Phương pháp kiểm tra không phá hủy NDT

1
NondestructiveTesting
TableofContents
Chapter
No:
NameoftheChapter Page
No
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
2
ChapterI
INTRODUCTION
NondestructiveTesting
The field ofNondestructive Testing(NDT)isaverybroad, that playsacritical rolein
assuringthatstructuralcomponentsandsystemsperformtheirfunctioninareliableand
costeffectivefashion.NDTtechniciansandengineersdefineandimplementteststhat
locate and characterize material conditions and flaws that might otherwise cause
seriousaccidentssuchas,planestocrash,reactorstofail,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usefulnessofthe
objectormaterial.Inotherwords,NDTallowspartsandmaterialstobeinspectedand
evaluatedwithoutdamagingthem.Becauseitallowsinspectionwithoutinterferingwith
a product's final use, NDT provides anexcellentbalancebetweenquality control and

costeffectiveness.
NondestructiveEvaluat ion
NondestructiveEvaluation(NDE)isatermthatisoftenusedinterchangeablywithNDT.
However,technically,NDEisusedtodescribemeasurementsthataremorequantitative
innature.Forexample,aNDEmethodwouldnotonlylocateadefect,butitwouldalso
be used to measure something about that defect such as its size, shape, and
orientation. NDE may be used to determine material properties such as fracture
toughness,ductility,conductivityandotherphysicalcharacteristics.
Useso f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
3
BackgroundonNondestructiveTesting(NDT)
Nondestructive testing has been practiced for many decades. One of the earliest
applicationswasthedetectionofsurfacecracksinrailcarwheelsandaxles.Theparts
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visualindicationindicating
that the component was flawed. This eventually led to oils that were specifically
formulatedforperformingtheseandotherinspectionsandtheseinspectiontechniques
arenowcalledpenetranttesting.
Xrayswerediscoveredin1895byWilhelmConradRoentgen(18451923)whowasa
Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen
produced the first industrial radiograph when he imaged a set of weights in a box to
show his colleagues. Other electronic inspection techniques such as ultrasonic and
eddy current testing started with the initial rapid developments 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 produced with a "safe life" design, and were intendedto be defect
free during their useful life. The detection of defects was automatically a cause for
removalofthecomponentfromservice.
The continuedimprovementofinspection technology,inparticulartheability todetect
smaller andsmallerflaws,ledto moreandmore parts being rejected. Atthistimethe
disciplineoffracturemechanicsemerged,whichenabledonetopredictwhetheracrack
of a given size would fail under a particular load if a particular 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 containing defects if the sizes of those defects were
known. This formed the basis for a new design philosophy called "damage tolerant
designs." Components having known defects could continue to be used aslong as it
couldbeestablishedthatthosedefectswouldnotgrowtoacriticalsizethatwouldresult
in catastrophic failure. A new challenge was thus presented to the nondestructive
testingcommunity.
Mere detection of flaws was not enough. One needed to also obtain quantitative
information about flaw size to serve as an input to fracturemechanics calculations 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emergenceofnondestructiveevaluation
(NDE)asanewdiscipline.
4
NDT/NDEMethods
The list of NDT methods that can be used to inspect components and make
measurementsislargeandcontinuestogrow.Researcherscontinuetofindnewways
of applying physics and other scientific disciplines to develop better NDT methods.
However, there are six NDT methods that are used most often. These methods are
Visual Inspection, Penetrant Testing, Magnetic Particle Testing, Electromagnetic or
EddyCurrentTesting,Radiography,andUltrasonicTesting.

