# The Mesoscopic Numerical Simulation of GAP/CL20/AP Composite Solid Propellant Based on MPM and FEM
## 1 Introduction  
With the rapid development of modern weapons, the comprehensive performance requirements of new solid propellants are getting higher and higher [1]. In order for the propellant to meet the demanding requirements of high energy and high performance, the most effective way is to use an energetic binder (such as a glycidyl azide polymer (GAP)) instead of a non-energetic binder. With high heat generation and high density, GAP is an excellent energetic adhesive. However, for a long time, research on the mechanical properties of gap-based composite solid propellants was limited to their macro-mechanical properties. For example, the macroscopic mechanical properties of propellants are studied via uniaxial tensile dumbbell-shaped specimens. However, this method can only providethe mechanical properties of a propellant in the static and macroscopic states. In the process of stretching, it is difficult to characterize the microstructure evolution of the propellant. Therefore, this method cannot meet the development needs of new weapon platforms. Meso-debonding is a common failure mechanism in composite solid propellants, initiated by the build-up of stress within the propellant due to thermal and mechanical loads. This stress can cause micro-cracks to form at the interface between the solid particles and the binder. Over time, these micro-cracks can grow and coalesce, leading to the formation of meso-cracks. Once meso-cracks form, the binder and the solid particles are no longer fully bonded, and the propellant may start to show signs of degradation. This can lead to reduced mechanical properties and a decrease in the overall performance of the rocket engine. The macro-mechanical behavior of composite solid propellants strongly depends on the microstructure, so under an external load, micro-cracks, the expansion of micro-holes, and local high stress and strain may destroy the microstructure of interface bonding, leading to interfacial debonding between the grains and the matrix and then affecting the macro-mechanical properties [2]. In recent years, scholars have carried out a lot of work in order to study the meso-mechanical characteristics and damage evolution processes of propellants in depth. [PERSON] et al. found that the interface between particles and matrix was the starting point of hydroxyl-terminated polybutadiene (HTPB) propellant failure [3; 4]. In order to describe the mechanical properties of propellant more accurately, [PERSON] et al. [5] learned through experiments that the mechanical properties of a propellant are related to its loading rate. Through uniaxial tensile tests at different temperatures and strain rates, [PERSON] et al. [6] found that a numerical simulation only considering interfacial debonding between the propellant particles and the matrix could not accurately predict the failure behavior of a propellant, so it was inferred that the failure of a propellant also included the damage to the matrix. Via scanning electron microscopy, [PERSON] [7] found that the failure of a propellant included debonding between the particles and the matrix, and the wire drawing failure of the matrix. However, due to the limitations of experimental methods, it is impossible to quantitatively analyze the meso-damage evolution process of high-energy materials, so the numerical simulation method has become an effective method of analyzing the interface failure of multiphase materials in recent years. By using a single particle axisymmetric mesoscopic model, [PERSON] et al. [8] studied the stress concentration factor and adhesion failure angle of a nitrate plasticized polyether (NEPE) propellant under different solid contents and bonding agents. [PERSON] et al. [9] used the molecular dynamics method to generate the representative volume element of a propellant and studied the damage and failure of a particle interface under the conditions of a small strain, which proved that the interface damage was the main reason for the nonlinear mechanical properties.