Physical,Chemical and Mechanical Properties

8/12/2019 Physical,Chemical and Mechanical Properties

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Physical and Chemical Properties

Physical properties are those that can be observed without changing the identity of

the substance. The general properties of matter such as color, density, hardness, are

examples of physical properties. Properties that describe how a substance changesinto a completely different substance are called chemical properties. Flammability

and corrosion/oxidation resistance are examples of chemical properties.

The difference between a physical and chemical property is straightforward untilthe phase of the material is considered. When a material changes from a solid to a

liquid to a vapor it seems like them become a difference substance. However, when

a material melts, solidifies, vaporizes, condenses or sublimes, only the state of thesubstance changes. Consider ice, liquid water, and water vapor, they are all simply

H2O. Phase is a physical property of matter and matter can exist in four phases

solid, liquid, gas and plasma.

Some of the more important physical and chemical properties from an engineering

material standpoint will be discussed in the following sections.

Phase Transformation Temperatures Density Specific Gravity Thermal Conductivity Linear Coefficient of Thermal Expansion Electrical Conductivity and Resistivity Magnetic Permeability Corrosion Resistance

Phase Transformation Temperatures When temperature rises and

pressure is held constant, a typical

substance changes from solid to

liquid and then to vapor.

Transitions from solid to liquid,

from liquid to vapor, from vaporto solid and visa versa are called

phase transformations or

transitions. Since some substanceshave several crystal forms,

technically there can also be solid

to another solid form phasetransformation.

Phase transitions from solid toliquid, and from liquid to vapor absorb heat. The phase transition

temperature where a solid changes to a liquid is called the melting point.The temperature at which the vapor pressure of a liquid equals 1 atm (101.3


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kPa) is called the boiling point. Some materials, such as many polymers, do

not go simply from a solid to a liquid with increasing temperature. Instead,

at some temperature below the melting point, they start to lose theircrystalline structure but the molecules remain linked in chains, which results

in a soft and pliable material. The temperature at which a solid, glassymaterial begins to soften and flow is called theglass transition temperature.

Density Mass can be thinly distributed as in a pillow, or tightly packed as in a block

of lead. The space the mass occupies is its volume, and the mass per unit of

volume is its density. Mass (m) is a fundamental measure of the amount of matter. Weight (w) is a

measure of the force exerted by a mass and this force is force is produced by

the acceleration of gravity. Therefore, on the surface of the earth, the massof an object is determined by dividing the weight of an object by 9.8

m/s2(the acceleration of gravity on the surface of the earth). Since we aretypically comparing things on the surface of the earth, the weight of anobject is commonly used rather than calculating its mass.

The density (r) of a material depends on the phase it is in and thetemperature. (The density of liquids and gases is very temperature

dependent.) Water in the liquid state has a density of 1 g/cm3= 1000g/m3at

4oC. Ice has a density of 0.917 g/cm3at 0oc, and it should be noted that thisdecrease in density for the solid phase is unusual. For almost all othersubstances, the density of the solid phase is greater than that of the liquid

phase. Water vapor (vapor saturated air) has a density of 0.051 g/cm3. Some common units used for expressing density are grams/cubic centimeter,

kilograms/cubic meter, grams/milliliter, grams/liter, pounds for cubic inch

and pounds per cubic foot; but it should be obvious that any unit of mass per

any unit of volume can be used.

Substance Density

(g/cm3)

Air 0.0013

Gasoline 0.7

Wood 0.85

Water (ice) 0.92Water (liquid) 1.0

Aluminum 2.7

Steel 7.8

Silver 10.5

Lead 11.3

Mercury 13.5

Gold 19.3

Specific Gravity


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Specific gravity is the ratio of density of a substance compared to the density of

fresh water at 4C (39 F). At this temperature the density of water is at its greatest

value and equal 1 g/mL. Since specific gravity is a ratio, so it has no units. Anobject will float in water if its density is less than the density of water and sink if

its density is greater that that of water. Similarly, an object with specific gravityless than 1 will float and those with a specific gravity greater than one will sink.Specific gravity values for a few common substances are: Au, 19.3; mercury, 13.6;

alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of 1 g/cm3,the specific gravity is the same as the density of the material measured in g/cm3.

The Discovery of Specific Gravity

The discovery of specific gravity makes for an interesting story. Sometime around

250 B.C., the Greek mathematician Archimedes was given the task of determining

whether a craftsman had defrauded King Heiro II of Syracuse. The king had

provided a metal smith with gold to make a crown. The king suspected that themetal smith had added less valuable silver to crown and kept some of the gold for

himself. The crown weighed the same as other crowns but due to its intricatedesigns it was impossible to measure the exact volume of the crown so its density

could be determined. The king challenged Archimedes to determine if the crownwas pure gold. Archimedes had no immediate answer and pondered this question

for sometime.

One day while entering a bath, he noticed that water spilled over the sides of the

pool, and realized that the amount of water that spilled out was equal in volume to

the space that his body occupied. He realized that a given mass of silver wouldoccupy more space than an equivalent mass of gold. Archimedes first weighed thecrown and weighed out an equal mass of pure gold. Then he placed the crown in a

full container of water and the pure gold in a container of water. He found that

more water spilled over the sides of the tub when the craftsmans crown wassubmerged. It turned out that the craftsman had been defrauding the King! Legend

has it that Archimedes was so excited about his discovery that he ran naked

through the streets of Sicily shouting Eureka! Eureka! (Which is Greek for I havefound it!).

Thermal Conductivity

Thermal conductivity () is the intrinsic property of a material which relates its

ability to conduct heat. Heat transfer by conduction involves transfer of energywithin a material without any motion of the material as a whole. Conduction takes

place when a temperature gradient exists in a solid (or stationary fluid) medium.

Conductive heat flow occurs in the direction of decreasing temperature becausehigher temperature equates to higher molecular energy or more molecular

movement. Energy is transferred from the more energetic to the less energetic

molecules when neighboring molecules collide.


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Thermal conductivity is defined as the quantity of heat (Q) transmitted through a

unit thickness (L) in a direction normal to a surface of unit area (A) due to a unit

temperature gradient (T)under steady state conditions and when the heat transferis dependent only on the temperature gradient. In equation form this becomes the

following:

Thermal Conductivity = heat distance / (area temperature gradient)

= QL/ (A T)

Approximate values of thermal conductivity for some common materials are

presented in the table below.

MaterialThermal Conductivity

W/m,o

K

Thermal Conductivity

(cal/sec)/(cm2

,o

C/cm)Air at 0 C 0.024 0.000057

Aluminum 205.0 0.50

Brass 109.0 -

Concrete 0.8 0.002

Copper 385.0 0.99

Glass, ordinary 0.8 0.0025

Gold 310 -

Ice 1.6 0.005

Iron - 0.163

Lead 34.7 0.083

Polyethylene HD 0.5 -

Polystyrene expanded 0.03 -

Silver 406.0 1.01

Styrofoam 0.01 -

Steel 50.2 -Water at 20 C - 0.0014

Wood 0.12-0.04 0.0001

Linear Coefficient of Thermal Expansion

When heat is added to most materials, the average amplitude of the atoms'

vibrating within the material increases. This, in turn, increases the separationbetween the atoms causing the material to expand. If the material does not go

through a phase change, the expansion can be easily related to the temperaturechange. The linear coefficient of thermal expansion ( a) describes the relative


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change in length of a material per degree temperature change. As shown in the

following equation, a is the ratio of change in length ( Dl) to the total starting

length (li) and change in temperature ( DT).

By rearranging this equation, it can be seen that if the linear coefficient of thermalexpansion is known, the change in components length can be calculated for each

degree of temperature change. This effect also works in reverse. That is to say, if

energy is removed from a material then the object's temperature will decreasecausing the object to contract.

Thermal expansion (and contraction) must be taken into account when designing

products with close tolerance fits as these tolerances will change as temperaturechanges if the materials used in the design have different coefficients of thermal

expansion. It should also be understood that thermal expansion can cause

significant stress in a component if the design does not allow for expansion andcontraction of components. The phenomena of thermal expansion can be

challenging when designing bridges, buildings, aircraft and spacecraft, but it canbe put to beneficial uses. For example, thermostats and other heat-sensitive sensors

make use of the property of linear expansion.

