Phương pháp kiểm tra không phá hủy NDT

Phương pháp kiểm tra không phá hủy (NonDestructive Testing) là tài liệu cung cấp các kiến thức về các phương pháp kiểm tra mối hàn: phương pháp thẩm thấu (Penetrant Testing), phương pháp từ tính (MPI or MT), phương pháp phóng xạ (RT), phương pháp siêu âm (UT), ...

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. [...]... 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 ... 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 ... When removal of the penetrant from the defect due to over­washing  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: ... 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... 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 ... 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 ­  20 ­  between the white developer that serves as a background as well as to pull the trapped  penetrant  from ... 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 ... 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 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 ... 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  . 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. 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. 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