Damage Criterions in Thermo-Mechanical Fatigue Models

In the present paper different damage criterions, recorded in literature, are compared, distinguishing the philosophical approach to thermo-mechanical fatigue (TMF) in energetic methods and phenomenological methods. TMF is the combination of mechanical and thermal stresses that takes place in most of mechanical components, especially in automotive engines. The presence of thermal together with mechanical cycling can worsen fatigue and ageing phenomena. In order to describe the TMF behavior of a material or of a component it is necessary to take into account different phenomena contributing to progressive damage as plastic strain, thermal strain, creep and oxidation. In literature it is possible to find different definitions of damage: strain accumulated energy, volume unit damage, residual life. Many papers on TMF are available like experimental studies, analytical investigations, analytical models of damage, life prediction models. The present paper aims to make a first selection of TMF life assessment criteria by means of literature critical analysis. Then, by means of numerical model of actual TMF component, i.e. an exhaust manifold, a comparison between the selected methods is done. Using a set of experimental and numerical analyses of exhaust manifolds collected in literature papers, a comparison and results discussion are done.