Transcript Stress and strain measurement

Done by :
Ahmad Al-Ameen
Osaid J. Matar
Definition of stress and strain.
 Tensile test theory (experiment to find stress
and strain).
 Clip on extensometers.

 Linear variable differential transformer (LVDT)
 Strain gage.

Non-contact extensometers .
• Non-contact laser extensometer.
• Non-contact video extensometer.
Summary.
 References.
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Stress is defined as force per unit area. It has
the same units as pressure, and in fact
pressure is one special variety of stress.
σ = Fn / A
(1)
where
σ = normal stress ((Pa) N/m2, psi)
Fn = normal component force (N, lbf)
A = area (m2, in2)

Strain is defined as the amount of
deformation an object experiences compared
to its original size and shape, Note that strain
is dimensionless.
ε = dl / lo = σ / E
(2)
where
dl = change of length (m, in)
lo = initial length (m, in)
ε = unitless measure of engineering strain
E = Young's modulus (Modulus of Elasticity)
(Pa, psi)
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
Robert Hooke found that, when the forces are
not too large (less than the yield force), the
amount of strain experience by an object was
directly proportional to the stress.
From Hooke's law.
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Objective:To determine the strength and several elastic
and plastic property of some material.
To do the tensile test.
To observe the behavior of material under
static load and to study fracture.
To take a good information about the
machine and how to use it .

If the load is divided by the original crosssection area and the elongation is divided by
the original gauge length, the size effects are
eliminated and the plot becomes known as an
engineering (Stress-Strain) diagram,
Stress= σ =P/Ao
Strain= ε =∆L/Lo
Tensile test machine (Tensometer):It is a machine used to determine the
strength and several elastic and plastic
properties of various materials; the test
specimen is installed between two large grips
of testing machine and then loaded in
tension.
Tensile test specimen:-
The ends of specimen are enlarged where
they fit in the grips so that the failure will not
occur near the grips themselves.
A failure at the ends will not produce the
desired information about the material
because the stress distribution in uniform
and the bar is subjected only to pure-tension.
Source of uncertainties:
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uncertainty in measuring the dimension of
specimen.
The material itself.
The temperature effects.
The crack in the specimen.
TENSILE TESTS are performed for several reasons.
•The results of tensile tests are used in selecting materials for
engineering applications.
•Tensile properties frequently are included in material
specifications to ensure quality.
•Tensile properties often are measured during development of
new materials and processes, so that different materials and
processes can be compared.
• Finally, tensile properties often are used to predict the
behavior of a material under forms of loading other than
uniaxial tension.
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Clip on extensometer:
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Non-contact laser extensometer.
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Linear variable differential transformer device (LVDT).
Strain gage.
Parallel scanner.
Cross scanner.
Laser Doppler scanner.
Biax scanner.
Double beam cross scanner.
Non-contact video extensometer.
Stress –Strain Measurements by Extensometers:The elongation of a specimen during load application can be
measured directly with various types of devices, such as clip-on
extensometers ,directly-mounted strain gages and various optical
devices.
Selection of a device for strain measurement depends on various
factors:
1- The useable range of the Extensometer .
2- Techniques for mounting the Extensometer.
3- Specimen size .
4- Environmental test conditions.
5- Electronic circuit configuration and analysis for signal
processing.
•Clip-on extensometers
•It can be attached to a test specimen to measure
elongation or strain as the load is applied.
•This is usually used for metals and similar materials that
exhibit high stiffness.
•Typical extensometers can measure for the gauge lengths
such as 25 or 50 mm (1 or 2 in.).
•There also are transverse strain measuring devices that
indicate the reduction in width or diameter as the
specimen is tested.
The two basic types of clip-on extensometers are :-
1-Linear variable differential transformer (LVDT)
devices
2- Strain-gage devices.
LVDT extensometers
•LVDT extensometers employ an LVDT with a
core, which moves from specimen deformation
and produces an electrical signal proportional
to amount of core movement .
•LVDT extensometers are small, lightweight,
and easy to use.
•They can be used on small specimens such as
thread components ,and on large test
specimens such as reinforcing bars, heavy steel
plate, and tubing up to 75 mm (3 in.) in
diameter.
Strain-gage Extensometer
•A device which measures mechanical deformation.
Which can convert a small mechanical motion to an
electrical signal ,when a metal (wire or foil) or
semiconductor is stretched, its resistance is
increased.
