Die vorliegende Arbeit beschäftigt sich mit der zweidimensionalen Simulation der Rissausbreitung mit Hilfe der adaptiven Finite-Elemente-Methode. Das Ziel war die Entwicklung von Algorithmen zur effizienten und automatisierten Modellierung des Risswachstums. Das zugrunde liegende Konzept besteht in der vollständigen Integration aller Teilschritte der Risswachstumssimulation in ein einziges FE-Programm. Während der gesamten Simulation erfolgt durch den Einsatz eines fehlergesteuerten h-adaptiven Verfahrens die automatische Anpassung der FE-Diskretisierung an das gestellte Rissproblem. Die Simulation der spröden Rissausbreitung erfolgt auf Basis der linear-elastischen Bruchmechanik. Die dafür benötigten Spannungsintensitätsfaktoren werden mit Hilfe des J-Integrals in Form der Interaction-Integral-Technik ermittelt. Die Simulation des duktilen Versagens in der Prozesszone an der Rissspitze wird mit Hilfe des Schädigungsmodells von Rousselier beschrieben. Das Kriterium für duktiles Risswachstum basiert auf der Auswertung des akustischen Tensors an der Rissspitze und legt den Beginn der makroskopischen Rissausbreitung mit dem Einsetzen der Lokalisierung fest.

<p>QC 20110523</p>

Cracking is an important potential failure mechanism for pavement structures. By combining a strain energy-based fracture criterion with conventional fracture mechanics based on the Energy Ratio (ER) concept, crack growth in asphalt can be investigated, and a low temperature Thermal Cracking model (TCMODEL) can be introduced. This thesis presents the implementation of the Florida cracking model into a Mechanistic-Empirical (ME) flexible pavement design framework. An improved analysis procedure for better converting raw data from the Superpave Indirect Tensile Test (IDT) into fundamental viscoelastic properties of the asphalt mixture allows for calibration of the TCMODEL. This thesis involves a detailed review of Florida cracking model and TCMODEL. Finally, a MATLAB tool is prepared for the thermal cracking model to investigate the cause and effect of the problems.

Rolling contact fatigue is a problem encountered with many machine elements.In the current report a numerical study has been performed in order to predictthe crack path and crack propagation cycles of a surface initiated rolling contactfatigue crack. The implementation of the contact problem is based on theasperity point load mechanism for rolling contact fatigue. The practical studiedproblem is gear contact. Different loading types and models are studied andcompared to an experimental spall profile. Good agreement has been observedconsidering short crack lengths with a distributed loading model using normalloads on the asperity and for the cylindrical contact and a tangential load on theasperity. Several different crack propagation criteria have been implemented inorder to verify the validity of the dominant mode I crack propagation assumption.Some general characteristics of rolling contact fatigue cracks have beenhighlighted. A quantitative parameter study of the implemented model hasbeen performed. / Utmattning med rullande kontakter är ett ofta förekommande problem för många maskinelement. I den aktuella rapporten utfördes en numerisk studieför att förutsäga sprickvägen hos utmattningssprickor som initierats i ytan vidrullande kontakter. Implementeringen av kontaktproblemet bygger på asperitpunktlastmekanismen för rullande kontakter. Studien av kontaktproblemetär tillämpad till kugghjul. Olika belastningstyper och modeller studeradesoch jämfördes med profilen hos en experimentell spall. Bra överensstämmelseobserverades för korta spricklängder när en modell med fördelad belastninganvänds för en belastningstyp där en normalbelastning agerar på asperiten ochvid cylindriska kontakten och en tangentialbelastning införs på asperiten. Olikakriterier för spricktillväxt implementerades för att verifiera giltigheten av antagandetatt mode I spricktillväxt är dominant. Några generella kännetecken avutmattningssprickor med rullande kontakter framhävdes. En kvantitativ parameterstudie för den implementerade modellen utfördes.

No description available.

The thesis deals with the study of the fracture behaviour of the pipe grades of high density polyethylene (HDPE). Detailed study of current state of knowledge was performed in the first part. The focus was put on those types of polymers used for pipeline applications, the limit states of the plastic pipes, and the key parameters affecting the fracture behaviour of plastic pipes’ materials. The methods used for testing the plastic pipes and native pipeline materials were summarized. In the second part of this work, the relationship between the processing conditions, internal structure and fracture behaviour of the native types of HDPE pipe grades was evaluated. Than, the correlation between short-term alternative tests and the standardized accelerated full notch creep test (FNCT) was studied. An experimental basis for numerical model for evaluation of the fracture behaviour of multilayer pipes was established. Two types of non-homogeneous specimens for evaluating the resistance of multilayer pipes against slow crack growth (SCG) were suggested. This evaluation was based on the parameters of linear elastic fracture mechanics (LEFM) and elasto-plastic fracture mechanics (EPFM).

