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08:05 May-12-1999
William Streett
Impact-Echo

I am writing in response to the article "Automated Impact-Echo: 2
and 3-D Imaging of Concrete Elements", by C. Colla, et al. in the
May 1999 issue of NDT.net. I am the co-author, with M. Sansalone,
of the book, "Impact-Echo: Nondestructive Evaluation of Concrete
and Masonry", Bullbrier Press, Ithaca, NY, 1997. (This book is
described on the web site http://www.impact-echo.com.)

The article by Colla, et al. is filled with so much misinformation
that one hardly knows where to begin. By ignoring a very large
body of published work on impact-echo, the authors have failed to
understand many fundamental aspects of the physics
of the method, and they have drawn conclusions that are unwarranted
and unsupported. Instead of taking the time to learn what is already
known about impact-echo, they have devoted time to the production of
of "pseudo 3-D re-elaborations" of impact-echo signals which they
have not correctly interpreted.

While the authors are certainly correct in pointing out that IE
can easily be misinterpreted, and that unskilled operators often make
mistakes, their description and analysis of their own IE tests
reveals a shallow and superficial understanding of the physics of
the method. They clearly do not understand how the choice of
equipment, including the transducer used to receive impact-echo
signals, the frequency range of the analog/digital converter, and the
sizes of the impactors used to produce stress waves, can affect the
results. They readily fit the description of unskilled and uninformed
operators of impact-echo.

On page 3 of their article they state that they used an A/D converter
with a frequency range of 10 Hz to 18 kHz. This suggests that the
instrument they are using employs an accelerometer as a transducer, but
this is not made clear. Regardless of the type of transducer used,
they have severely limited themselves by restricting
the frequency range to 18 kHz or less. The transducer developed
and used in the original instruments made by Sansalone and Carino
(reference 8 in the article), and used by other makers of impact-echo
instruments, is a broad-band transducer that responds to displacements
normal to the impact surface, and has an essentially flat response from
a few Hz to about 80 kHz.

By ignoring signals at frequencies above 18 kHz, they are unable
to identify signals from wave reflections at depths of about 10 cm
or less. In their "Results" section, for example, they state that
at the position of the pipe their signals indicated a greater
apparent thickness, but that the depth of the pipe was not indicated.
If the pipe is at a depth of about 9cm, as it appears to be, the
frequency of P-waves reflected from its top surface would be about
22 kHz. Naturally they cannot detect this part of the signal if
their upper frequency limit is 18 kHz. They could easily have avoided
this limitation through proper choice of equipment.

Theyrefer to the importance of geometry in impact-echo testing, but
in designing their specimens #2 and #3 they have ignored one of the
most fundamental and important geometrical principles of impact-echo.
A fundamental rule of impact-echo, observed by Sansalone and others
more than 10 years ago, is that, for impact-echo purposes, structures for which
the lateral dimensions are less than about 5 times the thickness, are bounded
structures. Such structures respond in an entirely different manner
from plates. Specimens #2 and #3 are clearly bounded structures.
When a bounded structure is subjected to an elastic impact, cross-sectional
modes of vibration are excited, as a result of stress waves reflected
from nearby vertical boundaries. (If the lateral dimensions are greater
than about 5 times the thickness, the impact-echo response is
over before P-waves reflected from the side boundaries arrive at
the transducer, and only wave reflections through the thickness are
important.) The physics of the response of bounded structures was
worked out and published by Lin and Sansalone at the beginning
of this decade (Y. Lin and M. Sansalone, Journal of Acoustical
Society of America, Vol 91, pp. 895-893 and pp. 2474-2685, 1992.)
When cross-sectional modes of vibration are excited, additional
frequency peaks appear in the spectrum, but if the cross-sectional
dimensions are known, these frequencies can be predicted, and the
results can be interpreted accordingly. These cross-sectional modes
of vibration are almost certainly responsible for many of the "spots"
the authors report observing in their signals for specimens 2 and 3.
They correctly attribute these to geometry, but they fail to understand
that it is simply the effect of the geometry of their bounded
structure. If their specimen had been a large section of a
concrete highway, or bridge deck, with very large lateral
dimensions compared to thickness, the "spots" would havebeen
largely absent, and the determination of thickness and
depth would have been less ambiguous.

