Abstract
The biaxial creep deformation of a thin-walled tube of celluloid softened by heating was tested under variable torsional stress consisting of stepwise increase, removal and reversal with superimposed constant axial stress. The results are summarized as follows.
(1) The axial creep strain is affected by the reversal of torsional stress under constant axial stress. The axial creep strain rate increases just after the reversal of torsional stress. Although axial creep strain increases in the resting stage of torsional stress, the increment of the strain is smaller than that in the loading stage under torsional stress.
(2) There appears the softening of torsional creep strain at the first reversal of torsional stress. The amount of variation of torsional creep strain decreases with the number of torsional stress reversal. The amount of recoverable torsional creep strain depends not on axial stress but on torsional stress, and the variation of the recoverable strain decreases with the number of stages.
(3) Just after the abrupt change of stress vector, the direction of creep strain increment vector inclines to show the direction of torsional stress from that of stress vector. The directions of creep strain increment vector and stress vector coincides with each other with the lapse of time.
(4) In the resting stage, the angle between creep strain increment vector and stress vector is large and the length of creep strain path is short. In the loading stage under torsional stress, the angle is smaller than that in the resting stage, and the length of creep strain path is long.
