Eddy Current Testing Parte 2

8/12/2019 Eddy Current Testing Parte 2

1/8

-4

Dur i ng t h a t t ime , o t h e r

n o n d e s t r u c t i v e

t e s t

t e c h n i q u e s

such

as u l t r a s o n i c s

and

r ad iog raphy

became

w e l l e s t a b l i s h e d

and

eddy c u r r e n t

t e s t i n g

p layed

a

s e c o n d a r y r o l e ,

m i n ~ y in the

aircraft

i n d u s t r y . Recent

r e q u i r e me n t s

p r t i c u l ~ l y f o r

he a t

exchange r tube

i n s p e c t i o n

in

t h e

n u c l e a r i n d u s t r y

-

have c o n t r i b u t e d s i g n i f i c a n t l y

t o

f u r t h e r development o f ET

as

a

f a s t , a c c u r a t e and

r e p r o d u c i b l e n o n d e s t r u c t i v e t e s t

t e c hn ique .

U

ntil r e c e n t l y , eddy

c u r r e n t t e s t i n g

was

a

t e chno logy where

t h e ba s i c p r i n i ~ l e s

wre

known

o n l y

to r e s e a r c h e r s , a n d a

b l

ack

box

approach t o i n s p e c t i o n was o f t e n

f o l lowed .

The

a u t h o r s '

o b j e c t i v e in c o m p i l i n g t h i s manua l

i s to

draw upon

r e s e a r c h ,

l a b o r a t o r y

and i n d u s t r i a l i n s p e c t i o n e x p e r i e n c e to

b r i d g e

t h a t gap

and t h e r e b y p e r mi t t h e full p o t e n t i a l

of eddy

c u r r e n t

t e s t i n g to

be r e a l i z e d .


2/8

2

.1

t

-5 -

CHAPTER 2 - EDDY CURRENT FUND MENT LS

BASIC EQUIPMENT

Basic eddy cu r ren t t e s t

equipment

cons i s t s of an

a l t e r n a t i n g

cu r ren t

sou r c e

o s c i l l a t o r ) , a probe co n ta in in g a c o i l

connected to

t he

cu r ren t so u rce , and a

vol tme te r

which

measu r

es

t h e v o l t ag e change ac ro s s t he co i l as shown

i n

F

gu re

2 1

OSCILL TOR

VOLTMETER

CURRENT t

PROBE

ROBE

MOVEMENT

CR CK

TEST

PL TE

Fig. 2 .1 :

Eddy Curren t T es t Equipment

The o s c i l l a t o r must be

capab le of

g en e ra t in g

a t ime va r y i ng

u s u a l l y

s

inuso ida l )

cu r ren t

a t

f requenc ies r ang ing

from

about

1 kHz

1000

cyc les

per

second)

to

about 2 MHz

2 ,000 ,000 cyc les pe r

second)

.

O s c i l l a t o r s which

opera te

a t

h ig h e r

or lower

f r eq u en c i e s ,

or with

pulsed cu r ren t s

a re

used

fo r s p e c i a l i z e d a p p l i c a t i o n s .

The c o i l w i th in the probe i s

~

i n s u l a t ed copper wire

wound

onto a s u i t a b l e fo rm.

The

wire diameter , the number

of

t u rn s

and c o i l dimensions

a re

a l l v a r i ab l e s which must be

de te rmined

i n o rde r to o b t a i n t he

d es i r ed i n s p ec t i o n r e s u l t s .

Coi l

v a r i ab l e s

a re di scussed in

l a t e r ch ap t e r s .


3/8

6

Depending upon

the

type of

i nspec t ion ,

an

eddy

cu r ren t probe

can

cons i s t

of a s i n g l e t e s t c o i l , an ex c i t a t i o n co i l with a

sepa ra t e

r ece iv e s e n s ~ n g ) c o i l , or an ex c i t a t i o n c o i l with a

Ha l l - e f f e c t

sens ing

d e t ec to r , as shown

in Figure 2:2 .

