Different approaches for the verification of force values measured with instrumented Charpy strikers

The Charpy test plays a fundamental role in the nuclear field for evaluating the neutron embrittlement of the reactor pressure vessel, specifically in the framework of the so-called Enhanced Surveillance Approach, developed at SCK•CEN and aimed at extracting as much information as possible from Charpy impact tests performed with an instrumented striker. Careful analysis of the instrumented force/deflection traces allows defining important parameters which can help investigating material characteristics such as flow properties, microcleavage fracture stress, crack arrest behaviour and alternative characteristic (index) temperatures. For this advanced approach to be successfully applied, confidence on the quality of instrumented force values must be high; as a consequence, extensive research has been performed in order to establish an optimal procedure for the verification of instrumented Charpy strikers. Various approaches will be described in this paper and their applicability and effectiveness discussed. A procedure based on the comparison between yield stresses measured from tensile tests and calculated from instrumented Charpy curves has recently been adopted at SCK•CEN as the recommended in-house procedure for verifying instrumented strikers. This method has shown that for all strikers investigated, the so-called “dynamic” calibration (based on the equalization of dial and calculated energies) yields the most accurate results.