Table Of ContentPergamon Titles of Related Interest
ANTHONY
Engineering Metrology
ASHBY
Engineering Materials 1 and 2
BROOK & HANSTEAD
Reliability in Non-Destructive Testing
DUMMER
An Elementary Guide to Reliability, 3rd Edition
FAN & MURAKAMI
Advances in Constitutive Laws for Engineering Materials, 2-Volume Set
GARDNER
Improving the Reliability of Non-Destructive Testing
HARRISON
Structural Analysis and Design, 2nd Edition
HEARN
Mechanics of Materials, 2-Volume Set, 2nd Edition
HULL & BACON
Introduction to Dislocations, 3rd Edition
INTERNATIONAL INSTITUTE OF WELDING
Physics of Welding, 2nd Edition
MOTTERSHEAD
Modern Practice in Stress and Vibration Analysis
SALAMA et al.
Advances in Fracture Research, 6-Volume Set
ZHENG
Applied Mechanics, 3-Volume Set
Pergamon Related Journals
Acta Metallurgica
Canadian Metallurgical Quarterly
Engineering Fracture Mechanics
Fatigue and Fracture of Engineering Materials and Structures
International Journal of Impact Engineering
International Journal of Solids and Structures
Journal of the Mechanics and Physics of Solids
Materials and Society
Materials Research Bulletin
Physics of Metals and Metallography
Progress in Materials Science
Scripta Metallurgica
Solid State Communications
Welding in the World
(free spec/men copy gladly sent on request)
Impact
of
Non-Destructive Testing
NDT-89
Proceedings of the 28th Annual British Conference
on Non-Destructive Testing,
Sheffield, UK, 18-21 September 1989
Organized by the British Institute of Non-Destructive Testing
Edited by
C. BROOK
Wells-Krautkramer Ltd, Minworth, Sutton Coldfield, UK
and
P. D. HANSTEAD
Consultant, Bristol, UK
PERGAMON PRESS
Member of the Maxwell Macmillan Pergamon Publishing Corporation
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Copyright © 1990 British Institute of Non-Destructive
Testing
All Rights Reserved. No part of this publication may be
reproduced, stored in a retrieval system or transmitted
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copyright holder.
First edition 1990
Library of Congress Cataloging in Publication Data
British Conference on Non-Destructive Testing (28th:
1989: Sheffield, England)
Impact of non-destructive testing, NDT-89: proceedings
of the 28th Annual British Conference on Non-
Destructive Testing, Sheffield, UK, 18-21 September
1989/edited by C. Brook and P. D. Hanstead. — 1st ed.
p. cm
I. Non-destructive testing—Congresses. I. Brook, C.
II. Hanstead, P. D. III. Title.
TA417.2.B66 1989 620.1Ί27—dc20 89-48956
British Library Cataloguing in Publication Data
British Conference on Non-Destructive Testing {28th:
1989: Sheffield, England)
Impact of non-destructive testing.
1. Non-destructive testing
I. Title II. Brook, C. (Christopher), 1945-
III. Hanstead, P. D. IV. British Institute of Non-
Destructive Testing
620.1Ί27
ISBN 0-08-040191-0
Printed in Great Britain by BPCC Wheatons Ltd, Exeter
Preface
The annual conference of The British Institute of Non-Destructive Testing
offers significant benefits to those privileged to attend. Perhaps one of these
is to take part in or overhear some of the many discussions and comments,
some formal following oral presentations, many informal gathered around
an interesting poster presentation or during the social activity of the
conference. Unfortunately these discussions are not recorded and for the
purposes of this publication we must be content with the written papers
recording the formal presentations. Nevertheless the proceedings of the
conference provide a valuable record for those who attended the conference,
an insight into the state-of-the-art for those who did not.
The theme for the conference was 'The Impact of NDT\ As in previous years
ultrasonics played a large part in the presentations but all the major methods
of NDT were represented. The research sessions of the conference were
particularly well represented which is fitting in view of the recent
establishment of an active Research Group within the Institute.
