ndt full course

8/10/2019 Ndt Full Course

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Introduction to Non-destructive TestingCourse Daily Schedule

Day & Date Session ProgrammedSunday04, 12, 05

Session I08 - 9.30

Introduction to NDT processes & their Uses

9.30 10.00 Tea breakSession II

10.00 11.30Penetrant Testing Method, Theory.

11.30 12.15 Salah & Lunch breakSession III

12.15 2.00Penetrant Testing Method, Practical

Monday05, 12, 05

Session I08 - 9.30

Identification of weld discontinuities

9.30 10.00 Tea break

Session II10.00 11.30 Magnetic Particle Testing Theory


12.15 2.00 Magnetic Particle Testing Practical

Tuesday06, 12, 05

Session I08 - 9.30

Ultrasonic Testing Theory


10.00 11.30Ultrasonic Testing Theory continued


12.15 2.00

Ultrasonic Testing Practical.

Wednesday07, 12, 05

Session I08 - 9.30

Radiographic Testing Theory.


10.00 11.30Radiographic Testing Theory continued


12.15 2.00 Radiographic Interpretation, practical.

Thursday08, 12, 05

Session I08 - 9.30

Eddy Current Testing Theory

9.30 10.00 Tea break

Session II10.00 11.30 Comparison and Selection of NDT Methods.


12.15 2.00 Eddy Current Testing & other methodPractical

Valedictory Function.


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Table of Contents

ChapterNo:

Name of the Chapter PageNo

1 Course daily schedule 1

2 Course Contents 2

3 Introduction NDT processes & their Uses 3 - 11

4 Identification of weld Discontinuities 12 - 20

5 Penetrant Testing 21- 30

6 Magnetic Particle Testing 31 48

7 Ultrasonic Testing 49 -60

8 Radiographic Testing 61 - 77

9 Eddy Current Testing 78 - 80

10 Comparison and Selection of NDT Methods 81


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Chapter I

INTRODUCTION

Nondestructive Testing

The field of Nondestructive Testing (NDT) is a very broad, that plays a criticalrole in assuring that structural components and systems perform their function ina reliable and cost effective fashion. NDT technicians and engineers define andimplement tests that locate and characterize material conditions and flaws thatmight otherwise cause serious accidents such as, planes to crash, reactors tofail, trains to derail, pipelines to burst, and a variety of troubling events.

These tests are performed in a manner that does not affect the future usefulnessof the object or material. In other words, NDT allows parts and materials to beinspected and evaluated without damaging them. Because it allows inspection

without interfering with a product's final use, NDT provides an excellent balancebetween quality control and cost-effectiveness.

Nondestructive Evaluation

Nondestructive Evaluation (NDE) is a term that is often used interchangeablywith NDT. However, technically, NDE is used to describe measurements that aremore quantitative in nature. For example, a NDE method would not only locate adefect, but it would also be used to measure something about that defect such asits size, shape, and orientation. NDE may be used to determine materialproperties such as fracture toughness, ductility, conductivity and other physical

characteristics.

Uses of NDE

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


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Background on Nondestructive Testing (NDT)

Nondestructive testing has been practiced for many decades. One of the earliestapplications was the detection of surface cracks in railcar wheels and axles. Theparts were dipped in oil, then cleaned and dusted with a powder. When a crack

was present, the oil would seep from the defect and wet the oil providing visualindication indicating that the component was flawed. This eventually led to oilsthat were specifically formulated for performing these and other inspections andthese inspection techniques are now called penetrant testing.

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) whowas a Professor at Wuerzburg University in Germany. Soon after his discovery,Roentgen produced the first industrial radiograph when he imaged a set ofweights in a box to show his colleagues. Other electronic inspection techniquessuch as ultrasonic and eddy current testing started with the initial rapiddevelopments in instrumentation spurred by technological advances and

subsequent defense and space efforts following World War II. In the early days,the primary purpose was the detection of defects. Critical parts were producedwith a "safe life" design, and were intended to be defect free during their usefullife. The detection of defects was automatically a cause for removal of thecomponent from service.

The continued improvement of inspection technology, in particular the ability todetect smaller and smaller flaws, led to more and more parts being rejected. Atthis time the discipline of fracture mechanics emerged, which enabled one topredict whether a crack of a given size would fail under a particular load if aparticular material property or fracture toughness, were known. Other laws were

developed to predict the rate of growth of cracks under cyclic loading (fatigue).With the advent of these tools, it became possible to accept structures containingdefects if the sizes of those defects were known. This formed the basis for a newdesign philosophy called "damage tolerant designs." Components having knowndefects could continue to be used as long as it could be established that thosedefects would not grow to a critical size that would result in catastrophic failure. Anew challenge was thus presented to the nondestructive testing community.

Mere detection of flaws was not enough. One needed to also obtain quantitativeinformation about flaw size to serve as an input to fracture mechanicscalculations to predict the remaining life of a component. These needs, led to the

creation of a number of research programs around the world and the emergenceof nondestructive evaluation (NDE) as a new discipline.


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NDT/NDE Methods

The list of NDT methods that can be used to inspect components and makemeasurements is large and continues to grow. Researchers continue to find newways of applying physics and other scientific disciplines to develop better NDTmethods. However, there are six NDT methods that are used most often. Thesemethods are Visual Inspection, Penetrant Testing, Magnetic Particle Testing,

Electromagnetic or Eddy Current Testing, Radiography, and Ultrasonic Testing.

Visual and Optical Testing (VT)

Visual inspection involves using an inspector's eyes to look for defects. Theinspector may also use special tools such as magnifying glasses, mirrors, orborescopes to gain access and more closely inspect the subject area. Visualexaminers follow procedures that range fm simple to very complex.

Penetrant Testing (PT)

Test objects are coated with visible or fluorescent dye solution. Excess dye isthen removed from the surface, and a developer is applied. The developer actsas blotter, drawing trapped penetrant out of imperfections open to the surface.With visible dyes, vivid color contrasts between the penetrant and developermake "bleedout" easy to see. With fluorescent dyes, ultraviolet light is used tomake the bleedout fluoresce brightly, thus allowing imperfections to be readilyseen.


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Magnetic Particle Testing (MT)

This method is accomplished by inducing a magnetic field in a ferromagneticmaterial and then dusting the surface with iron particles (either dry or suspendedin liquid). Surface and near-surface imperfections distort the magnetic field andconcentrate iron particles near imperfections, previewing a visual indication of theflaw.

Electromagnetic Testing (ET) or Eddy Current Testing

Electrical currents are generated in a conductive material by an inducedalternating magnetic field This electrical currents is called eddy currents becausethey flow in circles at and just below the surface of the material. Interruptions inthe flow of eddy currents, caused by imperfections, dimensional changes, orchanges in the material's conductive and permeability properties, are detected.


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Radiography (RT)

Radiography involves the use of penetrating gamma or X-radiation to examineparts and products for imperfections. An X-ray generator or radioactive isotope isused as a source of radiation. Radiation is directed through a part and onto filmor other imaging media. The resulting radiograph shows the dimensional featuresof the part. Possible imperfections are indicated as density changes on the film in

the same manner as a medical X-ray shows broken bones.

Ultrasonic Testing (UT)

Ultrasonics use transmission of high-frequency sound waves into a material todetect imperfections or to locate changes in material properties. The mostcommonly used ultrasonic testing technique is pulse echo, wherein sound isintroduced into a test object and reflections (echoes) are returned to a receiverfrom internal imperfections or from the part's geometrical surfaces

.

crack

0 2 4 6 8 1

Initialpulse

Crackecho

Back surfaceecho

Soundwaves

X-ray film

Source

Rays

Ob ect with defect

Film

Defect Image Film with image

Probe

Couplant

PlateScreen


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TestMethod

UT X-ray EddyCurrent

MPI LPT

Capital cost Medium tohigh

High Low tomedium

Medium Low

Consumablecost

Very low High Low Medium Medium

Time ofresults

Immediate Delayed Immediate Shortdelay

Shortdelay

Effect ofgeometry

Important Important Important Not tooImportant

Not tooImportant

Accessproblems Important Important Important Important Important

Type ofdefect

Internal Most External ExternalNearSurface

Surfacebreaking

Relativesensitivity

High Medium High Low Low

Operator

skill

High High Medium Low Low

Operatortraining

Important Important Important Important NotImportant

Trainingneeds

High High Medium Low Low

Portability ofequipment

High Low High tomedium

High tomedium

High

Capabilities Thicknessgauging,compositiontesting

Thicknessgauging Thicknessgauging,gradesorting

Defectsonly Defectsonly

Table 1 - Reference Guide to Major Methods for the NondestructiveExamination of Welds

The Relative Uses and Merits of Various NDT Methods


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InspectionMethod

EquipmentRequired

EnablesDetectiort of

Advantages Limitations Remarks

Visual MagnifyingglassWeld sizinggaugePocket ruleStraight edgeWorkmanshipstandards

Surface flaws -cracks,

porosity,unfilledcraters, slaginclusionsWarpage,underwelding,overwelding,

poorly formed beads,misalignments,improper fitup

Low cost.Can be appliedwhile work isin process,

permittingcorrection offaults.Givesindication ofincorrect

procedures.

