Thermomechanical fatigue – Mechanism-based considerations on the challenge of life assessment

The combination of cyclic mechanical and cyclic thermal loading leads to thermomechanical fatigue (TMF) which is considered to be the primary life-limiting factor for engineering components in many high-temperature applications. Extensive low-cycle fatigue (LCF) data, which is traditionally used for design purposes, has been generated isothermally on various high-temperature materials, and thus, it is tempting to try to predict TMF life based mainly on isothermal LCF data. In this contribution, studies on different metallic structural high-temperature materials, which have mainly been carried out in the author's laboratory, are reviewed addressing the question, in which way and to which extent a reliable, unerring and robust TMF life assessment is possible on the basis of isothermally obtained fatigue life data. It is shown by means of examples that a sound TMF life prediction first of all requires a detailed mechanistic understanding of the isothermal cyclic stress-strain response and the relevant damage mechanisms. Furthermore, the TMF-specific peculiarities in both the non-isothermal cyclic stress-strain behaviour and the non-isothermal damage evolution process must be known. If all these requirements are fulfilled and reflected in the TMF life assessment methodology applied, a reasonable predictive accuracy can be attained.