Mechanical properties of structural materials are affected by neutron irradiation. Irradiation induced hardening can be observed by an increase of the yield stress and concurrent reduction of the fracture strain. Irradiation induced embrittlement becomes manifest in the shift of the ductile-to-brittle transition towards higher temperatures.
Mechanical testing of neutron irradiated materials is done in our hot cell laboratories. Our fields of application are reactor pressure vessel (RPV) steels, with focus on long term irradiation effects, as well as advanced materials being developed for future reactor systems, in particular ODS alloys. The systematic investigation of RPV material from the decommissioned Greifswald Nuclear Power Plant offered a unique opportunity to understand the irradiation behaviour under real operation conditions.
Fracture toughness testing
Fracture toughness (FT) is a property describing a material's resistance against unstable crack propagation. Specimens for FT determination exhibit a well defined pre-crack. Upon loading of the specimen, stable or unstable crack initiation occurs. Dedicated evaluation procedures are used: the Master Curve concept for the ductile-to-brittle transition range (unstable crack propagation) and the crack extension resistance (J-R) curve for the ductile range (stable crack propagation). The FT specimen types include compact tension, C(T), and single edged bending, SE(B), specimens. The technology for using (non-standard) miniaturized C(T) specimens allows testing in cases of high radioactivity and shortage of material.
Supplementary mechanical testing
Other mechanical standard test methods are also available in the hot cells including the Charpy impact test to determine the ductile-to-brittle transition temperature, the tensile test and Vickers hardness.
The fracture surfaces of tested fracture mechanics samples are characterized by means of scanning electron microscopy (SEM) in order to identify the crack type and the crack initiation sites. Electron backscatter diffraction (EBSD) is used to gain additional insight into the effect of microstructure on fracture mechanisms.
Small punch test
The small punch test is used as a screening tool to estimate mechanical properties with smallest quantities of material. The following properties can be derived via correlations with standard test parameters: ductile to brittle transition temperature, yield stress, ultimate tensile stress, creep strength.