MT self-Study Note

8/16/2019 MT self-Study Note

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Nondestructive TestingStudy Note

MT

Leo.liu


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Content

I. Introduction

II. Physics

III. Equipment & MaterialsIV. Testing practices

V. Process control

VI. Example indications

VII. Quizzes


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I. Introduction

1. Introduction

. !asic o" concepts

#. $istory o" MPI


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1. IntroductionMagnetic particle inspection %MPI is a nondestructi'e testing method used "or de"ect

detection. MPI is "ast and relati'ely easy to apply( and part sur"ace preparation is not as

critical as it is "or some other )*T methods. These characteristics ma+e MPI one o" the most

,idely utilized nondestructi'e testing methods.

MPI uses magnetic "ields and small magnetic particles %i.e.iron "ilings to detect "la,s in

components. The only requirement "rom an inspecta-ility standpoint is that the component

-eing inspected must -e made o" a "erromagnetic material such as iron( nic+el( co-alt( or

some o" their alloys. erromagnetic materials are materials that can -e magnetized to a le'el

that ,ill allo, the inspection to -e e""ecti'e.

The method is used to inspect a 'ariety o" product "orms including castings( "orgings( and

,eldments. Many di""erent industries use magnetic particle inspection "or determining a

component/s "itness0"or0use. ome examples o" industries that use magnetic particle

inspection are the structural steel( automoti'e( petrochemical( po,er generation( and

aerospace industries. 2nder,ater inspection is another area ,here magnetic particleinspection may -e used to test items such as o""shore structures and under,ater pipelines.


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. !asic o" conceptsIn theory( magnetic particle inspection %MPI is a relati'ely simple concept. It can -e

considered as a com-ination o" t,o nondestructi'e testing methods3 magnetic "lux lea+age

testing and 'isual testing. 4onsider the case o" a -ar magnet. It has a magnetic "ield in and

around the magnet. 5ny place that a magnetic line o" "orce exits or enters the magnet is

called a pole. 5 pole ,here a magnetic line o" "orce exits the magnet is called a north pole

and a pole ,here a line o" "orce enters the magnet is called a south pole.

磁力线离开北极（

N），进入南极（

S）！（南进

北出）


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. !asic o" concepts6hen a -ar magnet is -ro+en in the center o" its length( t,o complete -ar magnets ,ith

magnetic poles on each end o" each piece ,ill result. I" the magnet is 7ust crac+ed -ut not

-ro+en completely in t,o( a north and south pole ,ill "orm at each edge o" the crac+. The

magnetic "ield exits the north pole and reenters at the south pole. The magnetic "ield spreads

out ,hen it encounters the small air gap created -y the crac+ -ecause the air cannot support

as much magnetic "ield per unit 'olume as the magnet can. 6hen the "ield spreads out( itappears to lea+ out o" the material and( thus is called a "lux lea+age "ield.


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. !asic o" conceptsI" iron particles are sprin+led on a crac+ed magnet( the particles ,ill -e attracted to and

cluster not only at the poles at the ends o" the magnet( -ut also at the poles at the edges o"

the crac+. This cluster o" particles is much easier to see than the actual crac+ and this is the

-asis "or magnetic particle inspection.

The "irst step in a magnetic particle inspection is to magnetize the component that is to -einspected. I" any de"ects on or near the sur"ace are present( the de"ects ,ill create a lea+age

"ield. 5"ter the component has -een magnetized( iron particles( either in a dry or ,et

suspended "orm( are applied to the sur"ace o" the magnetized part. The particles ,ill -e

attracted and cluster at the "lux lea+age "ields( thus "orming a 'isi-le indication that the

inspector can detect.


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#. $istory o" MPIhttps388,,,.nde0

ed.org8Education9esources84ommunity4ollege8MagParticle8Introduction8history.php

1:; Electyro0Magnetic teel Testing *e'ice %MPI made -y the Equipment and Engineering

4ompany


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II. Physics

1. Magnetism

. Magnetic Mat/ls

#. Magnetic *omains>. Magnetic ields

?. Electromag. ields

@. ield rom a 4oil

A. Mag Properties;. $ysteresis


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1. Magnetism

Magnets are 'ery common items in the ,or+place and household. 2ses o" magnets range "rom

holding pictures on the re"rigerator to causing torque in electric motors. Most people are "amiliar

,ith the general properties o" magnets -ut are less "amiliar ,ith the source o" magnetism. The

traditional concept o" magnetism centers around the magnetic "ield and ,hat is +no, as a dipole.

The term Cmagnetic "ieldC simply descri-es a 'olume o" space ,here there is a change in energy,ithin that 'olume. This change in energy can -e detected and measured. The location ,here a

magnetic "ield can -e detected exiting or entering a material is called a magnetic pole. Magnetic

poles ha'e ne'er -een detected in isolation -ut al,ays occur in pairs( hence the name dipole.

There"ore( a dipole is an o-7ect that has a magnetic pole on one end and a second( equal -ut

opposite( magnetic pole on the other.

5 -ar magnet can -e considered a dipole ,ith a north pole at one end and south pole at the other.

5 magnetic "ield can -e measured lea'ing the dipole at the north pole and returning the magnet at

the south pole. I" a magnet is cut in t,o( t,o magnets or dipoles are created out o" one. This

sectioning and creation o" dipoles can continue to the atomic le'el. There"ore( the source o"

magnetism lies in the -asic -uilding -loc+ o" all matter...the atom.


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1. Magnetism

The Source of Magnetism

5ll matter is composed o" atoms( and atoms are composed o" protons( neutrons and electrons. The

protons and neutrons are located in the atomDs nucleus and the electrons are in constant motion

around the nucleus. Electrons carry a negati'e electrical charge%电荷 and produce a magnetic "ieldas they mo'e through space. 5 magnetic "ield is produced ,hene'er an electrical charge is in

motion. The strength o" this "ield is called the magnetic moment%磁矩.

This may -e hard to 'isualize on a su-atomic scale -ut consider electric current "lo,ing through a

conductor. 6hen the electrons %electric current are "lo,ing through the conductor( a magnetic"ield "orms around the conductor. The magnetic "ield can -e detected using a compass. The

magnetic "ield ,ill place a "orce on the compass needle( ,hich is another example o" a dipole.

ince all matter is comprised o" atoms( all materials are a""ected in some ,ay -y a magnetic "ield.

$o,e'er( not all materials react the same ,ay. This ,ill -e explored more in the next section.


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. Magnetic Materials

Diamagnetic, Paramagnetic, and Ferromagnetic Materials6hen a material is placed ,ithin a magnetic "ield( the magnetic "orces o" the materialDs electrons

,ill -e a""ected. This e""ect is +no,n as aradays


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Diamagnetic抗磁

*iamagnetic materials ha'e a ,ea+( negati'e suscepti-ility to magnetic "ields. Diamagnetic

materials are slightly repelled by a magnetic field and the material does not retain the magnetic

properties when the external field is removed. In diamagnetic materials all the electron are

paired so there is no permanent net magnetic moment per atom. *iamagnetic properties arise

"rom the realignment o" the electron paths under the in"luence o" an external magnetic "ield. Most

elements in the periodic table, including copper, silver, and gold, are diamagnetic元素周期表的

大部分元素为抗磁性，如铜，银，金。

Paramagnetic顺磁

Paramagnetic materials ha'e a small( positi'e suscepti-ility to magnetic "ields. These materials are

slightly attracted by a magnetic field and the material does not retain the magnetic properties

when the external field is removed Paramagnetic properties are due to the presence o" some

unpaired electrons( and "rom the realignment o" the electron paths caused -y the externalmagnetic "ield. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum

顺磁材料如镁，钼，锂和钽


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Ferromagnetic铁磁

erromagnetic materials ha'e a large( positi'e suscepti-ility to an external magnetic "ield. They

exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties

after the external field has been removed erromagnetic materials ha'e some unpaired electrons

so their atoms ha'e a net magnetic moment. They get their strong magnetic properties due to the

presence o" magnetic domains%磁畴. In these domains( large num-ers o" atom/s moments %1B1 to1B1? are aligned parallel so that the magnetic "orce ,ithin the domain is strong. 6hen a

"erromagnetic material is in the unmagnitized state( the domains are nearly randomly organized

and the net magnetic "ield "or the part as a ,hole is zero. 6hen a magnetizing "orce is applied( the

domains -ecome aligned to produce a strong magnetic "ield ,ithin the part. !ron, nic"el, andcobalt are examples o" "erromagnetic materials. 4omponents ,ith these materials are commonly

inspected using the magnetic particle method.

铁、镍、钴


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#. Magnetic *omains

erromagnetic materials get their magnetic properties not only -ecause their atoms carry a

magnetic moment -ut also -ecause the material is made up o" small regions +no,n as magnetic

domains. In each domain( all o" the atomic dipoles are coupled together in a pre"erential direction.

This alignment de'elops as the material de'elops its crystalline structure during solidi"ication "rom

the molten state. Magnetic domains can -e detected using Magnetic orce Microscopy %MM andimages o" the domains li+e the one sho,n -elo, can -e constructed.


