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VISCOELASTIC PROPERTIES DETERMINATION

With the advent of "designed damping" in composite materials, and modern computer design tools, accurate three-dimensional material properties information is critically important. Further, design of advanced composite structures often requires knowledge of material properties over a range of temperatures. Conventional testing approaches for the determination of material properties present difficulties related to specimen preparation, fixturing and test apparatus. Consequently, engineers frequently make use of micromechanics generated properties. Unfortunately, micromechanics approaches do not usually account for manufacturing and temperature variations, which affect material properties. This research focuses on developing approaches to allow experimental characterization of elastic and viscoelastic properties of fiber reinforced composite laminae, over a range of temperatures. In addition to investigating the temperature dependence of the three-dimensional material properties, for the viscoelastic characterization, the effect of loading frequency is also addressed.

A technique for the determination of the three-dimensional elastic coefficients of transversely isotropic laminae has been developed, using a combination of laminate coefficient of thermal expansion (CTE) measurements and two elastic properties measured from a standard tensile test. The CTE measurements have been conducted in a laboratory thermo mechanical analyzer (TMA), using samples of simple geometry. PEEK/IM7 laminae were used to verify this approach and the computed room temperature elastic properties are in good agreement with quoted elastic material properties measured by standard techniques.



Viscoelastic material characterization is essential to designs that incorporate specified levels of damping, and to understand processing problems. This research has also resulted in an approach to determine the three-dimensional viscoelastic properties of transversely isotropic fiber reinforced materials. The minimum number of independent coefficients for three-dimensional viscoelastic characterization of transversely isotropic laminae is investigated and a reduced set of material coefficients, that specify the constitutive relationships, is proposed. The experimental approach developed is based only on flexural measurements allowing complete characterization using a dynamic mechanical analyzer (DMA). To verify the approach, viscoelastic properties of PEEK/IM7 laminae and laminates have been determined over ranges of temperature and frequency.

The mechanics relations and experimental techniques developed during this research effort have provided means for measuring the elastic and viscoelastic properties of fiber reinforced composites, and constitute a valuable contribution to the understanding of temperature and frequency dependence of these mechanical properties.







Mechanical Engineering, Fort Collins, CO 80523-1374