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ST 9122 EXPERIMENTAL TECHNIQUES

AND INSTRUMENTATION

FORCES AND STRAIN MEASUREMENT

Choice of Experimental stress analysis methods, Errors inmeasurements

Strain gauge, principle, types, performance and

uses. Photo elasticity - principle and applications - Hydraulic jacksand pressure gauges Electronic load cells Proving Rings Calibration of Testing Machines Long-term monitoring vibratingwiresensorsFibre optic sensors.

UNIT I

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Choice of Experimental stress analysis methods

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Stress

This is a measure of the internal resistance in a

material to an externally applied load. For

direct compressive or tensile loading the

stress is designated and is defined as:

stress = load Warea A

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Types of stress

Compressive

stress

Compressive loadTensile load

Compressive load

Tensile load

TensileStress

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Errors in measurements

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Some terminology

True valuestandard or reference ofknown value or a theoretical value

Accuracycloseness to the true value

Precision

reproducibility or agreementwith each other for multiple trials

PGCC CHM 103 Sinex

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Types of Errors

Determinate (or Systematic) Sometimes called biasdue to error in one

direction- high or low

Known cause Operator

Calibration of glassware, sensor, or instrument

When determined can be corrected May be of a constant or proportional nature

PGCC CHM 103 Sinex

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Types of Errors continued

Indeterminate (or Random) Cannot be determined (no control over)

Random nature causes both high and lowvalues which will average out

Multiple trials help to minimize

PGCC CHM 103 Sinex

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PGCC CHM 103 Sinex

samplingpreparation

analysis

Representativesample

homogeneous

vs.heterogeneous Loss

Contamination(unwanted addition)

Measurementof Analyte

Calibration ofInstrument orStandardsolutions

How about sampling achocolate chip cookie?

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How do you assess the total error?

One way to assess total error is totreat a reference standard as a sample.

The reference standard would becarried through the entire process tosee how close the results are to thereference value.

PGCC CHM 103 Sinex

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Expressing accuracy and precision

Mean (average)

Percent error

Range Deviation

Standard deviation

Percent coefficient of variation

precision

accuracy

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Strain gauge, principle, types,

performance and uses

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What is Strain?

Strain is the amount of deformation of a body due to an applied

force. More specifically, strain (e) is defined as the fractional change

in length.

, is strain

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What is Strain Gauge?

A strain gauge, a device whose electrical resistance varies inproportion to the amount of strain in the device. The most widelyused gauge is the bonded metallic strain gauge.

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How to Select Strain Gauge?Use Vishay Strain Gauge as Example

1. Grid length (250 = 0.250 inch)

2. Grid pattern and tab geometry (uniaxial, biaxial, three-element rosette, shear pattern)

3. Type of grid alloy and backing (strain gauge series EA, Constantan grid, polyimide backing)

4. Option features: protection, termination (L = pre-attached Leads, E= Encapsulated with

exposed tabs.)5. Strain gauge resistance (350=350 ohms)

6. Self temperature compensation (STC # = 06 for steel substrate)

Gauge selection

http://www.vishay.com/brands/measurements_group/strain_gages/mm.htm

Note : The S-T-C number is the approximate thermal expansion coefficient inppm/ deg F of

the structural material on which the strain gage will display minimumthermal out ut.

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How to Measure from Strain Gauge?

Wheatstone bridge with strain gaugeR3

When Vo is zero, R1*R3=R2*R4

When the strain gauge is strained,R3=R3+R

Vo=

C / IE = , where C = t/2, C is the distance to

the neutral axis. M is the bending moment = PL I is the moment inertia for a

cross section, I= b(t^3)/2 G = gauge factor,

(R/R)/(L/L)The gauge factor for various

alloys,A = 2.05, K = 2.1, D = 3.2, P = 2.0

(Vishay strain gauge)

1

2 3 1 4

s

R RV

R R R R

2 3 2 4 01 3 s

R R R R V

R R G V

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How to Install Strain Gauge?

Objective

Have strain gauge to produce the same amount of strain as the substrate.

Procedure

Align the grid direction with the uniaxial

Solder wires to termination which provides stress relief.

Apply adhesive to substrate and attach strain gauge with clamps and rubber pressure pads.

Cure the adhesive at room temperature or elevated temperature.

What happens to our plastic substrate when cure at elevated temperature?

Tape or secure wires to avoid stress from heavy instrument.

Substrate

WiresTerminationStrain gauge

Adhesive

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ECE 480, Prof. A. Mason

Fiber-optic Temperature Sensor

Sensor operation

small prism-shaped sample of single-crystal undoped GaAs attached to

ends of two optical fibers

light energy absorbed by the GaAs crystal depends on temperature

percentage of received vs. transmitted energy is a function oftemperature

Can be made small enough for biological implantation

GaAs semiconductor temperature probe