notes experimental stress analysis

8/12/2019 NOTES EXPERIMENTAL STRESS ANALYSIS

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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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