Analyses of Fracture Toughness Tests and Application to NESC II Thermal Shock Experiments

摘要

To verify fracture mechanics based methods for the assessment of components, in the NESC II project two cladded cylinders were tested under thermal shock loading conditions. One cylinder had a fully circumferential crack under the cladding, in the other cylinder two circumferential semi-elliptical cracks were introduced on the inner surface at locations 0° and 180°, respectively. During the test crack initiation and arrest were detected for the cylinder with the complete circumferential flaw, whereas no crack initiation occurred for the semi-elliptical surface cracks. This paper deals with finite element analyses of those experiments and their evaluation on the basis of available experimental data on fracture toughness for the material of the cylinders. The material fracture toughness was characterized using the master curve approach, and the related reference temperature, T0, was determined on Charpy sized SE(B) specimens having crack depths of a/W=0.5 and 0.1. The bending tests result in different reference temperatures for specimens with deep and shallow cracks which is in agreement with constraint considerations at the crack tip. After the tests the fracture surfaces were investigated to determine fracture origins and mechanisms. In addition, finite element simulation of the bending tests was performed and the material relevant fracture parameters were evaluated. The experimental data and the master curve concept are used further for the assessment of the component tests. Considering the constraint situation ahead of the crack tip as well as the different crack lengths in the tested cylinders, the analyses show a possibility for crack initiation in the cylinder with the complete circumferential crack, while no initiation is predicted for the surface cracked cylinder.