VisualandOpticalT esting(VT)
Visual inspectioninvolvesusinganinspector's eyestolookfordefects.Theinspector
mayalsousespecialtoolssuchasmagnifyingglasses,mirrors,orborescopestogain
accessandmorecloselyinspectthesubjectarea.Visualexaminersfollowprocedures
thatrangefmsimpletoverycomplex.
PenetrantTesting(PT)
Test objects are coated with visible or fluorescent dye solution. Excess dye is then
removed fromthesurface, and a developeris applied.The developer actsasblotter,
drawing trappedpenetrantoutofimperfectionsopentothesurface.Withvisibledyes,
vividcolorcontrastsbetweenthepenetrantanddevelopermake"bleedout"easytosee.
With fluorescentdyes,ultravioletlightisusedto make thebleedoutfluorescebrightly,
thusallowingimperfectionstobereadilyseen.
5
MagneticParticleTest ing(MT)
This methodis accomplished byinducingamagnetic fieldinaferromagnetic material
and then dusting the surface with iron particles (either dry or suspended in liquid).
Surfaceandnearsurfaceimperfectionsdistortthemagneticfieldand concentrateiron
particlesnearimperfections,previewingavisualindicationoftheflaw.
ElectromagneticTesting(ET)orEddyCurrentTesting
Electrical currents are generated in a conductive material by an induced alternating
magnetic field This electrical currents is called eddy currents because they flow in
circles at and just below the surface of the material. Interruptions in the flow of eddy
currents, caused byimperfections, dimensional changes, or changes in the material's
conductiveandpermeabilityproperties,aredetected.
6
Radiography(RT)
Radiography involves the use of penetrating gamma or Xradiation to examine parts
andproductsforimperfections.AnXraygeneratororradioactiveisotopeisusedasa
sourceofradiation.Radiationisdirectedthroughapartandontofilmorotherimaging
media. The resulting radiograph shows the dimensional features of 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T esting(UT)
Ultrasonics use transmission of highfrequency sound waves into a material to detect
imperfections or to locate changes in material properties. The most commonly used
ultrasonictestingtechniqueispulseecho,whereinsoundisintroducedintoatestobject
andreflections (echoes)are returned toareceiverfrominternalimperfectionsorfrom
thepart'sgeometricalsurfaces
.
crack
0 2 4 6 8 1
0
Initial
pulse
Crack
echo
Backsurface
echo
Sound
waves
Xrayfilm
Source
Rays
Objectwithdefect
Film
DefectImage
Filmwithimage
Probe
Couplant
Plate

Screen
7
AcousticEmissionTesting(AE)
Whenasolidmaterialisstressed,imperfectionswithinthematerialemitshortburstsof
acousticenergycalled"emissions."Asinultrasonictesting,acousticemissionscanbe
detectedbyspecialreceivers.Emissionsourcescanbeevaluatedthroughthestudyof
theirintensity,rate,andlocation.
LeakTesting(LT)
Severaltechniquesareusedtodetectandlocateleaksinpressurecontainmentparts,
pressure vessels, and structures. Leaks can be detected by using electronic listening
devices,pressuregaugemeasurements,liquidandgaspenetranttechniques,and/ora
simplesoapbubbletest.
8
Test
Method
UT Xray Eddy
Current
MPI LPT
Capitalcost
Mediumto
high
High Lowto
medium
Medium Low
Consumable
cost
Verylow High Low Medium Medium
Timeof
results
Immediate Delayed Immediate Short

delay
Short
delay
Effectof
geometry
Important Important Important Nottoo
Important
Nottoo
Important
Acc ess
problems
Important Important Important Important Important
Typeof
defect
Internal Most External External
Near
Surface
Surface
breaking
Relative
sensitivity
High Medium High Low Low
Operator
skill
High High Medium Low Low
Operator
training
Important Important Important Important Not
Important
Training

needs
High High Medium Low Low
Portabilityof
equipment
High Low Highto
medium
Highto
medium
High
Capabilities
Thickness
gauging,
composition
testing
Thickness
gauging
Thickness
gauging,
grade
sorting
Defects
only
Defects
only
The Relative Uses and Merits of Various NDT Methods
9
Table1ReferenceGuidetoMajorMethodsfortheNondestructive
ExaminationofWelds
Inspectio n
Method

