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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
(red line), and this can be exploited in design if a different mechanical
response is desired in orthogonal directions.
The semicircular curve
for random-orientation short fibers in the same matrix, shows that the maximum
modulus is reduced by about a factor of two but that the material modulus
is direction independent within the plane of the lamina. The cost of manufacturing
a chopped fiber lamina is also less than that of a uniaxial one, and cost
considerations may dominate the design choice, particularly if anisotropic
properties offer no performance advantage. |
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