BS DD ENV 583-6 Ed.2000

8/19/2019 BS DD ENV 583-6 Ed.2000

1/18

STD-BSI

D D

ENV 583-b-ENGL 2 0 0 0

L b 2 4 b b 9 0859070 3 b 3 =

DRAFT FOR DEVELOPMENT

Non-destructive

testing Ultrasonic

examination

Part

6:

Time-of-flight diffraction

technique

as a

method

for

detection and

sizing

of

discontinuities

ICs 19.100

DD ENV

583-6:2000

NO COPYING

WITHOUT

BSI PERM ISSION EXCEPT

AS

PERMIITED BY COPYBIGHT

LAW

opyright European Committee for Standardizationovided by IHS under license with CEN

Not for Resaleo reproduction or networking permitted without license from IHS

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S T D m B S I

D D E N V 5ö3-6-ENGL

2000

=

L b 2 4 b b 9 0859073

2 T T I m

having been prepared under the

direction of the Engineering

Amd.No. Date

Sector Committee,

was

published

under the authority of the

Standards Committee and comes

into effect on 15

July 2000

DD ENV 583-6:2000

Comments

National

foreword

This Draft for Development

is

the official English language version of

ENV 58362000. During the development of ENV 5836, the

UK

expressed concern

about some of its provisions. Particular attention is drawn to the points outlined in

national annex

NA.

Attention is also drawn to the related British Standard

BS 77061993.

This publication s not to be regarded as a British Standard.

The UK participation

in

its preparation was entrusted to Technical Committee

WEW46,

Non-destructive testing, which has the responsibility to:

id enquirers to understand the text;

resent to the responsible European committee any enquiries on the

onitor related international and European developments and promulgate

interpretation, or proposals for change, and keep the

UK

interests informed

them in the

UK

A

list

of organizations represented on

t s

committee can be obtained on request to

its secretary.

Cross-references

The British Standards which implement international or European publications

referred to in

t s

document may be found

in

the BSI Standards Catalogue under the

section entitled ?International Standards Correspondence Index?,or by using the

?Find? acility of the BSI Standards Electronic Catalogue.

Summary of pages

This

document comprises

a

front cover, an inside front cover, the ENV title page,

pages

2 to 15

and a back cover.

The BSI copyright notice displayed in

thi s

document indicates when the document

was last issued.

O BSI 07-2000

ISBN O 580 34871 7

opyright European Committee for Standardizationrovided by IHS under license with CEN


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~

~

~

STD-BSI D D ENV

583-b-ENGL 2 0 0 0 =

1 b 2 4 b b 9 0 8 5 7 0 7 2 1 3 b

=

EUROPEAN PRJESTANDAEI;D

ENV 583-6

PRENoRMEEuR0PEE E

EUROPAISCHE vomom January

2ooo

ICs

19.100

Engush

version

Nondestructive

testi ng ltrasonic

exambtion

Part

6 'Iirne-of-flq$t dZ&action technique as a method

for

detection

and sizing

of discontinuities

Essais non destructifs ontrôle Ultsasonore

Partie 6:Technique de difiiwtion du temps de vol

utilisée comme méthode de détection et de

dimensionnement des discontinuités

zerstörungsfeie

Prufung

ltraschaiiprufung

Teil6 Beugungsla.ufzei@chnik, ein Technik zum

Aunuiden und

Ausmessen

von Jnhomogenitäkn

This

European &standard (ENV)

was

approved by CEN on 21May1997 as

a

prospective standard for provisional application.

The period

of validity of ths

ENV

is

limited initially to

three

years. After

two

years

the members

of

CEN

will

be requested to submit their comments, particularly on

the question whether the ENV can be converted

into

a

European Standard.

CEN members are required to announce the existence of

t s

ENV in the same way

as for an EN and

to

make the ENV available promptly at

national

level in an

appropriate form. It

is

permissible

to

keep conflicting national

standards in

force

(in parallel to the ENV)

untü

the

f i n l

decision about the possible conversion of the

ENV into an EN is reached.

CEN members are the national standards bodies

of Austria,

Belgium, Czech

Republic, Denmark,

Finiand,

France,

Gennany,

Greece, Iceland, Ireland, Italy,

Luxembourg, Netherlands, Norway,

Portugai,

Spain, Sweden, Switzerland and

United Kingdom.

CEN

European Committee for Standardization

Comité Européen de Normalisation

Europäisches Komitee

fur

Normung

Central Secretariat:

rue

de

Stassart

36,

B-1060 Brussels

O

2000 CEN

All

rights

of

exploitation

in any

form and by

any means

reserved worldwide for CEN

national

Members.

Ref.

No.

ENV

583-6:2000

E



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Page 2

ENV

583 6:2000

Foreword

This European Prestandard has been prepared by

Technical Committee CENEC 138, Nondestructive

testing, the Secretariat of which is held by AFNOR.

According to the CENKENELEC Internal Regulations,

the national standards organizations of the following

countries are bound to announce t s European

Prestandard Austria, Belgium, Czech Republic,

Denmark, Finland, fiance, Germany, Greece, Iceland,

Ireland, Itaìy, Luxembourg, Netherlands, Norway,

Portugal, Spain, Sweden, Switzerland and the United

Kingdom.

