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7/25/2019 Flaw Characterization TOFD PISC
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C o m m i s s i o n of t h e E u r o p e a n C o m m u n i t i e s
Nuclear Science and Technology
Shared Cost Act ion
Reactor Safety Programme 1985-1987
PISC II:
PARAMETRIC STUDIES
Flaw C harac terisa tion using the TANDEM an d TOFD Techniques
Final Report
Work per form ed in the f rame of the
Shared Cost Action (SCA) programme 1985-87
CEC, JRC Shared Cost Action Contract
No . 28 71-85 -12 EN ISP - GB
Authors:
R.A. Murgatroyd, P.J. Highmore, T. Bann
UKAEA, RNL, Risley - United Kingdom, Atomic Energy Authority
Risley Power Development Laboratory, Risley, Warrington, Cheshire, WA3 6AT
S.F. Burch,A.T.Ramsey
UKAEA, AERE, Harwell - United Kingdom Atomic Energy Authority
Atomic Energy Research Establishment, Harwell, Didcot, Oxfordsh ire, OX 11 ORA
This repor t has bee n ap pro ve d a nd a uthor ized for pub l ica t ion
by the PISC III M an ag em en t B oard a t i ts mee ting of
D ec em be r 15, 1988 as PISC III - Rep. No. 4.
Di rectora te-Genera l for Science Research and D evelopm ent D/
Joint Research Ce ntre - Ispra Site ' '
Se ptem ber 1989 I C L A EUR 12431 EN
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Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
Batiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person
acting on behalf of the Commission is responsible for the use which might
be made of the following information.
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1989
ISBN 92-826-0989-8 Catalogue number: CD-NA-12431-EN-C
ECSC- EEC - EAEC, B russels-Luxembourg, 1989
Printed in Italy
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1 . INTRODUCTION
Studies on the effect of flaw characteristics and selected inspection parameters on the
detection and sizing of flaws in ferritic steel blocks have been performed by the
United Kingdom Atomic Energy Authority (UKAEA) as part of a larger Commission
of the European Communities (CEC) Parametric Study programme. The techniques in
cluded in the UKA EA studies were the 45 tandem and the tim e-of- f ligh t diffraction
(TOFD) techniques.
The purpose of the work was to acquire reliable experimental data that could be used
both to test and verify theoretical models and to contribute to the resolution of
anomalies encountered in PISC round-robin inspection exercises. To this end, the data
were gathered in a format compatible with that of the theoretical predictions and
scanning parameters were employed that would test the theoretical models over a range
of inspection conditions.
For the studies, eighteen test blocks were fabricated by Ispra in which a range of flaw
types were inserted covering flaw shape, size, roughness and orientation. Thirteen of
these were selected for the UKAEA programme on the basis of their relevance to the
validation of theoretical models and their value to flaw characterisation studies.
The work involved in the programme was shared between Risley Nuclear Laboratories
(RNL) and AERE Harwell, with the teams inspecting to similar procedures. The f irst
series of test blocks and probes for use with the tandem technique was received in June
1986 and commissioning of scanning and data gathering equipment commenced. This
part of the prog ram me was com pleted e arly in 1987. Tw o test plates suitable for use
with the TOFD technique were received in May 1987 and the experimental scanning
received high priority in the UKAEA to enable the plates to be despatched to
Association Vincotte at the end of May 1987, in accord with the overall programme.
This f inal report describes the programme, data gathering procedures and the results of
the studies.
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2 .
THE TANDEM INSPECTION PROGRAMME
Following a me eting at Association Vin cotte [1], it was clear that to obtain t and em data
which would test theoretical models adequately the extent of scanning of a given flaw
needed to be increased to include angles of skew between the flaw and the ultrasonic
beam. Since this would represent a significant increase in the work involved it was
agreed that a reduction would be made in the number of flaws examined to enable the
increased scanning to be performed within the agreed workload. An assessment was
made of the flaws of interest to the CEC Theoretical Modelling Action, and the
resulting list of flaws selected for inspection by the tandem technique is given in
Table 1. This results in a total of 90 tande m scans being required com pare d to the co n
tractual requirem ent of about 30 scans. Also, the data was gathered at 1 mm step inte r
vals rath er tha n 5 mm in order to improve the sensitivity of the data . Thus it is con
sidered that signif icantly more experimental work has been performed than contractu-
rally required.
2.1 Description of test blocks
The tes t b locks (EDC-2-0-9 and EDC-40-8) and a reference b lock (EDC-REF-2) were
inspected at RNL using the tandem technique. They were fabricated from reactor grade
ferriti c steel (type A SME SA 533 B Class 1) and were uncla d. Ea ch block conta ined
several machined artificial reflectors (side-drilled holes and flat-bottomed holes [fbh's])
simulating flaws of which only one in each block was suitable for scanning with the
tandem technique. Block ED C -R EF -2 contained a 6 mm dia fbh , E D C -2 -0- 9 con
tained a 25 mm dia re-ent ran t machined fbh and ED C- 40 -8 contained a 25 mm dia
meter shrink fit fbh. All the holes had smooth end faces and were untilted to ensure
specular reflection of ultrasound. The overall dimensions of the test blocks and relevant
flaws are shown in F igure 1 and details of each reflector scanne d are show n in
Figure 2.
