Determining the Master Curve of Aged Solid Propellant using a Dynamic Mechanical Analyzer

Understanding aging in solid propellants is important for predicting the service lifetime of solid rocket motors in aerospace and defense applications. The accurate measurement of the master curve of the propellant is necessary for input into finite element simulations of fracture and debonding in solid rocket motors that lead to their failure. The use of a dynamic mechanical analyzer over conventional stress relaxation tests presents several advantages including smaller specimen size, which simplifies the artificial aging procedure, and results in improved accuracy due to the application of smaller strains. A method is presented for measuring the master curve of a particular propellant, viz. hydroxyl-terminated polybutadiene (HTPB), using a dynamic mechanical analysis setup. Specimens were artificially aged at 50 and 60◦ C for durations ranging from one to eight weeks in order to approximate up to six years of natural aging at room temperature. The measured master curves show that increasing the artificial aging time at 60◦ C leads to a higher relaxation modulus for a given reduced time. This is attributed to higher cross-link density in aged HTPB specimens. Aging at 50◦ C for the given durations shows less of an effect, which indicates that 50◦ C is not a sufficiently high temperature for inducing aging in the time periods studied for the manufactured specimens. The measured master curves can be used to inform material models such as Prony series in finite element simulations for predicting the lifetime of solid rocket motors