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#. Magnetic *omains

*uring solidi"ication( a trillion or more atom moments are aligned parallel so that the magnetic

"orce ,ithin the domain is strong in one direction. erromagnetic materials are said to -e

characterized -y Cspontaneous magneti#ationC since they o-tain saturation magnetization in each

o" the domains ,ithout an external magnetic "ield -eing applied. E'en though the domains are

magnetically saturated( the -ul+ material may not sho, any signs o" magnetism -ecause thedomains de'elop themsel'es and are randomly oriented relati'e to each other.

erromagnetic materials -ecome magnetized ,hen the magnetic domains ,ithin the material are

aligned. This can -e done -y placing the material in a strong external magnetic "ield or -y passing

electrical current through the material. ome or all o" the domains can -ecome aligned. The moredomains that are aligned( the stronger the magnetic "ield in the material. 6hen all o" the domains

are aligned( the material is said to -e magnetically saturated. 6hen a material is magnetically

saturated( no additional amount o" external magnetization "orce ,ill cause an increase in its

internal le'el o" magnetization.“磁饱和”后外加磁场增加，不会增加磁化强度。


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>. Magnetic ields

Magnetic Field !n and $round a %ar Magnet

5s discussed pre'iously( a magnetic "ield is a change in energy ,ithin a 'olume o" space. The

magnetic "ield surrounding a -ar magnet can -e seen in the magnetograph -elo,. 5 magnetograph

can -e created -y placing a piece o" paper o'er a magnet and sprin+ling the paper ,ith iron "ilings.The particles align themsel'es ,ith the lines o" magnetic "orce produced -y the magnet. The

magnetic lines o" "orce sho, ,here the magnetic "ield exits the material at one pole and reenters

the material at another pole along the length o" the magnet. It should -e noted that the magnetic

lines o" "orce exist in three dimensions -ut are only seen in t,o dimensions in the image.

It can -e seen in the magnetograph that there are poles all along the length o" the magnet -ut that

the poles are concentrated at the ends o" the magnet. The area ,here the exit poles are

concentrated is called the magnet/s north pole and the area ,here the entrance poles are

concentrated is called the magnet/s south pole.


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>. Magnetic ieldsMagnetic Fields in and around &orseshoe and 'ing Magnets

Magnets come in a 'ariety o" shapes and one o" the more common is the horseshoe %2 magnet.

The horseshoe magnet has north and south poles 7ust li+e a -ar magnet -ut the magnet is cur'ed

so the poles lie in the same plane. The magnetic lines o" "orce "lo, "rom pole to pole 7ust li+e in the

-ar magnet. $o,e'er( since the poles are located closer together and a more direct path exists "or

the lines o" "lux to tra'el -et,een the poles( the magnetic "ield is concentrated -et,een the poles.

I" a -ar magnet ,as placed across the end o" a horseshoe magnet or i" a magnet ,as "ormed in the

shape o" a ring( the lines o" magnetic "orce ,ould not e'en need to enter the air. The 'alue o" such

a magnet ,here the magnetic "ield is completely contained ,ith the material pro-a-ly has limited

use. $o,e'er( it is important to understand that the magnetic "ield can "lo, in loop ,ithin a

material. %ee section on circular magnetism "or more in"ormation.


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>. Magnetic ields(eneral Properties of Magnetic )ines of Force

Magnetic lines o" "orce ha'e a num-er o" important properties( ,hich include3

• They see+ the path o" least resistance -et,een opposite magnetic poles. In a single -ar magnetas sho,n to the right( they attempt to "orm closed loops "rom pole to pole.

• They ne'er cross one another.

• They all ha'e the same strength.

• Their density decreases %they spread out ,hen they mo'e "rom an area o" higher

permea-ility%渗透性 to an area o" lo,er permea-ility.• Their density decreases ,ith increasing distance "rom the poles.

• They are considered to ha'e direction as i" "lo,ing( though no actual mo'ement occurs.

• They "lo, "rom the south pole to the north pole ,ithin a material and north pole to south pole

in air.


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?. Electromagnetic ieldsMagnets are not the only source o" magnetic "ields. In 1;B( $ans 4hristian =ersted%奥斯特disco'ered that an electric current "lo,ing through a ,ire caused a near-y compass to de"lect. This

indicated that the current in the ,ire ,as generating a magnetic "ield. =ersted studied the nature

o" the magnetic "ield around the long straight ,ire. $e "ound that the magnetic "ield existed in

circular "orm around the ,ire and that the intensity o" the "ield ,as directly proportional to the

amount o" current carried -y the ,ire. $e also "ound that the strength o" the "ield ,as strongestnext to the ,ire and diminished ,ith distance "rom the conductor until it could no longer -e

detected. In most conductors( the magnetic "ield exists only as long as the current is "lo,ing %i.e. an

electrical charge is in motion. $o,e'er( in "erromagnetic materials the electric current ,ill cause

some or all o" the magnetic domains to align and a residual magnetic "ield ,ill remain.


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?. Electromagnetic ields=ersted also noticed that the direction o" the magnetic "ield ,as dependent on the direction o" the

electrical current in the ,ire. 5 three0dimensional representation o" the magnetic "ield is sho,n

-elo,. There is a simple rule "or remem-ering the direction o" the magnetic "ield around a

conductor. It is called the right*hand clasp rule. I" a person grasps a conductor in one/s righthand ,ith the thum- pointing in the direction o" the current( the "ingers ,ill circle the conductor in

the direction o" the magnetic "ield.

右手定律：拇指指向电流方向，其余四指指向磁场方向。

还记得左手定律么，“左力右电”

左手定律的应用：电动机


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?. Electromagnetic ields$ word of caution about the right*hand clasp rule

or the right0hand rule to ,or+( one important thing that must -e remem-ered a-out the

direction o" current "lo,. tandard con'ention has current "lo,ing "rom the positi'e terminal to

the negati'e terminal. This con'ention is credited to !en7amin ran+lin ,ho theorized that electric

current ,as due to a positi'e charge mo'ing "rom the positi'e terminal to the negati'e terminal.

$o,e'er( it ,as later disco'ered that it is the mo'ement o" the negati'ely charged electron that isresponsi-le "or electrical current. 9ather than changing se'eral centuries o" theory and equations(

ran+lin/s con'ention is still used today.


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@. Magnetic ield Produced -y a 4oil6hen a current carrying conductor is "ormed into a loop or se'eral loops to "orm a coil( a magnetic

"ield de'elops that "lo,s through the center o" the loop or coil along its longitudinal axis and circles

-ac+ around the outside o" the loop or coil. The magnetic "ield circling each loop o" ,ire com-ines

,ith the "ields "rom the other loops to produce a concentrated "ield do,n the center o" the coil. 5

loosely ,ound coil is illustrated -elo, to sho, the interaction o" the magnetic "ield. The magnetic

"ield is essentially uni"orm do,n the length o" the coil ,hen it is ,ound tighter.


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@. Magnetic ield Produced -y a 4oilThe strength o" a coilDs magnetic "ield increases not only ,ith increasing current -ut also ,ith each

loop that is added to the coil. 5 long( straight coil o" ,ire is called a solenoid%螺线管 and can -eused to generate a nearly uni"orm magnetic "ield similar to that o" a -ar magnet. The concentrated

magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection

using the magnetic particle testing method. Please be aware that the field outside the coil is weak

and is not suitable for magnetizing ferromagnetic materials.


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A. Quanti"ying Magnetic Properties%Magnetic ield trength( lux *ensity( Total lux and Magnetization

2ntil no,( only the qualitati'e "eatures o" the magnetic "ield ha'e -een discussed. $o,e'er( it is

necessary to -e a-le to measure and express quantitati'ely the 'arious characteristics o"

magnetism. 2n"ortunately( a num-er o" unit con'entions are used %as sho,n in the ta-le -elo,. Iunits ,ill -e used in this material. The ad'antage o" using I units is that they are tracea-le -ac+ to

an agreed set o" "our -ase units 0 meter( +ilogram( second( and 5mpere.

QuantitySI Units

(Sommerfeld)SI Units

(Kennelly)

CGS Units

(Gaussian)

Field H A/m A/m oersteds

Flux Density

(Maneti! Indu!tion) B

tesla tesla auss

Flux f "e#er "e#er max"ell

Maneti$ation M A/m % er/&e%!m '


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A. Quanti"ying Magnetic Properties%Magnetic ield trength( lux *ensity( Total lux and Magnetization

The units "or magnetic field strength & are ampere8meter. A magnetic field strength of 1

amperemeter is produced at the center of a single circular conductor with a one meter diameter

carrying a steady current of 1 ampere.磁场强度用$表示，单位：58m，15的电流流经直径为1m的圆周导线所产生的磁场为158m。

The num-er o" magnetic lines o" "orce cutting through a plane o" a gi'en area at a right angle is

+no,n as the magnetic flux density, %. The "lux density or magnetic induction has the tesla as its

unit. =ne tesla is equal to 1 )e,ton8%58m. rom these units( it can -e seen that the "lux density is

a measure o" the "orce applied to a particle -y the magnetic "ield. The Fauss is the 4F unit "or "luxdensity and is commonly used -y 2 industry. =ne gauss represents one line o" "lux passing

through one square centimeter o" air oriented :B degrees to the "lux "lo,.

磁通密度（磁感应强度）用!表示，单位：Tesla，1TG1)8%58m，美国工业通常用Fauss做单

位，代表垂直穿过1平方厘米平面的一条线。1TG1B(BBBFauss


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A. Quanti"ying Magnetic PropertiesThe total num-er o" lines o" magnetic "orce in a material is called magnetic flux, f The strength o"

the "lux is determined -y the num-er o" magnetic domains that are aligned ,ithin a material. The

total "lux is simply the "lux density applied o'er an area. lux carries the unit o" a ,e-er( ,hich is

simply a tesla0 meter.