At no point in their article do the authors mention the importance
of the size of the impactor used in producing impact-generated stress
waves for impact-echo. The impact-echo method is based on the
physics of the impact of spheres on a flat surface, and the practical
method has been developed using small steel spheres (ball bearings),
typically mounted on the end of a spring rod, to produce the impact.
The physics of spherical impacts is discussed at some length in the
original work of Sansalone and Carino (reference 8 in the article)
and in the book cited at the beginning of these comments. The
diameter of the impacting sphere determines the distribution
of frequencies in the resulting stress waves, and the choice of this
diameter is a critical factor in successful impact-echo tests. The
sphere diameter determines the contact time -- the length of time
over which the sphere is in contact with the concrete surface -- and
this in turn determines the upper limit of useful frequencies in the
resulting stress waves. This information, together with many more
details of the physics of impact-echo, as well as practical aspects
of testing, are in the book cited above. A great deal of detailed
scientific work on the physics of impact-echo and on practical
aspects of impact-echo testing in the field, has been ignored in the
BAM-Berlin article.

At the very end of their article, Colla, et al. state, "Simulation of IE experiments
would be a very helpful tool for fully understanding the experimental
results." This is an astonishing statement, and reveals more than
anything else their ignorance of the field of impact-echo studies.
Beginning in about 1983, Sansalone adapted a finite-element program
developed at the Lawrence-Livermore Laboratories in the US, and
used it to simulate the propagation and reflection of elastic stress
waves in solids. In this method a continuum (concrete structure) is
divided into a finite number of discrete elements, and equations of motion for
the individual elements are combined to construct global equations describing
the dynamic behavior of the entire continuum. These equations are solved forward in time
using numerical methods. This work is described in detail in reference 8 in the article,
a US National Bureau of Standards Report that is essentially a reproduction of
Dr. Sansalone's PhD thesis. Indeed it was computer simulation of IE experiments
that guided virtually all of the subsequent development of practical applications
of the method, most of which the authors of this article have ignored.

There are other errors and misstatements in this article. Its publication
on NDT.net reveals the dangers in publishing papers that have not
been critically reviewed by others who are knowledgeable about the
topics covered.

Others interested in NDT methods for concrete and masonry would be well advised
to look elsewhere for additional informat




 
02:25 May-12-1999

Rolf Diederichs

Director, Editor, Publisher, Internet, PHP MySQL
NDT.net,
Germany,
Joined Nov 1998
602
Re: Impact-Echo William,

I hope that the authors of the Impact-echo article or other people of this field will
post their statement about your criticism of this article.

My concerns are more general. You wrote that it is dangerous that such an article
with "errors and misstatements" (your opinion) have not been critically
reviewed by experts of this field. First of all, the reference showed that this article
was presented at the DGZfP conference "ZfP - Bauwerksdiagnose, 1999 München",
which of course based on a professional committee as usual.
See here the DGZfP's NDT Civil Engineering Committee Homepage: http://www.dgzfp.de/fa/fa_b/

Anyway, I do not see it, as you criticized it, so dramatically, so dangerous, to put articles without
an editorial board on NDTnet. Of course it could be nice to have an scientific editorial board and it is already
in mind to establish that once. Nevertheless, is information published by reviewed publications really better?
Generally speaking - I thinknot!

One of your article was published in NDTnet 1998 No. 2
http://www.ndt.net/article/0298/streett/streett.htm, that was done without any screening board.
So far I am happy about all articles which have been published on NDTnet -
- I like this democracy in this new information society.