A total of nine test blocks (illustrated in Figure 7 is a representative block) containing
a variety of recta ngula r, sem i-infinite diffusio n-w elded strip flaws we re inspected at
AERE. The flaws were oriented at various angles of tilt relative to the inspection sur
face of the test block and had either a rough or smooth surface finish.
Details of all the flaws scanned using the tandem technique are given in Table 1.
3 . DATA GATHERING PROCEDURES FOR THE TANDEM TECHNIQUE
The work involved in the programme was shared between RNL and AERE as
described previously. Similar ultrasonic probes and scanning procedures were employed,
although there were some differences in equipment. The main aspects of each data
gathering system are outlined below.
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3.1 The RNL inspection procedure
A minicomputer based data-gathering system was used at RNL in conjunction with a 1
metre square rectilinear scanning frame to perform the specified scans. Scanning pro
ceede d in steps of 1 mm w ith the rectified an d smoothed ultra sonic signal being dig iti
sed and recorde d at each position. For one set of scans on one flaw the unrec tified R F
waveform was also recorded. The probes were Krautkramer WB45, 2 MHz, supplied by
JRC Ispra. A Reflectoscope S80 flaw detector provided the ultrasonic signals which
were digitised by a Tektronix Digital Storage Oscilloscope and subsequently analysed.
A single scan was made along the cent re-l ine of a 6 mm d iam eter flat bottom ed
calibration hole to provide a reference signal level (Figure 3). More extensive scanning,
including probes skewed at various angles to the reflector face was performed for the
25 mm diameter reflectors in blocks 2-0-9 and 40-8 (Figure 4). Scans were made along
the centre line of the reflector a t skew angles of 0, 5, 10 and 15 and at the same
angles off the centre line by +12 and +24 mm (except at 0 skew w hen only +12 and
+24 mm scans were re quire d since the -1 2 and -2 4 mm scans would be ide ntica l). In
total, there fore , 18 sets of results have been obtaine d for each flaw. How eve r, in order
to capture the complete echo-dynamic it was necessary to perform each scan in two
parts , giving a total of 36 scans. For the flaw in block 2-0-9 both the RF and rectified
waveforms were digitised giving a total of 72 scans for this flaw.
All scans were made in the same direction, namely in the +X direction, and the en
point of each scan was noted from the reflector so as to elim inate the effect of tak e-
up or backlash in the scanner and probe holder at the start of the scan. In order to
record the entire ultrasonic response (i.e. down to at least 20 dB below the peak value)
it was necessary to perform each scan in two halves, including the peak response in
both halves to serve as an added check on the X-location of the probes.
Data such as the equipment settings and scan details were recorded and a detailed log
book has been maintained.
3.2 The AERE inspection procedure
The two Kra utkr am er WB45 transducers (frequency = 2 MH z, crystal dimension =
20 x 22 mm ) supplied by Ispra w ere m ounte d in the tan dem config uration using a
specially designed holder , which maintained a constant separation between transducer
emission poin ts of 224 + 2 mm for blocks 20 -14 , 20 -16 and 20 -18 an d a separation of
280 + 2 mm for blocks 20-2 to 20-12.
The tandem probe holder was mounted in a steppe r-mo tor driven x -y scanning frame,
under computer control. Linear scans were made over the centre line of the three de
fects ,
with skew angles 0 and 15, i.e. a total of six linear scans were recorded from
the defects. Raster (x-y) scans (zero skew) over the calibration reflector were made
before and a fter the two sets of defect scans having ske w angles of 0 and 15 . The
steppin g interva l of the defect scans was 1 mm , while for the calibratio n scans the i n
terval was 1 mm in both the x and y directi ons.
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The equipment used for recording the ultrasonic data digitally was similar to that
described by Carte r and Slesenger [a]. The ultrasonic electronics w ere standard Harw ell
units mounted in a CAMAC crate. A LeCroy (type 2256) 20 MHz waveform digitiser
was used to digitise the unrectified (RF) ultrasonic signals. For each waveform, 1024
successive samples were stored to an accuracy of 8 bits, giving digital data for a con
tinuous period of 51.2 us. The initial time delay on the first digitised sample was ad
justed to 165 us, so that the signals of interest were centred in the digitised section of
the waveforms.
The operation of these units was controlled by an LSI 11/23 computer, which was also
used to average 64 independent waveforms from each scan position, thus reducing
random electrical and acoustic noise. After completion of each linear scan the digitised
RF waveform data were stored on computer disc in a single file, known as a B-scan.
