Influence of specimen type, crack length and evaluation method on quasi-static and dynamic fracture toughness properties

For up-to-date safety analyses and lifetime assessments of nuclear power plants (NPPs) fracture mechanical based methods are indispensable. When two Swiss NPPs were planned in the 1970s, no standards existed concerning the geometry or testing of pre-cracked Charpy-V-size surveillance specimens. Therefore, the guideline HSK-AN-425 was drafted for fracture toughness evaluation by dynamic testing. Before becoming a Swiss standard it is to be thoroughly checked for potential pitfalls. Furthermore, it has to be verified that surveillance specimens of those NPPs with shorter cracks than recommended in present standards are eligible for fracture mechanics assessment.

Experiments were performed on unirradiated commercial NPP steel 22NiMoCr3-7.
Quasi-static Master Curve (MC) reference temperature T0 were evaluated according to ASTM E1921 on Charpy-size (width B=0.4T, 1T=1 inch=25.4mm) single edge bend (SE(B)) specimens with crack lengths of 3 mm and 5 mm as well as 1T-C(T) specimens. Furthermore, transferability of surveillance specimen results to heavy-walled structures was checked with SE(B) specimens of B=0.4T, 0.8T, 1.6T and 3.2T. Dynamic MC results were retrieved from instrumented impact tests performed at 1.2 to 2.4 m/s in which T0 was calculated according to HSK-AN-425 and also by a modified ASTM 1921 method. Quasi-static J resistance curves were obtained according to standards ASTM E1820, ISO12135 and ESIS-P2.

In quasi-static MC tests it could be demonstrated that SE(B) specimens with 3 mm and 5 mm fatigue cracks yield the same T0 of ca. -80°C. Thus, surveillance 0.4T-SE(B) specimens with 3 mm crack length may be used for safety assessment without further modifications. The transferability of small-scale results to structural behaviour was proven. T0 obtained from 1T-C(T) specimens is 15K above T0 from 0.4T-SE(B) specimens.

The dynamic MC T0 (v=2.4m/s) is nearly equal for 0.4T-SE(B) specimens of both crack lengths but is ca. 80K above the quasi-static T0.

Small surveillance specimens do not yield valid quasi-static J resistance curve crack initiation values as requirements based on size unnecessarily invalidate all of the results. The idea of using a size criterion to limit J values should be re-evaluated. Ductile crack initiation values are highly dependent on the evaluation standard used. Ordered from highest to lowest, JIc(E1820) > J0.2BL(ISO 12135) > J0.2(ESIS-P2) > Ji(stretch zone width, ISO 12135). Ji is said to be a physically sound crack initiation value but numerous problems arise in stretch zone width determination. It is often not traceable throughout the whole crack front, affected by subjectivity and intricate for irradiated material.