Why poisson ration is less than 0.5?

The Poisson's ratio, which measures the ratio of lateral strain to axial strain in materials, is typically less than 0.5 because of the physical constraints imposed by the material's atomic structure. A value of 0.5 would imply incompressibility, where a material's volume does not change under deformation, which is not physically feasible for most materials under normal conditions. Additionally, exceeding this limit would lead to non-physical predictions, such as negative volume changes, which are not observed in real materials. Thus, the maximum practical Poisson's ratio is constrained to values less than 0.5.