Shock (mechanics)

Overview

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

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

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

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