Elastic and viscoelastic characterization of transversely isotropic composite laminae

Abstract

With the advent of "designed damping” in composite materials, and modem 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 o f transversely isotropic laminae is 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 are conducted in a laboratory thermo mechanical analyzer (TM A). using samples of simple geometry. PEEK/IM7 laminae are 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 work also presents an approach to determine the three-dimensional viscoelastic properties o f 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 o f PEEK/IM7 laminae and laminates are determined over ranges of temperature and frequency.

The mechanics relations and experimental techniques developed in this work provide 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.