Poisson's Ratio of Aluminium

Aluminium contracts laterally by about a third of its axial strain, a shade more than steel. The number is close enough to steel's that swapping materials rarely changes a hand calculation, but it is what makes G come out at 26 GPa rather than 25.7, and it feeds the same strain-gauge, plate-bending and pressure-vessel relations.

Poisson's ratio is bounded for real isotropic materials between −1 and 0.5, and almost everything metallic lands between 0.25 and 0.35: cork is famously near zero, rubber near 0.5, and engineered auxetic foams manage negative values by unfolding re-entrant cells. As with steel, this is an elastic-range value: plastic deformation is volume-conserving and drives the apparent ratio toward 0.5. Anisotropic materials such as rolled sheet, timber and composites have a different ratio for every pair of directions, so a single figure for them is a fiction.