Applicableto surfacedefects only.Provides no

permanentrecord.

Shouldalways be the

primarymethod ofinspection, nomatter whatothertechniques arerequired.Is the only"productive"type ofinspection.Is thenecessaryfunction ofeveryone whoin any waycontributes tothe making ofthe weld.

Radiographic CommercialX-ray orgamma unitsmadeespecially for

inspectingwelds,castings andforgings.Film and

processingfacilities.Fluoroscopicviewingequipment.

Interiormacroscopicflaws - cracks,

porosity, blowholes,

nonmetallicinclusions,incompleteroot

penetration,undercutting,icicles, and

burnthrough.

When theindications arerecorded onfilm, gives a

permanent

record.When viewedon afluoroscopicscreen, a low-cost method ofinternalinspection

Requiresskill inchoosingangles ofexposure,

operatingequipment,andinterpretingindications.Requiressafety

precautions. Notgenerallysuitable for

fillet weldinspection.

X-rayinspection isrequired bymany codesand

specifications.Useful inqualificationof weldersand welding

processes.Because ofcost, its useshould belimited tothose areas

where othermethods willnot providethe assurancerequired.

MagneticParticle

Specialcommercialequipment.

Excellent fordetectingsurface

Simpler touse thanradiographic

Applicable toferromagneticmaterials only.

Elongateddefects parallelto the magnetic


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Magnetic powders - dryor wet form;may befluorescentfor viewingunderultravioletlight.

discontinuities-especiallysurface cracks.

inspection.Permitscontrolledsensitivity.Relativelylow-costmethod.

Requires skillininterpretationof indicationsandrecognition ofirrelevant

patterns.Difficult to useon roughsurfaces.

field may notgive pattern;for this reasonthe field should

be appliedfrom twodirections at ornear rightangles to eachother.

LiquidPenetrant

Commercialkitscontainingfluorescent ordye penetrantsanddevelopers.Applicationequipment forthe developer.A source ofultravioletlight - iffluorescentmethod isused.

Surface cracksnot readilyvisible to theunaided eye.Excellent forlocating leaksin weldments.

Applicable tomagnetic andnonmagneticmaterials.Easy to use.Low cost.

Only surfacedefects aredetectable.Cannot beusedeffectively onhot assemblies.

In thin-walledvessels willreveal leaks notordinarilylocated byusual air tests.irrelevantsurfaceconditions(smoke, slag)may givemisleadingindications.

Ultrasonic Special

commercialequipment,either of the

pulse-echo ortransmissiontype.Standardreference

patterns forinterpretationof RF or

video patterns.

Surface and

subsurfaceflaws includingthose too smallto be detected

by othermethods.Especially fordetectingsubsurfacelamination-likedefects.

Very

sensitive.Permits probing of jointsinaccessibletoradiography.

Requires high

degree of skillin interpreting pulse-echo patterns.Permanentrecord is notreadilyobtained.

Pulse-echo

equipment ishighlydeveloped forweld inspection

purposes.Thetransmission-type equipmentsimplifies

patterninterpretation

where it isapplicable.


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Chapter II

IDENTIFICATION OF WELD DISCONTINUITIES

Discontinuities are interruptions in the typical structure of a material. These

interruptions may occur in the base metal, weld material or "heat affected" zones.Discontinuities, which do not meet the requirements of the codes or specificationused to invoke and control an inspection, are referred to as defects.

General Welding DiscontinuitiesThe following discontinuities are typical of all types of welding .Cracks:Crack is tight linear separations of metal that can be very short to very longindications. Cracks are grouped as hot or cold cracks. Hot cracks usually occuras the metal solidifies at elevated temperatures. Cold cracks occur after themetal has cooled to ambient temperatures ( delayed cracks).Cracks can be detected in a radiograph only when they are propagating in adirection that produces a change in thickness that is parallel to the x-ray beam.Cracks will appear as jagged and often very faint irregular lines. Cracks cansometimes appear as "tails" on inclusions or porosity.

Lack of Fusion:Lack of fusion (Cold Lap) is a condition where the weld filler metal does notproperly fuse with the base metal or the previous weld pass material (inter pass


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cold lap). The arc does not melt the base metal sufficiently and causes theslightly molten puddle to flow into base material without bonding.

Porosi ty: Porosity is the result of gas entrapment in the solidifying metal. Porosity can takemany shapes on a radiograph but often appears as dark round or irregular spotsor specks appearing singularly, in clusters or rows. Sometimes porosity iselongated and may have the appearance of having a tail This is the result of gasattempting to escape while the metal is still in a liquid state and is called


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wormhole porosity. All porosity is a void in the material it will have a radiographicdensity more than the surrounding area.

Cluster porosity:Cluster porosity is caused when flux coated electrodes are contaminated withmoisture. The moisture turns into gases when heated and becomes trapped inthe weld during the welding process. Cluster porosity appear just like regular

porosity in the radiograph but the indications will be grouped closetogether.

Slag inclusions:Slag inclusions are nonmetallic solid material entrapped in weld metal orbetween weld and base metal. In a radiograph, dark, jagged asymmetricalshapes within the weld or along the weld joint areas are indicative of slaginclusions.


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Incomplete penetration IP):Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weldmetal fails to penetrate the joint. It is one of the most objectionable weld

discontinuities. Lack of penetration allows a natural stress riser from which acrack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of theweldment.

Root concavity:Root or Internal concavity or suck back is condition where the weld metal hascontracted as it cools and has been drawn up into the root of the weld. On aradiograph it looks similar to lack of penetration but the line has irregular edgesand it is often quite wide in the center of the weld image.


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Internal or root undercut:Internal or root undercut is an erosion of the base metal next to the root of theweld. In the radiographic image it appears as a dark irregular line offset from thecenterline of the weldment. Undercutting is not as straight edged as LOP

because it does not follow a ground edge.

External or crown undercut:

External or crown undercut is an erosion of the base metal next to the crown ofthe weld. In the radiograph, it appears as a dark irregular line along the outsideedge of the weld area.


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Offset or mismatch:Offset or mismatch are terms associated with a condition where two pieces beingwelded together are not properly aligned. The radiographic image is a noticeable

difference in density between the two pieces. The difference in density is causedby the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.

Inadequate weld reinforcement:Inadequate weld reinforcement is an area of a weld where the thickness of weld

metal deposited is less than the thickness of the base material. It is very easy todetermine by radiograph if the weld has inadequate reinforcement, because theimage density in the area of suspected inadequacy will be more (darker) than theimage density of the surrounding base material.


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Excess weld reinforcement :Excess weld reinforcement is an area of a weld that has weld metal added inexcess of that specified by engineering drawings and codes. The appearance ona radiograph is a localized, lighter area in the weld. A visual inspection will easilydetermine if the weld reinforcement is in excess of that specified by the

engineering requirements.

Discontinuities in TIG weldsThe following discontinuities are peculiar to the TIG welding process. Thesediscontinuities occur in most metals welded by the process including aluminumand stainless steels. The TIG method of welding produces a clean homogeneousweld which when radiographed is easily interpreted.

Tungsten inclusions.Tungsten is a brittle and inherently dense material used in the electrode intungsten inert gas ( TIG ) welding. If improper welding procedures are used,tungsten may be entrapped in the weld. Radiographically, tungsten is denser


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than aluminum or steel; therefore, it shows as a lighter area with a distinct outlineon the radiograph.

