Abstract
The major objectives of this paper are to describe preliminary results from a systematic analytical study of the improvement of damping in polymer composites at the micromechanical level by the use of special fiber coatings. Since shear deformation is essential for viscoelastic damping in polymers, and large shear strains are generated near the fiber/matrix interface in composites, it is believed that the use of special viscoelastic polymer fiber coatings will be an effective way to improve damping in such composites. The analysis was carried out by developing finite element, theory of elasticity, and mechanics of materials models. A strain energy approach was implemented in both the finite element and theory of elasticity formulations to analyze the damping in the coated fiber composites. The models, based on a “representative volume element” or repeating element of a continuously reinforced fiber composite, are used to study damping and stiffness under longitudinal normal (LN), transverse normal (TN), transverse shear (TS) and longitudinal shear (LS) loading. Of particular interest are hybrid composites having a mixture of coated and uncoated fibers, since coating all the fibers would cause unacceptable reductions in other important properties like strength and stiffness. In order to analyze hybrid composites, two and three dimensional multi-cell finite element models have been developed. Parametric studies were done with particular emphasis on the effect of fiber coating thickness and coating volume fraction in order to improve the damping of the composite structure. Large shear strain deformations occur in the fiber coating material since the stiffness of the coating material is lower than that of the fiber or the matrix. This condition leads to improved damping in the coated fiber reinforced composite. A rapid increase in damping with increasing coating volume fraction was found for the cases of TN, TS, and LS loading. For the case of LN loading, damping does not change much with increasing coating volume fraction. Good agreement for the damping and stiffness results was obtained by using the finite element, elasticity, and mechanics of materials formulations.
