Programmed Instruction Handbook - Eddy Current

8/16/2019 Programmed Instruction Handbook - Eddy Current

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Pl 4 5

SECOND EDITION

P R O G R A M M E D

I N S T R U C T I O N

H A N D B O O K

NONDESTRU TIVE TESTING

eddy current

m•N•RAL DVNAMICll

Com air Division


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Pl 4 5

SECOND EDITION

P R O G R A M M E D I N S T R U C T I O N H A N D B O O K

NONDESTRU TIVE TESTING

eddy current

Copyright

@

1980

GENERAL DYNAMICS

onvair Division


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first printing 1983

second printing 02/88

third printing 09/89

fourth printing 02/91

fifth printing 03/94

sixth printing 03/97

seventh printing 02/01

eighth printing 04/06


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T BL E OF C ON T EN T S

Preface v

Acknowledgements

v1

Introduction . . vii

Instructions

. . vm

Chapter - Electromagnetic Induction.

1 1

Faraday's Experiment.

1 2

Definition of Variable 1 5

Coil to CoilInduction .

1 6

Magnet

to Coil

Induction 1 8

The Sine Wave 1 19

Review 1 22

Induction

with

Alternating Current

. 1 27

Inductive Reactance . 1 32

Ohm's Law 1 35

Resistance 1 37

Impedance

.

1 38

Vector Addition

1 42

Effect of Frequency on Inductive Reactance 1 49

Review

.

1 54

Chapter 2

-

Principles of Eddy Current Testing

2 1

Induction of Eddy Currents 2 3

Effect of Conductivity on Eddy Currents 2 7

Effect of Coil's Magnetic Field on

Eddy Currents

2 11

Lift-off 2 14

Effect

of

Material

Thickness on

Eddy Currents 2 16

Effect

of Magnetic Permeability on

Eddy Currents 2 19

Magnetic

Saturation 2 23

Review 2 26

International Annealed Copper Standard for Conductivity 2 31

1


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Factors

Affecting Conductivity

. 2 34

Effect of Conductive Coatings

.

2 38

Dimensional

Factors

. 2 48

Discontinuities. 2 51

Edge Effect 2 54

Review

. .

2 58

Chapter 3 - Eddy Current Test Circuits 3 1

SimpleTest Circuit 3 1

Elements of a Test Circuit

3 7

Basic Bridge Circuit 3-9

Bridge with Reference Coil

3 18

Induction Bridge 3 22

Through Transmission

System.

3 23

Reflection System

. . 3 23

Review 3 24

Inspection Coils

.

3 30

Surface Probes

. . .

3 30

Encircling Coils

.

3 31

Internal Coils . 3-36

Multiple CoilArrangements 3 37

Two-CoilArrangements

3 37

Absolute and Differential Arrangements 3 40

Four-CoilArrangements 3 43

Review

. .

3 48

Chapter

4 - Geometry of

Eddy Currents

4 1

Eddy Current Orientation 4 1

Coil Size and Shape . . . 4 9

Depth of Penetration

. . . .

4 11

Effect of Conductivity on Depth of Penetration . 4 12

Effect of Permeability on Depth of Penetration 4 14

Effect of Frequency on Depth of Penetration

4 15

Edge

Effect 4 21

Review . . . . . . 4 27

ii


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CKNOWLEDGMENTS

This handbook was originally prepared by the Convair Division of General

Dynamics Corporation under a joint arrangement with NASA's George C.

Marshall Space Flight Center. Convair's activities in the preparation of

nondestructive testing training materials were greatly enhanced and accel

erated by the MSFC technical and financial participation. Quality and

Reliability Assurance Laboratory personnel at NASA's MSFC were to a large

degree responsible for the successful completion of that program. Their

understanding of the problems involved in teaching difficult subject matter,

their realistic handling of NASA agency reviews, and their efficient transmit

tal of reviewer comments, made the publisher's task simpler than it might

have been. Convair considers itself fortunate to have been associated with

NASA on that project.

Additional assistance in the form of process data, technical reviews, and

technical advice was provided by a great many companies and individuals.

The following listing is an attempt to acknowledge this assistance and to

express our gratitude for the high degree of interest exhibited by the firms,

their representatives, and other individuals, many of whom gave considerable

time and effort to the project.

Aerojet-General Corp.; Automatiion Industries, Inc., Sperry Products Divi

sion; Avco Corporation; The Boeing Company; Dr. Foerster Institute; General

Electric Co.; Grumman Aerospace Corp.; Mr. H.L. Libby; Lockheed Aircraft

Corp.; Magnaflux Corp.; Magnetic Analysis Corporation; Martin Marietta

Aerospace, Denver Division; McDonnell Douglas Corp.; Rockwell Interna

tional North American Aerospace Group; Rohr Industries, Inc.; Southwest

Research Institute; St. Louis Testing Laboratories, Inc.

Vl


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INTRODU TION

During

the past

two decades eddy

current testing

has developed into one of

the important nondestructive testing

tools and

its

use is still growing. Inspec

tion with eddy

currents

is used to identify or differentiate between a wide

variety

of physical,

structural,

and metallurgical conditions in electrically con

ductive material.

In this handbook you will learn what eddy currents are, how they are introduced

into an article being inspected, and how they are affected by the physical, struc

tural, and metallurgical conditions in the material. You will also learn how these

effects are sensed and interpreted.

When you have completed

this

handbook you should be ready for

practical

demonstration

sessions

and

on-the-job

training that

will eventually qualify

you as an eddy current test technician.

Do not

rush through the

book. Take whatever time you need to

get the most

from the

material

presented. Depending on your background knowledge,

reading speed, etc., the reading time it takes to complete this book may vary

from 4 hours to 12 hours or more.

At the back of the book is a set of self-test questions that will help you in

evaluating

your newly-gained knowledge. Also included is a glossary of

terms

relating

to eddy

current testing.

vii


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INSTRUCTIONS

The

pages

in

this

book should

not

be read consecutively as in a conventional

book. You will be guided through the book as you read. For example, after

reading page 3-12, you may find an instruction similar to one of the following

at the bottom of the page -

• Turn to the next page

• Turn to page 3-15

•

Return

to page 3-10

On many pages you will be faced with a choice. For instance, you may find a

statement or question at the bottom of the page together with two or more

possible answers.

Each

answer will indicate a page number. You should

choose the answer you think is correct and turn to the indicated page. That

page will contain further instructions.

As you progress through the book, ignore the

back

of each page. THEY ARE

PRINTED UPSIDE DOWN. You will be instructed when to turn the book

around and read the upside-down pages.

As you will soon see, it's very simple - just followinstructions.

Turn to the next page.

viii


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1-1

CHAPTER

ELECTROMAGNETIC INDUCTION

Eddy current testing is based on the principles of electromagnetic induction.

"Electromagnetic Induction" -

two very scientific sounding words

that

are

used to identify a principle

that

allows you to use electricity

that

has been

generated hundreds of miles away; a principle upon which

the actual

genera

tion of the electriccurrent is based; a principle that causes your electric motor

to operate; and now a principle upon which a broad field of

nondestructive

testing

is based.

The word "electromagnetic" simply means that electricity and magnetism are

used.