EN

583,

Non-destructive testing ltrasonic

examination

consists of the

following

parts:

EN

583-1,


examination art : Generalprinciples.

EN 583-2, Non-destructive testing ltrasonic

examination art 2:Sensitivity and range

setting.

EN

583-3,


examination art 3: P a m i s s i o n technique.


examination art 4: Examinat ion

for

discontinuities

perpendicular to

the surface.

EN

583-5,


examination art

5:

C h a r a c M a t i o n and

sizing

of

discontinuities.

ENV 583-6, Non-destructive testing ltrasonic

examination art 6: 'Pime-offlight dìff mc tion

techniqw?as a method for detection and sizing of

discontinuities.

Contents

Foreword

1

Scope

2 Normative references

3 Definitions and symbols

4 General

4.1

Principle of the technique

4.2 Requirements for surface condition

4.3

Materials and process type

5 Qualification

of

personnel

6

Equipment requirements

6.1

Ultrasonic equipment and display

6.2 Uiîrasonic probes

6.3

Scanning mechanisms

7 Equipment Set-up procedures

7.1 General

7.2

7.3 Time window setting

7.4

Sensitivity setting

7.5

Scan resolution setting

7.6 Setting of scanning speed

7.7

Checking system performance

8

Interpretation and analysis of data

8.1

Basic analysis of discontinuities

8.2 Detailed analysis of discontinuities

9 Detection and sizing in complex

geometries

10 Limitations of the technique

10.1

Precision and resolution

10.2 Dead zones

11

TOFD examination without data

12

Examination procedure

13 Examination report

Annex A (normative) Reference blocks

and couplant

Probe choice and probe separation

recording

Page

2

3

3

3

4

4

4

6

6

6

6

6

8

8

8

9

9

9

9

9

9

10

10

11

12

12

12

13

13

13

13

14

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Page 3

ENV

583422000

1 scope

This European hestandard defines the general

principles for the application of the Timeof-flight

diffracton ('ï0FD) technique for both detection and

sizing

of discontinuities in low alloyed carbon steel

components. It could

also

be used for other

types

of

materials,

provided the application of the TOFD

technique is performed with necessary consideration of

geomeixy, acoustical properties of the materialsand the

sensitivity of the examination

Although it is applicable, in general terms,

to

discontinuities in materials and applications covered by

EN 583-1, it contallis references to the application on

welds.

This approach has been chosen for reasons of

clarity as o the ultmsonic probe positions and

directions of scanning.

Unless otherwise specified in the referencing

documents, the minimum requirements of ths

hestandard are applicable.

Unless explicitly stated otherwise, thi s hestandard is

applicable

to

the

following

product classes as defined

in EN 583-2

lass 1,without restrictions;

lasses 2 and 3, restrictions will apply as stated in

clause 9.

The inspection

of

products of classes

4

and

5 wiil

require special procedures. These are addressed in

clause

9

as

well.

2 Normative references

This European

hestandard

incorporates by dated or

undated reference, provisions from other publications.

These normative references are cited at the

appropriate places in the text and the publications are

listed hereafter. For dated references, subsequent

amendments to or revisions of any of these

publications apply to thi s European Fhstandard only

when incorporated in it by amendment

or

revision. For

undated references the latest edition of the publication

referred to applies.

EN

473,

Qud jfication and certjfic atio n of

NDT

EN 583-1,Non-destructive testing ltrasonic

examination art :

G M

rinciples.


examination art 2:S m s i t iv i t ~ nd range sett ing.

EN 126681,Ultrasonic exa min ation

Characterization and verification of ultrasonic

examina tion equipment art : Instruments.

EN 126682,Ultrasonic exa min ation

Characterization and verification of ultrasmic

examination equipment art 2:

h b e s .

EN 126683, Ultrasonic exam inatio n

chamcterization and v df ic at io n of ul trasonic

examin ation equipment art 3: C o mb in ed

equipment.

personnel

eneml

pr inÆìpb .

O

BSI 07-2000

3

Definitions and symbols

OFD

ïimeof-flight diffmdion.

h

Y

Y

hz

d

öd

Ddw

c

&

R

t

At

z

Dds

öt

td

tP

t

S

6s

W

dead zone

back wall

dead zone

A - s c ~ ~

k a n

n o n - p d e l

parallel scan

scan

coordinate parallel

to

the scanning

surface, and parallel to a

predetermined reference line.

In

case

of weld inspection this reference line

should coincide with the weld. The

origin

of the axes may be defined

as

best suits the specimen under

exanunah'on (see Figure 1);

imperfection length;

coordinate parallel to the scanning

surface, perpendicular

to

the

predetermined reference line

(*e

Figure

1);

error in

lateral

position;

coordinate perpendicular to the

scanning surface (see Figure i);

imperfection height;

depth of a imperfection tip below the

scanning surface;

error in depth;

scanning-surface dead zone;

backwail dead zone;

sound velocity;

error in sound velocity;

spatial resolution;

timeof-flight from the transmitter to

the receiver;

heof-flight difference between the

lateral

wave and a second uitrasonic

signai;

error in ümeof-flight;

time-of-fight

at

depth d;

length (in time) of the acoustical pulse

up to

an ampiitude

of

10

%

of the

maximum;

time-of-flight of the backwall echo;

half the distance between the index

poinîs of

two ultrasonic probes;

error in

half

the probe separation;

wall thickness;

zone where indications may be

obscured due to the presence of

signais of geometrical origin;

extra dead zone where signais may be

obscured by the presence of the back

wall echo;

display of the ultrasonic signal

amplitude

as

a

function of time;

display of the time-of-flight of the

ultrasonic signal

as

a function of

probe displacement;

scan perpendicular

to

the ultrasonic

beam direction (see

Figure 4);

scan parallel

to

the uitrasonic beam

direction (see Figure

5).