This conference returned to the practice of parallel sessions, allowing a
greater diversity and depth in the proceedings. This practice was again
enhanced by the extensive use of posters with participants appreciating the
opportunity of personal discussions with the authors on specialised subjects.
This publication contains the majority of the papers presented at the lecture
sessions and the poster presentations; it forms a permanent record of
important contributions to the non-destructive testing literature.
The work of the Secretariat in organising the conference under difficult
circumstances was again appreciated and the work of the authors of both the
oral papers and the poster presentations is recognised and will be apparent
to those who study these proceedings.
C.Brook
Chairman, Technical Committee
British Institute of NDT
VIII
Technical Committee
Mr C.Brook(Chairman)
Mr F.LAldridge
Mr B.Bridge
DrJ.M.Coffey
Miss A.Duncomb
Dr J.M.Farley
Mr R.Griffen
Dr R.Halmshaw
Dr PJ.Highmore
Mr A.Hunscott
Dr M.W.B.Lock
Mr P.Mudge
Prof.S.B.Palmer
Dr M.G.Silk
Dr R.L.Smith
Dr J.O.Taylor
Mr A.B.Wooldridge
IX
Achievable Flaw Sizing and
Monitoring Precisions
M. G. SILK, A.D. WHAPHAM and C. P. HOBBS
The National Nondestructive Testing Centre
Harwell Laboratory, U.K.
ABSTRACT
With the introduction of techniques such as ultrasonic TOFD1, based on flaw
tip diffraction, and the electrical ACPD2 technique, the past decade has
seen a dramatic improvement in the precision of flaw sizing in NDT. Ultra-
sonic work in other countries has resulted in other approaches such as
FET3, utilising a focussed transducer, the ALOK4 technique and the SLIC5
technique, all of which can take account of tip diffraction to infer flaw
size.
These developments are of undoubted value and there has been an improvement
in our ability to measure the size of flaws in structures compared with the
mid 1970's. However, what does this improvement truly amount to? Do we
know enough about the true flaw size distribution in structures to infer
what degree of precision is acceptable? A number of blind trials have been
conducted and much data published but these results seem only to confuse
the issue, because the errors quoted range from 0.1mm to over 10mm. This
throws doubt on what can really be achieved in the field and questions the
very high precisions currently being strived for.
This paper attempts to unravel the real situation as it would apply to the
inspection of structures. The experimental evidence presented for the flaw
size distribution is used to make an estimate of the precision required in
one practical situation. In addition, the nature of the component errors
is considered, and this is used to assess the potential performance of NDT
in tasks such as defect detection, sizing and flaw growth monitoring, in an
industrial environment.
3
4
KEYWORDS
Nondestructive Testing; reliability; error; assessment;
INTRODUCTION
Nondestructive testing is used both to locate and to size flaws. A typical
inspection requirement will be to isolate all flaws above a defined size.
A perfect technique would do this and give rise to no false calls.
However, a real inspection will be subject to errors, causing it to be im-
perfect in both of these respects. It is rarely adequate therefore to set
a discrimination level at the flaw size of concern because, inevitably,
some larger flaws will be sized wrongly and placed below the limit and vice
versa. Normally the discrimination level is set lower so that few flaws of
concern are missed. The penalty for this is that many of the flaws which
now exceed the chosen threshold are false calls. The more precise the
technique the closer it is likely to be possible to come to the ideal, but
attempts to produce a quantitative assessment fail because the distribution
of flaw sizes in structures is not well known.