"Induction"

is a form of the word "induce" which means

"to

bring

about"

or

"cause." In

fact,

the

flow of electricity, under

certain

cir

cumstances, can cause magnetism; and magnetism, under certain cir

cumstances, can cause

the

flow of electricity.

Now, if you already have a firm knowledge of

the

principles of electromagnetic

induction,

turn

to page 2-1.

If

your knowledge of electromagnetic induction is not

at

all

that

certain,

turn

to page 1-2.


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From page 1-1

1-2

OK,

let's take

a look

at

"electromagnetic induction."

By

the year

1820

scientists

had discovered

that

when

current

from a

battery

was sent through a coil of wire that a magnetic field was set up in the coil. The

magnetic field was

present

only

during the

time

the current

flowed

through

the

coil. They had discovered how to use electricity to make magnetism and

they thought that somehow magnetism could be used to make electricity.

Some 12 years later, in 1832, a man named Faraday was experimenting with

some coils of wire and a

battery.

He noticed

that

when he connected one coil to

the battery he got an electrical current through a second coil, placed near the

first, for just an instant. He also found that when he disconnected the battery

that he got an electrical current through the second coil for just an instant;

but, he noticed, the second current was in the opposite direction of the first

current.

He knew that somehow the two coils were affecting each other. The first coil

was inducinga current in the second coil,but only when he turned the battery

on and off. He reasoned

that the

magnetic field could be

the

coupling between

two coils.

But

since

the currents

occured only when

the battery

was

turned

on

and

off,

it

could only be

the change

in

the

magnetic field

that

caused

the

cur

rent

to flow in

the

second coil.

Electromagnetic induction is

the

name given to

the ..•

effect onecurrent carrying coil has on another

. . . . . . . . . . . . .

Page

1-3

coupling of two coils by a changing magnetic field . . . . . . . . . . . . Page 1-4

principle

that

a changing magnetic field will induce

an

electrical

current

in a coil

. . . . . . . . . . . . . . . . . . . . . . .

Page 1-5


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From page 1-2

1-3

We think you may have been a little

hasty

in selecting this answer. The state

ment Electromagnetic induction is the name given to the effect one current

carrying coil has on another (your selection) is

true

in

its

fashion

but it

is far

from being complete.

You will recall

that

electromagnetic means

that

electricity

and

magnetism

are

involved. The answer you selected mentioned only electricity (i.e., current).

Return to page

1-2

and see if there

isn't

a

better

answer.


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From page

1-2

1-4

We think you may have been a little hasty in selecting this answer. The state

ment Electromagnetic induction is the name given to the coupling of two

coils by a changing magnetic filed (your answer) is true in its fashion but it is

far from being complete.

You will recall

that

electromagnetic means

that

electricity

and

magnetism

are

involved. The answer you selected mentioned only magnetism (i.e., changing

magnetic field).

Return to page 1-2 and see if there isn't a better answer.


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From page

1-2

1-5

Excellent Of

the three

choices given,

this

was

the best

one to describe electro

magnetic induction.

Here's what Faraday's

experiment looked like.

SECONDARY

COIL

BATTERY

PRIMARY

COIL

AMMETER

The next logicalstep was to make different changes in the set up and seewhat

effect they had. For example:

Change

the

number of

turns

in the

primary

coil.

Change the physical size of the primary coil.

Change

the amount

of

current

in the

primary

coil.

Change

the

number of

turns

in

the

secondary coil.

Change the physical size of the secondary coil.

Change the spacing between the coils.

All of

these things

can be changed so

they

are called

"variables".

We won't,

at this

point, go into

the

effect

that

each of

these

variables had on

the amount

of

current that

was induced in

the

secondary coil.

It

is enough to

say that each and all of these variables had an effect on the current induced in

the

secondary coil.

It

changed

-

in one way or another.

In your best judgment,

is

the

following

statement true? ...

or false?

The

current

induced in

the

secondary coil is a variable.

True . . . . . . . . . . . . . . . . . . . . . . . . Page

1-6

False Page

1-7


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From page

1-5 1-6

Right The current in the secondary coil is a variable. Variables are anything

that

can be changed or

that

are changed as a

result

of

other

changes.

Now

that

we have

established what

a variable is,

let's get

on with electromag

netic induction. We have described how Faraday was able to produce an elec

trical current in a secondary coil by changing the magnetic fieldsurrounding a

primary coil.Faraday reasoned that the current was produced by the change

in

the

magnetic field and

not

by

the

simple presence of

the

field.

In other

words, so long as the magnetic field in the primary winding did not vary (was

held constant) no electrical current was induced in the secondary coil. Thus,

utilizing the theory of a magnetic field, current was induced only when the

lines of force of the magnetic field movedpast the coil. Here is an illustration

of that theory.

BUILDING FIELD

COLLAPSING FIELD

, ; , = . - - - ,

{,. \\

r.

1 1

11

11

I) J I

&~)'

\ LINES

OF FORCE

Now, if

this

were true, as

it

appeared to be,

then it

should be possible to induce

a current by moving a coil through a magnetic field.

Do you agree?

Yes Page 1-8

No Page

1-9


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From page

1-5

1-7

You believe that the statement "the current in the secondary coil is a

variable" is false. We're sorry but the statement is true.

Were going to assume that you know that "variable" means "subject to

change."

Since the amount of current in the secondary coil can be changed by varying

any one of several factors (number of turns, distance between coils, etc.), the

current is then, itself, a variable.

In fact there are relatively few constants in this world.

A "constant" is something that never changes. It is the opposite of

"variable."

Now

turn

to page

1-6

and continue.


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From page

1-6

1-9

You do not agree with the statement "If it is true that a current is induced

when a magnetic field moves past a coil, then it should be possible to induce a

current by moving a coil through a magnetic field.

You should have agreed

Look

at

it this way. The· induction of the electric current into a coil is due to

the relative motion between the magnetic field and the coil. It makes no dif

ference whether the magnetic field is expanding and contracting

past

the coil

or whether the coil is moving through the magnetic field. The

relative

motion

is the same. Thus, a current is induced in the coil in either case.

If you think about it, we're sure you'll agree.

Turn back to page 1-8.


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From page 1-8 1-10

You supposed correctly The direction of

the current

will change when

the

direction of

the

movement of

the

wire is changed. There is a rule for determin

ing

the

direction of

the current

induced in

the

wire

but

you will

not

have any

need for it in Eddy Current testing so we will not bother to learn it. It is suffi

cient that you know that there are three ways to cause the current to change

direction in

the

wire.

First,

you could change

the

direction of

the

magnetic

field (difficult to do in a permanent magnet); second, you could change the

direction that the wire is moving through the field; or third, you could swap

ends with the wire (which is exactly what happens when a coil is rotated in a

magnetic field).

Let's

bend

the

wire

into

a

"U"

shape,

insert it

into

the

magnetic field,

and

rotate it

around

the

axis as shown so

that the

segment of wire

A-Bis

coming

down through the field while segment C-D is coming up through the field.

Then which of the following statements is true?

The current flowing through segment D-C is subtracted

from the

current

flowing

through

Segment A-B Page 1-12

The

current

flowing

through

Segment D-C is added

to the current flowing through A-B Page 1-13


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From page

1-8

1-11

Your supposition was incorrect Changing the direction of movement of the

wire in the magnetic field does cause the current to change direction in the

wire.

