TRIBOLOGY..Assignment

7/30/2019 TRIBOLOGY..Assignment

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Beant college of engineering & technology

gurdaspur

INDUSTRIAL TRIBOLOGY

Assignment

INDUSTRIAL TRIBOLOGY

DEFINITION:


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The science offriction, lubrication and wearis called tribology.

FRICTIONFriction is theforce resisting the relative motion of solid surfaces, fluid layers, and

material elements sliding against each other. There are several types of friction:

Dry friction resists relative lateral motion of two solid surfaces in contact. Dry

friction is subdivided into static friction ("stiction") between non-moving surfaces,

and kinetic friction between moving surfaces.

Fluid friction describes the friction between layers within a viscous fluid that aremoving relative to each other.

Lubricated friction is a case of fluid friction where a fluid separates two solid

surfaces.

Skin friction is a component ofdrag, the force resisting the motion of a solid body

through a fluid.

Internal friction is the force resisting motion between the elements making up a

solid material while it undergoes deformation.

When surfaces in contact move relative to each other, the friction between the two

surfaces converts kinetic energyinto heat. This property can have dramatic

consequences, as illustrated by the use of friction created by rubbing pieces of wood

together to start a fire. Kinetic energy is converted to heat whenever motion with friction

occurs, for example when aviscousfluid is stirred. Another important consequence of

many types of friction can bewear, which may lead to performance degradation and/ordamage to components. Friction is a component of the science oftribology.

Friction is not itself a fundamental force but arises from fundamental electromagnetic

forces between the charged particles constituting the two contacting surfaces. The
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complexity of these interactions makes the calculation of friction from first

principles impossible and necessitates the use ofempirical methods for analysis and the

development of theory.

LUBRICATION

is the process, or technique employed to reduce wear of one or both surfaces in

close proximity, and moving relative to each other, by interposing a substance

called lubricant between the surfaces to carry or to help carry the load (pressure

generated) between the opposing surfaces. The interposed lubricant film can be a

solid, (e.g. graphite, MoS2) a solid/liquid dispersion, a liquid, a liquid-liquid

dispersion (a grease) or, exceptionally, a gas.

In the most common case the applied load is carried by pressure generated within

the fluid due to the frictional viscous resistance to motion of the lubricating fluid

between the surfaces.

Lubrication can also describe the phenomenon such reduction of wear occurs

without human intervention (hydroplaning on a road).

The science offriction, lubrication and wearis called tribology.

Adequate lubrication allows smooth continuous operation of equipment, with only

mild wear, and without excessive stresses or seizures at bearings. When

lubrication breaks down, metal or other components can rub destructively over

each other, causing destructive damage, heat, and failure.
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THE REGIMES OF LUBRICATION-

As the load increases on the contacting surfaces three distinct situations can be

observed with respect to the mode of lubrication, which are called regimes of

lubrication:

Fluid film lubrication is the lubrication regime in which through viscous

forces the load is fully supported by the lubricant within the space or gap

between the parts in motion relative to one another (the lubricated

conjunction) and solidsolid contact is avoided. [2]

Hydrostatic lubrication is when an external pressure is applied to

the lubricant in the bearing, to maintain the fluid lubricant film where itwould otherwise be squeezed out.

Hydrodynamic lubrication is where the motion of the contacting

surfaces, and the exact design of the bearing is used to pump lubricant

around the bearing to maintain the lubricating film. This design of bearing

may wear when started, stopped or reversed, as the lubricant film breaks

down.

Elastohydrodynamic lubrication: The opposing surfaces are separated,

but there occurs some interaction between the raised solid features

called asperities, and there is an elastic deformation on the contacting

surface enlarging the load-bearing area whereby the viscous resistance of the

lubricant becomes capable of supporting the load.

Boundary lubrication (also called boundary film lubrication): The bodies

come into closer contact at their asperities; the heat developed by the localpressures causes a condition which is called stick-slip and some asperities

break off. At the elevated temperature and pressure conditions chemically

reactive constituents of the lubricant react with the contact surface forming a
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highly resistant tenacious layer, or film on the moving solid surfaces (boundary

film) which is capable of supporting the load and major wear or breakdown is

avoided. Boundary lubrication is also defined as that regime in which the load

is carried by the surface asperities rather than by the lubricant.

