318 Ad ances in Elec ical and Elec onic Enginee ing
ADVANCES IN EDDY-CURRENT NON-DESTRUCTIVE EVALUATION
L. Janoušek, D. Gombá ska, K. ápo á
Depa men o Elec omagne ic and Biomedical Enginee ing, Facul y o Elec ical Enginee ing,
Uni e si y o Žilina, Uni e zi ná 1, 010 26 Žilina, Slo ak Republic, el.: +421 41 5135062,
e-mail: janousek@ el.uniza.sk, gomba ska@ el.uniza.sk, capo a@ el.uniza.sk
Summa y The pape p esen s a gene al o e iew o ecen de elopmen s in he eddy-cu en non-des uc i e e alua ion.
Basic p inciple o he eddy-cu en non-des uc i e es ing is explained and possible applica ions o he me hod in non-
in asi e e alua ion o conduc i e ma e ials a e summa ized. Ac ual issues o esea ch and de elopmen in his ield a e
discussed. A no el me hod o dep h e alua ion o a de ec ed c ack using eddy-cu en es ing signals is p oposed.
1. INTRODUCTION
Non-des uc i e e alua ion (NDE) o ma e ials is
de ined as a g oup o ac i i ies aiming o he
in es iga ion and he cha ac e iza ion o ma e ials as
well as s uc u es wi hou hei mechanical damage.
Di e en physical p inciples and phenomena a e
u ilised o he NDE o ma e ials. One o he
elec omagne ic me hods, o igina ing om he
p inciple o elec omagne ic induc ion, is an eddy
cu en non-des uc i e es ing (ECT) o conduc i e
ma e ials. Inc easing ends o u iliza ion o he
me hod in p ac ical inspec ions, especially in he las
pe iod, impose g adually aising demands on
esea ch and de elopmen (R&D) in his ield.
The pape summa izes ecen ad ances in eddy-
cu en non-des uc i e e alua ion. A e he
p inciple o he me hod is explained, possibili ies o
i s u iliza ion in di e en applica ions a e lis ed.
Ac ual issues o R&D ac i i ies in his ield a e
summa ized and discussed. A no el app oach in he
eddy-cu en non-des uc i e e alua ion is p oposed
and he p esen ed esul s p o e i s e ec i eness in
dep h e alua ion o a de ec ed de ec .
2. PRINCIPLE OF ECT
The p inciple o he ECT, shown in Fig. 1,
unde lies in he in e ac ion o induced eddy cu en s
wi h a s uc u e o an examined body [1], [2].
Fig. 1 P inciple o ECT
A p ima y al e na ing exci ing elec omagne ic ield
is gene a ed in he icini y o a coil d i en by an
al e na ing cu en . Elec omo i e o ce is induced in
a conduc i e objec which is in p oximi y o he coil
and eddy-cu en s low he e acco ding o he
elec omo i e o ce. A seconda y elec omagne ic
ield gene a ed by he eddy-cu en s coun e wo ks o
he p ima y exci ing elec omagne ic ield. The
induc ion coupling he e o e exis s be ween he coil
and he conduc i e objec . I can be simply
conside ed as an in e ac ion be ween he p ima y
and he seconda y elec omagne ic ield.
The esul ing elec omagne ic ield o he coil
and he conduc i e objec depends on geome ical
pa ame e s o he sys em as well as on he
elec omagne ic pa ame e s o he conduc i e objec .
Fo he gi en exci a ion, i.e. con igu a ion,
dimensions and o ien a ion o he coil(s) and i s
eeding, he coupling is in luenced by he ollowing
impo an pa ame e s:
• posi ion o he coil wi h espec o he objec ,
• geome ical con igu a ion o he objec ,
• dimensions o he objec , mainly i s hickness,
• he elec omagne ic pa ame e s o he objec
(conduc i i y, pe meabili y),
• na u e o he objec (homogenei y, linea i y,
aniso opy).
U iliza ion o he ECT in p ac ical applica ions
depends on a possibili y o de ec luc ua ions in he
esul ing elec omagne ic ield due o changes in he
impo an pa ame e s. The ECT is he e o e applied
in:
• hickness measu emen s o conduc i e ma e ials,
• hickness measu emen s o non-conduc i e
coa ings on conduc i e ma e ials,
• measu emen s o he elec omagne ic pa ame e s
(conduc i i y, pe meabili y) o conduc i e
ma e ials,
• e i ica ion o conduc i e ma e ial ea men ,
• e i ica ion o selec ed pa ame e s o p oduc s
(dimensions, e c.),
• de ec ion and e alua ion o discon inui ies
(de ec s) in conduc i e ma e ials, e c.
Rising employmen o he ECT in di e en echnical
applica ions imposes new challenging appeals on
R&D ac i i ies.
Ad ances in eddy-cu en non-des uc i e es ing … 319
3. ACTUAL ISSUES OF R&D ACTIVITIES
The p inciple o he ECT has been known o
se e al decades. Nowadays, he mos wide sp ead
applica ion a ea o he ECT is he de ec ion and
possible e alua ion o di e en discon inui ies in
conduc i e ma e ials.
