Compression at All Sides

Consider a cubical volume element of material with dimensions that are large relative to its structural units and small relative to the size of the whole sample. Any deformation of that volume element may be caused by forces acting on each of the six sides of the cube. Each of these forces has its own magnitude and direction and description of the relation between the overall deformation and the forces is very complex. For the sake of clarity, we consider three simple deformations equal compression at all sides of the cube, dilation in one direction, and shear. 

The value of K for condensed phases, solids and liquids, is on the order of 10 N but for gases it is much smaller, say, about 10 N m . Compression of condensed phases is energetically highly unfavorable as the short-range intermolecular and interatomic repulsive forces, especially the Bom repulsion (see Section 13.3.4), steeply increase with decreasing separations. 

Conversely, when the volume elanent is isotropically expanded (dilation). Equation 17.1 applies as well, with K now being called the dilation modulus. 

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