It seems possible, however, to produce a reasonable first estimate of the
quantitative distribution of flaws in structures and, when this is done, it
is possible to begin to assess the desirable sizing precisions for NDT. In
this discussion we concentrate on the effect of the precision of the in-
spection techniques. It is assumed that the flaws have been correctly lo-
cated using a suitable technique prior to the sizing step or that a
suitable technique is used for both location and sizing. Flaws may be
missed, mis-sized or misinterpreted because of the wrong choice of
technique or because of equipment or operator limitations6. These sources
of error must be treated separately and, as an example, the assessment of
human reliability6 suggests that a failure rate, of echoes missed for in-
stance, in the region of 5 in 104 is likely.
TYPES OF ERROR
A number of errors may contribute to the total error of measurement and
these can be placed into one of two groups: random errors and systematic
errors. In any inspection the random errors are the more obvious. They
arise from the process of measurement where repeated measurement of the
size of a particular flaw will give rise to similar but slightly differing
results. This represents the experimental error and defines the
reproducibility of the technique. It is well understood and normally
treated as following a normal (Gaussian) error distribution. It is
generally the smaller source of error in NDT sizing applications.
For most NDT techniques it is the systematic errors which are dominant and
5
these are often simply represented as a bias to the experimental data. In
other words, the distribution of errors is assumed to be the experimental
errors offset by a value representing the bias in the technique. A practi-
cal example of this assumption is the common statement that "the ACPD
technique always undersizes cracks". If this were true, and could be quan-
tified, there would be little problem with systematic errors. The trouble
is that the magnitude and sign of systematic errors is likely to vary from
flaw to flaw. A well known example is the variation in reflected pulse am-
plitude caused by variations in the orientation, roughness and transparency
of flaws. Measurements on any particular flaw show a high level of repro-
ducibility but the error may still be large as is shown up by inspection
trials which require measurements on a number of flaws. These variations
make amplitude a very poor estimator of flaw size, as is well known, but
systematic variations affect all NDT techniques in this manner and the ex-
istence of a simple bias is quite rare. Systematic error shows up in the
results of blind trials but may be misinterpreted as experimental error or
as simple bias. The effect that random errors and the two different types
of systematic error would have on a population of identically sized flaws
is indicated in figure 1.
How can the combined distribution of errors be represented? It is the sum
of the random and systematic errors and the former are assumed to follow a
normal error distribution. Turning to systematic errors, the comparative
rarity and the ability to calibrate makes it acceptable, as a first approx-
imation, to ignore simple bias. The treatment of the remaining systematic
errors is more complex. For a given flaw the errors represent a bias^ but
over a distribution of different flaws, which must be considered in
evaluating the general performance of a sizing technique, the errors will
operate in a more random manner. This is the basis behind our use of a
normal distribution as an approximate description of these errors. As
these are combined with random errors and simple bias is ignored, an over-
all normal distribution of errors is assumed in this paper. There is scope
for further refinement of the treatment as better data is assembled.
THE DEFECT SIZE DISTRIBUTION
It is usually assumed that there are larger numbers of small flaws than
large flaws with a schematic population of the kind shown in fig 2. The
form of this curve is of fundamental importance in inspection. Its steep-
ness defines the proportion of spuriously large indications which are ob-
served originating from small flaws and the proportion of defects which are
misclassified. In a particular inspection task the number of flaws will be
small and the distribution uneven. Figure 2 represents a global distribu-
tion over a large number of specimens and would be used to derive represen-
tative figures for assessing technique performance.
Is it possible to make a quantitative estimate of the flaw size
distribution? Many inspections are carried out to size the flaws in a
6
particular structure. It is usually assumed that this experimental distri-
bution of flaw sizes is not representative of the true distribution because
of measurement errors. While this is broadly true, the modern, accurate
sizing techniques will not produce large distortions in the distribution
and the experimental data will provide a reasonably accurate estimate of
the true flaw size distribution. To avoid bias the data sources have to be
chosen with care from general inspections but, if this is done, a picture
of the true distribution of flaws in structures should begin to emerge. We
believe that there is enough data in existence to enable this type of as-
sessment to be carried out. In this paper we concentrate on the height
(through-thickness penetration) of flaws as this is most closely linked
with flaw significance. As an example, for large chemical pressure ves-
sels, the data shown in figs 3a and 3b represent respectively the distribu-
tion of flaw heights of internal and surface-breaking, crack-like flaws as
derived from our archived data and based on TOFD measurements. It is
believed that none of these flaws is growing so the data represent an esti-
mate of the original population of flaws in the vessel.