The ha dwa e and he so wa e means o he
ECT can be di ided in o ollowing g oups:
• ECT ins umen s,
• ECT p obes,
• e alua ion o measu ed signals,
• posi ioning sys ems and manipula o s.
ECT ins umen s supply exci ing coils o he
p obes and sense he ECT signals. F equen ly,
se e al supe imposed ha monic exci ing signals a e
used o d i e he eddy cu en s. I helps o gain
in o ma ion abou a de ec unde se e al equencies
a he same ime. Mo eo e , be e sepa a ion
be ween he use ul signals and backg ound noises
can be ob ained. Nowadays, he R&D ac i i ies a e
inc easingly ocused on a pulse exci a ion [3]. I s
ad an age compa ing o he con en ional ha monic
exci a ion is in a wide equency spec um esponse
ob ained wi h one exci ing impulse [4].
ECT p obes a e one o he mos impo an
elemen s in he non-des uc i e es ing, because hey
ans e in o ma ion be ween an ECT ins umen and
a conduc i e objec h ough he induc ion coupling.
Usually, induc ance coils a e u ilized o build ECT
p obes. Howe e , magne ic senso s a e employed in
sensing low in ensi y ields [5].
Fea u es o ECT p obes depend on numbe , shape,
con igu a ion, o ien a ion, dimensions and
connec ions o coils as well as on pa ame e s o a
magne ic ci cui . Op imal ECT p obe should assu e
[6]:
• high sensi i i y o expec ed de ec s,
• high p obabili y o de ec ion o expec ed de ec s,
• possibili y o dis inguish pa ame e s (loca ion,
dimensions, e c.) o expec ed de ec s.
Many ypes o ECT p obes ha e been de eloped
e lec ing special demands o pa icula applica ions
[7]-[9]. ECT p obes wi h e omagne ic co es a e
some imes u ilized o p ecise localiza ion o he
elec omagne ic ield [10]. Howe e , he non-
linea i y o he co es b ings ano he unknown in o
he e alua ion.
Sensed signal is he in eg al alue. I s phase
depends mainly on a c ack dep h. Thus, a c ack
dep h can be oughly es ima ed om he signal
phase in o ma ion. Sol ing o in e se p oblems
ep esen s mo e sophis ica ed app oach in c ack
e alua ion. Selec ed pa ame e s o de ec s (dep h,
leng h, posi ion, p o ile, e c.) a e econs uc ed
based on he measu ed signals. Two app oaches a e
u ilized o he pu pose [11]:
• de e minis ic,
• s ochas ic.
Usually, wo a iables o a de ec a e es ima ed, i s
dep h and leng h, while a p o ile, a wid h and
elec omagne ic pa ame e s o a de ec a e adjus ed
in ad ance.
A di e ence be ween measu ed and simula ed
signals is minimized in he de e minis ic me hods
[12]. The p ocess is i e a i e and he e o e la ge
numbe o o wa d simula ions is equi ed.
Da abases o p e-compu ed signals [13] as well as
pa allel compu ing on supe compu e s [14] can help
o sho en he e alua ion ime.
The s ochas ic me hods u ilize so called e olu ion
algo i hms, o example neu al ne wo ks, gene ic
algo i hms, o he in e sion [15], [16].
A p esen , i is possible o de ec e en e y
small a i icial o a igue c ack and qui e p ecisely
es ima e i s dep h in homogeneous o non-
homogeneous s uc u es [17]. Cu en esea ch
ac i i ies a e mainly ocused on he ollowing a eas:
• design, de elopmen and op imisa ion o ECT
p obes o sa is y se e e demands o non-
des uc i e inspec ion o s uc u es wi h eal
de ec s,
• eliable de ec ion and localiza ion o unknown
and in mos cases in isible de ec s wi h a iable
o ien a ion, pa ame e s, p o ile and s uc u e,
• p ecise es ima ion o main pa ame e s o a
de ec , especially i s leng h and dep h,
• new possibili ies o p ac ical applica ions o he
ECT.
The issues lis ed abo e a e closely ela ed o each
o he and hey a e de e mined by ac ual p oblems
and p ac ical expe iences.
4. NOVEL EVALUATION METHOD
ECT signals a e in eg al alues and hey do no
ca y explici in o ma ion abou he c ack
dimensions. The e o e, e alua ing he dep h o a
de ec om he ECT signals is qui e di icul [18]. In
addi ion, he skin-e ec concen a es induced
cu en s on he su ace o a es ed ma e ial which
means ha ob aining in o ma ion abou he dep h is
essen ially di icul .
This sec ion p oposes a no el me hod o a c ack
dep h e alua ion om he ECT signals.
A new p obe [19] shown in Fig. 2 is used o he
inspec ion. The p obe consis s o ou ec angula
exci ing coils and a ci cula de ec ing coil placed in
he cen e be ween he exci ing coils. The exci ing
coils a e o ien ed angen ially ega ding he su ace
o a es ed sample and he pick-up coil has he
no mal o ien a ion. The inne exci ing coils and he
ou e ones a e connec ed in se ies, espec i ely.