Rolf Diederichs


------------
: I am writing in response to the article "Automated Impact-Echo: 2
: and 3-D Imaging of Concrete Elements", by C. Colla, et al. in the
: May 1999 issue of NDT.net. I am the co-author, with M. Sansalone,
: of the book, "Impact-Echo: Nondestructive Evaluation of Concrete
: and Masonry", Bullbrier Press, Ithaca, NY, 1997. (This book is
: described on the web site http://www.impact-echo.com.)

: The article by Colla, et al. is filled with so much misinformation
: that one hardly knows where to begin. By ignoring a very large
: body of published work on impact-echo, the authors have failed to
: understand many fundamental aspects of the physics
: of the method, and they have drawn conclusions that are unwarranted
: and unsupported. Instead of taking the time to learn what is already
: known about impact-echo, they have devoted time to the production of
: of "pseudo 3-D re-elaborations" of impact-echo signals which they
: have not correctly interpreted.

: While the authors are certainly correct in pointing out that IE
: can easily be misinterpreted, and that unskilled operators often make
: mistakes, their description and analysis of their own IE tests
: reveals a shallow and superficial understanding of the physics of
: the method. They clearly do not understand how the choice of
: equipment, including the transducer used to receive impact-echo
: signals, the frequency range of the analog/digital converter, and the
: sizes of the impactors used to produce stress waves, can affect the
: results. They readily fit the description of unskilled and uninformed
: operators of impact-echo.

: On page 3 of their article they state that they used an A/D converter
: with a frequency range of 10 Hz to 18 kHz. This suggests that the
: instrument they are using employs an accelerometer as a transducer, but
: this is not made clear. Regardless of the type of transducer used,
: they have severely limited themselves by restricting
: the frequency range to 18 kHz or less. The transducer developed
: and used in the original instruments made by Sansalone and Carino
: (reference 8 in the article), and used by other makers of impact-echo
: instruments, is a broad-band transducer that responds to displacements
: normal to the impact surface, and has an essentially flat response from
: a few Hz to about 80 kHz.

: By ignoring signals at frequencies above 18 kHz, they are unable
: to identify signals from wave reflections at depths of about 10 cm
: or less. In their "Results" section, for example, they state that
: at the position of the pipe their signals indicated a greater
: apparent thickness, but that the depth of the pipe was not indicated.
: If the pipe is at a depth of about 9cm, as it appears to be, the
: frequency of P-waves reflected from its top surface would be about
: 22 kHz. Naturally they cannot detect this part of the signal if
: their upper frequency limit is 18 kHz. They could easily have avoided
: this limitation through proper choice of equipment.

: They refer to the importance of geometry in impact-echo testing, but
: in designing their specimens #2 and #3 they have ignored one of the
: most fundamental and important geometrical principles of impact-echo.
: A fundamental rule of impact-echo, observed by Sansalone and others
: more than 10 years ago, is that, for impact-echo purposes, structures for which
: the lateral dimensions are less than about 5 times the thickness, are bounded
: structures. Such structures respond in an entirely different manner
: from plates. Specimens #2 and #3 are clearly bounded structures.
: When a bounded structure is subjected to an elastic impact, cross-sectional
: modes of vibration are excited, as a result of stress waves reflected
: from nearby vertical boundaries. (If the lateral dimensions are greater
: than about 5 times the thickness, the impact-echo response is
: over before P-waves reflected from the side boundaries arrive at
: the transducer, and only wave reflections through the thickness are
: important.) The physics of the response of bounded structures was
: worked out and published by Lin and Sansalone at the beginning
: of this decade (Y. Lin and M. Sansalone, Journal of Acoustical
: Society of America, Vol 91, pp. 895-893 and pp. 2474-2685, 1992.)
: When cross-sectional modes of vibration are excited, additional
: frequency peaks appear in the spectrum, but if the cross-sectional
: dimensions are known, these frequencies can be predicted, and the
: results can be interpreted accordingly. These cross-sectional modes
: of vibration are almost certainly responsible for many of the "spots"
: the authors report observing in their signals for specimens 2 and 3.
: They correctly attribute these to geometry, but they fail to understand
: that it is simply the effect of the geometry of their bounded
: structure. If their specimen had been a large section of a
: concrete highway, or bridge deck, with very large lateral
: dimensions compared to thickness, the "spots" would have been
: largely absent, and the determination of thickness and
: depth would have been less ambiguous.

