Crack propagation in a LMFBR elbow

Abstract

This paper provides results from a large scale elbow fracture mechanics fatigue test at LMFBR operating temperature and with sodium as fluid. The material, a Type 304 stainless steel, is one of the selected stainless steels used in breeder reactor design. Crack initiation, crack shape development, ligament instability and safety margins against gross plastic instability were predicted. Initial cracks were located at the crown of the elbow where the highest stresses occur under in-plane bending, which was the loading condition for the test. The nearly pure bending stress across the wall is regarded as a typical loading in LMFBR structures. Cracks under such unfavourable stress distribution extend preferably lengthwise before wall penetration, as compared with cracks under membrane stresses. The experiment, conducted at stress levels approaching the maximum design values, demonstrates low crack growth rates under plant conditions, showing that crack extension during service would be quite small. In fact, more than 28 times the expected transients would be required to advance a crack of 3 mm depth and 30 mm length to penetrate the wall in the region of highest stress.