WEAR

In materials science, wearis erosion or sideways displacement of material from

its "derivative" and original position on a solidsurfaceperformed by the action of

another surface.

Wear is related to interactions between surfaces and more specifically the

removal and deformation of material on a surface as a result of mechanical action

of the opposite surface. The need for relative motion between two surfaces and

initial mechanical contact between asperities is an important distinction between

mechanical wear compared to other processes with similar outcomes.

The definition of wear may include loss of dimension from plastic deformation if it

is originated at the interface between two sliding surfaces.

However, plastic deformation such as yield stress is excluded from the wear

definition if it doesn't incorporates a relative sliding motion and contact againstanother surface despite the possibility for material removal, because it then lacks

the relative sliding action of another surface.

Impact wear is in reality a short sliding motion where two solid bodies interact at

an exceptional short time interval. Previously due to the fast execution, the

contact found in impact wear was referred to as an impulse contact by the

nomenclature. Impulse can be described as a mathematical model of a

synthesized average on the energy transport between two travelling solids in

opposite converging contact.

Cavitation wear is a form of wear where the erosive medium or counter-body is a

fluid.
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Corrosion may be included in wear phenomenon, but the damage is amplified and

performed by chemical reactions rather than mechanical action.

Wear can also be defined as a process where interaction between two surfaces or

bounding faces of solids within the working environment results in dimensionalloss of one solid, with or without any actual decoupling and loss of material.

Aspects of the working environment which affect wear include loads and features

such as unidirectional sliding, reciprocating, rolling, and impact loads, speed,

temperature, but also different types of counter-bodies such as

solid, liquid orgas and type of contact ranging between single phase or

multiphase, in which the last multiphase may combine liquid with solid particles

and gas bubbles.

STAGES OF WEAR

Under normal mechanical and practical procedures, the wear-rate normally

changes through three different stages(ref.4):

Primary stage or early run-in period, where surfaces adapt to each otherand the wear-rate might vary between high and low.

Secondary stage or mid-age process, where a steady rate of ageing is in

motion. Most of the components operational life is comprised in this stage.

Tertiary stage or old-age period, where the components are subjected to

rapid failure due to a high rate of ageing.

The secondary stage is shortened with increasing severity of environmental

conditions such as higher temperatures, strain rates, stress and sliding velocities

etc.

Note that, wear rate is strongly influenced by the operating conditions.
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Specifically, normal loads and sliding speeds play a pivotal role in determining

wear rate. In addition, tribo-chemical reaction is also important in order to

understand the wear behavior. Different oxide layers are developed during the

sliding motion. The layers are originated from complex interaction among surface,

lubricants, and environmental molecules. In general, a single plot, namely wear

map. Demonstrating wear rate under different loading condition is used for

operation. This graph also represents dominating wear modes under different

loading conditions.

In explicit wear tests simulating industrial conditions between metallic surfaces,

there are no clear chronological distinction between different wear-stages due to

big overlaps and symbiotic relations between various friction

mechanisms. Surface engineering and treatments are used to minimize wear and

extend the components working life.

TYPES

The study of the processes of wear is part of the discipline of tribology. The

complex nature of wear has delayed its investigations and resulted in isolated

studies towards specific wear mechanisms or processes. Some commonly

referred to wear mechanisms (or processes) include:

1. Adhesive wear

2. Abrasive wear

3. Surface fatigue

4. Fretting wear

5. Erosive wear
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Adhesive wear

Adhesive wear can be found between surfaces during frictional contact and

generally refers to unwanted displacement and attachment of wear debris and

material compounds from one surface to another. Two separate mechanisms

operate between the surfaces.

Abrasive wear

Abrasive wear occurs when a hard rough surface slides across a softer

surface. ASTM International (formerly American Society for Testing and Materials)

defines it as the loss of material due to hard particles or hard protuberances that

are forced against and move along a solid surface.

