MATERIAL TESTING PDT 154 Week 11: Magnetic Particle ...portal.unimap.edu.my/portal/page/portal30/Lecture... · How Does Magnetic Particle Inspection Work? A ferromagne-c test specimen

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MATERIAL TESTING PDT 154

Week 11:

Magnetic Particle Testing

Dr. Norshah Aizat Shuaib

Introduction •  MPTusesmagne-c fields and smallmagne-c par-cles to

detectflawsincomponents.

•  Themethod isused to inspect a varietyofproduct formssuchascas-ngs,forgings,andweldments.

• Magne-cpar-cleinspec-oncandetectbothproduc-ondiscon-nui-es(seams,laps,grindingcracksandquenchingcracks)andin-servicedamage(fa-gueandoverloadcracks).

•  Testmethodforthedetec-onofsurfaceandslightlysub-surfaceindica-onsinferromagne-cmaterials

Three types of material

Diamagne-cParamagne-cFerromagne-c

Ferromagnetic Materials • Amaterialisconsideredferromagne-cifitcanbemagne-zed.Materialswith a significant Iron, nickel or cobalt content aregenerallyferromagne-c.

• Ferromagne-c materials are made up of many regions inwhichthemagne-cfieldsofatomsarealigned.Theseregionsarecallmagne-cdomains.

• Magne-c domains point randomly in demagne-zedmaterial,but can be aligned using electrical current or an externalmagne-cfieldtomagne-zethematerial.

Demagnetized Magnetized S N

Ferromagnetic Material

Surface Defect Subsurface Internal

MPT MPT CANNOT BE

DETECTED BY Magnetic Particle

Testing

Magnetic Particle Testing

Basic Principles of MPT

Consider a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole 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 bar magnets with magnetic poles on each end of each piece will result.

•  If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters the at the south pole.

•  The magnetic field spreads out when it encounter the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus, it is called a flux leakage field.

•  If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack.

•  This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

Magnetic Particle Testing Principle- Summary

•  Magnetic field

induced in component •  Defects disrupt the

magnetic flux •  Defects revealed by

applying ferromagnetic particles

How Does Magnetic Particle Inspection Work?

Aferromagne-ctestspecimenismagne-zedwithastrongmagne-cfieldcreatedbyamagnetorspecialequipment.Ifthespecimenhasadiscon-nuity,thediscon-nuitywillinterruptthemagne-cfieldflowingthroughthespecimenandaleakagefieldwilloccur.

How Does Magnetic Particle Inspection Work? (Cont.)

•  Finelymilledironpar-clescoatedwithadyepigmentareappliedtothetestspecimen.

•  Thesepar-clesareaKractedtoleakagefieldsandwillclustertoformanindica-ondirectlyoverthediscon-nuity.

•  Thisindica-oncanbevisuallydetectedunderproperligh-ngcondi-ons.

Procedure of magnetic particle testing

Surface preparation / Pre-cleaning

Magnetization

Application of magnetic particles

Viewing

Demagnetization

Clean

MPT Testing Procedure

Surface preparation •  Thesurfaceshouldberela-velycleanbutthisisnotascri-calasitiswithliquidpenetrantinspec-on.

•  The surface must be free of grease, oil or othermoisture that could keep par-cles from movingfreely. Any loose dirt, paint, rust or scale must beremoved

•  A thin layer of paint, rust or scale will reduce testsensi-vity but can some-mes be leL in place withadequateresults.

•  IMPORTANT:Thepart’ssurfaceshouldbecleananddrybeforeinspec-on.

Magnetization •  Apply the magnetizing force by using

permanent magnets, a electromagnetic yoke, prods, a coil or other means to establish the necessary magnetic flux. – a permanent magnet – an electromagnet – contact current flow method –  threading bar – coil –  induced current flow

Introduction of the Magnetic Field Therequiredmagne-cfieldcanbeintroducedintoacomponentinanumberofdifferentways.

1. Usingapermanentmagnetoranelectromagnetthatcontactsthetestpiece

2. Flowinganelectricalcurrentthroughthespecimen

3. Flowinganelectricalcurrentthroughacoilofwirearoundthepartorthroughacentralconductorrunningnearthepart.

Application of magnetic particles

•  There two class of magnetic available depending to vehicle or carrying agent: – Wet method: particles use a liquid as vehicle. – Dry method: particle are carried by air.

Viewing

•  Inspect for indications - Look for areas where the magnetic particles are clustered.

•  Indications are viewed under proper illumination.

Interpretation of Indications A"erapplyingthemagne0cfield,indica0onsthatformmustinterpreted.Thisprocessrequiresthattheinspectordis0nguishbetweenrelevantandnon-relevantindica0ons.

Demagnetisation

Required for: •  Aircraft parts •  Rotating parts •  Components to be welded,machined or

electroplated

Removal of residual magnetisation

Check for removal with Field strength meter (magnetometer)

Demagnetization • Partsinspectedbythemagne-cpar-clemethodmaysome-meshaveanobjec-onableresidualmagne-cfieldthatmayinterferewithsubsequentmanufacturingopera-onsorserviceofthecomponent.

• Possiblereasonsfordemagne-za-oninclude:– Mayinterferewithweldingand/ormachiningopera-ons

– Caneffectgaugesthataresensi-vetomagne-cfieldsifplacedincloseproximity.

– Abrasivepar-clesmayadheretocomponentssurfaceandcauseandincreaseinweartoenginescomponents,gears,bearingsetc.

Advantages of Magnetic Particle Inspection

•  Candetectbothsurfaceandnearsub-surfacedefects.

•  Caninspectpartswithirregularshapeseasily.•  Precleaningofcomponentsisnotascri-calasitisforsomeotherinspec-onmethods.Mostcontaminantswithinaflawwillnothinderflawdetectability.

•  Indica-onsarevisibledirectlyonthespecimensurface.

•  ConsideredlowcostcomparedtomanyotherNDTmethods.

•  Isaveryportableinspec-onmethodespeciallywhenusedwithbaKerypoweredequipment.

Limitations of Magnetic Particle Inspection

•  Cannotinspectnon-ferrousmaterialssuchasaluminum,magnesiumormoststainlesssteels.

•  Inspec-onoflargepartsmayrequireuseofequipmentwithspecialpowerrequirements.

•  Somepartsmayrequireremovalofcoa-ngorpla-ngtoachievedesiredinspec-onsensi-vity.

•  Limitedsubsurfacediscon-nuitydetec-oncapabili-es.Maximumdepthsensi-vityisapproximately0.6inch(underidealcondi-ons).

•  Postcleaning,andpostdemagne-za-onisoLennecessary.

• Alignmentbetweenmagne-cfluxanddefectisimportant