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Figure 1 oordinate definition

4 General

4.1 Principle of the technique

The TOW technique relies on the interaction of

ultmsonic waves with the tips of discontinuities. This

interaction results in the emission of diffracted waves

over a large

m r

range. Detection of the diffracted

waves makes it possible to establish the presence of

the imperfection. The time-of-fìight of the recorded

signals

is

a measure for the height of the imperfection,

thus enabhg sizing of the defect. The dimension

of

the imperfection is always determined kom the

timeof-flight of the difîracted signals. The signal

amplitude is not used in size estimation.

The basic conñguration for the

TOFD

technique

consists of

a

separate ultrasonic trammitter and

receiver (see Figure 2). Wide-angie beam compression

wave probes are normally used since the diffraction

of

ulhasonic waves is only weakly dependent on the

orientation of the imperfection tip. This enables the

inspection

of

a certain volume

in

one scan. However,

restrictions apply to the size of the volume that can be

inspected during

a

single scan (see

7.2 .

The

first

signal to arrive at the receiver after emission

of an acoustic pulse is

usually

the lateral wave which

travels

just

beneath the upper surface of the test

specimen.

In the absence of discontinuities the second signai to

arrive at the receiver is the backwail echo.

These

two

signals axe normally used for reference

purposes. If mode conversion is neglected, any signals

generated by discontinuities in the material should

arrive between the lateral wave and the backwail echo,

since the latter

two

correspond, respectively, to the

shortest and longest paths between fmnsmitter and

receiver. For similar reasons the diffracted signal

generated

at

the upper tip of

an

imperfection wili

arrive before the signal generated at the lower tip of

the imperfection.A typical pattern of indications

(A-scan)

is

shown

in

Figure 3. The height of the

imperfection can be deduced from the difference in

time-of-fiight of the two diffracted signals (see 8.1.6).

Note

the phase reversal between the lateral wave and

the backwall echo, and between echoes of the upper

and lower tip of the imperfection.

Where access to both surfaces of the specimen

is

possible and flaws are distributed throughout the

specimen thickness, scanning from both surfaceswili

improve the overall precision, particularly in regard to

flaws near the surfaces.

4.2

Requirements

for

surface condition and

couplant

Care

shaü

be taken that the surface condition meets

at

least the requirements stated in EN

583-1.

Since the

diffracted signals may be weak, the degradation of

signal quality due to poor surface condition

wili

have a

severe impact on inspection reliabiiiw.

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Legend

1 Itansmitter

2

Receiver

a Laterai wave

b Uppertip

c included angle

d Imperfection

e Lowertip

f

Backwallecho

Figure 2 asic TOFD configuration

Legend

X Amplitude

Y Time

a

Laterai

wave

a

-b

C

d

b

Uppertip

c

Backwallecho

d Lowertip

Figure

3

chematic A-scan

of

embedded imperfection

O BSI

07-2000

opyright European Committee for Standardization

ovided by IHS under license with CENNot for Resaleo reproduction or networking permitted without license from IHS

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Page 6

ENV 583-6.2000

Different coupling media can be used, but their type

shall be compatible with the materials to be examined.

Examples are: water, possibly containing an agent

(wetting, anti-freeze, corrosion inhibitor), contact paste,

oil,

grease, cellulose paste containing water, etc.

The characteristics of the coupling medium shall

remain constant throughout the examination. It shall

be suitable for the temperature range in which it

wili

be used.

4.3

Materials and process type

Due

to

the relatively low signal amplitudes that are

used in the TOFD technique, the method can be

applied routinely on materials with relatively low levels

of attenuation and scatter for ultrasonic waves. In

general, application on unalloyed and low alloyed

carbon steel components and welds

is

possible, but

also

on fine grained austenitic steels and duminium.

Coarse-grained materials and materials with significant

anisotropy however, such

as

cast iron, austenitic weld

materials and high-nickel alloys,

will

require additional

validation and additional data-processing.

By mutual agreement,

a

representative test specimen

with artificial and/or natural discontinuities can be

used to confirm inspectability. Remember that

ciifhction characteristics of artificial defects can differ

significantly from those of real defects.

5 Qualification

of

personnel

Personnel performing examinations with the TOFD

technique shall,

as a minimum,

be qualified in

accordance with

EN

473, and shall have received

additional

traini ng

and examhation on the use of the

TOFD technique on the product classes to be tested, as

specified in a written practice.

