Experimentally verified creep ratcheting analysis

In heat exchanging components amongst the primary stresses even thermally induced secondary stresses must be taken into account. Here fluctuating temperatures (e.g. hot streaks) are especially important. At very high temperatures above 800deg C a superposition of primary stresses and thermally induced fluctuating secondary stresses may lead to enhanced creep (creep ratcheting). At temperatures of 950deg C experiments were carried out to verify the creep ratcheting effect. Here INCOLOY 800 H tubes were loaded by an axial force and by cyclically changing temperatures. A creep ratcheting effect could be shown. Calculations were carried out by using a simple two-shell-model code and finite-element programme, respectively. It could be shown that the results obtained with the two-shell model are very conservative, while the results of the finite element analysis were in a good agreement with the experiments. Finally it is discussed, how the two shell model can be improved. (orig.)