Tensile behaviors of thermal aged HTPB propellant at low temperatures under dynamic loading

Abstract

With the purpose of further investigating the influence of aging on the tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at low temperatures under dynamic loading, uniaxial tensile stress responses of thermal accelerated aged propellant samples at different temperatures (223–298 K) and strain rates ($0.40\mbox{--}42.86~\mbox{s}^{-1}$) were obtained through the use of a new INSTRON testing machine. And scanning electron microscope (SEM) was employed to analyze the microscopic damage of HTPB propellant under the test conditions. Test results indicate that aging can significantly affect the characteristics of the stress-strain curves, mechanical properties and fracture mechanisms of HTPB propellant at low temperatures under dynamic loading. There are three regions in the tensile stress-strain curves of aged propellant when deforming at lower temperatures and the highest strain rate, however, there are five ones for unaged propellant. At lower temperatures and higher strain rates, the strain at maximum tensile stress of the propellant decreases more obviously after aging. Moreover, the variation of mechanical parameters for HTPB propellant with aging time are highly complex due to the occurrence of oxidative cross-linking during aging and the distinct changes of the fracture mechanisms. These variation were reasonably well described with linear model and the improved exponential model in this investigation. The fracture mechanism of aged propellant can change from dewetting, matrix tearing and AP particle fracture to only AP particle fracture with increasing strain rate for the entire test temperature range, and the strain rate for this transition is all at $4.00\mbox{--}14.29~\mbox{s}^{-1}$. In addition, the microscopic damage of HTPB propellant becomes more severe with the thermal aging time rising, however, this effect is weaker at higher strain rates after long-time thermal aging. Finally, the master curves of typical mechanical parameters for aged HTPB propellant were constructed according to the time-temperature superposition principle (TTSP).