A de ec ed c ack is inspec ed wice. A i s , only
he inne exci ing coils o he p obe a e d i en and
only he ou e exci ing coils a e d i en du ing he
second inspec ion. Di e en dis ances o he inne
and he ou e exci ing coils om he de ec ing coil
assu e a di e en p o ile o he eddy cu en densi y
along he ma e ial dep h unde he de ec ing coil.
320 Ad ances in Elec ical and Elec onic Enginee ing
Fig. 2 Pic u e o a new p obe
The p obe scan igh o e he c ack along i s leng h;
he windings o he exci ing coils a e pe pendicula
o he c ack leng h. The wo sensed signals o a same
c ack gained du ing wo inspec ions a e linea ly
supe imposed acco ding o he ollowing ela ions:
2211
2211
ImImIm
ReReRe
⋅−⋅=
⋅−⋅=
CC
CC
,
(1)
whe e Re
1
, Re
2
a e he eal pa s o wo complex
signals; Im
1
, Im
2
a e he imagina y pa s o wo
complex signals; and C
1
, C
2
a e eal posi i e
numbe s de ining a a io o he supe posi ion:
21
CC=
α
,
(2)
whe e max (C
1
, C
2
) = 1.0. The phase o he esul ing
supe imposed complex c ack signal
ϕ
co esponding
o he maximum absolu e alue is e alua ed wi h
espec o he alue o he a io. The phase is gi en
by he ollowing equa ion:
m
m
Re
Im
a c an=
ϕ
,
(3)
whe e Re
m
and Im
m
a e he eal and he imagina y
pa s o he supe imposed c ack signal
co esponding o he maximum absolu e alue o he
signal.
A pla e specimen made o he s ainless s eel
SUS316L is inspec ed o e alua e e ec i eness o
he p oposed me hod. The elec omagne ic
cha ac e is ics o he ma e ial include conduc i i y
o
σ
= 1.4 MS/m and ela i e pe meabili y o
µ
= 1.
The hickness o he specimen is d = 25 mm. Non-
conduc i e su ace b aking no ches o he
ec angula shape wi h a leng h o l
c
= 40 mm, a
wid h o w
c
= 0.5 mm and dep hs o d
c
= 10, 12, 15,
20 mm model he c acks. The equency o 10 kHz
is adop ed in he inspec ion.
Figu e 3 illus a es dependences o he
supe imposed c ack signal phase change on he a io
o supe posi ion o ou c acks. I can be obse ed
ha he phase o he supe imposed signal changes
wi h inc easing he a io o supe posi ion, while his
change depends on he c ack dep h. Thus, a unique
ea u e alue o he a io o supe posi ion can be
ex ac ed om he dependence gained o one c ack.
I is a alue o he a io whe e he c ack signal
o a es in a hal angle o i s o e all o a ion.
0
20
40
60
80
100
120
140
160
180
0 0.5 1 1.5 2 2.5 3
phase change [deg ee]
a io [-]
dc=10mm
dc=12mm
dc=15mm
dc=20mm
Fig. 3 Dependences o he supe imposed c ack signal
phase change on he a io o supe posi ion o he c acks
wi h dep hs o dc = 10, 12, 15 and 20 mm
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
a io [-]
c ack dep h [mm]
simula ion
measu emen
Fig. 4 Dependences o he ea u e alue o he a io o
supe posi ion on he c ack dep h
The dependences o he ea u e alue o he a io
o supe posi ion on he c ack dep h gained om
nume ical as well as expe imen al signals a e shown
in Fig. 4. The nume ical esul s a e plo ed o a
wide ange o he c ack dep h, om 0% o 100% o
he ma e ial hickness d = 25 mm, o highligh
ad an ages o he p oposed me hod. As i can be
seen, he e is a unique dependence be ween he
ea u e alue o he a io o supe posi ion and he
inne exci ing coils
de ec ing coil
ou e exci ing coils
Ad ances in eddy-cu en non-des uc i e es ing … 321
c ack dep h. I means ha he dep h o an inspec ed
de ec can be de e mined om he dependence based
on he ea u e alue o he a io ob ained om
measu ed signals acco ding o he s eps desc ibed
abo e. Mo eo e , he nume ically gained
dependence is almos linea wi hin he in es iga ed
ange and hus, he dep h o su ace b eaking de ec s
ha a e much deepe han he s anda d dep h o
pene a ion (
δ
= 4.25 mm in his case) can be
unambiguously e alua ed using he me hod.
4. CONCLUSION
The pape deal wi h ecen ad ances in eddy-
cu en non-des uc i e e alua ion. P inciple o he
me hod was explained in he beginning in o de o
demons a e a ie y o possible applica ions. Recen
esea ch and de elopmen ac i i ies in his ield
we e summa ized. No el me hod o non-des uc i e
e alua ion o a c ack dep h using eddy-cu en
es ing signals was p oposed. The nume ical and he
expe imen al esul s p o ed he e ec i eness o he
me hod e en o su ace b aking c acks ha a e
much deepe han he s anda d dep h o pene a ion.
Acknowledgemen
This wo k has been suppo ed by he g an
VEGA No. 1/0308/08 o he Slo ak Minis y o
Educa ion.
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