Non-Destructive Testing (CHAPTER 6)

8/13/2019 Non-Destructive Testing (CHAPTER 6)

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Non-Destructive Testing

allow parts and material to be inspected andmeasured w/o damaging them

Allow inspection w/o interfering with aproducts final use


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Six Most Common NDT Methods

Visual

Liquid Penetrant

Magnetic Ultrasonic

Eddy Current

X-ray


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Visual Inspection

Basic principle illuminate the test specimen with light Examine the specimen with the eye

Used to

To magnify defects which can not be detectedby the unaided eye

To assist in the inspection of defects To permit visual checks of areas not accessible

to unaided eye.

Most widely used of all the NDT tests Simple easy to apply, quickly carried out

and usually low in cost


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Visual Inspection Equipment Magnifying glass

Magnifying mirror

Microscope

Borescope Endoscope or endoprobes

Felxible Fiber Optic Borescope Working length are normally 60 to 365 cm with

diameters from 3-12.5 mm

Video Imagescope


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Borescope

Rigid Flexible fiber optic


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Robotic crawlers permit observation inhazardous or tight areas, such as airducts, reactors, pipelines.


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X-ray or Radiography Testing

This method consists in passing a mixedbeam of x-rays through the object andstudying the shadow formed either on

fluorescent screen or photographic film

Any imperfection will absorb the rays lessbecause of less density

The position of imperfection is marked onthe film by dark areas


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X-ray or Radiography Testing

The radiation used inradiography testing isa higher energy(shorter wavelength)

version of theelectromagneticwaves that we see asvisible light. Theradiation can comefrom an X-raygenerator or aradioactive source.


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Film Radiography

Top view of developed film

X-ray film

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.

= more exposure

= less exposure

The film darkness

(density) will vary with

the amount of radiation

reaching the film throughthe test object.


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X-Rays Images


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Liquid penetrant inspection

extension of visual inspection

for detection of surface flaws

two kinds ; dye penetrant and

fluorescent penetrants


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Liquid penetrant inspection is amethod that is used to reveal

surface breaking flaws bybleedout of a colored orfluorescent dye from the flaw.

Liquid Penetrant Testing


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The technique is based on the ability of

a liquid to be drawn into a "clean"surface breaking flaw by capillary action.

Fluorescentpenetrant onsurface seeps into

the crack.

PenetrantPenetration


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Emulsifies is applied to the penetrant. Emulsifier mixes

with penetrant on the surface, but penetrant in thecrack is not emulsified. Emulsifier makes the mixturewashable .


Penetrant only

Emulsifier and penetran

Emulsification


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Water spray removes the emulsified penetrant. Waterremoves the surface film as well.


Penetrant only

water

Rinse

Li id P t t T ti


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A developer is applied to pull the trapped penetrant out thedefect and spread it on the surface where it can be seen.


Penetrant

Development

developer


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Black light causes the penetrant to glow in

dark.


Penetrant

Inspection

Developer

Black Light


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Ultrasonic Testing

The propagation of sound waves throughsolid materials have been used to detecthidden crack, voids, porosity and other

internal discontinuities.

High frequency sound waves reflect fromflaws in predictable ways, producingdistinctive echo patterns that can bedisplayed and recorded by portableinstruments


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f

plate

crack

0 2 4 6 8 10

initial

pulse

crack

echo

back surface

echo

Oscilloscope, or flaw

detector screen

Ultrasonic testing

We can measure the distance/depthfor flaw in the sample by measuringthe echo distance from the initial


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Flaw size can also be interpreted

It is used on aircraft, the power stationsgenerating plant, or welds in pressure vessels atan oil refinery or paper mill.

technician training and skill is required.


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Eddy current

When an energized coil is broughtnear to the surface of a metal orconducting component, eddy current

set up a magnetic field that tend tooppose the original magnetic field.The impedance of coil in close

proximity to the specimen is affectedby the presence of the induced eddycurrents in the specimen


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when the eddy currents in the

specimen are distorted by thepresence of the flaws or materialvariations, the impedance in the coilis altered. This changes is measuredand displayed in a manner thatindicates the types of flaw ormaterial condition


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Eddy Current Testing

Conductivematerial

CoilCoil'smagnetic field

Eddycurrents

Eddy current'smagnetic field


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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 electricalconductivity 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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Application

Detect defect under coated material

Detect defect that are so small

To check welds

Application inside tube such as in heatexchanger

Detect surface crack, pit, subsurface

crack, corrosion inner surface


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Magnetic Particle Testing

Magnetic particle inspection is amethod that can be used to find

surface and near surface flaws inferromagnetic materials such assteel and iron.


