内圧厚肉円筒クリープおよび残留応力測定について

抄録

In recent years so much attention has been called to the problem of creep or creep rupture in pressure vessels and tubes that operate at high temperature, and numerous theoretical and experimental works on this problem have hitherto been reported. There are, however, several open questions that must be discussed from the standpoint of multiaxial creep behaviors.
The authors have taken interest in this subject and have been carrying out the creep tests of thick-walled cylinders under internal pressure at high temperature. The tests have been made with 0.19% carbon steel at the temperature of 450°C. The tubular specimens had a 400mm cylindrical gauge length, with the outside diameter about 50.45mm and the ratio 2 of outside to inside diameter (OD/ID). The specimens of uniaxial creep tests were cut from the same tubes. The internal pressure was supplied by the hand-operated water-pump and was maintained within ±1% by the pressure controller. The circumferential elongation on the outside diameter of the tubular specimen was measured by dial gauges and mirror-telescope instruments.
The results of the tests revealed that the circumferential strain on the outside diameter versus the time relations for the tubular specimens were similar as the strain versus the time relations for the uniaxial tension. The axial creep for the tubular specimens was negligible.
In addition to this, this report deals with the results of the measurement of residual stress distributions in the thick-walled cylinders after the creep tests. The measurement was carried out by the well-known Sachs' method. The X-ray method was also used for the residual stress determinations on the outer surface of the cylinder, and a fairly good agreement was obtained between the stress values measured by the two methods.
The results obtained were as follows: No residual stress in the cylinder before the creep tests can be measured except near the outer and inner surfaces. The residual stress distributions after the creep tests under 1000kg/cm² for 1 hour are similar as those under 1200kg/cm² for 104 hours; that is, the circumferential and axial residual stresses change monotonously from compression on the inner surface to the tension on the outer surface, and the radial residual stress, which is smaller than the others, is compressive in the wall. The stress distributions in creep conditions determined by these measured residual stresses correspond to the steadystate stress distributions. It was also found that the circumferential and axial residual stresses measured by means of X-rays remained constant on the surface of the cylinders after the creep tests under 1000kg/cm² for 1, 10 and 100 hours respectively.
From these results it is found that, as far as the creep of the cylinders at 450°C under 1000kg/cm² is concerned, the stress distributions change from the initial elastic state to a steady state in a short time. It is the authors' belief that the time required for the stress distributions almost to reach the steady state will be relatively short in the case of creep at high temperature and high stress. However, further discussions on a transient stage will be necessary from the point of view of multiaxial creep theory.