Linear Coefficient of Thermal Expansion for a Few Common Materials

Materiala

(m/m/oK)a (mm/m/oK)

Aluminum 23.8 x 10- 0.0238

Concrete 12.0 x 10 - 0.011

Copper 17.6 x 10 -6 0.0176

Brass 18.5 x 10 - 0.0185

Steel 12.0 x 10-

0.0115Timber 40.0 x 10 - 0.04

Quartz Glass 0.5 x 10 - 0.0005

Polymeric Materials 40-200 x 10 - 0.040-0.200

Acrylic 75.0 x 10 - 0.075

Electrical Conductivity and Resistivity

It is well known that one of the subatomic particles of an atom is the electron. The

electrons carry a negative electrostatic charge and under certain conditions canmove from atom to atom. The direction of movement between atoms is random


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unless a force causes the electrons to move in one direction. This directional

movement of electrons due to an electromotive force is what is known as

electricity.

Electrical ConductivityElectrical conductivity is a measure of how well a material accommodates themovement of an electric charge. It is the ratio of the current density to the electric

field strength. Its SI derived unit is the Siemens per meter, but conductivity values

are often reported as percent IACS. IACS is an acronym for International Annealed

Copper Standard or the material that was used to make traditional copper-wire .The conductivity of the annealed copper (5.8108 x 107S/m) is defined to be 100%IACS at 20C. All other conductivity values are related back to this conductivity of

annealed copper. Therefore, iron with a conductivity value of 1.044 x 107S/m, has

a conductivity of approximately 18% of that of annealed copper and this is

reported as 18% IACS. An interesting side note is that commercially pure copperproducts now often have IACS conductivity values greater than 100% because

processing techniques have improved since the adoption of the standard in 1913and more impurities can now be removed from the metal.

Conductivity values in Siemens/meter can be converted to % IACS by multiplyingthe conductivity value by 1.7241 x10-6. When conductivity values are reported inmicroSiemens/centimeter, the conductivity value is multiplied by 172.41 to convert

to the % IACS value.

Electrical conductivity is a very useful property since values are affected by suchthings as a substances chemical composition and the stress state of crystalline

structures. Therefore, electrical conductivity information can be used for

measuring the purity of water, sorting materials, checking for proper heat treatmentof metals, and inspecting for heat damage in some materials.

Electrical ResistivityElectrical resistivity is the reciprocal of conductivity. It is the is the opposition of a

body or substance to the flow of electrical current through it, resulting in a changeof electrical energy into heat, light, or other forms of energy. The amount of

resistance depends on the type of material. Materials with low resistivity are goodconductors of electricity and materials with high resistivity are good insulators.

The SI unit for electrical resistivity is the ohm meter. Resistivity values are morecommonly reported in micro ohm centimeters units. As mentioned above

resistivity values are simply the reciprocal of conductivity so conversion betweenthe two is straightforward. For example, a material with two micro ohm centimeter

of resistivity will have microSiemens/centimeter of conductivity. Resistivity

values in microhm centimeters units can be converted to % IACS conductivity

values with the following formula:


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172.41 / resistivity = % IACS

Temperature Coefficient of ResistivityAs noted above, electrical conductivity values (and resistivity values) are typically

reported at 20 oC. This is done because the conductivity and resistivity of materialis temperature dependant. The conductivity of most materials decreases astemperature increases. Alternately, the resistivity of most material increases with

increasing temperature. The amount of change is material dependant but has been

established for many elements and engineering materials.

The reason that resistivity increases with increasing temperature is that the number

of imperfection in the atomic lattice structure increases with temperature and this

hampers electron movement. These imperfections include dislocations, vacancies,interstitial defects and impurity atoms. Additionally, above absolute zero, even the

lattice atoms participate in the interference of directional electron movement asthey are not always found at their ideal lattice sites. Thermal energy causes theatoms to vibrate about their equilibrium positions. At any moment in time many

individual lattice atoms will be away from their perfect lattice sites and thisinterferes with electron movement.

When the temperature coefficient is known, an adjusted resistivity value can be

computed using the following formula:

R1= R2* [1 + a * (T1T2)]

Where: R1= resistivity value adjusted to T1R2= resistivity value known or measured at temperature T2a = Temperature Coefficient

T1= Temperature at which resistivity value needs to be known

T2= Temperature at which known or measured value was obtained

For example, suppose that resistivity measurements were being made on a hotpiece of aluminum. Normally when measuring resistivity or conductivity, the

instrument is calibrated using standards that are at the same temperature as the

material being measured, and then no correction for temperature will be required.However, if the calibration standard and the test material are at differenttemperatures, a correction to the measured value must be made. Presume that the

instrument was calibrated at 20oC (68oF) but the measurement was made at 25oC

(77oF) and the resistivity value obtained was 2.706 x 10-8ohm meters. Using theabove equation and the following temperature coefficient value, the resistivity

value corrected for temperature can be calculated.

R1= R2* [1 + a * (T1T2)]


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Where: R1= ?

R2= 2.706 x 10-8ohm meters (measured resistivity at 25 oC)

a = 0.0043/ oCT1= 20

oC

T2= 25

o

C

R1= 2.706 x 10-8ohm meters * [1 + 0.0043/ oC * (20 oC25 oC)]

R1= 2.648 x 10-8ohm meters

Note that the resistivity value was adjusted downward since this example involved

calculating the resistivity for a lower temperature.

Since conductivity is simply the inverse of resistivity, the temperature coefficient

is the same for conductivity and the equation requires only slight modification. Theequation becomes:

s1= s2/ [1 + a * (T1T2)]

Where: s1= conductivity value adjusted to T1s2= conductivity value known or measured at temperature T2a = Temperature Coefficient

T1= Temperature at which conductivity value needs to be knownT2= Temperature at which known or measured value was obtained

In this example lets consider the same aluminum alloy with a temperature

coefficient of 0.0043 per degree centigrade and a conductivity of 63.6% IACS at25 oC. What will the conductivity be when adjusted to 20 oC?

s1= 63.6% IACS / [1 + 0.0043 * (20oC25 oC)]

s1= 65.0% IASC

The temperature coefficient for a few metallic elements is shown below.

Material Temperature Coefficient (/ oC)

Nickel 0.0059

Iron 0.0060

Molybdenum 0.0046

Tungsten 0.0044

Aluminum 0.0043

Copper 0.0040

Silver 0.0038Platinum 0.0038


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Gold 0.0037

Zinc 0.0038

Magnetic Permeability

Magnetic permeability or simply permeability is the ease with which a material can

be magnetized. It is a constant of proportionality that exists between magnetic

induction and magnetic field intensity. This constant is equal to approximately1.257 x 10-6Henry per meter (H/m) in free space (a vacuum). In other materials itcan be much different, often substantially greater than the free-space value, which

is symbolized 0.

Materials that cause the lines of flux to move farther apart, resulting in a decrease

in magnetic flux density compared with a vacuum, are called diamagnetic.

Materials that concentrate magnetic flux by a factor of more than one but less thanor equal to ten are called paramagnetic; materials that concentrate the flux by afactor of more than ten are called ferromagnetic. The permeability factors of some

substances change with rising or falling temperature, or with the intensity of the

applied magnetic field.

In engineering applications, permeability is often expressed in relative, rather thanin absolute, terms. If o represents the permeability of free space (that is, 4p X10-

7H/m or 1.257 x 10-6H/m) and represents the permeability of the substance in

question (also specified in henrys per meter), then the relative permeability, r, isgiven by:

r= / 0

For non-ferrous metals such as copper, brass, aluminum etc., the permeability is

the same as that of "free space", i.e. the relative permeability is one. For ferrousmetals however the value of r may be several hundred. Certain ferromagnetic

materials, especially powdered or laminated iron, steel, or nickel alloys, have

rthat can range up to about 1,000,000. Diamagnetic materials have rless than

one, but no known substance has relative permeability much less than one. Inaddition, permeability can vary greatly within a metal part due to localized

stresses, heating effects, etc.

When a paramagnetic or ferromagnetic core is inserted into a coil, the inductance

is multiplied by rcompared with the inductance of the same coil with an air core.

This effect is useful in the design of transformers and eddy current probes.

