesium alloys reveal pseudoelastic behavior, leading to nonlinear unloading curves. Another interesting effect is the formation of local twin bands during compressive loading. Nevertheless, only little information can be found on the numerical fatigue analysis of wrought magnesium alloys up to now. The aim of this thesis is the investigation of the mechanical properties of wrought magnesium alloys and the development of an appropriate fatigue model. For this purpose, twin roll cast AM50 as well as AZ31B sheet metals and extruded ME21 sheet metals were used. Mechanical tests were carried out to present a comprehensive overview of the quasi-static and cyclic material behavior. The microstructure was captured on sheet metals before and after loading to evaluate the correlation between the microstructure, the texture, and the mechanical properties. Stress- and strain-controlled loading ratios and strain-controlled experiments with variable amplitudes were performed. Tests were carried out along and transverse to the manufacturing direction to consider the influence of the anisotropy. Special focus was given to sigmoidal hysteresis loops and their influence on the fatigue life. A detailed numerical description of hysteresis loops is necessary for numerical fatigue analyses. For this, a one-dimensional phenomenological model was developed for elasto-plastic strain-controlled constant and variable amplitude loading. This model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components. Considering different magnesium alloys, good correlation is reached between numerically and experimentally determined hysteresis loops by means of different constant and variable amplitude load-time functions. For a numerical fatigue life analysis, an energy based fatigue parameter has been developed. It is denoted by “combined strain energy density per cycle” and consists of a summation of the plastic strain energy density per cycle and the 25 % weighted tensile elastic strain energy density per cycle. The weighting represents the material specific mean stress sensitivity. Applying the energy based fatigue parameter on modeled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects. The combined strain energy density per cycle achieves significantly better results in comparison to conventional fatigue models such as the Smith-Watson-Topper model. The developed phenomenological model in combination with the combined strain energy density per cycle is able to carry out numerical fatigue life analyses on magnesium sheet metals.

Ultrafeinkörnige metallische Werkstoffe haben verstärkt wissenschaftliche Bedeutung erlangt. Um dieser neuartigen Werkstoffklasse über die grundlagenorientierte Forschung hinaus einen Einsatz in technischen Anwendungen zu ermöglichen, ist es notwendig, deren Verhalten unter verschiedenen einsatzrelevanten Belastungsbedingungen vorhersagen zu können. In der vorliegenden Arbeit wird das Schädigungsverhalten einer ultrafeinkörnigen Aluminiumlegierung in den Bereichen der hochzyklischen (HCF) und niedrigzyklischen (LCF) Ermüdung sowie des Rissfortschritts untersucht. Im Mittelpunkt steht dabei die Identifikation der mikrostrukturell wirksamen Mechanismen bei der Entstehung und Ausbreitung von Ermüdungsrissen. Es werden ein homogen ultrafeinkörniger und ein bimodaler Zustand sowie verschiedene duktilitätsoptimierte Zustände betrachtet und systematisch der Einfluss der Korngröße, der Korngrößenverteilung, der Ausscheidungscharakteristik sowie der Festigkeit und Duktilität auf das Ermüdungs- und Rissfortschrittsverhalten ermittelt. Die Untersuchungen zeigen, dass das Schädigungsverhalten der ultrafeinkörnigen Aluminiumlegierung insbesondere durch die Korngröße und Korngrößenverteilung sowie den Kohärenzgrad der festigkeitssteigernden Ausscheidungen beeinflusst wird.

The desire for energy and resource savings brings magnesium alloys as lightweight materials with high specific strength more and more into the focus. Most structural components are subjected to cyclic loading. In the course of computer aided product development, a numerical prediction of the fatigue life under these conditions must be provided. For this reason, the mechanical properties of the considered material must be examined in detail. Wrought magnesium semifinished products, e.g. magnesium sheet metals, typically reveal strong basal textures and thus, the mechanical behavior considerably differs from that of the well-established magnesium die castings. Magnesium sheet metals reveal a distinct difference in the tensile and compressive yield stress, leading to non-symmetric sigmoidal hysteresis loops within the elasto-plastic load range. These unusual hysteresis shapes are caused by cyclic twinning and detwinning. Furthermore, wrought magnesium alloys reveal pseudoelastic behavior, leading to nonlinear unloading curves. Another interesting effect is the formation of local twin bands during compressive loading. Nevertheless, only little information can be found on the numerical fatigue analysis of wrought magnesium alloys up to now. The aim of this thesis is the investigation of the mechanical properties of wrought magnesium alloys and the development of an appropriate fatigue model. For this purpose, twin roll cast AM50 as well as AZ31B sheet metals and extruded ME21 sheet metals were used. Mechanical tests were carried out to present a comprehensive overview of the quasi-static and cyclic material behavior. The microstructure was captured on sheet metals before and after loading to evaluate the correlation between the microstructure, the texture, and the mechanical properties. Stress- and strain-controlled loading ratios and strain-controlled experiments with variable amplitudes were performed. Tests were carried out along and transverse to the manufacturing direction to consider the influence of the anisotropy. Special focus was given to sigmoidal hysteresis loops and their influence on the fatigue life. A detailed numerical description of hysteresis loops is necessary for numerical fatigue analyses. For this, a one-dimensional phenomenological model was developed for elasto-plastic strain-controlled constant and variable amplitude loading. This model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components. Considering different magnesium alloys, good correlation is reached between numerically and experimentally determined hysteresis loops by means of different constant and variable amplitude load-time functions. For a numerical fatigue life analysis, an energy based fatigue parameter has been developed. It is denoted by “combined strain energy density per cycle” and consists of a summation of the plastic strain energy density per cycle and the 25 % weighted tensile elastic strain energy density per cycle. The weighting represents the material specific mean stress sensitivity. Applying the energy based fatigue parameter on modeled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects. The combined strain energy density per cycle achieves significantly better results in comparison to conventional fatigue models such as the Smith-Watson-Topper model. The developed phenomenological model in combination with the combined strain energy density per cycle is able to carry out numerical fatigue life analyses on magnesium sheet metals.