The B-scan files recorded on the LSI 11/23 data collection computer were transferred
to a VAX 11/750 minicomputer linked to International Imaging Systems display devices
for subsequent processing and analysis.
To facilitate comparison with theoretical predictions, the variation of peak signal
amplitude with transducer position was derived from the B-scan data using the follow
ing method. The pulse envelopes of the digitised RF waveforms were first computed
using the analytic signal method [s], using fast Fourier transforms to calculate the
necessary Hilbert transforms. This enabled the peak signal amplitude to be derived ac
curately for each transducer position, thus avoiding any under-estimates caused by the
20 MHz digitization.
The peak signal strengths from the calibration scans were then derived. The maximum
difference between the calibration signal strengths obtained before and after the defect
scans was only 0.1 dB . The averages of the very similar ca libration signal strengths
were then used to express the defect signal amplitudes in dB relative to the peak signal
from the standard 6 mm diameter calibration reflector.
4. RESULTS OF TANDEM INSPECTIONS
The results of scanning the test blocks listed in Table 1 with the tandem technique are
given in Tables 3 and 4.
4.1 Table 3 gives the peak signal amplitude in dB relative to a 6 mm FBH reflector
for the flaws in Blocks EDC 2-0-9 and 40-8. These flaws were respectively a re
entrant FB H and a shrink -fit FBH , both 25 mm diameter and at a depth of
82.5 mm (Figu res 1, 2). The variation in signal amplitude with horizontal scan
distance, X, is illustrated in Figure 5 for Block EDC 2-0-9. The results for skew
angles up to 15
s
are compared. The echodynamic curves obtained on Block EDC
40-8 are displayed in a similar manner in Figure 6.
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4.2 Table 4 gives the peak signal amplitude in dB relative to a 6 mm FBH reflector,
for the two sizes of strip flaws studied, and skew angles of 0 and 15. The
details of the flaws included in this part of the programme are illustrated in
Figure 7 and Table 1. The variation in signal amplitude with scan distance X, is
illustrated in Figures 8 to 13. In the Y -direction, the echodynamics for smooth
and rough flaws are shown in Figures 14 and 15 respectively. For the rough
flaws the curves show significant variability along the flaw edge with a tendency
for a minimum near the centreline position.
5. DISCUSSION OF TANDEM RESULTS
5.1 The amplitudes obtained for 0 skew and tilt scans on the re-e ntra nt 25 mm flat-
bottomed hole (block 2-0-9) and the 25 mm shrink-fit (block 40-8) defect agree
to better than 0.1 dB (Table 3). Differences exist in the data obtained for these
reflectors for other tilt and skew conditions but the trends observed are similar.
This is illustrated in Figures 16 and 17 for 0 and 15 skew angles respectively.
5.2 Skew has a pronounced effect on signal amplitude. The results for the re-entra nt
FBH in Block 2 -0 -9 , F igure 18, shows in excess of a 40 dB decrease in peak
signal amplitude on the flaw centreline as the skew angle increases from 0 to
15. A similar decrease occurs for the strip flaws. This is due to the loss of much
of the specular reflection as the beam skews with respect to the flaw. As the
tandem system scans parallel to the flaw in the skewed orientation, peaks appear
at the position of the flaw edges which are in excess of 30 dB below the
maximum centreline value. The peaks at the flaw edge are attributed to edge
diffracted waves. It is conceivable that this phenomenon could lead to incorrect
flaw characterisation and sizing with amplitude-based techniques.
5.3 Am plitude peaks are not observed for the strip flaws since transverse scanning
was not included in the studies. However, it is anticipated that the behaviour
would be similar at the edge of the flaws.
5.4 The studies on the strip-flaws (Table 4) included flaws with either rough or
smooth crack faces, and a significant difference in behaviour occurred between
the two types (Figure 19). The peak amplitude for the smooth flaws decreased by
over 40 dB along the flaw centreline as skew increased from 0 to 15. Flaw tilt
further decreased the signal. A combination of 15 skew and 15 tilt resulted in a
55 dB decrease in amplitude for scans along the flaw centreline.
5.5 Rough flaws, as defined in the sample studied , are less affected by skew than
are smooth flaws.
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5.6 A noteworth y feature of the results is that the signal amp litude from "rough"
flaws exceeds that from smooth flaws for skew angles above 5 to 10, for the
conditions studied.
5.7 The eff ect o f tilt along the centreline of the flaw is less pronou nced than that of
skew for the strip flaws (Figure 20) and decreases with increasing flaw skew.
5.8 The results for the 25 x 125 mm smooth strip flaws (Figure 21) show similar
trends for tilt and skew to those of the smaller strip flaws.
