静引張・変動ねじり組合応力下の多軸動クリープ

抄録

A number of experimental works have been carried out on the multiaxial creep problem, but most of them are concerned with the multiaxial creep under steady stress conditions. It is difficult to find creep data for non-steady multiaxial stress conditions, which will provide the basis for the analysis of multiaxial creep problem under the general stress state including the non-steady one. In view of such a situation, the authors have conducted a study of dynamic creep under combined static tension and alternating torsion, and have shown that a strain-hardening type stress-strain rate equation provides a good basis for the analysis of this kind of multiaxial dynamic creep, in the same way as it does for the case of uniaxial dynamic or non-steady stress creep.
This report deals with further extension of the previous work to the more general case of multiaxial dynamic creep, dynamic creep under combined static tension and repeated torsion. In this case, there appear biaxial creep strains, tensile and torsional, while the creep strain was uniaxial in the previous study, torsional creep being zero due to the alternating torsional stress.
The tests were conducted with a low carbon steel at the temperature of 450°C, with a fixed ratio 0.75 of alternating torsional stress to mean torsional stress and with various ratios of mean torsional stress to static tensile stress in the range from zero to infinity. Static creep tests under combined tension and torsion were also made with various ratios of torsional stress to tensile stress from zero to infinity for the purpose of comparison.
The results showed that the tensile creep was greatly accelerated when a repeated torsional stress was superimposed on a static tensile stress. Alternatively, torsional creep was considerably increased by superposing a static tensile stress on a repeated torsional stress. The figure of creep curves was not altered appreciably by combining a tensile stress with a repeated torsional stress as compared with the creep curve for simple tension, except that the period of transient creep was somewhat longer.
These results were discussed from the standpoint of the multiaxial creep theory which was employed successfully in the previous study. In doing this, an effective stress was derived from the information of multiaxial static creep tests under combined tension and torsion, in the same way as was done in the previous study. This effective stress, together with the strain hardening type stress-strain rate equation, was applied successfully also to the dynamic creep of this time, whereas the Mises or Tresca effective stress failed to give the results that agree with the experimental data. Slight discrepancy was observed between the theory and the experiments mainly as the result of the difference in transient period between the static and the dynamic case. However, when a prediction of creep under multiaxial dynamic stress is to be made from the information of static creep data, the results predicted is on the safe side for design, making this method of prediction applicable to the practical design purposes.