ASSESSMENT OF DYNAMICALLY LOADED SURFACE CRACKS IN CUBIC CONTAINERS

BAM is the German competent authority for design testing of packages for radioactive materials and develops in a research project improved fracture mechanical assessment methods for cracks in the highest stressed regions of cubic containers made of ductile cast iron. For that purpose postulated surface cracks in the centre of the container walls and grooves are investigated numerically. In the static case we found relations between the crack tip parameter (stress intensity factor or the Jintegral, respectively), stress load, crack depth, container geometry, and material behaviour. In the dynamic case it could be shown, that the dynamic crack tip parameter can be estimated by empirical formulas with the time-dependent “crack-free stress” inserted. This crack-free stress is calculated dynamically under drop test conditions without a crack, but at the position of interest in the container structure. Such a somewhat surprising result can be explained by the fact, that the drop event happens in the range of milliseconds. That is slowly enough for the crack to behave quasi-statically although the crack is loaded with a dynamic, i. e. time-dependent stress. Based on these calculations the critical crack depth is given as a function of the stress, the material quality (defined by the fracture toughness) and the wall thickness of the container for surface cracks in the centre of walls as well as in grooves of a cubically shaped container.