Multiscale damage evolution and fracture for thermal aging energetic material based on mesoscopic observation and molecular dynamics simulation

As a composite energetic material composed of filler and matrix, the mechanical properties of HTPB propellants change constantly due to the aging of each component and component interfaces during long-term storage. However, the study of a single component or performance parameter limits disclosure of the complex aging mechanism and mechanical property evolution. Therefore, based on accelerated thermal aging experiments, this study integrated gas detection, contact angle testing, in situ loading experiments, digital image correlation numerical analysis, and all-atom molecular dynamic simulations to explore the underlying mechanism of structural morphology and mechanical property changes during propellant aging. The results showed that the aging reaction was dominated by degradation chain breaking in the early stage and oxidative crosslinking in the later stage. Degradation chain breaking enhanced interfacial bonding between the two phases, while oxidative crosslinking deteriorated interfacial bonding. Compared with lossless specimens, short-term thermal aging weakened the strength of the matrix and enhanced bonding interfaces. During in situ loading, interfacial pore expansion and matrix damage and fracture occurred simultaneously and the matrix became the key damage source. However, with the extension of thermal aging time, the strength of the matrix increased, bonding interface weakened, and bonding interfaces became a weak point of loading. This made the high-strain area change from the matrix at the initial loading stage to the bonding interface and the damage of the bonding interface caused whole fracture of the specimen. The external loading resulted in the rearrangement of interfacial molecular chains, change of molecular spacing, and initiation and expansion of micropores between molecular chains. The tractor-separation curve conformed to the exponential cohesion model and the damage mode of the interface layer changed between adhesion and cohesion due to thermal aging.