Editing Compression (physics) (section)

{{Short description|Application of balanced forces which push inwards on an object}}

[[File:Compression applied.svg|thumb|right|50 px|Uniaxial compression]]
{{broader|Stress (mechanics)}}

In [[mechanics]], '''compression''' is the application of balanced inward ("pushing") [[force]]s to different points on a material or [[Structural system|structure]], that is, forces with no [[Net force|net sum]] or [[torque]] directed so as to reduce its size in one or more directions.<ref name=Beer>Ferdinand Pierre Beer, Elwood Russell Johnston, John T. DeWolf (1992), "Mechanics of Materials". (Book) McGraw-Hill Professional, {{ISBN|0-07-112939-1}}</ref> It is contrasted with [[tension (physics)|tension]] or traction, the application of balanced outward ("pulling") forces; and with [[shear stress|shearing]] forces, directed so as to displace layers of the material parallel to each other.  The [[compressive strength]] of materials and structures is an important engineering consideration.

In '''uniaxial compression''', the forces are directed along one direction only, so that they act towards decreasing the object's length along that direction.<ref>Erkens, Sandra & Poot, M. The uniaxial compression test.  Delft University of Technology. (1998). Report number: 7-98-117-4.</ref>  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 (geometry)|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 <math>x</math>, if the [[normal stress|normal component]] of the [[stress (mechanics)|stress]] vector across a surface with [[surface normal|normal direction]] <math>x</math> is directed opposite to <math>x</math>.  If the stress vector itself is opposite to <math>x</math>, the material is said to be under '''normal compression''' or '''pure compressive stress''' along <math>x</math>.  In a [[solid]], the amount of compression generally depends on the direction <math>x</math>, 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.<ref>Ronald L. Huston and Harold Josephs (2009), "Practical Stress Analysis in Engineering Design". 3rd edition, CRC Press, 634 pages. ISBN 9781574447132</ref> It affects the volume of the material, as quantified by the [[bulk modulus]] and the [[volumetric strain]].

{{Anchor|Dilation (physics)}}

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 wave|longitudinal]], the medium is displaced in the wave's direction, resulting in areas of compression and [[rarefaction]].