Previsão de vida em fadiga isotérmica e termomecânica em ferro fundido vermicular

Abstract

With outstanding casting properties, good thermal conductivity and appropriate mechanical performance, compacted cast irons are widely used in the cylinder head of diesel engine. The engine usually works under transient operating conditions, such as starting, suddenly loading and unloading and shutting down. The processes usually leads to large temperature variation and serious fatigue damage. Therefore, the components are submitted during long periods to a large number of heating and cooling cycles, making thermo-mechanical fatigue (TMF) one of the most common failure mechanism. In the present work predictions of mechanical behavior of vermicular cast iron, 450 class, are made, using an elasto-plastic constitutive model of the commercial finite element software ABAQUS ®. The parameters of this model are identified from the cyclic increasing loading test, isothermal fatigue, and tensile tests. The parameters of the Wöhler curve in isothermal fatigue are characterized, and with these parameters, a characterization is proposed for the case of thermomechanical fatigue of the same curve. The results of the computational simulation present good results in the case of constant loading in isothermal fatigue while in thermomechanical fatigue the constitutive model cannot represent the stress relaxation effect of the material, thus lacking a more adequate model that reproduce the viscous effects. The prediction of life presents a reasonable result that depends on further statistical studies to prove its efficiency. The best input method to the plasticity model that fits the fatigue tests is using parameters and , because these parameters present better flexibility to describe the hardening curve compared to the Ramberg-Osgood equation. It was possible to obtain the fatigue life prediction under isothermal conditions using the Wöhler equation. While in the thermomechanical loads, it was possible to predict life using the modified Wöhler equations, which depend on the variation of the mechanical deformation and the use of parameters found from isothermal tests performed at the maximum temperature of the thermomechanical cycle.