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Title of the thesis: Applicability of miniature-sized test specimens for determining fracture toughness and mechanical properties

Thesis defender: Laura Sirkiä
Opponent: Dr. Inge Uytdenhouwen, The Belgian Nuclear Research Center, Belgium
Custos: Adj. Prof. Iikka Virkkunen, Aalto University School of Engineering

As the operating lifetimes of nuclear power plants are extended, the availability of sufficient surveillance programme reference material has become a growing challenge. The reference material is exposed to the same radiation conditions as the reactor pressure vessel, and regular destructive mechanical tests are performed on it to monitor structural integrity. Radiation degrades the original mechanical properties of the material, causing radiation embrittlement among other effects, which is one of the most critical aspects to monitor from a structural integrity perspective. As material is consumed through testing and no new reference material is generated, ways must be found to ensure its sufficiency.

The aim of the dissertation is to investigate whether miniature compact tension specimens can be reliably used to determine the fracture toughness and mechanical properties of steel material. Manufacturing miniature specimens from the halves of tested specimens would enable more efficient material use and extend the lifespan of surveillance programmes. The research is positioned in the field of nuclear reactor pressure vessel structural integrity assessment and applied fracture mechanics, where the need for practical miniature specimen solutions is growing. In addition to experimental methods, FE modelling and microstructural characterization were utilized, and three weld materials from a decommissioned nuclear power plant were included as test subjects.

The main finding is that miniature C(T) specimens are reliably applicable for determining the reference temperature T0 through brittle fracture testing. Geometric variations or the absence of side grooves guiding crack growth did not significantly compromise result reliability. Specimen thickness can be further reduced from 4 mm to 3 mm without significant effect on stress-strain fields, as confirmed by FE modelling, allowing up to 50% more specimens from a single 10×10×55 mm³ Charpy specimen. The small specimen size proved particularly advantageous for weld material testing, and all tests were successfully conducted in a hot cell environment.

The results can be directly applied to nuclear power plant surveillance programmes, particularly at plants where reference material is limited due to lifetime extension. Reducing specimen size does not compromise result reliability and offers a concrete solution to the challenge of reference material sufficiency in the long-term operation of nuclear power plants. 

Keywords: miniature C(T) test specimen, fracture mechanics, brittle fracture, reference temperature T0, structural integrity

Thesis available for public display 7 days prior to the defence at . 

Contact information: laura.sirkia@stuk.fi, 040 658 7206