[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.

The aim of this dissertation work is the evaluation of the influence of nickel alloying on the structure and mechanical properties, both monotonic and dynamic, of nodular cast iron with ferritic and bainitic matrix. Two chock melts with 0.5 and 2.7 % Ni were used to study the nickel influence. The quantitative evaluation of structure of these melts using image analysis was done and basic tensile mechanical properties were determined. Subsequently, the time optimization of two-stage ferritic annealing and isothermal austempered heat treatment at 375 °C was performed with the aim to obtain optimal ferritic and bainitic structures with best static and dynamic mechanical properties. After ferritic annealing the nickel alloying contributes to substitution hardening of ferritic matrix which positively affects its strength and other mechanical properties. The higher nickel content in the bainitic structure causes the shift of phase transformation times to longer times which results in restricted production of small carbides and in bigger volume of retained austenite. These features were confirmed by observation in transmission electron microscope. Precise tensile and low cycle fatigue tests at temperatures 23 and – 45 °C were performed on the optimized structures of both nodular cast irons. As a result of the notch effect of graphite nodules, microplastic deformation of both nodular cast irons was observed at stresses which were lower than the yield stress. The Hollomon's equation very well describes the individual parts of tensile curves for both nodular cast irons including their mutual comparison. From the low cycle fatigue tests, the cyclic hardening/softening curves, the evolution of elastic modulus and hysteresis loop shape parameters, cyclic stress-strain curves and fatigue life curves were obtained for both temperatures and materials. Moreover, the decrease of retained austenite volume was measured by neutron diffraction and the evolution of surface relief was characterized during cyclic straining for both austempered nodular cast irons at both temperatures. On the basis of these results both cyclic plasticity and fatigue degradation mechanisms in relation to the cyclic strain localization were described for both nodular cast irons.