6

Equipment requirements

6.1 Ultrasonic equipment and display

Ultrasonic equipment used for the TOFD technique

shall,

as

a minimum,

comply with the requirements of

EN 126681, EN 126682 and EN 126683.

In

addition, the following requirements

shall

apply:

he receiver bandwidth

shall,as

a minimum,

range between 0,5 and

2

times the nominal probe

frequency at

-6 dB,

nless specific materials and

product classes require

a

larger bandwidth.

Appropriate band filters can be used;

he transmitting pulse can either be unipolar

or

bipolar. The rise time

shall

not exceed

0,25

times the

period corresponding

to

the nominal probe

frequency;

he unrectified

signals

shall be digitized with

a

sampling rate of

at

least four times the nominal

probe frequency;

or general applications combinations of

ultrasonic equipment and scanning mechanisms

(see

6.3)

shall be capable of acquiring and digitizing

signals with

a

rate of at least one A-scan per

1

mm

scan length. Data acquisition and scanning

mechanism movement

shall

be synchronized for th s

purpose;

o select an appropriate portion of the time base

within which A-scans are digitized,

a

window with

programmable position and length

shall

be present.

Window

start

shall be programmable between

O and 200

ps

from the transmitting pulse, window

length

shaii

be programmable between 5 and 100

p.

In thi s

way, the appropriate

signals

(iaterai or

creeping wave, backwall signal, one or more mode

converted signals

as

described in

4.1

can be

selected to be digitized and displayed;

igitized A-scans should be displayed in

amplitude related grey or single-colour levels, plotted

dacen tly to form

a

Escan. See Figures

4

and

5

for

typical B-scans of non-parallel and parallel scans

respectively. The number of grey or single-colour

scales should at least be

64;

or archiving purposes, the equipment shaii be

capable of storing all A-scans or B-scans

(as

appropriate) on a magnetic or optical storage

medium such as hard disk, floppy disk, tape or

optical disk For reporting purposes, it shall be

capable of making hard copies of

A-scans

or B-scans

(as

appropriate);

he equipment should be capable of performing

signal averaging.

In

order to achieve the relatively

high

gain settings

required for typical TOFD-signals,

a

preamplifier may

be used, which should have a

flat

response over the

frequency range of interest.

This

preamplifier

shall

be

positioned as close

as

possible to the receiving probe.

Additional requirements regarding features for basic

and advanced analysis of discontinuities are described

in clause

8.

6.2 Ultrasonic probes

Ulkasonic probes used for the TOFD technique s h d

comply with

at

least the following requirements:

umber of probes:

2

(transmitter and receiver);

ype: any suitable probe (see

7.2 ;

ave mode: usually compression wave; the use of

shear wave probes

is

more complex but may be

agreed upon in special cases;

oth probes shall have the same centre frequency

within

a

tolerance of 320 /o; frequency: for detajls on

probe frequency selection, see 7.2;

he pulse length of both the lateral wave and the

backwall echo shall not exceed two cycles,

measured at 10

%

of the peak amplitude;

ulse repetition rate shall be set such that no

interference occurs between acoustical signals

caused by successive transmission puises.

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583 6:2000

X

t

/

eference line

Transmitter

Direction o f Directiono f

probe probe

displacernent displacement

( X direction) X direction)

t

t

/

\

la

I

- Y

Transit time

(through wall extent)

Lateral

wave

irection o f probe

displacement (

X

direction)

\

Receiver

Backwall

reflection

I

Figure

4

on-parallel scan, with the typical direction of probe displacement

shown on the left, and

the

corresponding B-scan shown on the right

O BSI 07-2000

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583-6:2000

X

t

/

eference line

Transit time

(throug h wall extent)

Lateral

wave

displacement

\

Trancm tte r Receiver

Backwall

re f ect

o

n

irection of p robe

X direction)

Figure

6

arallel scan, with the typical direction of probe displacement shown

on the le ft, and the corresponding B-scan shown on the right

6.3

Scanning mechanisms

Scanning mechanisms shall be used to maintain

a

constant àistance and aiignment between the index

points of the

two

probes.

An

additional function of scanner mechanisms

is

to

provide the ultrasonic equipment with probe position

information, in order

to

enable the generation of

position-related B-scans. Information on probe position

can be provided by means of e.g. incremental magnetic

or optical encoders, or potentiometers.

Scanning mechanisms

in TOFD

can either be motor or

manually driven. They shall be guided by means of

a

suitable guiding mechanism (steel band, belt, automatic

track foliowing systems,

guiding

wheels etc.).

Guiding accuracy with respect to the centre

of a

reference h e e.g. the centre h e of

a

weld) should be

kept

within

a

tolerance

of

I10

% of the probe index

point separation.

7

Equipment Set-up procedures

7.1 General

Probe selection and probe configuration are important

equipment Set-up parameters. They largely detemine

the overall a~curacy,he

signal-to-noise

ratio and the

coverage

of

the region

of

interest of the

TOFD

technique.

The Set-up procedure described in

this

subclause

intends to ensure:

ufficient system gain

and

signal-to-noise ratio

to

detect the diffracted signals of interest;

cceptable resolution and adequate coverage of

the region of interest;

fficient use

of

the dynamic range of the system.