Equipment
Required
Enables
Detectiortof
Advantages Limitations Remarks
Visual
Magnifying
glass
Weldsizing
gauge
Pocketrule
Straightedge
Workmanship
standards
Surfaceflaws
cracks,
porosity,
unfilled
craters,slag
inclusions
Warpage,
underwelding,
overwelding,
poorlyformed
beads,
misalignments,
improperfitup
Lowcost.
Canbeapplied
whileworkis

inprocess,
permitting
correctionof
faults.
Gives
indicationof
incorrect
procedures.
Applicable
tosurface
defectsonly.
Providesno
permanent
record.
Should
alwaysbethe
primary
methodof
inspection,no
matterwhat
other
techniquesare
required.
Istheonly
"productive"
typeof
inspection.
Isthe
necessary
functionof

everyonewho
inanyway
contributesto
themakingof
theweld.
Radiographic
Commercial
Xrayor
gammaunits
made
especiallyfor
inspecting
welds,
castingsand
forgings.
Filmand
processing
facilities.
Fluoroscopic
viewing
equipment.
Interior
macroscopic
flawscracks,
porosity,blow
holes,
nonmetallic
inclusions,
incomplete
root

penetration,
undercutting,
icicles,and
burnthrough.
Whenthe
indicationsare
recordedon
film,givesa
permanent
record.
Whenviewed
ona
fluoroscopic
screen,alow
costmethodof
internal
inspection
Requires
skillin
choosing
anglesof
exposure,
operating
equipment,
and
interpreting
indications.
Requires
safety
precautions.

Not
generally
suitablefor
filletweld
inspection.
Xray
inspectionis
requiredby
manycodes
and
specifications.
Usefulin
qualification
ofwelders
andwelding
processes.
Becauseof
cost,itsuse
shouldbe
limitedto
thoseareas
whereother
methodswill
notprovide
theassurance
required.
10
Magnetic
Particle
Special

commercial
equipment.
Magnetic
powders dry
orwetform;
maybe
fluorescent
forviewing
under
ultraviolet
light.
Excellentfor
detecting
surface
discontinuities

especially
surfacecracks.
Simplerto
usethan
radiographic
inspection.
Permits
controlled
sensitivity.
Relatively
lowcost
method.
Applicableto
ferromagnetic

materialsonly.
Requiresskill
in
interpretation
ofindications
and
recognitionof
irrelevant
patterns.
Difficulttouse
onrough
surfaces.
Elongated
defectsparallel
tothemagnetic
fieldmaynot
givepattern;
forthisreason
thefieldshould
beapplied
fromtwo
directionsator
nearright
anglestoeach
other.
Liquid
Penetrant
Commercial
kits
containing

fluorescentor
dyepenetrants
and
developers.
Application
equipmentfor
thedeveloper.
Asourceof
ultraviolet
light if
fluorescent
methodis
used.
Surfacecracks
notreadily
visibletothe
unaidedeye.
Excellentfor
locatingleaks
inweldments.
Applicableto
magneticand
nonmagnetic
materials.
Easytouse.
Lowcost.
Onlysurface
defectsare
detectable.
Cannotbe

used
effectivelyon
hotassemblies.
Inthinwalled
vesselswill
revealleaksnot
ordinarily
locatedby
usualairtests.
irrelevant
surface
conditions
(smoke,slag)
maygive
misleading
indications.
Ultrasonic
Special
commercial
equipment,
eitherofthe
pulseechoor
transmission
type.
Standard
reference
patternsfor
interpretation
ofRFor
video

patterns.
Surfaceand
subsurface
flawsincluding
thosetoosmall
tobedetected
byother
methods.
Especiallyfor
detecting
subsurface
laminationlike
defects.
Very
sensitive.
Permits
probingof
joints
inaccessible
to
radiography.
Requireshigh
degreeofskill
ininterpreting
pulseecho
patterns.
Permanent
recordisnot
readily
obtained.

