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Bulk modulus measures a material's resistance to uniform compression. It is defined as the proportionality constant between a change in pressure and the resulting relative volume change. Materials with higher bulk modulus values resist compression more effectively than those with lower values.

Dilatation represents the total volume change of a material and is calculated as the sum of strains in all three directions. When a material undergoes uniform hydrostatic pressure, the dilatation becomes negative for stable materials. The bulk modulus is derived by substituting stress components into the dilatation equation.

An isotropic cube of unit volume deforms into a rectangular parallelepiped when subjected to normal stresses. The difference between the new volume and original volume is termed dilatation. Under uniform hydrostatic pressure, each stress component equals the negative of the pressure applied.

Poisson's ratio determines lateral contraction behavior during deformation. A material with zero Poisson's ratio can stretch without lateral contraction. Conversely, a Poisson's ratio of 0.5 signifies perfect incompressibility, meaning the material maintains constant volume during deformation under the relation between Poisson's ratio, modulus of elasticity and modulus of rigidity.

Stable materials decrease in volume when subjected to hydrostatic pressure, making dilatation negative. Since bulk modulus is the proportionality constant relating pressure change to relative volume change, the negative dilatation combined with negative pressure results in a positive bulk modulus value.

Bulk modulus is expressed in the same units as the modulus of elasticity, typically pascals (Pa) or megapascals (MPa). This consistency in units allows for direct comparison between bulk modulus and other elastic constants when analyzing material behavior under various loading conditions.

Bulk modulus specifically measures resistance to uniform volumetric compression, while other elastic constants address different deformation modes. The modulus of elasticity measures resistance to axial loading, and modulus of rigidity measures resistance to shear. Bulk modulus uniquely characterizes material behavior under hydrostatic pressure.