Shock (mechanics)

Information about Shock (mechanics)

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.

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

acceleration is defined as the rate of change of velocity, or, equivalently, as the second derivative of position. It is thus a vector quantity with dimension length/time². In SI units, acceleration is measured in metres/second² (m·s^-²).
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explosion is a sudden increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.

The most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation
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accelerometer is a measuring device for specific external force. Specific external force is the sum total of external forces acting on an object divided by the mass. Accelerometers do not measure internal forces such as gravity.
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The metre (or meter) per second squared is the SI derived unit of acceleration. It is a measure of magnitude and can be a scalar measure or, when associated with a direction, a vector, for example by having sign positive or negative.
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Gravitation is a natural phenomenon by which all objects with mass attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight.
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Shock Response Spectrum (SRS) is a graphical representation of an arbitrary transient acceleration input, such as shock in terms of how a Single Degree Of Freedom (SDOF) system (like a mass on a spring) responds to that input.
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A United States Defense Standard, often called a military standard, "MIL-STD", or "MIL-SPEC", is used to help achieve standardization objectives by the U.S. Department of Defense.
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incandescent light bulb (also spelled lightbulb) or incandescent lamp is a source of artificial light that works by incandescence. An electrical current passes through a thin filament, heating it until it produces light.
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brittle if it is liable to fracture when subjected to stress. That is, it has little tendency to deform (or strain) before fracture. This fracture absorbs relatively little energy, even in materials of high strength, and usually makes a snapping sound.
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A shear pin is the mechanical analogue of an electric fuse. Installed in a drive train, it is designed to break in the case of a mechanical overload, preventing other, more-expensive parts of the drive train from being damaged.
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Ductility is the mechanical property of being capable of sustaining large plastic deformations due to tensile stress without fracture (in metals, such as being drawn into a wire). It is characterized by the material flowing under shear stress. It is contrasted with brittleness.
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Metal fatigue redirects here. For the computer game, see Metal Fatigue.

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Mechanical failure modes
Buckling
Corrosion
Creep
Fatigue
Fracture
Melting
Thermal shock
Wear
Yielding
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In general, reliability (systemic def.) is the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances.

The IEEE defines it as ". . .
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worldwide view of the subject.
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Metrology (from Greek 'metron' (measure), and 'logos' (study of)) is the science of measurement.
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explosive material is a material that either is chemically or otherwise energetically unstable or produces a sudden expansion of the material usually accompanied by the production of heat and large changes in pressure (and typically also a flash and/or loud noise) upon initiation;
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Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. It is the more powerful of the two general classes of combustion, the other one being deflagration.
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In materials science and metallurgy, toughness is the resistance to fracture of a material when stressed. It is defined as the amount of energy that a material can absorb before rupturing, and can be found by taking the area (i.e.
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A shock absorber in common parlance (or damper in technical use) is a mechanical device designed to smooth out or damp a sudden shock impulse and dissipate kinetic energy. It is analogous to a resistor in an electric RLC circuit.
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Package cushioning is used to help protect fragile items during shipment. It is not uncommon for a transport package to be dropped, kicked, and impacted: These events may produce potentially damaging shocks.
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Fracture mechanics is a method for predicting failure of a structure containing a crack. It uses methods of analytical Solid mechanics to calculate the driving force on a crack and those of experimental Solid mechanics to characterize the material's resistance to fracture.
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An impact force is a high force or shock applied over a short time period. Such a force can have a greater effect than a lower force applied over a proportionally longer time period.
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response spectrum is simply a plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock.
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Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

Vibration is occasionally desirable.
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The MIL-STD-810 series of standards are issued by the United States Army's Developmental Test Command, to specify various environmental tests to prove that equipment qualified to the standard will survive in the field.
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