It is of interest to note that in both cases the data can be represented by
a curve of the form N(>h) = AExp(-Bh); where N(>h) represents the number
of flaws of height greater than h, and where both A and B are constants.
The deviation of the data at small flaw sizes from the fitted curves is
probably due to the increasing difficulties of resolving small flaws. In
both cases the deviation starts at a flaw height of about 1.5mm, which
would correspond to the estimated precision of about 1.0mm. Data is sparse
at large flaw sizes but there is some evidence of a deviation from the fit-
ted curve, possibly representing the location and removal of large flaws as
a result of pre-service inspection.
Some of the differences between the two distributions may also arise as a
result of pre-service inspection. For example, surface-breaking flaws
would be easier to locate. Moreover, they would be regarded as unaccept-
able and all those that are located would be repaired or removed. By com-
parison it may be that only large internal flaws would be located and
removed. If the differences in the flaw populations are assumed to arise
only from pre-service appraisal and repair, this would imply that the
detection probability for surface-breaking flaws ranges from about 80% for
flaws of height 4mm to 99% for flaws of height 8mm. These values are not
unreasonable.
Assuming that the fitted curves are valid representations of the true
original flaw distribution (as the vessels enter service) the data indi-
cated in table 1 may be derived. A typical vessel would contain between
0.15 and 0.3 km of weld. The number of defects and of false calls often
turns out to be small even when expressed as the number in 1 km of weld!
Fractional quantities are used in appropriate cases and, as an example, 3.3
10"2 defects per km would correspond to 1 defect in e\ery 30 km of weld
(about 150 vessels).
7
TABLE 1
The Height Distribution of Flaws
Internal Fl aws Surface-breaking Flaws
Flaw Hei ght No of Flaws/ km No of Flaws/km
(mm) of weld of weld
>2.5 110 14
>5.0 40 1.0
>7.5 16 0.06
>10.0 6.5 0.005
>15.0 1.0
>20.0 0.2
>25.0 0.03
DEFECT DISCRIMINATION
The objective of any NDT sizing technique is to separate flaws into accept-
able and unacceptable populations based on a defined discrimination level.
The degree of success will depend on precision as some flaws will be inter-
preted to be greater, and other flaws smaller, than their actual size. If
the flaw size is close to the discrimination level, the former will give
rise to false alarms while the latter will be defects misclassified by the
inspection. If the experimental flaw size distributions approximate to the
true situation, it becomes possible to begin to quantify the effect of the
error of a particular technique on the classification of flaws. The manner
in which this is done is now described; certain assumptions are made which
are intended to be realistic but which may not conform quantitatively to
actual practice.
The chemical pressure vessels for which this flaw size distribution data is
valid have wall thicknesses in the region of 100mm. It is assumed that the
data which can be extrapolated from table 1 are valid down to flaw sizes of
0.1mm and that flaws smaller than this will not affect the assessment. We
assume that an upper limit of flaw height must exist, because large flaws
would be unlikely to survive a proof test, and this has been arbitrarily
set at 40mm. We need to define a defect in this context and propose that
surface breaking flaws might cause concern if their height exceeded 10mm
and that internal flaws might cause concern if their height exceeded 20mm.
For the purposes of this demonstration, flaws larger than these limits are
classified as defects.
Taking the case of surface breaking flaws first, if a technique with an er-
ror (standard deviation) of 3mm is used and the discrimination level is set
Description:Book by British Conference on Non-Destructive Testing 1989 Sheffield, england, Brook, C., Hanstead, P. D
