EDDY CURRENT
STUDY GU1DE
ASNT.SG.ET3-83
The Eddy Current Level /11 Study Guide was prepared by:
A. L. Lucero
Publication and review 01 this Study Guide was under the direction 01 the Personnel Training and
Certification Committee of the American Society lor Nondestructive Testing:
Frank Sattler, Chairman
Robert Baker
Edward Briggs
Donald Dodge
George Pherigo
Ward D. Rummel
Carl Shaw
Jack C. Spanneι Vice.Chairman
John L. Summers
John H. Weiler
Published by the American Society lor Nondestructive Testing , Inc. ,
1711 Arlingate Lane , PO Box 28518 , Columbus , OH 43228
Copyright @ 1983 by the American Society for Nondestructive Testing , Inc.
AII rights reserved. Printed in the United States 01 America.
CONTENTS
Flgures/Tables
v
References
vi
Foreword
vii
Preface
viii
1.
Principles of Eddy Current Testing
1
Historical Background
1
Generation 01 Eddy Currents
2
Field Intensity
3
Current Density
6
Phase/Amplitude and CurrentlTime Relationships
2.
Test Coil Arrangements
10
Probe Coils
10
Encircling Coils
11
Bobbin Coils
12
Absolute Coils
13
Differential Coils
13
Hybrid Coils
14
Additional Coil Characteristics
3.
14
Test Coil Deslgn
16
Resistance
16
Inductance
16
Inductive Reactance
18
Impedance
19
Q or Figure 01 Merit
20
Permeability and Shielding Effects
Coil Fixtures
21
4.
21
Effects 01 Test Object on Test Coil
Electrical Conductivity
Permeability
26
Skin Effect
26
Edge Effect
26
End Effect
26
Li ft.Off
27
Fi 11 Factor
27
Discontinuities
28
Signal-to.Noise Ratio
8
24
24
29
111
5.
31
Selection 01 Test Parameters
Frequency Selec!ion
31
Single Frequency Sys!ems
31
Mul!ifrequency Sys!ems
36
6.
Instrument Systems
40
Impedance Tes!ing
41
Phase Analysis Tes!ing
42
Vector Poin!
42
Ellipse
43
Li near Time Base
44
Modula!ion Analysis Tes!ing
45
Tes! Objec! Handling Equipmen!
49
7.
52
Readout Mechanisms
Indica!or Li gh!s
52
Audio Alarms
52
Me!ers
52
Digi!al Displays
52
CRTs
53
Recorders
54
Compu!ers
55
8.
57
Applications
Flaw De!ec!ion
57
Dimensional Measuremen!s
58
Conduc!ivi!y Measuremen!s
58
Hardness Measuremen!s
59
Alloy Sor!ing
59
9
Eddy Current Test Procedures , Standards , and Specilications
61
ASTM
61
MIL.STD
62
ASME
62
72
ANSWERS to Review Questions
IV
FIGURES厅'ABLES
Figure
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
Arag 口 's Magnetic Experimentation , 1821
1
Induced Current with Coil and Magnet
1
Induced Current , Electromagnetic Technique
2
Induced Current Relationships
3
Electromagnetic Field Produced by Alternating Current
Generation 01 Eddy Current in a Test Object
4
. Induced Current Flow in a Cylindrical Part
4
a. Phasor Diagram 01 Coil Voltage without Test Object
b. Phasor Diagram 01 Coil Voltage with Test Object
Relative Eddy Current Density
6
Eddy Current Phase Angle Radians Lagging
8
2.1
2.2
2.3
2.4
2.5
2.6
10
Probe Coil
Encircling Coil
11
Bobbin Coil
12
Coil Conligurations
12
External Relerence Dillerential System
Hybrid Coil
14
3.1
3.2
Multilayer Coil
17
Impedance Diagram
4.1
4.2
Measured Conductivity Locus
Li ft.oll Conductivity Relationship
5.1
Ellect 01 Frequency Change
a. Primary Impedance Without Secondary Circuit
b. Primary Impedance with Secondary Circuit
Normalized Impedance Diagram
34
Impedance Variations , f!f g = 50
35
Impedance Variations , I/lg =; 15
36
5.2
5.3
5.4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
3
5
5
13
19
24
27
33
33
6.9
6.10
6.11
6.12
6.13
Internal Functions 01 the Electromagnetic Nondestructive Test
40
Four Types 01 Simple Eddy Current Instruments
41
Vector Point Method
42
Ellipse Method
43
CRT Displays lor Dimension and Conductivity
43
Li near Time Base Instrument Diagram
44
Screen Image 01 a Li near Time Base Instrument with Sinusoidal Signals
a. Null Balance Instrument with Amplitude.Phase Detectors
45
b. Typical Response to a Thin Wall Non-Ierromagnetic
46
Tube Calibration Standard
Instrument Providing Any One 01 Four Operating Frequencies
46
Multilrequency Instrument Operating at Three Frequencies Simultaneously
Commercial Multilrequency Instrument
48
Commercial Microprocessor-Based Instrument
48
Pulsed Wavelorm Excitation
49
7.1
7.2
7.3
7.4
Discontinuity Response in Thin Wall Non-Ierromagnetic Tubing
54
Commercial Strip Chart Recorder
Facsimile Recording 01 a Saw-cut Specimen
55
Computer-controlled Eddy Current System
55
Table 4.1
Electrical Resistivity and Conductivity 01 Several Common Metals and Alloys
V
44
47
53
25
REFERENCES
1. Recommended Practice. No. SNT.TC.1A (1980
Nondestructlve Testing , Columbus , Ohio.
Edit 旧时，
Supplement
巳
1980.
American Society lor
2. General Dynamics-CT-6-5 Eddy Current Testing, Programmed Instruction Handbooks, Volume 1, 1967;
Volume 2, 1967. General Dynamics Convair Division , San Diego , Calilornia.
3. General Dynamics-PI-4-5 Eddy Cu厅ent Testing, Programmed Instruction Handbooks, Volume 1, 1967;
Volume 2, 1967. General Dynamlcs Convair Division , San Diego , Calilornia
4. H. L. Ll bby , Introduction to Electromagnetic Nondestructive Test Methods. 1971. John Wiley & Sons , Inc.
New York , New York.
5. R.C. McMaster, ed. , Nondesfructive Testing Handbook. 1959. American Society lor Nondestructive Testing ,
Columbus , Ohio
6. W.J. McGonnagle , Nondestructive Testing, 2d ed. 1975. Gordon and Breach Publishlng Company, NewY。此，
New York
7. R. S. Sharpe , Research Techniques in Nondestructive Testing, Volume 1, 1970. Academic Press , New York,
New York
8. R.S. Sharpe , Research Techniques in Nondestructive Testing, Volume 2, 1973. Academic Press , New York ,
New York
9. Meta/s Handbook , Properties and Selection of Materials , 8th ed. 1961. American Society lor Metals , Metals
Park , Ohio.
10. J. L. Taylor, ed. , Basic Metallurgy for Nondestructive Testing. 1974. British Institute 01 Nondestructive
Testing , Essex , England
11. Nondestructive Evaluation in the Nuclear Industry (1980).1981. American Society lor Metals , Metals Park,
Ohio
12. Eddy Current Characterization of Materials and Structures-ASTM , STP 722. 1981. American Society lor
Testing and Materials , Phlladelphia , Pennsylvania.
13. Eddy Current Nondestructive Testing-NBS Special Publication 589.1981. National Bureau 01 Standards ,
Washington , D.C.
14. D.J. Hagemaier and A.P. Steinberg , "Low Frequency Eddy Current Inspection 01 Aircralt Structure."
Materials Evaluation , Vol. 40 , No. 2, Feb. 1982 , pp. 206-210. Amerlcan Society for Nondestructive Testing ,
Columbus. Ohi。
15. MIL-STD-1537A (USAF) "Electrical Conductivity Test lor Measurement 01 Heat Treatment 01 Aluminum
Alloys , Eddy Current Method." June 1981. U.S. Department of Defense , Washington , D.C
16. MIL-STD-271E (SHIPS) "Nondestructive Testing Requirements lor Metals." 1973. U.S. Department 01
Delense , Washington , D.C.
17. ASME Section VI, 1980 edition. American Society 01 Mechanical Engineers , New York , New York
18. 1982 Annual Book of ASTM Standards , Part 11 Metallography; Nondestructlve Testing. 1982. American
Society lor Testing and Materials , Phlladelphla , Pennsylvania
19. Metals Handbook, Nondestructive Inspection and Quality Control. 1976. American Society lor Metals ,
Metals Park , Ohi。
VI
FOREWORD
The Personnel Training and Certilication Committee 01 ASNT has prepared a series 01 Level 111 Study
Guides which are intended to present the major areas in each nondestructive testing method. The
Level 111 candidate should use this Study Guide only as a review , as it does not contain all 01 the in.
lormation necessary to pass a typical Level 111 examination.
In using this Study Guide , the reader will be given specilic relerences , including page numbers ,
where detailed inlormation can be obtained. Typical Level 111 queslions are available al Ihe end 01
each seclion 10 aid in delermining comprehension 01 Ihe malerial
A Iypical use 01 this Sludy Guide mighl include Ihe 1011 口wing sequence:
1. An individual should review Ihe queslions al Ihe end 01 each seclion in Ihe Sludy Guide 10
delermine il his or her comprehension 01 Ihe eddy currenl method is adequate. The ques.
tions will serve as an indicalor 01 Ihe individual's abilily 10 pass a Level 111 examination.
2. 1I Ihe individual linds queslions in a cerlain seclion 01 Ihe Sludy Guide 10 be dillicull , il is
suggesled Ihal Ihe individual carelully sludy Ihe inlormalion presented in Ihat seclion. This
review 01 Ihe inlormalion in Ihe Sludy Guide will serve 10 relresh one's memory 01 Iheory and
lorgotten lacls
3. 1I Ihe individual encounlers inlormalion Ihal is new or not clearly underslood , Ihen il is im.
porlant to nole Ihe specilic relerences given Ihroughoul Ihe Study Guide and carelully read
this inlormalion. Relerences are indicated by parentheses and Ihe relerence number: (N)
VII
PREFACE
Early experimenters in the lield 01 magnetlsm and electromagnetism established the basis lor the
principles 01 electromagnetic nondestructive examinati.on used today.
In 1824 , Arago discovered tha.t the vibration 01 a magneti.c needle was rapidly damped when it was
placed near a nonmagnetic conducting disk. In 1820 , Oersted discovered the magnetic lield surrounding a conductor when current was passed through the conductor. In 1820 , Ampere discovered that
equal currents Ilowing in opposite directions in adjacent conductors cancelled the magnetic effect
This discovery has led to development 01 modern coil arrangements and shielding techniques. Faraday discovered the principles 01 electromagnetic induction in 1831. Maxwell integrated the results
01 these and other works in a two-volume work published In 1873 , and Maxwell's equations are still
the basis for investigations of the magnetic and electromagnetic phenomena
The application of these laws and principles has led to the development of an industry whose purpose is to. qualitatively and quantltatively investigate the properties and characteristics 01 conducting materials using nondestructive electromagnetic techniques
As in any industry , controls and guidelines must be established to insure consistent and reproducible products or services. This Study Guide is intended to provide ASNT Level 111 Eddy
Current Method candidates with a concise relerence with which to prepare for the ASNT Level 111
examination.
VIII
1.
PRINCIPLES OF EDDY CURRENT TES Tl NG
HISTORICAL BACKGROUND
Belore discussing the principles 01 eddy current testing , it seems appropriate to discuss brielly
lacets 01 magnetism and electromagnetism that serve as the loundation lor our study of eddy cur.
rent testing.
In the period from 1775 to 19∞， scientific experimenters Coulomg, A f!l.pere ，哇 raday， Oersted ,
Arago , Maxwell , and Kelvin investigated and cataloged most 01 what is known about magnetism and
electromagnetism
Arago discovered that the oscillation 01 a magnet was rapidly damped when a nonmagnetic
conducting disk was placed near the magnet (Fi 日. 1.1). He also observed that by rotating the disk,
the magnet was attracted to the disk. In ellect , Arago had introduced a varying magnetic lield to the
disk causing eddy currents to Ilow in the disk producing a magnetic lield by the disk that atlracted
the magne t. Arago's simple model is a basis lor many automobile speedometers used today.
MOTION
'---I_../
CONDUCTING PLATE
Figure 1.1-Arago's Magnetic Experimentation , 1821 亿 etec ， Inc.)
Oersted discovered the presence 01 a magnetic lield around a current-carrying conductor , and he
c>bserved a magnetic lield developed in a卫星IQ钮豆Lc.u.LaLPlane to the direction of current flow in a
wire. Ampere observed that equal and opposite currents Ilowing in adjacent conductors cancelled
this magnetic elfec t. Ampere's observation is used in dillerential coil applications and to manufac.
ture noninductive , Q@.剑sien-resistors. Faraday's lirst experiments investigated induced currents by
the relative motion 01 magnet and a coil (Fig. 1.2)
qAJTq飞
N
5
S
N
(a)
Figure 1.2一 Induced Current with Coil and Magnet
(b)
Faraday's major contribution was the dlscovery 01 electromagnetic induction. His work can be sum.
marized by the example shown in Figure 1.3. Coil A is connected to a battery through a switch S. A
second coil B connected to a galvanometer G is nearby. When switch S is closed producing a cur.
rent in coil A in the direction shown , a momenta[y current is induced in coil B in a direction (• a) opposite to that in A. 1I S is now opened , a momentary current will appear in coil B having the direction
01 (• b). In each case , current Ilows in coil B only while the current in coil A is changing.
•
b
已11 1 1 1'=
Figure 1.3-lnduced Current , Electromagnetic Technique
The electromotive lorce (voltage) induced in coil B 01 Figure 1.3 can be expressed as follows:
E 一 卫生生
KÂt
where:
E
N
= Average induced voltage
= Number 01 turns 01 wire in coil B
A争=
Ât
K
Rate 01 change of magnetic lines
of force affecting coil B
= 10'
jv1 axwell produced a two-volumework "A Treatise on Electricity and Magnetism" lirst published in
1873:"Maxwell n 口 t only chronicled most 01 the work done in electricity and magnetism at that time ,
but he also developed and published a group 01 relations known as Maxwell's equations lor the electromagnetic lield. These equations lorm the base that mathematically describes most 01 what is
known about electromagnetism today (13).
In 1849 Lord Kelvin applied 旦旦$sel， 's equation to solve the elements 01 an electromagnetic lield.
The principles 01 eddy current testing depend on the process (j f electromagnetic induction. This pro
cess includes a test coil through which a varying or alternating current is passed. A varying current
I1 口 wing in a test coil produces a varying electromagnetic lield about the coi l. This lield is known as
the primary lield.
GENERATION OF EDDY CURRENTS
When an electrically conducting test object is placed in the primary lield , an electrical current will
be induced in the test objec t. This current is known as the eddy curren t. Figure 1.4 is a simple model
that illustrates the relationships 01 primary and induced (eddy) currents. Conductor A represents a
portion 01 a test coi l. Conductor 8 represents a portion 01 a test objec t.
2
CONDUCTOR B
反r
CONDUCTOR A
Figure 1.4 -lnduoed Current Relationshlps (Zeteo , Ino.)
Following Lenz's law and indicating the instantaneous direclion of primary currenl Ip , a primary field
也 is developed aboul Conduclor A. When Conduolor B is broughl inlo Ihe influenoe of 也， an eddy
currenl 1. is induced in Conduclor B. This eleclrical currenl 1. produces an eleclromagnelic field 骨g
Ihal opposes Ihe primary eleclromagnelic field 岳 p. The magnilude of 争. is direolly proportional 10
Ihe magnilude 01 IE •
Characlerislic changes in Conduclor B such as conduclivily , permeabilily , or geomelry will cause IE
10 change. When 1. varies ，争 E also varies. Varialions 01 弘 are reflecled 10 Conductor A by changes in
骨 p.
These changes are delecled and displayed on some Iype 01 readout mechanism Ihal relales Ihese
varialions 10 Ihe characlerislic Ihal is 01 inleres l.
FIELD INTENSITY
纠
O
-N
Aι/
11
I
、飞
，
、少
4
'布
1
tνat
vd
问 4r
J
qk
门
也叫 r
λνy
GENERATOR
」
γ w
Figure 1.5 presenls a schemalic view 01 an excited lesl coi l. The eleclromagnelic lield produced
aboul Ihe unloaded lest coil in Figure 1.5 can be described as decreasing in intensilywilh distance
from Ihe coil and also varying across Ihe coil's cr口 ss seclion. The eleclromagnelic field is mosl in.
lense near Ihe coil's surlace
\1,,"
~ 9,
主y 喝，主阳工(吃吟气
o
口
Figure 1.5-Eleotromagnetio Field Produoed by Alternating Current (Zet时， Ino.)
3
The lield produced about this coil is directly proportional to the magnitude 01 applied current , rate 01
change 01 current or Irequency , and the coil parameters. Coil parameters include inductance ,
diameter, length , thickness , number 01 turns 01 wire , and core materia l.
To bet!er understand the principles under discussion , we must again look at the instantaneous rela.
tionships 01 current and magnetic Ilux. The exciting current is supplied to the coil by an alternating
current generator or oscillator.
With a primary current Ip Ilowing through the coil , a primary electromagnetic lield 争 p is produced
about the coi l. When this excited test coil is placed on a conducting test object , eddy currents IE will
be generated in that test objec t. Figure 1.6 illustrates this concep t.
GENERATOR
Figure 1.6-Generation 01 Eddy Current in a Test Object (Zetec , Inc.)
Note the direction 01 Ip , <Þ p , and the resultant eddy current IE • Although Figure 1.6 shows IE by directional arrows on the surlace 01 the test object , IE extends into the test object some distance. Another
important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.7
emphasizes this point with a loop coil surrounding a cylindrical test object (4).
Figure 1.7一 Induced Current Flow in a Cylindrical Part (Zet时， Inc.)
4
A more precise method of describing the relationships of magnetic flux , voltage , and current is the
phase vector diagram or phasor diagrams (4).
•EEEf
(.)
W1THOUT TEST OBJECT
，。
ELECTRICAL
DEGREES
EXCITATION
CURRENTI
争p
岳E
PRIMARY
MAGNEγ10 FLUX
SECOND A. RY
MAGNETIC FLUX =。
EI~
(b)
En
p •+ E
....5 - E
'-r
,""
WITHγEST OBJECT
'"
争T
SECONDARY
MAGNETIC FLUX
EXCIτATl ON CURRENT
-----‘4>
p
PRIMARY
MAGNETl C FLUX
Figure 1.8-a. Phasor Diagram of Coil Voltage wlthout Test Object
b. Phasor Diagram of Coil Voltage with Test Object
(Reprinted with permission from Hugo L. Ll bby, Introduction to Electromagnetic Nondestrucfive
Tesf Methods, 'p. 20. Copyrighl@1971 , John Wiley & Sons , Inc.)
