What does Poisson's ratio of material mean?

What does Poisson's ratio of material mean?

Within the proportional limit of a material, the absolute value of the ratio of the transverse strain to the corresponding longitudinal strain caused by uniformly distributed longitudinal stress. For example, when a rod is stretched, its axial elongation is accompanied by transverse contraction (and vice versa), and the ratio of the transverse strain e 'to the axial strain e' is called Poisson's ratio v. the Poisson's ratio of a material is generally determined by experimental methods
It can be remembered as follows: the Poisson's ratio of air is 0, the Poisson's ratio of water is 0.5, and the one in the middle can be pushed out