Oxide inclusions:Oxide inclusions are usually visible on the surface of material being welded(especially aluminum). Oxide inclusions are less dense than the surroundingmaterials and, therefore, appear as dark irregularly shaped discontinuities in theradiograph.

Discontinuities in Gas Metal Arc Welds GMAW)The following discontinuities are most commonly found in GMAW welds.

Whiskers:Whiskers are short lengths of weld electrode wire, visible on the top or bottomsurface of the weld or contained within the weld. On a radiograph they appear aslight, "wire like" indications.

Burn-Through:Burn-Through results when too much heat causes excessive weld metal topenetrate the weld zone. Often lumps of metal sag through the weld creating athick globular condition on the back of the weld. These globs of metal arereferred to as icicles. On a radiograph, burn through appears as dark spots,which are often surrounded by light globular areas (icicles ).


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Chapter III

PENETRANT INSPECTION

Introduction

Liquid penetration inspection is a method that is used to reveal surface breakingflaws by bleedout of a colored or fluorescent dye from the flaw. The technique is

based on the ability of a liquid to be drawn into a "clean" surface breaking flaw bycapillary action. After a period of time called the "dwell," excess surfacepenetrant is removed and a developer is applied. This acts as a "blotter." It drawsthe penetrant from the flaw to reveal its presence.

Colored (contrast) penetrants require good white light while fluorescentpenetrants need to be viwed in darkened conditions with an ultraviolet "blacklight".


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A very early surface inspection technique involved the rubbing of carbon black onglazed pottery, whereby the carbon black would settle in surface cracksrendering them visible. Later it became the practice in railway workshops toexamine iron and steel components by the "oil and whiting" method. In thismethod, heavy oil commonly available in railway workshops was diluted withkerosene in large tanks so that locomotive parts such as wheels could besubmerged. After removal and careful cleaning, the surface was then coated with

a fine suspension of chalk in alcohol so that a white surface layer was formedonce the alcohol had evaporated. The object was then vibrated and stroked witha hammer, causing the residual oil in any surface cracks to seep out and stainthe white coating.

This method was in use from the latter part of the 19th century through toapproximately 1940, when the magnetic particle method was introduced andfound to be more sensitive for the ferromagnetic iron and steels. PenetrantInspection Improves the Detect ability of Flaws

The advantage that a liquid penetrant inspection (LPI) offers over an unaidedvisual inspection is that it makes defects easier to see for the inspector. Thereare basically two ways that a penetrant inspection process makes flaws moreeasily seen. First, LPI produces a flaw indication that is much larger and easierfor the eye to detect than the flaw itself. Many flaws are so small or narrow thatthey are undetectable by the unaided eye.

The second way that LPI improves the detectability of a flaw is that it produces aflaw indication with a high level of contrast between the indication and the background which also helps to make the indication more easily seen. When a

visible dye penetrant inspection is performed, the penetrant materials areformulated using a bright red dye that provides for a high level of contrast

between the white developer that serves as a background as well as to pull thetrapped penetrant from the flaw. When a fluorescent penetrant inspection isperformed, the penetrant materials are formulated to glow brightly and to give offlight at a wavelength that the eye is most sensitive to under dim lightingconditions.

Basic Processing Steps of aLiquid Penetrant Inspection


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6- Indication Development: The developer is allowed to stand on the partsurface for a period of time sufficient to permit the extraction of thetrapped penetrant out of any surface flaws. This development time isusually a minimum of 10 minutes and significantly longer times may benecessary for tight cracks.

7- Inspection: Inspection is then performed under appropriate lighting to

detect indications from any flaws that may be present.8- Clean Surface: The final step in the process is to thoroughly clean the

part surface to remove the developer from the parts that were found to beacceptable.

Penetrant Testing Materials

The penetrant materials used today are much more sophisticated than thekerosene and whiting first used by railroad inspectors near the turn of the 20thcentury. Today's penetrants are carefully formulated to produce the level ofsensitivity desired by the inspector.

1- Penetrant: Penetrant materials are classified in the various industry andgovernment specifications by their physical characteristics and their performancePenetrant materials come in two basic types. These types are listed below:

Type 1 - Fluorescent Penetrants Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce whenexposed to ultraviolet radiation. Visible penetrants contain a red dye that

provides high contrast against the white developer background. Fluorescentpenetrant systems are more sensitive than visible penetrant systems becausethe eye is drawn to the glow of the fluorescing indication. However, visiblepenetrants do not require a darkened area and an ultraviolet light in order tomake an inspection. Visible penetrants are also less vulnerable to contaminationfrom things such as cleaning fluid that can significantly reduce the strength of afluorescent indication.

Penetrants are then classified by the method used to remove the excesspenetrant from the part. The four methods are listed below:

Method A - Water Washable Method B - Post Emulsifiable, Lipophilic Method C - Solvent Removable Method D - Post Emulsifiable, Hydrophilic

Water washable (Method A) penetrants can be removed from the part by rinsingwith water alone. These penetrants contain some emulsifying agent (detergent)that makes it possible to wash the penetrant from the part surface with wateralone. Water washable penetrants are sometimes referred to as self-emulsifyingsystems.


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Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. Inpost emulsifiers, lipophilic systems (Method B), the penetrant is oil soluble andinteracts with the oil-based emulsifier to make removal possible. Postemulsifiable, hydrophilic systems (Method D), use an emulsifier that is a watersoluble detergent which lifts the excess penetrant from the surface of the partwith a water wash. Solvent removable penetrants require the use of a solvent toremove the penetrant from the part.

Properties of good PenetrantTo perform well, a penetrant must possess following important characteristics.

spread easily over the surface of the material being inspected to providecomplete and even coverage.

be drawn into surface breaking defects by capillary action. remain in the defect but remove easily from the surface of the part. remain fluid so it can be drawn back to the surface of the part through the

drying and developing steps. be highly visible or fluoresce brightly to produce easy to see indications. must not be harmful to the material being tested or the inspector.

2- Emulsifiers: When removal of the penetrant from the defect due to over-washing of the part is a concern, a post emulsifiable penetrant system can beused. Post emulsifiable penetrants require a separate emulsifier to break thepenetrant down and make it water washable. Most penetrant inspectionspecifications classify penetrant systems into four methods of excess penetrantremoval. These are listed below:

1. Method A: Water-Washable2. Method B: Post Emulsifiable, Lipophilic3. Method C: Solvent Removable4. Method D: Post Emulsifiable, Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant from the part beinginspected. Method A has emulsifiers built into the penetrant liquid that makes itpossible to remove the excess penetrant with a simple water wash. Method Band D penetrants require an additional processing step where a separateemulsification agent is applied to make the excess penetrant more removablewith a water wash. Lipophilic emulsification systems are oil-based materials thatare supplied in ready-to-use form. Hydrophilic systems are water-based andsupplied as a concentrate that must be diluted with water prior to use . Lipophilic


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emulsifiers (Method B) were introduced in the late 1950's and work with both achemical and mechanical action. After the emulsifier has coated the surface ofthe object, mechanical action starts to remove some of the excess penetrant asthe mixture drains from the part. During the emulsification time, the emulsifierdiffuses into the remaining penetrant and the resulting mixture is easily removedwith a water spray.Hydrophilic emulsifiers (Method D) also remove the excess penetrant with

mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solventsand surfactants. The hydrophilic emulsifier breaks up the penetrant into smallquantities and prevents these pieces from recombining or reattaching to thesurface of the part. The mechanical action of the rinse water removes thedisplaced penetrant from the part and causes fresh remover to contact and liftnewly exposed penetrant from the surface .

The hydrophilic post emulsifiable method (Method D) was introduced in the mid1970's and since it is more sensitive than the lipophilic post emulsifiable methodit has made the later method virtually obsolete. The major advantage of

hydrophilic emulsifiers is that they are less sensitive to variation in the contactand removal time. While emulsification time should be controlled as closely aspossible, a variation of one minute or more in the contact time will have littleeffect on flaw detectability when a hydrophilic emulsifier is used. However, avariation of as little as 15 to 30 seconds can have a significant effect when alipophilic system is used .

3- Developers The role of the developer is to pull the trapped penetrant material out of defectsand to spread the developer out on the surface of the part so it can be seen byan inspector. The fine developer particles both reflect and refract the incidentultraviolet light, allowing more of it to interact with the penetrant, causing moreefficient fluorescence. The developer also allows more light to be emitted throughthe same mechanism. This is why indications are brighter than the penetrantitself under UV light. Another function that some developers performs is to createa white background so there is a greater degree of contrast between theindication and the surrounding background.