6. THE TOFD INSPECTION PROGRAMME
6.1 Description of Test Blocks
The test blocks chosen for the UKAEA TOFD studies contained sharp-edged, circular
flaws wh ich in some cases were within 3 mm of the surface, in others were closely
spaced, multiple discs. It was recognised that this geometry presents difficulties to the
TOFD technique due to the weak diffracted signal or the proximity to the surface of
the test block, and the flaws were selected to enable a detailed assessment of the
capability of TOFD to be made under onerous conditions. The results are considered to
be representative of capability under extreme conditions.
6.1.1 Test Block EDC 20 24
The test block inspected by RNL (EDC-20-24) contained two untitled (i.e. normal to
inspection surface) 25 mm diameter near-surface d iffusion welded flaw s, one w ith a
"rough" surface finish and one "smooth".
There was insufficient information contained in the drawing to identify which of the
flaws was "rough" and which "smooth" and so the flaws were identified as "FLAW A"
and "FLAW B" with a test block ident ify mark acting as a reference point.
6.1.2 Test Block EDC 20 20
The test block inspected at Harwell contained two composite defects at equal depths
from op posite face s. Each defect appeared from the drawing (Figure 22) to contain
three coplanar vertical 10 mm discs separated from a parallel 40 mm diameter disc by
10 mm in the x-direction. The near-surface edges of the 40 mm discs were at a depth
of 15 mm from opposite faces. This specimen was originally constructed for pulse-echo
and tandem work and the defects were near one end. In order for TOFD to be used,
therefore, an extra block had been welded on to the end of the original specimen at
Ispra, thus extending the block by some 75% over that show n in Figure 22 .
Details of the flaws in both test blocks are given in Table 2.
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7. DATA GATHERING PROCEDURES FOR THE TOFD TECHNIQUE
7.1 The RNL inspection procedure
The same scanning equipment as used for the tandem studes was used for the TOFD
scans but a Harwell "Zipscan" digital ultrasonic inspection system was used to drive the
versatile scanner and rectilinear scanning frame via an independent microprocessor-
based scan controller developed at RNL. The ultrasonic probes were scanned over the
flaw in 1 mm steps and at each scan position the u nrectified RF ultrasonic waveform
was digitised by a 21 MHz 8-bit D/A converter into 512 points, resulting in a digitised
window approximately 25 ys long. A high speed hardware signal averager was used to
average 256 waveforms at each position to improve the signal-to-noise ratio of the
waveform. Such averaging is essential for processing the often weak diffracted signals
encountered in TOF inspection.
The digitised RF ultrasonic waveforms, i.e. A-scans, were displayed on-line as a grey-
scale B-scan and stored on an integral hard disc at the end of each linear scan.
Analysis of the data to extract flaw depth and dimensions was performed either on
Zipscan or the B-scan s w ere transferred to the ND T department's DE C Microvax II
multi-user computer system for subsequent processing, display and analysis. Hard
copies of the data were made using a video copier from a monochrome video display
terminal.
The test block was examined from the two opposite faces and therefore each flaw was
inspected twice: one as a near-surface flaw and once as a deeply-buried flaw, close to
the backwall of the test block.
Probe separation was optimised for the detection of signals from the top, middle or
bottom of each flaw. In order to obtain the best response from the flaws several types
of probes were employed. In general, 60 compression probes were used for the near-
surface flaw and 45 compression probes for the buried flaw. The probes used in the
exercise were short pulse length, 2
1
'
4
MH z immersion types w ith 1/2 inch diameter
crystals of a type normally used in TOFD inspections at RNL. Scan lengths were
su -
ficien t to capture the full extent of the flaw signals, with data being recorded at 1 mm
steps.
Stand-off immersion scanning was performed with the probes fixed at the correct
angles in teflon probe bodies. There were two types of scan in mutually orthogonal
direction s (Figure 23) . In on e, the probes were scan ned, together (at a fixed probe
separation) in a direction normal to the face of the flaw. This is termed a longitudinal
scan. In the other the scan direction was parallel to the face of the flaw with the
probes positioned symmmetrically about the centre of the flaw. This is termed a trans
verse scan. This was performed at skew angles of 0" and 15 relative to the flaw. All
the above scan patterns are shown in Figure 23.
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7.2 The AERE inspection procedure
A series of scans were carried out at Harwell using the Time of Flight Diffraction
technique on block PISC -ED C-20 -20. A pair of 12 mm diam eter, nominally 45 MHz
G5KB probes were mounted in perspex shoes which gave a beam angle in the steel of
60 degrees. The probes were acoustically coupled to the block with a light oil. The data
were gathered at 1 mm stepping intervals using the same hardwa re as in the tandem
measurements (see Section 3.1). The received data were amplified using a wide band (1
to 30 MH z) amplifier and digitized using a LeCroy 2256 20 MHz waveform digitizer
linked to an LSI 11/23 com puter. A 40 dB pre-amplifier was used where necessary to
increase the intensity of the received signal.
