Determinação da taxa de propagação de trinca por fadiga dos aços SAE-1050 e SAE-4130 empregados na fabricação de eixos ferroviários

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

On the finite element modeling of fatigue crack growth in pressurized cylindrical shells

A methodology for the computational modeling of the fatigue crack growth in pressurized shell structures, based on the finite element method and concepts of Linear Elastic Fracture Mechanics, is presented. This methodology is based on that developed by Potyondy [Potyondy D, Wawrzynek PA, Ingraffea, AR. Discrete crack growth analysis methodology for through crack in pressurized fuselage structures. Int J Numer Methods Eng 1995;38:1633-1644], which consists of using four stress intensity factors, computed from the modified crack integral method, to predict the fatigue propagation life as well as the crack trajectory, which is computed as part of the numerical simulation. Some issues not presented in the study of Potyondy are investigated herein such as the influence of the crack increment size and the number of nodes per element (4 or 9 nodes) on the simulation results by means of a fatigue crack propagation simulation of a Boeing 737 airplane fuselage. The results of this simulation are compared with experimental results and those obtained by Potyondy [1]. (C) 2008 Elsevier Ltd. All rights reserved.

Simulation on the process of fatigue crack initiation in a martensitic stainless steel

Die vorliegende Arbeit beschäftigt sich mit der Computersimulation des Rissinitiierungsprozesses für einen martensitischen Stahl, der der niederzyklischen Ermüdung unterworfen wurde. Wie auf der Probenoberfläche beobachtet wurde, sind die Initiierung und das frühe Wachstum dieser Mikrorisse in hohem Grade von der Mikrostruktur abhängig. Diese Tatsache wurde in mesoskopischen Schädigungsmodellen beschrieben, wobei die Körner als einzelne Kristalle mit anisotropem Materialverhalten modelliert wurden. Das repräsentative Volumenelement (RVE), das durch einen Voronoi-Zerlegung erzeugt wurde, wurde benutzt, um die Mikrostruktur des polykristallinen Materials zu simulieren. Spannungsverteilungen wurden mit Hilfe der Finiten-Elemente-Methode mit elastischen und elastoplastischen Materialeigenschaften analysiert. Dazu wurde die Simulation zunächst an zweidimensionalen Modellen durchgeführt. Ferner wurde ein vereinfachtes dreidimensionales RVE hinsichtlich des sowohl dreidimensionalen Gleitsystems als auch Spannungszustandes verwendet. Die kontinuierliche Rissinitiierung wurde simuliert, indem der Risspfad innerhalb jedes Kornes definiert wurde. Die Zyklenanzahl bis zur Rissinitiierung wurde auf Grundlage der Tanaka-Mura- und Chan-Gleichungen ermittelt. Die Simulation lässt auf die Flächendichten der einsegmentige Risse in Relation zur Zyklenanzahl schließen. Die Resultate wurden mit experimentellen Daten verglichen. Für alle Belastungsdehnungen sind die Simulationsergebnisse mit denen der experimentellen Daten vergleichbar.

The surface treatment influence on the fatigue crack propagation of Al 7050-T7451 alloy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Describing fatigue crack growth and load ratio effects in Al 2524 T3 alloy with an enhanced exponential model

The fatigue crack behavior in metals and alloys under constant amplitude test conditions is usually described by relationships between the crack growth rate da/dN and the stress intensity factor range Delta K. In the present work, an enhanced two-parameter exponential equation of fatigue crack growth was introduced in order to describe sub-critical crack propagation behavior of Al 2524-T3 alloy, commonly used in aircraft engineering applications. It was demonstrated that besides adequately correlating the load ratio effects, the exponential model also accounts for the slight deviations from linearity shown by the experimental curves. A comparison with Elber, Kujawski and "Unified Approach" models allowed for verifying the better performance, when confronted to the other tested models, presented by the exponential model. (C) 2012 Elsevier Ltd. All rights reserved.

Fatigue crack growth of Al 7475 T761 under constant and variable amplitude loading

Experimental programs in constant and variable amplitude loading were performed to obtain a x N curves and to study retardation in fatigue crack growth due to overloads. The main aim of this research program was to analyse the effect of overload ratio and number of overload peaks. The effect of underloads, before and after the overload blocks was also studied. The generalised equation of Paris-Erdogan type was used for modelling of obtained data on crack propagation under constant amplitude load.

Modelling of fatigue crack growth following overloads

In variable-amplitude loading there are interaction effects between the loading history and the crack propagation rate. The most important of these effects is the retardation in the crack propagation, which may raise the life of the cracked structureconsiderably. The main objective of this research is to analyse and quantify the retardation of crack propagation in a thin plate of the high-resistance aluminium alloy 2024-T3, comparing the results obtained from the mathematical models proposed to account for the retardation effect. The specimens were tested under high-low loading sequences, for different crack sizes and overload ratios. A simplified model was developed, based on crack closure theory, that could represent the crack behaviour during retardation very well. © 1991.

