Elastic constants and thermal expansivity of gel‐spun polyethylene fiber and its composites

The five independent stiffness constants, C11, C33, C44, C66, and C13, and the axial and transverse thermal expansivity of unidirectional gel‐spun polyethylene fiber reinforced composites have been measured as functions of fiber volume fraction Vf. The axial extensional modulus C33and axial Poisson's ratio v13follow the rule of mixtures, while the axial shear modulus C44, transverse shear modulus C66, and transverse plane‐strain bulk modulus Ct( = C11− C66) obey the Halpin‐Tsai equation. Extrapolation to Vf= 1 gives the five stiffness constants of gel‐spun polyethylene fiber. The tensile property of the fiber is highly anisotropic, with the axial Young's modulus about 40 times higher than the transverse Young's modulus. In contrast, the axial shear modulus exceeds the transverse shear modulus by only 5%. A similar treatment of the thermal expansivity data in terms of the Schapery equations gives an axial thermal expansivity of −1.25 × 10−5K−1and a transverse thermal expansivity of 11.7 × 10−5K−1for the fiber.