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[pt] Medindo a rigidez de corpos de prova em testes de propagação de trinca por fadiga, Elber identificou que a ponta da trinca só está totalmente aberta durante uma parte do ciclo de carregamento, e nomeou o carregamento onde a trinca abre totalmente de carga de abertura Pab. Baseado nisso, Elber assumiu que o dano à frente da trinca era induzido apenas pela fração do carregamento acima da Pab, propondo que a força motriz na propagação de trincas por fadiga é a gama do fator de intensidade de tensões efetivo (delta)Kef. Para verificar esta hipótese, neste trabalho foram examinados diferentes cenários em testes de propagação de trinca por fadiga. Primeiramente, em corpos DC(T) de aço AISI 1020 e em corpos DC(T) e C(T) de alumínio 6351-T6 foi propagada uma trinca com (delta)K e Kmax quase constantes, medindo Pab em campo próximo e em campo distante, com extensômetros e com um sistema 3D de correlação digital de imagens (DIC). Depois, usando novos corpos DC(T) de aço e de alumínio, foi propagada uma trinca com (delta)K e Kmax quase constantes, antes e depois de um evento de sobrecarga, medindo a Pab ao longo do teste, em campo próximo e campo distante. Cabe salientar que nesses testes as espessuras dos corpos de prova foram projetadas para que a trinca propagasse em tensão plana e em deformação plana. Finalmente, em testes de propagação de trinca com (delta)K e Kmax quase constantes, em corpos DC(T) de aço e de alumínio foi medido o campo de deformação à frente da ponta da trinca com um sistema de microscópio estéreo DIC, para analisar o comportamento dentro da zona plástica, medindo também a Pab com os métodos mencionados anteriormente. Dos resultados dos testes três comportamentos foram particularmente relevantes. Nos primeiros testes de propagação com deltaK e Kmax quase constantes, a razão Kab/Kmax diminuía enquanto a trinca propagava com uma taxa constante. Nos testes de sobrecarga o valor mínimo de (delta)Kef estava defasado em relação ao valor mínimo de taxa de propagação (da/dN). Já nos testes onde foi medido o campo deformação com o sistema de microscópio estéreo DIC, a deformação 0.1mm à frente da ponta da trinca mostrou que existia dano para cargas abaixo da Pab. Portanto, estes resultados não podem ser explicados pela hipótese de Elber, e contestam o (delta)Kef como a força motriz na propagação de trincas por fadiga. / [en] Measuring the stiffness of a fatigue-cracked plate during its loading cycle, Elber discovered that this crack only completely opened after reaching the crack opening load Pop. Based on this, Elber assumed that the damage ahead of the crack is induced by the loading part above the Pop. In this way, he proposed that the range of the effective stress intensity factor (delta)Keff is the driving force in fatigue crack growth. In order to verify this hypothesis, this study investigated different scenarios in fatigue crack growth tests. For this, DC(T) specimens of AISI 1020 steel, and DC(T) and C(T) specimens of 6351-T6 aluminum were tested. The fatigue crack growth tests were performed under quasi-constant K and Kmax conditions. The Pop measurements in the near and far field were obtained from strain gage readings and 3D Digital Image Correlation (DIC) analysis. In addition, simple variable amplitude tests were performed in steel and aluminum DC(T) specimens. A single tensile overload was introduced in the fatigue crack growth experiments under quasi-constant (delta)K and Kmax conditions, also measuring the Pop throughout the test in the near and the far field. It is important to note that the thickness of the specimens was designed to perform a crack propagation in plane stress and plane strain conditions. Moreover, near tip strain measurements in steel and aluminum DC(T) specimens were obtained with a stereo microscope DIC system, to analyze the behavior within the plastic zone, also measuring the Pop with the methods previously mentioned. From the experimental results, three behaviors were particularly relevant. In the first fatigue crack growth tests with quasi-constant {(delta)K, Kmax} conditions, the ratio Kop/Kmax decreased as the crack propagates under a constant rate. In the fatigue crack growth tests with overload application, the minimum value of (delta)Keff was lagged in relation to the minimum value of the propagation rate (da/dN). Finally, in the strain measurements performed with the stereo microscope DIC system, the de-formation at 0.1mm ahead of the crack tip showed the existence of damage in load values below Pop. Therefore, these results cannot be explained by Elber s hypothesis, and question the (delta)Keff as the driving force in fatigue crack growth.