Pulseecho
equipmentis
highly
developedfor
weldinspection
purposes.
The
transmission
typeequipment
simplifies
pattern
interpretation
whereitis
applicable.
11
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 specification used to invoke and
control an inspection, are referred to as defects.
General Welding Discontinuities
The following discontinuities are typical of all types of welding.
Cracks:
Crack is tight linear separations of metal that can be very short to very long indications.
Cracks are grouped as hot or cold cracks. Hot cracks usually occur as the metal
solidifies at elevated temperatures. Cold cracks occur after the metal has cooled to
ambient temperatures ( delayed cracks).
Cracks can be detected in a radiograph only when they are propagating in a direction
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 can sometimes appear as
"tails" on inclusions or porosity.
12
Lack of Fusion:
Lack of fusion (Cold Lap) is a condition where the weld filler metal does not properly
fuse with the base metal or the previous weld pass material (inter pass cold lap). The
arc does not melt the base metal sufficiently and causes the slightly molten puddle to
flow into base material without bonding.
13
Porosity:
Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many
shapes on a radiograph but often appears as dark round or irregular spots or specks
appearing singularly, in clusters or rows. Sometimes porosity is elongated and may
have the appearance of having a tail This is the result of gas attempting to escape while
the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in
the material it will have a radiographic density more than the surrounding area.
Cluster porosity:
Cluster porosity is caused when flux coated electrodes are contaminated with moisture.
The moisture turns into gases when heated and becomes trapped in the weld during the
welding process. Cluster porosity appear just like regular porosity in the radiograph but
the indications will be grouped close
together.
14
Slag inclusions:
Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld
and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or
along the weld joint areas are indicative of slag inclusions.
Incomplete penetration (IP):
Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal
fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack

of penetration allows a natural stress riser from which a crack 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 the weldment.
Root concavity:
15
Root or Internal concavity or suck back is condition where the weld metal has
contracted as it cools and has been drawn up into the root of the weld. On a radiograph
it looks similar to lack of penetration but the line has irregular edges and it is often quite
wide in the center of the weld image.
Internal or root undercut:
Internal or root undercut is an erosion of the base metal next to the root of the weld. In
the radiographic image it appears as a dark irregular line offset from the centerline of
the weldment. Undercutting is not as straight edged as LOP because it does not follow
a ground edge.
External or crown undercut:
16
External or crown undercut is an erosion of the base metal next to the crown of the
weld. In the radiograph, it appears as a dark irregular line along the outside edge of the
weld area.
Offset or mismatch:
Offset or mismatch are terms associated with a condition where two pieces being
welded together are not properly aligned. The radiographic image is a noticeable
difference in density between the two pieces. The difference in density is caused by 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:
17
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 to determine by
radiograph if the weld has inadequate reinforcement, because the image density in the

area of suspected inadequacy will be more (darker) than the image density of the
surrounding base material.
Excess weld reinforcement :
Excess weld reinforcement is an area of a weld that has weld metal added in excess of
that specified by engineering drawings and codes. The appearance on a radiograph is a
localized, lighter area in the weld. A visual inspection will easily determine if the weld
reinforcement is in excess of that specified by the engineering requirements.
Discontinuities in TIG welds
18
The following discontinuities are peculiar to the TIG welding process. These
discontinuities occur in most metals welded by the process including aluminum and
stainless steels. The TIG method of welding produces a clean homogeneous weld
which when radiographed is easily interpreted.
Tungsten inclusions.
Tungsten is a brittle and inherently dense material used in the electrode in tungsten
inert gas ( TIG ) welding. If improper welding procedures are used, tungsten may be
entrapped in the weld. Radiographically, tungsten is denser than aluminum or steel;
therefore, it shows as a lighter area with a distinct outline on 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 surrounding materials and,
therefore, appear as dark irregularly shaped discontinuities in the
radiograph.
Discontinuities in Gas Metal Arc Welds (GMAW)
The following discontinuities are most commonly found in GMAW welds.
19
Whiskers:
Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface
of the weld or contained within the weld. On a radiograph they appear as light, "wire
like" indications.