Developer FormsThe AMS 2644 and Mil-I-25135 classify developers into six standard forms.These forms are listed below:

1. Form a - Dry Powder2. Form b - Water Soluble3. Form c - Water Suspendible4. Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)5. Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)


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process if they have silicates in concentrations above 0.5 percent. Sodiummetasilicate, sodium silicate, and related compounds can adhere to the surfaceof parts and form a coating that prevents penetrant entry into cracks.Researchers in Russia have also found that some domestic soaps andcommercial detergents can clog flaw cavities and reduce the wettability of themetal surface, thus, reducing the sensitivity of the penetrant. Conrad and Caudillfound that media from plastic media blasting was partially responsible for loss of

LPI indication strength. Microphotographs of cracks after plastic media blastingshowed media entrapment in addition to metal smearing.

It is very important that the material being inspected has not been smearedacross its own surface during machining or cleaning operations. It is wellrecognized that machining, honing, lapping, hand sanding, hand scraping, shotpeening, grit blasting, tumble deburring, and peening operations can cause asmall amount of the material to smear on the surface of some materials. It isperhaps less recognized that some cleaning operations, such as steam cleaning,can also cause metal smearing in the softer materials. Take the link below tolearn more about metal smearing and its affects on LPI.

Common Uses of Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructiveevaluation (NDE) methods. Its popularity can be attributed to two main factors,which are its relative ease of use and its flexibility. LPI can be used to inspectalmost any material provided that its surface is not extremely rough or porous.Materials that are commonly inspected using LPI include the following:

Metals (aluminum, copper, steel, titanium, etc.) Glass Many ceramic materials

Rubber Plastics

LPI offers flexibility in performing inspections because it can be applied in a largevariety of applications ranging from automotive spark plugs to critical aircraftcomponents. Penetrant material can be applied with a spray can or a cottonswab to inspect for flaws known to occur in a specific area or it can be applied bydipping or spraying to quickly inspect large areas. At right, visible dye penetrantbeing locally applied to a highly loaded connecting point to check for fatiguecracking.


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Penetrant inspection systems have been developed to inspect some very largecomponents. In this picture, DC-10 banjo fittings are being moved into apenetrant inspection system at what used to be the Douglas Aircraft Company's Long Beach, California facility. These large machined aluminum forgings areused to support the number 3 engine in the tail of a DC-10 aircraft.

Liquid penetrant inspection is used to inspect of flaws that break the surface of

the sample. Some of these flaws are listed below: Fatigue cracks Quench cracks Grinding cracks Overload and impact fractures Porosity Laps Seams Pin holes in welds Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is thatflaws must be open to the surface.

Advantages and Disadvantages of Penetrant TestingLike all nondestructive inspection methods, liquid penetrant inspection has bothadvantages and disadvantages. The primary advantages and disadvantageswhen compared to other NDE methods are summarized below.Primary Advantages

The method has high sensitive to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic,

magnetic and nonmagnetic, and conductive and nonconductive materialsmay be inspected.

Large areas and large volumes of parts/materials can be inspected rapidlyand at low cost.

Parts with complex geometric shapes are routinely inspected. Indications are produced directly on the surface of the part and constitute

a visual representation of the flaw. Penetrant materials and associated equipment are relatively inexpensive .

Primary Disadvantages

Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected. Precleaning is critical as contaminants can mask defects.


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Metal smearing from machining, grinding, and grit or vapor blasting mustbe removed prior to LPI.

The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is require

Chapter IV

Magnetic Particle Inspection

Introduction:

Magnetic particle inspection is a nondestructive testing method used for surfaceand near surface defect detection. MPI is a fast and relatively easy to apply and

surface preparation is not as critical as it is for some other NDT methods. Thesecharacteristics make MPI one of the most widely utilized nondestructive testingmethods .

MPI uses magnetic fields and small magnetic particles, such as iron filings todetect flaws in components. The only requirement is that the component beinginspected must be made of a ferromagnetic material such iron, nickel, cobalt, orsome of their alloys.


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Ferromagnetic materials are materials that can be magnetized to a level that willallow the inspection to be effective .

The method is used to inspect a variety of product forms such as castings,forgings, and weldments. Many different industries use magnetic particleinspection for determining a component's fitness-for-use. Some examples ofindustries that use magnetic particle inspection are the structural steel,

automotive, petrochemical, power generation, and aerospace industries.Underwater inspection is another area where magnetic particle inspection maybe used to test items such as offshore structures and underwater pipelines.

Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept.Consider a bar magnet. It has a magnetic field in and around the magnet. Anyplace that a magnetic line of force exits or enters the magnet is called a pole. Apole where a magnetic line of force exits the magnet is called a north pole and apole where a line of force enters the magnet is called a south pole.

When a bar magnet is broken in the center of its length, two complete barmagnets with magnetic poles on each end of each piece will result. If the magnetis just cracked but not broken completely in two, a north and south pole will format each edge of the crack. The magnetic field exits the north pole and reentersthe at the south pole. The magnetic field spreads out when it encounter the smallair gap created by the crack because the air cannot support as much magneticfield per unit volume as the magnet can. When the field spreads out, it appears toleak out of the material and, thus, it is called a flux leakage field.

If iron particles are sprinkled on acracked magnet, the particles will be attracted to and cluster not only at the polesat the ends of the magnet but also at the poles at the edges of the crack. Thiscluster of particles is much easier to see than the actual crack and this is thebasis for magnetic particle inspection.


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The first step in a magnetic particle inspection is to magnetize the componentthat is to be inspected. If any defects on or near the surface are present, thedefects will create a leakage field. After the component has been magnetized,iron particles, either in a dry or wet suspended form, are applied to the surface ofthe magnetized part. The particles will be attracted and cluster at the flux leakagefields, thus forming a visible indication that the inspector can detect .

History of Magnetic Particle Inspection

Magnetism is the ability of matter to attract other matter. The ancient Greekswere the first to discover this phenomenon in a mineral they named magnetite.Later on Bergmann, Becquerel, and Faraday discovered that all matter including

liquids and gasses were affected by magnetism, but only a few responded to anoticeable extent.

The earliest known magnetic inspection an object took place as early as 1868.Cannon barrels were checked for defects by magnetizing the barrel then sliding amagnetic compass along the barrel's length. These early inspectors were able tolocate flaws in the barrels by monitoring the needle of the compass.

In the early 1920s, William Hoke realized that magnetic particles could be usedwith magnetism as a means of locating defects. Hoke discovered that a surfaceor subsurface flaw in a magnetized material caused the magnetic field to distort

and extend beyond the part. This discovery was brought to his attention in themachine shop. He noticed that the metallic grindings from hard steel parts, whichwere being held by a magnetic chuck while being ground, formed patterns on theface of the parts which corresponded to the cracks in the surface. Applying a fineferromagnetic powder to the parts caused a build up of powder over flaws andformed a visible indication.

Today, the MPI inspection method is used extensively to check for flaws in alarge variety of manufactured materials and components. MPI is used to checkmaterials such as steel bar stock for seams and other flaws prior to investing


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machining time during the manufacturing of a component. Critical automotivecomponents are inspected for flaws after fabrication to ensure that defectiveparts are not placed into service. MPI is used to inspect some highly loadedcomponents that have been in-service for a period of time. For example, manycomponents of high performance race cars are inspected whenever the engine,drive train and other systems are overhauled. MPI is also used to evaluate theintegrity of structural welds on bridges, storage tanks, pipelines and other critical

structures.

Magnetism

Magnets are very common items in the workplace and household. Uses ofmagnets range from holding pictures on the refrigerator to causing torque inelectric motors. The term " magnetic field " simply describes a volume of spacewhere there is a change in energy within that volume. This change in energy can be detected and measured. The location where a magnetic field can be detectedexiting or entering a material is called a magnetic pole. Magnetic poles havenever been detected in isolation but always occur in pairs and, thus, the name

dipole.

A bar magnet can be considered a dipole with a north pole at one end and SouthPole at the other. A magnetic field can be measured leaving the dipole at theNorth Pole and returning the magnet at the South Pole. If a magnet is cut in two,

two magnets or dipoles are created out of one. This sectioning and creation ofdipoles can continue to the atomic level. Therefore, the source of magnetism liesin the basic building block of all matter...the atom.

