Shock (mechanics)
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A mechanical or physical '''shock''' is a sudden [[acceleration]] or deceleration caused, for example, by impact, drop, [[earthquake]], or [[explosion]]. Shock is a transient physical excitation.
Shock is usually measured by an [[accelerometer]]. This describes a shock pulse as a plot of acceleration versus time. Acceleration can be reported in units of [[metre per second squared]]. Often, for convenience, the magnitude of a shock is stated as a multiple of the standard acceleration due to free fall in the Earth's [[gravity]], a quantity with the symbol '''''g''''' having the value 9.80665 m·s<sup>-2</sup>. Thus a shock of "20''g''" is equivalent to about 196 m/s<sup>2</sup>. A shock can be characterized by the peak acceleration, the duration, and the shape of the shock pulse (half sine, triangular, trapezoidal, etc). The [[Shock response spectrum]] is a method for further evaluating a mechanical shock. It is sometimes used as a [[defense standard]] for military equipment.
=== Effects of Shock ===
Mechanical shock has the potential for damaging an item (e.g., an entire [[light bulb]]) or an element of the item (e.g. a filament in an [[Incandescent light bulb]]):
* A [[brittle]] or fragile item can fracture. For example, two crystal wine glasses may shatter when impacted against each other. A [[shear pin]] in an engine is designed to fracture with a specific magnitude of shock.
* A [[ductile]] item can be bent by a shock. For example, a copper pitcher may bend when dropped on the floor.
* Some items may not be damaged by a single shock but will experience [[Fatigue (material)|fatigue]] failure with numerous repeated low-level shocks.
* A shock may result in only minor damage which may not be critical for use. However, cumulative minor damage from several shocks will eventually result in the item being unusable.
* A shock may not produce immediate apparent damage but might cause the service life of the product to be shortened: the [[reliability]] is reduced.
* A shock may cause in item to become out of adjustment. For example, when a precision scientific instrument is subjected to a moderate shock, good [[metrology]] practice may be to have it re[[calibrate]]d before further use.
* Some materials such as primary high [[explosive]]s may [[detonate]] with mechanical shock or impact.
=== Considerations ===
When laboratory testing, field experience, or engineering judgement indicates that an item could be damaged by mechanical shock, several courses of action might be considered:
* Reduce and control the input shock at the source.
* Modify the item to improve its [[toughness]] or support it to better handle shocks.
* Use [[shock absorber]]s or cushions to control the shock transmitted to the item. [[Cushioning]] reduces the peak acceleation by extending the duration of the shock.
* Plan for failures: accept certain losses. Have redundant systems available, employ insurance, etc.
=== See also ===
* [[Cushioning]]
* [[Fracture mechanics]]
* [[g-force]]
* [[Impact force]]
* [[Response spectrum]]
* [[Thermal shock]]
* [[Vibration]]
=== Further reading ===
* DeSilva, C. W., "Vibration and Shock Handbook", CRC, 2005, ISBN 0849315808
* Harris, C. M., and Peirsol, A. G. "Shock and Vibration Handbook", 2001, McGraw Hill, ISBN 0071370811
* ASTM D6537, Standard Practice for Instrumented Package Shock Testing for Determination of Package Performance.
* [[MIL-STD-810]]F, Environmental Test Methods and Engineering Guidelines, 2000
* MIL-S-901 D, Shock Tests, H. I. (High Impact) Shipboard Machinery, Equipment, and Systems, 1989
[[Category:Mechanics]]
[[Category:Packaging]]
[[es:Choque (física)]]
[[fr:Choc mécanique]]