: At no point in their article do the authors mention the importance
: of the size of the impactor used in producing impact-generated stress
: waves for impact-echo. The impact-echo method is based on the
: physics of the impact of spheres on a flat surface, and the practical
: method has been developed using small steel spheres (ball bearings),
: typically mounted on the end of a spring rod, to produce the impact.
: The physics of spherical impacts is discussed at some length in the
: original work of Sansalone and Carino (reference 8 in the article)
: and in the book cited at the beginning of these comments. The
: diameter of the impacting sphere determines the distribution
: of frequencies in the resulting stress waves, and the choice of this
: diameter is a critical factor in successful impact-echo tests. The
: sphere diameter determines the contact time -- the length of time
: over which the sphere is in contact with the concrete surface -- and
: this in turn determines the upper limit of useful frequencies in the
: resulting stress waves. This information, together with many more
: details of the physics of impact-echo, as well as practical aspects
: of testing, are in the book cited above. A great deal of detailed
: scientific work on the physics of impact-echo and on practical
: aspects of impact-echo testing in the field, has been ignored in the
: BAM-Berlin article.

: At the very end of their article, Colla, et al. state, "Simulation of IE experiments
: would be a very helpful tool for fully understanding the experimental
: results." This is an astonishing statement, and reveals more than
: anything else their ignorance of the field of impact-echo studies.
: Beginning in about 1983, Sansalone adapted a finite-element program
: developed at the Lawrence-Livermore Laboratories in the US, and
: used it to simulate the propagation and reflection of elastic stress
: waves in solids. In this method a continuum (concrete structure) is
: divided into a finite number of discrete elements, and equations of motion for
: the individual elements are combined to construct global equations describing
: the dynamic behavior of the entire continuum. These equations are solved forward in time
: using numerical methods. This work is described in detail in reference 8 in the article,
: a US National Bureau of Standards Report that is essentially a reproduction of
: Dr. Sansalone's PhD thesis. Indeed it was computer simulation of IE experiments
: that guided virtually all of the subsequent development of practical applications
: of the method, most of which the authors of this article have ignored.

: There are other errors and misstatements in this article. Its publication
: on NDT.net reveals the dangers in publishing papers that have not
: been critically reviewed by others who are knowledgeable about the
: topics covered.

: Others interested in NDT methods for concrete and masonry would be well advised
: to look elsewhere for additional informat

:

:




 
04:56 May-17-1999
Camilla Colla
Impact-Echo comments. we have not gone into the fundamentals of IE excitation:
the title of the paper is something else. Doing
scientific work, we have made very sure that the whole chain of
equipment used (excitation, sensor, coupling of the sensor to the
surface, amplifier, A/D converter, numerical algorithms) works correctly
in the frequency range we have stated.

- Yes, we do think that "Simulation of IE experiments" are very helpful
to fully understand our experimental results. We do not think that
research work stops after 1 research group has published in the area,
as proved by the varied citations in our "Reference" section. We do
think that independent confirmation is one of the fundamentals of
scientific work. Since we have not published anything on simulation of
IE and have not made any claims, we saw no need to discuss this rather
minor point in any greater detail in our publication.

We are confident that the readers will be able to form their own opinion
about the 2- and 3D-imaging of IE data presented in our publication.
We believe that, by easing data interpretation, a number of other benefits
follow, as we tried to explain in the paper's introduction. Those working
with IE performing point measurements have enough experience with
the interpretation of their data to find our experiments and results
valuable or not.

We are sorry to see that the "response to the article" has
actually said very little about our work - much more about its writer
whom we unfortunately never met knowingly.