Abrasive wear is commonly classified according to the type of contact and the

contact environment. The type of contact determines the mode of abrasive wear.

The two modes of abrasive wear are known as two-body and three-body abrasive

wear. Two-body wear occurs when the grits or hard particles remove material

from the opposite surface. The common analogy is that of material being removed

or displaced by a cutting or plowing operation. Three-body wear occurs when the

particles are not constrained, and are free to roll and slide down a surface. The

contact environment determines whether the wear is classified as open or closed.

An open contact environment occurs when the surfaces are sufficiently displaced

to be independent of one another

Deep 'groove' like surface indicates abrasive wear over cast iron (yellow arrow

indicate sliding direction)

There are a number of factors which influence abrasive wear and hence the

manner of material removal. Several different mechanisms have been proposed

to describe the manner in which the material is removed. Three commonly

identified mechanisms of abrasive wear are:

1. Plowing
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2. Cutting

3. Fragmentation

Plowing occurs when material is displaced to the side, away from the wearparticles, resulting in the formation of grooves that do not involve direct material

removal. The displaced material forms ridges adjacent to grooves, which may be

removed by subsequent passage of abrasive particles. Cutting occurs when

material is separated from the surface in the form of primary debris, or microchips,

with little or no material displaced to the sides of the grooves. This mechanism

closely resembles conventional machining. Fragmentation occurs when material

is separated from a surface by a cutting process and the indenting abrasive

causes localized fracture of the wear material. These cracks then freely propagate

locally around the wear groove, resulting in additional material removal by

spalling.

Abrasive wear can be measured as loss of mass by the Taber Abrasion Test

according to ISO 9352 or ASTM D 1044.

Surface fatigue

Surface fatigue is a process by which the surface of a material is weakened by

cyclic loading, which is one type of general material fatigue. Fatigue wear is

produced when the wear particles are detached by cyclic crack growth of

microcracks on the surface. These microcracks are either superficial cracks or

subsurface cracks.

Fretting wear

Fretting wear is the repeated cyclical rubbing between two surfaces, which is

known as fretting, over a period of time which will remove material from one or

both surfaces in contact. It occurs typically in bearings, although most bearings


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have their surfaces hardened to resist the problem. Another problem occurs when

cracks in either surface are created, known as fretting fatigue. It is the more

serious of the two phenomena because it can lead to catastrophic failure of the

bearing. An associated problem occurs when the small particles removed by wear

are oxidised in air. The oxides are usually harder than the underlying metal, so

wear accelerates as the harder particles abrade the metal surfaces further.

Fretting corrosion acts in the same way, especially when water is present.

Unprotected bearings on large structures like bridges can suffer serious

degradation in behavior, especially when salt is used during winter to deice the

highways carried by the bridges. The problem of fretting corrosion was involved in

the Silver Bridge tragedy and the Mianus River Bridge accident.

Erosive wear

Erosive wear can be described as an extremely short sliding motion and is

executed within a short time interval. Erosive wear is caused by the impact of

particles of solid or liquid against the surface of an object. The impacting particles

gradually remove material from the surface through repeated deformations and

cutting actions. It is a widely encountered mechanism in industry. A common

example is the erosive wear associated with the movement of slurries throughpiping and pumping equipment.

The rate of erosive wear is dependent upon a number of factors. The material

characteristics of the particles, such as their shape, hardness, impact velocity and

impingement angle are primary factors along with the properties of the surface

being eroded. The impingement angle is one of the most important factors and is

widely recognized in literature. For ductile materials the maximum wear rate is

found when the impingement angle is approximately 30, whilst for non ductile

materials the maximum wear rate occurs when the impingement angle is normal

to the surface.
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TYPES OF MOTION

SLIDING (MOTION)

Sliding is a type of frictional motion between two surfaces in contact. This can be

contrasted to rolling motion. Both types of motion may occur in bearings.