Corrosion

Corrosion involves the deterioration of a material as it reacts with its environment.Corrosion is the primary means by which metals deteriorate. Corrosion literally


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consumes the material reducing load carrying capability and causing stress

concentrations. Corrosion is often a major part of maintenance cost and corrosion

prevention is vital in many designs. Corrosion is not expressed in terms of a designproperty value like other properties but rather in more qualitative terms such as a

material is immune, resistant, susceptible or verysusceptible to corrosion.

The corrosion process is usually electrochemical in nature,

having the essential features of a battery. Corrosion is a

natural process that commonly occurs because unstablematerials, such as refined metals want to return to a morestable compound. For example, some metals, such as gold

and silver, can be found in the earth in their natural,

metallic state and they have little tendency to corrode. Iron

is a moderately active metal and corrodes readily in thepresence of water. The natural state of iron is iron oxide and

the most common iron ore is Hematite with a chemicalcomposition of Fe203. Rust, the most common corrosion

product of iron, also has a chemical composition of Fe2O3.

The difficulty in terms of energy required to extract metalsfrom their ores is directly related to the ensuing tendency to

corrode and release this energy. The electromotive force

series (See table) is a ranking of metals with respect to their

inherent reactivity. The most noble metal is at the top andhas the highest positive electrochemical potential. The mostactive metal is at the bottom and has the most negative

electrochemical potential.

Note that aluminum, as indicated by its position in the series, is a relatively reactive metal; among structural

metals, only beryllium and magnesium are more reactive. Aluminum owes its excellent corrosion resistance tothe barrier oxide film that is bonded strongly to the surface and if damaged reforms immediately in most

environments. On a surface freshly abraded and exposed to air, the protective film is only 10 Angstroms thick

but highly effective at protecting the metal from corrosion.

Corrosion involve two chemical processesoxidation and reduction. Oxidation is

the process of stripping electrons from an atom and reduction occurs when anelectron is added to an atom. The oxidation process takes place at an area known as

the anode. At the anode, positively charged atoms leave the solid surface and enter

into an electrolyte as ions. The ions leave their corresponding negative charge in

the form of electrons in the metal which travel to the location of the cathodethrough a conductive path. At the cathode, the corresponding reduction reactiontakes place and consumes the free electrons. The electrical balance of the circuit is

restored at the cathode when the electrons react with neutralizing positive ions,

such as hydrogen ions, in the electrolyte. From this description, it can be seen thatthere are four essential components that are needed for a corrosion reaction to

Partial Electromotive Force Series

Standard Potential Electrode Reaction(at 25oC), V-SHE

Au + 3e- -> Au 1.498

Pd + 2e- -> Pd 0.987

Hg + 2e--> Hg 0.854

Ag + e--> Au 0.799

Cu + e--> Cu 0.521

Cu + 2e--> Cu 0.337

2H + 2e--> H2 0.000 (Ref.)

Pb + 2e--> Pb -0.126

Sn + 2e--> Sn -0.136

Ni + 2e--> Ni -0.250

Co + 2e--> Co -0.277

Cd + 2e--> Cd -0.403

Fe + 2e--> Fe -0.440

Cr + 3e--> Cr -0.744

Cr + 2e--> Cr -0.910

Zn + 2e--> Zn -0.763

Mn + 2e--> Mn -1.180

Ti + 2e

-

-> Ti -1.630Al + 3e--> Al -1.662

Be + 2e--> Be -1.850

Mg + 2e--> Mg -2.363

Li + e--> Li -3.050


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proceed. These components are an anode, a cathode, an electrolyte with oxidizing

species, and some direct electrical connection between the anode and cathode.

Although atmospheric air is the most common environmental electrolyte, naturalwaters, such as seawater rain, as well as man-made solutions, are the environments

most frequently associated withcorrosion problems.

A typical situation might involve a piece

of metal that has anodic and cathodic

regions on the same surface. If thesurface becomes wet, corrosion may take

place through ionic exchange in the

surface water layer between the anode

and cathode. Electron exchange will

take place through the bulk metal.Corrosion will proceed at the anodic site

according to a reaction such as

M M+++ 2e-

where M is a metal atom. The resulting

metal cations (M++) are available at the metal surface to become corrosion productssuch as oxides, hydroxides, etc. The liberated electrons travel through the bulk

metal (or another low resistance electrical connection) to the cathode, where they

are consumed by cathodic reactions such as

2H++ 2e- H 2

The basic principles of corrosion that were just covered, generally apply to all

corrosion situation except certain types of high temperature corrosion. However,the process of corrosion can be very straightforward but is often very complex due

to variety of variable that can contribute to the process. A few of these variable arethe composition of the material acting in the corrosion cell, the heat treatment and

stress state of the materials, the composition of the electrolyte, the distance

between the anode and the cathode, temperature, protective oxides and coating, etc.

Types of CorrosionCorrosion is commonly classified based on the appearance of the corroded

material. The classifications used vary slightly from reference to reference butthere is generally considered to be eight different forms of corrosion. There forms

are:

Uniform or generalcorrosion that is distributed more or less uniformly over a

surface.


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Localizedcorrosion that is confined to small area. Localized corrosion often

occurs due to a concentrated cell. A concentrated cell is an electrolytic cell in

which the electromotive force is caused by a concentration of some components inthe electrolyte. This difference leads to the formation of distinct anode and cathode

regions.

Pittingcorrosion that is confined to small areas and take the form ofcavities on a surface.

Crevicecorrosion occurring at locations where easy access to the bulkenvironment is prevented, such as the mating surfaces of two components.

FiliformCorrosion that occurs under some coatings in the form ofrandomly distributed threadlike filaments.

Intergranularpreferential corrosion at or along the grain boundaries of a metal.

Exfoliationa specific formof corrosion that travels alonggrain boundaries parallel to

the surface of the part causing

lifting and flaking at thesurface. The corrosion

products expand between theuncorroded layers of metal to

produce a look that resembles

pages of a book. Exfoliationcorrosion is associated with sheet, plate and extruded products and usually

initiates at unpainted or unsealed edges or holes of susceptible metals.

Galvaniccorrosion associated primarily with the electrical coupling of materials

with significantly different electrochemical potentials.

Environmental Crackingbrittle fracture of a normally ductile material thatoccurs partially due to the corrosive effect of an environment.

Corrosion fatiguefatigue cracking that is characterized byuncharacteristically short initiation time and/or growth rate due to thedamage of corrosion or buildup of corrosion products.

High temperature hydrogen attackthe loss of strength and ductility of steeldue to a high temperature reaction of absorbed hydrogen with carbides. Theresult of the reaction is decarburization and internal fissuring.

Hydrogen Embrittlementthe loss of ductility of a metal resulting fromabsorption of hydrogen.

Liquid metal crackingcracking caused by contact with a liquid metal.

Stress corrosioncracking of a metal due to the combined action ofcorrosion and a residual or applied tensile stress.


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Erosion corrosiona corrosion reaction accelerated by the relative movement of

a corrosive fluid and a metal surface.

Fretting corrosiondamage at the interface of two contacting surfaces under load

but capable of some relative motion. The damage is accelerated by movement atthe interface that mechanically abraded the surface and exposes fresh material tocorrosive attack.

Dealloyingthe selective corrosion of one or more components of a solid solutionalloy.

Dezincificationcorrosion resulting in the selective removal of zinc fromcopper-zinc alloys.

Mechanical Properties The mechanical properties of a material are those properties that involve a

reaction to an applied load. The mechanical properties of metals determinethe range of usefulness of a material and establish the service life that can be

expected. Mechanical properties are also used to help classify and identify

material. The most common properties considered are strength, ductility,hardness, impact resistance, and fracture toughness.

Most structural materials are anisotropic, which means that their materialproperties vary with orientation. The variation in properties can be due todirectionality in the microstructure (texture) from forming or cold working

operation, the controlled alignment of fiber reinforcement and a variety of

other causes. Mechanical properties are generally specific to product formsuch as sheet, plate, extrusion, casting, forging, and etc. Additionally, it is

common to see mechanical property listed by the directional grain structureof the material. In products such as sheet and plate, the rolling direction iscalled the longitudinal direction, the width of the product is called the

transverse direction, and the thickness is called the short transversedirection. The grain orientations in standard wrought forms of metallic

products are shown the image.


