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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
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WITHOUT
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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.
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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
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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
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fur
Normung
Central Secretariat:
rue
de
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36,
B-1060 Brussels
O
2000 CEN
All
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any means
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Members.
Ref.
No.
ENV
583-6:2000
E
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STDmBSI
D D ENV 583-6-ENGL 2 0 0 0 b 2 1 i b b 9 0 8 5 9 0 7 3 0 7 2
W
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,
Non-destructive testing ltrasonic
examination art : Generalprinciples.
EN 583-2, Non-destructive testing ltrasonic
examination art 2:Sensitivity and range
setting.
EN
583-3,
Non-destructive testing ltrasonic
examination art 3: P a m i s s i o n technique.
EN 583-4, Non-destructive testing ltrasonic
examination art 4: Examinat ion
for
discontinuities
perpendicular to
the surface.
EN
583-5,
Non-destructive testing ltrasonic
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
O
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07 2000
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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.
EN 583-2, Non-destructive testing ltrasonic
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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ENV 583-6:2000
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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ENV 683-6:2000
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
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~
~
~
~
STD-BSI D D E N V
583-6 -ENGL
2ODü L b 2 4 6 6 9 Oô59077
718 m
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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Page 7
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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
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ENV
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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ENV 5 3- 2000
Centre Crystals ize Nominal
probe angle
Wa ll
mm MHZ
mm
thickness frequency
c
10 10 15
2-6 50
70
10to
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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.
O
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ENV 583-6:2000
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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ENV 583432000
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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ENV 583452000
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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DD ENV 583-6:2000
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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addressed to Customer Services. Tel 020 8996 9001.
Fax: 020
8996 7001.
in
response to orders for international standards, it
is
BSI policy to supply the BSI
implementation of those that have been published as British Standards, unless
otherwise requested.
Information on standards
BSI provides a wide range of information on national, European and international
standardsthrough its Library and its Technical Help to Exporters Service.
Various
BSI electronic information services are also available which give details on al its
products and services. Contact the Information Centre. Tel: 020 8996 7111.
Fax:
20 8996 7048.
Subscribing members of BSI are kept up to date with standards developments and
receive substantial discounts on the purchase price of standards.For details of
these and other benefits contact Membership Administsation. Tel:
020
8996 7002.
Fax:020 8996 7001.
Copyright
Copyright subsists in all BSI publications. BSI also holds the copyright, in the UK, of
the publications of the international standardization bodies. Except aspermitted
under the Copyright, Designs and Patents Act 1988no extsact may be reproduced,
dored in a retrieval system or transmitted in any form or by any means electronic,
photocopying, recording or otherwise without prior written permission from BSI.
l’his does not preclude the free use, in the course
of
implementing the standard, of
necessary details such
as
symbols, and size, type or grade designations.
If
these
details are to be used for any other purpose than implementation then the prior
written permission of BSI must be obtained.
[f permission is granted, the terms may include royalty payments or a licensing
greement. Details and advice can be obtained from the Copyright Manager.
Fel 020 8996 7070.
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