Figure 1.8 shows the effects of a non-ferromagnetic test object on a test coi l. Figure 1.8a shows an
encircling coil and the resultant phasor diagram for the unloaded coil. The components of phasor
diagram 1.8a are as follows:
= Coil Voltage
= Primary Voltage
1
= Excitation Current
争 p = Primary Magnetic Flux
4> s = Secondary Magnetic Flux
E
Ep
The current (1) and primary magnetic flux (4) p) are plotted in phase , and the primary voltage (E p) is
shown separated by 90 electrical degrees
Secondary magnetic flux 岳s is plotled at zero because without a test object no secondary flux exists
Figure 1.8b represents the action 01 placing a non-Ierromagnetic test object into the test coi l. The
components 01 phasor diagram 1.8b lor a loaded coil are as follows:
HMm ∞ m ∞甜UM
vvon
n
nuunuu3FrHhue
eehcuu
aagcent
aao
时 Fc
>
国 LN 』
四(
咿 ddu
副 nea
eneox
mVMum
A
川咄阳副
剧
门
岳骨岳
5
gdoyay.
ve
TPSPST
创 dγd
OD.
--2 一一一一一一===
EEEE
CLPSPSLE
MCCrMrc
eiHVguwl
UF
V
<
HUVA
Observlng Figure 1.8b we can see by vectorial addition 01 Ep and Es we arrive at a new coil voltage
(ET ) lor the loaded condition. The primary magnetic Ilux 骨p and secondary magnetic Ilux <þ s are also
combined by vectorial addition to arrive at a new magnetic Ilux 传T) lor the loaded coil. Notice that
lor the condition 01 the test object In the test coil , <Þ T is not in phase with the excitation current 1.
Also observe that the included angle between the excitation current and the new coil voltage Ep is
no longer 90 electrical degrees. These interactions will be discussed in detail later in this study
guide.
CURRENT DENSITY
The distribution 01 eddy currents in a test object varies exponentially. The current density in the test
object is most dense near the test coi l. This exponential current density lollows the mathematical
rules lor a natural exponential decay cu 阿e ('1.). Usually a natural exponential curve is illustrated by a
graph with the ordinate (Y axis) representing magnitude and the abscissa (X axis) representing time
or distance. A common point described on such a graph is the "knee" 01 the curve. The knee occurs
at the 37 percent value on the ordinate axis. This 37 percent point , or knee , is chosen because
changes in X axis values produce signilicant changes in Y axis values Irom 100 percent to 37 percent , and below 37 percent changes in X axis values produce less signilicant changes in Y axis
values
Applying this logic to eddy current testing , a term is developed to describe the relationship 01 cur.
rent density in the test objec t. Consider the eddy current generated at the surlace 01 the test object
nearest the test coil to be 100 percent 01 the available current , the point in the test object thickness
where this current is diminished to 37 percent is known as the standard depth o{ penetration (4).
Figure 1.9 is a relative eddy current density cu 阿e lor a plane wave 01 inlinite extent with magnetic
lield parallel to the conducting test object surlace.
1.0
6= 1.118 X~
如己
ONE STANDARD DEPTH OF PENETRATION
6
s
2
4
DEPTH X
DEPTH OF PENETRATION
'
Figure 1.9-Relative Eddy Current Denslty (Reprinted [adapted] with permlsslon Irom Hugo L. Ll bby ,
Introduction to Electromagnetic Nondestructive Test Methods, p. 26. Copyright@1971 , John Wlley
& Sons , Inc.)
The current density at any depth can be calculated as lollows:
J x = J o exp (-X.J召EZ丁
where:
luituT事--
= Current density at depth X
= Current density at surlace , amperes per square meter
= 3.1416
= Frequency in hertz
= Magnetic permeability , henries per meter (H/m)
= Depth Irom surlace , meters
= Electric conductivity , mhos per meter
xo
μ
X
σ
6
Magnetic permeabilityμis a combination 01 terms. For nonmagnetic materials:
μ=
4". • 10-'H/m
For magnetic materials:
μ=μrμ 。
where:
μr =
μ。=
Relative permeability
4". • 10-' H/m
The standard depth 01 penetration can be calculated as lollows:
1
b = 岳甲
where:
占=
Standard depth 01 penetration , meters
". = 3.1416
1 = Frequency in. hertz
Magnetic permeability , H/m
σ= Electric conductivity , mhos. per meter
μ=
It should be observed at this point that as Irequency, conductivity , or permeability is increased , the
penetration 01 current into the test object will be decreased.
We can use the graph in Figure 1.9 (p. 6) to demonstrate many eddy current characteristics. Using an
example 01 a very thick block 01 stainless steel being interrogated with a surlace or probe coil
operating at a test Irequency 01 100 kilohertz (kHz) , we can determine the standard depth 01 penetration and observe current densities at other depths
Stainless steel (300 Series) is non.lerromagnetic. Magnetic permeability 0<) is 4贸. 10-' H/m and the
conductivity is 0.14. 10' mhos per meter lor 300 Series stainless steel.
~.'vt'b(>t → e
川'飞
M 仙ψ0
多·川'LJo
T飞机叫
占百7
-
1
7
.,1 3.1416 • 10 ∞口 o .如. 1 口-， .τ百-;-'f百
b -= 一」一一
743.438
b = 0.00135 meters
b = 1.35 mm
Using 1.35 mm as depth X Irom surlace a ratio 01 depth/depth 01 penetration would be 1. Relerring to
Figure 1.9 , a depth/depth 01 penetration 01 1 indicates a relative eddy current density 01 0.37 or
37 percen t. What is the relative eddy current density at 3 mm 。
Depth X equals 3 mm and depth 01 penetration is 1.35 mm , therelore:
2.222
一旦=
1.35
This ratio indicates a relative eddy current density 01 about 0.1 or 10 percen t. With only 10 percent 01
the available current Ilowing at a depth 01 3 mm , detectability 01 variables such as conductivity ,
permeability , and discontinuities would be very difficult to detec t. The obvious solution lor greater
detectability at the 3 mm depth is to lower the test Irequency. Frequency selection will be covered in
detail later in this tex t.
7
PHAS ElAMPLITUDE AND CURRENT厅 IME RELATIONSHIPS
Figure 1.10 reveals another lacet 01 the eddy current. Eddy currents are not generated at the same in.
stant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and
time are analogous; i. e. , phase is an electrical term used to describe timing relationships 01 elec.
trical wavelorms.
e
s
4
EDDY CURRENT
PHASE ANGLE
RADIANS LAGGJNG
3
2
咂
。
。
'
2
3
4
5
8
DEPTH X
DEPTH OF PENETRATl ON
Figure 1.10-Eddy Current Phase Angle Radians Lagging (Reprinted [adapted] wlth permlssion from Hugo L
Li bby , Introduction to Electromagnetic Nondestructive Test Methods , p. 26. Copyrigh!@1971 ,
John Wiley & S。阳， Inc.)
Phase is usually expressed in either degrees or radians. There are 2... radians per 360 degrees. Each
radian therelore is approximately 57 degrees.
Using the surlace current phase angle near the test coil as a relerence , phase angle current deeper
in the test object lags the surlace curren t. The amount 01 phase lag is determined by
。 =X.r，r市百=
Depth/Depth 01 Penetration ,
where θequals the phase angle lag in radians
Figure 1.10 should be used as a relative indicator 01 phase lag. The exact phase relationship lor a
partlcular system may be different due to other variables , such as coil parameters and excitation
methods.
The amount 01 phase lag lor a given part thickness is an important lactor when considering resolu.
tion. Resolution is the ability to separate variables occurring in the test object; lor example ,
distinguishing two discontinuities occurring at dillerent depths in the same test objec t.
As an example , let us establish a standard depth 01 penetration at 1 mm in a 5 mm thick test objec t.
Reler to Figure 1.10 and observe the phase lag 01 the current at one standard depth 01 penetration.
Where depth 01 interest (X) is 1 mm and depth 01 penetration (的 is 1 mm , the XI占 ratio is 1 and the cur.
rent at depth X lags the surlace current by 1 radian.
Projecting this examination , let us observe the phase lag lor the entire part thickness. The standard
depth .~t pooet白ítion is 1 mm , the part thickness is 5 mm; therelore , the ratio Xló equals 5. This pro.
duces a phase lag 01 5 radians or approximately 287 degrees lor the part thickness. Having a
measurement capability 01 1 degree increments , the part thickness could be divided into 287 parts ,
each part representing 0.017 mm. That would be considered excellent resolution
There is an obvious limitation. Reler to Figure 1.9 and observe the resultant relative current density
with an Xló ratio 01 5. The relative current density is near O.
It should become apparent that the Irequency can be adjusted to achieve optimum results lor a pa严
ticular variable. These and other variables will be discussed in Section 5 01 this study guide
8
REVIEW OUESTIONS
0.1.1
Generation 01 eddy currents depends on the principle 01:
A.
wave guide theory.
B.
electromagnetic induction.
C.
magneto-restrictive lorces.
D. all 01 the above.
0.1-2
A secondary lield is generated by the test object and is:
A.
equal and opposite to the primary lield.
B.
opposite to the primary lield , but much smaller.
C.
in the same plane as the coil is wound.
D.
in phase with the primary lield
0.1-3
When a non-Ierromagnetic part is placed in the test coil , the co iJ 's voltage:
A.
increases.
B.
remains constant because this is essential
C. decreases.
D.
shilts 90 degrees in phase.
0_1-4
Reler 10 Figure 1.8b (p. 5): 1I ET was produced by the test object being stainless steel , what
would the ellect be il the test object were coppe r?
A.
ET would decrease and be at a dillerent angle.
B.
ET would increase and be at a ditterent angle.
C.
Because both materials are non-Ierromagnetic , no change occurs.
D.
None 01 the above
0.1-5
Eddy currents generated in a test object 110\ii:
人
in the same plane as magnetic Ilux.
B.
in the same plane as the coil is wound
C. 90 degrees to the coil winding plane.
D.
Eddy currents have no predictable direction.
0.1-6
The discovery 01 electromagnetic induction is credited to
A.
Arago.
B. Oersted.
C.
Maxwel l.
D.
Faraday.
0.1-7
A standard depth 01 penetration is delined as the point in a test object where the relative eddy
current density is reduced to:
A. 25 percen t.
B. 37 percen t.
C. 50 percen t.
D.
100 percen t.
0.1-8
Reler to Figure 1.9 (p. 6).11 one standard depth 01 penetration was established at 1 mm in an object 3 mm thick , what is the relative current density on the lar surlace?
A.
<0.1
C.
1/3
Indeterminate
D.
0.1-9
3
日
Refer to Flgure 1.10 (p. 8). Using the example in question 1.8, what is the phase dillerence be.
tween the near and lar surlaces?
A.
Far surlace leads near surlace by 57 。
B.
Far surlace leads near surlace by 171
C.
Far surlace lags near surlace by 171
D.
Far surlace lags near surlace by 57"
0
0
0.1-~O
Calculate the standard depth 01 penetration at 10 kHz in copper;σ= 5.7 • 107 mhos per meter.
A.
0.1 mm
B.
0.02 mm
C. 0.66 mm
D. 66 mm
9
2.
TEST COIL ARRANGEMENTS
Test coils can be categorized into three main mechanical groups: probe co118 , bobbln co118 , and en.
clrcllng col18 (5).
PROBE COILS
Surface coll , probe coll , f1 at col' , or pancake coll are all common terms used to describe the same
test coil type. Probe coils provide a convenient method 01 examining the surlace 01 a test objec t.
Figure 2.1 illustrates a typical probe coil used lor surlace scanning
Figure 2.1-Probe Coil (Zetec , Inc.)
Probe coils and probe coil lorms can be shaped to lit particular geometries to solve complex inspection problems. As an example , probe coils labricated in a pencil shape (pencil probe) are used to inspect threaded areas 01 mounting studs and nuts or serrated areas 01 turbine wheels and turbine
blade assemblies. Probe coils may be used where high resolution is required by adding coil
shielding (2)
When using a high-resolution probe coil , the test object surlace must be carelully scanned to assure
complete inspection coverage. This carelul scanning is very time consuming. For this reason , probe
coil inspections 01 large test objects are usually limited to critical areas. Probe coils are used extensively in aircralt inspection lor crack detection near lasteners and lastener holes. In the case 01
lastener holes (bolt holes , rivet holes) , the probe coil is spinning while being withdrawn at a unilorm
rate. This provides a helical scan 01 the hole using a "spinning probe" technique
10
ENCIRC Ll NG COILS
Encirc /i ng coil, 00 coil , and feed-through coil are terms commonly used to describe a coil that surrounds the test objec t. Figure 2.2 illustrates a typical encirclin 日 coi l.
Figure 2.2-Encircling Coil (Zetec , Inc.)
Encircling coils are primarily used to inspect tubular and bar-shaped products. The tube or bar is led
through the coil (Ieed-through) at relatively high speed. The cross section 01 the test object within
the test coil is simultaneously interrogated. For this reason , circumlerential orientation 01 discontinuities cannot be determined with an encircling coil (4)
The volume 01 material examined at one time is greater using an encircling coil than a probe coil;
the 时 ore ， the relative sensitivity is lower lor an encircling coi l. When using an encircling coil ，且与
important to keep Jhe tesJ object centered in the coi l. 1I the test object is not centered , a unilorm
H币币市市市苟百首τ油市宵节丽而盯节口市市õñ' practice io run the calibration standard
several times , each time indexing the artilicial discontinuities to a new circumlerential location in
the coi l. This procedure is used to insure proper response and proper centering.
11
BOBBIN COILS
Bobbin coil, 10 coil , and inside probe are terms that describe coils used to inspect from the inside
diameter (10) or bore of atubular test objec t. Bobbin coils are inserted and withdrawn from the tube
10 by long , semiflexible shafts or simply blown in with air and retrieved with an attached pull cable.
These mechanisms will be described later in the tex t. Bobbin coil information follows the same
basic rules stated for encircling coils. Figure 2.3 illustrates a typical bobbin coi l.
Figure 2.3-Bobbin Coil (Z刨出， Inc.)
Probe coils , encircling coils , and bobbin coils can be additionally classified (5). These additional
classifications are determined by how the coils are electrically connected. The three coil categories
are absolute , differential , and hybrid
Figure 2.4 shows various types of absolute and differential coil arrangements
ABSOlUTE
E
O
Z
Jω
凶
zt1i 蝠#，圃， ← J
•
ω。
O
zdZE
L
DIFFERENTIAL
工τ于-
;
令「恃
p.p :·-Iι「 3「j气
-T-
2
吕立3ZL斗}
ZEg、mpJ-。偏
Q
O
百-凶ZJ
kt士H-j;
4
JdETZ-
B工
- --一二主
。四
Z
。
adLM=
甘正
寻L
悍J
Figure 2 .4一 Coil Configurations 侣， p. 38-25)
12
::r-
..;，.i.. L_
‘
••+---
L与·阳工斗 JE气τf
.}
fEE王飞千二正旦. 事军，三王i♂
5二
』一
E
J。
ω
EL
"
ι
----
L二
-~二
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1国升自主」-7
J己
ι斗1_
- -:
ABSOLUTE COILS
An absolute coil makes its measurement without direct reference or comparison to a standard as
the measurement is being made (6). Some applications for absolute coll systems would be
measurements of conductivity , permeability , dimensions , and hardness.
DIFFEREN Tl AL COILS
Differential coils consist of two or more coils electrically connected to oppose each other. Differen.
tial coils can be categorized into two types. One is the self-comparison differentia/, and the other is
external reference differentia/.
The self-comparison differential coil compares one area of a test object to another area on the same
test objec t. A common use is two coils , connected opposing , so that if both coils are affected by
identical test object conditions , the net output is "0" or no signa l. The self-comparison arrangement
is insensitive t口 test object variables that occur gradually. Variables such as slowly changing wall
thickness , diameter, or conductivity are effectively discriminated against with the self-comparison
differential coil.
Only when a different condition affects one or the other test coils will an output signal be generated.
The coils usually being mechanically and electrically similar allows the arrangement to be very
stable during temperature changes. Short discontinuities such as cracks , pits , or other localized
discontinuities with abrupt bQundaries can be detected readily using the self-comparison differential coll.
The external reference differential co l/, as the name implies , is when an external reference is used to
affect one coil while the other coil is affected by the test object (4). Figure 2.5 illustrates this concep t. This system is used to detect differences between a standard object and test objects. It is particularly useful for comparative conductivity , permeability, and dimensional measurements. Obviously in Figure 2.5 it is imperative to normalize the system with one coil affected by the standard
object and the other co l/ affected by an acceptable test objec t. The external reference differential
coil system is sensitive to a 川 measurable differences between the standard object and test object
For this reason it is often necessary to provide additional discrimination to separate and define
variables present in the test object
'v
TE$T
OBJECT
STANDARD
OBJECT
Figure 2.5 一 External Reference Differential System (Reprinted with permission from Hugo L. Li bby , Introduction
to Electromagnetic Nondestructive Test Methods , p. 69. Copyright@1971 , John Wiley & Sons , Inc.)
13
HYBRID COILS
Hybrid coils may or may not be the same size and are not necessarily adjacent to each other (4).
Common types 01 the hybrid coil are Driver/Pickup , Through Transmission , or Primary/Secondary
coil assemblies. Figure 2.6 shows a typical hybrid arrangement
Figure 2.6-Hybrid Coil (Reprinted with permission Irom Hugo L. Li bby , Introduction to Electromagnetic
Nondestructive Test Me 价 ods， p. 198. Copyright@1971 , John WiJey & Sons , Jnc.)
A simpJe hybrid coil consists 01 an excitation coil and a sensing coi J. Jn the through transmission
coil , the excitation coil is on one side 01 the test object and the sensing coil is on the other. The
voJtage developed in the sensing coil is a lunction 01 the curr.ent magnitude and Irequency applied
to the excitation coil , coil parameters 01 the exciting and sensing coils , and test object
characteristics.
In Figure 2.6 an encircling coil induces circumlerential currents in a cylindrical test object , and the
disturbances 01 these currents are detected bya small probe coil.
ADDITIONAL COIL CHARACTERISTJCS
Coil conliguration is but one 01 many lactors to consider when setting up test conditions. Other coiJ
characteristics 01 importance are mechanical , thermal , and eJectrical stability; sensitivity; resoJution; and dimensions. The geometry 01 the coil is usuaJJy dictated by the geometry 01 the test object ,
and olten sensitivity and resolution are compromised. The relative importance 01 test coil
characteristics depends upon the nature 01 the tes t.
A blend 01 theory and experience usuaJJy succeeds in selection 01 proper coil parameters
Coil design and interactions with test objects wiJJ be discussed later in this study guide
REVIEW OUESTIONS
0.2-1
Dillerential coiJs are usuaJJy used in:
A. bobbin coils
B. probe coils.
C. OD coils.
D. any 01 the above.
0.2.2
When using a probe coil to scan a test object ，一一一一一一一一一
A. the object must be dry and poJished
B. the object must be scanned careluJJy to insure inspection coverage.
C. the object must be scanned in circuJar motions at constant speeds
D. the probe must be moving at aJJ times to get a reading
0.2-3
Aμspinning probe" wouJd
A.
B.
C.
D.
most likely be a (an)
bobbin coil
ID coi J.
OD coi J.
probe coi 1
14
0.2.4
A "feed.through" coil is:
A.
a coil with primarylsecondary windings connected so that the signal is led through
the primary to the secondary
B.
an encircling coi l.
C.
an 00 coi l.
O.
both B and C
0.2.5
When inspecting a tubular product with an encircling coil , which statement is not true?
A.
00 discontinuities can be found
B. Axial discontinuity locations can be noted.
C. Circumferential discontinuity locations can be noted.
O.
10 discontinuities can be lound.
