There are five elastic properties needed to represent the elastic behaviour of a unidirectional laminate under in-plane loading, but only four of these are independent.

The five properties are the modulus of the laminate parallel and perpendicular to the fibre direction (E₁ and E₂), the associated Poisson's ratios (n₁₂, n₂₁), and the in-plane shear modulus G12. The two different Poisson's ratios are defined as follows:

n₁₂, the larger value, gives the strain in the '2' direction associated with a mechanical strain in the '1' direction.

n₂₁, the smaller value, gives the strain in the '1' direction associated with a mechanical strain in the '2' direction.

Q4 A sample of unidirectional CFRP 2 mm thick and 20 mm wide, with elastic properties as above, is subjected to a single tensile load of 20 kN (parallel to the fibre direction). Determine the strains parallel and perpendicular to the fibre direction.