An Ultrasonic Testing Technique for Monitoring the Cure and Mechanical Properties of Polymeric Materials

Abstract

Large space booster solid rocket motors (SRMs) contain composite propellant materials based on either a hydroxyl terminated polybutadiene (HTPB) or the terpolymer of butadiene, acrylic acid, and acrylonitrile (PBAN) polymer matrix. Mechanical tests have revealed significant batch-to-batch variances and age related changes in the propellant modulil probably due to continued crosslinking of the system. This paper documents a work in progress directed toward testing the feasibility of using ultrasonic measurements of the propellant mechanical properties to monitor the condition and the degree of cure of the propellant. If the ultrasonic shear and longitudinal velocities, c_S and c_L, and the density, p, of the propellant are known, the Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio, E, μ, K and σ, respectively, can be calculated via the familiar relations

$$E = \frac{{\rho c_{L}^{2}(1 - 2\sigma )(1 + \sigma )}}{{1 - \sigma }}, \mu = \rho c_{s}^{2}, K = \frac{E}{{3(1 - 2\sigma )}}and \sigma = \frac{{c_{L}^{2} - 2c_{S}^{2}}}{{2c_{L}^{2} - 2c_{S}^{2}}}$$

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