 Fibre volume fraction

The fibre volume fraction is the fraction of a composite material's total volume occupied by reinforcing fibres:For a specimen with only fibres and a composite matrix, their fractions sum to one. Voids require a third fraction; they must not be silently counted as composite matrix. The fibre volume fraction is different from the fibre mass fraction because the constituents can have different mass densities.

For a void-free material, a measured fibre mass fraction $w_f$ giveswhere ${\rho }_f$ and ${\rho }_m$ are the constituent mass densities. For example, masses in the ratio $3:2$ and mass densities in the ratio $3:1$ give $V_f=1/3$, not $3/5$. Inserting $V_f$ into the longitudinal rule of mixtures estimates axial Young's modulus; orientation and fibre-matrix load transfer remain separate requirements.