Use of Johnson–Cook plasticity model for numerical simulations of the SNF shipping cask drop tests

Abstract

The paper presents the results of a numerical simulation for the strains and damage caused by the drop of a TUK-128 SNF shipping cask on a bar. This is a design basis accident in the SNF land transport outside hazardous sites (an NPP or a processing plant).Strains caused by a cask drop on a bar are calculated and simulated in the elastoplastic behavior region of structural materials. The simulation results depend heavily on the method used for defining the plastic properties of materials.The most common way for taking account of plasticity is to set a tabulated function that gives the link between stresses and plastic strains. Such plasticity definition fails to take into account not only the temperature dependence of plasticity properties but also the kinematic strengthening of the material (dependence of yield strength on intensity of strains). However, this plasticity model leads to a major decrease in the complexity of computations which is especially important when dozens of design cases need to be analyzed.Another (more adequate) approach to plasticity definition is to take into account the change in the yield strength not only depending on the strain magnitude and intensity but also on temperature (Johnson–Cook plasticity model). In this case, 5 to 7 parameters need to be determined for each type of structural material. Presently, there is no well-defined way to determine these parameters; for this reason, the authors suggest a procedure for finding them. A drawback of this approach is a much greater complexity of computations. Therefore, the paper presents a comparative analysis of the calculation results with the plasticity definition by the Johnson–Cook model and using a tabulated function.Simulations were performed for two design cases: a drop with an impact on the cask lid (stainless steel) and a drop with an impact on the cask bottom (high-strength cast iron). The considered options to define plastic properties of structural materials are compared against the TUK-128 cask drop test results.The impact was simulated by finite element method using the LS-Dyna code.