chapter2.5- mechanical tests
TRANSCRIPT
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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
SUMMARY: TENSILE TEST RESULTSSUMMARY: TENSILE TEST RESULTS
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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])
SUMMARY: TENSILE TEST RESULTSSUMMARY: TENSILE TEST RESULTS
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
BRINELL HARDNESSBRINELL HARDNESS
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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.
BRINELL HARDNESSBRINELL HARDNESS
BRINELL HARDNESSBRINELL HARDNESS
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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
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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VICKERS HARDNESSVICKERS HARDNESS
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
VICKERS HARDNESSVICKERS HARDNESS
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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
VICKERS HARDNESSVICKERS HARDNESS
ROCKWELL HARDNESSROCKWELL HARDNESS
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ROCKWELL HARDNESSROCKWELL HARDNESS
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]
ROCKWELL HARDNESSROCKWELL HARDNESS
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002.0100 hHR =
002.0130
hHR =
Different kind of Rockwell scales exist: Pre-load: 10kg
ROCKWELL HARDNESSROCKWELL HARDNESS
ScaleLoad
Diamond
Cone
Steelsphere
Steelsphere
ROCKWELL HARDNESSROCKWELL HARDNESS
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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
ROCKWELL HARDNESSROCKWELL HARDNESS
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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))