0.2.6
An absolute coil measurement is made 一一一一~一一一一
A.
by comparing one spot on the test object to another
日
without relerence 10 or direct comparison with a standard
C.
only with probe coils
D. by comparative measurement to a known standard
0.2.7
When coils in a differential arrangement are affected simultaneously with the same test object
variables , the output sig nal 一一-一一一一一A.
is directly proportional to the number of variables.
B.
is "0" or near-"Q."
C.
is indirectly proportional to the number of variables
D. is primarily a lunction of the exciting curren t.
0.2.8
Which coil type inherently has better thermal stability?
;
:;;:1;te
C目
00
t叭
ι《气叮刊叭飞
@ Oifferential
0.2:9
A hybrid coil is composed 01 two or more coils. The coils
A.
must be aligned coplanar to the driver axis;
B.
may be 01 widely dilferent dimensions.
C.
must be impedance.matched as closely as possible.
D.
are very temperature sensitive
0.2.10
Proper selection 01 test coil arrangement is determined by:
A.
shape 01 test objec t.
B.
resolution required.
C.
sensitivity required
D.
stability
E.
all 01 the above
15
3.
TEST COIL DESIGN
As discussed earlier , test coil design and selection is a blend 01 theory and experience. Many lac.
tors must be considered. These important lactors are determined by the inspection requirement lor
resolution , sensitivity , impedance , size , stability , and environmental considerations
In order to be t! er understand coil properties and electrical relationships , a short relresher in alter.
nating current theory is necessary.
First , we must examine electrical units-Ior example , current and its representative symbol 1. Cur.
rent not only suggests electron Ilow but also the amoun t. The amount 01 electrons Ilowing past a
point in a circuit in one second is expressed in amperes; 2.". • 10" electrons passing a point in one
second is called 1 ampere.
RESISTANCE
Resisfance is an opposition to the Ilow 01 electrons and is measured in ohms. Ohm's law is stated
by the equation:
E
- R
where:
1
R
E
= Current in amperes
= Resistance in ohms
= Electrical potential difference in volts
The resistance 01 a coil is determined primarily by the length 01 wire used to wind the coil; its
specilic resistance is determined 'by the type 01 wire (e.g. , copper , silver) and the cross.sectional
area 01 the wire
Resistance
一 S副司函卒. Lenoth ~f 去
where:
Resistance
Specilic Resistance
Area
Length
=
=
Ohms
Ohms/Circular miHoot
Circular mils
Feet
=
=
Thus , the resistance 01 a 10.loot length 0140 gauge copper wire with a specilic resistance 01 10.4 cir.
cular mil.loot at 20 'C would be lound as lollows:
10 .4 • 10
9.888
=一一一一一一一= 10.5180hms
In an alternating current circuit containing only resistance , the current and voltage are in phase. In
phase means the current and voltage reach their minimum and maximum values , respectively , at the
same time. The power dissipated in a resistive circuit appears in the lorm 01 hea t. For example , elec.
tric toasters are equipped with resistance wires that become hot when current Ilows through them ,
providing a heat source lor toasting bread
INDUCTANCE 例切
Heat generation is an undesirable trait lor an eddy current coi l. 1I the 10.loot length 01 wire used in
the previous example was wound into the shape 01 a coil , it would exhibit characteristics 01 alter.
nating current other than resistance. By lorming the wire into the shape 01 a coil , the coil also would
have the property 01 inductance. The role 01 inductance is analogous to inertia in mechanics ,
because inertia is the property 01 matter that causes a body to oppose any change in its velocity.
16
The unit 01 inductance is the henry (H). A coil is said to have the property 01 inductance when a
change in current through the coil produces a voltage in the coi l. More precisely , a circuit in which
an electromotive lorce 01 one volt is induced when the current is changing at a rate 01 one ampere
per second will have an inductance 01 one henry.
The inductance 01 a multilayer air core coil can be expressed by its physical properties , or coil para.
meters. Coil parameters such as length , diameter , thickness , and number 01 turns 01 wire allect the
coil's inductance.
Figure 3.1 i l/ ustrates typical coil dimensions required to calculate coil inductance.
也
MULTILAYER WOUND COILS
-_了
τ一十-土
• 1 b I•
Figure 3.1-Multilayer Coil (Zetec , Inc.)
An approximation 01 sma l/, multilayer, air core coil inductance is as lollows:
- _Q主立!::!f
L
where:
- 6r + 9
,+
10b
LNrlb
Sell.inductance in microhenries(I' H)
Total number 01 turns
Mean radius in inches
Length 01 coil in inches
Coil depth or thickness in inches
For example , a coil whose dimensions are as lo l/ ows
rlbN
一
0.1 inches
0.1 inches
0.1 inches
100 turns
would have an inductance 01:
L
坠旦旦1二旦旦1:
6.0.1 + 9 • 0.1 + 10.0.1
L
0.8(100)
80
=一一-= 32
2.5
0.6 + 0.9 + 1
L
32 microhenries inductance.
As stated earlier , this inductance is analogous to inertia in mechanical systems in that inductance
opposes a change in current as inertia opposes a change in velocity 01 a body. In alternating current
circuits the current is always changing; therelore inductance is always opposing this change. As the
current tries to change , the inductance reacts to oppose that change. This reaction is ca l/ ed índuc.
tíve reactance.
17
INDUCTIVE REACTANCE
The unit 01 inductive reactance (XL ) Is in ohms. Because the amount 01 reactance is a lunction 01 the
rate 01 change 01 current and rate 01 change can be described as Irequency, a lormula relating Ire.
quency, inductance , and inductive reactance is:
XL = 2...IL
(
ρ
where:
XL
1
L
=
=
=
Inductive reactance in ohms
Frequency in hertz
Inductance in henries
For example , using the 32 microhenry coil calculated earlier, operating at 100 kilohertz, its inductive
reactance would be lound as lollows:
XL
Inductive reactance
L
32μH or 0.000032H
100 kilohertz or 100,000 Hz
6.28
6.28 • 100,000 • 0.000032
20.0960hms
2π
XL
XL
Therelore , this coil would present an opposition 0120.096 ohms to currents with a rate 01 change 01
100 kilohertz due to its reactive componen t. Unlike a resistive circuit , the current and voltage 01 an
inductive circuit do not reach their minimum and ma刘 mum values at thesame time. In a pure induc.
tive circuit the voltage leads the current by 90 electrical degrees. This means that when the voltage
reaches a maximum value , the current is at "0."
Power is related to current and voltage as lollows:
P = EI
where:
P
E
I
=
=
=
Power in watts
Volts
Current in amperes
Notice that in a pure inductive circuit , when the voltage is maximum , the current is "0"; therelore ,
the product EI = O. Inductive reactances consume no alternating power where resistive elements
consume power and dissipate power in the lorm 01 hea t.
The opposition to current Ilow due to the resistive element 01 the coil and the reactive element 01 the
coil do not occur at the same time; therelore , they cannot be added as scalar quantities.
A scalar quantity is one having only magnitude; I. e. , it is a quantity lully described bya number, but
which does not involve any concept 01 direction. Gallons in a tank , temperature in a room , miles per
hour, lor example , are all scalars
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18
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IMPEDANCE
I
At.•
ve
In order to explain the addition 01 reactance and resistance with a minimum 01 mathematical calculations we can again use the vector diagram or phasor diagram to explain this addition (19). A phasor
diagram constructed with imaginary units on the ordinate, or (Y) axis and real units on the abscissa
or (X) axis is shown in Figure 3.2a.
〉Ed
『Z臼《E-
XL
Z
REAL
R
(a)
(b)
Figure 3.2-lmpedance Diagram (Zetec , Inc.)
二~二二
1
Substituting inductive reactance (X ,) and resistance (R) we can lind the resultant 01 the vector addition 01 XL and R. This resultant vector Z is known as impedance. Impedance is the total opposition to
current Ilow. Further observation 01 Figure 3.2b reveals XL , R, and Z appear to lorm the sides 01 a
right triangle. The mathematical solution 01 right triangles states the square 01 the hypotenuse is
equal to the sum 01 the squares 01 the other two sides , or
c 2 :::::; a 2 + b2
Substituting Z, XL , and R, the statement becomes
Z' = X,' + R'
lurther simplilied
Z = VJC'+ Fl'
Let's try an example. What is the impedance 01 a coil having an inductance 01 100 microhenries and
a resistance 015 ohms and being operated at 200 kilohertz?
First we must convert inductance to inductive reactance and then , by vector addition , combine inductive reactance and resistance to obtain the impedance.
V
〈 VAVA?ι7ι
LLL
2"IL
6.28 • 200 ,000 Hz • 0.0001 H
6.28 • 20 = 125.60hms
,,)
5' + 1~百万 =Æ丽江亘古
125.7 ohms
19
The importance 01 knowing the impedance 01 the test coil is more one 01 instrument consideration
than coil design.
Ml!-ximumjransle r_o! po响 er IS 旦旦旦旦rnplished whe旦 the drivin~ impedance and loadj旦j) edance are
ma汪里翌， 11 , ì 6i'l nstance , your eddy cu押ëñl丁 nstrumenγñãd a dr 百百矿阳市百aance 01 50 ohms , the
而石 st efficient test coils would also have impedances 01 50 ohms. Other, more common examples 01
impedance matching are home stereo systems rated at 100 watts per channel into 8 ohms.
We can discuss impedance in a more. detailed manner.by mathematically noting variables using
imaginary numbers (4). The indicated square root 01 a negative number is known as an imaginary
number. The imaginary number .J习"6 can be written .J(习Tf6 0 r .J=1 ..fffi 0 r .J习 '4. The notation
d习 is used extensively and is mathematically noted by ;. Since ; is also used in electrical terms lor
current , the ; notation lor electrical calculatlons is changed to j. The term j , olten called operator j , is
equal to the .J工1. Instead 01 noting .J=16 as .J=1 '4 we can slmply note .J工可"6 as j4. Since reac.
tance is known as an imaginary component , we can then note impedance Z = R + jXM = [Z[ Le. where
Tan e =总.
R
The term R + jXM is known as a rectangular notation. As an example , a resistance 01 4 ohms in
series with an inductive reactance 01 3 ohms could be noted as Z = 4 + j3 ohms. The impedance
calculation is then:
Z= .J万言3言= .J2ó
Z =
50hms
In coil design it is olten helplul to know also the included angle between the resistiye component
and impedance. A convenient method 01 nota阳1 is tt盯 01阳
町r 怕
a
1 om
川
T
In Cωluded angl怆
e
between
陌
r es
剖is
剖tance
and impedance. In the pr陀ev川l旧口 us exampl怡e our impedance
magni忱tude 怡
i s 5 ohms , but at what angle?
A proper lorm 01 notation is Z血 where Z is impedance and Le. is the included angle. Therelore , the
complete notation is
Tan e
=主=主= 0.750
R
Arc Tanθ= 36.9
Z
4
0
= 5l主6.9
0
Eddy current coils with included impedance angles 0160 0 to 90 0 usually make ellicient test coils. As
the angle between resistance and impedance approaches 0 , the test coil becomes very inefficient
with most 01 its energy being dissipated as hea t.
0
The term used to describe coil efficiency is Q or mer;t of the co;l. The higher the Q or merit 01 a coil ,
the more elliciently the coil perlorms as an inductor. The merit 01 a coil is mathematically stated as:
Q =告
也
where:
XL
R
= Inductive reactance
= Resistance
For example , a coil having an inductive reactance 01 100 ohms and a resistance 015 ohms would
have a Q 01 20.
20
PERMEABI Ll TY AND SHIELDING EFFECTS
The addilion 01 permeable core malerials in cerlain coil designs dramalically improves the Q lactor.
Permeable cores are usually constructed 01 high permeability "powdered iron." Probe coils , lor example , are wound on a lorm that allows a powdered iron rod or slug to be placed in Ihe center 01 the
coil (4). It is common to increase the coil impedance by a lactor 01 10 by the addition 01 core
materials. This increase in impedance without additional winding greatly enhances the Q 01 the coil
Some core materials are cylinder- or cup-shaped. A common term is cup core. The coil is wound and
placed in the cup core. In the case 01 a probe coil in a cup core , not only is the impedance increased ,
but the benelit 01 shielding is also gained. Shielding with a cup core prevents the electromagnetic
lield Irom spreading at the sides 01 the coi l. This greatly reduces the signals produced by edge ellect 01 adjacent members to the test area , such as lasteners on an aircrafl wing. Shielding , while im.
proving resolution , usually sacrilices some amount 01 penetration into the part.
Another method 01 shielding uses high conduclivity material , such as copper or aluminum , to suppress high Irequency interlerence Irom other sources and also to shape the electromagnetic lield 01
the test coi l. A copper cup would restrict the electromagnetic lield in much the same manner as the
"powdered iron cup core" discussed previously. A disadvantage 01 high conductivity , low or no per.
meability shielding is that the coil's impedance is reduced when the shielding malerial is placed
around the test coi l. The net ellect is , 01 course , that the coil's Q is less than il was when the coil
was surrounded byair.
Another coil design used lor inspection 01 lerromagnetic materials uses a saturation approach. A
predominant variable that prevents eddy current penetration in lerromagnetic material is called pe产
meability. Permeabilily effects exhibited by the test object can be reduced by means 01 magnetic
saturation. Saturation coils lor steels are usually very large and surround the lest object and test
coi l. A steady state current is applied to the saturation coi l. When the steel tesl objecl is magnetical.
Iy saturated it may be inspected in the same manner as a non-Ierromagnetic material. In the case 01
mild steel many thousands 01 gauss are required to produce saturation. In such other materials as
nickel alloys (monel and inconel) , the saturation required is much less and can usually be accomplished by incorporating permanent magnets adjacent to the test coi l.
COIL FIXTURES
Coil lixtures or holders may be as varied as the imagination 01 the designers and users. Alter the
Sl主e ， shape , and style have been decided upon , the next consideration should be the test environmen t.
Characteristics 01 wear, temperature , atmosphere , mechanical stress , and stability must be considered (4)
Normally wear can be reduced by selection 01 wear-resistant plastic compounds , or where severe
wear is expected , artilicial or genuine jewels may be used. Le.ss expensive and very ellective wear
materials , such as aluminum oxide or ceramics , are more commonly used.
Temperature stability may be accomplished by using coil holder material with poor heat transler
characteristics. Metals have high heat transler characleristics , and often coils made with metal
holders are sensitive to temperature variations caused by human touch. For high temperature appli.
cations , materials must be chosen carelully. Most common commercial copper coil wire may be
used up t口 150-200 C. For temperatures ab口ve 200 o C , silver or aluminum wire with ceramic or high
temperature silicone insulation must be used
0
Materials musl be chemically compatible with the test objec t. As extreme examples , a polystyrene
coil lorm would not be used to inspect an acetone cooler, or a lead or graphite housing allowed 10
come in contact with an inconel jet engine tail cone producing service-related stress cracks.
Mechanical and electrical stability 01 the test coil can be enhanced by an application 01 epoxy resin
between each layer 口 I coil winding. This accomplishes many objectives: (1) it seals the coil to ex.
clude moisture; (2) it provides additional electrical insulation; and (3) it provides mechanical
stability
21
Characteristics listed are not in order 01 importance. The importance 01 each characteristic is deter.
mined by specilic test requirements.
REVIEW OUESTIONS
0.3.1
A coil's resistance is determined by:
A. wire material.
B. wire length
C. wire cross.sectional area
D. all 01 the above.
0.3.2
Inductance is analogous t口
A. lorce.
B. volume
C. inertia.
D. velocity.
0.3.3
The unit 01 inductance is the:
A. henry.
B. maxwel l.
C. ohm.
D. larad
0.3.4
The inductance 01 a multilayer air core coil with the dimensions 1=0.2 , r=0.5 , b=0.1 , and
N=20, is:
A. 1.38 henries.
B. 13.8 microhenries
C. 13.8 ohms.
D. 1.380hms.
0.3.5
The inductive reactance 01 the coil in 0.3.4 , operating at 400 kHz , would be:
A. 13800hms.
B. 5520 ohms.
C. 34.66 ohms
D. 3466 ohms.
Q.3.6
The impedance 01 a 100 microhenry coil with a resistance 01 20 ohms operating at 100 kHz
would be:
A. 62.8 ohms.
B. 4343.8 ohms.
C. 6280hms.
D. 65.9 ohms.
Q.3.7
The 0 or merit 01 a coil is the ratio 01:
A.
Z
B.
泣
C.
总
XL
Z
R
D.
R
XL
0.3.8
The incorporation 01 magnetic shielding:
A. improves resolution
B. decreases lield extension.
C. increases impedance.
D. does all 01 the above.
Q.3.9
The purpose 01 a steady.state winding incorporated in a test coil is to:
A. reduce permeability effects.
B. provide magnetic saturation
C. provide a balance source for the sensing coil
D. both A and B.
22
Q.3.10
The most important consideration when selecting a test coil is:
A. sensitivity.
B. resolution.
C. stability.
D. test requirement and compatibility.
23
4
EFFECTS OF TEST OBJECT ON TEST COIL
As we have seen , the eddy current technique depends on the generation 01 induced currents within
the test objec t. Perturbations or disturbances in these small induced currents allect the test coil
The result is variance 01 test coil impedance due to test object variables. These are called operating
variables (19). Some 01 the operating variables are coil impedance , electrical conductivity , magnetic
permeability , skin ellect, lift-off, lill lactor, end ellect, edge effect, and signal-to-noise ratio.
Coil impedance was discussed at length in the previous Section. In this Section coil impedance
changes will be represented graphically to more elfectively explain the interaction 01 other operating
variables.
ELECTRICAL CONDUCTIVITY
In electron theory the atom consists 01 a positive nucleus surrounded by orbiting negative electrons
Materials that allow these electrons to be easily moved out 01 orbit around the nucleus are classified
as conductors. In conductors electrons are moved by applying an outside electrical lorce. The ease
with which the electrons are made to move through the conductor is called conductaJ1 ce. A unit 01
conductan 创 s t阳nho;The mho 阻 t 阳e即 rocal 01 the ohm or ∞ nductance G =古， whe 叫 IS
conductance in mhos and R is resistance in ohms.
,
In eddy current testing , instead 01 describing conductance in absolute terms , an arbitrary unit has
been assigned. Since the relative conductivity 01 metals and alloys varies over a wide range , the
need lor a conductivity benchmark is 01 prime importance. Th~JRternational王le c;_trochemical Comr:n ission established i f1_J 91~ a convenient method 01 comparing 0'11百而E豆.[ia_Uo anot 币， r. The com市IEEIE百 established t币市~万flïì百币ìlTity-copper，1币"ëfêrTrì丁画面iE百d unilorm section 01 1 mm' measuring 0.017241 ohms at 20.C would be arbitrarily considered 100 percent conductive. The symbol lor conductivity isσ(sigma) and the unit is % IACS or percent 01 the International
Annealed Conductivity Standard
青Table 4.1 lists m 叫由 by cond 叫 ivity and resistivity. A statement can be made abo阳 conductor
in terms 01 conductance or resistance. Note that a good conductor is a poor resistor. Conductance
and resistance are direct reciprocals as stated earlier. Conductivity and resistivity , however, have
different origins and units; therelore , the conversion is not so direc t. As previously discussed , conductivity is expressed on an arbitrary scale in % IACS. Resistivity is expressed in absolute terms of
micro ohm-centimeter. To convert to either unit, simply lollow the equation:
172.41
Resistivity in micro ohm-cm
%IACS
As the test coil is inlluenced by different conductivities , its impedance varies inversely to conductivity. A higher conductivity causes the test coil to have a lower impedance value. Figure 4.1 illustrates this concept
I I
卜\
υ恻刚
@盹
I I
「、
比
队
凶。zd
吨←
od
ME
飞
\
"、
"
k
叫、j
'"|比 "ωl 、
E
RESISTANCE -
i:SI Figure 4.1-Measu时 Cond叫i向 Loc叫ASM Comml阳。n Eddy Current Inspecti叫 "Eddy-C川ent Inspec-
tion ," Metals Handbook , Vo l. 11 , 8th E且， Howard E. Boyer, Editor, American Society for Metals,
1976 , p. 77.)
