Cyclic creep-rupture behavior of three high-temperature alloys

Some important characteristics of the cyclic creep-rupture curves have been studied for the titanium alloy 6Al-2Sn-4Zr-2Mo at 900° and 1100°F (755 and 865 K), the cobalt-base alloy L-605 at 1180°F <910 K), and for two hardness levels of 316 stainless steel at 1300°F (980 K). Such curves have been used successfully in a previous investigation for the evaluation and prediction of strain-cycling life at elevated temperatures within the creep range. The cyclic creep-rupture curve relates tensile stress and tensile time-to-rupture for strain-limited cyclic loading and has been found to be independent of the total strain range and the level of compressive stress employed in the cyclic creep-rupture tests. The cyclic creep-rupture curve was always found to be above and to the right of the conventional (constant load) monotonic creep-rupture curve by factors ranging from 2 to 10 in time-to-rupture. This factor tends to be greatest when the creep ductility is large. Cyclic creep acceleration was observed in every cyclic creep-rupture test conducted. The phenomenon was most pronounced at the highest stress levels and when the tensile and compressive stresses were completely reversed. In general, creep rates were found to be lower in compression than in tension for equal true stresses. The differences, however, were strongly material dependent.