The relative motion or tendency toward such motion between two surfaces is resisted

by friction. Friction may damage or 'wear' the surfaces in contact. However, wear can be

reduced by lubrication. The science and technology of friction, lubrication, and wear is

known astribology

Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the

frictional force associated with the rotational movement of a somewhat disclike or other

circular object along a surface. Generally the frictional force ofrolling friction is less than

that associated with slidingkinetic friction.[1]Typical values for the coefficient of rolling

friction are less than that of sliding friction.[2]

Correspondingly sliding friction typicallyproduces greater sound and thermal bi-products. One of the most common examples of

sliding friction is the movement ofbrakingmotor vehicletires on a roadway, a process

which generates considerable heat andsound, and is typically taken into account in

assessing the magnitude of roadwaynoise pollution.

ROLLING (MOTION)
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is a type of motion that combines rotation (commonly, of an axially

symmetric object) and translation of that object with respect to a surface (either

one or the other moves), such that, if ideal conditions exist, the two are in contact

with each other without sliding.

Rolling is achieved by a rotational speed at the line or point of contact which is

equal to the translational speed. When no sliding takes place the rolling motion is

referred to as 'pure rolling'. In practice, due to small deformations at the contact

area, some sliding does occur. Nevertheless, rolling resistance is much lower

than sliding friction, and thus, rolling objects, typically require much less energyto

be moved than sliding ones. As a result, such objects will more easily move, if

they experience a force with a component along the surface, for instance gravity

on a tilted surface; wind; pushing; pulling; an engine. Unlike most axially

symmetrical objects, the rolling motion of a cone is such that while rolling on a flat

surface, its center of gravity performs a circular motion, rather than a linearone.

Rolling objects are not necessarily axially-symmetrical. Two well known non-

axially-symmetrical rollers are the Reuleaux triangle and theMeissner bodies.

Objects with corners, such as dice, roll by successive rotations about the edge or

corner which is in contact with the surface.

One of the most practical applications of rolling objects is the use ofRolling-

element bearings, such asball bearings, in rotating devices. Made of a smooth

metal substance, the rolling elements are usually encased between two rings that

can rotate independently of each other. In most mechanisms, the inner ring is

attached to a stationary shaft (or axle). Thus, while the inner ring is stationary, the

outer ring is free to move with very little friction. This is the basis for which almost

all motors (such as those found in ceiling fans, cars, drills, etc.) rely on to operate.

The amount of friction on the mechanism's parts depends on the quality of the ball

bearings and how much lubrication is in the mechanism.

Rolling objects are also frequently used as toolsfortransportation. One of the

most basic ways is by placing a (usually flat) object on a series of lined-up rollers,
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orwheels. The object on the wheels can be moved along them in a straight line,

as long as the wheels are continuously replaced in the front (see history of

bearings). This method of primitive transportation is efficient when no other

machinery is available. Today, the most practical application of objects on wheels

are cars, trains, and other human transportation vehicles.

The velocity of a particle in the rolling object is given by: , where is

the distance between the particle and the rolling object's contact point (or line),

and is the rolling object's angular velocity.

Deformation

In materials science, deformation is a change in the shape or size of an object

due to an applied force (the deformation energy in this case is transferred through

work) or a change in temperature (the deformation energy in this case is

transferred through heat). The first case can be a result oftensile (pulling)

forces, compressive (pushing) forces, shear, bending ortorsion (twisting). In the

second case, the most significant factor, which is determined by the temperature,

is the mobility of the structural defects such as grain boundaries, point vacancies,

line and screw dislocations, stacking faults and twins in both crystalline and non-

crystalline solids. The movement or displacement of such mobile defects is

thermally activated, and thus limited by the rate of atomic diffusion. Deformation is

often described as strain.[1][2]

As deformation occurs, internal inter-molecular forces arise that oppose the

applied force. If the applied force is not too large these forces may be sufficient to
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completely resist the applied force, allowing the object to assume a new

equilibrium state and to return to its original state when the load is removed. A

larger applied force may lead to a permanent deformation of the object or even to

its structural failure.

In the figure it can be seen that the compressive loading (indicated by the arrow)

has caused deformation in the cylinderso that the original shape (dashed lines)

has changed (deformed) into one with bulging sides. The sides bulge because the

material, although strong enough to not crack or otherwise fail, is not strong

enough to support the load without change, thus the material is forced out

laterally. Internal forces (in this case at right angles to the deformation) resist the

applied load.