磁通量用" 表示，单位：6e-er，16e-erG1THm

The magnetization is a measure o" the extent to ,hich an o-7ect is magnetized. It is a measure o"

the magnetic dipole moment%磁偶极矩 per unit 'olume o" the o-7ect. Magnetization carries thesame units as a magnetic "ield3 amperes8meter.

4on'ersion -et,een 4F and I magnetic units.

; Th $ i < d M i


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;. The $ysteresis


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;. The $ysteresis


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;. The $ysteresis


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;. The $ysteresis


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:. Permea-ility5s pre'iously mentioned( permeability (m) is a material property that descri-es the ease with

which a magnetic flux is established in a component. It is the ratio o" the "lux density %! created

,ithin a material to the magnetizing "ield %$ and is represented -y the "ollo,ing equation3

m 4 %5&

It is clear that this equation descri-es the slope o" the cur'e at any point on the hysteresis loop.

The permea-ility 'alue gi'en in papers and re"erence materials is usually the ma&imum

permeability or the ma&imum relative permeability . The maximum permea-ility is the point ,here

the slope o" the !8$ cur'e "or the unmagnetized material is the greatest. This point is o"ten ta+en

as the point ,here a straight line "rom the origin is tangent to the !8$ cur'e.


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:. Permea-ilityThe relati'e permea-ility%相对磁导率 is arri'ed at -y ta+ing the ratio o" the material/spermea-ility to the permea-ility in "ree space %air.


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:. Permea-ilityThe shape o" the hysteresis loop tells a great deal a-out the material -eing magnetized. Thehysteresis cur'es o" t,o di""erent materials are sho,n in the graph.

9elati'e to other materials( a material ,ith a wider hysteresis loop has3

•


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:. Permea-ilityIn magnetic particle testing( the le'el o" residual magnetism is important. 9esidual magnetic "ieldsare a""ected -y the permea-ility( ,hich can -e related to the car-on content and alloying o" the

material. A component with high carbon content will have low permeability and will retain more

magnetic flu& than a material with low carbon content.

在MT检验中，剩磁量水平很重要，剩余磁场受磁导率影响，与碳含量及合金成分有关。碳含量高的材料磁导率低，比低碳材料有更多的剩磁。

In the t,o !0$ loops a-o'e( ,hich one ,ould indicati'e o" a lo, car-on steelJJ

!lue one.

1B Magnetic ield =rientation and


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1B. Magnetic ield =rientation and

la, *etecta-ilityTo properly inspect a component "or crac+s or other de"ects( it is important to understand that theorientation -et,een the magnetic lines o" "orce and the "la, is 'ery important. There are t,o

general types o" magnetic "ields that can -e esta-lished ,ithin a component.

$ longitudinal magnetic field has magnetic lines o" "orce that run parallel to the long axis o" thepart.


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la, *etecta-ilityThe type o" magnetic "ield esta-lished is determined -y the method used to magnetize thespecimen. !eing a-le to magnetize the part in t,o directions is important -ecause the -est

detection o" de"ects occurs ,hen the lines o" magnetic "orce are esta-lished at right angles to the

longest dimension o" the de"ect. This orientation creates the largest disruption o" the magnetic

"ield ,ithin the part and the greatest "lux lea+age at the sur"ace o" the part. 5s can -e seen in theimage -elo,( i" the magnetic "ield is parallel to the de"ect( the "ield ,ill see little disruption and no

"lux lea+age "ield ,ill -e produced.

1B Magnetic ield =rientation and


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la, *etecta-ility5n orientation o" >? to :B degrees -et,een the magnetic "ield and the de"ect is necessary to "orman indication. ince de"ects may occur in 'arious and un+no,n directions( each part is normally

magnetized in t,o directions at right angles to each other. I" the component -elo, is considered( it

is +no,n that passing current through the part "rom end to end ,ill esta-lish a circular magnetic

"ield that ,ill -e :B degrees to the direction o" the current. There"ore( de"ects that ha'e asigni"icant dimension in the direction o" the current %longitudinal de"ects should -e detecta-le.

5lternately( trans'erse0type de"ects ,ill not -e detecta-le ,ith circular magnetization.

'atch this short movie showing the effect of field direction on indication visibility. $(.1)%

11 Magnetization o" erromagnetic


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11. Magnetization o" erromagnetic

MaterialsThere are a 'ariety o" methods that can -e used to esta-lish a magnetic "ield in a component "ore'aluation using magnetic particle inspection. It is common to classi"y the magnetizing methods as

either direct or indirect.

• Magneti#ation 6sing Direct !nduction 0Direct Magneti#ation1

• Magneti#ation 6sing !ndirect !nduction 0!ndirect Magneti#ation1



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MaterialsMagneti#ation 6sing Direct !nduction 0Direct Magneti#ation1

6ith direct magnetization( current is passed directly through the component. 9ecall that ,hene'er

current "lo,s( a magnetic "ield is produced. 2sing the right0hand rule( ,hich ,as introduced earlier(

it is +no,n that the magnetic lines o" "lux "orm normal to the direction o" the current and "orm acircular "ield in and around the conductor. 6hen using the direct magnetization method( care must

-e ta+en to ensure that good electrical contact is esta-lished and maintained -et,een the test

equipment and the test component. Improper contact can result in arcing that may damage the

component. It is also possi-le to o'erheat components in areas o" high resistance such as the

contact points and in areas o" small cross0sectional area.

There are se'eral ,ays that direct magnetization is commonly accomplished. =ne ,ay in'ol'es

clamping the component -et,een t,o electrical contacts in a special piece o" equipment. 4urrent

is passed through the component and a circular magnetic "ield is esta-lished in and around the

component. 6hen the magnetizing current is stopped( a residual magnetic "ield ,ill remain ,ithin

the component. The strength o" the induced magnetic "ield is proportional to the amount o"

current passed through the component.

5 second technique in'ol'es using clamps or prods( ,hich are attached or placed in contact ,ith

the component. Electrical current "lo,s through the component "rom contact to contact. The

current sets up a circular magnetic "ield around the path o" the current.

11 Magnetization o" erromagnetic


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MaterialsMagneti#ation 6sing !ndirect !nduction 0!ndirect Magneti#ation1Indirect magnetization is accomplished -y using a strong external magnetic "ield to esta-lish a

magnetic "ield ,ithin the component. 5s ,ith direct magnetization( there are se'eral ,ays that

indirect magnetization can -e accomplished.

The use o" permanent magnets0永磁铁

1 is a lo, cost method o" esta-lishing a magnetic "ield.$o,e'er( their use is limited due to lac+ o" control o" the "ield strength and the di""iculty o" placing

and remo'ing strong permanent magnets "rom the component.

7lectromagnets0电磁铁

1 in the "orm o" an ad7usta-le horseshoe magnet %called a yo+e eliminate

the pro-lems associated ,ith permanent magnets and are used extensi'ely in industry.

Electromagnets only exhi-it a magnetic "lux ,hen electric current is "lo,ing around the so"t ironcore. 6hen the magnet is placed on the component( a magnetic "ield is esta-lished -et,een the

north and south poles o" the magnet( a magnetic "ield esta-lishes a magnetic "ield ,ithin the test

components.



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Materials5nother ,ay o" indirectly inducting a magnetic "ield in a material is -y using the magnetic "ield o" acurrent carrying conductor. 5 circular magnetic "ield can -e esta-lished in cylindrical components

-y using a central conductor. Typically( one or more cylindrical components are hung "rom a solid

copper -ar running through the inside diameter. 4urrent is passed through the copper -ar and the

resulting circular magnetic "ield esta-lishes a magnetic "ield ,ithin the test components.

The use o" coils and solenoids is a third method o" indirect magnetization. 6hen the length o" a

component is se'eral times larger than its diameter( a longitudinal magnetic "ield can -e

esta-lished in the component. The component is placed longitudinally in the concentrated

magnetic "ield that "ills the center o" a coil or solenoid. This magnetization technique is o"ten

re"erred to as a 8coil shot8


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1. Magnetizing 4urrent

5s seen in the pre'ious pages( electric current is o"ten used to esta-lish the magnetic "ield in

components during magnetic particle inspection. 5lternating current and direct current are the

t,o -asic types o" current commonly used. 4urrent "rom single phase 11B 'olts( to three phase

>>B 'olts( are used ,hen generating an electric "ield in a component. 4urrent "lo, is o"ten

modi"ied to pro'ide the appropriate "ield ,ithin the part. The type o" current used can ha'e an

e""ect on the inspection results( so the types o" currents commonly used ,ill -e -rie"ly re'ie,ed.

• Direct urrent

• $lternating urrent

• 'ectified $lternating urrent• &alf 9ave 'ectified $lternating urrent 0&9$1

• Full 9ave 'ectified $lternating urrent 0F9$1 0Single Phase1

• Three Phase Full 9ave 'ectified $lternating urrent

1 M i i 4


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1. Magnetizing 4urrentDirect urrent

*irect current %*4 "lo,s continuously in one direction at a constant 'oltage. 5 -attery is the most

common source o" direct current. 5s pre'iously mentioned( current is said to "lo, "rom the

positi'e to the negati'e terminal. In actuality( the electrons "lo, in the opposite direction. DC is

very desirable when inspecting for subsurface defects because DC generates a magnetic field

that penetrates deeper into the material. In "erromagnetic materials( the magnetic "ield produced-y *4 generally penetrates the entire cross0section o" the component. 4on'ersely( the "ield

produced using alternating current is concentrated in a thin layer at the sur"ace o" the component.

