Abstract
Structure members which are subjected to load at high temperature are, in general, not in a steady condition of applied stress and temperature. Therefore, the studies on creep of structures under cyclic temperatures or cyclic stressing are important problem. In this paper, the influence of temperature history on creep is discussed. Experimental and analytical relationships of the creep and the rupture life under temperature cycling or under combined variation of stress and temperature to the static creep and the rupture life are presented.
(1) The effect of temperature history on creep is composed of transitional change in internal yield stress, that is, recovery or aging, where the internal yield stress is defined as the local stress at which deformation begins without the aid of thermal fluctuation. The influence of the temperature history results in the existence of “induction period” or transitional increase in creep rate immediately after the change of temperature. However, these effects are negligible small in most metallic materials and the concept of “mechanical equation of state in solid”or the Robinson's hypothesis is applicable for preduction of creep or rupture life under cyclic temperatures.
(2) By adopting the assumption the creep strain in the stage of transient or steady state creep under temperature cycling can be predicted analytically from the data of static creep test in transient stage, by introducing the equivalent steady temperature. The equivalent steady temperature means the temperature at which the transient creep strain under cyclic temperatures is equivalent to the strain in the static creep test. In like manner, creep rupture life under temperature cycling can also be estimated analytically from the data of static creep rupture test by introducing the equivalent steady temperature for creep rupture life. These analysises are usefull if severe metallurgical change such as microstructural phase change or corrosion does not occur. The assumption is also applicable for prediction of creep or the rupture life under combined variation of stress and temperature. However, under the condition that the stress amplitude which is synchronized with temperature cycle is large, the influence of recovery must be considered.
