This study is focused on the performance evaluation of flash-butt-welded rail joints concerning their mechanical strength and fatigue life. Residual stresses created in the heat-affected zone were measured and the effects of the welding process and notch sensitivity on the fatigue behavior in the weld region were experimentally assessed. Aiming at clarifying the physical phenomenon behind the flash-butt welding, nonlinear transient thermomechanical finite-element analyses were performed to reconstitute the welding process and to simulate the subsequent formation of residual stresses. A new hypothesis for explaining higher temperatures in the rail web than in the rail foot and head was suggested and numerically verified. Experimental fatigue results showed high notch sensitivity and highlighted the effects of microstructure variations on the fatigue life. In general, a good agreement was achieved between numerical and experimental measurements of residual stresses, which was essential for understanding the fatigue phenomenon and the formation of residual stresses in flash-butt-welded rail joints. A relevant outcome is the simulation of the hot burrs from the welding process as prescribed heat inputs on a convenient part of the rail surface. This simulation strategy has shown adequate to explain the non-uniform temperature changes and the corresponding originated residual stresses. flashbuttwelded flash butt welded life heataffected affected assessed flashbutt finiteelement finite element verified general surface nonuniform non uniform

Resumo: Trilhos longos soldados por centelhamento elétrico (Flash Butt Welding - FBW) apresentam características desejáveis do ponto de vista do comportamento dinâmico da via, no entanto, as soldas são regiões de descontinuidade estrutural e mecânica onde se originam tensões residuais, e que estão associadas a falhas prematuras por fadiga. Ensaios de dilatometria foram empregados para simular os efeitos do tamanho de grão austenítico sobre a evolução microestrutural pós-soldagem. Simulações numéricas termomecânicas, não-lineares, no domínio do tempo, pelo Método dos Elementos Finitos (MEF), foram empregadas para avaliar os efeitos de modificações de parâmetros do processo de soldagem sobre as tensões residuais. Os resultados apresentados permitem compreender os mecanismos das alterações morfológicas da perlita na Zona Termicamente Afetada (ZTA) em termos de transformações de fases e podem ser utilizados para sistematicamente orientar mudanças nos parâmetros do processo ou controlar a taxa de resfriamento de modo a obter melhores condições metalúrgicas/mecânicas. Ademais, os resultados dos modelos numéricos mostram como a adoção de ZTA estreita influencia nas taxas de resfriamento e no desenvolvimento de tensões residuais em regiões críticas do componente.

Abstract: Long rails, welded by the Flash-Butt Welding process present desirable characteristics concerning the dynamic behavior of the track, however flash-butt-welded joints are regions of structural and mechanical discontinuity where high residual stresses originate, and, consequently, premature-fatigue failures may take place. This paper employs dilatometric analysis to simulate the austenitic grain size effects on the postweld microstructural evolution. Finite Element Method (FEM) is used to carry out transient, physically non-linear thermo-mechanical analyses to evaluate the effects of the welding parameters on residual stresses. The results allow understanding the mechanisms associated with the morphological changes of the pearlite in the HAZ in terms of phase transformation and can be used to systematically guide changes in the process parameters or to control the cooling rate in order to obtain better metallurgical/mechanical conditions. In addition, from the results of the numerical models, it is demonstrated how the adoption of narrow HAZ influences cooling rates and the development of residual stresses in critical regions of the welded component.

Resumo Trilhos longos, soldados pelo processo Flash Butt Welding (FBW), são a realidade das ferrovias brasileiras de média e alta carga por eixo. Embora apresentem características desejáveis do ponto de vista do comportamento dinâmico da via, as soldas são regiões de descontinuidade estrutural e mecânica onde se originam tensões residuais, e que estão associadas a falhas prematuras por fadiga. Simulações numéricas termomecânicas, fisicamente não-lineares, no domínio do tempo, pelo Método dos Elementos Finitos (MEF), foram empregadas para avaliar o desenvolvimento de tensões residuais originadas durante o processo de soldagem. Uma nova abordagem para a inclusão do aporte de calor envolvido no processo é proposta. Os resultados numéricos são comparados a medidas experimentais de tensões residuais superficiais e aspectos macrográficos das juntas, incluindo largura da Zona Termicamente Afetada (ZTA) e dados de taxas de resfriamento. Os resultados mostram boa correlação entre as análises numéricas e medidas experimentais de tensões residuais. Aspectos fundamentais relacionados ao desenvolvimento de tensões residuais são esclarecidos a partir da correlação entre análises numérica e experimental. Ademais, verifica-se que os modelos computacionais podem ser utilizados na previsão de pontos críticos para nucleação de trincas por fadiga e/ou avaliar efeitos de variáveis de processo sobre o campo de tensões residuais.

Abstract Long rails, welded by the Flash-Butt Welding (FBW) process, are the reality of the Brazilian railroads for medium and high axle loads. Although they present desirable characteristics concerning the dynamic behavior of the track, welded joints are regions of structural and mechanical discontinuity where high residual stresses originate, and, consequently, premature fatigue failures may take place. This paper employs the Finite Element Method (FEM) to carry out transient, physically non-linear thermo-mechanical analyses to evaluate residual stresses evolved in the welding process. A new approach is proposed to take into account the heat input involved in the process. The numerical results are compared to experimentally measured surface residual stresses, and to the macrographic joint aspects, including the HAZ width and cooling rate data. The results show a good correlation between the numerical and experimental measurements of residual stresses. Fundamental aspects related to the development of residual stresses are clarified, correlating numerical and experimental analyses. In addition, it is verified that the computational models can be used to predict critical crack nucleation points by fatigue, and/or to evaluate effects of process parameters on the residual stress field.

By coupling the Boundary Element Method (BEM) and the Finite Element Method (FEM) an algorithm that combines the advantages of both numerical processes is developed. The main aim of the work concerns the time domain analysis of general three-dimensional wave propagation problems in elastic media. In addition, mathematical and numerical aspects of the related BE-, FE- and BE/FE-formulations are discussed. The coupling algorithm allows investigations of elastodynamic problems with a BE- and a FE-subdomain. In order to observe the performance of the coupling algorithm two problems are solved and their results compared to other numerical solutions.