Eddy Current Testing Parte 1.2

7/27/2019 Eddy Current Testing Parte 1.2

1/4

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NOMEN

CLATU RE

QUANTITY

C r o s s - S e c t io n a l a r e a

Radius

Leng t h

T h i c k n e s s

Width

Diamete r

Magne t i c

f l u x

d e n s i t y

C a p a c i t a n c e

T e s t

f

r equency

Optimum

tube t e s t i n g f r equency

C h a r a c t e r i s t i c

o r L i m i t

f r equency

Res onan t

f r e q u e n c y

Magne t i c field

i n t e n s i t y

M a g n e t i z i n g

f o r c e )

C u r r e n t

C u r r e n t d e n s i t y

S e l f

I n d u c t a n c e

Number of

t u r n s

W

i n d i n g s )

C h a r a c t e r i s t i c

P a r a m e t e r

R e s i s t a n c e

R e s i s t i v e l o a d

E l e c t r i c

p o t e n t i a l

Depth be low t h e s u r f a c e

I n d u c t i v e

R e a c t a n c e

C a p a c i t i v e

Reac tance

mp ed

ance

S t a n d a r d

Depth

of

P e n e t r a t i o n

P e r m e a b i l i t y

R e s i s t i v i t y

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

Magne t i c f l u x

ill F a c t o r

Phase Lag

Angul a r

f r equency

Angle be t ween

Z R

. .;

2

met re

m e t r e

me t r e

metr e

met re

S I UN T

m e t r e

2

w

e b e r / m e t e r

fa

r ads

he r t z

he

rt

z

he

rt

z

o r

t e s l a

he r t z

a m p e r e s / m e t e r o r

l e n z e

amperes

2

amperes / mete r

henry

d i me n s i o n l e s s

d i me n s i o n l e s s

ohm

ohm

v o l t

met re

ohm

ohm

ohm

met re

henry /

me t e r

m i c r o h m c e n t i m e t r ~

s i emens / mete r

weber

d i m e n s i o n l e s s

r a d i a n s

r a d i a n s

/ second

degr ees


2/4

1 1

CHAPTER 1 - I NTRODUCTION

EDDY CURRENT

TESTING

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

ET)

is

a

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

based

on

i n d u c i n g

e l ~ c t r i c l

c u r r e n t s in

t h e

m a t e r i a

l

b e i n g

i n s p e c t e d and o b s e r v i n g t h e i n t e r a c t i o n

between

t h o s e

c u r r e n t s and t h e m a t e r i a l . Eddy c u r r e n t s a r e g e n e r a t e d by

e l e c t r o m a g n e t i c c o i l s

in th e test p r o b e , and

moni to r ed

s i m u l t a n e o u s l y by

measur ing

probe

electrical impedance .

S i n c e it s

an

e l e c t i o m a g n e t i c i n d u c t i o n p r o c e s s , d i r e c t

electrical c o n t a c t w i t h t h e

s ampl e

i s

not

r e qu i r e d ; however ,

t h e sample m a t e r i a l

has

to

be c o n d u c t i v e .

Eddy c u r r e n t

t e s t i n g is

a v e r s a t i l e t e c hn ique . It s main ly

used f o r

t h i n

m a t e r i a l s ; in t h i c k m a t e r i a l s ,

p e n e t r a t i o n

c o n s t r a i n t s l i m i t

t h e

i n s p e c t e d volume to

t h i n s u r f a c e

l a y e r s . In a d d i t i o n to f l aw

i n s p e c t i o n , ET can

be used

t o

i n d i r e c t l y

meas

ure

m e c h a n i c a l

and

m e t a l l u r g i c a l

c h a r a c t e r i s t i c s which c o r r e l a t e w i t h e l e c t r i c a l

and magne t ic

p r o p e r t i e s . Also , g e o me t r i c e f f e c t s such as t h i c k n e s s ,

c u r v a t u r e and

p r o b e - t o - m a t e r i a l s p a c i n g i n f l u e n c e eddy

c u r r e n t f low and can b e measured .

