NOTES EXPERIMENTAL STRESS ANALYSIS

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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. www.jdrr5.co.cc

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  

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

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

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EXTENSOMETERS

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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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 Mechanical  Optical

 Acoustical

 Electrical

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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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ELECTRICAL RESISTANCE STRAIN GAUGES

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Electrical Properties of the Resistance Gage R=ρL/A Where L= Length ρ= Resistively A= Cross sectional area 

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Constant Current Circuit



Ballast Circuit

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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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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 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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RS = total resistance of lead wires to gage

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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 is proportional 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 Circular Polarizer Polarizer (¼ wavelength)

Sample

Linear Circular Polarizer Polarizer Analyzer

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Observer

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Ether Particles Vibration Light Vector (Amplitude, Direction, Phase Angle)

Components

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•Polaroid: Energy Loss: damper

e

•Nicol’s Prism:

Calcium

Double Refraction

ni sin  i  nt sin  t no  ne

o Canada Balsam

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Phase Angle: 2

?

If

nl

o  e   

When  o  e  2

Another 1/4

 

V

no  ne

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 2

 

2

V

nl

V Wave Length (different for colors) •Different color light has different phase angle, Rainbow appears. If no stress When stressed www.jdrr5.co.cc

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Colors

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GFP 1000 is a strain measurement system based on photoelasticity

Aluminum Ring

Wrench

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Theory:

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Compression 

 Tension

•Orientation of ellipse measures direction of 1

• Ellipticity measures magnitude

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3 4

2

1

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NON – DESTRUCTIVE TESTING

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 Introduction

to NDT  Overview of Six Most Common NDT Methods  Selected Applications

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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. www.theo7.co.cc

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Visual

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    







Flaw Detection and Evaluation Leak Detection Location Determination Dimensional Measurements Fluorescent penetrant indication Structure and Microstructure Characterization Estimation of Mechanical and Physical Properties Stress (Strain) and Dynamic Response Measurements Material Sorting and Chemical Composition Determination www.jdrr5.co.cc

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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. Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines. Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines. www.jdrr5.co.cc

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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 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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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.

High Electrical Potential Electrons +

-

X-ray Generator or Radioactive Source Creates Radiation

Radiation Penetrate the Sample Exposure Recording Device

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The part is placed between the radiation 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.

X-ray film

The film darkness (density) will vary with the amount of radiation reaching the film through the test object. = less exposure = more exposure

Top view of developed film

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Coil

Coil's magnetic field

Eddy current's magnetic field Eddy currents Conductive material www.jdrr5.co.cc

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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 is scanned over the part surface in an attempt to detect a crack.

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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 f showing the depth of features that reflect sound. initial pulse

crack echo

back surface echo

crack 0

2

4

6

8

Oscilloscope, or flaw detector screen

10

plate

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High resolution images can be produced by plotting signal strength or time-of-flight using a computercontrolled 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 www.jdrr5.co.cc (inspected from “heads” side) www.theo7.co.cc

 Inspection

of Raw

Products  Inspection Following Secondary Processing  In-Services Damage Inspection www.jdrr5.co.cc

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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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Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.

Pipe with damage

Probe Signals produced by various amounts of corrosion thinning.

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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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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. www.jdrr5.co.cc

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• 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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• 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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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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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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Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.

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• The US has 578,000 highway bridges. • Corrosion, cracking and other damage can all affect a bridge’s 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. www.jdrr5.co.cc

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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. www.theo7.co.cc

Radiography of weld joints. www.jdrr5.co.cc

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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