TYPES OF DEFORMATION

Depending on the type of material, size and geometry of the object, and the forces

applied, various types of deformation may result. The image to the right shows the

engineering stress vs. strain diagram for a typical ductile material such as steel.

Different deformation modes may occur under different conditions, as can be

depicted using a deformation mechanism map.
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Typical stress vs. strain diagram with the various stages of deformation.

Elastic deformation

This type of deformation is reversible. Once the forces are no longer applied, the

object returns to its original shape. Elastomersand shape memory metals such

as Nitinol exhibit large elastic deformation ranges, as doesrubber. However

elasticity is nonlinear in these materials. Normal metals, ceramics and most

crystals show linear elasticity and a smaller elastic range.

Linear elastic deformation is governed by Hooke's law, which states:

Where is the applied stress, is a material constant called Young's

modulus, and is the resulting strain. This relationship only applies in the

elastic range and indicates that the slope of the stress vs. strain curve can be

used to find Young's modulus. Engineers often use this calculation in tensile
http://en.wikipedia.org/wiki/Elastomerhttp://en.wikipedia.org/wiki/Elastomerhttp://en.wikipedia.org/wiki/Shape_memoryhttp://en.wikipedia.org/wiki/Shape_memoryhttp://en.wikipedia.org/wiki/Shape_memoryhttp://en.wikipedia.org/wiki/Nitinolhttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Hooke's_lawhttp://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Young's_modulushttp://en.wikipedia.org/wiki/Young's_modulushttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/File:Stress_Strain_Ductile_Material.pnghttp://en.wikipedia.org/wiki/File:Stress_Strain_Ductile_Material.pnghttp://en.wikipedia.org/wiki/Elastomerhttp://en.wikipedia.org/wiki/Shape_memoryhttp://en.wikipedia.org/wiki/Nitinolhttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Hooke's_lawhttp://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Young's_modulushttp://en.wikipedia.org/wiki/Young's_modulushttp://en.wikipedia.org/wiki/Strain_(materials_science)


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tests. The elastic range ends when the material reaches its yield strength. At

this point plastic deformation begins.

Note that not all elastic materials undergo linear elastic deformation; some,

such as concrete, gray cast iron, and many polymers, respond nonlinearly. Forthese materials Hooke's law is inapplicable.

Plastic deformation

This type of deformation is irreversible. However, an object in the plastic

deformation range will first have undergone elastic deformation, which is

reversible, so the object will return part way to its original shape.Soft thermoplastics have a rather large plastic deformation range as do ductile

metals such as copper,silver, and gold. Steel does, too, but not cast iron.

Hard thermosetting plastics, rubber, crystals, and ceramics have minimal

plastic deformation ranges. One material with a large plastic deformation

range is wet chewing gum, which can be stretched dozens of times its original

length.

Under tensile stress plastic deformation is characterized by a strainhardening region and a necking region and finally, fracture (also called

rupture). During strain hardening the material becomes stronger through the

movement ofatomic dislocations. The necking phase is indicated by a

reduction in cross-sectional area of the specimen. Necking begins after the

ultimate strength is reached. During necking, the material can no longer

withstand the maximum stress and the strain in the specimen rapidly

increases. Plastic deformation ends with the fracture of the material.

Metal fatigue

Another deformation mechanism is metal fatigue, which occurs primarily

in ductile metals. It was originally thought that a material deformed only within
http://en.wikipedia.org/wiki/Linear_elasticityhttp://en.wikipedia.org/wiki/Yield_stresshttp://en.wikipedia.org/wiki/Thermoplasticshttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Chewing_gumhttp://en.wikipedia.org/wiki/Strain_hardeninghttp://en.wikipedia.org/wiki/Strain_hardeninghttp://en.wikipedia.org/wiki/Necking_(engineering)http://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Ductilehttp://en.wikipedia.org/wiki/Linear_elasticityhttp://en.wikipedia.org/wiki/Yield_stresshttp://en.wikipedia.org/wiki/Thermoplasticshttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Silverhttp://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Chewing_gumhttp://en.wikipedia.org/wiki/Strain_hardeninghttp://en.wikipedia.org/wiki/Strain_hardeninghttp://en.wikipedia.org/wiki/Necking_(engineering)http://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Metal_fatiguehttp://en.wikipedia.org/wiki/Ductile


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the elastic range returned completely to its original state once the forces were

removed. However, faults are introduced at the molecular level with each

deformation. After many deformations, cracks will begin to appear, followed

soon after by a fracture, with no apparent plastic deformation in between.