The Source of Magnetism

All matter is composed of atoms, and atoms are composed of protons, neutronsand electrons. The protons and neutrons are located in the atom's nucleus andthe electrons are in constant motion around the nucleus. Electrons carry anegative electrical charge and produce a magnetic field as they move throughspace. A magnetic field is produced whenever an electrical charge is in motion.

The strength of this field is called the magnetic moment .consider electric current flowing through a conductor. When the electrons(electric current) are flowing through the conductor, a magnetic field formsaround the conductor. The magnetic field can be detected using a compass. Themagnetic field will place a force on the compass needle .

Since all matter is comprised of atoms, all materials are affected in some way bya magnetic field. However, not all materials react the same way.


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Ferromagnetic materials become magnetized when the magnetic domains withinthe material are aligned. This can be done by placing the material in a strongexternal magnetic field or by passing electrical current through the material.Some or all of the domains can become aligned. The more domains are aligned,the stronger the magnetic field in the material. When all of the domains arealigned, the material is magnetically saturated and additional amount of externalmagnetization force will not cause any increase in its internal level of

magnetization.

Unmagnetized Material Magnetized Material

Magnetic Field Characteristics

Magnetic lines of force have a number of important properties, which include :

They seek the path of least resistance between opposite magnetic poles.In a single bar magnet as shown to the right, they attempt to form closedloop from pole to pole.

They never cross one another. They all have the same strength. Their density decreases (they spread out) when they move from an area

of higher permeability to an area of lower permeability. Their density decreases with increasing distance from the poles. They are considered to have direction as if flowing, though no actual

movement occurs. They flow from the south pole to the north pole withinthe material and north pole to south pole in air.

Electromagnetic Fields


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In most conductors, the magnetic field exists only as long as the current isflowing

the direction of the magnetic field is dependent on the direction of the electricalcurrent in the wire. A three-dimensional representation of the magnetic field isshown above. There is a simple rule for remembering the direction of themagnetic field around a conductor. It is called the right-hand rule . If a persongrasps a conductor in ones right hand with the thumb pointing in the direction ofthe current, the fingers will circle the conductor in the direction of the magneticfield.

Magnetic Field Produced by a Coil

When a current carrying conductor is formed into a loop or several loops to forma coil, a magnetic field develops that flows through the center of the loop or coilalong longitudinal axis and circles back around the outside of the loop or coil.The magnetic field circling each loop of wire combines with the fields from theother loops to produce a concentrated field down the center of the coil. A looselywound coil is illustrated below to show the interaction of the magnetic field. Themagnetic field is essentially uniform down the length of the coil when it is woundtighter.


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The strength of a coil's magnetic field increases not only with increasing currentbut also with each loop that is added to the coil. A long straight coil of wire is

called a solenoid and can be used to generate a nearly uniform magnetic fieldsimilar to that of a bar magnet. The concentrated magnetic field inside a coil isvery useful in magnetizing ferromagnetic materials for inspection using themagnetic particle testing method. Please be aware that the field outside the coilis weak and is not suitable for magnetize ferromagnetic materials.

The Hysteresis Loop and Magnetic Properties

A great deal of information can be learned about the magnetic properties of amaterial by studying its hysteresis loop. A hysteresis loop shows the relationshipbetween the induced magnetic flux density B and the magnetizing force H. It isoften referred to as the B-H loop. An example hysteresis loop is shown below.


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Plotting the change in magnetic flux B induced a ferromagnetic material while themagnetizing force H is changed generates the hysteresis loop. A ferromagneticmaterial that has never been previously magnetized or has been thoroughlydemagnetized will follow the dashed line as H is increased. As the linedemonstrates, the greater the amount of current applied (H+) , the stronger themagnetic field in the component (B+) . At point "a" almost all of the magneticdomains are aligned and an additional increase in the magnetizing force willproduce very little increase in magnetic flux. The material has reached the pointof magnetic saturation.

When H is reduced back down to zero, the curve will move from point "a" to point"b." At this point, it can be seen that some magnetic flux remains in the materialeven though the magnetizing force is zero, this is referred to as the point ofretentivity on the graph and indicates the remanence or level of residualmagnetism in the material. (Some of the magnetic domains remain aligned butsome have lost there alignment.) As the magnetizing force is reversed, the curve

moves to point "c", where the flux has been reduced to zero. This is called thepoint of coercivity on the curve. (The reversed magnetizing force has flippedenough of the domains so that the net flux within the material is zero.) The forcerequired to remove the residual magnetism from the material, is called thecoercive force or coercivity of the material .

As the magnetizing force is increased in the negative direction, the material willagain become magnetically saturated but in the opposite direction (point "d").


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Reducing H to zero brings the curve to point "e." It will have a level of residualmagnetism equal to that achieved in the other direction. Increasing H back in thepositive direction will return B to zero. Notice that the curve did not return to theorigin of the graph because some force is required to remove the residualmagnetism. The curve will take a different path from point "f" back the saturationpoint where it with complete the loop .

From the hysteresis loop, a number of primary magnetic properties of a materialcan be determined.

Retentivity - A measure of the residual flux density corresponding to thesaturation induction of a magnetic material. In other words, it is a material's abilityto retain a certain amount of residual magnetic field when the magnetizing forceis removed after achieving saturation. (The value of B at point B on thehysteresis curve.)

Residual Magnetism or Residual Flux - the magnetic flux density that remainsin a material when the magnetizing force is zero. Note that residual magnetism

and retentivity are the same when the material has been magnetized to thesaturation point. However, the level of residual magnetism may be lower than theretentivity value when the magnetizing force did not reach the saturation level.

Coercive Force - The amount of reverse magnetic field which must be applied toa magnetic material to make the magnetic flux return to zero. (The value of H atpoint C on the hysteresis curve.)

Permeability - A property of a material that describes the ease with which amagnetic flux is established in the component.

Reluctance - Is the opposition that a ferromagnetic material shows to theestablishment of a magnetic field. Reluctance is analogous to the resistance inan electrical circuit.

The shape of the hysteresis loop tells a great deal about the material beingmagnetized. The hysteresis curves of two different materials are shown in thegraph.

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important tounderstand that orientation between the magnetic lines of force and the flaw isvery important. There are two general types of magnetic fields that can beestablished within a component.


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A longitudinal magnetic field has magnetic lines offorce that run parallel to the long axis of the part.Longitudinal magnetization of a component can beaccomplished using the longitudinal field set up by acoil or solenoid. It can also be accomplished usingpermanent or electromagnets .

A circular magnetic field has magnetic lines of forcethat run circumferentially around the perimeter of apart. A circular magnetic field is induced in an articleby either passing current through the component or bypassing current through a conductor surrounded bythe component .

To magnetize the part in two directions is important because the best detection ofdefects occurs when the lines of magnetic force are established at right angles tothe longest dimension of the defect, if the magnetic field is parallel to the defect,the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect isnecessary to form an indication. Since defects may occur in various directions, each part is normally magnetized in two directions at right angles to each other.To determine most of the defects.


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Demagnetization

After conducting a magnetic particle inspection, it is usually necessary todemagnetize the component

Remanent magnetic fields can:

Affect machining by causing cuttings to cling to a component. Interfere with electronic equipment such as a compass. Create a condition known as "ark blow" in the welding process. Arc blow

may cause the weld arc to wonder or filler metal to be repelled from theweld.

Cause abrasive particle to cling to bearing or faying surfaces and increasewear.

Magnetizing Equipment for Magnetic Particle Inspection

To properly inspect a part for cracks or other defects, it is important to becomefamiliar with the different types of magnetic fields and the equipment used togenerate them. As discussed previously, one of the primary requirements fordetection of a defect in a ferromagnetic material is that the magnetic field inducedin the part must intercept the defect at a 45 to 90 degrees angle. Flaws that arenormal (90 degrees) to the magnetic field will produce the strongest indicationsbecause they disrupt more of the magnet flux.

A variety of equipment exist to establish the magnetic field for MPI. Someequipment is designed to be portable so that inspections can be made in the fieldand some is designed to be stationary for ease of inspection in the laboratory ormanufacturing facility.


