Characterization of Solid Composite Propellants Using Two-Dimensional Digital Image Correlation at Large and Small Scales

Solid composite propellants are used widely in the aerospace industry due to their desirable mechanical and performance characteristics, including their simplicity, high initial thrust and volume specific impulse. Knowledge of the mechanical properties is needed due to the stresses encountered by a solid rocket motor propellant during thermal cycling while in storage, during rapid ignition transients, and dynamic launch and flight phases. These stresses could damage the propellant grain, leading to an unplanned increase in burning surface area and subsequent catastrophic disassembly. Tensile testing with the conventionally used JANNAF Type C “dogbones” can be material-expensive and time-consuming, particularly if determining the propellant’s response to different strain rates and temperatures. The rapid development of propellants with novel ingredients or formulations is especially hampered by material and time constraints. Using small-scale tests, typically using “micro-dogbone” samples, tensile properties can be characterized with a strong correlation to standard JANNAF Type C samples and only use a fraction of the normally required material. The correlation between the two sample sizes can be demonstrated for a wide range of propellant formulations and environmental conditions, such as extreme temperatures used in test conditions. Propellant characterization can also be relatively slow due to the data analysis time required to ensure that samples did not contain voids or other defects. Using 2-D Digital Image Correlation (DIC) technology, a baseline behavior can be established for propellant samples that contain voids to help screen data faster, leading to a faster characterization time for propellants and reduced cost of the program. Overall, the DIC system is a promising method of non-contact strain measurement that can help characterize and screen solid composite propellants, while micro-dogbones show great promise in being able to reduce the time and cost required for characterizing novel solid propellants