Remember the building and collapsing magnetic fields? The current induced

went in one direction when the field was building and in the opposite direction

when the field was collapsing. We could expect the same effect if we passed

the wire in one direction through the field and then in the other direction

-

the current would change

its

direction through the wire.

Turn to page

1- 10

and continue.


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From page 1-10 1·12

You selected the wrong answer, but getting the right answer requires some

detailed thinking. Let's look at the setup once more.

Do you

understand

how

the

U-shaped wire is

rotating?

Segment A·B is mov

ing down through the field while segment C-D is moving up through the field.

We have just learned that the current in

these

segments will have to

travel

in

opposite directions. So let's assume that the current in the top segment is

going from A to B; then the current in the bottom segment is going in the

opposite direction, or, as shown on the diagram, from

C

to D.

Now, since

the

two

segments

are joined

at

one end by wire segment B-C,

the

current path

is A to B to C to D. See it?

So the two currents would be aiding each other and therefore would be added

together.

Now

turn

to page 1-13 and continue.


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From page 1-10

1- 13

That's

absolutely right The current through the segment C-D is added to the

current through A-B so

that

we have current flow now from A to D (in

that

direction).

Now,

let's

keep

rotating

the wire until the segment D-C is coming down

through the field and segment A-Bis moving up. The current in segment D-C

is flowing from D towards

C .

In

what direction is the current flowing in segment A-B?

From A towards B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 1-14

From B towards A

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

1 - 1 5


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From page 1-13

1-14

Be careful Remember

-

in the

setup

we've shown,

the

direction of

the

cur

rent in the segment depends on the direction that the segment is traveling.

We've told you

that

segment D-C is moving downward

through the

field and

that the current

is flowing from D to

C .

If

you

understood

the way

the

U-shaped wire is

rotating

around

the

axis,

it

must

follow

that segment A-Bis

moving upwards

through the

field and the

current is flowing from

B

to

A (in

the opposite direction as it was in segment

D-C).

Now

turn

to page 1-15.


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From page 1-13

1-15

True The current in that segment is flowing from B towards A. And since the

current in the other segment was flowing from D to C, we now have a current

in the wire from D to A.

Now let's compare the two situations.

In the first instance the current flowed through the wire from A to D; and in

the

second

instance

the

current

flowed

through the

wire from D to A. Imagine

that the wire is wrapped to form several loops and then spun on its axis in the

magnetic field. Can you now see

that the current through the

loop will change

directions at every half-turn that the coil makes as it rotates? If not, study the

diagrams again to see if you missed anything.

Now that you understand why the current

changes direction, we have to see

when. But

first,

let's

figure

out

how often

it

changes direction. (There's a clue

in the preceding paragraph.)

The

current

changes direction

after

180° of

rotation

of

the coil Page 1-16

The current changes direction after 360° of rotation of

the coil . . . . . . . . . . . . . . . . . . . . . . . . . . Page 1-17


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From page 6-28 6-29

You have just completed the programmed instruction course on Eddy Current

Testing.

Now you may want to

evaluate

your knowledge of

the material presented

in

this handbook. A set of self-test questions are included at the back of the

book. The answers can be found at the end of the test.

We

want to

emphasize

that the test

is for your

own evaluation of

your

knowledge of

the subject. If

you elect to

take

the

test,

be

honest

with yourself

- don't refer to the answers until you have finished.Then you will have a

meaningful measure of your knowledge.

Since

it

is a self evaluation,

there

is no

passing

score.

If

you find

that

you have

trouble in some

part

of

the test,

it is up to you to review the material

until

you

are

satisfied that

you know it.

Turn or rotate the book 180° and flip to page A-1 at the back.


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From page 1-15 1-16

Your choice was excellent. The current does change direction after 180

of coil

rotation. Now let's see exactly when the change occurs.

Here we show a coil in a position where the

plane

of the coil is across the

magnetic lines of force. Notice

that,

as

the

coil moves, the top windings and

the bottom windings are moving in a direction that is

parallel

to the lines of

force. Since

the

direction of movement is parallel

to

the lines of force,no lines

of force are being crossed - therefore no current is being induced in

the

coil.

In this view, the coil has rotated 90°. It now lies parallel to the lines of force

but the

movement of

the

coil sides is perpendicular

to the

lines of force.

At

this point, as the coil turns, it is passing through (or crossing) the maximum

number of magnetic lines of force.

You would expect then that the current induced in the coil as it passes

through this point would be at a ...

minimum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 1-18

maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 1-19


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6-28

4. frequency

5 . Once an eddy

current instrument

is calibrated,

the

controls are

not to

be

touched during ensuing tests. (True - False)

9.

data

•

Return to

page 6-24,

frame 6.

10. Reference standards often define the of acceptability of an

item

under test.

•

Return to page 6-24,

frame 11.

14. natural,

artificial

15. A fatigue crackthat has been induced by cyclicstresses in a laboratory is an

example of a (developed,accumulated) discontinuity

19. nonconductive

•


frame 16.

Turn to page 6-29.

'


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From page 1-15

1-17

You think that the current changes direction in the coil after 360

°

rotation of

the coil. In a way you are right - but not completely. The current does change

direction after 360° of travel but it also changes direction after only 180° of

travel.

Let's analyze the situation.

In view A the current flows from A to D while in view B the current flows from

D to A. The current has reversed direction, right?

Now

- the

U-shaped wire has been

rotated

112

turn about the axis in getting

from the position shown in view A to the position shown in view B. Since

112

turn about the

axis is equal to 180° of

rotation, it

follows

that the current

changes direction every 180 of rotation of the U-shaped wire.

A coil may be thought of as several of these U-shaped wires all connected

together - each turn acting in the same manner. So we can say that the cur

rent

in a coil

that

is

rotating

in a magnetic field reverses direction every 180°

of rotation of the coil.

Now,

turn

back to page 1-16 and continue.


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6-27

3. depth of penetration

4. Depth of penetration is controlled in an eddy current instrument by con-

trolling its operating _

•

8. reference

standard

9. In eddy current testing, the most useful test data is obtained by compar-

ing the data from an item under test with the-~· obtained from a

reference standard.

•

13. test

14. The two types of discontinuitystandards are n and a

-----

•

18. artificial

19. Lift-off

standards

are made from material.

•


47/375

From page 1-16

1-18

You expect the current induced in the coil to be at a minimum when the coil is

parallel to the magnetic lines of force. You have missed a very important concept.

Current is induced in a coil only when the coil is cutting across

the

magnetic

lines of force. By

"cutting across"

we mean

that

the

motion

of the coil is such

that the wires in the coil pass through the magnetic field in some direction

that

is not

parallel

to the lines of force. The more lines

that

are being

cut

in a

given period of time, the more current induced.

Lookat the illustration again. Can you see that as the coilrotates through this

position

it

is

cutting

across

the

maximum number of lines of force? Then,

since

the

maximum number of lines of force are being cut, the

current

induced

is

at

a maximum.

Turn

to page 1-19.


48/375

6-26

•

2 . standards

3. Even though the presence of a crack will effect the reading on a conduc

tivity tester, the tester cannot

be used as a crack

detector

because

the

d of p cannot be controlled.

