Application of the small-punch-test to irradiated reactor vessel steels in the brittle-ductile transition region

The warranty of the high safety standards of existing nuclear power plants requires the assessment and characterization of the actual material state changing during service, e.g. embrittlement due to irradiation. In the brittle and brittle-ductile transition region, ferritic steels fail due to transcrystalline or intercrystalline cleavage fracture starting at flaws resulting from microplastic deformations. The large scatter of fracture mechanical properties in the transition region originating from statistically distributed flaws can be described by Weibull theory. Using the Beremin local approach model, the probability of cleavage fracture at a certain load state can be quantified.
In the small-punch-testing, a quadratical specimen lying on a die is loaded centrically and deformed vertically by a spherical punch, hence starting at plate-like bending and ending in a deep drawing process. Large parts of the specimen are exposed to a biaxial stress state, making the test rather suitable to represent the actual stress state observed in pressure vessel steels in service than other miniaturized methods such as tension or bending tests. Due to the tiny specimen size, material remnants of ongoing surveillance programs (slices of Charpy-specimen) can be used. A special small-punch device was manufactured to be installed into a testing machine in hot cells. To enable testing at temperatures down to -185°C, a cooling system based on liquid nitrogen is used. Neural networks are used to solve the inverse problem of finding material parameters from measured load-displacement-curves. Having identified the parameters describing hardening, Weibull-parameters are determined from both experimental data, e.g. displacement of the punch at fracture, and calculated stress distribution inside the specimen, obtained from FE calculations using the identified plastic properties. The reactor vessel steel A508 (IAEA JFL) is characterized at different levels of irradiation and different temperatures, covering brittle and brittle-ductile transition region of the steel.