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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.

    11

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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.

    12

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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

    14

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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

    15

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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

    16

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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

    18

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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.

    19

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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 .

    20

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    Y rO m m SKEW ANGLE * 0*, 5* 10* 15

    >