3. Void Content

Unlike in other conventional materials, polymer and ceramic matrix materials have to be tested for one more physical property like void content. These composites have voids after fabrication. A composite with voids can affect the mechanical; thermal properties, strengths and resistance to fatigue and corrosion. A composite with less than 1% void content is treated as a well fabricated composite. Further, a composite upto 7% void content is regarded as a poorly fabricated composite.

The void content  is measured from experimental composite density  and theoretical composite density . The void content in percent is simply the ratio of difference between experimental and theoretical densities to theoretical density. If one knows the densities of the constituents and resin content then, theoretical density can be found. The methods are described in ASTM standard D2534–91, which requires use of ASTM standard D2584–68(1985) for determination of the resin content.
The theoretical density of the composite is of weight of the composite per unit its volume. The volume of the composite is sum of the volume of the fibre and resin. The volume of the fibre and resin can be found from their weights and respective densities. Thus,

(8.4)

Thus, using the volumes of fibre and resin,  and  

(8.5)

where,  is the weight of the fibre,  is the weight of the resin,  is the density of the fibre and  is the density of resin.  Now, percent the void content is given as:

(8.6)