直流电更适合检测近表面缺陷（探测深度比交流深）

$lternating urrent

5lternating current %54 re'erses in direction at a rate o" ?B or @B cycles per second. In the 2nitedtates( @B cycle current is the commercial norm -ut ?B cycle current is common in many countries.

ince 54 is readily a'aila-le in most "acilities( it is con'enient to ma+e use o" it "or magnetic particle

inspection. $o,e'er( ,hen 54 is used to induce a magnetic "ield in "erromagnetic materials( the

magnetic "ield ,ill -e limited to narro, region at the sur"ace o" the component. This phenomenon

is known as the skin effect and occurs because the changing magnetic field generates eddycurrents in the test ob!ect. The eddy currents produce a magnetic "ield that opposes the primary

"ield( thus reducing the net magnetic "lux -elo, the sur"ace. There"ore( it is recommended that 54

-e used only ,hen the inspection is limited to sur"ace de"ects.

交流电因为“集肤效应”，电流方向转化时产生涡流，抵消了一部分磁场，适合探测表面缺陷

*iew a short video on the A+ versus ,+. $(.1(

1 M i i 4


46/153

1. Magnetizing 4urrent'ectified $lternating urrent

4learly( the s+in e""ect limits the use o" 54 since many inspection applications call "or the detection

o" su-sur"ace de"ects. $o,e'er( the con'enient access to 54( dri'es its use -eyond sur"ace "la,

inspections.


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1. Magnetizing 4urrent&alf 9ave 'ectified $lternating urrent 0&9$1

6hen single phase alternating current is passed through a recti"ier( current is allo,ed to "lo, in

only one direction. The re'erse hal" o" each cycle is -loc+ed out so that a one directional(

pulsating%脉冲 current is produced. The current rises "rom zero to a maximum and then returns tozero. )o current "lo,s during the time ,hen the re'erse cycle is -loc+ed out. The $654 repeats at

same rate as the unrecti"ied current %@B hertz typical. ince hal" o" the current is -loc+ed out( theamperage is hal" o" the unaltered 54.

This type o" current is o"ten re"erred to as hal" ,a'e *4 or pulsating *4. The pulsation of the

"#$C helps magnetic particle indications form by vibrating the particles and giving them added

mobility . This added mo-ility is especially important when using dry particles. The pulsation is

reported to signi"icantly impro'e inspection sensiti'ity. $654 is most o"ten used to po,erelectromagnetic yo+es.半波整流提高了颗粒的流动性（振动），对于干法MT有很重要的意义，

常用于电磁轭。

1 M ti i 4 t


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1. Magnetizing 4urrentFull 9ave 'ectified $lternating urrent 0F9$1 0Single Phase1ull ,a'e recti"ication in'erts the negati'e current to positi'e current rather than -loc+ing it out.

This produces a pulsating *4 ,ith no inter'al -et,een the pulses. iltering is usually per"ormed to

so"ten the sharp polarity s,itching in the recti"ied current. #hile particle mobility is not as good

as half%wave $C due to the reduction in pulsation, the depth of the subsurface magnetic field is

improved.全波整流颗粒流动性不如半波，但是增加了磁场的渗透性，探测深度提高。

1 Magneti ing 4 rrent


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1. Magnetizing 4urrentThree Phase Full 9ave 'ectified $lternating urrentThree phase current is o"ten used to po,er industrial equipment -ecause it has more "a'ora-le

po,er transmission and line loading characteristics. This type o" electrical current is also highly

desira-le "or magnetic particle testing -ecause ,hen it is recti"ied and "iltered( the resulting

current 'ery closely resem-les direct current. &tationary magnetic particle e'uipment wired with

three phase $C will usually have the ability to magnetie with $C or DC (three phase full waverectified), providing the inspector with the advantages of each current form.

三相全波整流常用于固定设备（用于检测电力设备，更好的电力传输和负载能力），可以选择使用54或者*4（三相全波整流），为检验员提供方便。

1#.


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g g

*istri-ution and Intensity6hen the length o" a component is se'eral times larger than its diameter( a longitudinal magnetic"ield can -e esta-lished in the component. The component is o"ten placed longitudinally in the

concentrated magnetic "ield that "ills the center o" a coil or solenoid. This magnetization technique

is o"ten re"erred to as a Ccoil shot.K

The magnetic "ield tra'els through the component "rom end to end ,ith some "lux loss along its

length as sho,n in the image to the right. Leep in mind that the magnetic lines o" "lux occur in

three dimensions and are only sho,n in * in the image. The magnetic lines o" "lux are much more

dense inside the "erromagnetic material than in air -ecause "erromagnetic materials ha'e much

higher permea-ility than does air. 6hen the concentrated "lux ,ithin the material comes to the air

at the end o" the component( it must spread out since the air can not support as many lines o" "lux

per unit 'olume. To +eep "rom crossing as they spread out( some o" the magnetic lines o" "lux are

"orced out the side o" the component.

1#.


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*istri-ution and Intensity6hen a component is magnetized along its complete length( the "lux loss is small along its length.There"ore( ,hen a component is uni"orm in cross section and magnetic permea-ility( the "lux

density ,ill -e relati'ely uni"orm throughout the component. la,s that run normal to the

magnetic lines o" "lux ,ill distur- the "lux lines and o"ten cause a lea+age "ield at the sur"ace o" the

component.

1#.


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*istri-ution and Intensity6hen a component ,ith considera-le length is magnetized using a solenoid%螺线管( it is possi-leto magnetize only a portion o" the component. =nly the material ,ithin the solenoid and a-out

the same ,idth on each side o" the solenoid ,ill -e strongly magnetized. 5t some distance "rom

the solenoid( the magnetic lines o" "orce ,ill a-andon their longitudinal direction( lea'e the part at

a pole on one side o" the solenoid and return to the part at a opposite pole on the other side o"the solenoid. This occurs -ecause the magnetizing "orce diminishes ,ith increasing distance "rom

the solenoid. 5s a result( the magnetizing "orce may only -e strong enough to align the magnetic

domains ,ithin and 'ery near the solenoid. The unmagnetized portion o" the component ,ill not

support as much magnetic "lux as the magnetized portion and some o" the "lux ,ill -e "orced out

o" the part as illustrated in the image -elo,. There"ore( a long component must -e magnetized and

inspected at se'eral locations along its length "or complete inspection co'erage.

1>. 4ircular Magnetic ields


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*istri-ution and Intensity5s discussed pre'iously( ,hen current is passed through a solid conductor( a magnetic "ield "ormsin and around the conductor. The "ollo,ing statements can -e made a-out the distri-ution and

intensity o" the magnetic "ield.

• The "ield strength 'aries "rom zero at the center o" the component to a maximum at the sur"ace.

• The "ield strength at the sur"ace o" the conductor decreases as the radius o" the conductor

increases ,hen the current strength is held constant. %$o,e'er( a larger conductor is capa-le o"

carrying more current.

• The "ield strength outside the conductor is directly proportional to the current strength. Inside

the conductor( the "ield strength is dependent on the current strength( magnetic permea-ilityo" the material( and i" magnetic( the location on the !0$ cur'e.

• The "ield strength outside the conductor decreases ,ith distance "rom the conductor.



54/153

*istri-ution and IntensityIn the images -elo,( the magnetic "ield strength is graphed 'ersus distance "rom the center o" theconductor. It can -e seen that in a nonmagnetic conductor carrying D( the internal "ield strength

rises "rom zero at the center to a maximum 'alue at the sur"ace o" the conductor. The external "ield

strength decrease ,ith distance "rom the sur"ace o" the conductor. 6hen the conductor is a

magnetic material( the "ield strength ,ithin the conductor is much greater than it is in thenonmagnetic conductor. This is due to the permeability of the magnetic material0:1. The external

field is exactly the same for the two materials provided the current level and conductor radius

are the same相同

D的情况下，顺磁材料内部有更强的磁场（磁导率高），强度与距中心

距离成正比，同直径材料（不论磁导率）产生的外部磁场相同。



55/153

*istri-ution and Intensity6hen the conductor is carrying alternating current%$( the internal magnetic "ield strength rises"rom zero at the center to a maximum at the sur"ace. $o,e'er( the "ield is concentrated in a thin

layer near the sur"ace o" the conductor. This is +no,n as the Cs"in effect.C The s+in e""ect is e'ident

in the "ield strength 'ersus distance graph "or a magnetic conductor sho,n to the right. The

external field decreases with increasing distance from the surface as it does with D !t should beremembered that with $ the field is constantly varying in strength and direction

对于

$，内部磁场强度与距离不成正比，磁场更多聚集于表面（集肤效应），但最大值和

外部磁场同

D

注：区别

DC 和

$C 的方法，内部磁场

DC 近似直线，

$C 为加速上升曲线



56/153

*istri-ution and IntensityIn a hollo, circular conductor there is no magnetic field in the void area. The magnetic "ield is zeroat the inside ,all sur"ace and rises until it reaches a maximum at the outside ,all sur"ace. 5s ,ith a

solid conductor( ,hen the conductor is a magnetic material( the "ield strength ,ithin the conductor

is much greater than it ,as in the nonmagnetic conductor due to the permea-ility o" the magnetic

material. The external "ield strength decreases ,ith distance "rom the sur"ace o" the conductor.The external field is exactly the same for the two materials provided the current level and

conductor radius are the same

对于空心导体，内部空间（及内壁）磁场为零，外壁磁场最强，顺磁材料外壁磁场强度最

高，相同外径的材料（无论磁导率），外部磁场相同。

（下图有误，两个

F 应该一样高）



57/153

*istri-ution and Intensity9hen $ is passed through a hollow circular conductor, the s"in effect concentrates themagnetic field at the outside diameter of the component

