EXPERIMENTAL STRESS ANALYSIS

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EXPERIMENTAL STRESS ANALYSIS

• UNIT 1
• MEASUREMENTS

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• Measurement is the estimation of the magnitude of
  some attribute of an object, such as its length
  or weight, relative to a unit of measurement.
• Measurement usually involves using a measuring
  instrument, such as a ruler or scale, which is
  calibrated to compare the object to some
  standard, such as a meter or a kilogram. In
  science, however, where accurate measurement is
  crucial, a measurement is understood to have
  three parts first, the measurement itself,
  second, the margin of error, and third, the
  confidence level -- that is, the probability that
  the actual property of the physical 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

• Accuracy and precision can not be considered
  independently
• 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

• Accuracy can be defined as how close a number is
  to what it should be.
• Accuracy is determined by comparing a number to
  a known or accepted value.

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PRECISION

• The number of decimal places assigned to the
  measured number
• It is sometimes defined as reproducibility

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Accuracy vs. Precision for Example 1

• Each of these statements is more accurate and
  more precise than the one before it.
• Statement two is more accurate and more precise
  that statement one.
• Statement three is more accurate and more precise
  than statement two.

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Example 2

• How long is a piece of string?
• Johnny measures the string at 2.63 cm.
• Using the same ruler, Fred measures the string
  at 1.98 cm.
• Who is most precise?
• Who is most accurate?

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ACCURACY/PRECISION

• You can tell the precision of a number simply by
  looking at it. The number of decimal places
  gives the precision.
• Accuracy on the other hand, depends on comparing
  a number to a known value. Therefore, you cannot
  simply look at a number and tell if it is accurate

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SENSITIVITY

• Sensitivity is the study of how the variation in
  the output of a model (numerical or otherwise)
  can be apportioned, qualitatively or
  quantitatively, to different sources of
  variation.

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RANGE

• Range is used to indicate the difference between
  the largest and smallest measured values or set
  of data.

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UNIT 2

• EXTENSOMETERS

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INTRODUCTION

• Extensometer is a device that is used to measure
  small/big changes in the length of an object. It
  is useful for stress-strain measurements. Its
  name comes from "extension-meter". It was
  invented by Dr. Charles Huston who described it
  in an article in the Journal of the Franklin
  Institute in 1879. Huston later gave the rights
  to Fairbanks Ewing, a major manufacturer of
  testing machines and scales

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TYPES OF EXTENSOMETERS

• Mechanical
• Optical
• Acoustical
• Electrical

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MECHANICAL EXTENSOMETER
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OPTICAL EXTENSOMETER
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Acoustical Extensometer
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Electrical Extensometer
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Electrical Extensometer

• A thin plastic base supports thin ribbons of
  metal, joined in a zig-zag to form one long
  electrically conductive strip. The entire device
  is typically 10 mm long, with 16 or more parallel
  metal bands. When the plastic is stretched the
  wires become longer, and thinner. The electrical
  resistance therefore increases.

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Unit - III

• ELECTRICAL RESISTANCE STRAIN GAUGES

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Electrical Resistance Strain Gage Circuits

• Electrical Properties of the Resistance Gage
• R?L/A
• Where L Length
• ? Resistively
• A Cross sectional area

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Resistance Measuring Circuits

• Constant Current Circuit
• Ballast Circuit

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Wheatstone Bridge

• The Wheatstone Bridge is the most basic of a
  number of useful electrical bridge circuits that
  may be used to measure resistance, capacitance or
  inductance. It also finds applications in a
  number of circuits designed to indicate
  resistance changes in transducers such as
  resistance thermometers and moisture gages

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Wheatstone Bridge circuit diagram
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Wheatstone Bridge Circuit Considerations

• Temperature Effects in the Cage
• Fluctuations in ambient and in operating
  temperatures produce the most severe effects
  generally dealt with in strain measuring
  circuitry
• The problems arise primarily from two mechanisms
  
• (1) changes in the gage resistivity with
  temperature
• (2) temperature induced strain in the gage
  element

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Temperature Compensation in the Bridge Circuit

• Temperature compensation of the strain gage alone
  does not generally eliminate thermal problems
  entirely.
• Such compensation is rarely exact and the
  differences must usually be eliminated by careful
  configuration of the Wheatstone Bridge circuit.
  The ability to make such compensation is, in
  fact, one of the more desirable features of this
  circuit

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Half Bridge Configuration
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Quarter Bridge Configuration
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Lead wire Temperature/Resistance Compensation

• RS total resistance of lead wires to gage

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BRIDGE BALANCING
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CALIBRATION

• 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 is proportional to a known physical
  parameter.

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Calibration Circuits
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Unit IVPhotoelasticity
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Outline

• Theroy of Photoelasticty
• Example 1 Stress Opticon
• Example 2 GFP 1000

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Stress Opticon
Linear Polarizer Analyzer
Natural Light
Linear Polarizer
Circular Polarizer (¼ wave-length)
Circular Polarizer
Sample
Observer
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Natural Light
Ether Particles Vibration
Light Vector (Amplitude, Direction, Phase Angle)
Components
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Plane Polarizer

• Polaroid
• Energy Loss damper

e

• Nicols Prism
• Double Refraction

Calcium
o
Canada Balsam
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Circular Polarizer
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Circular Polarizer
Phase Angle
?
If
When
Another 1/4
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Colors
Wave Length (different for colors)

• Different color light has different phase angle,
  Rainbow appears.

