Measurements of Stress and Strain
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Transcript of Measurements of Stress and Strain
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MEASUREMENTSOFSTRESSAND STRAIN
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WHATIS STRAIN?
RE-CAP
Strain is defined as the amount of deformation anobject experiences compared to its original shape
and size.
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WHATISSTRESS?
RE-CAP
Stress is the amount of pull and push force applied over across-sectional area and right angle to the action of the
force.
= P/A
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BEHAVIOROFASUBJECTUNDERSTRESS
DEPENDSONTHE:
material
nature of the forces
shape
orientation of the subject
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STRAIN MEASUREMENT
The methods of measuring strain or deformation
based on various mechanical, optical, acoustical,
pneumatic, and electrical phenomena.
MECHANICAL DEVICES
OPTICAL METHODS
ELECTRICAL DEVICES
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MECHANICAL DEVICES
EXTENSOMETERMICROMETER
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OPTICAL METHODS
Uses the interference fringes
produced by optical flats to
measure strain.
interference fringe: a bright or
dark band caused by beams
of light that are in phase or out ofphase with one another.
The technique is so delicate that
laboratory conditions are required for
its use but the device is sensitive and
accurate.
interference fringe
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ELECTRICAL DEVICES
CAPACITANCE STRAIN
GAGE
BONDED RESISTANCE
STRAIN GAGE
SEMICONDUCTOR STRAIN GAGE
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PRINCIPLEOF
STRAIN GAGE
a device whose electrical
resistance varies in proportion to
the amount of strain in the device.
Edward Simmons and Arthur
Ruge in 1938.
consists of an insulating flexible
backing which supports a metallic
foil pattern.
attached to the object by a suitable
adhesive-cyanoacrylate.
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GAGECHARACTERISTICS AND
PARAMETERS
Strain-sensitive alloy
Backing material (carrier) Gage series
Gage pattern
Self-temperature compensation number
Gage length
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STRAIN-SENSITIVEALLOY
A: Constantan in self-temperature-compensated
form.
P: Annealed constantan.
D: Isoelastic. K: Nickel- ch romium al loy, a modified Karma in
self-temperature-compensated form.
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BACKINGMATERIAL
IMPORTANCE OF BACKING MATERIAL
provides a means for handling the foil pattern duringinstallation.
presents a readily bondable surface for adhering the
gage to the test specimen.
provides electrical insulation between the metal foiland the test object.
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GAGESERIES
Sets of test requirements
Type of strain measurement (static, dynamic, etc.).
Operating temperature of gage installation.
Test duration.
Accuracy required.
Cyclic endurance required.
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GAGELENGTH
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GAGEPATTERN
Gage Pattern depends primarily
on the following:
1. Solder tabs
2. Grid width
3. Gage resistance
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METALLIC RESISTANCE STRAIN GAGE
a very fine wire or, more
commonly, metallic foil
arranged in a grid pattern.
the grid pattern maximizes the
amount of metallic wire.
strain gauges are available
nominal resistance values
from 30 to 3000 , with 120,
350, and 1000 being themost common values
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SELECTIONOF BONDED METALLIC STRAIN
GAGES
determination of the frequency of vibration, or
comparison of relative amplitudes and frequencies
with different conditions of excitation.
determination of the maximum stress pattern
resulting from the vibration set up.
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MOUNTING STRAIN GAGESONTHE LOAD
CELL BODY
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TOOLSNEEDEDUPONINSTALLATION
1. Soldering iron with pointed tip (preferably new tip)
2. Illuminated magnifier (optional, but recommended)
3. Electrical grade Solder
4. Rosin soldering flux
5. Epoxy adhesive
6. Cyanoacrylate adhesive (optional)7. Lacquer thinner, acetone or alcohol
8. Masking tape
9. Toothpicks
10. Tweezers
11. Awl
12. Ruler
13. Fine gauge tinned-copper lead wires (30 awg wire-wrap wire
works well)
14. Ohmmeter
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STEPS IN MOUNTING STRAIN GAGE
ON A LOAD CELL BODY
1. Using the awl, lightly etch a line across the
side of the load cell body at the exact height
of the hole centre.
2. Clean the mounting area with lacquer thinner,
acetone or methyl alcohol.
3. Using tweezers, place the strain gage(s) in
position FOIL SIDE UP. Align the
longitudinal gage(s) such that the grid centre
alignment marks line up with the etched line.
Never touch the gages with your fingers.
4. For the 4 gage arrangement, similarly position
the two transverse gages above and below the
other two. The exact location of these gages is
not important, but it is important that they are
aligned in the transverse direction. See
Figure 4a.
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5. Cut off a strip of masking tape, and
carefully position the tape over the
gages. Press the tape down onto the load
cell body, making sure that the gages donot become disoriented. See Figure 4b.
6. Next, lift off one end of the tape such
that the gages are exposed. Leave the
other end attached to the load cell body.This is illustrated in Figure 4c.
7. Using a toothpick, apply a minimal
amount of epoxy adhesive (or
cyanoacrylate) to the underside of eachgage. Take care not to apply excessive
adhesive.
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8. Carefully re-apply the masking tape to
the load cell body. Press firmly down
over each strain gage, squeezing out
any excess adhesive.
9. Allow the adhesive to cure fully, then
remove the masking tape.
10. Strip 2 mm of insulation from the lead
wires. Place stripped ends of lead wires
onto the solder pads, then use a piece
of cellophane tape to hold lead wires in
place. Place the tape strip as shown in
Figure 5.
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11. Apply a small amount of epoxy to bond the lead
wires to the body of the load cell, as shown in
Figure 5. This serves to provide strain relief such
that movement of the wires will not induce strain(stress) to the strain gage solder pads (after the
wires are soldered to the pads).
12. Make certain the epoxy does not flow along the
lead wires (capillary action) such that it reachesthe solder pads. Allow the epoxy to fully cure
before proceeding to the next step.
13. Gently remove the cellophane tape. Make sure
the lead wire ends remain in contact with the solderpads. Using a toothpick, dab a small amount of
rosin flux onto the solder pads and wire.
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14. Allow the soldering gun to reach operating
temperature. Make sure the gun tip is clean (no
oxide). Apply a small amount of solder onto the
gun tip.15. Briefly touch the gun tip against the wire. Solder
will immediately flow onto the wire and solder pads,
forming a tiny solder mound, as shown in Figure 6
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16. Using an ohmmeter, confirm the resistance
for each gage is 3501 ohm.
17. Using alcohol (not lacquer thinner or
acetone) and a cloth or paper towel, carefully
clean away any excess solder flux.
18. Apply a thin layer of epoxy over the entirestrain gage to provide environmental
protection.