Determination of the complex elastic moduli of materials using a "free-free" bar technique
Bartlett, David L.
Brown, David A.
Gardner, David L.
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The objective of this research is to investigate the dynamic elastic properties of materials and their temperature and frequency dependence using an acoustic resonance based method. In this technique, the torsional, flexural, and/or longitudinal vibrational modes of a 'free-free' bar are selectively excited and tracked as a function of temperature. The resonance frequency of the torsional model is used to obtain the dynamic shear modulus. The dynamic Young's modulus is obtained from either the resonance frequency of the flexural or longitudinal mode of the bar. The quality factor, Q, of each mode is measured to obtain the damping properties of the material. A phase-locked-loop (PLL) is used to track the resonance frequency as a function of changing temperature for the particular vibrational model selected. Using this technique, the storage modulus and loss tangent may be obtained in a continuous fashion. Materials tested in this thesis include: Polyurethane PR-1592, a common sonar encapsulant, Polymethyl Methacrylate (PMMA) or plexiglass, and Polycarbonate. This research encompasses the theory, accuracy, limitations and applications of this measurement technique.
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