Lagrangian hydrocode modelling of hypervelocity impact on spacecraft

This thesis addresses the problem of modelling hypervelocity impact on spacecraft structures using a Lagrangian hydrocode. Lagrangian hydrocodes offer the advantages of computational efficiency and structural elements, but traditionally have been unable to model large material deformations. Analyses of impact on a thin plate were per¬formed using the finite element code DYNA3D. These analyses highlighted three areas where improvement was necessary: material modelling, element erosion criteria and modelling large deformations. To improve the material modelling the SESAME equa¬tion of state was implemented in the DYNA3D code. Two new element erosion criteria were then developed, one based on total element deformation, the second on element accuracy. The two criterion were then tested for modelling impacts onto semi-infinite and thin plate targets. The deformation criterion produced the best results for crater size and hole diameter, but can not be used to model the debris cloud. The element accuracy criterion allows a sufficient number of elements to survive to measure debris cloud velocities and spread angle. It was concluded that an alternative method for modelling the debris cloud is required. The Smoothed Particle Hydrodynamics (SPH) method was selected as it is a Lagrangian method, and allows modelling of large deformations as it does not use a computational mesh. The most recent SPH developments require the boundary conditions to be rigorously treated. A penalty contact algorithm for SPH was developed and tested in 1D and 2D. The tests revealed that for successful treatment of boundary conditions it was necessary to address the problem of zero-energy modes. A alternative discretisation method that calculates the velocity and stress at different points was proposed as a cure for the zero-energy mode problem. This method was tested in 1D, and was shown to be a solution to the zero-energy mode problem.