Initially, coarse raster scans were performed to find the position of the defect. Two
perpendicular sets of scans were performed with a raster spacing of 5 mm. From these
the probe position where the maximum in tensity of the signal from the 10 mm defect
occurred was found. This was 710 mm from the unwelded end of the original block
(before extending for TOFD) in the x-direction, and on the block centreline in the y-
direction. This position agreed well with that shown on the block drawings supplied by
JRC, Ispra (Figure 22).
In order to achieve accurate defect sizing for all depths in the block, different probe
separations were used for:
a) the top of the upper defect;
b) the bottom of the upper defect and;
c) the lower defect.
Transverse and longitudinal scans (see Figure 24) were then pe rformed at angles of
skew of zero and 15 degrees (achieved by skewing the block) for each separation - a
total of 12 B-scans. As seen from Figure 24 the transverse B -scans (at 0 degrees skew)
were performed across the block with the probes cen tred 710 mm from the original
end, and the longitudinal B-scans were performed along the centreline of the block.
Calibration scans were performed on blocks containing side-drilled holes at depths of
20 mm and 50 mm (close to the depth of the ends of the upper 40 mm disc).
The B-scans were transferred to the VAX 11/70 mini-computer for analysis using the
Image Processing hardware (International Imaging Systems I2S) and our specially de
veloped software. Techniques were available to measure accurately the arrival times of
the pulses and relate these to a depth measurement, assuming either two compression
paths or one compression path, one shear path.
Measurements on the calibration scans enabled the accuracy of the depth measurements
to be calculated. The calculated depths on these scans agreed to within 0.5 mm of the
actual depths.
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8 . RESULTS
OF TOFD INSPECTIONS
8.1
Results for Block
E D C - 2 0 - 2 4
Typical TOFD inspection data from the several scans performed on the test block are
presented as grey-scale B-scan displays (Figures 25 to 27), with the horizontal axis re
presenting probe movement and the vertical axis depth in the test block (both in mm).
Both axes are linear.
The average depths of the indications from the two flaws in the test block, extracted
from this data, are given in Table 5.
The B-scan display from the longitudinal scan over the near-surface f law (Figure 25a)
shows the bottom of the flaw located at a dep th of 30 mm , indicated by the a rc-l ike
signal response present at the expected depth of the f law extremity. The corresponding
transve rse scan (Figure 25b) shows the limite d region of the flaw over wh ich diffracted
signals are observed.
Th e indic ation from the top of the nea r-su rfac e flaw, althoug h visible as a distur
bance in the nea r-su rfac e wave (nsw) signal, was not resolved. This is to be expec ted
due to the close proximity of the top of the flaw to the surface of the test block (only
3 mm). However, the fact that the nsw was only attenuated and not totally interrupted
confirms that the f law is not surface-breaking. Further signal processing may enable
the indication from the top of the flaw to be extracted from the nsw.
The top of the buried flaw also appears as an arc-like signal (Figure 26), but this is
much weaker than for the near-surface flaw and is superimposed on horizontal signals
thought to be associated with the welds surrounding the flaw. Similarly extremely weak
signals were observ ed jus t in front of the back wall which suggest the loca tion of the
bottom of the buried flaw. However, these signals were on the limit of detection and
further processing and analysis is required to attempt to extract them from the large
backwall signals with which they merged.
In all cases the TOFD displays were complicated by extraneous signals, both arc-like
and planar, thought to be due to flaws in, and the properties of, the welds by which
the flaws were inserted in the test block.
When scans were performed with the probes skewed at an angle of 15 relative to the
face of the flaw, in order to simulate a flaw skewed relative to linear scanning axes,
multiple arc-like signals were observed instead of the usual single indication from the
tip of the buri ed flaw (F igure 27b ). No similar signals were observed for the ne ar-
surface flaw (Figure 27a).
Probe skewing also reduced the amplitude of the planar indications from the weld,
(also shown in Figure 27), easing observation of the arc-like signals and also enabling
the tip of the flaw to be located more accurately. It should be stressed, however, that
these complications with the fabrication welds are an artifact of the fabrication method
and are not necessarily a limitation of the TOFD technique.
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8.2 Results
for
Block EDC-20-20
The drawings of block 20-20 supplied by JRC, Ispra (Figure 22) showed that the upper
and lower mu ltiple defects each consisted of a 40 mm disc , separated from three
10 mm discs by 10 mm in the x -direction . The larger 40 mm disc was therefore
expected to shadow some or all of the signals from the smaller 10 mm discs. As shown
below, the TOFD results fully confirmed this interpretation of the defects.
Some recen t suggestions that the larger 40 mm discs were merely the (non -reflec ting)
peripheries of inserted plugs, with the defects comprising two groups of three 10 mm
discs are inconsistent with the TOFD results.
