Validity of strength hypothesis in damaged HT elbow evaluation

The influence of multiaxial load on long term creep behaviour was investigated in several research programs. The experimentally determined results of multiaxial loaded specimen were compared by way of a strength hypothesis with those of uniaxial data and the results of Finite Element calculation. It was found out that the validity of a strength hypothesis depends on the type of load and on deformation respectively failure criterions. Creep deformation is also influenced by the degree of multiaxiality. Full scale validation tests on bends of 0.6 Mo- and 12 Cr-steel, respectively, are being carried out. The obtained results from these tests confirm the results of the aforementioned tests with small scale specimens. To consider the effect of multiaxiality and additional (secondary) stresses in the life assessment of bends operating in the creep regime new approaches based on the aforementioned stress hypothesis are necessary. (authors). 2 refs., 12 figs