Tensile Test and Stress Tesis

8/14/2019 Tensile Test and Stress Tesis

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Tensile test and Stress-Strain DiagramDr. Dmitri Kopeliovich

Stress-Strain Diagramexpresses a relationship between a load applied to a material

and the deformation of the material, caused by the load .Stress-Strain Diagram is determined by tensile test.

Tensile tests are conducted in tensile test machines, providing controlled uniformly

increasing tension force, applied to the specimen.

The specimens ends are gripped and fixed in the machine and itsgauge length L0(a

calibrated distance between two marks on the specimen surface) is continuously

measured until the rupture.

Test specimen may be round or flat in the cross-section.

In the round specimens it is accepted, that L0= 5 * diameter.

The specimen deformation (strain) is the ratio of the increase of the specimen gauge

length to its original gauge length:= (L L0) / L0

Tensile stressis the ratio of the tensile load Fapplied to the specimen to its original

cross-sectional area S0:

= F / S0

The initial straight line (0P)of the curve characterizes proportional relationship between

the stress and the deformation (strain).

The stress value at the point Pis called the limit of proportionality:

p= FP/ S0

This behavior conforms to the Hooks Law:

= E*

Where Eis a constant, known as Youngs ModulusorModulus of Elasticity.

The value of Youngs Modulus is determined mainly by the nature of the material and is

nearly insensitive to theheat treatmentand composition.

Modulus of elasticity determines stiffness- resistance of a body to elastic deformationcaused by an applied force.
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The line 0Ein the Stress-Strain curve indicates the range of elastic deformation

removal of the load at any point of this part of the curve results in return of the

specimen length to its original value.

The elastic behavior is characterized by the elasticity limit(stress value at the

point E):

el= FE/ S0

For the most materials the points Pand Ecoincide and therefore el=p.

A point where the stress causes sudden deformation without any increase in the force is

called yield limit (yield stress, yield strength):

y= FY/ S0

The highest stress (point YU) , occurring before the sudden deformation is called upper

yield limit.

The lower stress value, causing the sudden deformation (point YL) is called lower yield

limit.

The commonly used parameter of yield limit is actually lower yield limit.

If the load reaches the yield point the specimen undergoesplastic deformationit does

not return to its original length after removal of the load.
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Hard steels and non-ferrous metals do not have defined yield limit, therefore a stress,

corresponding to a definite deformation (0.1% or 0.2%) is commonly used instead of

yield limit. This stress is calledproof stress or offset yield limit (offset yieldstrength):

0.2%= F0.2%/ S0

The method of obtaining the proof stress is shown in the picture.

As the load increase, the specimen continues to undergo plastic deformation and at a

certain stress value its cross-section decreases due to necking (pointSin the Stress-

Strain Diagram). At this point the stress reaches the maximum value, which is

called ultimate tensile strength(tensile strength):

t= FS/ S0

Continuation of the deformation results in breaking the specimen - the point B in the

diagram.

The actual Stress-Strain curve is obtained by taking into account the true specimen

cross-section instead of the original value.

Other important characteristic of metals is ductility- ability of a material to deform

under tension without rupture.

Two ductility parameters may be obtain from the tensile test:

Relative elongation - ratio between the increase of the specimen length before its

rupture and its original length:

= (LmL0) / L0

Where Lmmaximum specimen length.
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Relative reduction of area - ratio between the decrease of the specimen cross-section

area before its rupture and its original cross-section area:

= (S0Smin) / S0

Where Sminminimum specimen cross-section area.

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Optical propertiesMetals reflect equally nearly all visible electro-magnetic waves. Therefore the color of the

most of the metals is white or silvery-white (except copper and gold).

Metals are lustrous due to themetallic bonding,contributingfree electronsto the

metalcrystal structureand providing an ability of metals to reflect light when polished.

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Physical stateMetals are solid at normal temperatures (except mercury).

Metals transform to liquid from solid and to gas from liquid at definite

temperatures(melting and boiling points), which are high for most of metals (except

mercury, sodium and potassium).

Most of metals have relatively high densities (except sodium and potassium with

densities lower, than density of water).

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Electrical propertiesMetals have high electrical conductivity, provided by free electrons available in the metal

crystal structure.

Peltier effect:When there is an electric current, passing through a junction of two different metals, one

of them evolves heat and another absorbs heat.

Thomson effect:A current is produced in a metal conductor when there is a temperature gradient along

its length.

The Peltier and Thomson effects are widely used in thermocouples.

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Thermal propertiesThermal ConductivityThermal Conductivity ()is amount of heat passing in unit time through unit surface

in a direction normal to this surface when this transfer is driven by unite temperature

gradient under steady state conditions.

Thermal conductivity may be expressed and calculated from the Fouriers law:

Q/ t =*S *T/ x

Where

Q-heat, passing through the surface S;

t- change in time;

- thermal conductivity;

S- surface area, normal to the heat transfer direction;

T/x-temperature gradient along xdirection of the heat transfer.

Fouriers law is analogue of the First Ficks law, describing diffusion in steady state.

Metals have high thermal conductivity. Heat is transferred through the

metalcrystalbyfree electrons.Compare:

of alumina = 47 BTU/(lb*F) (6.3 W/(m*K)).

of Al = 1600 BTU/(lb*F) (231 W/(m*K)).

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Coefficient of Thermal ExpansionThermal Expansion (Coefficient of Thermal Expansion) is relative increase in length

per unite temperature rise:

= L/ (LoT)

Where

-coefficient of thermal expansion (CTE);

Llength increase;

Loinitial length;

Ttemperature rise.

Thermal expansion of metals is generally higher, than that ofceramics.

Compare:

CTEof SiC = 2.3 F (4.0 C).

CTEof Al = 13 F (23 C).

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Specific Heat CapacityHeat Capacityis amount of heat required to raise material temperature by one unit.

Specific Heat Capacity is amount of heat required to raise temperature of unit mass of

material by one unit:

c= Q/(mT)

Where

c-specific heat capacity;

Qamount of heat;mmaterial mass;
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Ttemperature rise.

Specific Heat Capacity of metals is lower, than that of ceramics.

Compare:

cof alumina = 0.203 BTU/(lb*F) (850 J/(kg*K)).cof steel = 0.115 BTU/(lb*F) (481 J/(kg*K)).

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Magnetic propertiesMost of metals are slightly magnetic, but only few of them (iron, nickel, cobalt and their

alloys) display pronounced magnetic properties, called ferromagnetism.

Magnetically soft metals metals, which are demagnetized after the magneticfield is removed. Magnetically soft metals are used in electric motors and

transformers.

Magnetically hard metals metals, retaining their magnetization after themagnetic field is removed.Magnetically hard metals are used for permanent magnets.

Magnetostriction effect of changing dimensions of a ferromagnetic metal when itsmagnetization is changed.

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Tensile Test and Stress Tesis