The longitudinal scan performed at the smallest separation (52.0 mm) shows a series of
defect signals (see Figure 28a). The,tops of the 40 mm diameter disc and the uppermost
10 mm disc (signal 1 & 2) are quite d istinct and appear at depths of 13.4 mm and
15.3 mm respec tively. The diffracted compression wave from the bottom of the
10 mm disc is not seen, presumably because it is shadowed by the 40 mm disc.
How ever, the mode-converted shear wave from the bottom of the 10 mm disc (signal
8) is observed (at a depth of 25.5 mm) as it travels at a steeper angle in the block and
emerges between the two discs. This gives a size of 10.2 mm for the small disc. Several
other signals appear whose arrival times and probable interpretations are summarised in
Figure 29 and Table 6. Some of these signals are reduced in amplitude (or do not ap
pear) on the scan performed under the same conditions when the block was skewed by
15 degrees (Figure 28b). This supported the suggestion tha t these signals involve
multiple reflections between the defects. Later analysis has shown that signal 5 could
also be due to a compression wave mode-converting to a Rayleigh wave which travels
up the 10 mm defect from bottom to top before reverting to a compression wave.
Although many signals appear on this scan, the defect locations and sizes can be
calculated only from signals 1, 2 and 8. All the other signals are fully consistent with
the presence of a large reflector separated in the x-direc tion by 10 mm from a 10 mm
high reflector.
At the intermediate probe separation (190.5 mm) strong signals from both the top and
bottom of the upp er defect are seen (signals 1 and 2 in Figures 30a and b) . The signal
from the bottom of this defect appears at a depth of 53.1 mm (see Table 7) giving a
size of 39.7 mm for the large disc . Signals 7 and 8 are the com pression-shear and
shear-compression waves from the bottom of the upper 40 mm disc, and signal 9 com
prises two signals formed by mode-conversions to Rayleigh waves which travel along
the upper 40 mm defect (one signal for a downwards travelling Rayleigh wave and one
for an upwards travelling wave). This twin signal (No. 9) disappears when the defect is
skewed by 15 degrees, suggesting that skewing the defect affects the generation of
Rayleigh waves. A vertical series of as yet unexplained signals appear on scans per
formed at both this and the largest probe separation (360 mm ) when either of the
probes is above the composite defect. It is possible that some kind of waveguide effect
may be occurring between the 40 mm and 10 mm discs. A few unexplained signals also
appear at apparent depths between the upper and lower composite defects. These dis
appear when the block is skewed, suggesting some form of multiple reflections have
occurred between the 40 mm and the three 10 mm discs.
10
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The scans performed on the lower defect (Figures 31a and b) show a signal at a depth
of 139 mm w hich is assumed to be from the upper edge of the comp osite defect (see
Table 8). Very weak signals appear at depths of 162 and 170 mm. A signal appears at a
depth of 180 mm on the skewed scans and corresponds to the bottom of a 41 mm high
def ect wh ose top is at 139 mm . This presumably does not appear on the scan p er
forme d at zero skew due to shadow ing by the low ermost "10 mm" disc.
Also on the two deeper sets of scans appear the mode-converted shear waves from the
bottom of the upper defect.
The TOFD results for Block 20-20 were fully consistent with both the upper and lower
mu ltiple defe cts being 4 0 mm diam eter discs saparated from (by 10 mm in the x-
direction) and shadowing 10mm discs.
Rega rding the up per or near-su rface def ect, the "40 mm" and upperm ost "10 mm" discs
were detec ted and sized as 39.7 mm and 10.2 mm re spective ly. T he "40 mm" disc was
located at a mean dep th of 33.3 mm and the "10 mm" disc was foun d to have a mean
depth of 20.4 mm . The depth measurement of the bottom of the "10 mm" disc was
achieved by studying the mode-converted signals, as the direct diffracted compression
signal was shad owed by the "40 mm" disc. No obvio us signals were observ ed from the
other two "10 mm" discs in this com posite d efect. The low er defe ct w as seen to contain
a 41 mm high vertical feature at a mean depth of 160.5 mm , but no unambiguous
signals were observed from the smaller defects in this cluster.
We would suggest that for a full evaluation of this defect, scans using pulse-echo and
tandem techniques in conjunction with TOFD be employed.
9. DISCUSSION OF TOFD RESULTS
The TOFD technique has been applied to the characterisation and sizing of flaw types
which were selected to provide an exacting examination of the capability of the
technique. It is concluded that:
9.1 The TO FD technique detected and located accurately the top and bottom of the
flaws accessible to the technique. Agreement with block fabrication data was
better than 2 mm.
9.2 Performance was limited by two inspection conditions. The first was where some
of the smaller flaws were obscured by larger ones, as seen in Block 20-20. This
would not necessarily occur with other techniques using single probes, such as
conventional pulse-echo. This indicates that whilst TOFD is a valuable sizing
technique, other diverse techniques should be included in an inspection for
reliable flaw characterisation. The second arose when defect edges lay close to a
surface making discrimination from the lateral wave difficult with standard
TOFD techniques. Further study on this aspect is planned by the UKAEA.
