Computational Modelling of Solid Propellants: Micro-Structural Analysis and Homogenization

Abstract

Energetic materials have widespread applicability in various fields of engineering as propellants, explosives, and pyrotechnics. The Netherlands Organisation for Applied Scientific Research (TNO) is investigating additive manufacturing of solid propellants. As part of this investigation, TNO developed a new polymer binder mix for solid propellants. The thermomechanical behavior of the solid propellants is not well understood yet. Therefore, TNO is interested in developing computational modelling schemes for characterizing their behavior. Computational schemes based on the finite element method (FEM) are developed in this thesis. They are calibrated with matrix-only and solid propellant uniaxial tension experiments provided by TNO. The developed schemes are compared with the experiments to better understand the materials behavior and improve the computational models.

The macro-structural behavior of solid propellants is significantly affected by the behavior of the micro-structure. It is computationally unfeasible to directly consider the micro-structure in macro-structural FEM. Therefore, computational homogenization (CH) is employed for idealizing the micro-structure with representative volume elements (RVEs) at a finite number of macroscopic locations. Micro-structural behavior consists of numerous nonlinear thermomechanical processes. This thesis focuses in characterizing micro-structural matrix viscoelasticity, continuum matrix damage, and debonding. The brittle and rate dependent behavior of the matrix and the solid propellants is described with these thermomechanical processes. Other relevant processes such as temperature effects, nonlinear elasticity, anisotropy, and large strains are not considered.

The experiments exhibit a clear rate dependence throughout the entire loading process, and viscoelasticity is hypothesized to play a major role in this dependence. The experimental samples experience a brittle failure that is believed to be caused by matrix micro-crack damage. Particle debonding is observed in the failure planes of the solid propellants and is hypothesized to significantly affect their behavior.

Matrix-only and solid propellant computational results confirm that viscoelasticity is a major source of rate dependence and that (continuum) matrix micro-crack damage causes brittle failure. They also suggest that continuum matrix damage is a significant source of rate dependence in the post-damage regime, and matrix-only experiments agree. Comparing the results for perfectly bonded solid propellants to the results for solid propellants with debonding and to the solid propellant experiments, it is clear that debonding indeed has a large effect in reducing the ultimate strength of the material. The full CH damage-debonding-viscoelastic scheme shows promise as a first step towards full characterization. Future work can improve the scheme by addressing the issue of free surface propagation and can build on it by including relevant thermomechanical processes that are not yet implemented. Recommendations on how to achieve this are given in the final chapter.