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The technique uses the principle thatmagnetic lines of force (flux) will be

distorted by the presence of a flaw in a

manner that will reveal it's presence.

S N S NN S

IRON POWDER

CRACK


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N S

IRON POWDER

CRACK

COIL

(COMPLETELY

CLOSED MAGNET)

M ti P ti l T ti


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The flaw (for example, a crack) is located from

the "flux leakage", following the application offine iron particles, to the area under examinatioThere are variations in the way the magnetic fielis applied.

N S

IRON POWDER

S N

N S

IRON POWDER

CRACK

CRACK


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The iron particles can be applied dry or wet;suspended in a liquid, colored or fluorescent.While magnetic particle inspection is primarily

used to find surface breaking flaws, it can also beused to locate sub-surface flaws. But it'seffectiveness quickly diminishes depending on theflaw depth and type.



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Surface irregularities and scratches can givemisleading indications. Therefore it is necessary toensure careful preparation of the surface before

magnetic particle testing is undertaken.


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Advantages and limitation

Advantages Simplicity of operation and application Complex shapes can be tested Detect small fine crack

Low cost

Disadvatages Restricted to ferromagnetic materials Restricted to surface or near surface flaws Required cleaning process Need more than two magnetization in order to

inspect complex shape.


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Common Application of

NDT

Inspection of RawProducts

Inspection FollowingSecondary Processing

In-Services DamageInspection


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Inspection of Raw Products

Forgings,Castings,Extrusions,etc.

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MachiningWeldingGrindingHeattreatingPlatingetc.

Inspection Following

Secondary Processing


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Cracking

CorrosionErosion/Wear

Heat Damage

etc.

Inspection For

In-Service Damage


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Power Plant Inspection

Probe

Signals produced

by various

amounts of

corrosion

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

Wi R I ti


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Wire Rope InspectionElectromagnetic devices andvisual inspections are used to

find broken wires and otherdamage to the wire rope thatis used in chairlifts, cranes andother lifting devices.

St T k I ti


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Storage Tank InspectionRobotic crawlers useultrasound to inspectthe walls of largeabove ground tanksfor signs of thinningdue to corrosion.

Cameras onlongarticulating

arms are usedto inspectundergroundstorage tanksfor damage.

Ai ft I ti


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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 duringoperation of the aircraft.

A fatigue crack that started at thesite of a lightning strike is shown

below.


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Jet Engine InspectionAircraft engines are overhauled after

being in service for a period of time.

They are completely disassembled,cleaned, inspected and thenreassembled.

Fluorescent penetrant inspection isused to check many of the parts for

cracking.

C h f U it d Fli ht 232


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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.

Crash of United Flight 232

Press re Vessel Inspection


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Pressure Vessel InspectionThe failure of a pressure vessel

can result in the rapid release of a

large amount of energy. To protectagainst this dangerous event, the

tanks are inspected using

radiography and ultrasonic testing.

Rail Inspection


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Rail Inspection

Special cars are used to

inspect thousands of miles ofrail to find cracks that couldlead to a derailment.


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Bridge Inspection Corrosion, cracking and

other damage can allaffect a bridgesperformance.

Bridges get a visualinspection about every 2years.

Some bridges are fittedwith acoustic emissionsensors that listen forsounds of cracks growing.

Pipeline Inspection


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NDT is used to inspect pipelines toprevent leaks that could damage the

environment. Visual inspection,radiography and electromagnetictesting are some of the NDTmethods 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.

Pipeline Inspection

Special Measurements


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Special Measurements

Boeing employees in Philadelphia were given the privilegeof evaluating the Liberty Bell for damage using NDT

techniques. Eddy current methods were used to measurethe electrical conductivity of the Bell's bronze casing atvarious points to evaluate its uniformity.

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Non-Destructive Testing (CHAPTER 6)