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The mechanical properties of a material are not constants and often changeas a function of temperature, rate of loading, and other conditions. For

example, temperatures below room temperature generally cause an increase

in strength properties of metallic alloys; while ductility, fracture toughness,and elongation usually decrease. Temperatures above room temperature

usually cause a decrease in the strength properties of metallic alloys.

Ductility may increase or decrease with increasing temperature dependingon the same variables

It should also be noted that there is often significant variability in the valuesobtained when measuring mechanical properties. Seemingly identical testspecimen from the same lot of material will often produce considerable

different results. Therefore, multiple tests are commonly conducted todetermine mechanical properties and values reported can be an average

value or calculated statistical minimum value. Also, a range of values aresometimes reported in order to show variability.


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Loading The application of a force to an object is known as loading. Materials can be

subjected to many different loading scenarios and a materials performance

is dependant on the loading conditions. There are five fundamental loading

conditions; tension, compression, bending, shear, and torsion. Tension is thetype of loading in which the two sections of material on either side of a

plane tend to be pulled apart or elongated. Compression is the reverse oftensile loading and involves pressing the material together. Loading by

bending involves applying a load in a manner that causes a material to curve

and results in compressing the material on one side and stretching it on theother. Shear involves applying a load parallel to a plane which caused thematerial on one side of the plane to want to slide across the material on the

other side of the plane. Torsion is the application of a force that causestwisting in a material.

If a material is subjected to a constant force, it is called static loading. If the

loading of the material is not constant but instead fluctuates, it is called

dynamic or cyclic loading. The way a material is loaded greatly affects itsmechanical properties and largely determines how, or if, a component will

fail; and whether it will show warning signs before failure actually occurs.


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Stress and Strain

Stress

The term stress (s) is used to express the loading in terms of force applied to acertain cross-sectional area of an object. From the perspective of loading, stress isthe applied force or system of forces that tends to deform a body. From the

perspective of what is happening within a material, stress is the internal

distribution of forces within a body that balance and react to the loads applied to it.

The stress distribution may or may not be uniform, depending on the nature of theloading condition. For example, a bar loaded in pure tension will essentially have auniform tensile stress distribution. However, a bar loaded in bending will have a

stress distribution that changes with distance perpendicular to the normal axis.

Simplifying assumptions are often used to represent stress as a vector quantity for

many engineering calculations and for material property determination. Theword " vector" typically refers to a quantity that has a "magnitude" and a

"direction". For example, the stress in an axially loaded bar is simply equal to theapplied force divided by the bar's cross-sectional area.

Some common measurements of stress are:Psi = lbs/in2 (pounds per square inch)ksi or kpsi = kilopounds/in2 (one thousand or 103pounds per square inch)

Pa = N/m 2 (Pascals or Newtons per square meter)kPa = Kilopascals (one thousand or 103Newtons per square meter)GPa = Gigapascals (one million or 106Newtons per square meter)

*Any metric prefix can be added in front of psi or Pa to indicate the multiplication

factor

It must be noted that the stresses

in most 2-D or 3-D solids are

actually more complex and needbe defined more methodically.

The internal force acting on a

small area of a plane can beresolved into three components:

one normal to the plane and twoparallel to the plane. The normal


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force component divided by the area gives the normal stress (s), and parallel force

components divided by the area give the shear stress (t). These stresses are average

stresses as the area is finite, but when the area is allowed to approach zero, thestresses become stresses at a point. Since stresses are defined in relation to the

plane that passes through the point under consideration, and the number of suchplanes is infinite, there appear an infinite set of stresses at a point. Fortunately, itcan be proven that the stresses on any plane can be computed from the stresses on

three orthogonal planes passing through the point. As each plane has three stresses,the stress tensor has nine stress components, which completely describe the state of

stress at a point.

Strain

Strain is the response of a system to an applied stress. When a material is loaded

with a force, it produces a stress, which then causes a material to deform.

Engineering strain is defined as the amount of deformation in the direction of theapplied force divided by the initial length of the material. This results in a unitless

number, although it is often left in the unsimplified form, such as inches per inchor meters per meter. For example, the strain in a bar that is being stretched in

tension is the amount of elongation or change in length divided by its originallength. As in the case of stress, the strain distribution may or may not be uniform

in a complex structural element, depending on the nature of the loading condition.

If the stress is small, the material may only strain a small amount and the materialwill return to its original size after the stress is released. This is called elastic

deformation, because like elastic it returns to its unstressed state. Elasticdeformation only occurs in a material when stresses are lower than a critical stress

called the yield strength. If a material is loaded beyond it elastic limit, the material

will remain in a deformed condition after the load is removed. This is called plasticdeformation.

Engineering and True Stress and Strain

The discussion above focused on engineeringstress and strain, which use thefixed, undeformed cross-sectional area in the calculations. Truestress and strain

measures account for changes in cross-sectional area by usingthe instantaneousvalues for the area. The engineering stress-strain curve does not


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give a true indication of the deformation characteristics of a metal because it is

based entirely on the original dimensions of the specimen, and these dimensions

change continuously during the testing used to generate the data.

Engineering stress and strain data is commonly used because it is easier to generatethe data and the tensile properties are adequate for engineering calculations. Whenconsidering the stress-strain curves in the next section, however, it should be

understood that metals and other materials continues to strain-harden until they

fracture and the stress required to produce further deformation also increase.

Stress Concentration

When an axial load is applied to a piece of

material with a uniform cross-section, thenorm al stress will be uniformly

distributed over the cross-section.However, if a hole is drilled in thematerial, the stress distribution will no

longer be uniform. Since the material thathas been removed from the hole is no

longer available to carry any load, the loadmust be redistributed over the remainingmaterial. It is not redistributed evenly over

the entire remaining cross-sectional area

but instead will be redistributed in an

uneven pattern that is highest at the edgesof the hole as shown in the image. This

phenomenon is known as stress

concentration.

Tensile Properties

Tensile properties indicate how the material will react to forces being applied intension. A tensile test is a fundamental mechanical test where a carefully prepared

specimen is loaded in a very controlled manner while measuring the applied loadand the elongation of the specimen over some distance. Tensile tests are used todetermine the modulus of elasticity, elastic limit, elongation, proportional limit,

reduction in area, tensile strength, yield point, yield strength and other tensile

properties.

The main product of a tensile test is a load versus elongation curve which is thenconverted into a stress versus strain curve. Since both the engineering stress and

the engineering strain are obtained by dividing the load and elongation by constantvalues (specimen geometry information), the load-elongation curve will have the

same shape as the engineering stress-strain curve. The stress-strain curve relatesthe applied stress to the resulting strain and each material has its own unique


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stress-strain curve. A typical engineering stress-strain curve is shown below. If the

true stress, based on the actual cross-sectional area of the specimen, is used, it is

found that the stress-strain curve increases continuously up to fracture.

Linear-Elastic Region and Elastic Constants

As can be seen in the figure, the stress and strain initially increase with a linear

relationship. This is the linear-elastic portion of the curve and it indicates that no

plastic deformation has occurred. In this region of the curve, when the stress isreduced, the material will return to its original shape. In this linear region, the line

obeys the relationship defined as Hooke's Lawwhere the ratio of stress to strain is

a constant.

The slope of the line in this region where stress is proportional to strain and is

called themodulus of elasticityor Young's modulus. The modulus of elasticity(E) defines the properties of a material as it undergoes stress, deforms, and thenreturns to its original shape after the stress is removed. It is a measure of thestiffness of a given material. To compute the modulus of elastic , simply divide the

stress by the strain in the material. Since strain is unitless, the modulus will havethe same units as the stress, such as kpi or MPa. The modulus of elasticity applies

specifically to the situation of a component being stretched with a tensile force.This modulus is of interest when it is necessary to compute how much a rod or

wire stretches under a tensile load.


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There are several different kinds of moduli depending on the way the material is

being stretched, bent, or otherwise distorted. When a component is subjected to

pure shear, for instance, a cylindrical bar under torsion, the shear modulusdescribes the linear-elastic stress-strain relationship.

Axial strain is always accompanied by lateral strains of opposite sign in the twodirections mutually perpendicular to the axial strain. Strains that result from an

increase in length are designated as positive (+) and those that result in a decrease

in length are designated as negative (-). Poisson' s ratiois defined as the negative

of the ratio of the lateral strain to the axial strain for a uniaxial stress state.