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Centre Crystals ize Nominal

probe angle

Wa ll

mm MHZ

mm

thickness frequency

c

10 10 15

2-6 50

70

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8

Interpretation and

analysis

of data

8.1

Basic analysis of discontinuities

8.1.1

General

Reporting or acceptance criteria shall be agreed upon

by contracting parties prior to inspection.

Discontinuities detected by TOFD

shall

be

characterized by at least:

heir position in the object x- nd y-

coordinates);

heir length

Ax);

heir depth and height (z, hz);

heir type, limited to: “top-surface breaking”,

“bottom-surfacebreaking” or “embedded.

8.1.2

Characterization of discontinuities

In order to characterize an imperfection, the phase of

the tip-diffraction associated with thi s imperfection

shall be determined

signal with same apparent phase as the lateral

wave shall be considered

to

originate from the lower

tip of an imperfection;

signal with the same apparent phase

as

the

backwall echo shall be considered to originate either

from an upper tip of an imperfection or from an

imperfection with no measurable height.

If the signal-tc-noise ratio is insufñcient to aUow the

phase of the signal to be detected, these identifications

are invalid.

8.1.2.1

Top-surface breaking impe@ection

An indication consisting of a lower-tip diffraction with

an associated weakening (check for couplant loss) or

interruption of the lateral wave shall be considered a

top-surïace breaking imperfection.

Sometimes a slight

shift

of the laterai wave towards

longer timeof-flight can be observed.

8.1.2.2 Bottom-surfàce breaking discontinuities

An

indication consisting of an upper-tip diffraction

with either an associated shift of the backwall echo

towards longer time-of-fight or an interruption (check

for couplant loss) of the backwall echo shall be

considereda bottom-surface breaking imperfection.

8.1.2.3 Embedded discontinuities

An

indication consisting of both

an

upper-tip and a

lower-tip diffraction shall be considered an embedded

imperfection.

An indication consisting solely of an apparent upper-tip

diffraction with no associated indications in either

lateral wave or backwall echo shall be considered an

imperfection with no height. Care must be taken

however, because the indications in the lateral wave or

backwall echo can be very weak, resulting in

misinterpretation of the imperfection.

In

case of doubt

appropriate action

shall

be taken, either by performing

multiple TOF’D-scans (see 8.2.1 or by applying other

techniques.

In case further characterization is required, reference

shaii be made to 8.2.

In case of doubt about the interpretation of a defect,

the worst possible interpretation

shall

be retained,

until

the interpretation can be verified

8.1.3

Estimation of imperfection position

In general it

will

be sufficiently accurate to assume

that

the imperfection

is

located on the intersection

between the x,z-plane mid-way between the two

ultrasonic probes and the y,z-plane through the

centre-lines of the two probes.

The time-of-fight of an indication generated by an

imperfection can

also

be used to estimate its position.

The surface of constant timeof-flight theoretically is an

ellipsoid centred around the index points of the

ultrasonic probes. The exact determination of the

position of the diffractor can only be achieved by

at

least two scans (see8.2.1 .

If

a more accurate assessment of the position andíor

orientation of the imperfection is required, multiple

TOFD-scans (non-parailel andíor parallel) will have to

be performed.

8.1.4

Estimation of imperfection length

The estimation of the length of an imperfection shall

be made directly from the probe displacement of a

non-parallel scan.

In

common with l l ultrasonic

techniques thi s record is iikely to be elongated because

of the f inte width of the dtrasonic beam, resulting in

conservative estimates of the imperfection length.

Indications with an apparent length of less than

1,5

times the size of the probe crystal used are too

small to be sized, in length, by normal TOFD

procedures, but see 8.2.2 for additional algorithms to

determine imperfection length.

8.1.5

Estimation of imperfection depth and height

It is assumed that the dtrasonic energy enters and

leaves the specimen at the index points of the probes.

In case the imperfection is assumed to be mid-way

between the two probes (see 8.1.3 , the depth of the

defect is given by:

where

d

=

[ (Ct)2

$1”

(1)

c is the sound velocity;

t is the time-of-fight of the t i m t i o n ignal;

d

is

the depth of the tip of the imperfection;

S

is

half the distance between the index points of

the ultrasonic probes.

The time-of-fight of the ultrasonic signai inside the

ultrasonic probes shall be subíracted before the

calculation of the depth is made. Failure to do

so

will

result in grave errors in the calculated depth.

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To avoid the errors th t may arise from probe delay

estimation the depth d shall be calculated, if possible,

from the timeof-flight differences,

At,

between the

lateral wave and the di fhcted pulse. Hence:

8.1.6.1 Top-suflme brealc.ng discontinuities

The height of a top-surface breaking imperfection

is

determined by the distance between the top surface

and the depth of the lower-tip diffrctction signal.

8.1.5.2 Bottom-surJime breaking disco ntinuitie s

The height of a bottom-suface b r e w mperfection

is

determined by the difference in depth between the

upper-tip diffraction and the bottom surface.

8.1.5.3

Embedded impe@ection

The height of an embedded imperfection

is

determined

by the difference in depth between the upper-tip and

lower-tip difñ-action.

