Mechanical Properties of Unidirectional Composites

It is worth noting that ideal arrangements of fibers of circular cross section with the highest packing, hexagonal arrays with the fibers touching, would lead to a theoretical volume fiber fraction of 0.9. This value is never 

As a consequence of the fiber s high modulus, the load on a composite will be carried mostly by the fibers, thus ensuring that y is higher than a critical value, = EJ Ef + E ). 

Since the volume fractions of fiber and matrix can be expressed as 

This equation indicates that the total strain is the weighted sum of strains in fibers and matrix. As we are assuming elastic behavior, Eq. (15.5) becomes 

Since the elastic modulus of fibers is higher than that of polymer matrices, Eq. (15.29) can be simplified as 

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Micromechanics are a study of mechanical properties of unidirectional composites in terms of those of constituent materials. In particular, the properties to be discussed are elastic modulus, hydrothermal expansion coefficients and strengths. In discussing composites properties it is important to define a volume element which is small enough to show the microscopic structural details, yet large enough to present the overall behavior of the composite. Such a volume element is called the Representative Volume Element (RVE). A simple representative volume element can consists of a fiber embedded in a matrix block, as shown in Fig. 9.3. 

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