Continuous fibres can carry loads very efficiently along their length, so a fibre-reinforced composite can be much stiffer and stronger in one direction than another.

This directional dependence is anisotropy.

Longitudinal loading

When continuous fibres and matrix experience approximately the same strain along the fibre direction, a simple rule-of-mixtures estimate for longitudinal modulus is

$$E_L\approx V_fE_f+V_mE_m,$$

where $V_f$ and $V_m$ are fibre and matrix volume fractions.

Transverse and shear response

Loading across the fibres depends much more strongly on the matrix and the fibre–matrix interface. The transverse modulus and shear properties therefore differ substantially from the longitudinal response.

Laminae and laminates

A thin composite layer with one fibre orientation is a lamina. Several laminae stacked at chosen angles form a laminate.

By changing the stacking sequence, engineers can distribute stiffness and strength among several directions instead of accepting the behaviour of one unidirectional layer.

Coordinates matter

Stress and strain must often be transformed between global component axes and local material axes. Composite mechanics therefore brings material anisotropy together with the stress-transformation ideas used in mechanics of materials.