Two multiaxial creep theories, a modified creep hardening surface model proposed by Murakamy and Ohno and a unified creep-plasticity model by Robinson, are evaluated by comparing their theoretical predictions with available creep test data of 304 stainless steel under variable stresses of combined tension and torsion at 593 [degrees] C. The material constants of each model were determined by trial and error methods for best fitting of theoretical curves to creep test data under constant stresses. The evaluation of each theory was made in view of its proper predictability of plastic and creep behavior of the material under step changes of stresses including unloading, reloading and stress reversals. Also, the prediction by a classical strain hardening theory was compared with the modified creep hardening surface model. The modified creep hardening surface model predicted much greater creep rates than the test data for most of reloadings and step-up stress changes, but the modified creep hardening surface model had good agreements for creep under stress reversals in torsion. The unified creep plasticity model predicted much larger plastic and creep strain than the test data upon reloading following complete unloading and some step-up stress changes as well as stress reversals. A good agreement was found for partial unloading and reloading to the same initial stress level.Includes bibliographical references
