Steady-State Biaxial Creep

Tensile and torsional stresses were applied simultaneously to each of a series of identical thin-walled aluminum-alloy tubes at an elevated temperature (400 F). Different combinations of stresses were chosen so that the combined stress intensity was kept substantially constant. Simultaneous measurements were made of the resulting tensile and torsional creep strains; the tests were not stopped at any specific time but were continued until the creep had definitely entered the final stage. This procedure permitted accurate estimates to be made of the steady-state creep rates which are independent of the time. It was found that (a) the splitting of the strain-rate intensity into its individual components was accurately predictable from the splitting of the known stress intensity into its components; and (b) the absolute magnitude of the individual steady-state creep rates was predictable from a knowledge of the applied stresses and the relation between tensile stress only and tensile steady creep rate, to an accuracy of about 20 percent. An analysis of the creep rates between the time of load application and the time of achievement of the steady state, necessarily somewhat crude, disclosed a strong probability that the relations which govern the splitting of the strain-rate intensity into its individual components persist even in the transient region. A theoretical Appendix gives the details for the computation of stresses under the conditions of the experiment.