Shock (mechanics)

Overview[edit | edit source]

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^-2. Thus a shock of "20g" is equivalent to about 196 m/s². 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[edit | edit source]

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 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 recalibrated before further use.
• Some materials such as primary high explosives may detonate with mechanical shock or impact.

Considerations[edit | edit source]

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 absorbers 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[edit | edit source]

• Cushioning
• Fracture mechanics
• g-force
• Impact force
• Response spectrum
• Vibration

Further reading[edit | edit source]

• 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-810F, Environmental Test Methods and Engineering Guidelines, 2000
• MIL-S-901 D, Shock Tests, H. I. (High Impact) Shipboard Machinery, Equipment, and Systems, 1989

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