Fracture Mechanics Testing of Materials
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 fracture. Specimens for FT determination exhibit a well defined pre-crack. Upon loading of the specimen, stable or unstable crack growth occurs. Dedicated evaluation procedures are used: the Master Curve concept for the ductile-to-brittle transition range (ASTM E 1921) and the crack extension resistance (JR) curve for the ductile range (ASTM E 1820). 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 is under development.
Supplementary mechanical tests
Other mechanical standard test methods are also available in the hot cells including Charpy impact test to determine the ductile-to-brittle transition temperature, tensile test and hardness test (HV 0.3 ... HV 10, HRC).
Fractography of tested samples by means of scanning electron microscopy (SEM) is used to identify the crack type and the 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 small quantities of material. The following properties can be derived: ductile to brittle transition temperature, yield stress, ultimate tensile stress, creep strength.