VOLT

HETER

VOLTMETER

OSCILLATOR OSCILLATOR

TEST ARTICLE

COIL

EXCITATION

COIL

SENSJNG

CO IL

1

/

EXCITATION

COIL

VOLTMETER

OSC LLATOR

TEST

ARTICLE

HALL

DETECTOR

A>

S E L F

I ~ O U C T

SE

ND RECEIVE

C)

MAGNETIC REACTION

Fi g . 2.2:

Eddy Curren t

In s p ec t i o n Systems

The

v o l tme te r measures changes

in

vol tage across t he c o i l

which

r e s u l t

from

changes

in

the

e l e c t r i c a l

condi t ions

and

proper t ies of

the eondueting

m a t ~ r i a l tes ted and/or changes

in r e l a t i v e pos i

t

on between

the co i l and

the

mat e r i a l

t e s t e d .

This

vol tage change c o n s i s t s

of

an ampl i tude

v a r i a t i o n and a phase

v a r i a t i o n

r e l a t i v e to t h e cu r ren t

p as s in g

through

t he c o i

.

The

reason fo r ampl i tude and phase

changes in

t h i s v o l t ag e

i s

d i scu s sed

in Chap

t e r 3.

2.2 GENERATION OF EDDY CURRENTS

2 .2 .1 In t roduc t ion

In

t h i s s e c t i o n t he t ap i e of t h e magnetic

f i e l d

sur rounding a

c o i l

ca r ry i n g

cur ren

t

i s

in t roduced

toge ther

with

t he

mechanism

by

which

eddy cu r

r

en t s

are

induce and how

t h

ey a r e

measured .

2 . 2 . 2 Magnet ic Fi e l d Around

A Coi l

Oers ted d i scovered t h a t whenever

c u r r e n t ,

a magnet ic f i e ld

e x i s t s .

di rec t ed

a long a w ire , a magnet ic

d i r ec t i o n t ha t i f

your

r i

ght -hand

d i r ec t i o n

of cur ren t ,

your

cur led

there

i s an e l e c t r i c

Consider e l e c t

r i

c curren t

f i e ld

i s c rea t ed in such a

thumb

p o in t s

in t h e

f ingers

poin t in the


4/8

-7 -

d i r e c t i o n

of t h e magne t ic

f i e l d .

r u l e .

This i s t h e r i g h t - h a n d

A s s o c i a t e d

w i t h a

magne t ic f i e l d i s magne t ic

f l u x d e n s i t y .

It

has

t h e

same d i r e c t i o n

as

t h e

magne t ic

f i e l d

and

its

magn i tude

depends

upon p o s i t i o n and c u r r e n t .

It

i s t h e r e f o r e

a f i e l d

v e c t o r q u a n t i ty

and is g i v e n t h e symbol B. I t s u n i t s

in t h e S I

sys tem

i s t h e t e s l a (T) o r weber s

p e r

s q u a r e m e t r e

Wb/m2) .

The B - f i e l d

d i s t r i b u t i o n

a round a

l ong

s t r a i g h t w i r e i s shown

in

F i g u r e

2 . 3 ( a ) . In F i g u r e 2 . 3 ( b ) t h e B- f i e l d in t h e a x i a l

d i r e c t i o n of a s i n g l e t u r n i s shown as a f u n c t i o n of r a d i u s .

As more wind ings

a r e added ,

each

c a r r y i n g t h e

same c u r r e n t ,

t h e f l u x

d e n s i t y r a p i d l y i n c r e a s e s and

its

a s s o c i a t e d

d i s t r i b u t i o n is

a l t e r e d .

