On the utilization of the instrumented Charpy impact test for characterizing the flow and fracture behavior of reactor pressure vessel steels

The Charpy impact test is extensively used in the surveillance programs to monitor neutron reactor pressure vessel degradation induced by neutron irradiation. The energy absorbed during the test is the main parameter used for engineering characterization of thematerials. At SCK·CEN, a large effort was put into taking advantage of the test instrumentation allowing for load versus time recording during the whole test. Analysis of the load-time trace allows to determine a number of key parameters, in particular, the flow properties, the microcleavage fracture stress, the crack arrest performance and the characteristic transition temperatures which aremore physically grounded than conventional indexing to fixed absorbed energy or lateral expansion levels. Combination of static tensile tests with instrumented Charpy impact tests allows for establishing the so-called stress diagram. This can be used as input to micromechanical modeling of the static as well as dynamic initiation fracture toughness trends. Furthermore, the Charpy arrest load correlates well to the nil ductility temperature as determined from the Pellini drop weight test and allows concurrent application of a crack arrest safety approach. As it will be illustrated by relevant examples, a significant advantage of the above considerations is that they allow to incorporate a physically grounded understanding of irradiation effects into the evaluation of reactor vessel integrity.