•

7. lift-off

8. In using any eddy current instrument the instrument must be calibrated

with a before conducting any tests

•

12.

IACS

13. An ideal discontinuity standard duplicates the situation as closely

as possible.

•

17. acceptability

18. Discontinuities which are machined into an article which has no

natural

discontinuities are called discontinuities.


49/375

From page 1-16

1-19

It's

maximum

-

You're

right

on

the button. At the

moment

the plane of the

coil is parallel to the lines of force the coil is crossing the maximum number of

magnetic lines of force - therefore, the current induced in the coil is at the

maximum.

Now,

let's

draw a

graph

so

that

we can visualize

what

is happening to

the

cur

rent as the coil rotates.

®

~ XE

: - r r

~ I I

~ I I

u I

--'

MAX- 0°

0

270

I

I

I

____ I _

I

90°

360°

I

I

I

- - - - - , - - - - -

I

900

360°70°

80°

COi L POSITION

The instant the coil is at position A

(0

°) the current is zero; the instant the coil

is at position B (90 of rotation) the current is maximum in one direction; at

position

C

(180° of rotation)

the current

is zero;

at

position D (270° of rotation)

the current is maximum in the opposite direction; and at position E (360 of

rotation) the current is back at zero. The curve that results from this plotting

of

current

values

against

coil position is called a

sine wave.

Now, looking at the sine wave, you can see that the current reaches its max

imum value in

either

direction

at

coil positions of

...

0° and 180° Page 1-20

90° and 270° Page 1- 2 1


50/375

6-25

1 . False

2. Calibration of a conductivity tester consists of setting the scale to read the

values stamped onboth the high and lowconductivity --------

•

. frequency

(depth of penetration)

7. By

varying the

frequency of

operation

of a crack

detector

we can

suppress

the effect of------

•

11.

sensitivity

12. Commercially

prepared conductivity standards have

a value

stamped

on

them. This value represents the conductivity of the block in %

-----

•

16. accumulated

1 7. A reference standard should have at least one discontinuity that is at the

limit of a

- - - - - - - - - ~

•


51/375

From page 1-19

1-20

It's

obvious

that

you

didn't understand the

graph.

~ X E

: - r r

~

I

I

w

~ 1 0

I

u I

I

0

MAX-0

90°

270°

I

I

_____ I _

I

I

360°

I

I

I

- - - - - , - - - - -

I

I

9 0 0

0

180

COi L POSITION

270°

360°

Let's take

a closer look

at the graph

of

the

sine wave. Notice

that

the zero posi

tion of

the

current is in the middle of the graph. The

upper

portion (above

the

zero line) gives values of

current

in one direction while

the

lower

portion

(below

the

zero line) gives

current

values in

the

opposite direction.

Thus,

the graph

shows

that the

maximum

current in one

direction occurs

at

90° of coil rotation and the maximum current in the other direction occurs at

270 of coil rotation. Do you see it?

Now turn to page 1-21.


52/375

6-24

From page 6-22

1 . The

meter

readings on a

conductivity tester

are

not

affected by

the

presence of discontinuities. (True

-

False)

•

5. True

6. Before any instrument can be used to detect discontinuities it must have

some means of controlling the

- - - - - - - - -

•

10. limits

11. A reference

standard

may also be used to make sure

that the test

equip-

ment provides consistant sen each time the equipment

is used.

•

15. developed

16. A sample which contains a discontinuity that developed during the

manufacturing process is an example of an _

referencestandard.

•


53/375

From page 1-19

1-21

Your eyesight is excellent. The current is at its maximum value in one direc

tion at 90° and at its maximum value in the other direction at 270°.

MAX

.r

~

I

I

~ o l

I

u I

--'

0

MAX- 0

I

I

I _

-----1

~

/1

I

- - - - - - - - - -

I

I

000

180°

COi L POSITION

270°

360°

Now, here is a concept of utmost importance. On our graph we have labeled

the

horizontal axis in degrees (0°, 90°, 180°, 270°, 360°) which refer to the

position of

the

coil. These could

just

as well have been

units

of time.

In

fact,

they

are

units

of time

-

90 being a

measurement

of

the amount

of time

it

took the coil to

travel

from 0° to 90°.

You will, as we progress,

run

into such

statements

as

"the

voltage lags behind

the current in time by 15°." It is by virtue of the relationship between the

rotating

coil and time elapsed

that

we can measure time in degrees.

We have, essentially, already measured time in degrees when we said

that

maximum

current

is

obtained at

90°.

It

is easier

to

work in degrees of

rotation

when explaining electromagnetic induction than it is to work in units of time

(seconds, milliseconds, etc.).

Now

turn

to page 1-22 for a review.


54/375

From page 6-21

6-23

You feel

that the

lift-off

standard

has to be

constructed

from

the

same

material as was used for the coating of the article. This is incorrect.

You must understand that to an eddy current probe one nonconductive

material looks like any other nonconductive material. So it makes no dif

ference what kind of material we use to construct a lift-off standard so long as

the

material

is nonconductive.

The firm

requirement

for a lift-off

standard

is

that the thickness

of

the

non

conductive

material

be known.

It

is

the thickness that

determines the

measurement

of

the amount

of lift-off.

Now turn back to page 6-22 and continue.


55/375

1-22

From page 1-21

1 . The

next

few

pages are different

from

the

ones

that

you

have

been

reading.

There are arrows on this page. (Write in the correct number of

arrows.)

Do not

read the frames

below. FOLLOW

THE

ARROW

and turn

to the TOP

of

the next

page.

There

you will find

the correct

word for

the

blank line above. •

4. changing

5. Current is induced in a coil rotating in a magnetic field by the principle of

el

.:.:in"'-------

MAX+~

-- I

f f i

T

I

~i~

I

I I

MAX - o o o o o

0 90 180 270 360

8. magnetic field

9.

The current induced

in a coil

that

is

rotating

in a

magnetic

field

travels

first in one direction and then in the other d as the coil

•

otates through 360°.

12.0, 180, 360

13. The current induced in a coil rotating in a magnetic field is maximum when

the

coil is

passing through the maximum number

of

magnetic

lines of

force. Maximum current, then, is induced at coil positions of __

0

and

0


56/375

From page 6-21

6-22

You are so right. The firm requirement for constructing lift-off standards is

that the thickness of those standards be known.

Layers of paper, mylar, or cellophane may be built up to the required

thickness for the standard.

As we have stated, reference standards are used to correlate the reading on

the test set to the conditions that we know exist in the reference standard.

Standards are also used in another way. If, after the equipment has been

calibrated to the standard, the electrical characteristics of the test set have

been inadvertantly changed, the test results will not be accurate. For this

reason it is wise to recheck the equipment against the reference standard

whenever an

unexpected result

is

obtained

in order

to

be

assured that the

cause of the unexpected reading is not due to a fault in the equipment.

During long, production runs it is wise to periodically recheck the instrument

against the reference standards to make sure that the electrical

characteristics of the test set have not "drifted", thus causing erroneous test

results.

Now turn to page 6-24 for a short review.


57/375

1 - 2 3

1 3.

90 270

This is

the

answer to

the

blank in Frame number 1 .

1.

six

~ 2

is next.

2~es will provide a review of the material you have covered to this

point. There will be one or more blanks in each

f

----


Follow the arrow.