•The strain gage attached to the beam is an
electrically conductive small-sized grid that changes
its resistance when deformed in tension,
compression, bending, or torsion.
•Strain-gage Extensometer are also common and
lighter in weight and smaller in size, but strain
gages are somewhat more fragile than LVDTs.
•Thus, strain gages can be used to supply the
information necessary to calculate strain, stress,
angular torsion.
•These gages typically measure 9.5 to 13 mm ( 3/8
to 1 ⁄ 2 in.) in width and 13 to 19 mm ( 1 ⁄ 2 to 3 ⁄ 4
in.) in length.
How does the Strain Gauge work?
•Operation of strain-gage extensometers is based on gages that are
bonded to a
metallic element and connected to a bridge circuit.
•Deflection of the element, due to specimen strain, changes the
gage’s resistance that produces an output signal from a bridge
circuit.
•This signal is amplified and processed by signal conditioners before
being displayed on a digital readout, chart recorder, or computer.
•Foil strain gages currently are the most widely used, due to the ease
of their attachment
Test specimen with bonded resistance strain gages and a 25 mm (1 in.) gage length.
Calibration, Classification, and Verification of
Extensometers.
•All types of extensometers for materials testing
must be verified, classified, and calibrated in
accordance with applicable standards.
• Calibration of extensometers refers to the
procedure of determining the magnitude of
uncertainty in strain measurements.
•Several calibration devices can be used, including an
interferometer, calibrated standard gage blocks and
an indicator, and a micrometer.
•Verification is a calibration to ascertain whether the
errors are within a predetermined range.
•Verification also implies certification that an
extensometer meets stated accuracy requirements,
which are defined by classifications such as those in
ASTM.
Non-contact extensometer:
 Laser extensometer .
• parallel scanner .
•Stereo angle scanner.
•Cross scanner.
•Laser Doppler scanner.
•Biax scanner.
•Double beam parallel scanner.
Video extensometer.
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Non contact extensometers (video and laser
scanning)
The main advantage of non-contact video
and laser scanning extensometers is that they
can be used up to break without damage
even when testing specimens that exhibit
whiplash. They require measurement marks
to be attached to the specimen which are
optically distinct from the surrounding area
of the specimen.
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The measurement marks are clipped, tacked,
or glued onto the specimen, or the specimen
is marked with a colored pen. In every case,
this introduces additional sources of error as
the marks can become indistinct, move, or
fall off the specimen surface as it deforms
during loading. The application of the
measurement marks is also an additional
process by the operator and can introduce
higher costs as well as inaccuracies to the
test results.
Laser extensometer
Scanning Laser Extensometers for measuring deformation at
static, quasistatic and cyclic loading
Parallel Scanner
high resolution and accuracy, insensitive to small variations of the
working distance
Stereo Angular Scanner
measurement of large strain, scanning range up to 500 mm
Cross Scanner
simultaneous measurement of longitudinal and lateral strain,
determination of Poisson's ratio
Parallel Scanner
The laser extensometer - type parallel scanner is used for
non-contact measurement of strain or compression of
specimen at uniaxial load.
Before starting the experiment, two marks are put on the
sample. A laser beam is directed onto a rotating optical
flat. During entering and leaving the laser beam is
refracted at two opposite planes of the optical flat which
results in an identical refractive angle. By the rotation of
the optical flat, the laser beam is deflected in parallel to
itself and is directed onto the specimen. The laser
extensometer scans the measuring range with a visible
laser beam. After automatically determining the reference
length at the beginning, the positions of the marks are
continuously observed throughout the experiment
Technical Data
P-50
P-100
P-130
Scan Range on
Specimen
50 mm
100 mm
130 mm
100..300 mm
100..300 mm
100..300
mm
0.1 um
0.25 um
0.3 um
Class 0.2; 0.5; 1
Class 0.5; 1
Class 0.5;
1
50; 200 Hz
50; 200 Hz
50; 200 Hz
Duration of each
Scan at Specimen
5.0 ms
5.0 ms
5.0 ms
Scan Speed On
Specimen
10 m/s
20 m/s
27 m/s
plane or
structured
(components)
plane or
structured
(components)
plane or
structured
(componen
ts)
Working Distance
Resolution
(micron)
Accuracy (DIN EN
10002-4)
Scanning Rate
Specimen Surface
Stereo Angular Scanner:
The SWS-300 is a deformation meter for all materials. Its high
accuracy allows non-contact strain control from elasticity range
till breaking elongation.
