Characterization of the behaviour of heat-resistant steels under cyclic creep conditions. Final report

The creep behaviour of characteristic, heat-resistant steels under conditions of rectangular, cyclic creep loading can be derived from the standard creep behaviour under constant creep by applying the modified life-fraction rule. The relative life as the characteristic constant of this rule can be determined by means of a factor set composed of a cycle factor, a materials factor, and a temperature factor. The cycle factor depends on the variation of stress or temperature, or both, and on the value and extent of changes in loading. The materials factor depends on the steel type and the temperature factor is determined by the temperature level exceeding a steel-type-specific temperature close to the lowest service temperatures. It is possible to extend the factor set by an additional strain factor describing the relative tensile yield strength up to a given plastic strain. The product of the materials factor, temperature factor and, if applicable, the strain factor has to be interpolated towards the value of one for the case of transition to constant creep. By means of scatter band evaluations, average time-to-rupture curves and limiting creep stress curves have been measured for the steels subjected to cyclic loads, and after fitting to standard values, these curves have been incorporated into a LARA user program that can calculate expected values of the limiting creep time and the time-to-rupture under conditions of rectangular, cyclic loading, based on the modified life-fraction rule and the factor set approach. (orig.)