B

B

'

r

M a g n e t ~ c

~ e t d

l

~

(a ) S t r a i g h t

Wire

(b) S i n g l e

Turn

C o i l

~ e n t

towing ~ n t o page

F ig . 2

. 3 :

Magne t i c

F lux

D i s t r i b u t i o n

Flux d e n s i t y v a r e s l i n e a r l y with

e l e c t r i c c u r r e n t

in t h e

c o i l , i.e. i f c o i l c u r r e n t d o u b l e s , f l u x d e n s i t y doub le s

eve rywhere . The total

magne t ic

f l u x ,

0

,

c o n t a i n e d

w i t h i n

t h e loop i s t h e p roduc t

of

B

and

a r e a o t h e c o i l . The

u n i t

in the S I

sys tem

f o ~ m a g n e t i c f l u x i s t h e weber

Wb).

r


5/8

-8-

2 . 2 . 3 E q u a t i o n s

Govern ing

G e n e i a t i o n

of

Eddy C ur r e n t s

In

any

e l e c t r i c a l

c i r ~ u i t

c u r r e n t

f l ow

s

governed

by Ohm s

Law

and i s

equa l to t h e d r i v i n g (p r imary c i r c u i t ) vo l t a ge

d i v i de d by

pr imar y

c i r c u i t

impedance .

I

p

V / Z

p p

2 .1 )

The

eddy

c u r r e n t c o i l

i s

p a r t o f t he

pr imar y c i r c u i t .

The

e u r r e n t p a s s i n g t h r ou i h

t he

c o i l normal .ly v a r i e s

s i n u s o i d a l l y

w i t h

t ime and

i s

g i v e n by:

I I s i n ( w t )

p o

2 . 2

where

1

0

i s

t he

peak c u r r e n t

va lue

in

t he

c i r c u i t and w

(omega) i s

t he f r equency

i n r a d i a n s / s

(w

equa l s 2rrf when f

s f r equency in h e r t z ) .

From O e r s t e d ' s d i s c o v e r y , a magne t ic f l u x ~ p )

e x i s t s

around

a c o i l c a r r y i n g

c u r r e n t

( s e e

F igu r e

2 . 4 ) p r o p o r t i o n a l to t he

number

of t u r n s i n t he

c o i l (Np)

and

t he

c u r r e n t t i p )

~ p a: N I

p p

(

2 .3)

PROBE

(pt imary

c i r c u i t )

S MPLE

( .secondary

c i r c u i t )

Fig . 2 .4 :

Coi

Car ry ing

A l t e r n a t i n g

Curren t Ad jacen t

To a

T e s t

Sample


6/8

- 9 -

Farad ay s Law s t a t e s a v o l t ag e (Vs) i s c rea t ed

or

induced in

a reg ion o f

space

when the re i s a changing

magnet ic

f i e l d .

When

we apply

t h i s to

our c o i l ,

where

d P

d t

V

S

-

~

p d t

i s

t he ra t e

of

change in

9

with t ime.

p

( 2 . 4)

Since

c o i l cu r ren t v a r i e s s i n u s o i d a l l y with t i m e ~

t o t a l

magnet ic f l u x i n t h e c o i l a l s o v ar i e s s i n u s o i d a l l y ,

= P

s in (wt )

o

where i s

the magnet ic

f lux

cor responding to

1

0

The induced

vol t age as

d esc r ib ed

by e q u a t i o n

2.4

V

= -

N w P

cos (wt )

S p O

r e s u l t s i n

(

2 . 5)

which

a l s o

v ar i e s

p e r i o d i c a l l y with

t ime.

I f

we br ing

the

c o i l c l a s e

to

a t e s t

sample, Ohm s

Law

s t a t e s

t ha t

i f t h e re

i s a d r iv in g v o l t ag e (V

5

) and t he samp le s

impedance

i s

f i n i t e , c u r r e n t

w i l l f low,

I

S

V Z

S S

( 2 .

6)

where

. I

5

i s

cu r ren t

f lowing

th rough

the

sample ,

V

5

i s

induced v o l t ag e and Z

5

i s

the

sa m p l e s

impedance

o r o p p o s i t i o n to t he f low of

c u r r e n t .