24
The coil's inductive reactance is represented by the Y axis and coil resistance appears on the X axis.
The 0 percent conductivity point, or air point , is when the coil's empty reactance (X w ) is maximum.
Figure 4.1 represents a measured conductivity locus (4). Conductivity is inlluenced by many lactors.
Table 4.1 (19 , p. 206) lists conductivities 01 materials with dillerent chemical compositions.
Table 4.1-Electrical Resistivity and Conductivity 01 Several Common Metals and Alloys (ASM Commlttee on
Eddy Current Inspection , "Eddy-Current Inspection ," Metals Handbook , Vo l. 11 , 8th E止， Howard E.
Boyer, Editor, American Soclety lor Metals , 1976, p. 79.)
Metal or a l/ oy
SlI ver
Copper, annealed
Gold
Aluminum
Aluminum alloys:
6061-T6
7075-T6
2024-T4
Magnesium
70-30 brass
Phosphor bronzes
Monel
Resistivity
ohm-cm
X 10-6
Conductivity
%IACS
1.63
1.72
2.44
2.82
105
100
70
61
4.1
5.3
5.2
4.6
6.2
16
48.2
50
72
54.8
172
70
98
115
123
Zirconiur啊
Zircaloy-2
Tltaniu 阿1
Ti-6AI-4V alloy
304 stainless steel
Inconel600
Hastelloy X
Waspaloy
42
32
30
37
28
11
3.6
3.4
2.4
3.1
1.0
2.5
1.7
1.5
1.4
80me other lactors affecting conductivity are temperature , heat treatment , grain size , hardness , and
residual stresses. A change in the temperature 01 the test object will change the electrical conductivity 01 that object
In metals , as the temperature is increased , the conductivity is decreased. Carbons and carbon compounds have negative temperature coellicients; therelore , their conductivity increases as temperature is increased.
Heat treatment also allects electrical conductivity by redistributing elements in the material
Dependent upon materials and degree 01 heat treatment , conductivity can either increase or decrease as a resu It 01 heat treatment
8tresses in a material due to cold working produces lattice distortion or dislocation (2). This mechanical process changes the grain structure and hardness 01 the material , changing its electrical
conductivity.
Hardness in "age hardenable" aluminum alloys changes the electrical conductivity 01 the alloy. The
electrical conductivity decreases as hardness increases. As an example , a Brinell hardness 0160 is
represented by a conductivity 01 23 , and a Brinell hardness 01 100 01 the same alloy would have a
conductivity 01 19.
25
PERMEABILITY
Permeability of any material is a measure of the ease with which its atoms can be aligned , or the
ease with which it can establish lines of force (2). Materials are rated on a comparative basis. Air is
assigned a .p ermeability of 1. A basic determinalion of permeability ， μ(pronounced "mu") , is:
Number of Li nes Produced with lhe Malerial as a Core
μ= Number of Li nes Produced with Air as a Core
Ferromagnetic metals and alloys including nickel , iron , and coball lend 10 concenlrale magnelic
flux li.nes (19). Ferromagnelic malerial or sinlered ionic compounds are also useful in concenlrating
magnelic flux (4).
Magnetic permeability is nol conslanl for a given maleria l. The permeabilily depends more upon lhe
magnetic field acling upon it. As an example , consider a magnetic sleel bar placed in an encircling
coi l. As lhe coil currenl is increased , lhe magnelic field of lhe coil will increase. The magnelic flux
wilhin the sleel will increase rapidly al firsl , and then will lend 10 level offas lhe sleel approaches
magnetic saluralion. This phenomenon is called lhe Barkhausen effect (4)
When increases in lhe magnelizing force produce little or no change on lhe flux wilhin lhe sleel bar,
lhe bar is magnetically salurated. When ferromagnelic malerials arè saturaled , permeability be.
comes conslan t. Wilh magnetic permeabilily conslanl , ferromagnelic malerials may be inspecled
using lhe eddy currenl melhod. Wilhoul magnelic saluration , ferromagnelic materials exhibil such a
wide range of permeabilily varialion thal signals produced by disconlinuilies or conduclivily varia.
tions are masked by lhe permeabilily signal (19)
Permeabilily effecls are mosl predominanl at lower frequencies (5). 01her magnelic effecls include
diamagnetic and paramagnetic (4).
SKIN EFFECT
Eleclromagnetic tesls in many applicalions are mosl sensilive to tesl objecl variables nearesl lhe
lesl coil due 10 skin effec t. Skin effecl is a resull of mulual interaclion of eddy currenls , operaling
frequency , lesl objecl conduclivily , and permeability. The skin effecl, lhe concenlralion of eddy cur.
renls in lhe lesl objecl nearesl lhe lesl coil , becomes more evidenl as lesl frequency , lesl objecl
conduclivity , and permeabilily are increased (4). For currenl densily or eddy currenl dislribulion in
lhe lesl objecl , refer 10 Figure 1.9 in Seclion 1 目
EDGE EFFECT
The eleclromagnelic field produced by an exciled lesl coil exlends in all directions from lhe coi l. As
lesl objecl geomelrical boundaries are approached by lhe lesl coil , lhey are sensed by lhe coil prior
10 lhe coil's arrival al the boundary.
The coil's field precedes lhe coil by some dislance (2) delermined by coil paramelers , operaling fre.
quency, and lesl object characlerislics. As lhe coil approaches lhe edge of a lesl objecl , eddy cur.
renls become dislorled by lhe edge signal. This is known as edge effect. Response 10 lhe edges of
tesl objecls can be reduced by lhe incorporation of magnetic shields around lhe lesl coil or by
reducing lhe tesl coil diamele r. Edge effecl is a term mosl applicable to lhe inspeclion of sheets or
plales wilh a probe coi l.
END EFFECT
End effecf follows lhe same logic as edge effec 1. End effecl is lhe signal observed when lhe end of a
producl approaches lhe lesl coi l. Response 10 end effecl can be reduced by coil shielding or reduc.
ing coil length in 00 encircling or 10 bobbin coils. End effecl is a lerm mosl applicable 10 lhe in.
spection of bar or lubular producls
26
Ll FT-OFF
Electromagnetic coupling between test coil and test object is of prime importance when conducting
an eddy current examination. The coupling between test coil and test object varies with spacing between the test coil and test objec t. This spacing is called I;ft-off (4). The effect on the coil impedance
is called lift-off effec t.
flll
RES1STANCE ___
Figure 4.2 一Li ft-off Conductivity Relationship (ASM Committee on Eddy Current lnspeclion , "Eddy.Current lnspection ," Meta/s Handbook , Vo l. 11 , 8th Ed. , Howard E. Boyer, Editor, American Society for Metals ,
1976 ，口 .79.)
Figure 4.2 shows the relationship between air, conductive malerials , and lifl.off. The electromagnetic field , as previously discussed , is strongest near Ihe coil and dissipales with dislance from the
coi l. This fact causes a pronounced lift-off effect for small varialions in coil-to-object spacing. As an
example , a spacing change from contact to 0.001 in. will produce a lift-off effect many limes greater
than a spacing change of 0.010 in. 100.011 in. (19). Li ft-off erfect is generally an undesired effect
causing increased noise and reduced coupling resulting in poor measuring ability (12). In some instances , equipmenl having phase discrimination capability can readily separale lift-off from conduclivily or olher variables. Li fl-off can be used 10 advanlage when measuring nonconductive
coatings on conduclive bases. A nonconductive coating such as paint or plastic causes a space between Ihe coil and conducting base , allowing lifl-off 10 represent the coating thickness. Li ft-off is
also useful in profilometry and proximity applications. Lift-off is a lerm mosl applicable 10 tesling
objects with a surface or probe coi l.
…
CTOR
l 悦 4生头吃-
1~ 有元叫
<"
1
"-
I
F;II factor is a lerm used 10 describe how well a lesl objecl will be electromagnelically coupled 10 a
lest coil Ihat surrounds or is inserled inlo Ihe lest objec t. Fill faclor Ihen perlains to inspeclions using bobbin or encircling coils. Li ke lifl-off , eleclromagnetic coupling between lesl coil and lesl objecl is mosl efficienl when Ihe coil is nearesl Ihe surface of the par t.
Fill laclor can be described as Ihe ralio 01 lesl objecl diameler to coil diameler squared. The
diamelers squared is a simplilied equation resulling in the division 01 effective coil and parl areas
The area 01 a circle (A) is delermined using Ihe equation:
A -一
= ,..d'
-
4
fappears in both nume 剧。 r and denominator 01 Ihe Iractional equation; Iherelore ~ cancels ， 怡
1 e卧
I咱
n吨9 阳
1 he盯叫叫「阻剧叫
剖甜巾
a
li口
ω01
们川
1创
d la凹
meters 叫叩U盹a时 在 = η = lil川11门laω
蚓
c创t川
O
Fi 川 lactor will always be a number less than 1 , and efficienl fill lactors approach 1. A lill lactor 01
0.99 is more desirable than a lill lactor 01 0.75. The elfect 01 fill laclor on the lest syslem is that poor
lill lactors do nol allow the coil 10 be sufficiently loaded by Ihe lest objec t. This is analogous to Ihe
eflecl 01 drawing a bow only slighlly and releasing an arrow. The result is , with the bow slighlly
drawn and released , little ellect is produced 10 propel the arrow
27
In electrical terms , we say the coil is loaded by the test objec t. How much the coil is loaded by the
test object due to fill factor can be calculated in relative terms. A test system with constant current
capabilities being affected by a conductive nonmagnetic bar placed into an encircling coil can be
used to demonstrate this effec t.
For this example , the system parameters are as follows:
(a) Unloaded coil voltage equals 10 volts
(b) Test object effective permeability (5) equals 0.3
(c) Test coil inside diameter equals 1 inch
(d) Test object outside diameter equals 0.9 inches
Fill Factor ~ =
(生旦)2=0.81
1
An equation demonstrating coil loading is given by:
E = Eo (1-η+ημeft)
where:
Eo
E
= Coil voltage with coil affected by air
= Coil voltage with coil affected by test object
η= Fill factor
μeff
= Effective permeability
When the nonmagnetic test object is inserted into the test coil , the coil's voltage will decrease
川剧
.
。υ 。。
0
1+
飞，
只unos
才tnunuυv
414d
0003
EEEE
)93t
(((
1114
+0 -)
====
- AUqG)
-··c
川
This allows 10 - 4.3 or 5.7 volts available to respond to test object changes caused by discontinuities
or decreases in effective conductivity of the test objec t.
It is suggested that the reader calculate the resultant loaded voltage developed by a 0.5 inch bar of
the same material and observe the relative sensitivity difference.
DISCONTIN UITIES
Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discon.
tinuities , such as cracks , pits , gouges , vibrational damage , and corrosion , generally cause the effec.
tive conductivity of the test object to be reduced.
Discontinuities open to the surlace are more easily detected than subsurlace discontinuities (19)
Discontinuities open to the surface can be detected with a wide range 01 frequencies; subsurface investigations require a more careful Irequency selection. Discontinuity detection at depths greater
than 0.5 inch in stainless steel is very dillicul t. This is in part due to the sparse distribution 01
magnetic Ilux lines at the low Irequency req川 red lor such deep.penetrations
Figure 1.9 (p. 6) is again uselul to illustrate discontinuity response due to current distribution. As an
example , consider testing a non-Ierromagnetic tube at a Irequency that establishes a standard
depth 01 penetration at the midpoint 01 the tube wal l. This condition would allow a relative current
density 01 approximately 20 percent on the lar surlace 01 the tube. With this condition , identical near
and lar surlace discontinuities would have greatly different responses. Due to current magnitude
alone , the near surlace discontinuity response would be nearly 5 times that 01 the lar surlace
discontinuity
28
Discontinuity orientation has a dramatic ellect on response. As seen earller, discontlnuity response
is maximum when eddy currents and discontinuities are at 90' , or perpendicular. Discontinuities
parallel to the eddy current Ilow produce little or no response. The easiest method to insure detect.
ability 01 discontinuitles is to use a relerence standard or model that provides a consistent means 01
adjusting instrumentation (12).
SIGNAL.TO.NOISE RATIO
Signal.to.noise ratio Is the ratio 01 signals 01 interest to unwanted signals (4). Common noise
sources are test object variations 01 surlace roughness , geometry, and homogenelty. Other elect时，
cal noises can be due to such external sources as welding machines , electric motors and genera.
tors. Mechanical vibrations can increase test system noise by physlcal movement 01 test coil or test
objec t. In other words , anything that Interferes with a test system's ability to deline a measurement
is considered noise.
Signal.to.nolse ratios can be improved by several methods. 1I a part is dirty or scaly; signal.to.noise
ratio can be Improven t> y cleaning the part. Electrical interlerence can be shielded or isolated. Phase
discrimination and II;tering can improve signal.to.noise ratio.
飞飞
It is common practice in nondestructive testing to require a minimum signal.to.noise ratio 01[3 to jl
This means a signal 01 interest must have a response at least three times that 01 the nolse anñat
p口 in t.
REVIEW QUESTIONS
Q.4.1
Materials that hold their electrons loosely are classilied as:
A.
reslstors.
B.
conductors.
C. semiconductors.
D.
insulators.
0.4.2
100% IACS is based on a specilied copper bar having a resistance 01:
A. 0.01 ohms.
B.
1000hms
C. 0.017241 ohms.
D.
172.41 ohms.
0.4.3
A resistivity 01 13 micro ohm.cm is equlvalent to a conductivity in % IACS 01 一一一一一--一·
A. 11.032
B. 0.0625
C. 16.52
D. 13.26
0.4.4
A prime lactor allecting conductivity is:
A.
temperature
B.
hardness.
C.
heat treatmen t.
D. all 01 the above.
0.4.5
Materials that tend to concentrate magnetic Ilux lines are_一一一一-一·
A.
conductive
B.
permeable
C.
resistive
D.
inductive
0.4.6
Diamagnetic materials have_一一一一一一_.
A.
a permeability greater than air
B.
a permeability less than air
C.
a permeability greater than lerromagnetic materials
D.
no permeability
29
0.4.7
When an increase in field intensity produces little or no additional flux in a magnetic test ob.
ject, the object is considered:
A.
stabilized.
B. balanced.
C. saturated.
D. at magnetic threshold.
0.4.8
Edge effect can be reduced by:
A. shielding.
B. selecting a lower frequency.
C.
using a smaller coi l.
D.
both A and C.
0.4.9
Li ft.off signals produced by a 0-10 mil spacing change are approximately 一一一-一一一_ times
greater than a 80-90 mil spacing change
A. 10
B. 2
C.
5
D.
100
0.4.10
Calculate the effect of fill factorwhen a conducting barO.5 inches in diameterwith an effective
permeability of 0.4 is placed into a 1.inch diameter coil with an unloaded voltage of 10 volts.
The loaded voltage is 一一一一一一
A. 2 volts
B. 4.6 volts
C. 8.5 volts
D. 3.2 volts
0.4.11
Laminations are easily detected with the eddy current method
A. True
B.
False
0.4.12
Temperature changes , vibration , and environmental effects are test coil inputs that generate
A. unwanted signals.
B.
magnetic fields
C. eddy currents.
D. drif!.
30
5.
SELECTION OF TEST PARAMETERS
As NDT engineers and technicians , it is our responsibility to industry to pr口vide and perlorm non.
destructive examinations that in some way assure the quality or uselulness 01 industry products. In
order to apply a nondestructive test , it is essential that we understand the parameters allecting the
tes t. Usually , industry establishes a product or component and then seeks a method to inspect it.
This practice establishes test object geomet印， conductivity , and permeability prior to the applica.
tion 01 the eddy current examination. Instrumentation , test coil , and test Irequency selection be.
come the tools used to solve the problem 01 inspection. Test coils were discussed previously , and
instrumentation will be discussed later in this tex t. Test Irequencies and their selection will be ex.
amined in detail in this Section.
FREQUENCY SELECT川
J
In Section 1, we observed that eddy currents are exponentially reduced as they penetrate the test objec t. We also observed a time or phase dillerence in these currents. The currents near the test coil
happen lirst，口 r lead the current that is deeper in the objec t. A high current density 剖 lows good
detectability , and a wide phase difference between near and lar surlaces allows good resolution.
Single Frequency Systems
Aιn时阳川
10巾巾
D町r
phase difference between near and 怡
1 ar surlace is reduced.
Selection 01 Irequency olten becomes a compromise. It is common practice in in-service inspection
01 thin wall , non-Ierromagnetic tubing to establish a standard depth 01 penetration just past the midpoint 01 the tube wall (17). This permits about 25 percent 01 the available eddy current to Ilow at the
outside surlace 01 the tube wall. In addition , this establishes a phase difference 01 approximately
150 to 170 degrees between the inside and outside surlace 01 the tube wal l. The combination 01 25
percent outside , or surlace current , and 170 degrees included phase angle provides good detectability and resolution lor thin wall tube inspection
The depth 01 penetration lormula discussed in Section 1, although correct , has rather cumbersome
units. Conductivity is usually expressed in percent 01 the "International Annealed Copper Standard"
(% IACS). Resistivity is usually expressed in terms 01 micro-ohm-centimeter (μOcm) (9). Depths 01
penetration are normally much less than 0.5 inch. A lormula using these units may be more appropriate and easier to use.
A depth 01 penetration lormula using resistivity , Irequency, and permeability can be expressed as
lollows:
ó=K~
where:
8
K
E
μrel
= Depth 01 penetration in inches
= Constant = 1.98
= Resistivity inμOcm
= Frequency in hertz
= 1 lor non-Ierromagnetic materials
31
For non-Ierromagnetic materials the term I'rol is ignored. The equation lor non-Ierromagnetic materials then becomes:
=K J丰
8
As technicians and engineers , our prime variable is Irequency. By adjusting Irequency we can be
selectively responsive to test object variables. Solving the non-Ierromagnetic depth 01 penetration
lormula lor Irequency requires a simple algebraic manipulation as lollows:
(a)
(b)
(c)
= K ....n-
占
d子
S
K
ó'
-
Q
K'
(d)
K'
ô'
(e)
K'P
E
ó'
e
= 1 or 1 = (1.9B)'
ó'
As an example 01 how this may be used consider inspecting an aluminum plate 0.3 inch thick ,
lastened to a steel plate at the lar surlace. Ellects 01 the steel part are undesirable and require discrimination or elimination. The aluminum plate has a resistivlty 01 5 micro-ohm-cm. Byestabllshing
a depth 01 penetration at 0.1 inch , the lar surlace current will be less than 10 percent 01 the available
current , thus reducing response to the steel part. The Irequency required lor this can be calculated
by applying:
(1.9B}'(5)
0.1'
1960 hertz
1I detection 01 the presence 01 the steel part was required , the depth 01 penetration could be reestablished at 0.3 inch in the aluminum plate , and a new Irequency could be calculated.
