Editing Compression (physics)

{{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]].

== 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]].

== Uses ==
[[File:Compression test.jpg|right|thumb|150px|Compression test on a [[universal testing machine]]]]

Compression of solids has many implications in [[materials science]], [[physics]] and [[structural engineering]], for compression yields noticeable amounts of [[Stress (physics)|stress]] and [[tension (mechanics)|tension]].

By inducing compression, mechanical properties such as [[compressive strength]] or [[modulus of elasticity]], can be measured.<ref>Hartsuijker, C.; Welleman, J. W. (2001). Engineering Mechanics. Volume 2. Springer. ISBN 978-1-4020-412</ref>

Compression machines range from very small table top systems  to ones with over 53 MN capacity.

Gases are often stored and shipped in highly [[compressed gas|compressed]] form, to save space.  Slightly compressed air or other gases are also used to fill [[balloon]]s, [[rubber boat]]s, and other [[inflatable structure]]s.  Compressed liquids are used in [[hydraulic equipment]] and in [[fracking]].

== In engines ==

=== Internal combustion engines ===
In [[internal combustion engine]]s the explosive mixture gets compressed before it is ignited; the compression improves the efficiency of the engine. In the [[Otto cycle]], for instance, the second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke.<ref>{{Cite book |last=Heywood |first=John |url=https://books.google.com/books?id=OmJUDwAAQBAJ |title=Internal Combustion Engine Fundamentals 2E |date=2018-05-01 |publisher=McGraw Hill Professional |isbn=978-1-260-11611-3 |language=en}}</ref>

=== Steam engines ===
The term is applied to the arrangement by which the exhaust valve of a [[steam engine]] is made to close, shutting a portion of the exhaust steam in the [[cylinder (engine)|cylinder]], before the stroke of the piston is quite complete. This steam being compressed as the stroke is completed, a cushion is formed against which the [[piston]] does work while its velocity is being rapidly reduced, and thus the stresses in the mechanism due to the inertia of the reciprocating parts are lessened.<ref name="Wiser">{{cite book|title=Energy resources: occurrence, production, conversion, use|last= Wiser |first= Wendell H.|year= 2000|publisher= Birkhäuser|isbn= 978-0-387-98744-6|page= 190|url= https://books.google.com/books?id=UmMx9ixu90kC&dq=steam&pg=PA190}}</ref> This compression, moreover, obviates the shock which would otherwise be caused by the admission of the fresh steam for the return stroke.

== See also ==
* [[Buckling]]
* [[Container compression test]]
* [[Compression member]]
* [[Compressive strength]]
* [[Longitudinal wave]]
* [[P-wave]]
* [[Rarefaction]]
* [[Strength of materials]]
* [[Résal effect]]
* [[Plane strain compression test]]

== References ==
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[[Category:Continuum mechanics]]
[[Category:Mechanical engineering]]