Creep of metal torsion-tension members subjected to nonproportionate load changes

The creep behavior of torsion-tension members subjected to nonproportionate loading was studied by testing members made of SAE 1035 steel at 975° F and of copper alloy 360 at 700° F. Creep curves for these members were predicted by an incremental theory using both the strain-hardening rule and the time-hardening rule and a total-strain theory using the incremental-hardening rule. Both incremental theories were based on a creep law which assumed that the total strain in specimens subjected to constant stress was made up of an elastic component and a creep component given by a power function of stress and of time. The total-strain theory was based on isochronous stress-strain curves which were approximated by an arc hyperbolic sine relation. The creep tests were limited to a total duration of 60 min. The nonproportionate loading was obtained by stepped increases of either the axial load, the torque, or both at 30 min. Except for the incremental theory based on the time-hardening rule, good agreement was found between theory and experiment.