抄録
Concerning the problems respecting the creep under non-steady multiaxial stress conditions, there are two cases in the state of stress encountered. The first is the case in which the principal stress directions are kept constant, and the second is the case in which those are variable with varying stresses. In order to investigate a creep behavior under such conditions, the creep of thick-walled cylinders of low carbon steel and 21/4Cr-1Mo steel under stepwise changing internal pressure of periodic rectangular wave was studied. This is one of the fundamental problems on non-steady multiaxial stress conditions under constant direction and constant ratio of principal stresses. Furthermore, such a situation is present in boiler tubes and pressure vessels subjected to internal pressure. It is necessary, therefore, to consider the problem with the aim of predicting the creep strength of thick-walled pressurized cylinders under varying stress.
As the results of the experimental and analytical study. It was found that the law of“the mechanical equation of state in solid”was not valid for the creep of the materials tested both in the case of varying uniaxial tension and also in the case of varying multiaxial stresses. The strain rate at the period of reloading was about 20% higher than that under steady load, and the rupture life in the tests under cyclic stress was about 20% shorter than that of the reference tests under constant stress. In the stage of steady state creep, creep recovery came to an almost constant value independent of the amount of strain. These tendencies were quite the same in the creep of the thick-walled cylinders under varying internal pressure. It is considered that residual stresses in the cylinder at the period of pressure removal have little effect on the stress distribution after reloading, and it seems to be appropriate to assume in the analysis that there is no rotation in the principal directions of creep rate and that the material is isotopic under cyclic loading and unloading. This leads to the conclusion that the creep strength under non-steady multiaxial stress condition of constant stress direction and constant ratio of principal stresses can be estimated from the results of the creep tests under varying uniaxial tension by the use of the multiaxial creep theory based on the relationship between the static creep strength of the uniaxial stress and that of the multiaxial stresses.