The surface of the specimen is scanned by a laser beam. A set of
stripes applied to the surface reflects the laser light. The receiver
evaluates the reflected light and changes the signals into digital
impulses. The positions of stripes and the measured length are
determined from the time process of the signals. A novel
deflection system allows the SWS-300 to measure changes in
distance between specimen and scanner.
Advantages
•highest precision up to a big measuring area of 300mm
•accuracy class (1) referring to DIN EN ISO 9513 .
•non-contact measurement with measuring marks
•no influence of specimen by extensometer
•easy integration in testing machine software
•measurement of pressure tests and bending tests
•measurement from smallest to big strains
•determination of E-module without additional receiver or
rebuilding
•measurement of changes in distance between specimen and SWS300
Optional extensions
The whole experimental run is traceable on screen
strain-regulated experiments are possible
local dissolving at measurement of breaking elongation
at welds and assembling components
force recording with evaluation software for calculation of
characteristic values
applicable to climatic chambers
Technical Data
SWS-300
measuring range
300 mm
accuracy class (DIN EN ISO 9513)
1
resolution
1 μm
lowest measuring length
2 mm
working distance
(scanner<>specimen)
250 mm
scanning rate
100 Hz
number of stripes
2 / optional locally resolved
Laser safety class
2 M (no additional protective
arrangements necessary)
dimensions/weight
310 mm x 240 mm x 250 mm / 8
kg
Cross Scanner
The laser extensometer - type cross-scanner is used for
simultaneous contact free measurement of longitudinal and
lateral strain of specimens at uniaxial load. This scanner type is
optimised for determining Possion's ratio ..
Before starting the experiment, two marks for longitutinal
strain and two marks for lateral strain are put on the sample.
One laser beam for each measuring direction is directed onto
rotating optical flats. During entering and leaving the laser
beam is refracted at two opposite planes of the optical flat
which results in an identical refractive angle. By the rotation of
the optical flat, the laser beam is deflected in parallel to itself
and is directed onto the specimen. The laser extensometer
scans the measuring range with a visible laser beam. After
automatically determining the reference length at the
beginning, the positions of the marks are continuously
observed throughout the experiment .
Technical Data
Working Distance
Scan Range on Specimen
Resolution (micron)
Accuracy (DIN EN 10002-4)
Scanning Rate
Duration of each Scan at
Specimen
Scanning Speed at Specimen
K-50
K-100
K-130
100100-300
100-300 mm
300
mm
mm
130
50 mm
100 mm
mm
0.1 um
0.25 um
0.3 um
Class 0.2; 0.5;
Class
Class 0.5; 1
1
0.5; 1
50; 100
50; 100 Hz
50; 100 Hz
Hz
5.0 ms
5.0 ms
5.0 ms
10 m/s
20 m/s
27 m/s
Lateral Strain
50 mm
50 mm
Resolution (micron)
0.1 um
Accuracy (DIN EN 10002Class 0.5
4)
0.1 um
0.1 um
Class 0.5
Class 0.5
Scanning Rate
50; 100 Hz
50; 100 Hz
Duration of each Scan at
5.0 ms
Specimen
5.0 ms
5.0 ms
Scanning Speed at
Specimen
10 m/s
10 m/s
Scan Range on Specimen
Specimen Surface
50 mm
50; 100 Hz
10 m/s
plane or
plane or
plane or
structured
structured structured
(Component (Component (Component
Laser Doppler Extensometer
The Laser Doppler Extensometer is typically used at
high speed tensile tests with deformation speed up
to 50 m/s. Based on the high resolution in strain and
time of our system, even the elastic range can be
analyzed with a reasonable amount of data points
(5000 points with an aluminum alloy at 10 m/s). The
strain is measured directly at the sample. Because of
the contact free and therefore non intrusive
measuring principle, the samples are not influenced
by the measuring equipment. Furthermore the Laser
Doppler Extensometer is not endangered during the
experiment and at rupture of the sample.
Technical Data
Measurement Length on
Sample
Working Distance:
Accuracy:
Sample Rate:
Deformation Speed:
any
200 mm
1%
up to 50 MHz
0.01 .. 1 of vmax; vmax = 1 .. 50
m/ Several parameters are default
s
settings for a typical setup of a Laser
Doppler Extensometer. Most of them
can be chosen for the device to be
delivered.
