抄録
In gas turbines the rotor disks are highly stressed by centrifugal force caused by high speed rotation and are required to run for a long time at high temperature. So it is important for designers to predict the stress and the creep strain. Research on the creep of rotating disk, both theoretical and experimental, is indispensable for development of gas turbine and many theoretical papers have been published. But, as for experimental papers, the number is quite small, as the experiment necessitates complicated methods and is expensive and moreover requires time and efforts.
Wahl has developed an analytical study on steady state creep of rotating disks extending Bailey's method. In his study, creep rate at the given temperature is assumed to be a function of stress multiplied by a function of time i.e.ε=F(σ)f(t) and as a key expression between stress and creep rate the power function relation i.e. F(σ)=σn is adopted. Two dimensional stress state is assumed because axial stress is thought negligible as compared with radial and tangential stresses which are constant across the thickness of the disk. Elastic and transient creep strains are assumed to be negligible as compared with the steady state creep strain. Calculations are carried out in the following three cases: (1) Mises criterion and Mises flow rule, (2) Tresca criterion and Mises flow rule and (3) Tresca criterion and its associated flow rule. The analytical results have been compared with the experimental results carried out by him and his collaborators. In the experiments, allegheny 418 stainless steel disks (12% Cr, 3%W) were tested at an uniform temperature of 1000°F (538°C), and the test time exceeded 900 hours. Disks were 12 inches (305mm) O.D., 2.5 inches (63.5mm) I.D. and 1 inch (25.4mm) in thickness. According to him, the analytical results obtained by means of Tresca criterion and its associated flow rule generally showed the closest agreement with the test data. Based upon this result he is continuing the analysis further.
Manson has presented an analytical method, in which effects due to elastic and plastic strains, disk contour and temperature gradient of disk are taken into account as well as the effect of creep strain. Deformation theory in plasticity is applied for the calculation of creep strain, and Mises criterion and flow rule are employed. Other assumptions are similar to those of Wahl. In numerical calculation the finite difference approach is used, but it is so complicated that the computation is almost impossible without electronic computors.
In this paper, the authors have adopted the assumptions similar to those of Wahl. Elastic and transient creep strains are assumed to be negligible as compared with steady state creep strains. Mises and Tresca criteria with Mises flow rule are applied in the calculation. In solving stress equations Runge-Kutta's method or finite difference approach is used. The results obtaind are close to those of Wahl, but the calculation procedure is simpler. Analytical results are compared with the test data obtained by means of hot spin tester manufactured by the authors. S 816 disk (41% Co, 20% Ni, 20% Cr) was tested and its sizes were 300mm O.D., 90mm I.D. and 20mm in thickness. Disk temperature was uniform over the disk and was 820°C. The results of the authors' analysis and experiments are compared with those of other researchers. Tresca criterion has given a good agreement with the authors' test data as it did in Wahl's experiment. However, further test data are necessary before a general conclusion can be drawn.