Poisson's ratio is sometimes also defined as the ratio of the absolute values of

lateral and axial strain. This ratio, like strain, is unitless since both strains areunitless. For stresses within the elastic range, this ratio is approximately constant.

For a perfectly isotropic elastic material, Poisson's Ratio is 0.25, but for most

materials the value lies in the range of 0.28 to 0.33. Generally for steels, Poissonsratio will have a value of approximately 0.3. This means that if there is one inch

per inch of deformation in the direction that stress is applied, there will be 0.3

inches per inch of deformation perpendicular to the direction that force is applied.

Only two of the elastic constants are independent so if two constants are known,

the third can be calculated using the following formula:

E = 2 (1 + n) G.

Where: E = modulus of elasticity (Young's modulus)

n = Poisson's ratio

G = modulus of rigidity (shear modulus).

A couple of additional elastic constants that may be encountered include the bulkmodulus (K), and Lame's constants (m and l). The bulk modulus is used describethe situation where a piece of material is subjected to a pressure increase on all

sides. The relationship between the change in pressure and the resulting strainproduced is the bulk modulus. Lame's constants are derived from modulus ofelasticity and Poisson's ratio.

Yield Point

In ductile materials, at some point, the stress-strain curve deviates from the

straight-line relationship and Law no longer applies as the strain increases fasterthan the stress. From this point on in the tensile test, some permanent deformation


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occurs in the specimen and the material is said to react plastically to any further

increase in load or stress. The material will not return to its original, unstressed

condition when the load is removed. In brittle materials, little or no plasticdeformation occurs and the material fractures near the end of the linear-elastic

portion of the curve.

With most materials there is a gradual transition from elastic to plastic behavior,

and the exact point at which plastic deformation begins to occur is hard to

determine. Therefore, various criteria for the initiation of yielding are used

depending on the sensitivity of the strain measurements and the intended use of thedata. (See Table) For most engineering design and specification applications, theyield strength is used. The yield strength is defined as the stress required to

produce a small, amount of plastic deformation. The offset yield strength is the

stress corresponding to the intersection of the stress-strain curve and a line parallel

to the elastic part of the curve offset by a specified strain (in the US the offset istypically 0.2% for metals and 2% for plastics).

To determine the yield strength using this offset, the pointis found on the strain axis (x-axis) of 0.002, and then a

line parallel to the stress-strain line is drawn. This linewill intersect the stress-strain line slightly after it beginsto curve, and that intersection is defined as the yield

strength with a 0.2% offset. A good way of looking at

offset yield strength is that after a specimen has been loaded to its 0.2 percent

offset yield strength and then unloaded it will be 0.2 percent longer than before thetest. Even though the yield strength is meant to represent the exact point at whichthe material becomes permanently deformed, 0.2% elongation is considered to be a

tolerable amount of sacrifice for the ease it creates in defining the yield strength.

Some materials such as gray cast iron or soft copper exhibit essentially no linear-

elastic behavior. For these materials the usual practice is to define the yield

strength as the stress required to produce some total amount of strain.

True elastic limi tis a very low value and is related to the motion of a fewhundred dislocations. Micro strain measurements are required to detectstrain on order of 2 x 10 -6 in/in.

Proportional limi tis the highest stress at which stress is directlyproportional to strain. It is obtained by observing the deviation from thestraight-line portion of the stress-strain curve.

Elastic limi tis the greatest stress the material can withstand without anymeasurable permanent strain remaining on the complete release of load. It is

determined using a tedious incremental loading-unloading test procedure.With the sensitivity of strain measurements usually employed in engineering

studies (10 -4in/in), the elastic limit is greater than the proportional limit.With increasing sensitivity of strain measurement, the value of the elastic

In Great Britain, the yield

strength is often referred

to as the proof stress.

The offset value is either

0.1% or 0.5%


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limit decreases until it eventually equals the true elastic limit determined

from micro strain measurements.

Yield strengthis the stress required to produce a small-specified amount ofplastic deformation. The yield strength obtained by an offset method is

commonly used for engineering purposes because it avoids the practicaldifficulties of measuring the elastic limit or proportional limit.

Ultimate Tensile Strength

The ultimate tensile strength (UTS) or, more simply, the tensile strength, is the

maximum engineering stress level reached in a tension test. The strength of amaterial is its ability to withstand external forces without breaking. In brittlematerials, the UTS will at the end of the linear-elastic portion of the stress-strain

curve or close to the elastic limit. In ductile materials, the UTS will be well outside

of the elastic portion into the plastic portion of the stress-strain curve.

On the stress-strain curve above, the UTS is the highest point where the line ismomentarily flat. Since the UTS is based on the engineering stress, it is often not

the same as the breaking strength. In ductile materials strain hardening occurs andthe stress will continue to increase until fracture occurs, but the engineering stress-

strain curve may show a decline in the stress level before fracture occurs. This isthe result of engineering stress being based on the original cross-section area andnot accounting for the necking that commonly occurs in the test specimen. The

UTS may not be completely representative of the highest level of stress that a

material can support, but the value is not typically used in the design of

components anyway. For ductile metals the current design practice is to use theyield strength for sizing static components. However, since the UTS is easy todetermine and quite reproducible, it is useful for the purposes of specifying a

material and for quality control purposes. On the other hand, for brittle materials

the design of a component may be based on the tensile strength of the material.

Measures of Ductility (Elongation and Reduction of Area)The ductility of a material is a measure of the extent to which a material will

deform before fracture. The amount of ductility is an important factor when

considering forming operations such as rolling and extrusion. It also provides anindication of how visible overload damage to a component might become before

the component fractures. Ductility is also used a quality control measure to assessthe level of impurities and proper

processing of a material.

The conventional measures of ductility

are the engineering strain at fracture

(usually called the elongation ) and thereduction of area at fracture. Both of

these properties are obtained by fittingthe specimen back together after


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fracture and measuring the change in length and cross-sectional area. Elongation is

the change in axial length divided by the original length of the specimen or portion

of the specimen. It is expressed as a percentage. Because an appreciable fraction ofthe plastic deformation will be concentrated in the necked region of the tensile

specimen, the value of elongation will depend on the gage length over which themeasurement is taken. The smaller the gage length the greater the large localizedstrain in the necked region will factor into the calculation. Therefore, when

reporting values of elongation , the gage length should be given.

One way to avoid the complication from necking is to base the elongationmeasurement on the uniform strain out to the point at which necking begins. Thisworks well at times but some engineering stress-strain curve are often quite flat in

the vicinity of maximum loading and it is difficult to precisely establish the strain

when necking starts to occur.

Reduction of area is the change in cross-sectional area divided by the originalcross-sectional area. This change is measured in the necked down region of the

specimen. Like elongation, it is usually expressed as a percentage.

As previously discussed, tension is just one of the way that a material can beloaded. Other ways of loading a material include compression, bending, shear and

torsion, and there are a number of standard tests that have been established tocharacterize how a material performs under these other loading conditions. A very

cursory introduction to some of these other material properties will be provided on

the next page.

Compressive, Bearing, & Shear Properties

Compressive Properties

In theory, the compression test is simply the opposite of the

tension test with respect to the direction of loading. Incompression testing the sample is squeezed while the load and

the displacement are recorded. Compression tests result inmechanical properties that include the compressive yieldstress, compressive ultimate stress, and compressive modulus

of elasticity.

Compressive yield stress is measured in a manner identical tothat done for tensile yield strength. When testing metals, it is

defined as the stress corresponding to 0.002 in./in. plastic

strain. For plastics, the compressive yield stress is measured atthe point of permanent yield on the stress-strain curve. Moduliare generally greater in compression for most of the commonly used structural

materials.


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Ultimate compressive strength is the stress required to rupture a specimen. This

value is much harder to determine for a compression test than it is for a tensile test

since many material do not exhibit rapid fracture in compression. Materials suchas most plastics that do not rupture can have their results reported as the

compressive strength at a specific deformation such as 1%, 5%, or 10% of thesample's original height.

For some materials, such as concrete, the compressive strength is the most

important material property that engineers use when designing and building a

structure. Compressive strength is also commonly used to determine whether aconcrete mixture meets the requirements of the job specifications.