Optical Clipon
The High Frequency Laser Extensometer (Optical Clipon) was
developed for strain measurement at UHCF applications and
optimized for determination of hysteresis loops and dynamic
characteristics .
Technical Data
Measurement Range
Resolution
Testing Frequency
Sample Frequency
Measurement Length
± 500
± 200 µm
µm
0.25 µm
0.1 µm
± 100 µm
0.05 µm
1 - 1000 Hz
1 MHz
5 mm - 100 mm
Biax Scanner
The laser extensometer - type biax-scanner is used for non-contact
measurement of strain or compression of specimen at biaxial load.
Two laser extensometers of the type parallel-scanner are combined to
measure strain in X- and Y-direction. The scanning sequence of the two
systems is synchronized to each other. A single receiver with special
optical components is used for both scanners.
Technical Data
Scanning Range
on Specimen
Working Distance
Resolution
(micron)
Accuracy (DIN EN
10002-4)
Scanning Rate
Duration of each
Scan at Specimen
Scanning Speed
at Specimen
Specimen Surface
B-50
B-100
B-130
50 mm
100 mm
130 mm
250..400 mm
250..400 mm
250..400 mm
0.1 um
0.25 um
0.3 um
Class 0.2; 0.5; 1
Class 0.5; 1
Class 0.5; 1
50; 100 Hz
50; 100 Hz
50; 100 Hz
5.0 ms
5.0 ms
5.0 ms
10 m/s
20 m/s
27 m/s
plane or
structured
(components)
plane or
structured
(components)
plane or
structured
(components)
Double Beam Parallel Scanner
The laser extensometer P-50D is used for non-contact
measurement of strain, compression, or motion of
specimen along two scanning lines.
A laser beam is directed onto a rotating optical flat. During
entering and leaving the laser beam is refracted at two
opposite planes of the optical flat which results in an
identical refractive angle. By the rotation of the optical flat,
the laser beam is deflected in parallel to itself and scans
along the specimen.
The P-50D includes an optical beam splitter which
generates two scan lines. One scan line is fixed, the other
is adjustable in horizontal position. The distance between
the scan lines can be adjusted by the user between 0 and
20 mm - other ranges are available on request.
The extensometer is optimized for experiments like shear
strain, crack opening and bending tests
Technical Data
P-50D
Scan Range on Specimen
50 mm
Distance between Scan Lines
Working Distance
Resolution (micron)
Accuracy (DIN EN 10002-4)
Scanning Rate
Duration of each Scan at
Specimen
Scanning Speed at Specimen
Specimen Surface
0 - 20 mm
100..300 mm
0.1 um
Class 0.2; 0.5; 1
50; 200 Hz
5.0 ms
10 m/s
plane or structured
(components)
Along with integrated software that
facilitates setup and consistent calibration,
advanced video extensometer (AVE) uses digital
camera and real-time image processing to make
strain measurements on material test samples.
Non-contact measurement design ensures
extensometer has no influence on specimen,
eliminating possibility of damage. Solid-state
unit is available with 3 fields of view and
transverse strain measurement capabilities.
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Contact type extensometers measure extension
extremely accurately and are very cost effective.
However, clip-on extensometers require much
more manual intervention and without care can
introduce scatter in the test results.
Feeler arms extensometers offer extremely
high accuracy, excellent repeatability, and ease
of use due to fully automatic operation which
includes the setting of variable gauge lengths.
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Non contact extensometers are required when
the specimen is sensitive to notching knife edges
or when the extensometer might be damaged at
specimen break. They are also still relatively
expensive and time consuming to set up and
calibrate especially when testing different
specimen types.
In short, there is no such device as a universal
extensometer. The large range of applications
demands various devices with different functions
and characteristics, and the extensometer must
be selected for each application.
[1] Davis, J. R. (Editor). Tensile Testing (2nd
Edition).Materials Park, OH, USA.
[2] http://www.fiedler-oe.de/en/index.html .
[3] 4 ASTM E 83-02 Standard Practice for Verification
Extensometer System . and Classification of
[4] Anwander, M., Zagar, B.G., Weiss, B., and Weiss, H.,
‘‘Non-Contacting Strain Measurements at High
Temperatures by the Digital Laser Speckle Technique,’’
Experimental Mechanics .