对于

$，集肤效应导致外壁磁场强度升高，（加速上升曲线）



58/153

*istri-ution and Intensity补充：相同 $C 和DC 强度情况下，曲线上升速度不一样，DC 近似直线， $C 为加速上升曲线，开始缓慢，近表面迅速提升

%%%%%中文教材

另外，考虑到能量守恒，相同

$C 和

DC 能达到的最大磁感应强度不同

$C 部分损失变为涡流

和反向磁场）

图

%*可理解为非磁性材料的磁场，图

%+ 可理解为铁磁材料的磁场

那么，非磁性材料为何不能用直接磁化法进行检验？是否因为表面磁场依然很弱？）



59/153

*istri-ution and Intensity5s can -e learned "rom these three "ield distri-ution images( the "ield strength at the inside sur"aceo" hollo, conductor is 'ery lo, ,hen a circular magnetic "ield ,as esta-lished -y direct

magnetization. There"ore( the direct method of magneti#ation is not recommended when

inspecting the inside diameter wall of a hollow component for shallow defects. The "ield strength

increases rapidly as one mo'es out %into the material "rom the I*( so i" the de"ect has signi"icantdepth( it may -e detecta-le.直接磁化法不能用于探测空心导体的内壁缺陷。

$o,e'er( a much -etter method o" magnetizing

hollo, components "or inspection o" the I* and

=* sur"aces is ,ith the use o" a central conductor.5s can -e seen in the "ield distri-ution image to

the right( ,hen current is passed through a

nonmagnetic central conductor %copper -ar( the

magnetic "ield produced on the inside diameter

sur"ace o" a magnetic tu-e is much greater and

the "ield is still strong enough "or de"ect detection

on the =* sur"ace.

空心导体内部穿过通电棒可以实现内外壁同时

MT检测。

1?. *emagnetization


60/153

1?. *emagnetization

5"ter conducting a magnetic particle inspection( it is usually necessary to demagnetize the

component. 9emanent magnetic "ields can3残余磁场会导致：

• a""ect machining -y causing cuttings to cling to a component.%机加工铁屑吸附

• inter"ere ,ith electronic equipment such as a compass. %影响电气设备• create a condition +no,n as Karc -lo, in the ,elding process. 5rc -lo, may cause the ,eld

arc to ,onder or "iller metal to -e repelled "rom the ,eld.%焊接磁偏吹

• cause a-rasi'e particles to cling to -earing or "aying sur"aces and increase ,ear.%颗粒吸附导致

磨损

9emo'al o" a "ield may -e accomplished in se'eral ,ays. This random orientation o" the magnetic

domains can -e achie'ed most e""ecti'ely -y heating the material a-o'e its curie temperature. The

curie temperature "or a low carbon steel is ;;<℃

or -/=


61/153

1?. *emagnetizationIt is o"ten incon'enient to heat a material a-o'e its curie temperature to demagnetize it( soanother method that returns the material to a nearly unmagnetized state is commonly used.

u-7ecting the component to a re'ersing and decreasing magnetic "ield ,ill return the dipoles to a

nearly random orientation throughout the material. This can -e accomplished -y pulling a

component out and a,ay "rom a coil with $ passing through it. The same can also -e

accomplished using an electromagnetic yo+e ,ith 54 selected. 5lso( many stationary magnetic

particle inspection units come ,ith a demagnetization "eature that slo,ly reduces the 54 in a coil

in ,hich the component is placed.

方法二：交流线圈退磁法，穿过交流线圈，或者使用交流磁轭，或者放于交流线圈中缓慢减小电流强度。

5 "ield meter is o"ten used to 'eri"y that the residual "lux has -een

remo'ed "rom a component. Industry standards usually require

that the magnetic "lux -e reduced to less than # gauss a"ter

completing a magnetic particle inspection.工业标准要求剩余磁场强度低于#F，则认为MPI后退磁完成。

场强计时常被用于测量残余磁场强度。（右图有点看不懂）

1@. Measuring Magnetic ields


62/153

g g6hen per"orming a magnetic particle inspection( it is 'ery important to -e a-le to determine thedirection and intensity%强度 o" the magnetic "ield. 5s discussed pre'iously( the direction o" themagnetic "ield should -e -et,een >? and :B degrees to the longest dimension o" the "la, "or -est

detecta-ility. The "ield intensity must -e high enough to cause an indication to "orm( -ut not too

high to cause nonrele'ant indications to mas+ rele'ant indications. To cause an indication to "orm(

the "ield strength in the o-7ect must produce a "lux lea+age "ield that is strong enough to hold the

magnetic particles in place o'er a discontinuity. lux measurement de'ices can pro'ide important

in"ormation a-out the "ield strength.

ince it is impractical to measure the actual "ield strength ,ithin the material( all the de'ices

measure the magnetic "ield that is outside o" the material. There are a num-er o" di""erent de'ices

that can -e used to detect and measure an external magnetic "ield. The t,o de'ices commonly

used in magnetic particle inspection are the field indicator and the &all*effect meter( ,hich is also

called a gauss meter. Pie gauges and shims are de'ices that are o"ten used to pro'ide an indication

of the field direction and strength but do not actually yield a 'uantitative measure. They ,ill -e

discussed in a later section.



63/153

g gField !ndicatorsield indicators are small mechanical de'ices that utilize a so"t iron 'ane%叶片 that is de"lected -ya magnetic "ield. The N0ray image -elo, sho,s the inside ,or+ing o" a "ield meter loo+ing in "rom

the side. The 'ane is attached to a needle that rotates and mo'es the pointer "or the scale. ield

indicators can -e ad7usted and cali-rated so that quantitati'e in"ormation can -e o-tained.

$o,e'er( the measurement range o" "ield indicators is usually small due to the mechanics o" thede'ice. The one sho,n to the right has a range "rom plus B gauss to minus B gauss. This limited

range ma+es them -est suited "or measuring the residual magnetic "ield a"ter demagnetization.

场强表因量程较小，常用于测量剩余磁场，下图测量范围为±B高斯

5 "ield indicator is sho,n chec+ing "or residual magnetism

in this mo'ie. %.1@ mo'

影片.1@所示为剩余磁场测量



64/153

g g&all*7ffect 0(auss5Tesla1 Meter

高斯计

5 $all0e""ect meter is an electronic de'ice that pro'ides a digital readout o" the magnetic "ield

strength in gauss or tesla units. The meters use a very small conductor or semiconductor element

at the tip o" the pro-e. Electric current is passed through the conductor. In a magnetic "ield( a "orce

is exerted on the mo'ing electrons ,hich tends to push them to one side o" the conductor. 5

-uildup o" charge at the sides o" the conductors ,ill -alance this magnetic in"luence( producing ameasura-le 'oltage -et,een the t,o sides o" the conductor. The presence o" this measura-le

trans'erse 'oltage is called the $all0e""ect a"ter Ed,in $. $all( ,ho disco'ered it in 1;A:.

磁场影响通电导体的原理，测量的是电压，算出来的是磁场



65/153

g gThe 'oltage generated >h can -e related to the external magnetic "ield -y the "ollo,ing equation.

>h 4 ! % 'h 5 b

6here3

>h is the 'oltage generated.

! is the applied direct current.

% is the component o" the magnetic "ield that is at a right angle to the direct current in the $all

element.

'h is the $all 4oe""icient o" the $all element%霍尔元件.

b is the thic+ness o" the $all element.

“右手法则”



66/153

g gPro-es are a'aila-le ,ith either tangential %trans'erse or axial sensing elements. Pro-es can -epurchased in a ,ide 'ariety o" sizes and con"igurations and ,ith di""erent measurement ranges.

The pro-e is placed in the magnetic "ield such that the magnetic lines o" "orce intersect the ma7or

dimensions o" the sensing element at a right angle. Placement and orientation o" the pro-e is 'ery

important and ,ill -e discussed in a later section.

探头分为横向和轴向，探头的方向和放置非常重要，在后面章节会讨论。

III. Equipment and materials


67/153

q p

1. Porta-le Equipment

. tationary Equipment

#. . ield trength Indicators

?. Magnetic Particles

@. uspension


68/153

To properly inspect a part "or crac+s or other de"ects( it is important to -ecome "amiliar ,ith the

di""erent types o" magnetic "ields and the equipment used to generate them. 5s discussed

pre'iously( one o" the primary requirements "or detecting a de"ect in a "erromagnetic material is

that the magnetic "ield induced in the part must intercept the de"ect at a >? to :B degree angle.

la,s that are normal %:B degrees to the magnetic "ield ,ill produce the strongest indications

-ecause they disrupt more o" the magnet "lux.

There"ore( "or proper inspection o" a component( it is important to -e a-le to esta-lish a magnetic

"ield in at least t,o directions. 5 'ariety o" equipment exists to esta-lish the magnetic "ield "or MPI.

=ne ,ay to classi"y equipment is -ased on its porta-ility. ome equipment is designed to -e

porta-le so that inspections can -e made in the "ield and some is designed to -e stationary "orease o" inspection in the la-oratory or manu"acturing "acility. Porta-le equipment ,ill -e discussed

"irst.

