29867193 Magnetic Particle Testing

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MAGNETIC PARTICLE TESTING


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Introduction

This module is intended to presentinformation on the widely used method of

magnetic particle inspection.

Magnetic particle inspection can detectboth production discontinuities (seams,

laps, grinding cracks and quenching

cracks) and in-service damage (fatigue

and overload cracks).


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Outline

Magnetism and Ferromagnetic Materials Introduction of Magnetic Particle

Inspection

Basic Procedure and ImportantConsiderations

1. Component pre-cleaning2. Introduction of magnetic field3. Application of magnetic media4. Interpretation of magnetic particle

indications

Examples of MPI Indications


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Magnetic lines of force

around a bar magnet

Opposite poles attracting Similar poles repelling

Introduction to Magnetism

Magnetism is the ability of matter toattract other matter to itself. Objectsthat possess the property ofmagnetism are said to be magnetic ormagnetized and magnetic lines of

force can be found in and around theobjects. A magnetic pole is a pointwhere the a magnetic line of forceexits or enters a material.

Magnetic field lines: Form complete loops. Do not cross. Follow the path of least

resistance.

All have the same strength. Have a direction such that

they cause poles to attractor repel.


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Ferromagnetic Materials

A material is considered ferromagnetic if it can be

magnetized. Materials with a significant Iron, nickel or

cobalt content are generally ferromagnetic.

Ferromagnetic materials are made up of many regions inwhich the magnetic fields of atoms are aligned. These

regions are call magnetic domains.

Magnetic domains point randomly in demagnetized

material, but can be aligned using electrical current or an

external magnetic field to magnetize the material.

Demagnetized Magnetized

S N


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How Does Magnetic Particle

Inspection Work?A ferromagnetic test specimen is magnetized witha strong magnetic field created by a magnet orspecial equipment. If the specimen has adiscontinuity, the discontinuity will interrupt the

magnetic field flowing through the specimen and aleakage field will occur.


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How Does Magnetic Particle

Inspection Work? (Cont.)Finely milled iron particles coated with a dye pigmentare applied to the test specimen. These particles areattracted to leakage fields and will cluster to form anindication directly over the discontinuity. This

indication can be visually detected under properlighting conditions.


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Basic Procedure

Basic steps involved:

1. Component pre-cleaning

2. Introduction of magnetic field

3. Application of magnetic media

4. Interpretation of magnetic particle indications


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Pre-cleaning

When inspecting a test part with the magneticparticle method it is essential for the particles tohave an unimpeded path for migration to bothstrong and weak leakage fields alike. The partssurface should be clean and dry beforeinspection.

Contaminants such as oil,grease, or scale may notonly prevent particles from

being attracted to leakagefields, they may alsointerfere with interpretationof indications.


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Introduction of the Magnetic Field

The required magnetic field can be introduced into acomponent in a number of different ways.

1. Using a permanent magnet or an electromagnet thatcontacts the test piece

2. Flowing an electrical current through the specimen

3. Flowing an electrical current through a coil of wirearound the part or through a central conductor runningnear the part.


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Direction of the Magnetic Field

Two general types of magnetic fields (longitudinaland circular) may be established within thespecimen. The type of magnetic field established isdetermined by the method used to magnetize thespecimen.

A longitudinal magnetic field hasmagnetic lines of force that run

parallel to the long axis of the

part. A circular magnetic field has

magnetic lines of force that run

circumferentially around the

perimeter of a part.


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Importance of Magnetic Field Direction

Being able to magnetize the part in twodirections is important because the bestdetection of defects occurs when the lines ofmagnetic force are established at right angles tothe longest dimension of the defect. This

orientation creates the largest disruption of themagnetic field within the part and the greatestflux leakage at the surface of the part. Anorientation of 45 to 90 degrees between themagnetic field and the defect is necessary toform an indication.

Since defects mayoccur in various andunknown directions,each part is normallymagnetized in twodirections at rightangles to each other.

Flux Leakage

No Flux Leakage


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Question

?? From the previous slide regarding the optimum

test sensitivity, which kinds of defect are easily

found in the images below?

Longitudinal (along the axis) Transverse (perpendicular the axis)


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Producing a Longitudinal Magnetic

Field Using a Coil

A longitudinal magnetic fieldis usually established byplacing the part near the

inside or a coils annulus.This produces magneticlines of force that areparallel to the long axis ofthe test part.