Microstructural effects on fatigue crack growth behavior of a microalloyed steel

Thermal transformations on microalloyed steels can produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect the crack path or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of the present work is to evaluate the effects of microstructure on the tensile strength and fatigue crack growth (FCG) behaviour of a 0.08%C-1,5%Mn (wt. pct.) microalloyed steel, recently developed by a Brazilian steel maker under the designation of RD480. This steel is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Various microstructural conditions were obtained by means of heat treatments followed by water quench, in which the material samples were kept at the temperatures of 800, 950 and 1200 °C. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results allowed correlating the tensile properties and crack growth resistance to the microstructural features. It is also shown that the Region II FCG curves of the dual and multiphase microstructural conditions present crack growth transitions that are better modeled by dividing them in two parts. The fracture surfaces of the fatigued samples were observed via scanning electron microscopy in order to reveal the fracture mechanisms presented by the various material conditions. © 2010 Published by Elsevier Ltd.

Physical integration of Mercosur-Bolivia-Chile-Peru: The potential contribution of the railway systems

This edition of the Bulletin highlights sections of a recent study carried out by the Transport Unit of ECLAC on behalf of the Institute for Latin American Integration (INTAL) entitled Physical Integration of Mercosur-Bolivia- Chile-Peru: the potential contribution of the railway systems. One of the conclusions stresses the need for the railway and other companies involved in the construction of any major project to sign formal agreements relating to the allocation of freight, track utilization fees and other factors on which operation of the railway to be constructed will depend.

The current state of metro and railway systems in Latin America

This issue of the FAL Bulletin describes how metro and railway systems contribute to sustainable mobility, looking at some of the current challenges and their technical solutions.

Fatigue crack growth rate in mode I of a carbon fiber 5HS weave composite laminate processed via RTM

Delamination or crack propagation between plies is a critical issue for structural composites. In viewing this issue and the large application of woven fabrics in structural applications, especially the ones that requires high drapeability to be preformed in a RTM mold cavity such as the asymmetric ones, e.g HS series, this research aimed in dynamically testing the carbon fiber 5HS/RTM6 epoxy composites under opening mode using DCB set up in order to investigate the crack growth rate behavior in an irregular surface produced by the fabric waviness. The evaluation of the energy involved in each crack increment was based on the Irwin-Kies equation using compliance beam theory. The tests were conducted at constant stress ratio of R=0.1 with displacement control, frequency of 10 Hz, in accordance to ASTM E647-00 for measurement of crack growth rate. The results showed large scatter when compared to unidirectional carbon fiber composites due to damage accumulation at the fill tows.

Describing fatigue crack growth and load ratio effects in Al 2524 T3 alloy with an enhanced exponential model

The fatigue crack behavior in metals and alloys under constant amplitude test conditions is usually described by relationships between the crack growth rate da/dN and the stress intensity factor range Delta K. In the present work, an enhanced two-parameter exponential equation of fatigue crack growth was introduced in order to describe sub-critical crack propagation behavior of Al 2524-T3 alloy, commonly used in aircraft engineering applications. It was demonstrated that besides adequately correlating the load ratio effects, the exponential model also accounts for the slight deviations from linearity shown by the experimental curves. A comparison with Elber, Kujawski and "Unified Approach" models allowed for verifying the better performance, when confronted to the other tested models, presented by the exponential model. (C) 2012 Elsevier Ltd. All rights reserved.

Fatigue crack growth assessments in welded components including crack closure effects: Experiments and 3-D numerical modeling

This work provides a numerical and experimental investigation of fatigue crack growth behavior in steel weldments including crack closure effects and their coupled interaction with weld strength mismatch. A central objective of this study is to extend previously developed frameworks for evaluation of crack clo- sure effects on FCGR to steel weldments while, at the same time, gaining additional understanding of commonly adopted criteria for crack closure loads and their influence on fatigue life of structural welds. Very detailed non-linear finite element analyses using 3-D models of compact tension C ( T ) fracture spec- imens with center cracked, square groove welds provide the evolution of crack growth with cyclic stress intensity factor which is required for the estimation of the closure loads. Fatigue crack growth tests con- ducted on plane-sided, shallow-cracked C ( T ) specimens provide the necessary data against which crack closure effects on fatigue crack growth behavior can be assessed. Overall, the present investigation pro- vides additional support for estimation procedures of plasticity-induced crack closure loads in fatigue analyses of structural steels and their weldments