Complex stress relaxation of metals at elevated temperatures

Abstract

Four engineering metals have been investigated under complex stress relaxation conditions at elevated temperatures. These are a 0·24%C steel tested at 450°C, a commercially pure copper at 250° C, an aluminium alloy at 200°C, and a magnesium alloy at 50° and 20°C. The objects of the investigation were to determine whether simple tensile relaxation tests could provide an acceptable prediction of complex stress relaxation behaviour, and whether simple tensile creep data couldform the basis of a similar prediction of relaxation under complex stresses. Combined tension-torsion relaxation tests were made on thin-walled tubular specimens of each metal; these tests could be considered representative of complex stress conditions in general. The study has shown that tensile relaxation tests can provide a satisfactory prediction of complex stress relaxation. Alternatively, where only tensile primary creep data are available, the mechanical equations of state in association with tensile creep data lead to reasonably accurate predictions of the complex stress relaxation results, the strain-hardening form proving marginally superior to the time-hardening form.