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9.3 The limited data obtained at a skew angle of 15 indicates the relative insensitiv-
ity of the TOFD technique to flaw skew. In view of this it is considered that
further studies should be performed on the Ispra test blocks over a wider range
of skew and on other flaw types.
10.REFERENCES
[l] Special Meeting of Param etric Studies Effect of the Defect Characte ristics. As
sociation Vincotte, February '86. 173/40-97/86/SC/rm.
[2] P. Carter and T. Slesenger - UKAEA Harwell Report AERE-R 10386, 1981.
[3]
P.M. Gammell
- Ultrasonics, 18, 73-76 1981.
11.
ACKNOWLEDGEMENT
The valuable assistance of A.J. Plevin and N. Bealing in the experimental measurements
is acknowledged.
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TABLE
1 -
Flaw
a nd
scanning detaits
f or
TANDEM
technique.
TEST BLOCK
IDENTITY
SIZE
(mil)
DEPTH
(mm)
FLAW PARAMETERS
TANDEM SCANNING PARAMETERS
COMMENTS
TYPE
TILT
SURFACE SKEW
RASTER
2-0-9
40-8
is 25
0 25
82.5
82.5
FBH, Re-entrant
FBH,
shrink-fit
5, 10, 15
5 , 10, 15
Centre,
+12, +24
Centre, +12, +24
Circular flaws
20-14
20-16
20-18
25 x 125
25 x 125
25 x 125
10 x 50
10 x 50
10 x 50
10 x 50
10 x 50
10 x 50
82.5
82.5
82.5
55
55
55
55
55
55
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip, DW
0
7
10
0,
o,
0,
0,
0,
0.
0,
o.
0,
15
15
15
15
is-
is
0
is-
is
15
Centre
Centre
Centre
Tilted strip flaws
20-2
20-4
20-6
20-8
20-10
20-12
0*
0
7
7
15
15
Centre
Centre
Centre
Centre
Centre
Centre
Smooth and rough tilted
strip flaws
DW = Diff usi on Welded Flaw ; S = Smooth Flaw Surfa ce ; R = Rough Flaw Surface
TABLE 2 - Flaws for TOFD techn ique studies.
TEST BLOCK
IDENTITY
SIZE
(mm)
DEPTH
(imO
FLAW PARAMETERS COMMENTS
TYPE
TILT
SURFACE
20- 20
3 - 18 37.5 Composit e, DW 0 S
Composite flaw
20-24 25 15.5 Compos ite, DW 0 R,S Near surfac e, sharp -edge d flaw
DW Dif fus ion Welded Flaw ; S = Smooth Flaw Surfa ce ; R = Rough Flaw Surface
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TABLE 3 - Results of TANDEM inspection of Test Blocks EDC 2- 0-9 and 40-8: peak flaw signal amplitude
relative to 6 mm diameter FBH reference reflector.
Flaws: 26 mm dia, 82.5 mm deep (Table 1).
S K E W A N G L E
(De,
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
jrees)
0
0
0
5
5
6
5
5
10
10
10
10
10
15
15
15
16
15
Y + A X I S O F F S E T
(mm)
0
+ 12
+ 24
0
+ 12
+ 24
- 12
- 24
0
+ 12
+ 24
- 12
- 24
0
+ 12
+ 24
- 12
- 24
P E A K S I G N A L A M P L I T U D E
E D C - 2 - 0 - 9
21
12
- 11
0
3
- 5
2
- 17
- 13
- 6
- 14
- 7
- 21
- 21
- 11
- 22
- 12
- 27
(dB)
E D C - 4 0 - 8
21
14
- 13
2
8
- 7
- 3
- 5
- 15
- 11
- 13
- 12
- 28
- 28
- 15
- 22
- 18
- 3
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TABL E 4 - R esults of TANDEM inspection of strip flaws: peak flaw signal amplitude relative to 6 mm FBH
reference reflector.
T E S T B L O C K
I D E N T I T Y
20- 2
20 - 2
2 0 - 4
2 0 - 4
2 0 - 6
2 0 - 6
2 0 - 8
2 0 - 8
2 0 - 1 0
20 - 10
20 - 12
20 - 12
20 - 14
20 - 14
20 - 16
20 - 16
20 - 18
2 0 - 1 8
SIZE
(mm)
1 0 x 5 0
1 0 x 6 0
1 0 x 5 0
1 0 x 5 0
1 0 x 5 0
1 0 x 5 0
25 x 125
25 x 125
25 x 125
D E P T H
(mm)
55
65
55
65
55
55
82.6
82.5
82.5
F L A W P A R A M E T E R S
T Y P E
Strip, DW
Strip, DW
Strip, DW
Strip,
D W
Strip, DW
Strip, DW
Strip, DW
Strip, DW
Strip,
D W
TI LT
(deg.)