1. Porta-le Equipment "or MPI


69/153

• Permanent magnets

• 7lectromagnets

• Prods

• Portable oils and onductive ables

• Portable Power Supplies



70/153

Permanent magnets

Permanent magnets are sometimes used "or magnetic particle inspection as the source o"

magnetism. The t,o primary types o" permanent magnets are bar magnets and horseshoe 0yo"e1

magnets. These industrial magnets are usually 'ery strong and may require signi"icant strength to

remo'e them "rom a piece o" metal. ome permanent magnets require over 3< pounds o" "orce to

remo'e them "rom the sur"ace. !ecause it is difficult to remove the magnets from the component

being inspected, and sometimes difficult and dangerous to place the magnets( their use is not

particularly popular. $o,e'er( permanent magnets are sometimes used -y di'ers "or inspection in

underwater en'ironments or other areas( such as explosive environments( ,here electromagnets

cannot -e used. Permanent magnets can also -e made small enough to "it into tight areas ,here

electromagnets might not "it.



71/153

7lectromagnets

Today( most o" the equipment used to create the magnetic "ield used in MPI is -ased on

electromagnetism. That is( using an electrical current to produce the magnetic "ield. 5n

electromagnetic yo+e is a 'ery common piece o" equipment that is used to esta-lish a magnetic

"ield. It is -asically made -y ,rapping an electrical coil around a piece o" so"t "erromagnetic steel. 5

s,itch is included in the electrical circuit so that the current and( there"ore( the magnetic "ield can

-e turned on and o"". They can -e po,ered ,ith alternating current "rom a ,all soc+et or -y direct

current "rom a -attery pac+. This type o" magnet generates a 'ery strong magnetic "ield in a local

area ,here the poles o" the magnet touch the part -eing inspected. ome yo+es can lift weights in

excess of 2< pounds



72/153



73/153

Prods

Prods are handheld electrodes that are pressed against the sur"ace o" the component -eing

inspected to ma+e contact "or passing electrical current through the metal. The current passing

-et,een the prods creates a circular magnetic "ield around the prods that can -e used in magnetic

particle inspection. Prods are typically made "rom copper and ha'e an insulated handle to help

protect the operator. =ne o" the prods has a trigger s,itch so that the current can -e quic+ly and

easily turned on and o"". ometimes the t,o prods are connected -y any insulator %as sho,n in the

image to "acilitate one hand operation. This is re"erred to as a dual prod and is commonly used "or

,eld inspections.

I" proper contact is not maintained -et,een the prods and the component sur"ace( electrical

arcing can occur and cause damage to the component. or this reason( the use o" prods are not

allo,ed ,hen inspecting aerospace and other critical components. To help pre'ent arcing( the

prod tips should be inspected fre?uently to ensure that they are not oxidized( co'ered ,ith scale

or other contaminant( or damaged.

The "ollo,ing applet sho,s t,o prods used to create a current through a conducting part. The

resultant magnetic "ield roughly depicts the patterns expected "rom an magnetic particle

inspection o" an un"la,ed sur"ace. The user is encouraged to manipulate the prods to orient the

magnetic "ield to Ccut acrossC suspected de"ects.



74/153

注：环形磁场的方向按照右手定律确定，可见叠加

后的磁场方向与两个电极连线呈

* 度，即电极分布

焊缝两侧，探测横向缺陷，分布在焊缝长度方向，

探测纵向缺陷



75/153

Portable oils and onductive ables4oils and conducti'e ca-les are used to esta-lish a longitudinal magnetic "ield ,ithin a component.9hen a preformed coil is used, the component is placed against the inside surface on the coil.

4oils typically ha'e three or "i'e turns o" a copper ca-le ,ithin the molded "rame. 5 "oot s,itch is

o"ten used to energize the coil. 4onducti'e ca-les are ,rapped around the component. The ca-le

used is typically BB extra "lexi-le or BBBB extra "lexi-le. The num-er o" ,raps is determined -y the

magnetizing "orce needed and o" course( the length o" the ca-le. )ormally( the ,raps are +ept as

close together as possi-le. 6hen using a coil or ca-le ,rapped into a coil( amperage is usually

expressed in ampere*turns. 5mpere0turns is the amperage sho,n on the amp meter times the

num-er o" turns in the coil.

6atch these short mo'ies sho,ing a ca-le -eing used to esta-lish magnetic "ields in parts. 4a-le

,rapped around part %#.1.mo'. 4a-le ,rapped through the part %#.1.#mo'.



76/153

Portable Power SuppliesPorta-le po,er supplies are used to pro'ide the necessary electricity to the prods( coils or ca-les.Po,er supplies are commercially a'aila-le in a 'ariety o" sizes. Small power supplies generally

pro'ide up to -,3


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tationary magnetic particle inspection equipment is designed "or use in la-oratory or production

en'ironment. The most common stationary system is the ,et horizontal %-ench unit. 6et

horizontal units are designed to allo, "or -atch inspections o" a 'ariety o" components. The units

ha'e head and tail stoc+s %similar to a lathe ,ith electrical contact that the part can -e clamped

-et,een. 5 circular magnetic "ield is produced ,ith direct magnetization. The tail stoc+ can -e

mo'ed and loc+ed into place to accommodate parts o" 'arious lengths. To assist the operator in

clamping the parts( the contact on the headstoc+ can -e mo'ed pneumatically 'ia a "oot s,itch.

. tationary Equipment "or MPIMost units also ha'e a mo'a-le coil that can -e mo'ed into place so the indirect magnetization can


78/153

Most units also ha'e a mo'a-le coil that can -e mo'ed into place so the indirect magnetization can

-e used to produce a longitudinal magnetic "ield. Most coils ha'e "i'e turns and can -e o-tained in

a 'ariety o" sizes. The ,et magnetic particle solution is collected and held in a tan+. 5 pump and

hose system is used to apply the particle solution to the components -eing inspected. Either the

'isi-le or "luorescent particles can -e used. ome o" the systems o""er a 'ariety o" options in

electrical current used "or magnetizing the component. The operator has the option to use *4( hal"

,a'e 54( or "ull ,a'e 54. In some units( a demagnetization "eature is -uilt in( ,hich uses the coiland decaying 54.

To inspect a part using a head0shot( the part is clamped -et,een t,o electrical contact pads. The

magnetic solution( called a -ath( is then "lo,ed o'er the sur"ace o" the part. The -ath is then

interrupted and a magnetizing current is applied to the part "or a short duration( typically B.? to 1.?

seconds. %Precautions should -e ta+en to pre'ent -urning or o'erheating o" the part. 5 circular"ield "lo,ing around the circum"erence o" the part is created.


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Multidirectional units allo, the component to -e magnetized in t,o directions( longitudinally and

circum"erentially( in rapid succession. There"ore( inspections are conducted ,ithout the need "or a

second shot. In multidirectional units( the t,o "ields are -alanced so that the "ield strengths are

equal in -oth directions. These quic+ly changing -alanced "ields produce a multidirectional "ield in

the component pro'iding detection o" de"ects lying in more than one direction.

Oust as in con'entional ,et0horizontal systems( the electrical current used in multidirectional

magnetization may -e alternating( hal"0,a'e direct( or "ull0,a'e. It is also possi-le to use a

com-ination o" currents depending on the test applications.

Multidirectional magnetization can -e used "or a large

num-er o" production applications( and high 'olume

inspections.

To determine adequate "ield strength and -alance o" the

rapidly changing "ields( technique de'elopment requires a

little more e""ort ,hen multidirectional equipment is used.It is desira-le to de'elop the technique using a component

,ith +no,n de"ects oriented in at least t,o directions( or

a manu"actured de"ect standard. Quantitati'e Quality

Indicators %QQI are also o"ten used to 'eri"y the strength

and direction o" magnetic "ields.

#.


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Magnetic particle inspection can -e per"ormed using particles that are highly 'isi-le under ,hite

light conditions or particles that are highly 'isi-le under ultra'iolet light conditions. 6hen an

inspection is -eing per"ormed using the 'isi-le color contrast particles( no special lighting is

required as long as the area o" inspection is ,ell lit. $ light intensity of at least -


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g

2ltra'iolet light or C-lac+ lightC is light in the 1(BBB to >(BBB 5ngstroms %1BB to >BBnm ,a'elength

range in the electromagnetic spectrum. It is a 'ery energetic "orm o" light that is in'isi-le to the

human eye. 9avelengths above 2,BBnm以上的可见光为紫光（>BBnm以下不可见，紫外线），紫外线分为2V05( 2V0!(

2V04，随着波长依次减小，能量依次升高，对人体细胞危害也增大。

Class

U%A

U%U%C

*a+elent, -ane

'.0012.000 Anstroms

.300

1'.00 Anstroms.30014.000 Anstroms

#.


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a pea" wavelength at about /,@3BBnm，峰值波长为#@?nm，属于2V05的范围。2V0!可以更有效激发荧光，但对皮肤和眼睛伤害更大。2V04可以杀死细胞，用于消毒。

The desired ,a'elength range "or use in )*T is o-tained -y "iltering the ultra'iolet light generated

-y the light -ul-. The output o" a 2V -ul- spans a ,ide range o" ,a'elengths. The short

,a'elengths o" #(1B to #(#>B5 are produced in lo, le'els. 5 pea+ ,a'elength o" #@?B5 isproduced at a 'ery high intensity. 6a'elengths in the 'isi-le 'iolet range %>B?B5 to >#?B5( green0

yello, %?>@B5( yello, %@B5 and orange %@AAB5 are also usually produced. The "ilter allo,s

only radiation in the range o" #BB to >BBB5 and a little 'isi-le dar+ purple to pass.

