Fracture mechanics analysis of stable crack growth under sustained load and instability of circumferential cracks in straight pipes of a helium-high temperature-reactor

In order to determine the component behaviour of pipes of X20CrMoV121 for the water-steam circulation system of a helium high-temperature reactor component tests were carried out under load conditions similar to those of actual operations. The aim of the tests was to determine the stable and unstable crack growth in the less tough filler metal of the girt-welded pipes with 20 mm wall thickness and 400 mm inside diameter. The tests were carried out with filler materials of two varying toughness. The stable crack growth is of particular importance in the case of residual heat removal. For this reason two tests were carried out on pipes with a circumferential crack under sustained load. In the second part of the test program pipes with defects were stressed under sustained internal pressure, and additionally a bending moment was applied in a continuous fashion. The aim of this investigation was to determine the boundary conditions for failure occuring by leak or by fracture with regard to the different toughness of the filler metal. The fracture mechanics analysis was carried out using a modified version of the flow-stress criterion for circumferential cracks (Battelle formalism) and using also the so-called 'effective fracture toughness Ksub(Ieff)'. The investigation has revealed that the component toughness is adequately high and component failure trough fracture need not to be expected. The experimental results can be described theoretically using the Battelle formalism or the formulas of linear-elastic fracture mechanics if the data obtained in the test charcterising component behaviour (flow stress or effective fracture toughness) are used as a basis. The results show that an evaluation of component behaviour using Ksub(J)sub(o) values determined by CT specimens according to the J integral procedure is too conservative. (orig.)