Coil on Wet Horizontal Inspection Unit

PortableCoil


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Producing a Longitudinal Field Using

Permanent or Electromagnetic Magnets

Permanent magnets andelectromagnetic yokesare also often used toproduce a longitudinalmagnetic field. Themagnetic lines of forcerun from one pole to theother, and the poles arepositioned such that anyflaws present run normal

to these lines of force.


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Circular Magnetic Fields

Circular magnetic fields are produced bypassing current through the part or byplacing the part in a strong circularmagnet field.

A headshot on a wet horizontal test unitand the use of prods are several commonmethods of injecting current in a part toproduce a circular magnetic field.Placing parts on a central conductorscarrying high current is another way toproduce the field.

Magnetic Field

ElectricCurrent


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Application of Magnetic

Media (Wet Versus Dry)MPI can be performed using eitherdry particles, or particlessuspended in a liquid. With thedry method, the particles are

lightly dusted on to the surface.With the wet method, the part isflooded with a solution carryingthe particles.

The dry method is more portable.

The wet method is generally moresensitive since the liquid carriergives the magnetic particlesadditional mobility.


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Dry Magnetic Particles

Magnetic particles come in a variety of colors. A

color that produces a high level of contrast

against the background should be used.


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Wet Magnetic Particles

Wet particles are typically supplied

as visible or fluorescent. Visible

particles are viewed under normal

white light and fluorescent particlesare viewed under black light.


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Interpretation of Indications

After applying the magnetic field, indications that

form must interpreted. This process requires that

the inspector distinguish between relevant and

non-relevant indications.

The following series of images depict

relevant indications produced from a

variety of components inspectedwith the magnetic particle method.


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Crane Hook with

Service Induced Crack

Fl orescent, Wet Particle Method


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Gear with




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Drive Shaft with

Heat Treatment Induced Cracks



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Splined Shaft with

Service Induced Cracks



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Threaded Shaft with




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Large Bolt with




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Crank Shaft with

Service Induced Crack Near Lube Hole



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Lack of Fusion in SMAW Weld

Visible, Dry Powder Method

Indication


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Toe Crack in SMAW Weld



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Throat and Toe Cracks in

Partially Ground Weld



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Advantages of

Magnetic Particle Inspection

Can detect both surface and near sub-surfacedefects.

Can inspect parts with irregular shapes easily.

Precleaning of components is not as critical as itis for some other inspection methods. Mostcontaminants within a flaw will not hinder flawdetectability.

Fast method of inspection and indications are

visible directly on the specimen surface. Considered low cost compared to many otherNDT methods.

Is a very portable inspection method especiallywhen used with battery powered equipment.


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Limitations of

Magnetic Particle Inspection

Cannot inspect non-ferrous materials such asaluminum, magnesium or most stainless steels.

Inspection of large parts may require use ofequipment with special power requirements.

Some parts may require removal of coating orplating to achieve desired inspection sensitivity.

Limited subsurface discontinuity detectioncapabilities. Maximum depth sensitivity is approximately0.6 (under ideal conditions).

Post cleaning, and post demagnetization is oftennecessary.

Alignment between magnetic flux and defect isimportant


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Glossary of Terms

Black Light: ultraviolet light which is filtered toproduce a wavelength of approximately 365nanometers. Black light will cause certain materials tofluoresce.

Central conductor: an electrically conductive barusually made of copper used to introduce a circularmagnetic field in to a test specimen.

Coil: an electrical conductor such a copper wire orcable that is wrapped in several or many loops that

are brought close to one another to form a stronglongitudinal magnetic field.


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Glossary of Terms

Discontinuity: an interruption in the structure of thematerial such as a crack.

Ferromagnetic: a material such as iron, nickel andcobalt or one of its alloys that is strongly attracted toa magnetic field.

Heads: electrical contact pads on a wet horizontalmagnetic particle inspection machine. The part to beinspected is clamped and held in place between theheads and shot of current is sent through the part

from the heads to create a circular magnetic field inthe part.

Leakage field: a disruption in the magnetic field.This disruption must extend to the surface of the partfor particles to be attracted.


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Glossary of Terms

Non-relevant indications: indications produced dueto some intended design feature of a specimen sucha keyways, splines or press fits.

Prods: two electrodes usually made of copper oraluminum that are used to introduce current in to atest part. This current in turn creates a circularmagnetic field where each prod touches the part.(Similar in principal to a welding electrode andground clamp).

Relevant indications: indications produced fromsomething other than a design feature of a testspecimen. Cracks, stringers, or laps are examples ofrelevant indications.


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For More Information

The Collaboration for

NDT Education

www.ndt-ed.org

The American Society

for NondestructiveTesting

www.asnt.org

Documents

29867193 Magnetic Particle Testing