Rule of Mixtures

The density of the composite in terms of volume fraction can be found by considering the weight of the composite to be composed of the weights of their constituent, wc=wf+wm.
The weights can be expressed in terms of their respective densities and volumes,?c vc= ?m vm+ ?f vf
After fabrication, composites often contain voids. Voids must be treated as a constituent with volume but with no weight. To determine the volume fraction of voids, Vv , express the void volume as voids actual theoretical v voids = v actual- v theortical
where v theroretical is the volume of the solid constituents that contribute to the weight of the composite and v actual v is the measured or actual volume of the composite including the voids. The volume fraction is then found to be theroetical actual v theoretical
V = v theroretical - v actual ? v theortical

From practical considerations it is important to minimize void content in a composite to achieve the best mechanical and environmental properties. For instance to minimize water penetration and fatigue void volume fraction should be less than 0.01.
Strength and stiffness parallel to fibers
The strength of a composite when measured in the direction of the fibers is referred to as the longitudinal strength. The stresses at which the fibers and the matrix fail determine the strength of a composite. The stresses in a composite can be determined if a few simplifying assumptions can be made. These assumptions are:
· The fibers are continuous, that is they extend for the length of the composite
· The fibers are aligned in one direction only
· The load is applied parallel to the direction of the fibers
· There is perfect bonding between the fiber and the matrix thereby preventing interfacial slip