Bearing Properties

Bearing properties are used when designing mechanically fastened joints. The

purpose of a bearing test is to determine the the deformation of a hole as a functionof the applied bearing stress. The test specimen is basically a piece of sheet or

plate with a carefully prepared hole some standard distance from the edge. Edge-

to-hole diameter ratios of 1.5 and 2.0 are common. A hardened pin is insertedthrough the hole and an axial load applied to the specimen and the pin. The bearing

stress is computed by dividing the load applied to the pin, which bears against theedge of the hole, by the bearing area (the product of the pin diameter and the sheetor plate thickness). Bearing yield and ultimate stresses are obtained from bearing

tests. BYS is computed from a bearing stress deformation curve by drawing a line

parallel to the initial slope at an offset of 0.02 times the pin diameter. BUS is the

maximum stress withstood by a bearing specimen.

Shear Properties

A shearing stress acts parallel to the stress plane, whereas a tensile or compressivestress acts normal to the stress plane. Shear properties are primarily used in thedesign of mechanically fastened components, webs, and torsion members, and

other components subject to parallel, opposing loads. Shear properties are

dependant on the type of shear test and their is a variety of different standard shear

tests that can be performed including the single-shear test, double-shear test,

blanking-shear test, torsion-shear test and others. The shear modulus of elasticity isconsidered a basic shear property. Other properties, such as the proportional limit

stress and shear ultimate stress, cannot be treated as basic shear properties becauseof form factor effects.

Hardness

Hardness is the resistance of a material to localized deformation. The term can

apply to deformation from indentation, scratching, cutting or bending. In metals,ceramics and most polymers, the deformation considered is plastic deformation of

the surface. For elastomers and some polymers, hardness is defined at theresistance to elastic deformation of the surface. The lack of a fundamental


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definition indicates that hardness is not be a basic property of a material, but rather

a composite one with contributions from the yield strength, work hardening, true

tensile strength, modulus, and others factors. Hardness measurements are widelyused for the quality control of materials because they are quick and considered to

be nondestructive tests when the marks or indentations produced by the test are inlow stress areas.

There are a large variety of methods used for determining the hardness of a

substance. A few of the more common methods are introduced below.

Mohs Hardness TestOne of the oldest ways of measuring hardness was devised by the German

mineralogist Friedrich Mohs in 1812. The Mohs hardness test involves observingwhether a materials surface is scratched by a substance of known or defined

hardness. To give numerical values to this physical property, minerals are rankedalong the Mohs scale, which is composed of 10 minerals that have been givenarbitrary hardness values. Mohs hardness test, while greatly facilitating the

identification of minerals in the field, is not suitable for accurately gauging thehardness of industrial materials such as steel or ceramics. For engineering

materials, a variety of instruments have been developed over the years to provide aprecise measure of hardness. Many apply a load and measure the depth or size ofthe resulting indentation. Hardness can be measured on the macro-, micro- or

nano- scale.

Brinell Hardness TestThe oldest of the hardness test methods in common use on engineering materials

today is the Brinell hardness test. Dr. J. A. Brinell invented the Brinell test in

Sweden in 1900. The Brinell test uses a desktop machine to applying a specifiedload to a hardened sphere of a specified diameter. The Brinell hardness number, orsimply the Brinell number, is obtained by dividing the load used, in kilograms, by

the measured surface area of the indentation, in square millimeters, left on the test

surface. The Brinell test is frequently used to determine the hardness metal

forgings and castings that have a large grain structures. The Brinell test provides a

measurement over a fairly large area that is less affected by the course grainstructure of these materials than are Rockwell or Vickers tests.

A wide range of materials can be tested using a Brinell test simply by varying the

test load and indenter ball size. In the USA, Brinell testing is typically done on ironand steel castings using a 3000Kg test force and a 10mm diameter ball. A 1500kilogram load is usually used for aluminum castings. Copper, brass and thin stock

are frequently tested using a 500Kg test force and a 10 or 5mm ball. In Europe

Brinell testing is done using a much wider range of forces and ball sizes and it iscommon to perform Brinell tests on small parts using a 1mm carbide ball and a test

force as low as 1kg. These low load tests are commonly referred to as baby Brinelltests. The test conditions should be reported along with the Brinell hardness


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number. A value reported as "60 HB 10/1500/30" means that a Brinell Hardness of

60 was obtained using a 10mm diameter ball with a 1500 kilogram load applied for

30 seconds.

Rockwell Hardness TestThe Rockwell Hardness test also uses a machine to apply a specific load and thenmeasure the depth of the resulting impression. The indenter may either be a steel

ball of some specified diameter or a spherical diamond-tipped cone of 120 angle

and 0.2 mm tip radius, called a brale. A minor load of 10 kg is first applied, which

causes a small initial penetration to seat the indenter and remove the effects of anysurface irregularities. Then, the dial is set to zero and the major load is applied.Upon removal of the major load, the depth reading is taken while the minor load is

still on. The hardness number may then be read directly from the scale. The

indenter and the test load used determine the hardness scale that is used (A, B, C,

etc).

For soft materials such as copper alloys, soft steel, and aluminum alloys a 1/16"

diameter steel ball is used with a 100-kilogram load and the hardness is read on the"B" scale. In testing harder materials, hard cast iron and many steel alloys, a 120

degrees diamond cone is used with up to a 150 kilogram load and the hardness isread on the "C" scale. There are several Rockwell scales other than the "B" & "C"scales, (which are called the common scales). A properly reported Rockwell value

will have the hardness number followed by "HR" (Hardness Rockwell) and the

scale letter. For example, 50 HRB indicates that the material has a hardness

reading of 50 on the B scale.

A -Cemented carbides, thin steel and shallow case hardened steel

B -Copper alloys, soft steels, aluminum alloys, malleable iron, etc.C -Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened

steel and other materials harder than B 100

D -Thin steel and medium case hardened steel and pearlitic malleable iron

E -Cast iron, aluminum and magnesium alloys, bearing metals

F -Annealed copper alloys, thin soft sheet metals

G -Phosphor bronze, beryllium copper, malleable ironsH -Aluminum, zinc, lead

K, L, M, P, R, S, V -Bearing metals and other very soft or thin materials,including plastics.

Rockwell Superficial Hardness TestThe Rockwell Superficial Hardness Tester is used to test thin materials, lightly

carburized steel surfaces, or parts that might bend or crush under the conditions of

the regular test. This tester uses the same indenters as the standard Rockwell testerbut the loads are reduced. A minor load of 3 kilograms is used and the major load

is either 15 or 45 kilograms depending on the indenter used. Using the 1/16"diameter, steel ball indenter, a "T" is added (meaning thin sheet testing) to the


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superficial hardness designation. An example of a superficial Rockwell hardness is

23 HR15T, which indicates the superficial hardness as 23, with a load of 15

kilograms using the steel ball.

Vickers and Knoop Microhardness TestsThe Vickers and Knoop Hardness Tests are a modification of the Brinell test andare used to measure the hardness of thin film coatings or the surface hardness of

case-hardened parts. With these tests, a small diamond pyramid is pressed into the

sample under loads that are much less than those used in the Brinell test. The

difference between the Vickers and the Knoop Tests is simply the shape of thediamond pyramid indenter. The Vickers test uses a square pyramidal indenterwhich is prone to crack brittle materials. Consequently, the Knoop test using a

rhombic-based (diagonal ratio 7.114:1) pyramidal indenter was developed which

produces longer but shallower indentations. For the same load, Knoop indentations

are about 2.8 times longer than Vickers indentations.

An applied load ranging from 10g to 1,000g is used. This low amount of load

creates a small indent that must be measured under a microscope. Themeasurements for hard coatings like TiN must be taken at very high magnification

(i.e. 1000X), because the indents are so small. The surface usually needs to bepolished. The diagonals of the impression are measured, and these values are usedto obtain a hardness number (VHN), usually from a lookup table or chart. The

Vickers test can be used to characterize very hard materials but the hardness is

measured over a very small region.

The values are expressed like 2500 HK25 (or HV25) meaning 2500 Hardness

Knoop at 25 gram force load. The Knoop and Vickers hardness values differ

slightly, but for hard coatings, the values are close enough to be within themeasurement error and can be used interchangeably.