Burn-Through:
Burn-Through results when too much heat causes excessive weld metal to penetrate
the weld zone. Often lumps of metal sag through the weld creating a thick globular
condition on the back of the weld. These globs of metal are referred to as icicles. On a
radiograph, burn through appears as dark spots, which are often surrounded by light
globular areas (icicles).
20
ChapterIII
PENE TRANTINSPECTION
Introduction
Liquidpenetrationinspectionisamethodthatisusedtorevealsurfacebreakingflaws
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bycapillaryaction.
After a period of time called the "dwell," excess surface penetrant is removed and a
developer is applied. This acts as a "blotter." It draws the penetrant from the flaw to
revealitspresence.
Colored(contrast)penetrantsrequiregoodwhitelightwhilefluorescentpenetrantsneed
tobeviwedindarkenedconditionswithanultraviolet"blacklight".
A very early surface inspection technique involved the rubbing of carbon black on
glazedpottery,wherebythecarbonblackwouldsettleinsurfacecracksrenderingthem
visible. Later it became the practice in railway workshops to examine iron and steel
components by the "oil and whiting" method. In this method, heavy oil commonly
available in railway workshops was diluted with kerosene in large tanks so that
locomotive parts such as wheels could be submerged. 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formedoncethealcoholhadevaporated.Theobjectwasthen
vibrated and stroked withahammer, causingtheresidualoilinany surfacecracksto
seepoutandstainthewhitecoating.
Thismethodwasinusefromthelatterpartofthe19thcenturythroughtoapproximately
1940, when the magnetic particle method was introduced and found to be more

sensitive for the ferromagnetic iron and steels. Penetrant Inspection Improves the
DetectabilityofFlaws
The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual
inspectionisthatit makesdefectseasiertosee for theinspector. Thereare basically
twowaysthatapenetrantinspectionprocessmakesflawsmoreeasilyseen.First,LPI
producesaflawindicationthatismuchlargerandeasierfortheeyetodetectthanthe
flawitself.Manyflawsaresosmallornarrowthattheyareundetectablebytheunaided
eye.
ThesecondwaythatLPIimprovesthedetectabilityofaflawisthatitproducesaflaw
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 are formulated
usingabrightreddyethatprovidesforahighlevelofcontrast
21
betweenthewhitedeveloperthatservesasabackgroundaswellastopullthetrapped
penetrant from the flaw. When a fluorescent penetrant inspection is performed, the
penetrantmaterialsareformulatedtoglowbrightlyandtogiveofflightatawavelength
thattheeyeismostsensitivetounderdimlightingconditions.
BasicProcessingStepsofaLiquidPenetrantInspectio n
1. Surfac e Preparation: One of the most critical steps of a liquid penetrant
inspection is the surface preparation. The surface must be free of oil, grease,
water,orothercontaminantsthatmaypreventpenetrantfromenteringflaws.The
sample may also require etching if mechanical operations such as machining,
sanding, or grit blasting have been performed. These and other mechanical
operationscansmearthesurfaceofthesample,thusclosingthedefects.
2. PenetrantApplication:Oncethesurfacehasbeenthoroughlycleanedanddried,
the penetrant material is applied on the surface by spraying, brushing, or
immersingthepartsinapenetrantbath.
3. Penetrant Dwell:Thepenetrantisleftonthesurfaceforasufficienttimetoallow
as much penetrant as possible to be drawn from or to seep into a defect.

Penetrantdwelltimeisthetotaltimethatthepenetrantisincontactwiththepart
surface.Dwelltimesareusuallyrecommendedbythe
penetrant producers or required by the specification being followed. The times
varydependingontheapplication,penetrantmaterialsused,thematerialbeing
inspected,andthetypeofdefectbeinginspected.Minimumdwelltimestypically
range from 5 to 60 minutes. Generally, there is no harm in using a longer
22
penetrant dwell time as long as the penetrant is not allowed to dry. The ideal
dwelltimeisoftendeterminedbyexperimentationandisoftenveryspecifictoa
particularapplication.
4 Excess Penetrant Removal: This is a most delicate part of the inspection
procedure becausethe excess penetrant mustberemoved from the surface of
the sample while removing as little penetrant as possible from defects.
Dependingonthepenetrantsystemused,thisstepmayinvolvecleaningwitha
solvent, direct rinsing with water, or first treated with an emulsifier and then
rinsingwithwater.
5 DeveloperApplication:Athinlayerofdeveloperisthenappliedtothesample
to draw penetrant trapped in flaws back to the surface where it will be visible.
Developers come in a variety of forms that may be applied by dusting (dry
powdered),dipping,orspraying(wetdevelopers).
6 IndicationDevelopment:Thedeveloperisallowedtostandonthepartsurface
foraperiodoftimesufficienttopermittheextractionofthetrappedpenetrantout
ofanysurfaceflaws.Thisdevelopmenttimeisusuallyaminimumof10minutes
andsignificantlylongertimesmaybenecessary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beacceptable.
PenetrantTestingMaterials 
The penetrant materials used today are much more sophisticated than the kerosene

