Shock (mechanics)
Encyclopedia
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 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/s2. Thus a shock of "20 g" 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.
):
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
or deceleration caused, for example, by impact, drop, kick, earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
, or explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
. Shock is a transient physical excitation.
Shock is usually measured by an accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
. This describes a shock pulse as a plot of acceleration versus time. Acceleration can be reported in units of metre per second squared
Metre per second squared
The metre per second squared is the unit of acceleration in the International System of Units . As a derived unit it is composed from the SI base units of length, the metre, and the standard unit of time, the second...
. 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/s2. Thus a shock of "20 g" 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
Shock response spectrum
A Shock Response Spectrum is a graphical representation of an arbitrary transient acceleration input, such as shock in terms of how a Single Degree Of Freedom system responds to that input. Actually, it shows the peak acceleration response of an infinite number of SDOFs, each of which have...
is a method for further evaluating a mechanical shock. It is sometimes used as a defense standard
Defense Standard
A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or "MilSpecs", is used to help achieve standardization objectives by the U.S...
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 bulbIncandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
):
- A brittleBrittleA material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
or fragile item can fracture. For example, two crystal wine glasses may shatter when impacted against each other. A shear pinShear pinA shear pin is a safety device designed to shear in the case of a mechanical overload, preventing other, more-expensive parts from being damaged...
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 superpositionTime-temperature superpositionThe Time–temperature superposition principle is a concept in polymer physics and in the physics of glass-forming liquids. Some materials, polymers in particular, show a strong dependence of viscoelastic properties on the temperature at which they are measured...
. - 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 fatigueFatigue (material)'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
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 reliabilityReliability engineeringReliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
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 metrologyMetrologyMetrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
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 bottlesGlass BottlesA glass bottle is a bottle created from glass. Glass bottles can vary in size considerably, but are most commonly found in sizes ranging between about 10ml and 5 liters....
of liquid are dropped or subjected to shock, the water hammerWater hammerWater hammer is a pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly . Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe...
effect may cause hydrodynamic glass breakage.
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 toughnessToughnessIn materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
or support it to better handle shocks. - Use shock absorberShock absorberA shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
s or cushions to control the shock transmitted to the item. CushioningCushioningPackage cushioning is used to help protect fragile items during shipment. It is common for a transport package to be dropped, kicked, and impacted: These events may produce potentially damaging shocks. Transportation vibration from conveyors, trucks, railroads, or aircraft can also damage some...
reduces the peak acceleration by extending the duration of the shock. - Plan for failures: accept certain losses. Have redundant systems available, etc.
See also
- CushioningCushioningPackage cushioning is used to help protect fragile items during shipment. It is common for a transport package to be dropped, kicked, and impacted: These events may produce potentially damaging shocks. Transportation vibration from conveyors, trucks, railroads, or aircraft can also damage some...
- Fracture mechanicsFracture mechanicsFracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. 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.In...
- Fracture toughnessFracture toughnessIn materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. The fracture toughness of a material is determined from the...
- g-forceG-forceThe g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
- Impact (mechanics)
- Jerk (physics)
- Modal testingModal testingModal testing is a form of vibration testing of an object whereby the natural frequencies, modal masses, modal damping ratios and mode shapes of the object under test are determined.A modal test consists of an acquisition phase and an analysis phase...
- Response spectrumResponse spectrumA response spectrum is simply a plot of the peak or steady-state response of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. The resulting plot can then be used to pick off the response of any linear system, given its natural...
- Thermal shockThermal shockThermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
- VibrationVibrationVibration 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"...
- Water hammerWater hammerWater hammer is a pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly . Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe...
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
- 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.