Measurements of Stress and Strain

7/27/2019 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.

Measurements of Stress and Strain

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