Unstable ductile fracture under combined load of thermal shock and tension

In nuclear power plants, it is an important problem to estimate the structural safety margin of a reactor vessel under a pressurized thermal shock, which may occur in an accident such as small break loss of coolant. Therefore, from the view point of fracture mechanics, this paper deals both experimentally and numerically with the behavior of cracks existing in the A508 class 3 pressure vessel steel plate under the combined load of thermal shock and tension, which simulates the above mentioned phenomenon in an actual vessel. In order to estimate simply the structural integrity of the nozzle corner as a special case, the initial cracks located at the edge of the hole in the specimen were considered. To take into account the influence of the compliance of the pressure vessel itself and pressurized water, the pre-load was mechanically applied with a spring which was set in series with the specimen. By holding the pre-load at the initial elevated temperature, the specimen was quenched with cold water rapidly. The experimental results showed that the initial cracks extended unstably due to the thermal shock loading after a stable extension of a few milimeters. (author)