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Permanent magnets

Permanent magnets are sometimes used for magnetic particle inspection as thesource of magnetism. The two primary types of permanent magnets are barmagnets and horseshoe (yoke) magnets. These industrial magnets are usuallyvery strong and may require significant strength to remove them from a piece ofmetal. Some permanent magnets require over 50 pounds of force to remove

them from the surface. Because it is difficult to remove the magnets from thecomponent being inspected, and sometimes difficult and dangerous to place themagnets, their use is not particularly popular. However, a diver for inspection inan underwater environment or other areas sometimes uses permanent magnets,such as in an explosive environment, where electromagnets cannot be used.Permanent magnets can also be made small enough to fit into tight areas whereelectromagnets might not fit

Electromagnets Today, most of the equipment used to create the magnetic field used in MPI isbased on electromagnetism. That is, using an electrical current to produce themagnetic field. An electromagnetic yoke is a very common piece of equipmentthat is used to establish a magnetic field. It is basically made by wrapping anelectrical coil around a piece of soft ferromagnetic steel. A switch is included inthe electrical circuit so that the current and, therefore, also the magnetic field canbe turn on and off. They can be powered with alternating current from a wallsocket or by direct current from a battery pack. This type of magnet generates a

very strong magnetic field in a local area where the poles of magnet touch thepart to be inspected. Some yokes can lift weights in excess of 40 pounds.


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Portable yoke with battery pack Portable magnetic particle kit

ProdsProds are handheld electrodes that are pressed against the surface of thecomponent being inspected to make contact for passing electrical currentthrough the metal. The current passing between the prods creates a circular

magnetic field around the prods that is can be used in magnetic particleinspection. Prods are typically made from copper and have an insulated handleto help protect the operator. One of the prods has a trigger switch so that thecurrent can be quickly and easily turned on and off. Sometimes the the two prodsare connected by any insulator as shown in the image to facilitate one handoperation. This is referred to as a dual prod and is commonly used for weldinspections.

If proper contact is not maintained between the prods and the componentsurface, electrical arcing can occur and cause damage to the component. Forthis reason, the use of prods are not allowed when inspecting aerospace and

other critical components. To help to prevent arcing, the prod tips should be

inspected frequently to ensure that they are not oxidized, covered with scale orother contaminant, or damaged.


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Portable Coils and Conductive Cables Coils and conductive cables are used to establish a longitudinal magnetic fieldwithin a component. When a preformed coil is used, the component is placedagainst the inside surface on the coil. Coils typically have three or five turns of acopper cable within the molded frame. A foot switch is often used to energize thecoil. Conductive cables are wrapped around the component. The cable used is

typically 00 extra flexible or 0000 extra flexible. The number of wraps isdetermined by the magnetizing force needed and, of course, the length of thecable. Normally the wraps are kept as close together as possible. When using acoil or cable wrapped into a coil, amperage is usually expressed in ampere-turns.

Ampere-turns is the amperage shown on the amp meter times the number ofturns in the coil.

Portable coil Conductive Cable

central conductor.


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This type of a setup is used to inspect parts that are hollow such as gears, tubes,and other ring-shaped objects. A central conductor is an electrically conductivebar that is usually made of copper or aluminum. The bar is inserted through thecenter of the hollow part and the bar is then clamped between the contact pads.When current is passed through the central conductor, a circular magnetic fieldflows around the bar and enters into the part or parts being inspected.

Lights for Magnetic Particle Inspection

Magnetic particle inspection can be performed using particles that are highlyvisible under white lighting conditions or particles that are highly visible underultraviolet lighting conditions. When an inspection is being performed using thevisible color contrast particles, no special lighting is required as long as the areaof inspection is well lit. A light intensity of at least 1000 lux (100 fc) isrecommended when a visible particles are used, but a variety of light sourcescan be used.

When fluorescent particles are used, special ultraviolet light must be used.Fluorescence is defined as the property of emitting radiation as a result of andduring exposure to radiation. Particles used in fluorescent magnetic particleinspections are coated with a material that produces light in the visible spectrumwhen exposed to the near-ultraviolet light. This "particle glow" provides highcontrast indications on the component anywhere particles collect. Particles thatfluoresce yellow-green are most common because this color matches the peaksensitivity of the human eye under dark conditions. However, particles thatfluoresce red, blue, yellow, and green colors are available.

Ultraviolet Light


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Ultraviolet light or "black light" is light in the 1,000 to 4,000 Angstroms (100 to400 nm) wavelength range in the electromagnetic spectrum. It is a very energeticform of light that is invisible to the human eye. Wavelengths above 4,000

Angstroms fall into the visible light spectrum and are seen as the color violet. UVis separated according to wavelength into three classes: A, B, and C. The shorterthe wavelength, the more energy that is carried in the light and the moredangerous it is to the human cells.

ClassUV-AUV-BUV-C

Wavelength Range3,2004,000 Angstroms2,8003,200 Angstroms2,8001,000 Angstroms

The desired wavelength range for use in nondestructive testing is between 3,500and 3,800 Angstroms with a peak wavelength at about 3,650 A. This wavelengthrange is used because it is in the UV-A range, which is the safest to work with.UV-B will do an effective job of causing substances to fluoresce, however, itshould not be used because harmful effects such as skin burns, and eye damage

can occur. This wavelength of radiation is found in the arc created during thewelding process. UV-C (1,000 to 2,800) is even more dangerous to living cellsand is used to kill bacteria in industrial and medical settings.

The desired wavelength range for use in NDT is obtained by filtering theultraviolet light generated by the light bulb. The output of a UV bulb spans a widerange of wavelengths. The short wave lengths of 3,120 A to 3,340 A areproduced in low levels. A peak wavelength of 3650 A is produced at a very highintensity. Wavelengths in the visible violet range (4050 A to 4350 A), green-yellow (5460 A), yellow (6220 A) and orange (6770 A) are also usually produced.The filter allows only radiation in the range of 3200 to 4000 angstroms and a little

visible dark purple to pass.

Magnetic Particles

As mentioned previously, the particles that are used for magnetic particleinspection are a key ingredient as they form the indications that alert theinspector to defects. Particles start out as tiny milled (a machining process)pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to theirsurfaces to give the particles color. The metal used for the particles has highmagnetic permeability and low retentivity. High magnetic permeability isimportant because it makes the particles attract easily to small magnetic leakage

fields from discontinuities, such as flaws. Low retentivity is important because theparticles themselves never become strongly magnetized so they do not stick toeach other or the surface of the part. Particles are available in a dry mix or a wetsolution.

Dry Magnetic Particles


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Dry magnetic particles can typically be purchased in red, black, gray, yellow andseveral other colors so that a high level of contrast between the particles and thepart being inspected can be achieved. The size of the magnetic particles is alsovery important. Dry magnetic particle products are produced to include a range ofparticle sizes. The fine particles are around 50 mm (0.002 inch) in size are aboutthree times smaller in diameter and more than 20 times lighter than the coarseparticles (150 mm or 0.006 inch), which make them more sensitive to the leakage

fields from very small discontinuities. However, dry testing particles cannot bemade exclusively of the fine particles. Coarser particles are needed to bridgelarge discontinuities and to reduce the powder's dusty nature. Additionally, smallparticles easily adhere to surface contamination, such as remanent dirt ormoisture, and get trapped in surface roughness features producing a high level ofbackground. It should also be recognized that finer particles will be more easilyblown away by the wind and, therefore, windy conditions can reduce thesensitivity of an inspection. Also, reclaiming the dry particles is not recommendedbecause the small particle are less likely to be recaptured and the "once used"mix will result in less sensitive inspections.

The particle shape is also important. Long, slender particles tend alignthemselves along the lines of magnetic force. However, research has shown thatif dry powder consists only of long, slender particles, the application processwould be less than desirable. Elongated particles come from the dispenser inclumps and lack the ability to flow freely and form the desired "cloud" of particlesfloating on the component. Therefore, globular particles are added that are shorter. The mix of globular and elongated particles result in a dry powder thatflows well and maintain good sensitivity. Most dry particle mixes have particlewith L/D ratios between one and two.

Wet Magnetic ParticlesMagnetic particles are also supplied in a wet suspension such as water or oil.The wet magnetic particle testing method is generally more sensitive than the drybecause the suspension provides the particles with more mobility and makes itpossible for smaller particles to be used since dust and adherence to surface


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contamination is reduced or eliminated. The wet method also makes it easy toapply the particles uniformly to a relatively large area.