Depending on the material, shape, and how close to the elastic limit it is

deformed, failure may require thousands, millions, billions, or trillions of

deformations.

Metal fatigue has been a major cause of aircraft failure, such as the De

Havilland Comet accidents, especially before the process was well

understood. There are two ways to determine when a part is in danger of

metal fatigue; either predict when failure will occur due to the

material/force/shape/iteration combination, and replace the vulnerable

materials before this occurs, or perform inspections to detect the microscopic

cracks and perform replacement once they occur. Selection of materials not

likely to suffer from metal fatigue during the life of the product is the best

solution, but not always possible. Avoiding shapes with sharp corners limits

metal fatigue by reducing stress concentrations, but does not eliminate it.

Compressive failure

Usually, compressive stress applied to bars, columns, etc. leads to shortening.

Loading a structural element or a specimen will increase the compressive

stress until the reach ofcompressive strength. According to the properties of

the material, failure will occur as yield for materials with ductile behavior

(most metals, some soils and plastics) or as rupture for brittle behavior

(geomaterials,cast iron, glass, etc.).

In long, slender structural elements such as columns ortruss bars an

increase of compressive force Fleads to structural failure due to bucklingat

lower stress than the compressive strength.
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Fracture

Diagram of astress-strain curve, showing the relationship between stress (force applied) and

strain (deformation) of a ductile metal.

This type of deformation is also irreversible. A break occurs after the material

has reached the end of the elastic, and then plastic, deformation ranges. At

this point forces accumulate until they are sufficient to cause a fracture. All

materials will eventually fracture, if sufficient forces are applied.

Misconceptions

A popular misconception is that all materials that bend are "weak" and those

that don't are "strong." In reality, many materials that undergo large elastic and

plastic deformations, such as steel, are able to absorb stresses that would

cause brittle materials, such as glass, with minimal plastic deformation ranges,

to break.

Surface energy
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Contact anglemeasurements can be used to determine the surface energy of a material. Here, a

drop of water on glass.

SURFACE ENERGYquantifies the disruption of intermolecular bonds thatoccur when a surface is created. In the physics ofsolids, surfaces must be intrinsically

less energetically favorable than the bulk of a material (the molecules on the surface

have more energy compared with the molecules in the bulk of the material), otherwise

there would be a driving force for surfaces to be created, removing the bulk of the

material (see sublimation). The surface energy may therefore be defined as the excess

energy at the surface of a material compared to the bulk.

For a liquid, thesurface tension (force per unit length) and the surface energy density are

identical. Water has a surface energy density of 0.072 J/m2and a surface tension of 0.072

N/m; the units are equivalent.

Cutting a solid body into pieces disrupts its bonds, and therefore consumes energy. If the

cutting is done reversibly (see reversible), then conservation of energy means that the

energy consumed by the cutting process will be equal to the energy inherent in the two

new surfaces created. The unit surface energy of a material would therefore be half of its

energy ofcohesion, all other things being equal; in practice, this is true only for a surface
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freshly prepared in vacuum. Surfaces often change their form away from the simple

"cleaved bond" model just implied above. They are found to be highly dynamic regions,

which readily rearrange or react, so that energy is often reduced by such processes

as passivation oradsorption.
http://en.wikipedia.org/wiki/Passivation_(chemistry)http://en.wikipedia.org/wiki/Adsorptionhttp://en.wikipedia.org/wiki/Adsorptionhttp://en.wikipedia.org/wiki/Passivation_(chemistry)http://en.wikipedia.org/wiki/Adsorption

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