Correlation of ultrasonic measurements to the mechanical and physical properties of an hydroxy-terminated polybutadiene composite solid propellant

The objectives of this study were to first demonstrate the equivalence of the ultrasonic and dynamic mechanical analysis (DMA) methods as regards engineering property measurement and then to form a general assessment and development plan for the use of ultrasonics on propellant materials. To this end, measurements of shear and longitudinal velocity were made on several inert propellant materials that used hydroxy-terminated polybutadiene as the base polymer. These measurements were converted using linear elastic theory to engineering moduli. The ultrasonically-derived shear moduli were compared to more direct measurements made using dynamic torsion. Both the DMA and ultrasonic shear measurements were made over an extended range of temperatures and individually superposed using the method of reduced variables.; The findings of this study show that the ultrasonic and DMA methods yield equivalent results in spite of the known disparity in measurement strains. Therefore, ultrasonic methods can successfully discriminate between propellant materials with varying solids/porosity content. However, these results are the first to show that this degree of discrimination is not extended to include propellants with varying amounts of crosslinking. Both data sets show that the shear moduli of propellants showing a factor of two difference at quasi-static, room temperature conditions converge at ultrasonic frequencies.; While this places a significant limitation on the use of ultrasonics as a means of determining a useful mechanical stiffness of a solid propellant for the structural analyst, it does not negate its potential as a health monitor. Relative measurement of longitudinal ultrasonic attenuation show sensitivity to changes in crosslink density. Additionally, the longitudinal velocities measured for the various sample sets studied indicate a large variability in propellant bulk modulus. While variations in bulk modulus do not influence the overall stress state within a solid rocket motor as much as variations in shear modulus, it still affects the accuracy of the stress determination.; As missile and space launch systems push the design envelope further towards its limit, the need for more accurate physical properties will increase. The successful use of nondestructive characterization methods on solid rocket motor materials and bondlines can provide a number of potential benefits which will lead to increased SRM reliability.