0
0
7
7
15
15
0
7
10
S U R F A C E
R
S
S
R
S
R
S
S
S
S C A N
S K E W
(deg. )
0
16
0
15
0
15
0
15
0
15
0
16
0
15
0
16
0
15
M A X A M P
(dB)
+ 2.7
- 22.7
+ 1S.S
- 29.1
+ 0.6
- 36.5
- 7.6
- 15.8
- 8.2
- 41.0
13.6
- 23.6
- 18.0
- 27.0
- 4.0
- 37.0
- CO
- 34.0
DW = Di f fusion Welded Flaw ; S = Sm ooth Flaw Surface ; R = Ro ugh Flaw Surface
TA BLE 5 - S i t i ng r e s u l t s f r om TO FD i ns pe c t i on o f Te s t B l oc k ED C - 20- 24 .
F L A W
F L A W T I P D E P T H F R O M I N S P E C T I O N S U R F A C E ( m m )
A S N E A R - S U R F A C E F L A W A S D E E P L Y - B U R I E D F L A W
T O P
Not resolved
165.0
B O T T O M
30.0 + 1.0
Not resolved
T O P Not resolved 166.6 + 1.5
BO T TO M 28 . 0 + 0 .5
Not resolved
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T A B L E 6 - Interpretat ion of s ignals appearing in Figures 28a and b .
SIGNAL N o .
1
2
S
4
6
6
ARRIVAL TIME
p s e c
1.10
1.40
1.85
4.50
5.90
6.35
x / m m
3
- 10
-
0
- 3 to 5
- 14
- 7
C OMME N T
c / c from C
c / c from A
c / c
diffracted
from
A on t o C
c reflected off 40 m m
defect on to B , cto R
c to C,s t oA , c to R
c toB , diffracted to 40 m m
IN FE R R E D D E P T H
mm
13.4
15.3
defect, reflected to 10 m m ,
c toR al l compress ion
7.65
13 c toB , s to 40m m defect ,
c to 10 mm defect , ct o R
8.05
8.70
- 22
- 27
diffracted swave off B
c to B , c t 40mm defect ,
e
to
A ,
diffracted
shear
wave t o R
25.5
10
9.35
- 12 c to B , s to40m m defect ,
reflection to 10 mm defect ,
mode
convers ion
to
c
t o
R
c
=
compress ion
wave
s = shear wave
A = top of uppermost 10 m m defect
B = bo t t omof 10 mm defect
C = top of upper40 mm defect
*
s ignal
5
could
also
be
due
t o
a
Rayle igh
wave
travel l ing
up
th e
10
m m
defect
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TABLE 7 - Interpretat ion of s ignals appearing in Figures 28a and b.
SI G N A L N o .
1
2
3
4
5
6
7
8
9
10
11
A R R I V A L T I M E
( V"
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TA BL E 8 - Interpretat ion of s ignals appearing in Figu re 31a and b.
SI G N A L N o .
1
2
3
4
6
6
7
8
9
10
A R R I V A L T I M E
( u s e e )
2.8
7.4
15.9
14.6
14.6
18.1
17.7
22.7
20.9
25.2
x / m m
- 14 to 29
46 to 74
- 13 to + 7
- 144
147
- 170
174
- 13
6
10
C O M M E N T
e/c to bot tom of upper
40 mm defect
c/c from top of lower
40 mm defect
c / s to bot tom of upper
40 mm defect
s /c to bot tom of upper
40 mm defect
(0 deg. skew only)
(15 deg. skew only)
c /c from bot tom of lower
I N F E R R E D D E P T H
( m m )
65.6
138.7
52.8
62.8
170
162
180
40 mm defect
(15 deg. skew only)
c = compress ion wav e
s = shear wa ve
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L
W
T
D
REF-2
459.5
2C0
193.5
82 .5
50
2-0 -9
449.5
299
192.5
82 .5
30
40-8
600
299 .5
194
82 .5
30
Figure 1
- Dim ensions of test blocks and flaws e xam ined by R N L using the
TANDEM technique.
3J'
PLUG END-FACE
REFLECTOR
^
A
i
1
Of
>
RE ENTRANT HOLE
Figure 2 - Details of flaw sizes and shapes.
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PLAN VIEW
- X - *
centreline
scan only
Figure 3 - Scan for 6 mm dia. fla t-bo ttom ed hole (FBH ) in Block re f-2 and
coordinate conventions.
PLAN VIEW
Skew ang e
f
G=o:5; iO; i5 '
Figure 4 - Scan paths for Blocks 40- 8 and 2 -0 -9 .
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Y rO m m SKEW ANGLE * 0*, 5* 10* 15
>