ELASTIC-CONSTANTS AND THERMAL EXPANSIVITY OF GEL-SPUN POLYETHYLENE FIBER AND ITS COMPOSITES

The five independent stiffness constants, C-11, C-33, C-44, C-66, and C-13, and the a;dal and transverse thermal expansivity of unidirection al gel-spun polyethylene fiber reinforced composites have been measure d as functions of fiber volume fraction V-f. The axial extensional mod ulus C-33, and axial Poisson's ratio nu(13) follow the rule of mixture s, while the axial shear modulus C-44, transverse shear modulus C-66, and transverse plane-strain bulk modulus C-t (= C-11 - C-66) obey the Halpin-Tsai equation. Extrapolation to V-f = I gives the five stiffnes s constants of gel-spun polyethylene fiber. The tensile property of th e fiber is highly anisotropic, with the axial Young's modulus about 40 times higher than the transverse Young's modulus. In contrast, the ax ial shear modulus exceeds the transverse shear modulus by only 5%. A s imilar treatment of the thermal expansivity data in terms of the Schap ery equations gives an axial thermal expansivity of -1.25 x 10(-5) K-1 and a transverse thermal expansivity of 11.7 x 10(-5) K-1 for the fib er.