5 . electromagnetic

induction

6. When we say

that

the spacing between coils is a

"variable"

we mean

that

the spacing between coils is subject to =ch=-----

14. Time may be measured in degrees. In the diagram of the sine wave the 90°

point represents the time it takes for the coil to rotate __

•

•

9. direction

10. The current induced in a coil rotating in a magnetic field is zero when the

plane of the coil is across the m f

----

MAX+~

-

I

i E T I

~;~

I

I I

MAX- o o o o o

0 90 180 270 360


58/375

From page 6-19

6-21

Good choice. The reference

standard

should have

discontinuities that

repre

sent the

limit of acceptability.

There is a

test situation,

however, where we need a

standard that represents

perfection.

When

external

comparison techniques are being used,

the standard under the

reference coil should

represent

perfection for

that

article.

It must

be free of

discontinuities. However, even then, the limits of acceptability must be

established

by placing a reference

standard containing the

required discon

tinuity

under

the test

coil to

obtain

a reading

that represents the

limit of

acceptability.

LIFT OFF

S T

AND ARDS

Since lift-offamounts to having a nonconductive space between the test coil

and the article, lift-off standards are easy to construct. The application of a

known

thickness

of any nonconductive

material

to a sample of the

material

under test willconstitute a lift-offstandard. Paper, mylar, and cellophane are

examples of nonconductive

materials

often used.

If

we are

measuring the thickness

of a nonconductive

coating

over a conduc

tive article, we need to construct lift-off standards that represent both the

maximum and the minimum acceptable thicknesses.

The firm requirement for the lift-offstandards we construct is that ...

the thicknesses be known . . . . . . . . . . . Page 6-22

the material be the same as the coating Page 6-23


59/375

1-24

2.

frame

3.

By following the arrows or instructions you will be directed to the frame

that follows in sequence. Each frame presents information and requires

the

filling in of

_

6. change

7. Anything that is subject to change is called a " "

10. magnetic field

11. When the rotating coil reaches a position so that the plane of the coil is

E . to the magnetic lines of force, the current induced is at the

maximum value.

14. 90

15. 0°, 90°, 180°, 270°, and 360° are all

measurements

of when

considering a coil rotating in a magnetic field.


60/375

From page 6-19

6-20

You felt

that the fabricated discontinuity

should be

greater than the

limit of

acceptability. This is incorrect.

You must keep in mind that the reference standard is most often used to

establish limits of acceptability so that we can record its effect on the test set.

Once the reading of the acceptability limit is taken and recorded, any reading

taken

on

the test

items

that

exceeds

this reading

is cause for

further

investigation.

Now

turn

to page 6-21.


61/375

1-25

3.

blanks (spaces, words)

4.

Electromagnetic induction is the principle by which a ch

--------

magnetic field will induce a current in a coil.


frame 5

7 .

variable

8. An electric

current

may be induced in a coil in two ways

-

1)

when a magnetic field moves past the coil, and

2) when a coil is moved through a _

.• Return to page 1-22,

..,. frame

9

MAX+

11. parallel I ~

t

~ 0

a t

M A X - a a

a a

a

0 90 180 270 360

12. The current output from a coilrotating in a magnetic field is in the form of

a

sine

wave. The sine wave shows that the current is at a maximum at 90

and 270°; and

at

a minimumat ,

,

an.d

•


frame 13

15. time



62/375

From page 6-16 6-19

You are

right.

The reading on

the instrument that

we

obtain

when

the test

coil

is placed over

the

crack

that

is

at the

limit of

acceptability

is

the highest

reading that

we can

get

and still accept

the

article we are

testing.

ARTIFICIAL DISCONTINUITY STANDARDS

Artificial discontinuity standards are standards that are prepared in the shop

by machining artificial

discontinuities

into an article

that

has no

natural

discontinuities. Several samples may have to be run through the inspection

system to find one that does not produce any appreciable indications of

natural discontinuities.

Once such a sample is located, standard referencediscontinuities that are per

tinent to the required specification are then fabricated into the sample. Types

of standard referencediscontinuities used to simulate natural discontinuities

are longitudinal notches, circumferential notches, drilled holes, file cuts,

pits,

diameter steps, and indentations.

The discontinuities fabricated into the sample should represent a natural

discontinuity that is ...

greater than the

limit of

acceptability

. . . . . . . . . . . . . . . . . . . Page 6-20

at the

limit of

acceptability.

. . . . . . . . . . .

.

. Page 6-21


63/375


64/375


65/375

From page 1-26

1-27

Now that we know what happens when a coil is rotating in a magnetic field,

let's go back and look at electromagnetic induction between two coils; but

instead

of using a

battery,

we will supply

the primary

coil with a source of

alternating current.

AC

SOURCE

PRIMARY

COIL

SECONDARY

COIL

The alternating current from the power source is in the form of the sine wave

that was generated by rotating a coil in a magnetic field. The important point

is that the current in the primary coil is constantly varying. It goes from zero

to maximum and back to zero in one direction and then to maximumand back

to

zero in

the

opposite direction.

Since the current in the primary coil is constantly varying, what is happening

to the magnetic field produced by the primary coil?

The magnetic field is

constantly changing. . . . . . . . . .

Page 1-28

The magnetic field going one way cancels

the

magnetic

field going

the other

way

. . . . . . . . . . . . . .

.

.

Page 1-29


66/375

From page 6-14

6-17

Sorry, producing a fatigue crack in a test sample is an example of a developed

discontinuity and not an example of an accumulated discontinuity.

The difference between the developed discontinuity and the accumulated

discontinuity is the source. The developed discontinuity is one that is pro

duced by our own action taken to achieve our goal of having a discontinuity in

the test sample.

An accumulated discontinuity is one that has occurred at some point in the

manufacturing process and we have merely collected it as a sample.

Now turn back to page 6-16 and continue.


67/375

From page 1-27

1-28

That is correct The magnetic field in the primary coil is varying in exactly the

same manner as the current. We now have a situation where the magnetic

field is building up in one direction, collapsing, building up in the opposite

direction, collapsing, and so on. Since this field intercepts the secondary coil, a

current

is

constantly

being induced in

the

secondary coil because

the

lines of

force are

cutting

across

the

wires forming

the

secondary coil.

BUILDING FIELD COLLAPSING FIELD

r ; : . . . '

{,.\

\\

rrl

I

: :

1 ·

1 1

P, I

~;;;,

',\c .,

\ LINES OF FORCE

0

r

AMMETER

In

order for the secondary coil to carry the current, it

must

be made of a material

that

will conduct electricity

-

for example, no current would be induced in a coil

made of cotton

string

since cotton is not a conductor of electricity.

In

general,

metals

are

the best

conductors of electricity

but there

is a dif

ference in

conductivity

even between metals. Silver has

the best conductivity

of all

the

metals while

titanium

has

the

lowest conductivity. This means

that

silver has less

resistance to the

flow of electricity

than titanium.

In your opinion, would the conductivity of the material in the secondary coil

have any effect on

the amount

of

current

induced in

it

by

the primary

coil?

Yes Page 1-30

No Page 1-31


68/375

From page 6-14

6-16

Right

on Since we have

taken

action

to

deliberately introduce a

discontinuity

into the test sample, the discontinuity is called a "developed" discontinuity.