The l a r g e number o f p 9 t e n t i a l l y s i g n i f i c a n t v a r i a b l e s in ET

i s bo th

a

s t r e n g t h and

a

weakness o f ~ h e

t e c h n i q u e s i n c e

e f f e c t s

o f

o t h e r w i s e trivial

p a r a me t e r s

can

mask

impor t an t

i n f o r m a t i o n

o r

be m i s i n t e r p r e t e d .

V i r t u a l l y

e v e r y t h i n g

t h a t

a f f e c t s eddy

c u r r e n t f low o r o

t h e r w i

s e

i n f l ue nc e s probe

impedance

has

to

be

t aken i n t o

accoun t

t o o b t a i n r e l i a b l e

r e s u l t s .

Thus , c r e d i b l e

eddy

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

h i g h

level of operator

tra1n1ng and awareness.

1 . 2

PURPOSE

OF

THIS M NU L

The

purpose o f

t h i s

manual

is

to promote the deve l opmen t and

use

o f eddy c u r r e

n t

t e s t i n g

by

pr ov id ing

a s c i e n t i f i c a l l y

sound

t r a i n i n g

and

r e f e r e n c e

manual . The

s e l e c t i o n

of

m a t e r i a l

pr e s e n t e d

i s based on t h e

premise t h a t a sound

u n d e r s t a n d i n g of ba s i c

p r i n c i p i e s

i s e s s e n t i a l

t o

o b t a i n i n g

v a l i d

da t a and i n t e r p r e t i n g it c o r r e c t l y .

A

non

-

r i g o r o u s

approach has

been

used

to p r e s e n t

complex

p h y s i c a l

phenomena

in

a

document

o r i e n t e d t owards

a p p l i c a t i o n

o f eddy c u r r e n t

t e c h n i q u e s ,

e s p e c i a l l y

f o r

d e f e c

t

d e t e c t i o n

and

d i a g n o s i s .

The p r e s e n t a t i o n moves from t h e o r y ( i n c l u d i n g

a

rev iew of

b a s i c

e l e c t r i c a l

c o n c e p t s

)

t o t e s t

methods

an

d

s i g n a

a n a l y s i s .

S i m p l i f i e d d e r i v a t i o n s of pr obe r e sponse to test

p a r a me t e r s a r e p r e s e n t e d

to deve lop

a

b a s i c u n d e r s t a n d i n g of

eddy c u r r e n t t e s t p r i n c i p l e s . Thus , eddy c u r

r e n t

s i g n a l s a r e

. ~


3/4

1 .3

-2 -

c o n s i s t e n t l y i l l u s t r a t e d on impedance plane diagrams

( the

d i s p l ay used

in

modero eddy cu r ren t t e s t i n s t ru men t s ) and to

a i d ex p lan a t io n ,

t he

paramete r "eddy cu r ren t

phase ,

lag" i s

i n t roduced .

Since

probes

p lay a key r ole in

eddy

cu r ren t t e s t i n g ,

t e chn ica l

as p ec t s

of

probe

d es ig n a re

i n t r oduc e d

wi th

p e r t i n e n t e l e c t r i c a l impedan ce

ca l cu l a t i o n s .

While knowledge

o f b as i c e l e c t r i c a l c i r c u i t s i s requ i red fo r a complete

unde r s t a nd i ng

of eddy cu r ren t t e s t p r in c ip i e s , a good

t e ch n i ca l

base fo r

i n s p ec t i o n can still be obta ined if

s e c t i o n s

of t h i s

manual

r

eq u i r i n g

such a background a re

sk i ppe d .

From an ap p l i ca t i o n s p o i n t

of

view, the mat e r i a l in t h i s

manual prov ides an i n s p ec t o r wi th the necessa ry background to

decide :

1)

what

probe(s )

to use ,

2) what t e s t f requencies a re s u i t a b l e ,

3)

what ca l i b r a

t i o n d e fec t s or s tandards

a re

requ i red

for

s i g n a

c a l i b r a t i o n and / or s imu la t io n ,

4) what t e s t s a re req u i red

to

d i f f e r e n t i a t e between

s i g n i f i c a n t

s i g n a l s

and f a l s e

i n d i c a t i o n s .

