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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