chapter2.5- mechanical tests

7/29/2019 Chapter2.5- Mechanical Tests

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Mechanical Tests

Tensile Test Hardness Test

Impact Test


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TENSILE TESTTENSILE TEST

Main pourpose: to investigate the behaviour of a metallic material

under an uniaxial tensile stress

F F

The test must be carried out with a standard specimen.

During the test we can record and plot the values of force and elongation of

the sample so obtaining the so called TensileTensile curvecurve.. Analizing theseAnalizing these datadata itsits

possible to calculate parameters very important forpossible to calculate parameters very important for thethe designer.designer.

During the test a force is applied along the main axis of the sample, pulling it

until fracture.


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StandardsStandards

UNI EN 10002 Metallic Materials - Tensile test

Section 1: test at room temperature

Section 5: test at high temperature

UNI 8899-1 Mechanical test for non ferrous materials (Al, Mg)

ASTM E 8 - 00b Standard test methods for tension testing of metallic materials

ISO 6892 Metallic materials - Tensile test at room temperature

ISO 783 Metallic materials - Tensile test at high temperature


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Fixed Crosshead

Load Cell

Grippers

Controller

Moving Crosshead

Tensile MachineTensile Machine

Sample -

Extensometer


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ProportionalProportional samplesample::

THE SAMPLE (1)THE SAMPLE (1)

00 SkL =

For proportional samples withFor proportional samples with roundround sectionsection::

K = 5.65K = 5.65 shortshort

K = 11.3K = 11.3 normalnormal

000 5dSkL ==

000 10dSkL ==

Threaded head

Zone with uniform cylindrical sectionFillet

LLtt== total lengthtotal lengthLLcc== length of the cylindrical tractlength of the cylindrical tract

LLoo== useful tract lengthuseful tract length

LLee== reference length of the extensometerreference length of the extensometer

Le


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ForFor squaresquare sectionsection proportionalproportional samplessamples::

K = 5.65K = 5.65 shortshort

K = 11.3K = 11.3 normalnormal

ProportionalProportional SampleSample::

00 SkL =

a

Le

THE SAMPLE (2)THE SAMPLE (2)


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0S

F=

Inside the useful tract we have an uniform value of the stress on the

whole section.

F F


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[

[N/m

m2]

[%]

TENSILE CURVETENSILE CURVE

[ ]20

/mmNS

F=

==mmmm

LLL

LL

0

0

0

stressstress

Engineering strainEngineering strain

l [mm]

[F[N

]

From theLoad Cell

From the

extensometer


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Elastic field - small deformations

Plastic field - large deformations

Plastic Field - Necking

[%]

[N/m

m2]

TENSILE CURVETENSILE CURVE


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In this zone the tensile curve can beapproximated by a line:

I. Elastic Field

= E Hooke Law

E = Young Modulus

[%]

[N

/mm2]

E

The Young Modulus is very important for the designer, because it allows to calculate,

inside the elastic field, the deformation of a structure under some loads.

The Young Modulus depends on temperature:E [M pa] 20C 200C 400C

A cciaio al carbo nio 207000 186000 155000

A cciaio inox 193000 176000 159000

Legh e di al lum inio 72000 66000 54000


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Elastic DeformationElastic Deformation

If the forces on a metallic body cause stresses lower than e, the lattice can deform,

but the energy is not enough for a permanent deformation; when the applied forces

are removed the deformation come back to zero.


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II. Plastic Field - High deformations

[%]

[N/mm2]

During the plastic deformation two phenomena are important:

The resistant Area decreases and so the force would tend to decrease too (its like to pull

a smaller sample)During the deformation the Strain-Hardening occurs: the material becomes stronger andthis makes the force increase

In this step the effect of the strain-hardening is strong and so the force increases

Increasing the stress, deformations tend tobecame greater and greater


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s

Between small and high deformations we can identify an

important load value: the Yield Stress s

ReH ReL

This zone of the tensile curve can have different shapes

Rp0.2

0.2%

Alloyed Steels

Plain Carbon Steels

(%C


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III. Plastic Field - Necking

This time the section reduction is the main phenomenon and so the force decreases until

the final rupture.