It still is defined as a "natural" discontinuity since cyclic stresses could be

naturally

applied when

the part

is in service.

An accumulated discontinuity is one which might occur during the manufac

turing processes applied to the part. Articles that contain this type of discon·

tinuity may be accumulated over a period of time during routine testing of

articles.

Samples containing natural discontinuities, either developed or accumulated,

may be machined to produce a surface crack or hole of a known depth as

shown below.

LOCATION OF

INDUCED

FATIGUE

CRACK

0

SMALLSLOTTOINDUCE

FATIGUE AT THIS

POINT

0

0

FATIGUE

SPECIMEN

MACHINE

TO

LEAVE

CRACK ON SURFACE

SECTION

CONTAINING FATIGUE

CRACK

MACHINED

FROM

FATIGUE

SPECIMEN

At least

one of

the

cracks in

the

reference

standard

should be

at the

limit of

acceptability.

Having

a crack in

the standard that

is

at the

limit of

acceptability

is useful in

defining ...

the lowest acceptable eddy current test reading Page 6-18

the highest acceptable eddy current test reading Page 6-19


69/375

From page 1-27 1-29

You felt that the magnetic field caused by the alternating current through the

primary coil would be canceled out because the current reversed direction.

It

is true

that

the field changes direction and therefore could be thought of as

cancelling the original field but, to be sure that you understand, the idea that

we are emphasizing here is

that

during a span

of

time

the magnetic field caused

by the alternating current is varying just as the current is varying.

When

alternating

current is applied to the primary coil the magnetic field,

over a period of time, goes from zero to a maximum and back to zero in one

direction, then goes to maximum and back to zero in the opposite direction.

The magnetic field is constantly varying

just

as the current is varying.

Turn to page 1-28.


70/375

From page 6-13 6-15

No The fact that both the test piece and the reference standard are both

made from copper is no guarantee that they are both the same type of

material.

Both may look alike but one may be an alloy of copper and some other metal.

In

order for us to know

that they

are

exactly the

same

material

we should

check the conductivity of each. If they both have the same conductivity then

we know

they

are made from

the

same material.

Of course, if we have some other reason for knowing that they are made from

the

same

material

we

won't

have to check

the

conductivity.

Now

turn



71/375

From page 1-28 1-30

Yes, the

amount

of

current

induced in

the

secondary coil is affected by

the

conductivity of the material in the secondary coil. A higher conductivity

allows more current to be induced than a lower conductivity. This is an impor

tant point to remember in eddy current testing.

Now let's look at another point of extreme importance in eddy current testing.

Let's see what occurs when an alternating current is applied to a coil.

900

180°

270°

360°

MAX - ·o

0

90°

180° 270°

/

/

/

/

, , , , " '

360°

If we connect a voltmeter to measure the voltage across the coil and put an

ammeter

in

the

circuit to measure the

current

and

then

plot the

instantaneous

readings of the instruments on a graph, we find that the voltage rises to a

maximum before any current begins to flow. Then, while the voltage is

decreasing to zero,

the current

is increasing to a maximum as shown on the

graph above.

The

graph

shows

that the current

lags behind

the

voltage by 90°.

True Page 1-32

False . . . . . . . . . . . . . . . . . . . . . . . . Page 1-33


72/375

From page 6-13

6-14

Very good You seem to have realized that, though two pieces of material may

look alike,

the test

of whether

they

are alike is to measure

their

respective con

ductivities.

If their

conductivities are

the

same

they

are made of

the

same

type

of material.

The

material

in the

standard,

then,

must

be of

the

same

type

as

the material

to

be

tested.

The reason

that the

geometry of

the standard

should be

the

same as

the

geometry of the test articles is fairly obvious for pieces that have exotic

shapes. Geometry is also very

important

for

thin

pieces since

thickness

in

those

ranges

has such an effect on

the results.

As you shall see,

obtaining

samples of test articles for use as reference standards is not a great problem.

Discontinuity

standards

fall under two types

- natural

and artificial

-

depend

ing on their source.

NATURAL DISCONTINUITY S TAND ARDS

Natural discontinuity standards consist

of duplicates of

the test

piece con

figuration that contain discontinuities of a known size and shape that have

occurred from natural causes.

Natural discontinuity standards

can be developed or accumulated. By submit

ting

a

test

sample to cyclic

stresses,

a

natural

fatigue crack can be produced in

the

sample. This would be an example of

...

a developed

discontinuity . . . . . . . . . . . .

Page 6-16

an accumulated discontinuity . . . . . . . Page 6-1 7


73/375

From page 1-28

1-31

Apparently

we

haven't

clarified

the

meaning of conductivity.

Conductivity is

the ability to carry electrical current.

Here we show two batteries hooked up to light bulbs. The batteries and bulbs

are identical; the difference between the two circuits is the type of material

used in the hookup wiring.

IRON

W I R E

-

COPPER

W I R E

I

I I/

I


74/375

From page 6-11

6-13

Your selection is correct. Since

the "high"

block has 101.0

stamped

on

it

you

know

that the conductivity

of

that

block is exactly 101%

IACS.

If the

meter is

adjusted to read 101% when the test coil is placed on that block then the

"high"

end of

the

meter is calibrated.

The

next

step, you will recall, is to

calibrate the meter

so

that the

low end of

the meter will read the value stamped on the "low" block. In this case, 13.5%.

After these two steps have been accomplished the meter has been fully

calibrated and

is

ready

for use in

the test situation.

Calibration blocks are also available in the mid-range - 25 to 50% IACS - for

use with aluminum alloys.

DISCONTINUITY S T

ANDA

RDS

Ideally a discontinuity standard should duplicate the test situation as closely

as possible. Duplication of the

test situation

includes

material

type and

geometry as well as duplication of

the type

of

discontinuity

sought.

This means

that

if copper pipe is to be

tested that

a sample of copper pipe can

be used as a

standard

provided

that

the sample and the material to be

tested ...

have the same conductivity . . . . . . . . . . . . . . Page 6-14

are made of copper Page 6-15


75/375

From page 1-30

1-32

Excellent From the chart you can see that the current through the coil lags

behind the

voltage by 90 .

To show why this occurs

let's

look for a moment

at

a coil with one

turn

slightly

separated

from the

rest

of

the

coil and consider what is happening in the coil

when ac is applied to it.

Here we show one

turn

of

the

coil

separated

from the

other turns.

The alter

nating current through that

one

turn

produces a

constantly varying

magnetic

field

that cuts

across all of

the other turns

in the coil

thereby

inducing a cur

rent

in each of

the other turns

of

the

coil. This self-induced

current opposes

the originalcurrent in part of the cycle and aids the originalcurrent in another

part

of

the

cycle so

that the net

effect is

that

the

resultant current

is

shifted

out

of

phase

with the voltage.

(It

is delayed in time).

In

the same manner, every

turn

in

the

coil induces the same effect in every

other turn.

The overall effect is

that the current through the

coil lags behind

the voltage by 90°. This effect that causes the current to lag behind the

voltage is called

inductive reactance.

In

a circuit

containing

pure inductive

reactance

the maximum

voltage

occurs

at 90° and 270°, and the maximum current occurs at ...

90° and 270° Page 1-34

180 and 360 .

