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- Ductile cast iron (4) (entfernen)
Modern structural integrity assessment procedures in the field of nuclear related technology incorporate fracture
mechanical concepts. Therefore, they inevitably require the availability of both, loading parameters as well as material
characteristics in terms of fracture mechanical quantities. Especially in case of dynamic loading conditions, the methods
for the determination of the loading parameters need further improvement and there is a lack of material characteristics
as well. In Germany, ductile cast iron (DCI) is used for heavy-sectioned casks for radioactive materials. New developments
in cask design and efforts to extend the application limits require further investigations. The present study is part
of an ongoing fracture mechanics research programme of BAM which is focused on the systematic mechanical and
fracture mechanical material characterisation of DCI materials under dynamic loading conditions. In this study, results
of fracture mechanics investigations on ductile cast iron from an original DCI container with a wide variety of microstructure
under dynamic loading conditions in the temperature range from -50 °C to +22 °C are presented. Large scale
as well as small scale single edge crack bend specimens SE(B) with thicknesses of 140 mm and 15 mm, respectively
were tested. Furthermore, it is reported on the results of a finite element simulation of the dynamic large scale fracture
mechanics tests. Strength and deformation characteristics were determined in dynamic tensile tests. They are discussed
with respect to the influence of pearlite content and test temperature. The material specific experimental difficulties in
the determination of reliable dynamic crack initiation toughness values of DCI are outlined.
In the present study, ductile cast iron from an original DCI container with a wide variety of microstructure was
investigated in order to determine the materials fracture toughness under impact loading conditions. Three-point bending
specimens with thickness of 140 mm from a cubic DCI container were investigated at elevated loading rates to
provide reliable fracture toughness parameters for the assessment procedures. In contrast to static fracture behaviour,
the fracture toughness values of thick-walled DCI at higher loading rates show a remarkable reduction with decreasing
temperature up to 50 °C and a significant shift of the transition range. The lower bound fracture toughness value used
in the BAM DCI safety concept was confirmed for impact loading conditions by these fracture mechanics tests using
large specimens. These measurements are relevant to the licensing tests for storage containers without shock absorbers,
especially to the assessment of the dynamic behaviour of crack-like defects inside a cask structure under drop test conditions.
Using extensive dynamic numerical analyses it could be shown, that for special crack configurations the dynamic
crack tip parameter (KI, or J) may be estimated by static formulae even under mechanical impact. These results
of the safety assessment were verified according to the German Konrad repository acceptance criteria by a drop test
with a cubic waste container with artificial crack-like defects from 5 m height onto a representative storage facility
foundation.