After the maximum load the deformations

concentrate in a small region and so thearea decrease quite fastly. This

phenomenon is called Necking. The state

of stress is no more uniaxial

[%]

[N/mm2]

Rm

Rm (o

m) is called UpperUpperTensileTensile StressStress. This value, as the yield stress, is referred tothe initial area of the sample section


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SUMMARY: TENSILE TEST RESULTSSUMMARY: TENSILE TEST RESULTS

Young Modulus:

UTS:

[N/mm2]

[%]

[N/mm2]

[%]

Yield Stress: [ ]20

/mmNS

FR SSs ==

[ ]20

/mmNS

FR mrm ==

( )[ ]2

0

0

0

/mmNLL

LSFE

==


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Percentage elongation at fracture:

dove:

Lu = length of the useful tract after fracture

L0 = initial length of the useful tract

A% depend on the relations between the geometrical dimensions of the sample; its

necessary to indicate some of these relations: ex. A11.3; A5.65;A80mm. Its possible to

compare elongations only if the samples have the same ratio L0

/d0

100

)(

%0

0

= L

LL

Au

Percentage Necking Coefficient:

S0 = initial value of the area

Su = final value of the area

100)(

%

0

=

S

SSZ uo



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g s

Ag = elongation at maximum load

s = permanent deformation due tonecking

A = elongation after fracture

Area under the stress-strain curve: it represents the worknecessary for sample fracture ([mJ/mm3])


Work before necking; sample shape

is uniformly cylindrical

Work after necking; the

deformation concentrates in

a small region

TRUE STRESSTRUE STRESS TRUE STRAIN CURVETRUE STRAIN CURVE


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,

[N

/mm2]

, [%]

TRUE STRESSTRUE STRESS--TRUE STRAIN CURVETRUE STRAIN CURVE

istS

F=

*

Engineering Curve

True curveIf I referred the deformationsto the istantaneous values of

length (List) and area (Sist) of

the sample, I woul find the

true stress and the true strain.

Plotting these data I can find

the true tensile curve; the

stress continously increasesbecause its referred to the

istantaneous area.

=

0

* lnL

List

TRUE STRESSTRUE STRESS TRUE STRAIN CURVETRUE STRAIN CURVE


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From the yield stress till the maximum load, the true tensile curve can be fitted by thefollowing equation * = K *n where: K = strength coefficient e n = strain-hardening coefficient (for steels usually 0.1< n < 0.3). In a bilogarithmic plot, this

expression can be written as : ln(*)=ln(K)+n ln(*).

Using the volume constancy principle during plastic deformation, its possible to

relate engineering stress and strain with the true ones.

* = ln (+1) * = (+1)

ln *

ln *

ln(K)

n=tg()

n represents the true strain

at necking

TRUE STRESSTRUE STRESS--TRUE STRAIN CURVETRUE STRAIN CURVE


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The hardness tests are based on the resistance to the indentation of a

material; hardness tests are always carried out using an indenter with

different shapes.

Brinell Hardness (UNI EN ISO 6506)

Vickers Hardness (UNI EN ISO 6507)

Rockwell Hardness (UNI EN ISO 6508)

HARDNESS TESTSHARDNESS TESTS

BRINELL HARDNESSBRINELL HARDNESS


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BRINELL HARDNESSBRINELL HARDNESSP

TheThe BrinellBrinell HardnessHardness isis proportionalproportional toto thethe ratioratio betweenbetween thethe appliedapplied

loadload andand thethe imprintimprint area.area.

( )22

2

dDDD

PHB

=

P [kg]: applied load

D [mm]: sphere diameter

d [mm]: imprint diameter

( )222

102.0dDDD

PHB

=

If P is given in [N]

Sample

Test Indenter Imprint shape

Sphere of steel

or tungsten

carbid



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The test must be carried out on a flat quite clean surface

The test needs an optical measurement and so the sample must bepolished enough

In order to have meaningful results, the sphere diameter must be as higher

as possible, consistently with the load and sample thickness (t > = 8h)

P

D

From 4 to 6d

d h

> than 3 d

> than 8 hThe load must be

applied for 10 - 15 s

Its possible to perform different hardness tests on the same surface, but

its important to pay attention to the distance between two imprints and tothe distance of an imprint and the sample boundary.




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When we choose the sphere diameter, the applied load is automatically determined

Its important that the imprinting angle is about 136. This allows to have a similitude

condition between different tests. This condition is verified if we use the right value of P/D2

P/D2 =


BRINELL HARDNESSBRINELL HARDNESS -- ProcedureProcedure


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To choose a sphere diameter

To find the right load value using the previous table

To perform the test

To measure the imprint diameter (d) and to verify that d/D =(cos /2) = 0.25 - 0.50

The test is not valid if HB>650 because the hardnesses of the sample and of thesphere are too close

Its possible to estimate the UTS according to the following equation Rm = c*HB

where c= 3.3 for quench and tempered steels

BRINELL HARDNESSBRINELL HARDNESS -- ProcedureProcedure

VICKERS HARDNESSVICKERS HARDNESS


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P

2854.1

d

PHV =

P [kg]: applied load

d [mm]: imprint diagonal

TheThe VickersVickers HardnessHardness isis proportionalproportional toto thethe ratioratio betweenbetween thethe

appliedapplied loadload andand thethe imprintimprint areaarea

21891.0

d

PHV = If P is given in [N]

