Editing Compression (physics) (section)

== Effects ==
When put under compression (or any other type of stress), every material will suffer some [[deformation (physics)|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.<ref name  =ONE>Fung, Y. C. (1977). A First Course in Continuum Mechanics (2nd ed.). Prentice-Hall, Inc. ISBN 978-0-13-318311-5.</ref>

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]].

[[File:Corset 1900.jpg|thumb|100px|Tightening a [[corset]] applies biaxial compression to the waist.]]
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.<ref name  =ONE/>  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]].