Poisson's ratio

A measure of a material's stiffness, defined as the ratio of stress to strain in the elastic region of loading. A higher modulus means less deflection under a given load; for plastics and composites it is typically measured in tension to ISO 527.

The deformation of a material in response to stress, expressed as change in length divided by original length. Strain is dimensionless, often quoted as a percentage, and may be elastic (recoverable) or plastic (permanent).

The variation of a material's properties with direction. Fibre-reinforced composites are strongly anisotropic, with much higher stiffness and strength along the fibre direction than across it, so ply orientations must be matched to the expected load paths.