Scleroscope and Rebound Hardness TestsThe Scleroscope test is a very old test that involves dropping a diamond tipped

hammer, which falls inside a glass tube under the force of its own weight from afixed height, onto the test specimen. The height of the rebound travel of the

hammer is measured on a graduated scale. The scale of the rebound is arbitrarilychosen and consists on Shore units, divided into 100 parts, which represent theaverage rebound from pure hardened high-carbon steel. The scale is continued

higher than 100 to include metals having greater hardness. The Shore Scleroscopemeasures hardness in terms of the elasticity of the material and the hardnessnumber depends on the height to which the hammer rebounds, the harder the

material, the higher the rebound.

The Rebound Hardness Test Method is a recent advancement that builds on the

Scleroscope. There are a variety of electronic instruments on the market thatmeasure the loss of energy of the impact body. These instruments typically use a


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spring to accelerate a spherical, tungsten carbide tipped mass towards the surface

of the test object. When the mass contacts the surface it has a specific kinetic

energy and the impact produces an indentation (plastic deformation) on the surfacewhich takes some of this energy from the impact body. The impact body will lose

more energy and it rebound velocity will be less when a larger indentation isproduced on softer material. The velocities of the impact body before and afterimpact are measured and the loss of velocity is related to Brinell, Rockwell, or

other common hardness value.

Durometer Hardness TestA Durometer is an instrument that is commonly used for measuring the indentationhardness of rubbers/elastomers and soft plastics such as polyolefin, fluoropolymer,

and vinyl. A Durometer simply uses a calibrated spring to apply a specific pressure

to an indenter foot. The indenter foot can be either cone or sphere shaped. An

indicating device measures the depth of indentation. Durometers are available in avariety of models and the most popular testers are the Model A used for measuring

softer materials and the Model D for harder materials.

Barcol Hardness TestThe Barcol hardness test obtains a hardness value by measuring the penetration ofa sharp steel point under a spring load. The specimen is placed under the indenterof the Barcol hardness tester and a uniform pressure is applied until the dial

indication reaches a maximum. The Barcol hardness test method is used to

determine the hardness of both reinforced and non-reinforced rigid plastics and to

determine the degree of cure of resins and plastics.

Creep and Stress Rupture

Properties

Creep Properties

Creep is a time-dependent deformation

of a material while under an applied

load that is below its yield strength. It

is most often occurs at elevatedtemperature, but some materials creep

at room temperature. Creep terminates

in rupture if steps are not taken tobring to a halt.

Creep data for general design use are

usually obtained under conditions of constant uniaxial loading and constanttemperature. Results of tests are usually plotted as strain versus time up to rupture.

As indicated in the image, creep often takes place in three stages. In the initial

stage, strain occurs at a relatively rapid rate but the rate gradually decreases until itbecomes approximately constant during the second stage. This constant creep rate


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is called the minimum creep rate or steady-state creep rate since it is the slowest

creep rate during the test. In the third stage, the strain rate increases until failure

occurs.

Creep in service is usually affected by changing conditions of loading andtemperature and the number of possible stress-temperature-time combinations isinfinite. While most materials are subject to creep, the creep mechanisms is often

different between metals, plastics, rubber, concrete.

Stress Rupture Properties

Stress rupture testing is similar to creep testing except that the stresses are higher

than those used in a creep testing. Stress rupture tests are used to determine the

time necessary to produce failure so stress rupture testing is always done untilfailure. Data is plotted log-log as in the chart above. A straight line or best fit

curve is usually obtained at each temperature of interest. This information canthen be used to extrapolate time to failure for longer times. A typical set of stressrupture curves is shown below.

Toughness

The ability of a metal to deform plastically and to absorb energy in the process

before fracture is termed toughness. The emphasis of this definition should beplaced on the ability to absorb energy before fracture. Recall that ductility is a

measure of how much something deforms plastically before fracture, but just

because a material is ductile does not make it tough. The key to toughness is agood combination of strength and ductility. A material with high strength and high

ductility will have more toughness than a material with low strength and high

ductility. Therefore, one way to measure toughness is by calculating the area underthe stress strain curve from a tensile test. This value is simply called material

toughness and it has units of energy per volume. Material toughness equates to aslow absorption of energy by the material.


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There are several variables that have a profound influence on the toughness of a

material. These variables are:

Strain rate (rate of loading) Temperature Notch effect

A metal may possess satisfactory toughness under static loads but may fail under

dynamic loads or impact. As a rule ductility and, therefore, toughness decrease asthe rate of loading increases. Temperature is the second variable to have a major

influence on its toughness. As temperature is lowered, the ductility and toughness

also decrease. The third variable is termed notch effect, has to due with thedistribution of stress. A material might display good toughness when the applied

stress is uniaxial; but when a multiaxial stress state is produced due to the presence

of a notch, the material might not withstand the simultaneous elastic and plasticdeformation in the various directions.

There are several standard types of toughness test that generate data for specific

loading conditions and/or component design approaches. Three of the toughnessproperties that will be discussed in more detail are 1) impact toughness, 2) notch

toughness and 3) fracture toughness.


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Impact Toughness

The impact toughness (AKA Impact strength) of a material can be determined with

a Charpy or Izod test. These tests are named after their inventors and weredeveloped in the early 1900s before fracture mechanics theory was available.Impact properties are not directly used in fracture mechanics calculations, but the

economical impact tests continue to be used as a quality control method to assess

notch sensitivity and for comparing the relative toughness of engineering materials.

The two tests use different specimens

and methods of holding the specimens,

but both tests make use of a pendulum-testing machine. For both tests, the

specimen is broken by a single overload

event due to the impact of the pendulum.A stop pointer is used to record how far

the pendulum swings back up afterfracturing the specimen. The impact

toughness of a metal is determined bymeasuring the energy absorbed in thefracture of the specimen. This is simply

obtained by noting the height at which

the pendulum is released and the heightto which the pendulum swings after it has struck the specimen . The height of the

pendulum times the weight of the pendulum produces the potential energy and thedifference in potential energy of the pendulum at the start and the end of the test is

equal to the absorbed energy.

Since toughness is greatly affected by

temperature, a Charpy or Izod test is oftenrepeated numerous times with each specimen

tested at a different temperature. This producesa graph of impact toughness for the material as a

function of temperature. An impact toughnessversus temperature graph for a steel is shown inthe image. It can be seen that at lowtemperatures the material is more brittle and

impact toughness is low. At high temperaturesthe material is more ductile and impacttoughness is higher. The transition temperature

is the boundary between brittle and ductile

behavior and this temperature is often an extremely important consideration in the

selection of a material.


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Notch-Toughness

Notch toughness is the ability that a material possesses to absorb energy in the

presence of a flaw. As mentioned previously, in the presence of a flaw, such as anotch or crack, a material will likely exhibit a lower level of toughness. When aflaw is present in a material, loading induces a triaxial tension stress state adjacent

to the flaw. The material develops plastic strains as the yield stress is exceeded in

the region near the crack tip. However, the amount of plastic deformation is

restricted by the surrounding material, which remains elastic. When a material isprevented from deforming plastically, it fails in a brittle manner.

Notch-toughness is measured with a variety of specimens such as the Charpy V-notch impact specimen or the dynamic tear test specimen. As with regular impact

testing the tests are often repeated numerous times with specimens tested at a

different temperature. With these specimens and by varying the loading speed andthe temperature, it is possible to generate curves such as those shown in the graph.

Typically only static and impact testing is conducted but it should be recognizedthat many components in service see intermediate loading rates in the range of the

dashed red line.

Fracture Toughness

Fracture toughness is an indication of the amount of stress required to propagate a

preexisting flaw. It is a very important material property since the occurrence of

flaws is not completely avoidable in the processing, fabrication, or service of amaterial/component. Flaws may appear as cracks, voids, metallurgical inclusions,weld defects, design discontinuities, or some combination thereof. Since engineers

can never be totally sure that a material is flaw free, it is common practice toassume that a flaw of some chosen size will be present in some number of


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components and use the linear elastic fracture mechanics (LEFM) approach to

design critical components. This approach uses the flaw size and features,

component geometry, loading conditions and the material property called fracturetoughness to evaluate the ability of a component containing a flaw to resist

fracture.

A parameter called the stress-intensity factor (K) is used to

determine the fracture toughness of most materials. A

Roman numeral subscript indicates the mode of fracture and

the three modes of fracture are illustrated in the image to theright. Mode I fracture is the condition in which the crack

plane is normal to the direction of largest tensile loading.