19.6
(0.3)'
f
_ 19.6
- 0.09
218 hertz
Another approach to Irequency selection uses argument "A" 01 the Bessel lunctlon (5) where arguπlent "A" is equal to unity or 1.
A
fμ..，σd'
5066
where:
fσd
μrel
=
Frequency in hertz
Conductivity meter/ohm-mm'
Diameter 01 test object, cm
Relative permeability
32
A Irequency can always be selected to establish lactor "A" equal to 1. This Irequency is known as
the fimit frequency and is noted by the term Ig. By substituting 1 lor lactor "A" and Ig for f , the equa.
tion becomes:
主旦旦豆2
5066
or
5066
fg
μ<0'σd'
Li mit frequency (1 0 ) is then established in terms 01 conductivity , permeabiiity dimension , and a con.
stant (5066).
Since IImit frequency is based on these parameters , a method of frequency determination using a
test frequency to limit frequency ratio fff n can be accomplished. High fff n ratios are used for near su 卜
face tests , and lower fffg ratios are used- for subsurface tests.
Often results of such tests are represented graphically by diagrams. These diagrams are calied im.
pedance diagrams (4). Impedance illustrated by vector diagrams in Section 3 shows inductive re.
actance represented on the ordinate axis and resistance on the axis of abscissa.
The vectorsum 01 the reactive and resistive components is impedance. This impedance is a quantity
with magnitude and direction that is directly proportional to frequency. In order to construct a uni.
versal Impedance diagram valid for all frequencies , the jmpedance must be normalized (4). Figure
51illustrates a normalization process-/时 z →年Lυ'.\ 叫 ιA酬<(/
吨'-
t l oul
F~1
p
A
p t sul
?4
XIUUZ
〈」FU
〈MU
x-MUZ〈F
」U
〈ME
z_p (6..1
'.-1
E
D
Z_p (4
., ..1
-1
c
Z_p "-1
(2ωl
zpel ul l
B
。
。
R
1
RES ISTANCE - R
(b)
RES I STANCE - R
(a)
Figure 5.1-Effect 01 Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary Impedance with Secondary Circuit (Reprlnted with permlssion from Hugo L. Li bby ,.lntroduction to
Electromagnetic Nondestructive Test Methods , p. 37. Copyright@1971 , John Wiley & Sons , Inc.)
Figure 5.1a shows the effect on primary impedance Zp with changes in frequency (w = 2贺句. Figure
5.1a represents primary impedance without a secondary circuit or test objec t.
Figure 5.1b Iliustrates the effect of frequency on primary impedance with a secondary circuit or test
object presen t. The primary resistance R, in Figure 5.1a has been subtracted in Figure 5.1b since resistance is not affected by frequency. The term ωLsG in Figure 5.1b represents a reference quantity
for the secondary impedance. The units are secondary conductance G and ωLs secondary reactance.
33
Further normalization is accomplished by dividing the reactive and resistive components by the
term ωLo or the primary inductive reactance without a secondary circuit present
Figure 5.2 shows a typical normalized impedance diagram (19).
1.0
0.2
0.4
0.9
。
A叫
ωEZ 。 J
。
j3ASVMO
←Z
od
d
飞MKM〉巳030至
kt
8
nwnunv
7
CYLI
…
\08
v…
Ll TUBE
ARk
6
2.6
2.8
巨
1.5
。14
飞， O:~正ëG
3 .9
/
。 B
。 2~------~----~
/
JI
。
。
。
v
0 .1
0.2
0.3
/
1. 4
0 .4
05
。6
。?
0.8
NORMALJZED RESISTANCE (阳ωLo) OHMS
Figure 5.2-Normalized Impedance Dlagram lor a long coil encircling a solid cylindrical non.lerromagnetic b町，
showing also the locus lor a thin.wall tube. (k = electromagnetic.wave propagation constant lor a
conducting material or -J高σ; r = radius 01 conducting cylinder , meters;ω= 2霄1; 1 = Irequency;
,;WC;;G = equivalent 01 -J wp.a lor simplilied electrical circuits;μ= magnetic permeability
01 bar , or = 4". • 10.7 henries/meter il bar is nonmagnetic;σ= electrical conductivity 01 bar,
mhos/meter; 1.0 = coil lill lactor.) (ASM Committee on Eddy Current Inspection , "Eddy.Current In
spection ," Metals Handbook, Vo l. 11 , 8th Ed. , Howard E. Boyer, Editor, American Society lor Metals ,
1976 , p. 82.)
The termsωυωLo and RIωLo represent the relative impedance 01 the test coil as affected by the test
objec t.
34
Signals generated by changes in ωL or R caused by test object conditions such as surlace and subsurlace discontinuities may be noted by t.ωL or t. R. The t.ωLo and t. R notation indicates a change in
the impedance.
Figure 5.3 shows the impedance variation in a non-Ierromagnetic cylinder caused by surface and
subsurlace discontinuities (5)
t. R
"， L。
INSTlγυT DR. FOERSTER
Figure 5.3一 Impedance Varia!ions caused by surface and subsurface cracks in non.ferromagne!ic cylinders , a!
a frequency ra!io fllg = 50 (5 , p. 37-21)
Figure 5.3 also illustrates a sensitivity ratio for surface and subsurface discontinuities. Notice with
an f/fa ratio 01 50 , a relatively high frequency , the response to subsurface discontinuities is not very
pronóunced
35
Figure 5 .4 shows responses to the same discontinuities with an I/ln ratio 01 15. This lower Irequency
allows better detection 01 subsurlace discontinuities as shown in -Figure 5 .4
ωLn
0.16
0.14
0 .1、唱~';__U/o'j
0.08
0.06
\
~I '<(
\- t3"件Y?~一→ l
lUUVMXl!llB
电 1，
\
\ '\1
I ,\ '.,'A
\
II
.~
0.04
0.02
。
INSTITUT DR. FOERSTER
Figure 5.4 -lmpedance Variations caused by surface and subsurface cracks in non.ferromagnetic cylinders , at
a frequency ratio f/fg = 15 (5 , p. 37.20)
Multifrequency Systems
It becomes obvious that the technician mus! have a good working knowledge 01 current density and
phase relationships in order.to make intelligent Irequency choices. The Irequency choice discussed
to date deals with coil systems driven by only one Irequency. Test systems driven by more than one
Irequency are calied multifrequency 口 r multiparameter systems. It is common lor a test coil to be
driven with three or m 口 re Irequencies. Although several Irequencies may be applied simultaneously
or sequentially to a test coil , each 01 the individual Irequencies lollows rules established by single
Irequency methods. Signals generated at the various Irequencies are olten combined or mixed in
electronic circui!s that algebraically add or subtract signals to obtain a desired result
36
One multilrequency approach is to apply a broadband signal , with many Irequency components , to
the test coil (4). The inlormation transmitted by this signal is proportional to its bandwidth , and the
logarithm 01 1 plus the signal.t口.noise power ratio. This relationship is stated by the equation:
C=WL咱们一音)
where:
C
W
= Rate 01 inlormation transmitted in bits per second
= Bandwidth 01 the signal
…
告= Signal.to.noise pow
tio
This is known as the Shannon.Hartley law.
Another approach to multiparameter methods is to use a multiplexing process (12). The multiplexing
process places one frequency at a time on the test coi l. This results in zero cross.talk between Ire.
quencies and eliminates the need lor bandpass lilters. The major advantages 01 a multiplex system
are (1) lower cost , (2) greater Ilexibility in Irequency selection , and (3) no cross.talk between Ire.
quency channels.
1I the multiplex switching rate is sufficiently high , both broadband and multiplex systems have
essentially the same results. The characterization of eddy current signals by their phase angle and
amplitude is a common practice and provides a basis for signal mixing to suppress unwanted sig.
nals Irom test data (12). Two Irequencies are required to remove each unwanted variable
Practical multiparameter frequency selection can be demonstrated by the following example:
Problem: Eddy current inspection 01 installed thin.wall non.ferromagnetic heat exchanger tubing.
TUbing is structurally supported by ferromagnetic tube supports at several locations. It is desired to
remove the tube support response signal Irom tube wall data.
Solullon: Apply a multiparameter technique to suppress tube support signal response
First , a Irequency is selected to give optimum phase and amplitude inlormation about the tube wal l.
We shall call this the prime frequency. At the prime Irequency , the response to the tube support and
a calibrating through.wall hole are equal in amplitude response.
A second frequency called the subtractor frequency is selected on the basis 01 tube support
response. Since the tube support surrounds the OD 01 the tube , a low Irequency is selected. At the
subtractor frequency the tube support signal response is approximately 10 times greater than the
calibrating through.wall hole
1I the mixing unit amplitude adjustments are set so that both prime and subtractor tube support
signal amplitudes are equal and phased in a manner to cause signal subtraction , the tube support
signals cancel , leaving only slightly attenuated prime data inlormation. F::or$.uppressi旦旦旦 nside or
near surl号电些EFh共且嗖旦旦旦旦121freq 坚旦旦旦旦且旦旦旦旦A combination 01 prime , 1 口w ， and high subtractor Irequencies is olten used to suppress both near
and lar surlace signals , leaving only data pertaining to the part thickness and its condition
Optimization 01 frequency then depends on the desired measurement or parameter 01 interest (11 ,
12 , 4)
37
REVIEW QUESTl ONS
0.5.1
What Irequency is required to establish one standard depth 01 penetration 01 0.1 inch in Zir.
conium?
A. 19.6 kHz B. 196 Hz )
C. 3.4 kHz
D. 340 Hz
0.5.2
In order to reduce effects 01 lar surlace indications , the test frequency 一一一一一一一一·
A. must be mixed
B. must be raised
C. must be lowered 守
D. has no ellect
0.5.3
The frequency required to establish the Bessel lunction Argument "A" equal to 1 is called
A.
B.
C.
D.
optimum frequency
resonanj Irequency
limit Irequency
penetration Irequency
J
0.5.4
Calculate the limit frequency lor a copper bar (ó = 50.6 mete时 ohm mm') 1 cm in diameter. The
correct limit Irequency is 一一一一一一一一·
A. 50 kHz
B. 50.6 Hz
C. 100 Hz
D. 100 kHz
0.5.5
Using the example in Question 5.4, what is the I/lg ratio il the test frequency is 60 kHz?
A.
1.2
B. 120
C. 60
D. 600
0.5.6
In Figure 5.1 b the valueωLsG equaling 1.4 would be indicative of 一一一一一一一一_.
A. a high resistivity material
B. a high conductivity material
C. a low conductivity material
D. a nonconductor
0.5.7
Primary resistance is subtracted Irom Figure 5.1b because _ _ _ __
A. resistance is always constant
B. resistance is not frequency dependent
C. resistance does not add to the impedance
D. none 01 the above
0.5.8
The relerence quantity is different for solid cylinder and thin-wall tube in Figure 5.2 because
A,
B.
C.
D.
0.5.9
the Irequency is different
the conductivity is different
the skin elfect is no longer negligible
the thin-wall tube has not been normalized
A 25 percent deep crack open to the near surlace gives a response 一一一一一一一 times greajer
than the same crack 3.3 percent 01 diameter under the surface (rel. Figure 5.4).
A. 10
B. 2.4
C. 1.25
D.
5
38
0.5.10'"
When using multifrequency systems , low subtractor frequencies are used to suppress
A.
(
C.
D.
conductivity changes
far su 归 ce signals
near surface signals
permeability changes
39
6.
INSTRUMENT SYSTEMS
Most eddy current instrumentation is categorized by its linal output or display mode. There are basic
requirements common to all types 01 eddy current instrumentation
Five dillerent elements are usually required to produce a viable eddy current instrument (4). These
lunctions are excitation, modulation, signal preparation, signal analysis, and signal display. An optional sixth component would be test object handling equipmen t.
Figure 6.1 illustrates how these components interrelate
EXCITATION
MQDUlATION
SIGNAL
PREPARATION
BAιANCE OR
COMPENSATING
NETWORKS
FILTERS
SIGNAL SHAPING
CIRCUITS
。 EMODULATION
AND ANALYSIS
DISPLAY OR
INDICATION
RECORDERS
ALAAMS
RELAYS
AUTOMATIC
MECHANISMS
-------.--- -------_-------- --- -
」一一--一-一'…
---'
Figure 6.1-lnternal Functions 01 the Electromagnetic Nondestructive Test (Reprinted with permission Irom
Hugo L. Li bby , Introduotion to Eleotromagnetio Nondestructive Test Methods , p. 60. Copyright @
1971 , John Wiley & Sons, Ino.)
The generator provides excilalion signals 10 the lest coi l. The signal modulation occurs in Ihe electromagnetic lield 01 Ihe lesl coil assembly. Next , Ihe signal preparalion section , usually a balancing
network , prepares the signal lor demodulation and analysis. In the signal preparation stage , balance
networks are used 10 "null" out sleady-value alternating currenl signals. Ampliliers and lilters are
also part 01 this section 10 improve signal-to-noise ratio and raise signal levels lor Ihe subsequent
demodulation and analysis stage
The demodulation and analysis section is made up 01 detectors , analyzers , discriminators , lillers ,
and sampling circuils. Deteclors can be a simple amplitude type or a more sophisticaled phasel
amplitude or coherent Iype
The signal display section is the key link between the lest equipmenl and its intended purpose. The
signal can be displayed many dillerent ways. Common displays include cathode ray lube (CRT)
oscilloscopes , melers , recorders , visual or audible alarms , computer lerminals , and automatic si 日'
nalling or rejecl equipmen l.
40
A series of simple eddy current instruments is shown in Figure 6.2 a , b , c , and d (19).
'电
VOLTMETER { V
GROUND (a)
GROUND
GROUND G俩。 UND (b)
(a)
GROUND (c)
GROUNO
(b)
GROUNO
(c)
GROUND 阳}
GROUND
SAMPLE
(d)
Figure 6.2-Four Types of Simple Eddy Current Instruments (ASM Committee on Eddy Current Inspection ,
"Eddy-Current Inspection ," Meta/s Handbook , Vol 11 , 8th Ed. , Howard E. Boyer, Editor, American
Society for Metals , 1976 ，口 .86.)
In Figure 6.2a , the voltage across the inspection coil is monitored by an ac voltmete r. This type of instrument could be used to measure large lift-off variations where accuracy was not critica l. Figure
6.2b shows an impedance bridge circui t. This instrument consists of an ac exciting source , dropping
resistors , and a balancing impedance
Figure 6.2c is similar to Figure 6.2b. In Figure 6.2c a balance coil similar to the inspection coil is
used to provide a balanced bridge. Figure 6.2d illustrates a balancing coil affected by a reference
sample. This is commonly used in external reference differential coil tests. In all cases , since only
the voltage change or magnitude is monitored , these systems can all be grouped as impedance
magnitude types (5).
Eddy current testing can be divided into three broad groups (2). The groups are impedance testing ,
phase analysis testing , and modulation analysis testing. Impedance testing is based on gross
changes in coil impedance when the coil is placed near the test objec t. Phase analysis testing is
based on phase changes occurring in the test coil and the test object's effect on those phase
changes. Modulation analysis testing depends on the test object passing through the test coil's
magnetic field at a constant rate. The amount of frequency modulation observed as a discontinuity
passes through the test coil's field and is a function of the transit time of the discontinuity through
the coil's field. The faster the transit time , the greater the modulation
IMPEDANCE TESTING
With impedance magnitude instrumentation it is often difficult to separate desired responses , such
as changes in conductivity or permeability , from dimensional changes. A variation of the impedance
magnitude technique is the reactance magnitude instrument (5). In reactance magnitude tests , the
test coil is part of the fundamental frequency oscillator circui t. This operates like a tuned circuit
where the oscillatorfrequency is determined by the test coil's inductive reactance. As the test coil is
affected by the test object , its inductive reactance changes , which in turn changes the oscillator frequency. The relative frequency variation Ll f/f is , therefore , an indication of test object condition.
Reactance magnitude systems have many of the same limitations as impedance magnitude
systems (5)
41
PHASE ANALYSIS TESTING
Phase analysis techniques are divided into many subgroups depending on the type 01 data display
Some 01 the various types are vector point, impedance plane, e Jl ipse , and linear time base (2). The
vector point circuit and display are illustrated in Figure 6.3.
Vector Point
FROM HERE
10 HERE
CONDUCTIVITY
HORIZONTAL
"A"
FROM HERE
γo HERE
OIMENSIONAL CHG
VER Tl CAL
"B"
90"
STANDARD
REFERENCE
PADCESSING
CIRCUITS
GENERATOR
。-
1E51
ARTICLE
270.
CRT
,.0
V.
A 引 C
M 目阻
πW
UN 呐
剧。扪
四旧时
μDg
PDC EUU
Z==
Y
一一-...
/ I POINT
?~
/l
90"
/
/
/
I
~X
。
VOLTAGE PLANE
Figure 6.3-Vector Point Method (2 , p. 3-15) Reprinted with permission
42
LIGm
l
V,
The vector point display is a point 01 light on a CRT. The point is the vector sum 01 the Y and X axis
voltages present in the test coil (2). By proper selection 01 Irequency and phase adjustment, voltage
V, could represent dimensional changes and voltage V, could represent changes in conductivity.
Ellipse
The ellipse method is shown in Figure 6.4.
PROCESSING
CIRCUITS
GENERATOR
七兰RTICLE
PHASE
SHIFTER
Figure 6.4 -Ellipse Method (2 , p. 3-16) Reprinted with permisslon
As with the vector point method , the test object and relerence standard are used to provide a balanced outpU t. A normal balanced output is a straight horizontal line. Figure 6.5 shows typical ellipse
responses (2).
DIMENSION
SMALL CHANGE
ιARGE CHANGE
SMALL CHANGE
LARGE CHANGE
SMALL CHANGE
LARGE CHANGE
CONOUCTIVITγ
80TH DIMENSION
AND CONDUCTIVITY
Figure 6.5-CRT Displays for Dimension and Conductivity (2 , p. 3-17) Reprinted with permission.
43
With the ellipse method the vertical deflection plates 01 a CRT are energized by an amplilied voltage
Irom the secondary test coils (5). The horizontal dellection plates are energized by a voltage that cor.
responds to the primary magnetizing curren t. With this arrangemenl, an ellipse opening occurs
when a discontinuity signal is perpendicular to a dimensional variation in the impedance plane
The ellipse method can be used to examine many test object variables such as conductivity , permeability , hardness , dimensions, and discontinuities. When testing lerromagnetic parts with the
ellipse and vector point methods , the relative permeability 01 the part will vary due to the nonlinear
magnetization 01 the magnetizing lield. This nonlinear magnetization creates odd harmonic Irequencies to appear in the output data (5).
Linear Time Base
A test instrument system that is well suited 10 determine harmonic distortions present in the lundamental Irequency uses the linear time base method 01 analysis (5).
Figure 6.6一Li near Time Base Instrument Diagram 侣， p. 40-29)
The Ii near time base unit applies a sawtooth shaped voltage 10 the horizontal dellection plates 01 a
CRT. This provides a linear trace 01 Ihe CRT beam Irom left to right across the CRT screen. The.linear
trace is timed so lhat it is equal to one cycle 01 the magnetizing curren t. This allows one cycle 01 the
magnetizing sine wave voltage to appear on the CRT. Figure 6.7 illustrates a linear time base
display.