5) how

to

es t ma te

depth

of r e a l

d e fec t s

.

To

supplement theory , p r a c t i c a

examples

a re

presen ted to

develop prof ic iency in per forming

i n s p ec t i o n s , and

to

i l l u s t r a t e how bas ic p r i n c i p i e s a re app l ied to d iagnose r ea l

s i g n a l s .

A

number

of

l ab o ra t o ry

demons t ra t ions ,

p r a c t i c a

t e s t s and

mu l t ip l e choice q u es t io n s a re i nc luded in Volume 2 , "Eddy

C

u r r e n t

Course Supplement" . They a re d iv ided

i n to

groups

co

r r esponding to th e ch ap te r s in t h i s manual . The

demonst ra t ions a re i n t ended fo r use in eddy cu r ren t cour ses

to he lp c la

r

i f y

some

o f

t he more

d i f f i c u l t

concep t s . The

p r a c t i c a t e s t s a re to give s t u d en t s prac t i ce in us ing

equipment

and per forming t y p i ca l t e s t s . The mu l t ip l e

choice

ques t ions a re i n t ended to check s tu d en t s unders t and ing

of

the course mat e r i a l and prepare fo r c e r t i f i c a t i o n exams.

H STORICAL

PERSPECTIVE

E l e c t r om a gne t i c t e s t i n g the i n t e r ac t i o n

of

magnet ic f i e ld s

wi th c i r c u l a t i n g e l e c t r i c a l cu r r e n t s had its o r i g i n in

1831 when M.

Faraday

di scovered e l ec t ro mag n e t i c i n d u c t io n .

He

induced cu r ren t

f low in a

secondary c o i l

by

swi tch ing

a

ba t t e ry

on and o f f , D.E. Hughes pe rformed

the

f i r s t re corded

eddy

cu r ren t

t e s t

in 1879 . e

was

ab le to

d i s t i n g u i s h

between d i f f e r e n t

metals

by no t i ng a change in ex c i t a t i o n

f requency

r e s u l t i n g from e f f ec t s of t e s t

mat e r i a l r e s i s t i v i

ty

and magnet ic p ermeab i l i t y .


4/4

-3 -

THER

MUST

E

DEFECTS

IN

THB SE TUBES SOMEWHERE

I

S W SQU

IGGLES ON

THE

EDDY

CURRENt SCREEN

/

F i g .

l . l: M i s i n t e r p r e t e d

S i g n a l s

Initially t he

ex t r eme

s e n s i t i v i t y

to

many m a t e r i a l

p r o p e r t i e s and c o n d i t i o n s made

ET

difficult and u n r e l i a b l e .

F i g u r e

1 . 1

illustrates

t h i s

p o i n t .

It t o o k

u n t i l

926 b e f o r e

t h e first eddy c u r r e n t i n s t r u m e n t

was

d e v e lo p e d

to measure

samp le

t h i c k n e s s .

By t he

end

of ~ o r l War

II

f u r t h e r

r e s e a r c h and improved

e l e c t r o n i c s

made i n d u s t r i a l

i n s p e c t i o n

p o s s i b l e ,

and many

p r a c t i c a

i n s t ru me n t s

were d e v e l o p e d .

A

majo r

b re a k th ro u g h

came

in t h e

1 9 5 0 ' s

when F o r s t e r

d e v e lo p e d

i n s t ru me n t s

wi th

impedance

p l a n e s i g n a l d i s p l a y s . These made

it

p o s s i b l e

to

d i s c r i m i n a t e

be t w een d i f f e r e n t

p a ra me t e r s ,

though t h e p r o c e d u r e

was

still e m p i r i c a l .

D ur t ng

t h e

1 9 6 0 ' s

p r o g r e s s in t h e o r e t i c a l and p r a c t i c a u se s

of

edd

y

c u r r e n t

t e s t i n g advanced t h e t e c h n o l o g y f rom an e m p i r i c a l a r t

t o

an

a c c e p t e d e n g i n e e r i n g d i s c i p l i n e .

~

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Eddy Current Testing Parte 1.2