Wet method magnetic particles products differ from dry powder products in anumber of ways. One way is that both visible and fluorescent particle areavailable. Most nonfluorescent particles are ferromagnetic iron oxides, which areeither black or brown in color. Fluorescent particles are coated with pigments that

fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yelloware most common to take advantage of the peak color sensitivity of the eye butother fluorescent colors are also available. (For more information on the colorsensitivity of the eye, see the penetrant inspection material.)

The particles used with the wet method are smaller in size than those used in thedry method for the reasons mentioned above. The particles are typically 10 mm(0.0004 inch) and smaller and the synthetic iron oxides have particle diametersaround 0.1 mm (0.000004 inch). This very small size is a result of the processused to form the particles and is not particularly desirable, as the particles arealmost too fine to settle out of suspension. However, due to their slight residualmagnetism, the oxide particles are present mostly in clusters that settle out ofsuspension much faster than the individual particles. This makes it possible tosee and measure the concentration of the particles for process control purposes.Wet particles are also a mix of long slender and globular particles. The carriersolutions can be water- or oil-based. Water-based carriers form quickerindications, are generally less expensive, present little or no fire hazard, give offno petrochemical fumes, and are easier to clean from the part. Water-basedsolutions are usually formulated with a corrosion inhibitor to offer some corrosionprotection. However, oil-based carrier solutions offer superior corrosion andhydrogen embrittlement protection to those materials that are prone to attack bythese mechanisms.

Chapter IV


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Ultrasonic Testing

Basic Principles of Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conductexaminations and make measurements. Ultrasonic inspection can be used forflaw detection/evaluation, dimensional measurements, material characterization,

and more.

The sound energy is introduced and propagates through the materials in the formof waves. When there is a discontinuity (such as a crack) in the wave path, partof the energy will be reflected back from the flaw surface. The reflected wavesignal is transformed into electrical signal by the transducer and is displayed on ascreen.

SCREEN

Ultrasonic Inspection is a very useful and versatile NDT method for detectingboth surface and subsurface volumetric defects and is widely used in pipeline, oiland gas and processing industry.

Plate Crack

0 2 4 6 8 10

Initialpulse

Crackecho

Back surfaceecho

Oscilloscope, or flaw

Probe


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Advantages of Ultrasonic Inspection

Some of the advantages of ultrasonic inspection that are often cited include:

It is sensitive to both surface and subsurface discontinuities. The depth of penetration for flaw detection or measurement is superior to

other NDT methods. Only single-sided access is needed when the pulse-echo technique is

used. It is high accuracy in determining reflector position and estimating size and

shape. Minimal part preparation required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. It has other uses such as thickness measurements, in addition to flaw

detection.

Disadvantages of Ultrasonic Inspection

16H z 20 kH z 200 kH z 15 MH z

256 H z 70 kH z

Audiblerange

Ultrasonic testing range

1-5 MH z

Usual steel testingrange

Sound Spectrum


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As with all NDT methods, ultrasonic inspection also has its limitations, whichinclude:

Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods.

It normally requires a coupling medium to promote transfer of soundenergy into test specimen. Materials that are rough, irregular in shape, very small, exceptionally thin

or not homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect due to

low sound transmission and high signal noise. Linear defects oriented parallel to the sound beam may go undetected. Reference standards are required for both equipment calibration, and

characterization of flaws.

Properties of sound wave

Wave PropagationUltrasonic testing is based on time-varying deformations or vibrations inmaterials, which is generally referred to as acoustics. All material substances arecomprised of atoms, which may be forced into vibrational motion about theirequilibrium positions.

In solids, sound waves can propagate in four principle modes that are based onthe way the particles oscillate. Sound can propagate as longitudinal waves, shearwaves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic

testing. The particle movement responsible for the propagation of longitudinaland shear waves is illustrated below.

Longitudinal waves:


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In longitudinal waves the oscillations occur in the longitudinal direction of thedirection of wave propagation. Since compressional forces are active in thesewaves, they are also called compressional waves. Compression waves can begenerated in liquids, as well as solids because the energy travels through theatomic structure by a series of comparison and expansion (rarefaction)

movements.Transverse or shear wave :In the transverse or shear wave, the particles oscillate at a right angle ortransverse to the direction of propagation. Shear waves require an acousticallysolid material for effective propagation and, therefore, are not effectivelypropagated in materials such as liquids or gasses. Shear waves are relativelyweak when compared to longitudinal waves

Surface or Rayleigh waves :Surface or Rayleigh waves travel on the surface of a relative thick solid materialpenetrating to a depth of one wavelength. The particle movement has an elliptical

orbit. Raleigh waves are useful because they are very sensitive to surfacedefects and since they will follow the surface around curves, therefore can beused to inspect areas that other waves might have difficulty in reaching.

Plate waves: Plate waves can be propagated only in very thin metals. Lamb waves are themost commonly used plate waves in NDT. Lamb waves are a complex vibrationalwave that travels through the entire thickness of a material. Propagation of Lambwaves depends on density, elastic, and material properties of a component, andthey are influenced by a great deal by selected frequency and material thickness.

Velocity: How quickly a sound wave will travelFrequency: How many vibrations per second


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Wave length:

How far a sound wave will advance in completing one cycle The wavelength isdirectly proportional to the velocity of the wave and inversely proportional to thefrequency of the wave. This relationship is shown by the following equation .

A change in frequency will result in a change in wavelength. In ultrasonic testing,the shorter wavelength resulting from an increase in frequency will help in thedetection of smaller discontinuities.

Sensitivity :Sensitivity is the ability to locate small discontinuities. Sensitivity generallyincreases with higher frequency (shorter wavelengths).

Resolution :

1 SecondA 2H z

1 SecondB=5H z

Speed of sound in air = 332 m/sec


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materials. The velocity of sound in each material is determined by the materialproperties (elastic modules and density) for that material.

Snell's Law describes the relationship between the angles and the velocities ofthe waves. Snell's law equates the ratio of material velocities v1 and v2 to the

ratio of the sine's of incident ( Q 1) and refraction ( Q 2) angles, as shown in thefollowing equation.

Where:

VL1 is the longitudinal wave velocity in material 1.

VL2 is the longitudinal wave velocity in material 2.

Ultrasonic Probes

The conversion of electrical pulses to mechanical vibrations and the conversionof returned mechanical vibrations back into electrical energy is the basis forultrasonic testing. The active element is the Probe. It converts the electricalenergy to acoustic energy, and vice versa.

Characteristics of Probes

The probe is a very important part of the ultrasonic instrumentation system. Theprobe converts electrical signals into mechanical vibrations (transmit mode) andmechanical vibrations into electrical signals (receive mode). Many factors,including material, mechanical and electrical construction, and the externalmechanical and electrical load conditions, influence the behavior a transducer.Mechanical construction includes parameters such as radiation surface area,mechanical damping, housing, connector type


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.

Types of Probes

Ultrasonic transducers are manufactured for a variety of application and can becustom fabricated when necessary. Careful attention must be paid to selectingthe proper transducer for the application It is important to choose transducersthat have the desired frequency, bandwidth, and focusing to optimize inspectioncapability. Most often the transducer is chosen either to enhance sensitivity orresolution of the system.

Transducers are classified into groups according to the application.

Contact transducers are used for direct contact inspections, and aregenerally hand manipulated. They have elements protected in a ruggedcasing to withstand sliding contact with a variety of materials. Thesetransducers are designed so that they are easy to grip and move along asurface. They also often have replaceable wear plates to lengthen theiruseful life. Coupling materials of water, grease, oils, or commercial


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materials are used to remove the air gap between the transducer and thecomponent inspected. Contact probes are classified as.

Single crystal probe Twin crystal probe Normal beam or zero degree probe Angle beam probe

Contact transducers are available in a variety of configurations to improve theirusefulness for a variety of applications.

Single crystal probe normal probe:

The flat contact transducer shown above is used normal beam inspections ofrelatively flat surfaces, and where near surface resolution is not critical. If thesurface is curved, a shoe that matches the curvature of the part may need to beadded to the face of the transducer.