SL1T
SCREEN
_/
/
/
/0
Figure 6.7-Screen Image 01 a Li nearTime Base Inslrumenl with Sinusoidal Signals 侣， p. 40.31)
44
A slit or narr口w vertical scale is provided to measure the amplitude 01 signa'ls present in the slit (5)
The base voltage is normally adjusted to cross the slit at "0" volts , the 180' point on the sine wave
The slit value "M" is used to analyze results. The slit value "M" is described by the equation:
M = A sin e
where:
M
A
= Slit value
= Amplitude 01 the measurement in the slit
θ=
Angle between base signal and measurement ellect
In Figure 6.7 , the angle dillerence A to B is approximately 90 degrees.
MODULATION ANALYSIS TESTING
Test instruments may also be classilied by mode 01 operation (4). The mode 01 operation is determined by two lunctional areas within the instrument type.
The lirst consideration is the method 01 test coil excitation. The second area is the degree 01 compensation , or nulling , and the type 01 detector used.
The types 01 excitation include single frequency or multifrequency sinusoidal, single or repetitive
pulse, and swept frequency.
Compensation and detection can be accomplished by three modes. The three main input-detector
modes are:
1.
2.
3.
null balance with amplitude detector,
null balance with amplitude-phase detectors , and
selected oll-null balance with amplitude detector.
Mode 1 responds to any signal irrespective 01 phase angle. Mode 2, using amplitude-phase detectors , can discriminate against signals having a particular phase ang 怡. With this system , the total
demodulated signal can be displayed on an X-Y oscilloscope to show amplitude and phase relationships. Figure 6.8a shows a commercial null balance instrument with amplitude phase detectors
Figure 6.8a-Null Balanoe Instrument with Amplitude-Phase Deteotors (Zeteo , Ino.)
45
5 回%
ID
3 陌%
。D
Figure 6.8b-Typlcal Response to a Thin Wall Non.ferromagnetic Tube Calibration Standard (Zetec , Inc.)
Mode 3 is a phase.sensitive system although it has 口 nly amplitude detectors. It achieves phase sen.
sitivity by operating at a selected off.balance condition. This off.null signal is very large compared
with test object variations.. Under this condition , the amplitude detector output varies in accordance
with the test object signal variation on the large off-null signal. Two off.null systems are required to
present both components 01 the test coil output signa l.
Figure 6.9 shows a block diagram 01 a stepped single Irequency phase-amplitude instrument (4)
LJ一-一-E=:r-一一-1
i
i
~-一--'
I
!
~一一一-，
Figure 6.9一 Instrument Providing Any One of Four Operating Frequencies (Reprinted with permission from
Hugo L. Li bby , Introduction to Electromagnetic Nondestructive Test Methods , p. 65. Copyright @
1971 , John Wiley & Sons , Inc.)
46
The circuit in Figure 6.9 is capable 01 operating at any 01 the lour Irequencies. 1I the lour frequencies
are over a wide range , several different test coils may be required to use the instrument over the entire range. Most modern single Irequency instruments use this principle; however, the lour individual
generators are usually replaced by one variable Irequency generator with a wide operating range. A
typical Irequency range lor such an instrument is 100 Hz to several megahertz.
Figure 6.10 shows a block diagram lor a multilrequency instrument operating at three Ireqυencìes
simultaneously.
----,
MULTIPLE
READ.QUTS
Fìgure 6.10-Multilrequency Instrument Operatìng at Three Frequencies Sìmultaneously (Reprìnted with permis
sion from Hugo L. Li bby , Introduction to Electromagnetic Nondestructive Test Methods , p. 65.
Copyright @ 1971 , John Wiley & Sons , Inc.)
In Figure 6.10 , excìtation currents at each Irequency are impressed on the coìl at the same time.
Multìple cìrcuìts are used throughout the instrument (4). The test coìl output carrìer Irequencìes are
separated by lilters. Multiple dual phase amplitude detectors are used and theìr outputs summed to
provìde separation 01 several test object parameters. A system sìmìlar to thìs ìs described in "In.
Service Inspectìon 01 Steam Generator Tubìng Usìng Multìple Frequency Eddy Current Technìques"
(12).
Another approach to the multìlrequency technique uses a sequentìal coìl drìve called multiplexing
(12). The Irequencìes are changed by a step.by.step sequence wìth such rapìdìty that the tes.t para.
meters remaìn unchanged. The multìplex technìque has the advantages 01 lower cost , contìnuously
varìable Irequencies , and little or no cross-talk between channels.
47
Figure 6.11 illustrates a commercial multilrequency instrument capable 01 operating at lour dillerent Irequencies sequentially. Each 01 the Irequency modules may be adjusted over a wide range
01 Irequencies. In addition , two mixing modules are used to combine output signals 01 the various
channels lor suppression 01 unwanted variables. Results 01 such suppression are described in
"Multi-Frequency Eddy Current Method and the Separation 01 Test Specimen Variables" (12).
E骂=罩
_i ， il昌
圃 II! 噩
噩 11圄
Figure 6.11 一 Commercial Multifrequency Instrument (Zetec , Inc.)
Instruments are being developed that are programmable , computer or microprocessor based. With
microprocessor controlled instruments , test setups can be stored in a programmable memory
system. This allows complicated , preprogrammed test setups to be recalled and used by semiskilled
personne l. Systems are designed with preprograms having superviso叩 code interlocks that prevent
reprogramming by other than authorized personnel. Microprocessor-based instrume r1ts can inter.
lace with larger computer systems lor control and signal analysis purposes. Figure 6.12 shows a
single Irequency portable microprocessor-based instrumen t. The CRT display applies the phase
analysis technique lor signal interpretation.
Figure 6.12-Commercial Microprocessor-Based Instrument (Nortec Corporation)
48
Other instruments being developed will be microprocessor based with the ability to excite several
coils .at several frequencies. This would allow automatic suppression of unwanted variables and a
direct link to larger computers for computer enhancement of test signal information.
A test system using pulsed excitation is shown in Figure 6.13 (剑，
Figure 6.13-Pulsed Waveform Excitation (Reprinted with þermission from Hugo L. Li bby , Introduction to Elec.
tromagnetic Nondestructive Test Methods , p. 66. Copyright @ 1971 , John Wiley & Sons , Inc.)
A pulse is applied to the test cOil , compensating networks , and analyzers simultaneously. Systems
having analyzers with one or two sampling points perform similar to a single frequency tester using
sinusoidal excitation.
Pulsed eddy current systems (7) having multiple sampling points perform more like the multifre.
quency tester shown in Figure 6.10.
TEST OBJECT HANDLl NG EQUIPMENT
Test object handling equipment is often a necessary component 01 a test system (4). Bars and tubes
can be led through encircling coils by means of roller fed assemblies. The stock being led through
the coil is usually transported at a constant speed. The transport speed is selected with instrument
response and reject system response being 01 prime ìmportance to the tes t. Pen marking and auto
matic sorting devices are common in automaled systems. Spinning probes are used where the
probe is rotated and the lube or bar is Iranslated. Probe rotational speeds must be compalible wilh
instrument response and Iranslation speeds in order to obtain the desired inspeclion coverage and
results.
Small parls are often gravity fed through coil assemblies. A major problem wilh small parls is load.
ing , inspecting , and unloading. A speed effect (4) occurs when a conducting objecl is passed
through a coi l. As the object moves through the coil's magnetic field , an addilional induced vollage
within the object occurs. This addilional induced voltage has the same Irequency as Ihe exciting
current , and it causes a currenl flow and associated magnetic fields that produce signals proportional to Ihe speed of the objecl Ihrough the coi l.
For larger or stationary structures , test probes are scanned over the part surlace by manual or remotely operated systems. Scanning considerations are the same as lor tube and bar slock: instrument response , marking or reject system response , and desired coverage. In the case 01 large heal
exchangers , a probe positioning device is used 10 position the test probe over each lube opening 10
be inspected. Tubes 10 be inspecled are identilied by manual templales , ortheir coordinates are pro
grammed into computer memory. Positive feedback is supplied to computer positioning syslems by
encoder devices. In manual lemplale syslems the tube end is viewed by a video camera. Tube identilication and control feedback are supplied 10 Ihe operalor via a video display system.
In each syslem , as the probe guide is positioned correctly , Ihe probe is inserted and withdrawn from
Ihe heal exchanger tube bore , and results 01 the scan are recorded on charl paper and magnetic
lape
49
REVIEW QUES Tl UN::;
0.6.1
Signal preparation is usua Jl y accomplished by
A. detectors.
B. samplers.
C. balance networks.
D. discriminators.
0.6.2
M 口 st eddy current instruments have 一一一一一一一_ coil excitation.
A.
B.
C.
D.
sQuare wave
triangular wave
sine wave
sawtooth wave
0.6.3
When only coil vOltage is monitored , the system is considered a(an) 一一一一一一一_ type system.
A.
impedance magnitude
B.
phase analysis
C. reactance magnitude
D. resistance magnitude
0.6.4
It is easy to distinguish dimensional variations from discontinuities in a reactance magnitude
system.
A. True
B. False
0.6.5
Eddy current systems can be grouped by:
A. output characteristics.
B. excitation mode
C. phase analysis exten t.
D.
both A and B
0.6.6
In modulation testing the test object must be
A. stationary
B. moving
C. pOlarized
D. saturated
0.6.7
Using the vector point method , undesired responses appear 一一一一一一一一_ on the CRT
A. vertical
B. horizontal
C. at 45. to horizontal
D. random and cannot be set
0.6.8
When ellipse testing a rod , the f1f g ratio is lowered from 50 to 5 percen t. The response from a 5
percent surface flaw:
A. will appear more e Jl iptical.
B. will appear less elliptical
C. is unchanged.
D. rotates 90. clockwise.
0.6.9
Using the linear time base , harmonics appear
A. as phase shifts of the fundamental waveform.
B. as distortions of the fundamental waveform.
C. to have no effect on the fundamental waveform.
D. as modified sawtooth signals.
0.6.10
Calculate the slit value "M" for a signal phase shift of 45 degrees at 10 divisions vertical
amplitude.
A. 14
日
7
C.
0.7
1.4
D.
50
0.6.11
A multifrequency instrument that excites the test coil with several frequencies simultaneously
uses the 一一一一一一-一_ concept
A.
multiplex
B. time share
C.
broadband
D. synthesized
0.6.12
A multifrequency instrument that excites the test coil with several frequencies sequentially
uses the 一一一一一一-一_ concep t.
A. multiplex
B. time base
C. broadband
D. Cartesian
0.6.13
In a pulsed eddy current system using a short duration and a long duration pulse , the short
duration pulse is used to reduce 一一一-一一一一-一
A.
edge effect
B. skin effect
C. lift.off effect
D. conductivity variations
0.6.14
When selecting feed rates for automatic inspection of tube and bar stock, consideration is
given to:
A.
instrument response.
B. automatic sorting response.
C. speed effect
D. all of the above
51
7.
READOUT MECHANISMS
Eddy current test data may be displayed or indicated in a variety 01 ways. The type 01 display or readout depends on the test requirements (4). Test records may require archive storage on large inservice components so that corrosion or discontinuity rates 01 change can be monitored and projected. In some production tests , a simple GO/NO-GO indicator circuit is all that is required.
Some common readout mechanisms are indicator lights , audio alarms , meters , digital displays ,
cathode ray tubes , recorders , and computer printout or displays.
INDICATOR LIGHTS
A simple use 01 the indicator light is to monitor the eddy current signal amplitude with an amplitude
gate circui t. When the signal reaches a preset amplitude limit , the amplitude gate switches a relay
that applies power to an indicator light or automatic sorting device. With the amplitude gate circuit ,
high-Iow limits could be preset to give GOII叶 O-GO indications.
AUDIO ALARMS
Audio alarms can be used in much the same manner. Usually the audio alarm indicates only the abnormal condition. Alarm lights and audio alarms are commonly incorporated in eddy current test
equipmen t. The indicator light and audio alarm give only qualitative inlormation about the item ,
whether a condition is present or no t. The degree 01 condition cannot normally be determined with
these devices. Indicator lights and audio alarms are relatively inexpensive and can be interpreted by
semiskilled personnel
METERS
Meters can present quantitative inlormation about a test objec t. Meters operate on the D'Arsonval
galvanometer principle. The principle is based on the action between two magnetic lields. A common meter uses a strong permanent magnet to produce one magnetic lield; the other magnetic lield
is produced by a movable coil wound on a core. The coil and core are suspended on jewelled bearings and attached to a pointer or "needle." The instrument output current is passed through the coil
and produces a magnetic lield about the coil that reacts to the permanent magnetic lield surrounding the assembly. The measuring coil is dellected , moving the meter pointer. The degree 01 pointer
movement can be related to test object variables as presented by the tester output signals.
DIGITAL DISPLAYS
Numerical digital displays or indicators provide the same type 01 inlormation as analog meter
systems. Many eddy current instruments have analog output circuits.
Data handling (7) 01 analog inlormation in digital lorm requires analog inlormation to be processed
through analog-to-digital (A-D) converters. The A-D converter translorms analog voltages to numerical values lor display
52
CRTs
Cathode ray tubes (CRTs) play.an important role in the display 01 eddy current inlormation. Most
CRTs are the "electrostatic" type. Three main elements comprise a cathode ray tube: (1) electron
gun , (2) dellection plates , and (3) a Iluorescent screen. The electron gun generates , locuses , and
directs the electron beam toward the lace or screen 01 the CRT. The dellection plates are situated
between the electron gun and the screen. They are arranged in two pairs , usually called horizontal
and vertical , or X and Y. The plane 01 one pair is perpendicular to the other pair and therelore considered X and Y.
The screen is the imaging portion 01 the CRT. The screen consists 01 a co a. ting or coatings that produce photochemical reactions when struck by the electron beam. The photochemical action ap
pears in two stages. Fluorescence occurs as the electron beam strikes the screen. Phosphorescence
enables the screen to continue to give 011 light after the electron beam has been removed
or has passed over a section 01 the screen. AII screen materials possess both Iluorescence and
phosphorescence. Screen materials are relerred to as phosphors. The color 01 Iluorescence and
phosphorescence may differ as the case lor zinc sullide: the Iluorescence is blue-green , and the
phosphorescence is y创 low-green. Fluorescence may appear blue , white , red , yellow , green , orange ,
。 r a combination 01 colors , depending on the chemical makeup 01 the screen. The amount 01 light
output Irom the Iluorescent screen depends on the electron beam accelerating potential , screen
chemical composition , thickness 01 screen material , and writing speed 01 the electron beam.
‘
The duration 01 the photochemical effect is called persistance. Persistance is grouped as to low ,
medium , or high persistance. To display repetitive signals , a low or medium persistance CRT may be
used. To display nonrecurrent or single events , a high persistance CRT should be used. Many
modern CRTs have the capability 01 both low or medium and high persistance. Storage or memory
CRTs have the ability to display nonrecurrent signals. The image Irom a single event may remain
visible on the CRT lor πlany hours , il desired
Figure 7.1 illustrates a typical eddy current signal response on a storage CRT.
Figure 7.1 一 Discontinuity Response in Thin-Wall Non-ferromagnetic Tubing (Zetec , Inc.1
The amplitude 01 the signal in Figure 7.1 is an indicator 01 the volume 01 the discontinuity. The phase
angle with respect to the X axis represents discontinuity depth and origin , origin indicating whether
the discontinuity originated on the inside or outside surlace 01 the tube (13).
53
RECORDERS
Recorders are also used to display data and to provide a convenient method 01 data storage. Record.
ing is accomplished on paper strip charts , lacsimile paper , lacsimile photosensitive , magnetic tape
(AM , FM , or video) , or digital memory disks.
Strip chart recordings are common in testing tubing or nuclear luel rods where the discontinuity's
location down the length 01 rod or tube is critica l. The strip chart length is indexed to time or
distance and pen response indicates normal or abnormal conditions.
?咱磨司最
Figure 7.2一 Commercial Strip Chart Recorder (Gould Instruments)
Fascimile recording (12) is a technique 01 displaying data signals as a raster 01 lines which have
varying levels 01 blackness which correspond to data.signal voltage changes. Facsimile recording is
commonly relerred to as C-scan recording. 1I no data is transmitted to the lacsimile recorder, a
unilorm light or dark (depending on prelerence) line or series 01 lines (raster) would be recorded. In
the case 01 light rasters , the incoming data signal would produce areas 01 different darkness. The
darkness would be dependent on the incoming data signa l. Facsimile recorders are used in conjunc.
tion with scanning mechanisms and scan rates , and locations are synchronized with the lacsimile
recorder to present an image 01 the object variances. Figure 7.3 illustrates a typical lacsimile record.
mg.
54
Figure 7.3-Facsimile Recording 01 Saw-cut Specimen (Copyright , American Society 10rTesting and Materials ,
1916 Race Street, Philadelphia , PA. 19103. Reprinted , with permission.)
Another common type recorder is the X.Y recorder. X-Y recorders are usually used to present scanning type data. In X-Y systems , only data signals are printed; no raster is produced in a conventional
X-Y recorder system.
Magnetic tape recorders , usually Irequency-modulated multichannel types , are used to provide a
permanent record 01 test results. In the case 01 eddy current equipment with X-Y outputs , quadrature
inlormation is recorded and played back into analyzers lor post inspection analysis (13)
COMPUTERS
Computers may be used to control data acquisition and analysis processes. Data handling techniques (7) take a wide variety 01 approaches. Dodd and Deeds (12) describe a computer-controlled
multilrequency system. Figure 7.4 shows a computer-controlled eddy current system
Figure 7.4 -Computer-controlled Eddy Current System (Oak Ridge National Laboratory , No. 1747-49)
55
REVIEW OUESTl ONS
0.7.1
Display requirements are based on:
A. test applications.
B. records requiremen t.
C. need lor automatic contro l.
D. all 01 the above
0.7-2
Amplitude gates provide a method 01 controlling:
A.
reject or acceptance limits.
日
instrument response.
C. displayamplitude.
D. all 01 the above.
0.7-3
Alarms and lights oller only:
A. qualitative inlormation
B. quantitative inlormation.
C. reject inlormation.
D. accept inlormation
0.7.4
The galvanometer principle is the basis lor 一一一一一一一一一·
A. corrosion rates
B. metallographic deterioration
C. a voltmeter
D. light source illumination
0.7.5
In order lor analog inlormation to be presented to a digital computer , it must be processed
through
A. an A.D converter
B. a microprocessor
C. a phase detector
D. an amplitude detector
0.7.6
In a cathode ray tube , the electron gun:
A. directs the beam.
B. locuses the beam.
C. generates the beam.
D. all 01 the above.
0.7.7
Photochemical reactions produced by electrons striking a CRT screen cause:
A. photosynthesis.
B. phosphorescence.
C. Iluorescence.
D. both B and C.
0.7.8
High persistance CRT screens are normally used lor repetitive signal display.
A. True
B. False
0.7.9
Length 01 a strip chart can indicate:
A. Ilaw severity.
B. distance or time.
C. orthogonality.
D. all 01 the above.
0.7.10
A series 01 lines produced in lacsimile recording is/are called:
A. grid lines.
B. raster.
C. crosshatch.
D. sweep display.
56
8.
APP Ll CATIONS
Electromagnetic induction and the eddy current principle can be affected in many different ways.
These effects may be grouped by discontinuity detection , measurement 01 material properties ,
dimensional measurements , and other special applications (4).
