Fracture Mechanical Analysis of a Cylindrical Cast Iron Cask

The safety evaluation of cask components made of ductile cast iron includes investigations to prevent brittle fracture. Generally, ductile cast iron is endangered by brittle fracture especially at low temperatures (down to -40°C) and in combination with existing crack-like material defects. An applicable method is the assessment of fracture resistance using fracture mechanics according to the IAEA guidelines. The approach is based on the prevention of fracture initiation. For application of these principles for drop loads, account must be taken both of dynamic stresses within the component and dynamic material behavior. Basically the dynamic stress intensity factor of postulated pre-existing crack-like defects is compared with the dynamic fracture toughness of the material. Applicable numerical and experimental methods for the safety assessment of cask components are demonstrated for the case of an artificially pre-cracked cylindrical cast iron cask which undergoes dynamic loading conditions as result of the hard impact between the cask and a concrete target. The proposed evaluation procedure is a combination of numerical and experimental steps. At first a numerical simulation of the test scenario with expected highest loads for the cask without any cracks and with realistic material models should be carried out. If necessary, dynamic material properties dependent on temperature and strain rate must be determined. From the calculation results, the locations of high major principal stresses in the structure can be found. At these locations a high fracture mechanical load of crack-like defects is expected. A postulated crack is positioned there in a way that a normal stress is acting on the crack faces. The maximum size of postulated crack-like defects depends on the requirements for non-destructive testing. The number of similar crack configurations to investigate can be reduced by restriction to cases with highest load. The dynamic crack tip parameter is calculated by means of a dynamic numerical simulation of the cask (or cask component) containing the crack. Exemplarily, the calculated stress intensity factor is compared with measured fracture toughness values from single edge notched bending specimens. Additional safety factors must be considered in this procedure to cover uncertainties, applicable values are discussed.