andwhitingfirstusedbyrailroadinspectorsneartheturnofthe20thcentury.Today's
penetrants are carefully formulated to produce the level of sensitivity desired by the
inspector.
1 Penetrant: Penetrant materials are classified in the various industry and
governmentspecificationsbytheirphysicalcharacteristicsandtheirperformance
Penetrant 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 whenexposed to
ultraviolet radiation. Visible penetrants contain a red dye that provides high contrast
against the white developer background. Fluorescent penetrant systems are more
sensitive than visible penetrant systems because the eyeis drawn to the glow of the
fluorescingindication.However,visiblepenetrantsdonotrequireadarkenedareaand
an ultraviolet light in order to make an inspection. Visible penetrants are also less
vulnerable to contamination from things such as cleaning fluid that can significantly
reducethestrengthofafluorescentindication.
23
Penetrantsarethenclassifiedbythemethodusedtoremovetheexcesspenetrant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rinsingwith
wateralone.Thesepenetrantscontainsomeemulsifyingagent(detergent)thatmakesit
possibletowashthepenetrantfromthepartsurfacewithwateralone.Waterwashable
penetrantsaresometimesreferredtoasselfemulsifyingsystems.
Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post
emulsifiers,lipophilicsystems(MethodB),thepenetrantisoilsolubleandinteractswith
the oilbased emulsifier to make removal possible. Post emulsifiable, hydrophilic

systems(MethodD),useanemulsifierthatisawatersolubledetergentwhichliftsthe
excess penetrant from the surface of the part with a water wash. Solvent removable
penetrantsrequiretheuseofasolventtoremovethepenetrantfromthepart.
PropertiesofgoodPenetrant
Toperformwell,apenetrantmustpossessfollowingimportantcharacteristics.
· spread easily over the surface of the material being inspected to provide
complete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 be used. Post
emulsifiablepenetrantsrequireaseparateemulsifiertobreakthepenetrant downand
make it water washable. Most penetrant inspection specifications classify penetrant
systemsintofourmethodsofexcesspenetrantremoval.Thesearelistedbelow:
1. MethodA:WaterWashable
2. MethodB:PostEmulsifiable,Lipophilic
24
3. MethodC:SolventRemovable
4. MethodD:PostEmulsifiable,Hydrophilic
Method C relies on a solvent cleaner to remove the penetrant from the part being
inspected.MethodAhasemulsifiersbuiltintothepenetrantliquidthatmakesitpossible
toremovetheexcesspenetrantwithasimplewaterwash.MethodBandDpenetrants
requireanadditionalprocessingstepwhereaseparateemulsificationagentisapplied
to make the excess penetrant more removable with a water wash. Lipophilic
emulsification systems are oilbased materials that are supplied in readytouse form.
Hydrophilic systems are waterbased and supplied as a concentrate that must be