.

. . .

. .

. . .

. .

. . .

. .

. . .

. . .

. . .

. . .

. .

. . . .

. .

. . .

. . . Page 1-35


76/375

From page 6-11

6-12

Be careful The value

stamped

on

the "high"

block was 101

% not

100%.

If

you

adjust the conductivity tester

to read 100% while

the test

probe is on a

piece of metal

that

is known to have a

conductivity

value of 101% you will

introduce an error into all subsequent readings of conductivity.

The conductivity tester is always adjusted to read the value stamped on the

block

when

calibrating the instrument.



77/375

From page 1-30

1-33

You have selected

the

answer

that

indicates

that

you do

not understand the

graph.

0

180 270

I

. . • . .

'

'

,

'

360°

MAX - "o

0

90°

180°

270°

/

/

/

/

//

360°

Here's the graph

again. Note

the

sine wave marked

"voltage."

The

graph

shows that the voltage is zeroat 0°, maximumat 90°, zero again at 180°, max

imum in the opposite direction at 270°, and zero at 360°.

Now, look at the sine wave marked "current." See how the current is zero at

90°, maximum at 180°, zero again at 270°, and maximum at 360°.

See how the voltage is

at

its maximum 90° before the current is

at

its maximum?

Thus we say

that

the voltage across a coil is 90° ahead of

the current,

or, con

versely, the current lags behind the voltage by 90°.

Turn to page 1-32 to find out why

this

occurs.


78/375

From page 6-10 6-11

In

eddy

current testing, standards

are

most

often

manufactured at the test

site to fit a

particular test situation.

However, commercially

prepared

conduc

tivity standards

are available and are

usually

supplied with

conductivity

measuring instruments.

CONDUCTIVITY S T

AND

ARDS

Two metal blocks

representing

specific values of

conductivity

in % IACS are

supplied with

conductivity measuring instruments.

One block

represents the

high level of

conductivity

while

the other represents the

low level. The per

centage value in IACS is

stamped

on

the

blocks as shown here.

101.0%

13.5%

HIGH LOW

With the test probe of the conductivity tester placed on the conductivity

standard representing the high level of conductivity, the tester is adjusted so

that the tester reads exactly ...

100% Page 6-12

101% Page 6-13


79/375


80/375

From page 6-8

6-10

That is correct Once the instrument has been calibrated to the reference

standard any adjustment of the frequency and scale controls will upset the

calibration and invalidate the test results.

Some discontinuity testers are also equipped with light and buzzer systems

which may be preset to alert the operator to any readings which exceed the

values established by the readings taken on the reference standard. Refer to

the manufacturer's handbook for the procedure to set these alarms.

D V N C E D TEST EQUIPMENT

The operation of more advanced test equipment, such as resistance and induc

tive reactance measuring testers and such testers as use CRT and strip-chart

recorders, are of such complexity that an explanation of how they are

operated is better left to labs where the equipment is available.

S T N D R D S

As in other types of nondestructive testing the most useful test data is

obtained by comparing the data from an item under test with data obtained

from a reference standard. Standards furnish an exact value that has been

established by authority, custom, or agreement as the norm by which other

like articles may be judged. Standards also help in the design of procedures

developed to measure those quantities that are represented by the standard.

Standards often define the limits of acceptability of an item and serve to

ascertain that the equipment being used is capable of measuring that quan

tity to the required degree of accuracy. A standard is also used to make sure

that the equipment provides consistent sensitivity each time the equipment is

used.



81/375

From page 1-34

1-35

Very good.

Apparently

you

understand the

time

relationship

involved when

we say

that

in a

purely inductive circuit the

current

lags behind

the

voltage by

90° because of inductive reactance.

Now

let's take

a look

at another factor that

affects

the

flow of

current through

a circuit.

In any circuit there is a resistance (opposite of conductance) that opposes the

flow of current in the circuit. Here we show a battery (a source of direct cur

rent) hooked up to a coil.If wethen place an ammeter in the circuit to measure

the amount

of

current

flowing we have a circuit like this.

A

3 AMPERES

BATTERY

COIL

RESISTANCE

As soon as

the direct current through the

coil reaches its maximum value

there is no inductive reactance from the coil and the only factor opposing the

flow of current is the resistance of the wire.

We can compute

the amount

of

resistance

in

the

circuit from Ohm's Law

which

states that the resistance (R)

in a circuit is equal to

the

voltage

(V)

divided by the current (I). Since we know the voltage of the battery, and the

meter tells us how much

current

is flowing, we can compute

the resistance

in

the

circuit.

In

our circuit the voltage of the

battery

is 16 volts and

the ammeter

reads 2

amperes,

the total resistance

of

the

circuit is

...

32 ohms . . . . . . . . . . . . . . . . . . . . . . . . . Page 1-36

8 ohms Page 1-37


82/375

From page 6-8

6-9

We were afraid that you might have gotten the wrong idea. Once the instru

ment has been calibrated on the reference standard, the frequency and scale

controls are not to be adjusted during testing. To do so will invalidate all the

results of the tests.

The object of performing the set-up procedure is to adjust the instrument so

that readings taken during tests can be compared with the readings estab

lished on the reference standard. Since the reference standard contains exam

ples of the type of discontinuity sought, the meter readings then are mean

ingful in terms of discontinuities.

Now turn to page 6-10 and continue.


83/375

From page 1-35

1-36

Hold up You selected the answer you could only have reached by multiplying

16 X

2.

You

must

remember

that

the

resistance

is equal

to the

voltage divided

by

the current ...

in

this

case, 16 volts divided by 2 amperes equals 8 ohms.

The

total resistance

of

the

circuit is

eight

ohms.

To see if you understand, try this problem.

3 AMPERES

12V

BATTERY

COIL

RESISTANCE

What

is the voltage of the

battery?

How much current is flowing in the circuit?

What

is

the total resistance

of the circuit?

We hope

that

you can see from

the diagram

above

that

the voltage of

the

bat

tery

is 12

volts

and

that the current

measured by

the ammeter

is

3

amperes.

The total resistance of the circuit is 12 divided by 3 which equals

4

ohms. The

total resistance of the circuit is

4

ohms.

Now turn to page 1-37 and continue.


84/375

From page 6-6

6-8

With

the instrument turned

on, a frequency is selected and

the

probe is placed

on sound, bare metal.

If

a meter

reading cannot

be

obtained

by

adjustment

of

the

scale control,

another

frequency is selected and

the

scale control

adjusted.

This procedure is

repeated until

a

reading

is obtained.

The next step is to fine-tune the instrument to find the frequency that will

suppress the

lift-off variable. This is accomplished by placing a sheet of

paper

between

the

probe and

the

material,

noting the

reading on

the

dial, and com

paring it with the reading obtained on bare metal. The frequency must be fine

tuned until there

is no change, or a minimum change, between

the

two

readings. The

instrument

is now

set

up for

detecting

discontinuities.

The

next step

is to calibrate

the instrument

with

the

reference

standard.

This

is accomplished as follows: With the probe over the required discontinuities,

the meter readings

are noted.

At this

time

the

scale may be

contracted

or

expanded

to adjust the reading

from significant

discontinuities

to

particular

scale divisions on

the

meter.

