A strong need exists for quantitative nondestructive assessment of complex polymer systems, particularly in polymer systems such as solid rocket motor (SRM) propellants that are susceptible to post-cure and aging. Ultrasonic methods were applied to the problem because of their well-established and fundamental relationship between wave propagation and material properties. Measurements of longitudinal and shear velocity were made from 30-500 kHz, converted to an effective linear elastic modulus and compared to linear viscoelastic dynamic mechanical measurements. The results indicate that ultrasonics and dynamic mechanical analysis yield equivalent material stiffnesses in spite of the disparity in applied strains. However, the results also show that the shear moduli of differently crosslinked materials converge at or near ultrasonic frequencies. Thus, the ability of ultrasonics to discriminate between these materials is limited. In spite of this limitation, there is still strong experimental evidence that damping factor (mechanical loss factor) may be used as an indicator of polymeric aging.
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