dilutedwithwaterpriortouse .Lipophilicemulsifiers(MethodB)wereintroducedinthe
late1950'sandwork withbothachemical andmechanicalaction.Aftertheemulsifier
has coated the surfaceofthe object, mechanicalaction starts to remove some of the
excesspenetrantasthemixturedrainsfromthepart.Duringtheemulsificationtime,the
emulsifier diffuses into the remaining penetrant and the resulting mixture is easily
removedwith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 solvents and surfactants.
The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents
thesepiecesfromrecombiningorreattachingtothesurfaceofthepart.Themechanical
action of the rinse water removes the displaced penetrant from the part and causes
freshremovertocontactandliftnewlyexposedpenetrantfromthesurface.
Thehydrophilicpostemulsifiablemethod(MethodD)wasintroducedinthemid1970's
andsinceitismoresensitivethanthelipophilicpostemulsifiable method ithasmade
the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is
that they are less sensitive to variation in the contact and removal time. While
emulsificationtimeshouldbecontrolledascloselyaspossible,avariationofoneminute
ormoreinthecontacttimewillhavelittleeffectonflawdetectabilitywhenahydrophilic
emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a
significanteffectwhenalipophilicsystemisused.
3Developers
Theroleofthedeveloperistopullthetrappedpenetrantmaterialoutofdefectsandto
spreadthedeveloperoutonthesurfaceofthepartsoitcanbeseenbyaninspector.
Thefinedeveloperparticlesbothreflectandrefracttheincidentultravioletlight,allowing
more of it to interact with the penetrant, causing more efficient fluorescence. The
developer also allows more light to be emitted through the same mechanism. This is
why indications are brighter than the penetrant itself under UV light. Another function
that some developers performs is to create a white background so there is a greater
degreeofcontrastbetweentheindicationandthesurroundingbackground.

DeveloperForms
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The AMS 2644 and MilI25135 classify developers into six standard forms. These
formsarelistedbelow:
1. FormaDryPowder
2. FormbWaterSoluble
3. FormcWaterSuspendible
4. FormdNonaqueousType1Fluorescent(SolventBased)
5. FormeNonaqueousType2VisibleDye(SolventBased)
The developer classifications are based on the method that the developer is applied.
The developer can beappliedas a drypowder,ordissolvedor suspendedin aliquid
carrier.Eachofthedeveloperformshasadvantagesanddisadvantages.
A)DryPowder
Dry powder developer is generally considered to be the least sensitive but it is
inexpensivetouseandeasytoapply.Drydevelopersarewhite,fluffypowdersthatcan
be applied to a thoroughly dry surface in a number of ways. The developer can be
applied bydippingpartsinacontainerofdeveloper,orbyusinga puffertodustparts
withthedeveloper.Partscanalsobeplacedina dustcabinetwherethedeveloperis
blown around and allowed to settle on the part. Electrostatic powder spray guns are
also available to apply the developer. The goal is to allow the developer to come in
contactwiththewholeinspectionarea.
Unlessthepartiselectrostaticallycharged,thepowderwillonlyadheretoareaswhere
trapped penetrant has wet the surface of the part. The penetrant will try to wet the
surface of the penetrant particle and fill thevoids between the particles, which brings
more penetrant to the surface of the part where it can be seen. Since dry powder
developersonlysticktothepartwherepenetrantispresent,thedrydeveloperdoesnot
provide a uniform white background as the other forms of developers do. Having a
uniform light background is very important for a visible inspection to be effective and
sincedrydevelopersdonotprovideone,theyareseldomusedforvisibleinspections.
When a dry developer is used, indications tend to stay bright and sharp since the

penetranthasalimitedamountofroomtospread.
B) - Water Soluble
Asthenameimplies,watersolubledevelopersconsistofagroupofchemicalsthatare
dissolvedinwaterandformadeveloperlayerwhenthewaterisevaporatedaway.The
bestmethodforapplyingwatersolubledevelopersisbysprayingitonthepart.Thepart
can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is
sometimes used but these methods are less desirable. Aqueous developers contain
wettingagentsthatcausethesolutiontofunctionmuchlikedilutehydrophilicemulsifier
and can lead to additional removal of entrapped penetrant. Drying is achieved by
placingthewetbutwelldrained
partinarecalculatingwarmairdryerwiththetemperatureheldbetween70and75°F.If
thepartsarenotdriedquickly,theindicationswillwillbeblurredandindistinct.Properly
developedpartswillhaveaneven,palewhitecoatingovertheentiresurface.