If no stress
When stressed
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ReviewStress Opticon
Colors
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GFP 1000--Grey field polariscope
GFP 1000 is a strain measurement system based on
photoelasticity
Aluminum Ring
Wrench
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GFP 1000
Theory
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GFP 1000

• Orientation of ellipse measures direction of e1
• Ellipticity measures magnitude

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GFP 1000
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Coating
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Unit - V

• NON DESTRUCTIVE TESTING

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Introduction to Nondestructive Testing
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Outline

• Introduction to NDT
• Overview of Six Most Common NDT Methods
• Selected Applications

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Definition of NDT
The use of noninvasive techniques to determine
the integrity of a material, component or
structure or quantitatively measure some
characteristic of an object.
i.e. Inspect or measure without doing harm.
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Methods of NDT

• Visual

Thermography
Microwave
Magnetic Particle
Tap Testing
X-ray
Acoustic Microscopy
Acoustic Emission
Liquid Penetrant
Magnetic Measurements
Replication
Ultrasonic
Eddy Current
Laser Interferometry
Flux Leakage
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What are Some Uses of NDE Methods?

• 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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When are NDE Methods Used?
There are NDE application at almost any stage in
the production or life cycle of a component.

• 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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Six Most Common NDT Methods

• Visual
• Liquid Penetrant
• Magnetic
• Ultrasonic
• Eddy Current
• X-ray

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Visual Inspection
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Liquid Penetrant Inspection

• The excess liquid is removed from the surface of
  the part.

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Magnetic Particle Inspection

• The part is magnetized. Finely milled iron
  particles coated with a dye pigment are then
  applied to the specimen. These particles are
  attracted to magnetic flux leakage fields and
  will cluster to form an indication directly over
  the discontinuity. This indication can be
  visually detected under proper lighting
  conditions.

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Magnetic Particle Crack Indications
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Radiography
The radiation used in radiography testing is a
higher energy (shorter wavelength) version of the
electromagnetic waves that we see as visible
light. The radiation can come from an X-ray
generator or a radioactive source.
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Film Radiography
The film darkness (density) will vary with the
amount of radiation reaching the film through the
test object.
X-ray film
Top view of developed film
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Radiographic Images
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Eddy Current Testing
Coil
Conductive material
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Eddy Current Testing
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 is scanned over the part surface in an
attempt to detect a crack.
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Ultrasonic Inspection (Pulse-Echo)
High frequency sound waves are introduced into a
material and they are reflected back from
surfaces or flaws. Reflected sound energy is
displayed versus time, and inspector can
visualize a cross section of the specimen showing
the depth of features that reflect sound.
f
initial pulse
back surface echo
crack echo
crack
plate
Oscilloscope, or flaw detector screen
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Ultrasonic Imaging
High resolution images can be produced by
plotting signal strength or time-of-flight using
a computer-controlled scanning system.
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)
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Common Application of NDT

• Inspection of Raw Products
• Inspection Following Secondary Processing
• In-Services Damage Inspection

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Inspection of Raw Products

• Forgings,
• Castings,
• Extrusions,
• etc.

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Inspection Following Secondary Processing

• Machining
• Welding
• Grinding
• Heat treating
• Plating
• etc.

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Inspection For In-Service Damage

• Cracking
• Corrosion
• Erosion/Wear
• Heat Damage
• etc.

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Power Plant Inspection
Periodically, power plants are shutdown for
inspection. Inspectors feed eddy current probes
into heat exchanger tubes to check for corrosion
damage.
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Wire Rope Inspection
Electromagnetic devices and visual inspections
are used to find broken wires and other damage to
the wire rope that is used in chairlifts, cranes
and other lifting devices.
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Storage Tank Inspection
Robotic crawlers use ultrasound to inspect the
walls of large above ground tanks for signs of
thinning due to corrosion.
Cameras on long articulating arms are used to
inspect underground storage tanks for damage.
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Aircraft Inspection

• Nondestructive testing is used extensively during
  the manufacturing of aircraft.
• NDT is also used to find cracks and corrosion
  damage during operation of the aircraft.
• A fatigue crack that started at the site of a
  lightning strike is shown below.

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Jet Engine Inspection

• Aircraft engines are overhauled after being in
  service for a period of time.
• They are completely disassembled, cleaned,
  inspected and then reassembled.
• Fluorescent penetrant inspection is used to check
  many of the parts for cracking.

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Crash of United Flight 232
Sioux City, Iowa, July 19, 1989
A defect that went undetected in an engine disk
was responsible for the crash of United Flight
232.
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Pressure Vessel Inspection
The failure of a pressure vessel can result in
the rapid release of a large amount of energy.
To protect against this dangerous event, the
tanks are inspected using radiography and
ultrasonic testing.
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Rail Inspection
Special cars are used to inspect thousands of
miles of rail to find cracks that could lead to a
derailment.
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Bridge Inspection

• The US has 578,000 highway bridges.
• Corrosion, cracking and other damage can all
  affect a bridges performance.
• The collapse of the Silver Bridge in 1967
  resulted in loss of 47 lives.
• Bridges get a visual inspection about every 2
  years.
• Some bridges are fitted with acoustic emission
  sensors that listen for sounds of cracks
  growing.

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Pipeline Inspection
NDT is used to inspect pipelines to prevent leaks
that could damage the environment. Visual
inspection, radiography and electromagnetic
testing are some of the NDT methods used.
Remote visual inspection using a robotic crawler.
Magnetic flux leakage inspection. This device,
known as a pig, is placed in the pipeline and
collects data on the condition of the pipe as it
is pushed along by whatever is being transported.
Radiography of weld joints.
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Special Measurements

• 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.