Test Indenter Imprint shape

Diamond pyramid

with vertex angle

equal to 136



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Diamonds allows to perform test even on very hard materials

Its possible to use very low loads in order to perform amicrohardness test

Its necessary to take more care in the finishing, above all for

microhardness tests

Brinell and Vickers hardness teoretically have the same value

untill 500HB, if the Brinell hardness imprint satisfies the rule of

=136

There arent any limits on the applied load (except the one given

from the test machine), being the similitude condition automaticallyverified (the indenter angle is 136)

The lowest distance between two imprints is 4 x d, while the

minimum distance from sample boundary is 3 x d

Sample thickness must be >1.5 x d


ROCKWELL HARDNESSROCKWELL HARDNESS


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According to this scale, the hardness value has not a physical meaning; its

evaluated from the indenter sinking under a certain load

Procedure:

the indenter must touch the sample surface and the machine applies a pre-load Fo

(10 kg);

the comparatore (used for measuring the sinking) must be set to zero;

the test load F1 is applied (It is different according to the kind of test - In this way

the total applied load is Fo+ F1);

after 10s F1 must be removed and its possible to measure the sinking;

hardness value can be calculated as:

N=100 for Rockwell A, C, D

N=130 for Rockwell B, E, F, G, H, KS

hNHR =

h: sinking[mm]S: [0.002 mm]



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002.0100 hHR =

002.0130

hHR =

Different kind of Rockwell scales exist: Pre-load: 10kg


ScaleLoad

Diamond

Cone

Steelsphere

Steelsphere



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Very fast

Itnot necessary to measure the imprint and so a good finishing is

not requested

The Rockwell hardness cannot be related to Brinell and

Hardness ones except with empirical tables

Rockwell scales A, C, D are suitable for very hard materials; Rockwell C is not

suggested for very very hard materials because the diamond could damage

If the hardness decreases under 20 HRC its suggested to use HRB.

Lowest distance between two imprints : 4 x d

Lowest distance between one imprint and the boundary: 2.5 x d

Minimun sample thickness : 10 x h (sphere) or 15 x h (cone)

Advantages

Disadvantages



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Pre-load: 3KgSuperficialSuperficial ROCKWELLROCKWELL hardnesshardness

001.0100 hHR =

Scale Load

DiamondCone

Steel

sphere

IMPACT TESTIMPACT TEST ResilienceResilience TestTest


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IMPACT TESTIMPACT TEST -- ResilienceResilience TestTest

Resilience: its a measure of the material resistance to an impact

It can be evaluated measuring the work ([J]) spent to break a notched sample underan Impact Machine (Charpy Pendolum).

Standard: UNI - EN 10045

High absorbed energy highhigh resilienceresilience high deformation (tough fracture)

Low absorbed energy Low resilienceLow resilience low deformation (brittle fracture)

Tensile and hardness tests are not enough to investigate the behaviour of a material

2 materials can have the same tensile behaviour, but completely different results if

submitted to an impact test

Samples


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Mesnager

Charpy U-notched

Izod

Mesnager

Charpy V-notched

Charpy pendolumCharpy pendolumTEST MACHINETEST MACHINE


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Charpy pendolumCharpy pendolum

Main features:

Maximun Energy = 300 J

Distance between supports: 40 mm

Hammer speed at impact: 5 - 7 m/s

TEST MACHINETEST MACHINE

After the pendolum

broke tha sample, its

movement go on on the

other side of themachine until a certain

height; this height is

related to its residual

energy. The differencebetween the initial

height and the height

after fracture gives the

energy absorbed by thesample.

Some general information about Impact Test


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The results of this test depend on the proof conditions and on sample shape. In

particular they depend on:

Test temperature

load application speed

sample geometry and dimensions

sample machining

Some general information about Impact Test

Nevertheless this test is widely used because its fast, easy and give an idea of

material toughness.

Varying the tese temperature its possible to find the so called TRANSITION

TEMPERATURE.

Its defined as the test temperature at which a great variation of K (resilience

[J]) -T curve slope appears; the TT divides the zone where brittle fractures

occur from the zone where the material is ductile.

Not all the materials have a transition temperature. Some of them may be

ductile even at very low temperature

Influence of test condition:


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Here you can see a resiliece curve varying the sample geometrical dimensions

a

b

b/a

K[J]

1.4 1.8 21

Ductile fracture

Brittle fracture

Scattering zone

Influence of test condition:


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KCU

KV

[C]

K[J]

0 20-20

Transition

scattering

Transition

Temperature

Here you can see a resiliece curve varying the temperature and the sample shape (two

different notches: U-notch (KCU) and V-notch (KV))

chapter2.5- mechanical tests

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