Modulus development in oriented short‐glass‐fiber‐reinforced polymer composites

The increase in modulus obtained in a short‐glass‐fiber‐reinforced polymer composite as a result of uniaxial deformation may be related to the observed increase in both fiber and matrix orientation. Quantitative measurements of both fiber and matrix orientation are presented for a series of samples of short‐glass‐fiber‐reinforced polyoxymethylene copolymer, processed to various substantial deformation ratios by solid‐phase hydrostatic extrusion. The polymer matrix becomes highly oriented at modest deformations, but the glass fibers orient in a slower pseudo‐affine manner and dominate the development of modulus in the composite. A simple “law‐of‐mixtures” model is used to demonstrate that perfect uniaxial orientation of the fibers is not achieved, but a better fit to measured modulus data is obtained by using an “aggregate” model applied to oriented fibers in an oriented matrix. The development of modulus with deformation ratio may be predicted very well if it is assumed that: the composite consists of a series‐coupled array of sub‐units, each containing continuous and fully oriented fibers in a fully oriented matrix; and orientation of the sub‐units develops with deformation in a pseudo‐affine manner.