Twin crystal normal probe:

contain two independently operating elements in a single housing. One of theelements transmits and the other receives. Active elements can be chosen fortheir sending and receiving capabilities providing a transducer with a cleanersignal, and transducers for special applications, such as inspection of coursegrain material. Dual element transducers are especially well suited for making


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Standard blocks are used to calibrate the instrument and to calculate differentfeatures of probe and the instrument. These blocks consists accurately cut andfine polished surfaces, holes ,angles etc.

Inspection of Welded Joints

The most commonly occurring defects in welded joints are porosity, slaginclusions, lack of side-wall fusion, lack of inter-run fusion, lack of rootpenetration, undercutting, and longitudinal or transverse cracks.


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Ultrasonic weld inspections are typically performed using a straight beam probein conjunction with an angle beam probe A straight beam probe, producing alongitudinal wave at normal incidence into the test piece, is first used to locateany laminations in or near the heat-affected zone. This is important because anangle beam transducer may not be able to provide a return signal from a laminar

flaw.

Chapter VI

RADIORGAPHIC TESTING


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Introduction:

In this method of Non-destructive testing the penetration property of X-ray andGamma rays to detect the discontinuities. The object to be inspected is placedbetween the radiation source and a piece of film. X-rays or gamma rays passthrough the object. The object will stop some of the radiation. Thicker and denserarea will stop more of the radiation and show on the film lighter than thinner orless dense area. Most weld defects will show on the film darker than thesurrounding area.

Nature of Penetrating Radiation

X-rays and gamma rays are part of the electromagnetic spectrum. They arewaveforms as are light rays, microwaves, and radio wave, but x-rays and gammarays cannot been seen, felt, or heard. They possess no charge and no mass and,therefore, are not influenced by electrical and magnetic fields and will alwaystravel in straight lines. They can be characterized by frequency, wavelength, andvelocity

The Electromagnetic Spectrum


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The International System (SI) unit for activity is the Becquerel (Bq), w

Wavelengths of Electro Magnetic Spectrum

Electro Magnetic Radiation Type Wave length in nm

Visible Light 700-400 Ultraviolet light 400-100X-RaysGamma -Rays

1 nm =10 -9 Meters

Advantage of Radiography

1. Gives a permanent record2.Detects internal Flaws3.Detects volumetric flaws readily4.Can be used on most materials5.Can check for correct assembly6.Gives direct Images7.Real time Image is possible


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Disadvantages of Radiography

1-Radiation Health2-Can be sensitive to defect orientation and could miss planar flaws3-Has limited ability to detect fine cracks

4-Access is required to both sides of the object5-Limited thickness of the material can be penetrated6-Skilled radiographic interpretation is required7-Require high capital cost8-Relatively slow process9-Require high capital cost10-Require high running cost

Properties of X-rays and gamma rays

1.They have no effect on the human senses

2.They have adverse effect on the body tissues and blood3.They penetrate matter4.They move in straight line5.They are part of electromagnetic spectrum 6.They travel at the speed of light 7.They obey the inverse square law8.They ionize gases9.They may be scattered10.They make certain materials fluoresce11.They may be refracted, diffracted and polarized

X-ray Tube

Electrons-+

X-ray Generator or Radioactive SourceCreates Radiation

Exposure Recording Device

RadiationPenetrate

the Sample


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1.Gamma rays are emitted from artificial radio active isotope 2.Radio active isotope is an unstable state of element which has different number

of neutrons to the normal state of the same element3.The mass number of Radio active Isotope will be different from same element4.The radio active isotope disintegrate continuously releasing electromagnetic

energy (gamma rays)5-Gamma ray sources are usually disc,cylindrical or spherical shape6-The discs: 3.0 mm diameter and 1 mm thick, stacked together7-Cylindrical: Typically upto 4 mm in length8-Spherical: 0.6 3.0 mm diameter9-Sources are encapsulated in the capsules of 316 \ S12 grade Stainless steel

Isotope Decay Rate (Decay of the Gamma Source)

Loss of activity of a radioactive nuclease due to Disintegration

Half Life of Gamma source:

Time taken for a radio active Isotope to reduce its out put by half

Source Half-life Penetration rangesteel

60Cobalt 26 Year s 75- 150 mm

192Iridium 74 da s 20 45 mm

Ytterbium 169 31 days 1-15 mm


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Advantages of Gamma rays over X-rays

1.No electrical or water supply are needed

2.Gamma equipment is usually smaller and lighter and therefore more portable

3.The equipment is more simple

4.Places inaccessible to x-ray equipment are accessible to gamma equipment

5.Because of high energy there is less scatter

6.Gamma equipment is less expensive than x-ray equipment

7.Greater penetrating power than x-rays

Disadvantages of gamma rays over x-rays 1.Due to the higher energy, poorer contrast and definition

2.Exposure times are longer

3.Sources need replacing at regular intervals

4.The radiation cannot be switched off

5.SFD is shorter, resulting in poorer geometric unsharpness

6.Remote handling is necessary

Radiographic Techniques1) SWSI : ( Film Inside Source Outside )2) SWSI : ( Film Outside Source Inside )3) DWSI : ( Film Outside Source Outside )4) DWDI : (Film Outside Source Outside


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Radiographic Contrast

Radiographic contrast describes the differences in photographic density in aradiograph. The contrast between different parts of the image is what forms theimage and the greater the contrast, the more visible features become.Radiographic contrast has two main contributors: subject contrast and detector orfilm contrast.

Subject contrast is determined by the following variables:- Absorption differences in the specimen- Wavelength of the primary radiation- Scatter or secondary radiation

Film contrast is determined by the following:- Grain size or type of film- Chemistry of film processing chemicals- Concentrations of film processing chemicals- Time of development- Temperature of development- Degree of mechanical agitation (physical motion)

Exposing the film to produce higher film densities will generally increase contrast.

In other words, darker areas will increase in density faster than lighter areasbecause in any given period of time more x-rays are reaching the darker areas.

Reasons for low contrastRadiation wave length too shortOver exposureProlonged developmentToo cold developer


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Insufficient fixingFog on the film

Reasons for High contrast

Radiation wave length too longIncorrect developerUnder exposure

Definition

Radiographic definition is the abruptness of change in going from one density toanother. There are a number of geometric factors of the X-ray equipment and theradiographic setup that have an effect on definition. These geometric factorsinclude:- Focal spot size, which is the area of origin of the radiation.The focal spot sizeshould be as close to a point source as possible to produce the most definition.- Source to film distance, which is the distance from the source to the part.Definition increases as the source to film distance increase.

- Specimen to detector (film) distance, which is the distance between thespecimen and the detector. For optimal definition, the specimen and detectorshould be as close together as possible. .- Abrupt changes in specimen thickness may cause distortion on the radiograph.- Movement of the specimen during the exposure will produce distortion on theradiograph.- Film graininess, and screen mottling will decrease definition.

The grain size of the film will affect the definition of the radiograph. Wavelengthof the radiation will influence apparent graininess. As the wavelength shortensand penetration increases, the apparent graininess of the film will increase. Also,increased development of the film will increase the apparent graininess of theradiograph.

Radiographic Density


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Degree of blackening of a radiograph. Density is measured by adensitometer.High density area is a dark area and low density area is a lightarea. A high density area absorbs more light than the low density areaDensity is the log of the intensity of light incident on the film to the intensity of

light transmitted through the film. A density reading of 2.0 is the result of only 1

percent of the transmitted light reaching the sensor.Density required in the area of interest should be between 1.5 and2.5.Radiographs with very low density and with very high density are notacceptable.

Reasons for low density Under exposure to radiation Insufficient development time Development temperature too low Incorrect developer

Reasons for Excessive density. Over exposure to Radiation Excessive development time Development temperature too high Incorrect Developer

sensitivity The ability of the radiographic technique to detect the smallest possibledefect.Sensitivity is measured by using Image Quality Indicators ( IQI ),alsocalled as Penetrameters. Sensitivity depends on Radiographic contrast andDensity.

Controlling Radiographic QualityOne of the methods of controlling the quality of a radiograph is through the use ofimage quality indicators (IQI). IQIs provide a means of visually informing the filminterpreter of the contrast sensitivity and definition of the radiograph. The IQIindicates that a specified amount of material thickness change will be detectable

in the radiograph, and that the radiograph has a certain level of definition so thatthe density changes are not lost due to unsharpness. Without such a reference

point, consistency and quality could not be maintained and defects could goundetected.

Image quality indicators t

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