These induced

cur ren t s

a re

known as

eddy

cur ren t s

because

of

t h e i r c i r c u l a t o ry p a th s . They, in

t u rn ,

g en e ra t e t h e i r own

magnet ic

f i e l d

accord ing

to

L en z s Law, which

opposes

the

pr imary f i e l d ,

a: -

I

S

( 2 7)

and

(

2 .

8 )

where E i s

the

equ i l ib r ium magnet ic

f l u x

s u r ~ o u n i n g

the

c o i l

in

the presence

of a t e s t sample .

The

flow

of

eddy cur ren t s r e s u l t s in r e s i s t i v e (Ohmic) l o s ses

and a decrease in magnet ic f l u x . This i s r e f l e c t e d as a

decrease in probe impedance. In equa t ion form ,

and

z a: tf

' E

V

=

ZI

p

( 2. 9 )

2 .10)


7/8

-10-

Equa t ion 2 .9 i n d i c a t e s

a

c o i l ' s impedance

i s a

fu n c t i o n of

t h e

magnet ic f i e l d s u r ro u n d i n g

it

and i n t u r n t he magne t i c

f i e l d

i s governed by i r lduced

c u r r e n t

in the spec imep

( eq u a t i o n s 2 .8

and 2 . 7 ) .

The

r e l a t i o n s between

probe

impedance

and

sample

p r o p e r t i e s

w i l l

be

d er i v ed

in

Chapte r

3 .

To

summarize ,

f l u x i s s e t up by p as s i n g

a l t e r n a t i n g

c u r r e n t

th rough t he t e s t c o i . When t h i s c o i l i s brought c lo s e to a

c o n d u c t i v e sample , eddy c u r r e n t s a re

induced.

In a d d i t i o n ,

the magne t i c f l u x a s s o c i a t ed w i t h t he eddy cur r en t s oppose t h e

coil s magnet ic

f l u x , t h e r e b y d ec r eas i n g n e t f l u x .

This

r e s u l t s in a change in c o i

impedance

and v o l t ag e drop . I t i s

t h e o p p o s i t i o n between t h e pr imary ( c o i ) and

secondary

eddy

c u r r e n t ) f i e l d s t h a t prov ides t h e b as i s fo r e x t r a c t i n g

i n f o r ma t i o n dur ing

eddy

c u r r e n t t e s t i n g .

I t

shou ld

be noted t h a t

if

a sample i s

f e r r o mag n e t i c ,

eq u a t i o n

2 .9 still

a p p l i e s

but

t h e

magne t i c

f l u x

i s

s t r en g t h en ed

d e s p i t e

oppos ing eddy c u r r e n t e f f e c t s . The

high

magne t i c p e rm eab i l i t y

o f f e r r o mag n e t i c

m a t e r i a l s

d i s t i n g u i s h e s them f rom

n o n - f e r ro m a g n e t i c

m a t e r i a l s and

s t r o n g l y

i n f l u e n c e s

eddy

c u r r e n t t e s t p ar ame t e r s .

Fer romagne t ic

sp ec i aen i n s p e c t i o n i s d i scu s sed in

Chap te r

and

u n l es s

s p e c i f i e d t b e r e s t

o f

t h e manual

i s

r e s t r i c t e d to

n o n - f e r ro ma g n e t i c

m a t e r i a l s .

2 .3 FUNDAMENTAL P ~ O P E R T I E S OF E Y CURRENT FLOW

Eddy

c u r r e n t s a re c l o sed

loops

o f induced cu r ren t c i r c u l a t i n g

in

p l an es

p e rp en d i cu l a r

to

the

a a g n e t i c

f l u x .

