notes experimental stress analysis
TRANSCRIPT
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UNIT 1
MEASUREMENTS
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Measurement is the estimation of themagnitude of some attribute of an object, such
as its length or weight, relative to a unit ofmeasurement. Measurement usually involves using a
measuring instrument, such as a ruler or scale,which is calibrated to compare the object tosome standard, such as a meter or a kilogram.In science, however, where accuratemeasurement is crucial, a measurement isunderstood to have three parts: first, themeasurement itself, second, the margin oferror, and third, the confidence level -- that is,
the probability that the actual property of thephysical object is within the margin of error. For example, we might measure the length of an
object as 2.34 meters plus or minus 0.01 meter,with a 95% level of confidence.
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Accuracy and precision can not be consideredindependently
A number can be accurate and not precise
A number can be precise and not accurate The use of the number determines the
relative need for accuracy and precision
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Accuracy can be defined as how close anumber is to what it should be.
Accuracy is determined by comparing anumber to a known or accepted value.
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Each of these statements is more accurateand more precise than the one before it.
Statement two is more accurate and moreprecise that statement one.
Statement three is more accurate and moreprecise than statement two.
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How long is a piece of string? Johnny measures the string at 2.63 cm.
Using the same ruler, Fred measures the string at1.98 cm.
Who is most precise? Who is most accurate?
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You can tell the precision of a number simplyby looking at it. The number of decimalplaces gives the precision.
Accuracy on the other hand, depends oncomparing a number to a known value.Therefore, you cannot simply look at anumber and tell if it is accurate
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Sensitivity is the study of how thevariation in the output of a model (numericalor otherwise) can be apportioned,
qualitatively or quantitatively, to differentsources of variation.
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Range is used to indicate the differencebetween the largest and smallest measuredvalues or set of data.
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EXTENSOMETERS
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Mechanical Optical Acoustical Electrical
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A thin plastic base supports thin ribbonsof metal, joined in a zig-zag to form onelong electrically conductive strip. Theentire device is typically 10 mm long,with 16 or more parallel metal bands.
When the plastic is stretched the wiresbecome longer, and thinner. Theelectrical resistance therefore increases.
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ELECTRICAL RESISTANCE STRAIN GAUGES
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Electrical Properties of the ResistanceGageR=L AWhere L= Length
= ResistivelyA= Cross sectional area
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Constant Current Circuit
allast Circuit
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The Wheatstone Bridge is the most basic of anumber of useful electrical bridge circuitsthat may be used to measure resistance,capacitance or inductance. It also finds
applications in a number of circuits designedto indicate resistance changes in transducerssuch as resistance thermometers andmoisture gages
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Temperature Effects in the CageFluctuations in ambient and in operating
temperatures produce the most severeeffects generally dealt with in strain
measuring circuitry The problems arise primarily from twomechanisms:
(1) changes in the gage resistivity withtemperature
(2) temperature induced strain in thegage element
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Temperature compensation of the straingage alone does not generally eliminatethermal problems entirely.
Such compensation is rarely exact andthe differences must usually be eliminatedby careful configuration of the WheatstoneBridge circuit. The ability to make such
compensation is, in fact, one of the moredesirable features of this circuit
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The output from a strain gage bridge is
proportional to changes in resistance of all of the arms.
In most situations, only one or two arms are active and
it is desirable to be able to provide some means of
assurance that the circuit is working properly.
The Wheatstone Bridge circuit is ideally suited for
this purpose because it is relatively easy to affect a
change in resistance in one or more arms that isproportional to a known physical parameter.
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Theroy of Photoelasticty
Example 1: Stress Opticon
Example 2: GFP 1000
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Natural
Light
Linear
PolarizerCircular
Polarizer
( wave-
length)
Sample Circular
Polarizer
LinearPolarizer
Analyzer
Observer
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Ether Particles Vibration
Light Vector (Amplitude, Direction,
Phase Angle)
Components
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Nicols Prism:
Double Refraction
o
e
ttii nn sinsin
eo nn
Calcium
Canada
Balsam
Polaroid:
Energy Loss: damper
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When stressed
nl
V
2
V
Wave Length (different for colors)
If no stress
Differentcolorlighthasdifferent
phaseangle, Rainbow appears.
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Colors
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AluminumRing
Wrench
GFP 1000is a strain measurement system based on photoelasticity
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Theory:
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Tension
Compression
Orientation of ellipse measures direction of 1
Ellipticity measures magnitude
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1
2
3
4
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NON DESTRUCTIVE TESTING
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Introduction to NDT Overview of Six MostCommon NDT Methods
Selected Applications
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Visual
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Flaw Detection and Evaluation Leak Detection
Location Determination
Dimensional Measurements
Structure and Microstructure Characterization Estimation of Mechanical and Physical
Properties
Stress (Strain) and Dynamic Response
Measurements Material Sorting and Chemical Composition
Determination
Fluorescent penetrant indication
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To assist in product development
To screen or sort incoming materials
To monitor, improve or control manufacturing
processes To verify proper processing such as heat treating
To verify proper assembly
To inspect for in-service damage
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Visual
Liquid Penetrant
Magnetic
Ultrasonic
Eddy Current
X-ray
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Most basic and common
inspection method.