Herbert Wiggenhauser
Camilla Colla

Federal Institute for Materials Research and Testing
Division VII.3 - Building Diagnosis; NDT in Civil Engineering, - Berlin, Germany
WWW: http://www.bam.de/g3_vii.html




 
01:27 May-21-1999

Alex gibson

Consultant
Chased Technologies,
Australia,
Joined Feb 2008
1
Re: Impact-Echo comments. atly amplified. However, with
a 6mm ball bearing I obtained distinct peaks in the
range of 20 - 120 KHz, which I associated with
cross-sectional modes of the beam rather than
consecutive reflections. I think there is a hazy area
here where the two concepts overlap, and tend to think
more attention should be focused here.

I hope things are going well for you in Berlin.


 
03:32 Jun-06-1999
William Streett
Re: Impact-Echo comments. This is in reply to the comments of Alex Gibson, posted under
the subject of impact-echo on 21 May.

It is puzzling that those engaged in research on impact-echo
use accelerometers rather than displacement transducers.
Mr. Gibson correctly points out that the displacement
response provides the information sought from an
impact-echo test. The transducer used in the development
of impact-echo, and in the most successful commercial
instruments, produces a voltage proportional to displacement,
with a response that is essentially flat over the frequency
range of interest in impact-echo tests on concrete. A description
of this transducer can be found in a paper by its inventor,
Thomas Proctor, in "Some Details of the NBS Conical Transducer",
Journal of Acoustic Emission, Vol. 1, No. 3, pp. 173-78
(1982). This transducer is commercially available in the
U.S. for a price less than $50 per crystal. It is far
more satisfactory for impact-echo than accelerometers.

In commenting on the distinction between cross-sectional
and thickness vibrations in bounded structures, Mr. Gibson
states, "I think there is a hazy area here where the
two concepts overlap, and tend to think more attention
should be focused here."

The transition from a plate to a bounded structure has
been investigated at length, and is described in the papers
of Lin and Sansalone (see, for example, "Transient Response
of Thick Rectangular Bars Subjected to Transverse Elastic
Impact", J. of the Acoustical Society of America, Vol. 91,
No. 4, pp. 2674-2685 (1992).) The impact-echo response
of rectangular bars, and the transition from bars to
plates, is described in detail in the book,
"Impact-Echo: Nondestructive Evaluation of Concrete and
Masonry", by M. J. Sansalone and W. B. Streett, Bullbrier
Press, Jersey Shore, PA, 17740, USA (1997).

Those engaged in research on impact-echo could benefit
from the extensive work done by Professor Sansalone and
her colleagues, rather than attempting to reinvent the method.




 
07:34 Jun-09-1999
Alex Gibson
Re: Impact-Echo comments. In reply to William Streett's comments;

I fully agree with the article posted by Christian
Grosse, under the heading "The art of discussion",
and will not waste any time repeating issues raised
there.

Regarding the subject of the use of accelerometers.
Of the equipment immediately available to me, they
were the most suitable, having a linear response over
the frequency range of interest. With an
understanding of the concepts involved, I see no
disadvantage in their use. I also find that the
acceleration response is of greater value in
determining one sided velocity measurements. There
is an interesting paper on this subject:
"One-Sided Stress Wave Velocity Measurements in
Concrete" (Popovics, J. S. et al. Journal of
Engineering Mechanics, December 1998. pp 1346-1353).

I would welcome any criticisms with scientific basis.

I have read Prof. Sansalone's work extensively,
including the Impact-Echo book and the rather vainly
titled paper "Impact-Echo: The Complete Story"
(ACI Structural Journal, V.94, No.6, pp 777-786. 1997).
However, I feel that there is still a lot of research
to be done in this field, going beyond simply
"reinventing the method" developed by Sansalone.


--------------------
Ing. Alex Gibson
Pueyrredon 256, 15"D"
5000 Córdoba
Argentina


 


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