With discontinuity, or the Ilaw detection group , we are concerned with locatin 日 cracks , corrosion ,
erosion , and mechanical damage. The material properties group includes measurements 01 co忏
ductivity , permeability , hardness , alloy sorting or chemical composition , and degree 01 heat treat.
men t.
Dimensional measurements commonly made are thickness , prolilometry , spacing or location , and
coating or cladding thickness.
Special applications include measurements 01 temperature , Ilow metering 01 liquid metals , sonic vi.
brations , and anisotropic conditions.
FLAW OETECTION
The theoretic a:1response to discontinuities has been discussed in previous Sections 01 this guide.ln
this Section , some actual practice examples are given to enhance the understanding 01 applied
theory
A problem common to the chemical and electric power industries is the corrosion 01 heat exchanger
tubing. This tubing is installed in large vessels in a high density array. It is not uncommon lor a
4-loot diameter heat exchanger to contain 3000 tubes. This high density and limited access to the inspection areas olten preclude the use 01 other NOE methods.
Heat exchanger inspection systems and results are described by Denton (13) , Wehrmeister (13) , Li bby (8) , Dodd , Sagar, and Davis (12).
In most 01 these cases , the severity 01 the discontinuity is determined by analyzing the eddy current
signal phase and amplitude. The signal amplitude is an indicator 01 the discontinuity volume. The
phase angle determines the depth 01 the discontinuity and also the originating surlace (10 or OD) 01
that discontinuity. (See Figure 6.8 , pp. 45-46.)
Phase angle and amplitude relationships are usually established by using a relerence standard with
artilicial discontinuities 01 known and documented values.
The geometry 01 real discontinuities may differ Irom relerence standard discontinuities. This dil.
lerence produces interpretation errors as discussed by Sagar (12). Placement 01 real discontinuities
near tube support members causing a complex coil impedance change is also a source 01 error
This , 01 course , is dependent upon the size 01 the discontinuity and its resultant eddy current signal
in relation to the tube support signa l. Thislollows the basic principle 01 signal-to-noise ratio.
The signal.to-noise ratio can be improved at tube-to-tube support intersections by the use 01 mult 卜
Irequency techniques (12 , 11). In multilrequency applications , an optimum Irequency is chosen lor
response to the tube wall and a lower than optimum Irequency is chosen lor response to the tube
suppor t. The two signals are processed through comparator circuits called mixers where the tube
support response is subtracted Irom the tube wall response signal , leaving only the response to the
tube wall discontinuity.
Another industry that uses eddy current testing extensively is the aircralt industry. Many eddy cur.
rent examinations are conducted on gas turbine engines and airlrame structures. A common prob.
lem with gas turbines is latigue cracking 01 the compressor or exhaust turbine blade roots (13).
57
Usually these inspections are pe 时 ormed with portable instruments with meter response capability
The meter response is compared to the response of known discontinuities in a reference specimen.
A determination is then made of the part's acceptance
The reference specimen and its associated discontinuities are very critical to the success of the
tes t. Often models are constructed with artificial discontinuities that are exact duplicates of the
item being inspected
The low frequency eddy current inspection of aircraft structures is explained by D.J. Hagemaier (14)
The low frequency (100-1000 Hz) technique is used to locate cracks in thick or multiple layer , bolted
or riveted aircraft structures. Again , models are constructed with artificial cracks , and their
responses are compared to responses in the actual test objec t. Pulsed eddy current systems also
are used for crack detection in thick structures.
DIMENSIONAL MEASUREMENTS
Dimensional measurements , such as thickness , shape , and position , or proximity of one item to
another , are important uses of the eddy current technique
Often materials are clad with òther materials to present a resistance to chemical altack or to provide
wear resistance. Cladding or plating thickness then becomes an important variable to the serviceability of the unit (6).
For nonconductive coatings on conductive bases , the "probe-to-specimen spacing" (6) , or Ii ft-off
technique can be applied.
The case of conductive plating or cladding on conductive bases requires more refinemen t. The
thickness loci respond in a complex manner on the impedance plane (4). The loci for multilayered
objects with each layer consisting of a material with a different conductivity follow a spiral paltern.
In certain cases , two frequency or multifrequency systems (6) are used to stabilize results or minimize lift-off variations on the thickness measurement
The depth of case hardening can be determined by measuring the nitride case thickness in stainless
steel (11). The nitride case thickness produces magnetic permeability variations. The thicker the
nitride , the greater the permeability. The coil's inductive reactance increases with a permeability increase. This variable is carefully monitored and correlated to actual metallographic results
Eddy current profilometry is another common way to measure dimensions; for example , the measurement of inside diameters of tubes using a lilt-off technique (11). For this measurement, several
small probe coils are mounted radially in a coil form. The coil form is inserled into the tube and each
coil's proximity to the tube wall is monitored. The resultant output 01 each coil can provide information about the concentricity 01 the tube.
An obvious problem encountered with this method is centering of the coil holder assembly. The cen.
ter of the coil holder must be near the center of the tube. When inspecting for localized dimensional
changes , a long coil holder is effective in maintaining proper centering. Another function 01 the long
coil lorm is to keep the coils Irom becoming "cocked" or tilted in the tube.
CONDUCTIVITY MEASUREMENTS
Conductivity is an important measured variable. In the aircraft industry , aluminum is used extensive.
Iy. Aluminum conductivity varies not only with alloy but also with hardness and tensile strength.
Eddy current instruments scaled in % IACS are normally used to inspect for conductivity variations.
Secondary conductivity standards (12) are commonly used to check instrument calibration. Common secondary conductivity standards range from 8% IACS to approximately 100% IACS. The
58
secondary standards are usually certilied accurate within :!: 0.35 percent or :!: 1 percent 01 value ,
whichever is less. Temperature is an important variable when making conductivity measurements.
Most instruments and standards are certilied at 20 o C. Primary conductivity standards are maintained at a constant temperature by oil bath systems.
Primary standards are measured by precision Maxwell bridge type instruments. This circuit increases measurement accuracy and minimizes Irequency dependence 01 the measurement (12).
HARDNESS MEASUREMENTS
Hardness 01 steel parts is often measured with low Irequency comparator bridge instruments (19)
The relerence and test coil are balanced with sample parts 01 known hardness. As parts 01 unknown
hardness affect the test coil , the instrument outþut varies. The amount 01 output variation depends
upon the degree 01 imbalance created by the unknown test object hardness.
Signal output is then correlated to test object hardness by comparing to known hardness samples 01
the same geometry. For example , il a cathode ray tube were used to display hardness inlormation ,
the "balance" hardness could be adjusted to center screen , lower hardness values could appear
below center , and higher hardness values could appear above center on the CRT.
ALLOY SORTING
Alloy sorting can be accomplished in the same comparator bridge manner as hardness. A major consideration in both cases is the selection 01 correct and accurate relerence specimens. Since most
eddy current instruments respond to a wide range 01 variables , the relerence s8ecimen parameters
must be controlled carelully.
Test object and relerence specimens must be the same or very similar in the lollowing characteristics:
1.
2.
3.
4.
5.
geometry,
heat treatment ,
surlace linish ,
residual stresses , and
metallurgical structure
In addition , it is advisable to have more than one relerence specimen lor backup in case 01 loss or
damage. In the case 01 steel parts , they should be completely demagnetized to remove the ellects 01
residual magnetism on instrument readings. As in most comparative tests , temperature 01 specimen
and test object should be the same or compensated.
Many other measurements can be made using eddy current techniques. The electromagnetic technique produces so much inlormation about a material , its application is only limited by our ability to
decipher this inlormation (13).
REVIEW QUESTIONS
Q_8-1
Conductivity , hardness , and composition are part 01 the 一一一一一一_ group.
A. delect detection
B. material properties
C. dimensional
D. special
Q_8-2
Using an ID coil on tubing and applying the phase/amplitude method 01 inspection , a signal appearing at 90 on a CRT would be caused by:
A.
ID Ilaw.
B. OD Ilaw.
C. den t.
D. bulge
0
59
0.8.3
Discontlnuitles In heat exchangers at tube support locations are easier to detect because the
support plate concentrates the electromagnetic f!e!d at that poin t.
A. True
B.
False
0.8.4
Using multifrequency techniques on installed heat exchanger tubing , a tube support plate
signal can be suppressed by adding a 一一--一__ frequency signal to the optimum frequency
slgnal.
A.
low
B.
high
C.
A O{ B
D.
none of the above
0.8.5
In the aircraft industry , a common problem in gas turbine engines is:
A.
corrosion
B.
fatigue cracking.
C.
vibration damage.
D. erosion.
0.8.6
Thick or multilayered aircraft structures are normally inspected by:
A.
low frequency sinusoidal continuous wave instruments.
B.
high frequency sinusoidal continuous wave instruments.
C.
pulsed systems
D. A and C.
0.8.7
Response to multilayer varying conductivity structures follow 一一一一一一_Ioci.
A.
orthogonal
B.
spiral
C.
linear
D. stepped
0.8.8
Nitride case thickness can be monitored in stainless steel cylinders by measuring 一一一-一一一·
A.
conductivity
B.
dimensions
C.
permeabillty
D.
none of the above
0.8.9
Conductivity is not affected by temperature.
A.
True
B.
False
0.8.10
Residual stresses in the test part produce such a small effect that they are usually ignored
when selecting reference specimens.
A.
True
B.
False
60
9.
EDDY CURRENT TEST PROCEDURES, STANDARDS , AND SPECIFICATIONS
Procedures , specilications , and standards are produced to provid.. a means 01 controlling product
or service quality. Written instructions that guide a company or individual to a desired end result ,
and are acceptable to industry, are the basis 01 procedures , specilications , and standards.
Many publications are available to guide or instruct us. Some 01 the most Irequently used relerences
are the American Society lor Testing and Materials (ASTM) , American Society 01 Mechanical Engineers (ASME) , American National Standards Institute (ANSI) , and Military Standards
(MIL-STD-XXXX).
These publications are laboriously produced by committees made up 01 scientilic and technical
people. Usually alter a committee produces a draft document, it is submitted to industry and the
scientilic community lor comment and subsequent revision (17).
In certain cases , standards combine to assist each other. As an example , ASME Section V Appendix
A (17) uses ASTM E268(18) to provide "STANDARD DEFINITIONS OF TERMS RELATING TO ELEC.
TROMAGNETIC TESTING." The military standard , MIL-STD-1537A "Electrical Conductivity Test lor
Measurement of Heat Treatment 01 Aluminum Alloys , Eddy Current Method ," relerences ASTM 8193
"Resistivity of Electrical Conductor Materials" and ASTM #18 "Rockwell Hardness and Rockwell
Superlicial Hardness 01 Metallic Materials."
ASTM
5tandards or Standard Practices , ASTM , usually include in the written instructlons headings such
as scope , relerences , method , significance , definitions , apparatus , calibration standards , calibration , and procedure. The Scope heading makes a general statement about the document's applica.
bilityand inten t. The Applicable Document heading relers to other publications used as references
for the standard. Method is usually a summary 01 how the test method is applied to the test object
and what type 01 measurements can be made. Under the Significance heading is a more detailed discussion 01 test results and probable causes 01 indications expected during the examination. The
Definition section usually contains delinitions 01 key words or key phrases associated with the in.
spection. Apparatus describes the general requirements for the inspection system including instrumentation , coils , positioning , and driving mechanisms. Under Calibration Standards the labrication
requirements lor artilicial discontinuity calibration standards are discussed. Included is usually a
discussion 01 the reference specimen and the geometrical requirements 01 the artilicial discontinuities in i t. The Calibration section provides instructions lor adjustment a. nd standardization 01 the
apparatus used lor the examination. The response to known discontinuities is usually described in
this section. Under the Procedure heading , detailed instmctions are given to implement the inspection. These instructions usually include acceptancll limits and how to handle components that are
not acceptable.
The A5TM publishes several standards pertaining to the eddy current method. These standards are
numbered; lor example , E268-81. "E268" relers to the standard , and "81" relers to the year 01 origin
or the year 01 last revlsion.
50me A5TM standards that pertain to the eddy current method are as follows:
E 309-77 Eddy-Current Examination 01 5teel Tubular Products Using Magnetic Saturation
E 571-82 Electromagnetic (Eddy-Current) Examination 01 Nickel and Nickel Alloy Tubular Products
E 703-79 Electromagnetic (Eddy-Current) Sorting 01 Nonlerrous Metals
E 426-76 (1981) Elecromagnetic (Eddy-Current) Testing 01 5eamless and Welded Tubular Products ,
Austenitic 5tainless 5teel and Similar Alloys
E 243-80日 Electromagnetic (Eddy-Current) Testing 01 Seamless Copper and Copper Alloy Tubes
E 566-82 Electromagnetic (Eddy-Current) 50rtlng 01 Ferrous Metals
61
E 215-67 (1979)自 Electromagnetic Testing 01 Seamless Aluminum-Alloy Tube , Standardizing
Equipment lor
E 690-79 In Situ Electromagnetic (Eddy-Current) Examination 01 Nonmagnetic Heat Exchanger
Tubes
E 376-69 (1979) Measuring Coatlng Thickness by Magnetic-Field or Eddy-Current (Electromagnetic)
Test Methods
E 268-81 Electromagnetic Testing
MIL-STD
The United States Military uses the Military Standard document to control testing and materials
Standard procedures are provided by a series 01 MIL-STD-XXXXX documents. Special requirements
are specilied by the Military Specllication system. For example , MIL-STD-1537A relers to "Electrical
Conductivity Test lor Measurement 01 Heat Treatment 01 Aluminum Alloys , Eddy Current Method."
The "Calibration System Requirements" lor MIL-STD-1537A are contained in Military Specilication
MIL-C-45662.
The MIL-STD usually contains several parts and is very descriptive. These parts normally include
Scope, Applicable Documents, Definitions, General Requirements, Detail Requirements, and Notes.
The Scope contains a general statement 01 applicability and intent 01 the Standard. Applicable
Documents pertains to other relerence or controlling documents such as other MIL-STD , Military
Specilication , or ASTM publications. The Definition part contains precise delinitions 01 key words
and phrases used in the Standard. Under General Requirements, equipment , relerence specimen ,
and personnel requirements are described in sullicient detail to implement the Standard. Included
in this part is instrument sensitivity and response , test object variables , relerence specimen requirements , and personnel qualilication requirements. The Detail Requirements part describes the
specilic procedure to implement the Standard. The Notes part contains pertinent statements about
the process and guidelines lor reporting 01 results.
ASME
In 1911 the American Society 01 Mechanical Engineers set up a committee to establish rules 01 salety lor design , labrication , and inspection 01 boilers and pressure vessels. These rules have become
known throughout industry as the ASME Code. The committee designated as ASME Boiler and Pres.
sure Vessel Committee is a very large group 01 people Irom industry and the scientilic community
The "Committee" has many subc口 mmittees ， groups , subgroups, and working groups. Currently,
there are approximately 3 committees , 15 subcommittees , 50 subgroups , and 37 working groups.
Each subcommitlee, subgroup , and working group combines as a unit lor a specilic area 01 interest
For example , the Subcommitlee on Pressure Vessels (SCVIII) has three working groups and lour subgroups reporting to i t. The purpose 01 these groups is t口 interlace with industry to keep pace wlth
changing requirements and needs 01 industry and public salety
The ASME Boiler and Pressure Vessel Code is divided into eleven sections. ASME Sec t1 0n V is
"Nondestructive Examination." Section V is divided into two subsections , "A" and "B". Subsection
"A" deals with "Nondestructive Methods 01 Examination." Article 8 is "Eddy Current Examination 01
Tubular Products." Subsection "B" contains "Documents Adopted by Section V."
Eddy current standards are described in Article 26. In this case , the ASTM -E 215 document has been
adopted by the ASME and reassigned the designation "SE215."
ASME Section V Article 8 - Appendix I gives detailed procedure requirements lor "Eddy Current Examination Method lor Installed Non-Ierromagnetic Steam Generator Heat ExchangerTubing." A procedure designed to meet this requirement can be illustrated by the lollowing example , Document
QA3.
62
Procedure No. QA 3
11-1
EDDY CURRENT INSPECTION OF
NONFERROUS TUBING BY
SINGLE FREQUENCY TECHNIQUES
A.
PURPOSE
This procedure describes the equipment and methods as well as the personnel qualifications to be
utilized lor the perlormance 01 the eddy current examination 01 steam generator tubes , and meets
the requirements 01 the N.R.C. Regulatory Guide 1.83 , ASME Section XI Appendix IV , and ASME Sec.
tion V Article 801 the ASME Boiler and Pressure Vessel Code.
B.
SCOPE
The scope 01 the examination to be pe时 ormed is contained in the eddy current inspection program
document applicable to the specilic plant to be Inspected.
C.
PREREQUISITES
1.
Plant Condition
The plant must be shut down wlth the primary system drained. The steam generators shall be
open on the prima叩 side lor access to the channel head and the shell cool.down sequence
shall be complete. Air movers shall be attached to circulateair through the generator to dry the
tube shee t.
2.
Equipment
The examinations shall be performed utilizing an XXXX/ XX eddy current instrument with bobbin
coil probes designed lor testing Irom the inside 01 the tubes. The inspection perlormance shall
be monitored by the use 01 the oscilloscope with a phase sensitive vector display and recorded
lor later evaluation on both magnetic tape and strip chart.
a.
Equipment utilized shall be:
i.
XXXX/ XX eddy current instrument with integral series YYYY two.channel storage
dlsplay oscilloscope.
i i.
Dillerential coil probes.
ii i. XYZ or YZA two.channel Irequency modulated magnetic tape recorder.
iv. Brush Mark 220 strip chart recorder.
v.
Four.channel communications systems.
vi. Calibration standard
The calibration standard shall be manulactured lrom a length 01 tubing 01 the same
size and type 01 material that is to be examined in the vesse l. The standard shall con.
tain 6 intentional delect areas as lollows:
aa. 100% drill hole (0.052" lor 3/4" OD tubing and smaller, and 0.067" lor larger tub.
ing).
bb. Flat.bottomed drill hole 5/64" diameter X 80% through Irom the outer tube wall
surlace.
cc. Flat.bo t! omed drill hole 7/64" diameter X 60% through Irom the outer tube wall
surlace
dd. Flat.bo t! omed drill hole 3/16" X 40% diameter through Irom the outer tube wall
surlace.
ee. Four Ilat.bottom holes , 3/16" diameter, spaced 90 degrees apart around the tube
circumlerence , 20% through the tube wal l. Each standard shall be identilied by
a serial number etched on one end and be traceable to the master standard
stored at the lacility.
63
Procedure No. QA 3
11.2
ff.
b.
c.
3.
Circumferential groove 20% deep by 1/16" long by 360 degrees on the inside
tube wall surface
gg. Circumferential groove 10% deep by 1/8" long by 360 degrees on the outer tube
wall surface
Probe positioning and feeding shall be accomplished remotely for in.service inspection
whenever practica l. Baseline inspection may be done manually.
Communications shall be provided by the use of a Model 1 four.channel mixer system.
Personnel Qualifications
Personnel collecting data in accordance with this procedure shall be qualified to Level I or
higher in accordance with Document QA 101 目 Personnel interpreting data collected in accor.
dance with procedure shall be qualified to LevelllA or higher in accordance with Document QA
101. Prior to receiving a certification , the applicants shall have completed the program recom.
mended by SNT.TC.1A (1968 editi 口时， Supplement E
D.
E.
