Validation of blast simulation models via drop-tower tests

Abstract: This study aims to validate a screw joint simulation model used by BAE Systems in LS-DYNA during blast simulations. It is important that the screw joint simulation model is physically correct, since the simulation results can influence major design decisions. The study provides a short overview on the subject of bolts and screws, material deformation and stress and strain in materials, of the finite element method (FEM) and on some specific numerical methods used in this study. BAE Systems started a validation project of the screw joint simulation model in 2015, but it was not finished due to other priorities. In this older project some drop-tower tests measuring the axial force in a screw joint were conducted. These old tests can now serve as validation data for the screw joint simulation model. The screw joint simulation model used by BAE Systems is dependent on a special kind of finite element formulation; a so called beam element. This study provides a finite element analysis on this simulation model, which is implemented through an established industry FEM solver called LS-DYNA. The validation of the screw joint simulation model is done against three drop-tower experiments performed at 900, 1000 and 1100mm drop height respectively. The drop-tower experiments were replicated in LS-DYNA, with a prescribed velocity on the falling parts rather than simulating a free fall and non-elastic impact. A comparison between the simulation model using beam elements, that is used by BAE Systems, and a similar simulation model using solid elements is presented as part of the validation. To make sure that the result of the study is confident, a local mesh convergence study and a study of the mass scaling numerical method in LS-DYNA is also presented. The results show that the screw joint simulation model using beam elements is valid according to the available experimental data. In one of the experiments, where the drop-test was performed twice, an average maximum force on the screw was measured to be 33.5+-4.8 kN. Simulations of the same case, under the same conditions, using beam elements resulted in a maximum force on the screw of 35.4 kN, well within the experimental result range. In the other two drop-tower experiments, the simulated results showed correlation considering the error sources in the simulation model and the statistical spread that is present in the experimental results. The simulation model using beam elements is also similar to the results using solid elements, which also indicates that the beam model is valid. All in all, it is shown that the beam model can be used to produce safe results that either overestimate or place the simulations of the axial force in the screw in the upper spread of the measurements.