低炭素鋼の静引張繰返ねじり組合せ応力下の多軸動クリープ

抄録

Numerous articles have been published on the multiaxial creep problem since 1935 when bailey presented his extensive theoretical work together with some experimental results on this subject. However, these articles are concerned mainly with theories, and experimental works, so far made, are not sufficient to verify the theories. Especially, the experimental works on the nonsteady stress problem are two few, there being only those conducted by the Johnson and his colleagues.
This report deals with the two dimensional dynamic creep of thin-walled cylinder under axial static tension and alternating torsion, as a special case of the nonsteady-state multiaxial creep. The tests were conducted with a low carbon steel at the temperature of 450°C. The static tensile load was applied to the specimen by dead weight through a lever mechanism, while the alternating torsion was given by vibrating the end of a lever, attached to the specimen grip perpendicularly to the axis of the specimen, with the aid of an eccentric driven by an induction motor. The vibrating frequency was 1500 cycles per minute. These loading assemblies were realized by modifying the elevated temperature fatigue testing machine for combined tension and bending, which was used by the authors previously in making the combined stress dynamic creep test under static tension and alternating bending. Heating of the specimen was carried out by a conventional heating system consisting of an electric furnace and an automatic temperature controller. The axial elongation of the specimen was measured by dial gauges.
The results of the tests revealed that the material employed shows similar creep curves under combined static tension and alternating torsion as those in the case of simple tension creep tests. The magnitude of creep under a combined stress, however, was considerably greater than under the static tensile stress that is equal in magnitude to the static tensile component of the combined stress.
These experimental results were discussed from the standpoint of the multiaxial creep theory, and a fairly good agreement was obtained between theory and experiment. In this discussion, the multiaxial creep theories based on von Mises and Tresca criterion for the effective stress were first taken up by using the strain hardening criterion for the stress-strain relation, but they failed to agree with the experimental results. As the second step, the authors used an effective stress, derived from the information of multiaxial static creep tests made with the same material under the combined tension and torsion, and obtained a fairly good agreement between theory and experiment for dynamic creep. Finally, the effect of alternating torsional creep strain on strain hardening was also discussed, and it was shown that a better agreement between theory and experiment can be obtained by taking this effect into account. However, this effect was rather small and may be neglected for practical purpose.