8.2 Detailed analysis of discontinuities

Detailed imperfection analysis can be performed on

discontinuties already detected by basic TOFD-scam.

In addition, the application

of

other NDT-techniques

can be considered in order

to

arrive at a more detailed

characterization

The motivation for detailed imperfection anal ysi s can

be:

d =

[(CAt)z

+

kAtS]'

(2)

ore accurate assessment of imperfection length,

depth and height;

ssessment of imperfection orientation;

etailed estimation of imperfection type.

The detailed imperfection analysis nvolves performing

additional scans with different probe angles,

frequencies and/or probe separation. Also parallel

scans can be performed. The detailed

analysis

can

also

involve the application of additional computer

algorithmsto analyse the data

8.2.1Additional scans

8.2.1.1 Scans wit h

lower

testfreq.uency

Scans with lower test frequencies can be performed

if

the signal-to-noise ratio is too low to permit detailed

imperfection

analysis

even with considerable averaging.

In general

thi s will

be at the expense of an increased

dead zone, and a decreased resolution.

The equipment Set-up parameters shall be optimized

(see clauses 6 and

7).

8.2.1.2 Scans with

higher

testfrequency

Scans

with higher test frequencies can be performed to

obtain increased resolution, increased s i z i i accuracy

and

a

reduced dead zone, at the expense of

a

reduced

signal-tenoise

ratio,

due to increased

grain

noise. The

equipment Set-up parameters

shall

be optimized

(see clauses 6 and 7).

8.2.1.3 Scans

with

redwed probe ang k

Scans with

a

reduced probe angle and an associated

decreased probe separation

can

be performed

to

obtain increased resolution, increased s i z i i accura~y

and a reduced dead zone

at

the expense of a smalier

honified volume

of

the specimen. The equipment

Set-up parameters shall be optimized (see clauses 6

and 7).

8.2.1.4 Scans

with

d i f f m t

probe

offset

In

order

to

obtain the lateral position of the

imperfection (ydirection) and/or its orientation, either

a parailel scan or an additional non-parailel scan with

different probe distance (offset) can be made. The

equipment Set-up parameters sh llbe optimized

(see clauses

6

and

7).

It

shall

be checked that the phase relationship of the

signals

observed in these scans is identical to the

phase relationship in the initial scans.

The surface of constant timeof-fight for

a

tipdifhction

signal

(locus curve)

is

an ellipsoid. If we

consider oniy the

y z-phne

through the probes, the

ellipse describing a constant path

is

expressed by:

From

this

expression it

is

clear th t

a

different offset

of the diffractor

h m

he cenire plane between the

probes (ie.

a

different y-value)

will resuit

in

a

different

time-of-flight of the t ipdi fhct ion. Therefore the

apparent depth of the imperfection-tip

will

change in

scans with different probe positions.

The lateral position of

a

imperfection-tip (y-direction)

can be determined directly from

a

parallel scan by the

position of minimum apparent depth. A number of

adjacent parallel scans at different x-coordinates

will

be required

to

find the position of real

minimal

depth

of the imperfection.

Once the position and depth of both tips of an

imperfection are known its orientation

can

be

determined from the axis throughthe

two

imperfection-tips.

in

principle, two non-parallel scam, offset with respect

to each other,

also

suffice for

the

accurate

determination of imperfection depth, length and

orientaiion, provided that the overlap of the insonified

volumes

is

sufficient.

However, the determination of the position of the

imperfection-tip íkom two non-parallel scans

is

less

straightforward and

will

involve the

drawing

of locus

curves by additional so h a r e , (see

8.2.2).

Additional parallel scam

may also

be used

to

detect

near=surface defects, th t are poorly resolved because

of the proximity of the lateral wave or the backwall

echo. The apparent depth of the defect

will

change in

each scan and

thi s

will

enable resolving it from the

lateral

wave or the backwall echo.

ct = [d2+

S

+ [ d 2+

S

+

(3)

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8.2.2Additional algorithms

Computer algorithms can be useful in analysing the

data recorded in a TOFDscan.

For example:

urve fitting overlays for accurate determination

of imperfection length (see also 8.1.4);

ubtraction of lateral wave andíor backwall echo

in order to detect indications otherwise obscured

due to interference (see 10.2). If the surface

is

rough

or pitted, the effectiveness of thi s technique should

be demonstrated in trials;

inearization algorithms to linearize complete

&scans to accurately determine the depth or the

height of the imperfection;

odelling algorithms enabling the drawing of

locus curves and the analysis of mode converted

signais. This can provide additional insight in the

position, depth and orientation of the imperfection.

Detailed understanding of the physics and modelling

software are required

The algorithms to be used in analysing the data shall

be agreed upon by the contracting parties prior

to

inspection.

9 Detection and sizing in complex

geometries

For class

2

objects, if the surface between the two

probes

is

flat, no further restrictions apply.

Otherwise for class 2 objects and for all class

3

objects,

a modified inspection and interpretation procedure will

be required to allow for the curvature of the object.

For class4 nd

5

objects special data processing

techniques and operating conditions will apply.

Computer algorithmswill be useful in analysing the

data in these cases.

To

confirm imperfection detection capabilities, the use

of representative test specimens

with natural flaws or

artificial defects is strongly recommended in these

cases as well.