PRECAUTIONS
1.
AII personnel to be engaged in eddy current inspection programs at operating plants shall have
received instructions in and understand the radiation protection rules and guidelines in effect
on the plant site.
2.
. AII personnel to be engaged in the test program shall wear protective clothing to the extent 01
the type delined by the exclusion area work permit
3.
AII personnel entering a radiation work area will have proven their ability to work in a lace mask
by successlully passing the pulmonary lunction test during their annual physical
4.
No entries shall be m a: de into the steam generator channel head without the presence 01 a
qualilied health physics technician.
5.
Ensure that nozzle covers (when applicable) are securely in place inside the vessel prior to com.
mencement 01 the eddy current inspection program
PERFORMANCE
1.
Preparation
a.
Establish location 01 control'operation center.
b.
Arrange power distribution box at control.operation center.
c.
Install the lour.channel communications system control box at the control-operation cen.
ter
d.
Establish communication with one or more headsets at the steam generator
e.
Install XXXX/XX eddy current test instrument, tape recorder , and Brush recorder at control.
operation center.
f.
Connect the horizontal and vertical outputs Irom the XXXX/XX eddy current test instru.
ment to the H & V input jacks 01 the FM tape recorçier
g.
Connect the H & V output jacks 01 the FM tape recorder to the two.channel inputs 01 the
日 rush recorder
h.
Connect the microphone to the FM tape recorder.
64
Procedure No. QA 3
2.
n.3
i.
Connect Amphenol plug 01 the probe to receptacle on XXXXIXX eddy current test instm.
men t. Be sure to use a 100' extension cable in series with the probe.
j.
Position the trace 10 the center horizontal line 01 Ihe CRT graticule and sweep the trace
horizontally across the screen , utilizing the posilion knob on the XXXX.
NOTE: Trace is parallel to the horizontalline on graticule. 1I necessary, adjust thetrace
rotation knob on the back 01 the XXXXIXX.
k.
Turn XXXXJXX sensitivity to zem and center oscilloscope spot at the center 01 the graticule
using the position knobs
1.
Place all Iront panel switches on the FM tape recorder in the "up" position , start the tape
recorder in the recordπlode. Advance the record knobs until both meters are in the upper
position 01 the red scale. Run at low speed (3.314 inlsec)
m.
Depress the microphone key and speak into the microphone. Temporarily , use the com.
munication headset to assure that the voice recorded on the tape is loud and clear.
n.
With the tape recorder operating in the record mode and the oscilloscope spot centered ,
center the Brush chart pens with both channels set on 200 M.V .l div. and in the calibrated
position.
o.
With the position controls on the oscilloscope , make sure that the channels are correctly
connec!ed and the equipment will swing lull scale both directions. One volt dellection 01
XXXXIXX should give one major division dellection on Brush recorder
Equipment Calibration
a.
Prior to the commencement 01 the eddy current examination 01 the steam generator tubes
and alter the replacement 01 any component , the equipment shall be calibrated in accord.
ance with the lollowing steps:
i.
llisert the bobbin coil probe into a section 01 the calibration standard which is Iree 01
delects.
i i.
Select the desired operation Irequency (as per the specilic calibration procedure) by
operating lirst the "coarse" and then the "Iine" FREQUENCY control knobs on the
Iront panel
ii i. Set the balance selector swilch to "L" or "C" as required lor the specific test Ire.
quencylprobe combination.
iv. Set the sensitivity to zem and center the oscilloscope spot with the position controls
v.
Set the sensitivity adjustment knob to 050.
vi. Set the R and X balance knobs to 500目
vi i. Depress the automatic null push button
vii i. Release the automatic null button when spot motion ceases or reaches a minimum
This indicates null condition.
NOTE: As proper null is achieved , the X and R BALANCE controls will appear to
‘ 'hun t." This is a normal occurrence. 1I rotation 01 the X and R BALANCE
controls is not observed , verily that the dial.locking mechanisms are in their
luHy unlocked positions (Iull counterclockwise). Manually check Ireedom 01
rotation.
ix. Set the sensitivity knob as stated in the specilic calibration procedures
x.
Re.balance with the automatic nul l.
x i. Set the oscilloscope attenuators to 1 voltldivision
xi i. Rotate the phase control until the probe motion signal is horizontal (as per the spe.
cilic calibration procedure) and the lirst lobe 01 the 100% drill hole goes down lirst as
the probe is withdrawn Irom the standard
xii i. Position the trace to the center 01 the CRT display by rotating the position control
knobs (inner red knobs on the attenuators).
65
Procedure No. QA 3
3.
11.4
Tube Inspection General
(Reler to Specilic Calibration Procedure QA 2)
4.
5.
a.
Eddy current inspection activities shall be perlormed with equipment sensitivities and
speeds se! as lollows:
i.
Oscilloscope attenuator settings shall be 1 or 2 volts/division on both the vertical and
horizontal ampliliers
i i.
The tape recorder shall be run at low speed.
ii i. The Brush recorder sensitivity shall be set as per the Specilic Calibration Procedure.
iv. The Brush recorder shall be run at 5 mm/second lor all delect inspections and 25
mm/second lor all sludge measurement inspections.
v.
Insert probe completely through the calibration standard. Record pertinent data. at
beginning on both sides 01 magnetic tape and corresponding brush paper. Start tape
recorder and Brush chart recorder and record calibration standards while withdraw.
ing probe through standard past the delects. The system is now ready lor operation.
b.
Visual verilication 01 the identity 01 the specific tube being inspected shall be perlormed
before and after each fix!ure change and at the beginning and end of each side of
magnetic tape. Verilication of !he positive iden!ifica!ion 01 !ube location shall be recorded
on both the magnetic tape and strip charts.
c.
Should the performance 01 the tube identity verification reveal an error has occurred in the
recording 01 probe location , all tubes examined since the previous verification of location
shall be reexamined.
d.
The equipment calibration shall be verified and recorded at the beginning and end of each
side 01 the reel 01 magnetic tape. The calibration shall be checked Irequently by observing
that the tube support plates are at the proper phase angle. At a minimum , the calibration
will be verified at 4.hour intervals and after any equipment change.
e.
Should the equipment be lound to be outol calibration , the equipmen! will be recalibrated
as per Section E.2 of this procedure. The recalibration will be noted on both the magnetic
tape and strip char t. The data interpreter will de!ermine if it is necessary to reinspect any
01 the tubes.
Tube Inspection Manual
a.
The da!a recording shall be made during probe withdrawa l. Withdrawal speed is 14" per
second maximum. No minimum speed specilication is required , but a good unilorm pull 01
12" per second is prelerred
b.
Since no inspection is perlormed during probe insertion , the speed should be as rapid as
possible.
c.
The prober shall have con!inuous communication with the equipment operator and both
personnel must concur on the tube number being examined.
d.
Due to radiation exposure or endurance factors , combinations 01 probe guides and probe
pusher/pullers may be used !o lacilitate the inspection.
Tube Inspection Automatic Remote
NOTE:
a.
Ensure !hat all probe positioner , probe leeder, and probe and communication connect.
ing cables are clear 01 access walkways and secured to any available supports
Install remotely operated probe leeder local to steam generator with "on.off ," "Iorward.
reverse" control at control-operation center.
66
Procedure No. QA 3
F.
11.5
b.
Verify correct direction of rotation of probe feeder (make sure slow speed retracts probe)
c.
Check the operation of the remotely operated eddy current positioner and connect the
flexible probe conduits to the probe guide tube and the probe pusher.
d.
Install remotely operated probe positioner on tube sheet of the steam generator to provide
coverage of the area to be examined.
e.
Connect power supply.control cable assembly to remotely operated probe positioner
1.
Connect power supply-control cable assembly to the control console at the control.opera.
tion center.
g.
Verily the correct operation and control 01 the remotely operated probe positioner.
h.
Utilizing the probe pusher, feed theprobe through the Ilexible conduit up to the guide tube
01 the positioner
i.
Operate the positioner to locate the probe beneath the tube to be examined.
j.
Utilizing the probe pusher , leed the probe into and up the tube to the desired height
Monitor the extent of insertion by reference to impedance signals Irom tube supports on
the oscilloscope screen.
k.
Withdraw the eddy current probe Irom the tube until the impedance signal on the oscillo.
scope screen indicates that the probe is clear 01 the tube shee t. Concurrent with the probe
withdrawal , visually monitor the signals displayed on the oscilloscope screen while re.
cording all data on the magnetic tape and strip chart recorder.
1.
Reposition the probe beneath the next tube selected lor examination.
m.
Repeat the procedures described in the preceding steps until a 川 the tubes selected lor in.
spection have been examined.
INSPEC Tl ON RESULTS AND DOCUMENTA Tl ON
1.
Requirements
a.
The data interpreter shall be certilied to Level IIA or 111 as per Procedure QA 101.
b.
Data shall be collected with an eddy current test system with a current certification 01 cali.
bration as per procedure CSP.
c.
Data shall be interpreted with an interpretation system with a current certification 01 cali.
bratioπas per procedure CSP.
d.
The data collection station shall be calibrated with an approved standard which is serial.
ized and traceable to master calibration standard
e.
The identily 01 the plant site , the steam generator, the date , the test Irequency , the calibra.
tion standard serial numbers , and reel side numbers shall be recorded at the start 01 each
new reel and side 01 magnetic tape and its companion roll 01 chart paper
f.
The data collection station shall be set up and calibrated as per Procedure QA 3.
67
Procedure No. QA 3
2.
11.6
Performance
a.
The dala inlerpreter shall:
i.
Delermine Ihat all dala collecled is properly marked.
Reporl lubes whose data is incomplele or uninlerprelable.
i i.
ii i. Require a retesl al allernale frequency(ies) for any lube(s) Ihal has(have) a signal
whose origin is difficull 10 ascerlain.
iv. In.service inspeclions
aa. Reporl all defecls > 19% which occur on Ihe ouler surface.
bb. Reporl all olher indicalions which appear 10 be relevan l.
cc. Idenlify Ihe approximale axial position of Ihe indicalion wilh respecl 10 a known
slruclural member
v.
b.
Preservice inspeclions
aa. Reporl all indicalions observed. Include the approximale axial posilion of Ihe indicalion wilh respecl 10 a known slruclural member.
Inlerprelalion
i.
AII dala shall be recorded on form A3 or equivalenl
The conversion from signal phase angles 10 defecl deplhs shall be accomplished via
i i.
Ihe following:
Table Number
OA 4.1
OA 4.2
OA 4.3
ii i.
iv.
v.
G.
Equivalenl graphs , programmed compulers , elc. , may be used.
AII slrip charl recordings shall be examined in Iheir enlirely.
Any abnormal signals observed on Ihe slrip charls shall be recorded on Ihe dala
sheels. The magnelic lape recording shall be examined for each lube Ihal is recorded. In some cases , il may be necessary 10 examine Ihe magnelic lape in its enlirely. (This condilion would normally require addilional dala inlerprelalions 10 meel
scheduling requiremenls.)
The slrip charl recording is nol considered abnormal unless Ihe verlical excursion is
more Ihan 3 minor divisions wilh bolh channels having significanl phasing.
REFERENCES
The foliowing documenls are required for Ihe performance of eddy currenl inspeclion programs
ulilizing Ihe melhods described in Ihis procedure.
1.
Required Documents
a.
Eddy currenl inspeclion specific calibralion procedure documenl applicable 10 Ihe planl
10 be inspecled.
b.
Tube sheel maps marked 10 designale Ihe extenl of examinalion 10 be performed
c.
Eddy currenl inspection program fixlures placemenl delail documenl applicable 10 model
。f sleam generalor.lo be inspecled (when applicable)
68
11.7
FORM NUMBER
TITLE: SPECIFIC CALIBRATION PROCEDURE
SITE
STEAM GENERATOR#.一一一一一一一一 TUBE MATERIAL_一_ DIA._一一_WALL_一­
DATE
HOTLEG
COLDLEG
XXXX/XX
1. Frequency
士
2. Sensitivity
3. Balance
Manual 一一一一 Auto 一--一 L 一一一_C 一一一-
4. Phase
士
PROBE
1. Style Number
2. Length of Shaft
3. Length of Extension Cable
BRUSH RECORDER MARK 220
1. Vertical Sensitivity
2. Horizontal Sensitivity
YZA
1. Record Meters
Vertical Sensitivity
Horizontal Sensitivity
CA Ll BRATION STANDARD
1. Serial Numbers
2. Data Conversion Table/Curve #
INSPECTION TECHNIQUE (state probe retraction speed in applicable blank)
1. H and Probe Local
2. Hand Probe Remote
3. Hand Manipulate with Probe Pusher
4. Remote Control Fixture
5. Remote Control Fixture with Probe Pusher
6. Fixture Procedure Document Required?
69
YES__NO
A 2 Rev. 2
REVIEW aUESTIONS
a.9.1
A precise statement 01 a set 01 requirements to be satlslied by a material , product, system , or
service is a 一一一一-一
A. standard
B. specilication
C. procedure
D.
practice
a.9.2
A statement that comprises one or more terms with explanation is a 一一一一一-一
A.
practice
B. classilication
C. delinition
D.
proposal
a.9.3
A general statement 01 applicability and intent is usually presented in the 一一一一一一_ola
standard?
A. summary
B. scope
C. slgnllicance
D. procedure
a.9.4
Military Standards are designated by "MIL-C-(numbe吼"
A. True
B. False
a.9.5
In the structure 01 ASME the subcommittee reports to the subgroup.
A. True
B. False
a.9.6
In example QA 3, personnel Interpreting results must be:
A.
Level I or higher
B. Levelll or higher.
C.
Level IIA or higher.
D. Levelll l.
a.9-7
The prime artilicial discontinuity used to calibrate the system described in QA 3 is:
A. 20% ID
B. 50% OD
C. 100%
D. 50% ID
a.9-8
In QA 3, equipment calibration must be verilied at least 一一一一一一一一
A. every hour
B. each day
C. every 4 hours
D. every 8 hours
a.9.9
QA 3 specilies a maximum probe traverse rate 01
A.
12"/sec
B. 14"/sec
C. 6"/sec
D. not specilied
a.9.10
The system in QA 3 is calibrated with an approved standard that is traceable to 一一一-一一一一
A. NBS
B. ASME
C. a master standard
D. ASTM
70
0.9.11
In accordance with QA 3, tubes whose data are incomplete or uninterpretable must be
A.
B.
C.
D.
0.9.12
reinspected
reported
reevaluated
removed from service
Referring to QA 3 , QA 4.1 is a 一一一一-一一一一·
A. calibration form
B. data interpretation table
C. data report form
D. certification form
71
ANSWERS TO REVIEW QUESTIONS
Numbers in oarentheses indicate where answers may be checked and verified.
For Questions 1-1 through 9.5 , numbers in parentheses are keyed to the
references on page vi 01 this Study Guide. For Questlons 9.f!. th~o~gh 9.12,
numbers in parentheses reler to the reprint of Procedure No. QA 3, lound on
pages 63.68 of this Study Gulde.
,
Q 1-10
(4 , p. 45)
Q2-2
Q 2-3
Q 2-4
Q 2-5
Q 2-6
Q 2-7
Q 2-8
Q 2-9
Q 2-10
但但"
Q 4-2
Q 3-9
Q 3-10
Q 5-4
Q 5-5
Q 5-6
Q 5-7
Q5回S
Q 5-9
Q 5-10
Q 6-2
Q 8-3
Q 8-4
Q 8-5
Q 8-6
Q 8-7
Q 8-8
Q 8-9
Q 8-10
RMnu口MAREURURM户uvnuEU
Q 8-1
Q 4-3
Q 4-4
Q 4-5
Q 4-6
Q 4-7
Q 4-8
Q 4-9
Q 4-10
Q 4-11
Q 4-12
(5 , p. 38.25)
帆， p. 194)
(4 , p. 71)
侣， p. 40.1)
(4 , p. 195)
侣， p. 353)
(4 , p. 69)
(4 , p. 21创
(4 , p. 198)
例， p.211)
(2 , p.8)
(12 , p. 95)
(9 , p. 56)
(2 , p. 13)
(19 , p. 78)
(4 , p. 171)
(4, p. 173)
(2, p. 26)
(6 , p. 360)
(5 , p. 36.17)
(19 , p, 88)
(4 , p. 27)
、'\;.，.，
Q 6-2
CCABDBACBBCACD
Q 7:'1
(4 , p. 60)
Q 7-2
(4 , p. 60)
Q6圄 S
Q 7-3
(19 , p. 79)
(5 , p. 37.20)
(5 , p. 36.13)
(5 , p. 36.13)
(5 , p. 36.13)
(4 , p. 37)
(4 , p. 3η
(19 , p. 82)
(5 , p. 37.20)
(12 , p. 289)
Q 6-1
(4 , p. 270)
(13 , p. 59)
(12 , p. 282)
(12 , p. 256)
(13 , p. 4η
(12 , p. 129)
(4 , p. 51)
(11 , p. 631)
(12 , p. 121)
(19 , p. 102)
Q 9-1
(5 , p. 40.1)
(5 , p.40.14)
(4 , p. 64)
(2, p. 4.26)
(5 , p. 40.23)
(5 , p. 42.4)
(5 , p. 42.35)
(5 , p. 40.31)
(4 , p. 215)
(12 , p. 219)
(7, p. 38η
(4, p. 77)
Q6-4
Q 6-5
Q 6-6
Q 6-7
Q 6-8
Q 6-9
Q 6-10
Q 6-11
Q 6-12
Q 6-13
Q 6-14
BCBBBCCCEGBB
(18 Pa时 11， p. 111)
(18 Part 11 , p. 111)
(18 Pa民 I~ p.28四
(15 , p. 1)
(17 Sectlon V, p. x)
(QA.3 , p. 2)
(QA.3 , p. 3)
(QA.3 , p. 4)
(QA.3 , p. 4)
(QA.3 , p. 5)
(QA.3 , p. 6)
(QA.3 , p. 6)
Q 9-2
Q 9-3
Q 9-4
Q 9-5
Q9罔 S
Q 9-7
Q 9-8
Q 9-9
Q 9-10
Q 9-11
Q 9-12
72
Q7回 4
Q 7-5
Q 7-6
Q 7-7
Q 7-8
Q 7-9
Q 7-10
户U
nUA"nA
RAmnunuRununu
。 5-3
unununu
户U 口M 口M
AHEDF户
U
Q 5-2
Q 4-1
(2 , p. 38)
(19 , p. 78)
斜， p. 212)
(4 , p. 195)
(4 , p. 173)
(4 , p. 211)
Q 3-8
。 5-1
。。、e'巧，h
n
DCABCDCDDD
nvnvnv
, ))
-eono
,,,
eo 内dno
Q 3-1
Q 3-2
Q 3-3
Q 3-4
Q 3-5
Q 3-6
Q 3-7
。 2-1
户UEMAH
unMAR
RUFUhununun户
u
BBCABDBBCC
(4 , p. 19)
(4 , p. 19)
侣， p. 20)
(4 , p. 20)
侣， p. 23)
(13 , p. 4)
(4 , p. 25)
(4 , p. 26)
(4 , p. 26)
wE
nunununu
户ununununu』
Q 1-1
Q 1-2
Q 1-3
Q 1-4
Q 1-5
Q 1-6
Q 1-7
Q 1-8
Q 1-9
(4 , p. 76)
(4 , p. 76)
(7 , p.238)
(4 , p.250)
(12 , p. 450)