Shock (mechanics)

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A mechanical or physical shock is a sudden acceleration or deceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation.

Shock is a term for extreme forces that matter is subjected to. A shock is usually measured as acceleration vs time. A shock can be characterised by its 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.

Shock is a vector quantity, with both magnitude and direction.

Effects of shock[edit]

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. Note that a soft ductile material may sometimes exhibit brittle failure during shock due to time-temperature superposition.
  • A ductile item can be bent by a shock. For example, a copper pitcher may bend when dropped on the floor.
  • Some items may appear to be not damaged by a single shock but will experience 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 an 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 recalibrated before further use.
  • Some materials such as primary high explosives may detonate with mechanical shock or impact.
  • When glass bottles of liquid are dropped or subjected to shock, the water hammer effect may cause hydrodynamic glass breakage.[1]

Considerations[edit]

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:[2]

  • 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 absorbers or cushions to control the shock transmitted to the item. Cushioning [3] reduces the peak acceleration by extending the duration of the shock.
  • Plan for failures: accept certain losses. Have redundant systems available, etc.

See also[edit]

Notes[edit]

  1. ^ Saitoh, S (1999). "Water hammer breakage of a glass container". International glass journal (Faenza Editrice,). ISSN 1123-5063. 
  2. ^ Burgess, G (March 2000). "Extensnion and Evaluation of fatigue Model for Product Shock Fragility Used in Package Design". J. Testing and Evaluation 28 (2). 
  3. ^ Package Cushioning Design. MIL-HDBK 304C. DoD. 1997. 

Further reading[edit]

  • DeSilva, C. W., "Vibration and Shock Handbook", CRC, 2005, ISBN 0-8493-1580-8
  • Harris, C. M., and Peirsol, A. G. "Shock and Vibration Handbook", 2001, McGraw Hill, ISBN 0-07-137081-1
  • ISO 18431:2007 - Mechanical vibration and shock
  • ASTM D6537, Standard Practice for Instrumented Package Shock Testing for Determination of Package Performance.
  • MIL-STD-810F, Environmental Test Methods and Engineering Guidelines, 2000
  • Brogliato, B., "Nonsmooth Mechanics. Models, Dynamics and Control", Springer London, 2nd Edition, 1999.