Master thesis and internship[BR]- Master's thesis : Methodology for numerical simulation of shaped charges[BR]- Integration Internship : GD Tech

Abstract

A shaped charge is a penetration device where a metal liner is inverted from its original shape due to extremely high pressure from a detonation of an explosive. The aim of this master’s thesis is to establish a methodology of numerical simulations of shaped charges. The Johnson-Cook flow stress model with Grüneisen equation of state was chosen for the material model of the metal liner and target. Jones-Wilkins-Lee equation of state was chosen for the modelling of the explosives. Analytical solutions for jet velocity and mass, as well as others were found and implemented to validate the numerical method. Three literatures of different shaped charge configuration and material with experimental and numerical results were selected for validation.

One literature was chosen for validation of the numerical model as it had the most quantitative experimental results. In 2D, a Lagrangian, Eulerian, and ALE mesh formulations were investigated. Two key assumptions were made: axisymmetry and target alloy material. It was seen that the Lagrangian is inadequate for a shaped charge simulation due to the large deformation. The Eulerian and ALE mesh were successful in modelling shaped charges to a reasonable accuracy. However, it was speculated that the explosive detonation energy per unit volume from literature was inaccurate. A study on the detonation energy was done and it was concluded that the explosives used in the experiment could have been less than optimal. Both Eulerian and ALE mesh showed similar accuracy, but since ALE mesh requires more computational power, the Eulerian mesh is best for a two-dimensional simulation. The axisymmetry assumption was valid as the overall behaviour was accurate, but the target alloy assumption was incorrect and the penetration behaviour was not similar to the literature.

For 3D, an Eulerian mesh and SPH, a meshfree method was investigated to validate the material parameters, validate axisymmetry assumption once more, and analyse the jet characteristics more in-depth. The Eulerian mesh showed closer result to literature than the 2D model, but required a lot more time for computation. A study of the effect of number of processor was done for efficient parallel computation. It was seen that there is a limit on the reduction of computational power due to background calculations that has to be carried out. There were no correlation between increasing number of processors and computational time or decomposition time. However, if the decomposition took a longer time, then the computational time was shorter. This could be due to the geometry of the model, where the high aspect ratio and centred ‘activities’ impact the efficiency. The SPH method required careful selection of parameters, and needs further investigation to achieve a similar accuracy to the Eulerian models.

Two other literature was used to validate the models that were constructed. It was confirmed that the detonation energy per unit volume can be different from the literature values, and the mesh sensitivity of the liner and explosive is high. Nonetheless, the material models selected for the liner and the target showed reasonable agreement for all three literature.

It was concluded that the Eulerian mesh formulation is a good balance between correct modelling of the shaped charge while being relatively simple to implement. Mesh convergence and explosive energy study is critical for an accurate simulation. The explosive study is especially critical since it is the energy input into the system and the deformation mechanism of the liner. The explosives could impart less energy to the system due to poor storage, incomplete detonation, and so on. The analytical solutions should be used for verification if no experimental results are available. The 2D models should be used for mesh convergence and explosive material parameter study, and the 3D model should be used for a more in-depth analysis of the behaviour of the jet.