10

Limitations of the

technique

This

clause considers the limitations of the

TOFD

technique and

is

e q d y applicable to basic

TOFD-detectionas

well

as

to TOFD-sizing. The

limts

of achievable accuracy under normal conditions are

defined and the influence of dead zones, which can

affect detectabihty, is discussed. It is important to

realize that the overall reliability of the technique is

determined by a large number of contributing factors

and the overall error will not be less than the

combined errors discussed in thi s clause.

Defects which are highiy tilted or skewed, such as

transverse cracks in non-parallel scans, are likely to be

more àifñcuit to detect and it is recommended that

specific demonstrations

of

capability are carried out in

such cases. In addition flaws which are not serious,

such as point defects, have some ability to mimic more

serious flaws such

as

cracks. Once again it is

recommended that the abiìity to distinguish

smal l

cracks

is

demonstrated, where appropriate.

Demonstrations of capability can be specific to the

inspection or can be referred back to other

documented data.

10.1 Precision and resolution

A distinction should be made between precision and

resolution. Precision is the degree to which the

position of a reflector or diffractor can be determined,

whereas resolution defines the degree to which closely

spaced W actors c m be distinguished from one

another.

The precision of a TOFD-measurementwiii be

influenced by timing errors, errors in the sound

velocity, probe separation errors and errors in the

assumed lateral position of an indication. Under

normaì circumstances the overall precision will be

dominated by the latter,

10.1.1Errors in the lateral position

As stated in 8.1.3, the lateral position of an indication

is n o d y ssumed to be mid-way between the two

probes.

In

reality the indication will be located on

an

eìlipse [equation

(3)].

The error in depth

(Sd)

due

to

the error in lateral position (Sy) can be calculated by:

6d=(c2 t2 -@)

( 6 ~ 2 / ~ 2 t 2 ) l [ ( 0 , 2 5 - 6 ~ 2 / ~ 2 t 2 ) ] ' / 2

4)

In principle, the lower edge of the acoustic beams

determines

6y.

If no reliable information on the lower

beam edge is available,

Sy

=

S shall be used

10.1.2 riming errors

The limit of precision in the depth of an indication,

due to

t i mng

errors (Et), can be estimated from:

where

ôd = c6t[d2

+

$1 1 2 d

(5)

6d

is the error in

d.

The timing error can be reduced by using a shorter

pulse

andíor a higher frequency.

10.1.3Errors in sound velocity

The limit of precision in the estimate of the depth of

an indication, due to errors in the sound velocity k) ,

is given by:

This error

is

reduced if the probe separation

is

reduced. Independent calibration

of

the velocity by

measurement of the delay of the backwali echo, with a

known wall thickness, greatly reduces ths error.

d

=

6c[d2+9

S(d2

+ $) I I Cd

(6)

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10.1.4 Errors in probe separation

Errors in the distance between the index points (ss)

will result in errors in depth-measurement. The error in

depth 6d can be calculated by:

o

It should be noted th t errors in probe separation can

arise from both measurement errors in the distance

between the probes,

as

well

as

errors in the index

point caiibration.

When the probe separation is smaller than twice the

specimen thickness, the index point can no longer be

considered a

fixed point, but it becomes a function of

depth In this case, if accurate sizing is required, the

depth measurement shallbe caiibrakd with the aid of

a

representative test specimen.

10.1.6 Spatial resolution

The spatial resolution (R)s

a

function of depth and

can be calculated by:

where

6d

= w ( d 2 +@) s] l d

R

=

[C2 td

+ $J2/4 1

d

8)

tp is the length of the acoustic pulse and t is the

timeof-flightat depth d .

The resolution increases with increasing depth, and

can be improved by decreasing the probe separation

or

the acoustic pulse length.

10.2 Dead

zones

Near the scanning surface there

is a

dead zone (D&)

due to presence of the lateral wave. Interference

between the lateral wave and the imperfection

indication can obscure the indication. The depth of the

socalied scanning-swface dead zone

is

given by:

(9)

Near the backwall there is also a dead zone ( w) due

to presence of the backwall+xho. The depth of the

backwali dead zone

is

given by:

where

D = [ c2 t2 1 4

+

scfpp

DdW= [$(& +

fp 2

/

4

1 - w

(10)

is

the time-of-flight of the backwall echo and

W

is

the wall thickness.

11 T O D examination without data

recording

In

manualiy applied TOFD, where interpretation is

obtained directly from the A-scan, unrectifíed display

of the signais shall be

used

This

form of the

TOF'D

technique should

only

be used

on product classes with simple geometries, and the

equipment Set-up shali comply with the requirements

of 7.2,7.3 and 7.4.

In general it

will

not be possible to perîorm the

detailed investigation of any response

th t is

possible

with recorded data It

will

be more difficult to detect

phase changes, slight changes in transit m e nd

defect echoes close

to

the lateral wave.

12 Examination procedure

TOFD examination procedures shall comply with the

requirements given in EN 583-1,

as

applicable.

Specific conditions of application and use of the

TOFD

technique

will

depend on the type of product

examined

and

specific requirements, and

will

be

described in written procedures.