紫外灯可以发出很多种波长的光，只有#B>BBnm的光和一点可见的暗紫色可以过滤后通过。

Class

U%A

U%

U%C

*a+elent, -ane

'.0012.000 Anstroms

.3001'.00 Anstroms

.30014.000 Anstroms


83/153

#.


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inside the -ul-. This tu-e contains t,o electrodes that esta-lish an arc. The distance -et,een

electrodes is such that a starting electrode must -e used. 5 resister limits the current to the

starting electrode that esta-lishes the initial arc that 'aporizes the mercury in the tu-e. =nce this

lo, le'el arc is esta-lished and the mercury is 'aporized( the arc -et,een the main electrodes is

esta-lished !t ta"es approximately five minutes to 8warm up8 and establish the arc between the

main electrodes. This is ,hy speci"ications require a C,arm up timeC -e"ore using the highpressure mercury 'apor lights. lood and spot -lac+ lights produce large amounts o" heat and

should -e handled ,ith caution to pre'ent -urns. This condition has -een eliminated -y ne,er

designs that include cooling "ans. The arc in the -ul- can -e upset ,hen exposed to an external

magnetic "ield( such as that generated -y a coil. 4are should -e ta+en not to -ring the lamp close to

strong magnetic "ields( -ut i" the arc is upset and extinguished( it must -e allo,ed to cool -e"ore it

can -e sa"ely restarted.

高压汞灯或反光灯，紫外线由灯泡里的石英管产生，要使用

起弧电极（电流）将汞汽化，然后正式点燃电弧，这就是

为何需要大约?分钟的“预热”才可以使用高压汞灯。

因为是电弧发热，要注意散热和避免强磁场的影响，一旦受到磁场干涉，要等待冷却后再次启动。

#.


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The >BB ,att metal halide -ul-s or Csuper lightsC can -e "ound in some "acilities. This super -right

light ,ill pro'ide adequate lighting o'er an area o" up to ten times that co'ered -y the 1BB ,att

-ul-. *ue to their high intensity( excessi'e light re"lecting "rom the sur"ace o" a component is a

concern. Mo'ing the light a greater distance "rom the inspection area ,ill generally reduce this

glare. 5nother type o" high intensity light a'aila-le is the micro0discharge light. This particular light

produces up to ten times the amount o" 2V light con'entional lights produce. 9eadings o" up to@B(BBB u68cm at 1? inches can -e achie'ed.

高强度紫外灯，要注意“反射光”的影响，>BB6的灯可以照亮1B倍于1BB6灯的范围。

离远一点会好些。

另外还有一种叫“micro0discharge light”微发电J的灯，

可以产生1B倍于普通方法的紫外线

@B(BBBu,8cm读数于1?in距离

>. Magnetic ield Indicators*etermining ,hether a magnetic "ield is o" adequate strength and in the proper direction is critical


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,hen per"orming magnetic particle testing. 5s discussed pre'iously( +no,ing the direction o" the"ield is important -ecause the field should be as close to perpendicular to the defect as possible

and no more than 23 degrees from normal. !eing a-le to e'aluate the "ield direction and strength

is especially important ,hen inspecting ,ith a multidirectional machine( -ecause ,hen the "ields

are not -alanced properly( a 'ector "ield ,ill -e produced that may not detect some de"ects.

There is actually no easy0to0apply method that permits an exact measurement o" "ield intensity at

a gi'en point ,ithin a material. In order to measure the "ield strength( it is necessary to intercept

the "lux lines. This is impossi-le ,ithout cutting into the material and cutting the material ,ould

immediately change the "ield ,ithin the part. $o,e'er( cutting a small slot or hole into the

material and measuring the leakage field that crosses the air gap with a -auss meter is probablythe best way to get an estimate of the actual field strength within a part . )e'ertheless( there are

a num-er o" tools and methods a'aila-le that are used to determine the presence and direction o"

the "ield surrounding a component.

>. Magnetic ield Indicators(auss Meter or &all 7ffect (age


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5 Fauss meter ,ith a $all E""ect pro-e is commonly used to measure the tangential "ield strength

on the sur"ace o" the part. 5s discussed in some detail on the CMeasuring Magnetic ieldsC page(

the $all e""ect is the trans'erse electric "ield created in a conductor ,hen placed in a magnetic "ield.

-auss meters, also called Tesla meters, are used to measure the strength of a field tangential to

the surface of the magnetied test ob!ect . The meters measure the intensity o" the "ield in the airad7acent to the component ,hen a magnetic "ield is applied.

The advantages of "all effect devices are they provide a 'uantitative measure of the strength of

magnetiing force tangential to the surface of a test piece, they can be used for measurement of

residual magnetic fields, and they can be used repetitively.

Their main disadvantages are that they must be periodically calibrated and they cannot be used

to establish the balance of fields in multidirectional applications.

>. Magnetic ield IndicatorsBuantitative Buality !ndicator 0BB!1


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The Quantitati'e Quality Indicator %QQI or 5rti"icial la, tandard is o"ten the pre"erred method

o" assuring proper "ield direction and adequate "ield strength. The use o" a QQI is also the only

practical ,ay o" ensuring -alanced "ield intensity and direction in multiple0direction magnetization

equipment. QQIs are o"ten used in con7unction ,ith a Fauss meter to esta-lish the inspection

procedure "or a particular component. They are used ,ith the ,et method only( and li+e other "luxsharing de'ices( can only -e used ,ith continuous magnetization.

• QQI或者人造缺陷标准块是最常用来确定磁场方向和磁场强度是否合适的方法

• QQI是唯一可以确定平衡场（复合场）方向和强度的方法

• QQI通常和高斯计一起使用来确定某构件的检测工艺

• QQI仅适用于湿法和连续磁化法。

>. Magnetic ield IndicatorsBuantitative Buality !ndicator 0BB!1


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The QQI is a thin strip o" either B.BB or B.BB> inch thic+ 5II 1BB? steel. 5 photoetch process is

used to inscri-e a speci"ic pattern( such as concentric circles or a plus sign. QQIs are nominally #8>

inch square( -ut miniature shims are also a'aila-le. QQIs must -e in intimate contact ,ith the part

-eing e'aluated. This is accomplished -y placing the shim on a part etched side do,n( and taping

or gluing it to the sur"ace. The component is then magnetized and particles applied. 6hen the "ieldstrength is adequate( the particles ,ill adhere o'er the engra'ed pattern and pro'ide in"ormation

a-out the "ield direction. 6hen a multidirectional technique is used( a -alance o" the "ields is

noted ,hen all areas o" the QQI produce indications.

>. Magnetic ield Indicatorsome o" the ad'antages o" QQIs are3 they can -e quanti"ied and related to other parameters( they

can accommodate 'irtually any con"iguration ,ith suita-le selection and they can -e reused ,ith


90/153

can accommodate 'irtually any con"iguration ,ith suita-le selection( and they can -e reused ,ithcare"ul application and remo'al practices. ome o" the disad'antages are3 the application process

is some,hat slo,( the parts must -e clean and dry( shims cannot -e used as a residual magnetism

indicator as they are a "lux sharing de'ice( they can -e easily damaged ,ith improper handling( and

they ,ill corrode i" not cleaned and properly stored.

!elo, le"t is a photo o" a typical QQI shim. The photo on the right sho,s the indication produced-y the QQI ,hen it is applied to the sur"ace a part and a magnetic "ield is esta-lished that runs

across the shim "rom right to le"t.

>. Magnetic ield IndicatorsPie (age


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The pie gage is a disk of highly permeable material divided into four# si or eight sections by non-

ferromagnetic material. The divisions serve as artificial defects that radiate out in different

directions from the center. The diameter o" the gage is #8> to 1 inch. The divisions between the low

carbon steel pie sections are to be no greater than 1( inch. The sections are "urnace -razed and

copper plated. The gage is placed on the test piece copper side up and the test piece is magnetized.5"ter particles are applied and the excess remo'ed( the indications provide the inspector the

orientation of the magnetic field

>. Magnetic ield IndicatorsThe principal application is on "lat sur"aces such as ,eldments or steel castings ,here dry po,der

is used ,ith a yo+e or prods The pie gage is not recommended "or precision parts ,ith complex


92/153

is used ,ith a yo+e or prods. The pie gage is not recommended "or precision parts ,ith complexshapes( "or ,et0method applications( or "or pro'ing "ield magnitude. The gage should be

demagneti#ed between readings

e'eral o" the main ad'antages o" the pie gage are that it is easy to use and it can -e used

inde"initely ,ithout deterioration. The pie gage has se'eral disad'antages( ,hich include3 it retainssome residual magnetism so indications ,ill pre'ail a"ter remo'al o" the source o" magnetization( it

can only -e used in relati'ely "lat areas( and it cannot -e relia-ly used "or determination o"

-alanced "ields in multidirectional magnetization.

6atch this short mo'ie to see a Pie "ield gage in action %#.>.

>. Magnetic ield IndicatorsSlotted Strips


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lotted strips( also +no,n as !urmah04astrol trips( are pieces o" highly permea-le "erromagnetic

material ,ith slots o" di""erent ,idths. They are placed on the test o-7ect as it is inspected. The

indications produced on the strips gi'e the inspector a general idea o" the "ield strength in a

particular area.

