Thermomechanical Loading Constraints

Abstract

There is a great deal of concern for thermomechanical fatigue (TMF) durability as it is an important technological problem. TMF is an unavoidable consequence of operating equipment at high temperatures. To get to and from high service temperatures, it is necessary to undergo temperature variation. If the variation is rapid enough, which is the case for high performance equipment, thermal stresses and strains are imposed by the transient thermal gradients in the alloy as heat flows in and out of the component. It is the cyclic thermal stresses and strains that cyclically deform the material and impart TMF damage. TMF has been approximated by isothermal fatigue studies for a number of years, but there is a growing concern regarding the adequacy of this approach. Examples, such as the one shown in figure 1, have been found over the past few years, show TMF damage to be more severe than would be expected from isothermal life prediction methods. How and why TMF damage differs from isothermal damage and how these differences might be predicted by advanced life prediction schemes is the subject of this paper.