Further adjustments

of frequency may be

required to

obtain the

required

sensitivity

to

the

discontinuities.

If

so,

the

lift

off suppression procedure

must

be repeated.

Based on

the

information you now have, will

the

frequency and scale controls

have to be

adjusted during tests

on

the

specimens?

Yes .

.

. .

.

. .

.

. .

.

. . . . . Page 6-9

No Page 6-10


85/375

From page 1-36

1-37

8 ohms, very good. Now that you seem to understand how voltage, current,

and

resistance

are

interrelated, let's

hook up

the resistance to

a source of

alter

nating current, add an ammeter and voltmeter as we did before, and plot the

results.

00

90°

M r +

, ; , " '° ' ' < , ~

~ o / ~ . . ( , .

'/

~ « . ;

~

Vl>R l G V

MAX-

00

900

180° 270°

360°

180°

270°

360°

This time we find that when the voltage is at the maximum, the current is also

at the maximum; and when the voltage is zero, the current is also at zero.

There is no leading voltage or lagging

current.

In other words, the applied voltage and the resultant current are exactly in

phase with each other through a resistance.

Resistance in an ac circuit does not cause the current to lag behind the

voltage.

True

.

.

. . .

.

. . . . . . . . . . . .

.

. .

Page 1-38

False

. . .

.

. . . . . . . . .

.

. . . . . . . . . . . . . .

Page 1-39


86/375

From page 6-5 6-7

We realize

that

you probably

had

to guess

at this

answer

but

you guessed

wrong. The effects of lift-off can be suppressed by selecting the right fre

quency. Notice

that

we did

not

say

that the

effects of lift-off are eliminated

but they can be greatly reduced.

Why are the effects of lift-off reduced? Remember the impedance-plane

diagram and

the

effect

that

changing

the

frequency had.

In adjusting the

frequency we are looking for

that point

where lift-off has

the

least effect yet where the depth of penetration is still adequate to do the job.

Now

turn



87/375

From page

1-3

7

1-38

True, very good. Now

that

you seem to

understand

how voltage,

current,

and

resistance

are

interrelated, let's

hook up

the

coil to a source of

alternating

current.

r-

-1

I X L I

COIL

I I

I

R

L- _J

First

of all you

must understand that the resistance

( R ) we found in

the

circuit

is still there. It resists the flow of alternating current just as it did the flow of

direct current. The factor that has been added is the inductive reactance of the

coil. The inductive

reactance

(indicated by

the letters

XL) causes the

current

to lag behind

the

voltage by 90°, i.e., out of phase by 90°.

In

an ac circuit the combination of resistance and inductive reactance is called

impedance (designated by the letter

Z).

When we speak of the impedance in an

alternating current circuit we mean the total opposition to current flow through

the circuit and we are including both resistance and inductive reactance.

The total impedance

(Z)

of the circuit is the sum of the resistance ( R ) and the

inductive reactance (XL). However, the two cannot be added directly because

their effect on the voltage is out of phase. The maximum

current

due to

resistance

does

not

occur

at the

same

instant that the

maximum

current

due

to inductive reactance occurs.

Turn to page 1-40.


88/375

From page

6-5

6-6

Very good The variable frequency can be used to suppress the lift-off variable

in addition to

obtaining the

proper

depth

of

penetration.

Here we show a typical crack detector.

FREQUENCY

CONTROLS

20 40 60 80 100

1

OBATTERY TESb

2

FINE

LEVEL

O

I I SCALE

__µ..----

CONTROL

PROBE

EDDY CURRENT INSTRUMENT

e

The frequency of crack

detectors

is generally controlled by means of two con

trols -

a course control and a fine control. The course control is used

to adjust

the instrument

to

the

frequency required to

detect the

cracks in a reference

standard. The frequency is then fine-tuned to suppress lift-off.

The meter scale control provides the means to expand or contract the scale of

the

meter so

that readings

which are too

slight

to be read may be

stretched

across the scale or, conversely, readings which are too large to appear on scale

may be reduced to values which are on scale.

To set up this type of instrument for operation the test coil is first placed on a

reference standard that represents the type of material to be tested and that

contains

the type

and size of

the discontinuity

sought.

Turn to page 6-8 and continue.


89/375

From page 1-3 7

1-39

Sorry,

the statement

was true.

It

is very

important that

you realize

that

when

alternating current

is applied

to a purely

resistive

circuit,

the current

is always in-phase with

the

voltage.

The presence of

resistance

does not

cause the current to lag behind the

voltage.

I

j

90°

270° 360°

0

MAX+

MAX-

180°

270° 360°

You must also remember that whenalternating current is applied to a purely

reactive circuit the inductive reactance causes the current to lag behind the

voltage by 90°. The voltage and current are 90° out of phase as shown here.

MAX+

O o

j

900

180° 270° 360°

M A X

-

o

0

90°

0

180

270°

/

/

/

/

.

360°

Nowlet's seewhat happens when we have both resistance and inductive reac

tance in a circuit when alternating current is applied. Turn to page 1-38.


90/375

From page 6-2

6-5

Right The next step is to calibrate the low end of the scale to make sure that

the

meter

reads

as

it

should

at the

low end as well as

at the

high end.

Once

the instrument

is calibrated,

the

high and low controls are not touched

during the ensuing tests.

Tests

are conducted by placing

the test

coil firmly on

the test

specimen and

rotating the IACS% dial to a position where the meter is centered. The

reading on

the

dial

at that

point is

the conductivity

of

the

specimen.

Most conductivity testers have some arrangement to calibrate the instrument

that

is similar

to the

one we've explained here.

CRACK DETECTORS

Crack, or discontinuity, detectors are more complex instruments than conduc

tivity testers. First, there is the requirement for a variable frequency to allow

adjustment for depth of penetration. And second, the scale on the meter must

be expandable to allow for a wide variation of meter deflection.

Since the frequency applied to the coil may be varied, we can suppress lift-off

effects with this instrument.

True . . . . . . . . . . . . . . Page 6-6

False Page 6-7


91/375

From page 1-38

1-40

Here we show a plot of both the current due to resistance and the current due

to inductive reactance.

CURRENT

DUE TO

CURRENT DUE TO

INDUCTIVEREACTANCE

1-

z

w

a:

a:

: : : : >

o

MAX - o

0

1 8 0 °0°

180°

270°

0

360

0 0

0

270

360°

0 0

Since the total current due to the impedance

(lz)

is the algebraic sum of the

current due to resistance

(IR)

and the current due to inductive reactance

(I

XL),

we can plot the current due to impedance by adding the resistance cur

rent to the reactive current. The results are as shown here.

TOTALCURRENT CURRENT

DUE TO DUE TO

CURRENT DUE TO

INDUCTIVE

REACTANCE

/

'

,

0

180 360°

0 0

9 0 ° 1 8 0 °

210°

70°

360°

0 0

To obtain the impedance current curve shown it is necessary to add instan

taneous values of

IR

and

I XL·

For example, at 90 °IR is at its maximum while

IxL

is

at

zero. The impedance curve

must pass through

this maximum IR

value since at that point the IxL value is zero.

Continue on the next page.


92/375

From page 6-2

6-4

Sorry, we cannot begin testing yet. In this type of instrument the high end

and the low end of the scale have to be calibrated.

What we are accomplishing is this -

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