5d'antages o" these strips are3 they are relati'ely easily applied to the component( they can -e

used success"ully ,ith either the ,et or dry method ,hen using the continuous magnetization(

they are repeata-le as long as orientation to the magnetic "ield is maintained( and they can -e

used repetiti'ely. ome o" the disad'antages are that they cannot -e -ent to complex

con"iguration and they are not suita-le "or multidirectional "ield applications since they indicatede"ects in only one direction.

?. Magnetic Particles5s mentioned pre'iously( the particles that are used "or magnetic particle inspection are a +ey

ingredient as they "orm the indications that alert the inspector to de"ects. Particles start out as tiny


94/153

ingredient as they "orm the indications that alert the inspector to de"ects. Particles start out as tinymilled %a machining process pieces o" iron or iron oxide. 5 pigment %some,hat li+e paint is

-onded to their sur"aces to gi'e the particles color. The metal used for the particles has high

magnetic permeability and low retentivity. $igh magnetic permea-ility is important -ecause it

ma+es the particles attract easily to small magnetic lea+age "ields "rom discontinuities( such as

"la,s.


95/153

*ry magnetic particles can typically -e purchased in red( -lac+( gray( yello, and se'eral other

colors so that a high le'el o" contrast -et,een the particles and the part -eing inspected can -e

achie'ed. The size o" the magnetic particles is also 'ery important. *ry magnetic particle products

are produced to include a range o" particle sizes. The fine particles are around /) um $).))( inch%

in size# and are about three times smaller in diameter and more than () times lighter than the

coarse particles $1/) um or ).))0 inch%. This ma+e them more sensiti'e to the lea+age "ields "rom'ery small discontinuities. $o,e'er( dry testing particles cannot -e made exclusi'ely o" the "ine

particles. 4oarser particles are needed to -ridge large discontinuities and to reduce the po,der/s

dusty nature. 5dditionally( small particles easily adhere to sur"ace contamination( such as remnant

dirt or moisture( and get trapped in sur"ace roughness "eatures. It should also -e recognized that

"iner particles ,ill -e more easily -lo,n a,ay -y the ,ind there"ore( windy conditions can reduce

the sensitivity of an inspection. 5lso, reclaiming the dry particles is not recommended -ecause

the small particles are less li+ely to -e recaptured and the Conce usedC mix ,ill result in less

sensiti'e inspections.

干粉由粗细颗粒组成，粗颗粒大小为1?Bum，细颗粒为

?Bum，粗颗粒重量约为细颗粒的B倍。

?. Magnetic ParticlesThe particle shape is also important.


96/153

g ( y p y g(particles( the application process ,ould -e less than desira-le. Elongated particles come "rom the

dispenser in clumps and lac+ the a-ility to "lo, "reely and "orm the desired CcloudC o" particles

"loating on the component. There"ore( glo-ular particles are added that are shorter. The mix o"

glo-ular and elongated particles result in a dry po,der that "lo,s ,ell and maintains good

sensiti'ity. Most dry particle mixes ha'e particles ,ith


97/153

g p pp p g

particle testing method is generally more sensiti'e than the dry -ecause the suspension pro'ides

the particles ,ith more mo-ility and ma+es it possi-le "or smaller particles to -e used since dust

and adherence to sur"ace contamination is reduced or eliminated. The ,et method also ma+es it

easy to apply the particles uni"ormly to a relati'ely large area.

6et method magnetic particles products di""er "rom dry po,der products in a num-er o" ,ays.

=ne ,ay is that -oth 'isi-le and "luorescent particles are a'aila-le. Most non"luorescent particles

are "erromagnetic iron oxides( ,hich are either -lac+ or -ro,n in color. luorescent particles are

coated ,ith pigments that "luoresce ,hen exposed to ultra'iolet light. Particles that "luoresce

green0yello, are most common to ta+e ad'antage o" the pea+ color sensiti'ity o" the eye -ut other"luorescent colors are also a'aila-le. %or more in"ormation on the color sensiti'ity o" the eye...see

the material on penetrant inspection.

?. Magnetic ParticlesThe particles used ,ith the ,et method are smaller in size than those used in the dry method "or

the reasons mentioned a-o'e. The particles are typically 1B um %B.BBB> inch and smaller and the


98/153

synthetic iron oxides ha'e particle diameters around B.1 um %B.BBBBB> inch. This 'ery small size is

a result o" the process used to "orm the particles and is not particularly desira-le( as the particles

are almost too "ine to settle out o" suspension. $o,e'er( due to their slight residual magnetism(

the oxide particles are present mostly in clusters that settle out o" suspension much "aster than the

indi'idual particles. This ma+es it possi-le to see and measure the concentration o" the particles"or process control purposes. 6et particles are also a mix o" long slender and glo-ular particles.

The carrier solutions can -e ,ater or oil0-ased. 6ater0-ased carriers "orm quic+er indications( are

generally less expensi'e( present little or no "ire hazard( gi'e o"" no petrochemical "umes( and are

easier to clean "rom the part. 6ater0-ased solutions are usually "ormulated ,ith a corrosioninhi-itor to o""er some corrosion protection. $o,e'er( oil0-ased carrier solutions o""er superior

corrosion and hydrogen em-rittlement protection to those materials that are prone to attac+ -y

these mechanisms

水基悬浮液更加安全，通过添加腐蚀抑制剂达到防蚀效果。

油基悬浮液防蚀效果更好，且避免氢脆，但不利于清理和安全考虑。

@. uspension


99/153

most desira-le carriers -ecause they pro'ided good ,etting o" the sur"ace o" metallic parts.

$o,e'er( ,ater0-ased carriers are used more -ecause o" lo, cost( lo, "ire hazard( and the a-ility

to "orm indications quic+er than sol'ent0-ased carriers. 6ater0-ased carriers must contain ,etting

agents to disrupt sur"ace "ilms o" oil that may exist on the part and to aid in the dispersion o"

magnetic particles in the carrier. The ,etting agents create "oaming as the solution is mo'ed a-out(so anti0"oaming agents must -e added. 5lso( since ,ater promotes corrosion in "errous materials(

corrosion inhi-itors are usually added as ,ell.

Petroleum -ased carriers are primarily used in systems ,here maintaining the proper particle

concentration is a concern. The petroleum -ased carriers require less maintenance -ecause they

e'aporate at a slo,er rate than the ,ater0-ased carriers. There"ore( petroleum -ased carriers

might -e a -etter choice "or a system that gets only occasional use or ,hen regularly ad7usting the

carrier 'olume is undesira-le. Modern sol'ent carriers are speci"ically designed ,ith properties

that ha'e "lash points a-o'e BBo and +eep nocuous 'apors lo,. Petroleum carriers are required

to meet certain speci"ications such as 5M @>1.

油基悬浮液性能更好，不易干，用于偶然检测以及经常调配磁悬液

比例的情形，要符合例如5M@>1的要求（安全）

水基悬浮液更便宜，要添加润湿剂，防泡剂和防腐蚀剂，容易干燥。

IV. Testing Practices

1 *ry Particles


100/153

1. *ry Particles

. 6et uspension

#. Magnetic 9u--er

>. 4ontinuous & 9esidual Magnetization?. ield *irection & Intensity

@.


101/153

conducted on rough sur"aces. 6hen an electromagnetic yo+e is used( the 54 or hal" ,a'e *4

current creates a pulsating magnetic "ield that pro'ides mo-ility to the po,der. The primary

applications "or dry po,ders are unground ,elds and rough as0cast sur"aces.

干法用54或半波整流*4，保持颗粒的流动性，粗糙表面的检测。

*ry particle inspection is also used to detect shallo, su-sur"ace crac+s. *ry particles ,ith hal",a'e *4 is the -est approach ,hen inspecting "or lac+ o" root penetration in ,elds o" thin

materials. $al" ,a'e *4 ,ith prods and dry particles is commonly used ,hen inspecting large

castings "or hot tears and crac+s.

干法半波*4也用于检测薄板的根部未焊透，及大型铸件的热裂纹。

1. *ry ParticlesSteps in performing an inspection using dry particles


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Prepare the part surface 0 the sur"ace should -e relati'ely clean -ut this is not as critical as it is

,ith liquid penetrant inspection. The sur"ace must -e "ree o" grease( oil or other moisture that

could +eep particles "rom mo'ing "reely. 5 thin layer o" paint( rust or scale ,ill reduce test

sensiti'ity -ut can sometimes -e le"t in place ,ith adequate results. peci"ications o"ten allo, up

to B.BB# inch %B.BA@ mm o" a nonconducti'e coating %such as paint and B.BB1 inch max %B.B?mm o" a "erromagnetic coating %such as nic+el to -e le"t on the sur"ace. 5ny loose dirt( paint( rust

or scale must -e remo'ed.

$pply the magneti#ing force 0 2se permanent magnets( an electromagnetic yo+e( prods( a coil or

other means to esta-lish the necessary magnetic "lux.

Dust on the dry magnetic particles 0 *ust on a light layer o" magnetic particles.(ently blow off the excess powder 0 6ith the magnetizing "orce still applied( remo'e the excess

po,der "rom the sur"ace ,ith a "e, gentle pu""s o" dry air. The "orce o" the air needs to -e strong

enough to remo'e the excess particles -ut not strong enough to dislodge particles held -y a

magnetic "lux lea+age "ield.

Terminate the magneti#ing force 0 I" the magnetic "lux is -eing generated ,ith an electromagnet oran electromagnetic "ield( the magneti

MT self-Study Note

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