They

normal l

y

t r a v e l

p a r a l l e l to t h e c o i l ' s winding and

p a r a l l e l

to t he

s u r f a c e . Eddy c u r r e n t f low

is

l i m i t e d t ~ t he

a r ea

o f the

i nducing

magne t i c

f i e l d .

Tes t f r equency de te rmines dep th o f p e n e t r a t ~ o n

i n t o

t he

spec imen;

as f r equency

i s

i n c r e a s e ~

p e n e t r a t i o n

d ec r eases

and

the

eddy

cur r en t

d i s t r i b u t i o n

becomes

d en se r

nea r t h e

s p e c i m e n ' s s u r f a c e .

Tes t

f requency a l s o

a f f e c t s

t h e

s e n s i t i v i t y

to

changes in

m a t e r i a l

p r o p e r t i e s and

d e f e c t s .

Figure 2 . 5 ( a ) shows t h e a l g e b r a i c r e l a t i o n s h i p s and Fi g u re

2 .5 (b ) th e o s c i l l o s c o p e d i s p l ay

o f

eddy

c u r r e n t

and

magnet ic

f i e l d d i s t r i b u t i o n

with dep th

i n t o

t he specimen. Both t he

eddy

c u r r e n t s

and

magnet ic f l u x get v e a k e r v i t h dep th

because

of Mskin

e f f e c t .

In a d d i t i o n

to

t h i s a t t e n u a t i o n , t he eddy

c u r r e n t s l ag in phase

v i t h

d ep t h .

Eddy c u r r e n t s ' phase l ag

i s

t he

key p a ra me t e r t h a t makes

eddy

c u r r e n t

t e s t i n g a

u s e f u l

N T method. The paramete rs s k i n depth and

phase

l a g

a r e

d i s cu s sed in t he next s e c t i o n .


8/8

-11-

< >

Ol L

4>=

< / > i wtl

1=-ls x}sin wt/3)

~ < > < >

l h i

wt

l r-:..--- 7.L----......-

a)

( b)

Fig .

2 . 5 : Eddy

Curren t

and

Magnet ic

Flux D i s t r i b u t i o n

With

Depth

I n t o a Conductor

2 .4 SKIN EFFECT

Eddy

cur r en t s

induced

by a changing

magnet ic f i e l d concen t r a t e

near the

su r f ace

ad j acen t

to

t h e

e x c i t a t i o n

c o i l .

The

depth

of

pene t ra t i on

decreases with t e s t f requency

and

i s a

func t ion

of

e l e c t r i c a l

cond u c t i v i t y

and

magnet ic p e rm eab i l i t y o f t h e

specimen. This phenomenon

i s known as the sk in e f f e c t and i s

ana logous to

t he

s i t u a t i o n in t e r r e s t r i a l heat conduct ion where

da i ly

su r f ace t empera tu re f l u c t u a t i o n s are n o ~ apprec iab le ~ l o w

the e a r t h s

s u r f a c e .

Skin e f f e c t a r i s e s as fo l lows : t he

eddy

cur r en t s f lowing in the

t e s t

ob jec t a t any depth produce

magnet ic f i e l d s

which

oppose

the

pr imary

f i e l d , t hus r educing

net magnetic f lux and

caus ing a

decrease

in

c u r r e n t f low as

depth

i n c r e a s e s . A l t e r n a t i v e l y , eddy c u r r e n t s

n ea r t he su r f

ace

can

be viewed as s h i e l d i n g the co i

l s

magnet ic

f i e l d

t hereby

weakening the magnet ic f i e l d a t g r e a t e r dep ths and reducing

induced

c u r r e n t s .

The equa t ion fo r f low of induced cur r en t s i s

i/ 2J

=

OJJ cU

at

(2 .11)

where

J

i s

cu r re

n t

dens i ty ,

i s

c o n d u c t i v i t y ,

u

i s

magnet ic

p e rm eab i l i t y and

i s

a d i f f e r e n t i a l ope r a t or

of

second orde

r .

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Eddy Current Testing Parte 2