Approach on creep deformation and thermal stress state of a uniformly deformed high temperature component under a graded temperature condition

Abstract

Application of sophisticated cooling systems and thermal barrier coatings to advanced gas turbine components impels the structural materials be in service under a significantly graded temperature condition. Whereas the graded temperature enhances thermal or internal stress which leads to thermo-mechanical fatigue failure in conjunction with external load, the thermal stress can also be relaxed time-dependently by creep. Since the temperature gradation may get more pronounced in future, a material design concept will be required under such circumstances. In this work a simple two-dimensional model is constructed to semi-quantitatively estimate the creep deformation and internal or thermal stress state for a high temperature component which uniformly deformed under a graded temperature condition. Based on the model the numerical calculations were carried out for some representative cases, by employing elastic-creep constitutive equation: one is a case without no external load, and the other is a case when an external tensile load is applied. The calculation indicated that in the former case the thermal stress deforms the component in compression when the stress relaxation is more significant at the higher temperature side than at the lower temperature side. Not only the range of graded temperature, but also the distribution shape is found to remarkably affect the behavior. It is also indicated, as a reflection of the above behavior that the component can deform in compression at early stage even when the external stress in tension is applied to the component, followed by tensile deformation. Of particular importance for structural design is that the stress promotion always takes place at the lower temperature side in the component. These behaviors are summarized as a function of relative magnitude between the thermal stress and external stress