Tools include
fiberscopes,
borescopes, magnifying
glasses and mirrors.
Robotic crawlers permit
observation in hazardous or
tight areas, such as air
ducts, reactors, pipelines.
Portable video inspection
unit with zoom allows
inspection of large tanks
and vessels, railroad tank
cars, sewer lines.
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A liquid with high surface wetting characteristics
is applied to the surface of the part and allowed
time to seep into surface breaking defects. The excess liquid is removed from the surface
of the part.
A developer (powder) is applied to pull the
trapped penetrant out the defect and spread it
on the surface where it can be seen.
Visual inspection is the final step in the
process. The penetrant used is often loaded
with a fluorescent dye and the inspection is
done under UV light to increase test
sensitivity.
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The part is magnetized. Finely milled iron particles coated with a dyepigment are then applied to the specimen. These particles areattracted to magnetic flux leakage fields and will cluster to form anindication directly over the discontinuity. This indication can bevisually detected under proper lighting conditions.
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The radiation used in radiographytesting is a higher energy (shorter
wavelength) version of theelectromagnetic waves that wesee as visible light. The radiation cancome from an X-ray generator or aradioactive source.
High Electrical Potential
Electrons
-+
X-ray Generator
or Radioactive
Source Creates
Radiation
Exposure Recording Device
Radiation
Penetrate
the Sample
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Top view of developed film
X-ray film
The part is placed between theradiation source and a piece of film.
The part will stop some of the
radiation. Thicker and more dense
area will stop more of the radiation.
= more exposure
= less exposure
The film darkness
(density) will vary with
the amount of radiation
reaching the filmthrough the test object.
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Conductivematerial
CoilCoil'smagnetic field
Eddycurrents
Eddy current'smagnetic field
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Eddy current testing is particularly well suited for detecting surface
cracks but can also be used to make electrical conductivity and
coating thickness measurements. Here a small surface probe isscanned over the part surface in an attempt to detect a crack.
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High frequency sound waves are introduced into amaterial and they are reflected back from surfaces or
flaws.
Reflected sound energy is displayed versus time, andinspector can visualize a cross section of the specimenshowing the depth of features that reflect sound.
f
plate
crack
0 2 4 6 8 10
initial
pulse
crack
echo
back surface
echo
Oscilloscope, or flaw
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Gray scale image produced using
the sound reflected from the front
surface of the coin
Gray scale image produced using the
sound reflected from the back surface
of the coin (inspected from heads side)
High resolution images can be produced by plotting
signal strength or time-of-flight using a computer-
controlled scanning system.
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Inspection of RawProducts
Inspection FollowingSecondary Processing
In-Services DamageInspection
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Forgings, Castings, Extrusions, etc.
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Machining Welding Grinding Heat treating
Plating etc.
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Cracking Corrosion Erosion/Wear Heat Damage etc.
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Probe
Signals produced
by various
amounts ofcorrosion
thinning.
Periodically, power plants are
shutdown for inspection.Inspectors feed eddy current
probes into heat exchanger
tubes to check for corrosion
damage.
Pipe with damage
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Electromagnetic devicesand visual inspections are
used to find broken wiresand other damage to thewire rope that is used inchairlifts, cranes and otherlifting devices.
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Robotic crawlersuse ultrasound toinspect the walls oflarge above groundtanks for signs ofthinning due tocorrosion.
Cameras onlongarticulatingarms are usedto inspectundergroundstorage tanksfor damage.
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Nondestructive testing is usedextensively during the
manufacturing of aircraft. NDT is also used to find cracks
and corrosion damage duringoperation of the aircraft.
A fatigue crack that started atthe site of a lightning strike is
shown below.
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Aircraft engines are overhauledafter being in service for a period
of time. They are completely disassembled,
cleaned, inspected and thenreassembled.
Fluorescent penetrant inspection
is used to check many of the partsfor cracking.
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Sioux City, Iowa, July 19, 1989A defect that went
undetected in anengine disk was
responsible for
the crash of
United Flight 232.
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The failure of a pressure vessel
can result in the rapid release of
a large amount of energy. Toprotect against this dangerous
event, the tanks are inspected
using radiography and
ultrasonic testing.
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Special cars are used to
inspect thousands of milesof rail to find cracks thatcould lead to a derailment.
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The US has 578,000
highway bridges. Corrosion, cracking and
other damage can allaffect a bridgesperformance.
The collapse of the SilverBridge in 1967 resulted inloss of 47 lives.
Bridges get a visualinspection about every 2years.
Some bridges are fittedwith acoustic emissionsensors that listen forsounds of cracks growing.
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NDT is used to inspect pipelinesto prevent leaks that coulddamage the environment. Visualinspection, radiography andelectromagnetic testing are someof the NDT methods used.
Remote visual inspection usinga robotic crawler.
Radiography of weld joints.
Magnetic flux leakage inspection.This device, known as a pig, isplaced in the pipeline and collectsdata on the condition of the pipe as itis pushed along by whatever is beingtransported.
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Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell
for damage using NDT techniques. Eddy current methods were used to measure the
electrical conductivity of the Bell's bronze casing at various points to evaluate its
uniformity.
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