Experimental and simulation study on micro damage of HTPB propellant under multi angle tensile shear loading

The damage evolution of composite solid propellants is influenced by the stress state. In order to investigate the in-situ damage evolution mechanism of hydroxyl terminated polybutadiene (HTPB) propellant under tensile shear conditions, computer tomography (CT) technology was used to scan and reconstruct micro samples of HTPB propellant loaded at different angles. The variation of propellant internal damage with loading process and the influence of different representative volume element (RVE) sizes on porosity were analyzed. Subsequently, numerical simulations of relaxation loads were conducted using 12 different finite element models with 4 RVE sizes and 3 mesh sizes. The experimental results show that under tension shear loading conditions, the porosity increases exponentially with the equivalent effect, and the propagation direction of macroscopic cracks formed by the convergence of microcracks tends to be perpendicular to the tensile stress component. When the side length of RVE reaches and exceeds 600 μm, the porosity tends to stabilize. The numerical simulation study of variable angle tension shear loading found that when the RVE size is 800 μm and the grid size is 10 μm, the calculation effect considering calculation accuracy and efficiency is the best. As the loading angle increases, the dewetting stress first decreases and then increases, the dewetting strain shows a linear increasing trend. • This study innovatively designed and prepared microstructure samples of solid propellants with different loading angles, and systematically verified the feasibility of the experimental scheme using finite element method. By conducting micro mechanical experiments under variable angle tensile shear composite loading conditions, the propagation behavior of macroscopic cracks in complex stress fields was revealed for the first time: the propagation direction of macroscopic cracks formed by the convergence of microcracks is orthogonal to the tensile stress component. The experimental results show that solid propellants exhibit significant microscopic damage evolution characteristics under load, and their porosity increases exponentially with the increase of nominal strain. It is worth noting that there are significant differences in the evolution of porosity under different loading angles: as the loading angle decreases, the shear component in the stress field decreases accordingly, and the loading state gradually approaches uniaxial tension, at which point the growth rate of porosity significantly accelerates; • Based on advanced micro CT scanning technology and multiple segmentation algorithms, a high-precision 3D digital model of the propellant bonding interface was successfully constructed by processing the raw data. By systematically studying the influence of representative volume element (RVE) size on porosity characterization, the critical threshold for RVE size has been determined for the first time: when the RVE side length reaches 600 μm, the measured porosity value tends to stabilize. This discovery provides important theoretical basis for the study of the micro mechanical properties of HTPB composite solid propellants. It is recommended to use RVE with a side length of not less than 600 μm for quantitative characterization analysis in subsequent mechanical modeling and numerical simulation to ensure the reliability and accuracy of the research results; • This study constructed 12 sets of finite element models containing 4 RVE sizes and 3 mesh sizes, and systematically conducted numerical simulation research under relaxed load conditions. Research has found two key patterns: firstly, under the condition of fixed RVE size, as the grid size increases, the calculation accuracy shows a characteristic of first rapidly decreasing and then stabilizing; Secondly, under fixed grid size conditions, an increase in RVE size will significantly improve computational accuracy, and this effect is more pronounced at larger grid sizes. Through optimization analysis, it was determined that the optimal computational performance can be achieved when the RVE size is 800 μm and the grid size is 10 μm. This discovery provides important parameter optimization basis for numerical simulation research of solid propellants; • This study established a three-dimensional micromechanical model of composite solid propellants under variable angle tensile shear loads for the first time, and conducted systematic numerical simulation research. Several innovative findings have been made in the study: the critical dewetting stress first decreases then increases with loading angle, reaching minimum values near 60° loading angles, while critical dewetting stress under 90° pure shear loading exceeds that under 0° uniaxial tension; critical dewetting strain shows approximately linear growth, measuring 6.1% under 0° uniaxial tension and 10.8% under 90° pure shear loading.