Development of In Situ SEM Mechanical Testing Instrument for Solid Propellants Under Near-Service Conditions

The mechanical properties of solid propellants play a crucial role in ensuring the safety and stability of solid rocket motors (SRMs). During actual service, the propellants are subjected to various mechanical and thermal loads, resulting in complex failure modes. Unfortunately, the relationship between macromechanical properties and microscopic damage evolution of propellants under service conditions remains unclear due to the lack of suitable testing devices. A scanning electron microscope in situ mechanical testing instrument specifically designed for solid propellants under near-service conditions (SEIM-PSC) was developed to address the issue. The instrument enables quasi-static multimechanical loading (tension, shear, and bending), fatigue loading (maximum amplitude of 3.5 mm and a maximum frequency of 20 Hz), and high/low-temperature loading ( $- 55~^{\circ }$ C to $70~^{\circ }$ C) while providing simultaneous in situ monitoring capability. Variable tensile rate tests, pretension fatigue tests, tensile-shear tests, and high/low-temperature tensile tests were conducted on NEPE propellants using the instrument. In situ data such as microstructure, strain distribution, and stress-strain curves were comprehensively analyzed during loading. The results indicated that stress concentration at the particle/matrix interface initiated crack formation and microvoids’ expansion, leading to the eventual development of macrovoids, which were synchronously reflected in the deterioration of mechanical properties. These studies confirmed that SEIM-PSC exhibited excellent capability for constructing near-service environments of propellants while demonstrating broad potential applications.