In addition to being dependent on the fiber volume fraction the Young's modulus of a uniaxial composite is dependent on the angle the fibers make with respect to the tensile stress direction. The diagram shows the modulus behavior for a unidirectional lamina

of glass fibers in a polyester resin matrix. The volume fraction of the fibers is 0.3 . The Young's modulus is highly directional, and this can be exploited in design if different mechanical response is desired in orthogonal directions. The semicircular curve for random orientation short fibers, shows that the maximum modulus is reduced by about a factor of two but the material modulus is direction independent within the plane of the lamina. The cost of manufacturing a chopped fiber lamina is less than that of a uniaxial one, and cost considerations may dominate the design choice, particularly if anisotropic properties offer no advantage.

From: Hull, "An Introduction to Composite Materials,"
Cambridge (1992)