Compression (physics)

In uniaxial compression, the forces are directed along one direction only, so that they act towards decreasing the object's length along that direction. The compressive forces may also be applied in multiple directions; for example inwards along the edges of a plate or all over the side surface of a cylinder, so as to reduce its area (biaxial compression), or inwards over the entire surface of a body, so as to reduce its volume.

Technically, a material is under a state of compression, at some specific point and along a specific direction ${\displaystyle x}$, if the normal component of the stress vector across a surface with normal direction ${\displaystyle x}$ is directed opposite to ${\displaystyle x}$. If the stress vector itself is opposite to ${\displaystyle x}$, the material is said to be under normal compression or pure compressive stress along ${\displaystyle x}$. In a solid, the amount of compression generally depends on the direction ${\displaystyle x}$, and the material may be under compression along some directions but under traction along others. If the stress vector is purely compressive and has the same magnitude for all directions, the material is said to be under isotropic compression, hydrostatic compression, or bulk compression. This is the only type of static compression that liquids and gases can bear. It affects the volume of the material, as quantified by the bulk modulus and the volumetric strain.

The inverse process of compression is called decompression, dilation, or expansion, in which the object enlarges or increases in volume.

In a mechanical wave, which is longitudinal, the medium is displaced in the wave's direction, resulting in areas of compression and rarefaction.

Effects

When put under compression (or any other type of stress), every material will suffer some deformation, even if imperceptible, that causes the average relative positions of its atoms and molecules to change. The deformation may be permanent, or may be reversed when the compression forces disappear. In the latter case, the deformation gives rise to reaction forces that oppose the compression forces, and may eventually balance them.

Liquids and gases cannot bear steady uniaxial or biaxial compression, they will deform promptly and permanently and will not offer any permanent reaction force. However they can bear isotropic compression, and may be compressed in other ways momentarily, for instance in a sound wave.

Every ordinary material will contract in volume when put under isotropic compression, contract in cross-section area when put under uniform biaxial compression, and contract in length when put into uniaxial compression. The deformation may not be uniform and may not be aligned with the compression forces. What happens in the directions where there is no compression depends on the material. Most materials will expand in those directions, but some special materials will remain unchanged or even contract. In general, the relation between the stress applied to a material and the resulting deformation is a central topic of continuum mechanics.