13 Examination report

TOFD examination reports shall comply with the

requirements given in EN

583-1, as applicable.

In addition,

TOF D

examination reports shall contain

the following information:

description of the test specimen or reference

block, if a test specimen or reference block has been

used;

robe type, frequency, angle(s), separation and

position with respect to a reference line (e.g. weld

centre line);

lotted images (hard copies) of at least those

locations where relevant indications have been

detected. Details of equipment

settings

and method

of setting test sensitivity.

Furthermore,

al

raw data recorded during the

examination, stored on a magnetic or optical storage

medium such ashard disk, floppy disk, tape or optical

disk shall be kept for later reference.

Both dead zones can be reduced by decreasing the

probe separation or by using probes

with shorter puise

length.

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O

Annex

A

(normative)

Reference blocks

The purpose of reference blocks

is

to set the system

sensitivity correctly and to establish sufficient

volumetric coverage.

The

minimm

requirements of a reference block are

the following:

a) it should be made of similar materiai as the object

under inspection (e.g. with regard to sound velocity,

grain noise and surface condition);

b) the wall thickness

shall

be equal

to

or greater

than the nominal wall thickness of the object under

inspection;

c) the width and the length of the scanning surface

shall be adequate for probe movement over the

reference diffractom.

.--a

Measurements

shall

be based on the diffracted signals

from reference diffractom These are either:

a) machined notches, open to the scanning surface

of the reference block or

b) side drilled holes with a diameter of at least twice

the wavelength of the nominal frequency of the

probes utilized in the inspection. The holes should

be cut to the scanning surface in order to block the

direct reflection í3om the top of the hole,

see F'igure A.1.

Reference diffractors should be present at

approximately 10 %,

25

%, 50 %, 75 % and 90 % of the

nominal thickness of the object under inspection.

O

O

Legend

a Sawcut

b

Side drilled hole

Figure A . l Sketch of a reference block, using side drilled holes, connected

to the scanning surface by means of a scan cut, as reference reflectors

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National annex

NA

(informative)

UK comments

on

the

text

of

ENV583-6

NA.l General

During the development of ENV 583-6, the UK

expressed concern about some

of

its provisions.

Particular attention

is

drawn to the points outlined

in

NA.2 to NA.9. In

addition, throughout the text the

t e m defect , fiaw , discontinuity and crack are

used usually, but not always,

to

describe imperfections,

and there

is

concern

that thi s

might cause confusion

NA.2 Coordinate definitions

(see Figures 1 , 4 and 5)

The labelling of the axis in Figures

1,4

and5

is

not

consistent.

Within

the

UK

it

is

common practice

to

adopt the convention shown in

Fïgure 1,

with the

addition of

defuied

+Y/-Y

directions.

This

would

avoid, for example, possible ambiguity for parallel

scans

carried out transverse

to

the weld directions.

NA.3 Personnel qualincation

(see clause

6)

Clause

6

should permit the use of suitable schemes

other thanEN 473; thi s should be by agreement

between the contracting parties.

NA 4 Probe selection

(see Tables

1

and

2)

The recommended crystal sizes and probe angie ranges

in Tables and

2

do not fuily represent current

practice within the

UK.

For

example, for wall

thicknesses from

10

mm up to

30mm,

crystal sizes

of

10mm

and nominal probe angies of

45

have been

used.

NA.6 Probe separation

(see

7.2.2)

For

guidance, when

inspecting

the

fulì test

piece

thickness with

a

single pair

of

probes, the probe beam

centses should intersect at

half

to

two-thirds

of the

thickness.

NA.6 Sensitivity setting

(see

7.4)

ENV

583 6

attempts

to

detail

a

specific method

for

sensitivity setting based on

mated grain

noise.

BS

T706

includes alternative methods which UK

industry may wish to retain. If the sensitivity setting

is

based on material

grain

noise, it

is

essential that it

is

checked

as

described in annex

A

NA.7 Checking system performance

(see

7.7)

The

use of

calibration blocks

is a

suitable method for

pre- and post-inspection sensitivity checks, in which

case the maximum allowable Merence in signai

amplitude should be agreed between the contracthg

parties.

NA.8 Characterization

of

discontinuities

(see

8.1.2)

It should be noted th t parallel scans may be needed

in addition

to

non-paraiiel

scans

for characterization of

certain types of indication, e.g. for embedded

discontinuities.

NA.9 Errors in the lateral position

There

is

an error in equation

(4),

which should read.

NA.10 Reference blocks

(see annex

A)

It is important to establish the percentage

full

screen

height to which the

maximum

amplitude response

from the reference

diffractors

should be set. This is

important to ensure repeatability between different

inspections.

It should be noted

that

ENV

5û3-6

permits the use of

blocks with either machined notches or side-drilled

holes.

Figure

A l is

not drawn

to

scale and attention

is

drawn

to

the fact

that

the horizontal separation between holes

will be much larger

than

shown and it

may

be more

practical

to

produce multiple blocks.

6d

=

(c2t2

4s2) 63

c2t2 / (1 42 2CY)

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greement. Details and advice can be obtained from the Copyright Manager.

Fel 020 8996 7070.

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Documents

BS DD ENV 583-6 Ed.2000