Measurement of changes in the bulk material properties of solid rocket motor propellant

Abstract

Solid rocket motor propellant’s bulk viscoelastic material properties change with age and service history. Safety considerations preclude the use of propellant after an estimated service life. An ultrasonic method that characterizes the material by tracking attenuation and velocity, as a function of frequency, over time is presented. No record of service history was assumed. A simple numerical simulation, based on the Kelvin–Voigt model, guided by a scalar expression, matched experimental attenuation over the limited transducer frequency band and provided a viscoelastic damping coefficient, which is a measure of viscosity. Ultrasonic nondestructive evaluation was used on a sample of inert propellant at the 5th and 11th months after casting. A matched pair of 50 kHz transducers, wavelength in the material of 3.2 cm, were used in 120 tests. The preliminary experimental results indicate a decreasing attenuation and associated viscoelastic damping coefficient as time increases. Further testing will be used to ascertain an empirical relationship between attenuation, viscoelastic damping coefficient, and Young’s modulus over time, and their relationship to the safe life for motors.