Untersuchungen zum Kriechratchetingverhalten von Rohrproben

By superimposing cyclic secondary stresses $\sigma$$_{T}$ additionally to a constant primary stress $\sigma$$_ {p}$, significant enhanced creep rates of structural components at very high temperatures may be the result. This effect is known as "Creep Ratcheting" or "Incremental Growth". It may occur due to cyclic variations of thermal stresses in the walls of heat exchanger - or chemical reformertubes for example. The creep ratcheting effect was evaluated experimentally by comparing the creep behaviour of metallic tube specimens under constant axial load P with and without superimposed cyclic thermal stresses across the tube wall. For theoretical analysis a numerical computer programme has been developed. The tests on lead specimens at temperatures of up to 100 ° C and bn the high temperature alloy X10 NiCrAlTi 32 20 H of up to 950 ° C showed no significant enhanced stationary creep rates for the stress relations $\sigma$$_{T}$/$\sigma$$_{p}$ 2) thecreep rate increased up to a factor of 15 compared to the creep behavior with the same primary stress as the only effective load. The results of the numerical computer programme developed for this paper and of a Finite Element Analysis showed good consistencywith the experimental data only when the temperature dependence of the stationary creep rate and the shape of the time-temperature cycles of the thermal load variations were considered