This is the most commonly encountered mode and,

therefore, for the remainder of the material we will consider

KI

The stress intensity factor is a function of loading, crack

size, and structural geometry. The stress intensity factor maybe represented by the following equation:

Where:KIis the fracture toughness in

s is the applied stress in MPa or psi a is the crack length in meters or inches

Bis a crack length and component geometry factor that is different for

each specimen and is dimensionless.

Role of Material Thickness

Specimens having standardproportions but different

absolute size produce different

values for KI. This results

because the stress statesadjacent to the flaw changes

with the specimen thickness (B)until the thickness exceeds some

critical dimension. Once the

thickness exceeds the criticaldimension, the value of

KIbecomes relatively constantand this value, KIC, is a true

material property which is

called the plane-strain fracture toughness. The relationship between stressintensity, KI, and fracture toughness, KIC, is similar to the relationship between


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stress and tensile stress. The stress intensity, KI, represents the level of stress at

the tip of the crack and the fracture toughness, KIC, is the highest value of stress

intensity that a material under very specific (plane-strain) conditions that a materialcan withstand without fracture. As the stress intensity factor reaches the KICvalue,

unstable fracture occurs. As with a materials other mechanical properties, KICiscommonly reported in reference books and other sources.

Plane-Strain and Plane-Stress

When a material with a crack is loaded in tension, the

materials develop plastic strains as the yield stress is

exceeded in the region near the crack tip. Material withinthe crack tip stress field, situated close to a free surface,

can deform laterally (in the z-direction of the image)because there can be no stresses normal to the freesurface. The state of stress tends to biaxial and the

material fractures in a characteristic ductile manner, witha 45oshear lip being formed at each free surface. This

condition is called plane-stress" and it occurs inrelatively thin bodies where the stress through thethickness cannot vary appreciably due to the thin

section.

However, material away from the free surfaces of arelatively thick component is not free to deform

laterally as it is constrained by the surrounding

material. The stress state under these conditionstends to triaxial and there is zero strain

perpendicular to both the stress axis and the

direction of crack propagation when a material is

loaded in tension. This condition is called plane-

strain and is found in thick plates. Under plane-

strain conditions, materials behave essentiallyelastic until the fracture stress is reached and then

rapid fracture occurs.Since little or no plasticdeformation is noted,

this mode fracture istermed brittle fracture.

Plane-Strain Fracture

Toughness Testing

When performing afracture toughness test,

Plane Strain- a condition of a

body in which the

displacements of all points in

the body are parallel to a givenplane, and the values of theses

displacements do not depend on

the distance perpendicular to the

plane

Plane Stressa condition of a

body in which the state of stress

is such that two of the principal

stresses are always parallel to a

given plane and are constant in

the normal direction.


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the most common test specimen configurations are the single edge notch bend

(SENB or three-point bend), and the compact tension (CT) specimens. From the

above discussion, it is clear that an accurate determination of the plane-strainfracture toughness requires a specimen whose thickness exceeds some critical

thickness (B). Testing has shown that plane-strain conditions generally prevailwhen:

Where: Bis the minimum thickness that produces a condition where plastic

strain energy at the crack tip in minimal

KICis the fracture toughness of the material

sy is the yield stress of material

When a material of unknown fracture toughness is tested, a specimen of full

material section thickness is tested or the specimen is sized based on a predictionof the fracture toughness. If the fracture toughness value resulting from the test

does not satisfy the requirement of the above equation, the test must be repeatedusing a thicker specimen. In addition to this thickness calculation, test

specifications have several other requirements that must be met (such as the size ofthe shear lips) before a test can be said to have resulted in a KICvalue.

When a test fails to meet the thickness and other test requirement that are in placeto insure plane-strain condition, the fracture toughness values produced is given

the designation KC. Sometimes it is not possible to produce a specimen that meetsthe thickness requirement. For example when a relatively thin plate product with

high toughness is being tested, it might not be possible to produce a thicker

specimen with plain-strain conditions at the crack tip.

Plane-Stress and Transiti onal -Stress States

For cases where the plastic energy at the crack tip is not negligible, other fracture

mechanics parameters, such as the J integral or R-curve, can be used tocharacterize a material. The toughness data produced by these other tests will bedependant on the thickness of the product tested and will not be a true material

property. However, plane-strain conditions do not exist in all structuralconfigurations and using KICvalues in the design of relatively thin areas may resultin excess conservatism and a weight or cost penalty. In cases where the actual

stress state is plane-stress or, more generally, some intermediate- or transitional-stress state, it is more appropriate to use J integral or R-curve data, which accountfor slow, stable fracture (ductile tearing) rather than rapid (brittle) fracture.


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Uses of Plane-Strain Fracture Toughness

KICvalues are used to determine the critical crack length when a given stress is

applied to a component.

Where: sc is the critical applied stress that will cause failure

KIC is the plane-strain fracture toughness

Y is a constant related to the sample's geometry

ais the crack length for edge cracksor one half crack length for internal crack

KICvalues are used also used to calculate the critical stress value when a crack of agiven length is found in a component.

Where: ais the crack length for edge cracks

or one half crack length for internal crack

s is the stress applied to the material

KIC is the plane-strain fracture toughness

Y is a constant related to the sample's geometry

Orientation

The fracture toughness of a material commonly varies with grain direction.Therefore, it is customary to specify specimen and crack orientations by an ordered

pair of grain direction symbols. The first letter designates the grain direction

normal to the crack plane. The second letter designates the grain direction parallelto the fracture plane. For flat sections of various products, e.g., plate, extrusions,

forgings, etc., in which the three grain directions are designated (L) longitudinal,(T) transverse, and (S) short transverse, the six principal fracture path directions

are: L-T, L-S, T-L, T-S, S-L and S-T.


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Fatigue Properties

Fatigue cracking is one of the primary damage mechanisms of structuralcomponents. Fatigue cracking results from cyclic stresses that are below theultimate tensile stress, or even the yield stress of the material. The name fatigue

is based on the concept that a material becomes tired and fails at a stress level

below the nominal strength of the material. The facts that the original bulk designstrengths are not exceeded and the only warning sign of an impending fracture is

an often hard to see crack, makes fatigue damage especially dangerous.

The fatigue life of a component can be expressed as the number of loading cycles

required to initiate a fatigue crack and to propagate the crack to critical size.Therefore, it can be said that fatigue failure occurs in three stages crack

initiation; slow, stable crack growth; and rapid fracture.

As discussed previously, dislocations play a major role inthe fatigue crack initiation phase. In the first stage,

dislocations accumulate near surface stress concentrationsand form structures called persistent slip bands (PSB)after a large number of loading cycles. PSBs are areas

that rise above (extrusion) or fall below (intrusion) the

surface of the component due to movement of material

along slip planes. This leaves tiny steps in the surface thatserve as stress risers where tiny cracks can initiate. These

tiny crack (called microcracks) nucleate along planes of

high shear stress which is often 45oto the loadingdirection.

In the second stage of fatigue, some of the tiny microcracks join together and beginto propagate through the material in a direction that is perpendicular to the

maximum tensile stress. Eventually, the growth of one or a few crack of the largercracks will dominate over the rest of the cracks. With continued cyclic loading, the


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growth of the dominate crack or cracks will continue until the remaining uncracked

section of the component can no longer support the load. At this point, the fracture

toughness is exceeded and the remaining cross-section of the material experiencesrapid fracture. This rapid overload fracture is the third stage of fatigue failure.

Factors Affecting Fatigue LifeIn order for fatigue cracks to initiate, three basic factors are necessary. First, the

loading pattern must contain minimum and maximum peak values with large

enough variation or fluctuation. The peak values may be in tension or compression

and may change over time but the reverse loading cycle must be sufficiently greatfor fatigue crack initiation. Secondly, the peak stress levels must be of sufficientlyhigh value. If the peak stresses are too low, no crack initiation will occur. Thirdly,

the material must experience a sufficiently large number of cycles of the applied

stress. The number of cycles required to initiate and grow a crack is largely

dependant on the first to factors.

In addition to these three basic factors, there are a host of other variables, such as

stress concentration, corrosion, temperature, overload, metallurgical structure, andresidual stresse

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Physical,Chemical and Mechanical Properties