In January 2002 Cambridge Ultrasonics along
with other partner organisations within Europe started working
together to develop and evaluate new ultrasonic inspection methods
for concrete structures.
The project acronym is SGIM, which stands for second generation
inspection methods; this name is used because the partners plan
to develop instruments that are better than two successful ‘first-generation’
instruments developed some years ago by Cambridge Ultrasonics.
We will be using the best ideas of those first-generation instruments
and adding new functionality to enhance performance.
Cambridge Ultrasonics is the co-ordinator of the project as well
as the main supplier of the novel technology and developer of
one of the instruments. An important partner is Sonatest Plc.
It is now the second largest manufacturer in the World of industrial
ultrasonic inspection equipment. Sonatest will be the main route
to market for at least one of the instruments, which it is developing
based upon know-how transferred to it from Cambridge Ultrasonics.
Patents are still in preparation so it is not possible to divulge
details of the operation at this stage. As soon as prototypes
are available other partners in the project will start tests to
evaluate them. Other partners are: Necso (Spain), IETcc (Spain),
Provnings-och forskningsinstitut (Sweden), and Queen’s University
Belfast.
A number of organisations have expressed interest in becoming
associated with the project, mainly to have some chance to evaluate
the instruments. If your organization is interested then please
contact camben@cambridge-en.com.
Cambridge Ultrasonics has developed a new
database for managing the ordering, storing and assembly of low-volume
prototype electronic equipment.
When a prototype instrument contains more than a few tens of components
and particularly if it has a few hundred components then keeping
track of them for the purpose of making the several needed for
evaluation and testing to international standards can be quite
a problem.
That’s why we wrote a new Access database program to manage
the bill of materials. It holds details for each component: value,
rating, manufacturer, two suppliers, price/volume data, delivery
times and location in store. It then associates components with
parts in drawings, it also associates one or more drawings to
make an assembly and, finally, each product will comprise one
or more assemblies. It can be used to locate parts to make an
assembly. It can also be used to order parts to make a specific
number of prototypes. Finally, it works out the prime cost of
components according to volume made.
The database bill of materials has two other benefits: it helps
simplify the transfer of information to manufacturing and it has
an audit trail of responsibility for data entry and checking,
which helps for ISO 9000 purposes.
The photograph on the left (figure 1)was taken using Cambridge
Ultrasonics’ photoelastic visualization equipment. It shows
compression-waves travelling from top to bottom in glass past
a thin, horizontal slot (black line). Residual, static, stresses
are visible as brightness at the tip of the slot. The purpose
of the slot is to represent a crack in the material and the large
static stress visible at the tip of the slot illustrates the danger
of cracks: they concentrate stresses at their tips that can cause
the crack to extend, sometimes rapidly and catastrophically. Crack-tip
diffraction can be used to find cracks and their tips.
The compression waves interact with the
slot or crack in a variety of ways:
The cylindrical shear-waves (4) are a feature
of crack-tip diffraction that can be used to advantage for inspection
purposes.
Firstly, a transducer with greater sensitivity
to compression-waves should be used as a transmitter and a transducer
with greater sensitivity to shear should be used as a receiver.
No crack means no signal at the shear-sensitive receiver—that
makes for simple decision-making by an operator.
Secondly, the shear-wave is cylindrical
and centred on the tip of the crack. That means the operator can
position the receiver in many different locations and still get
a signal from the crack tip—the receiver position is not
(in principle) sensitive to position.
Thirdly, shear-waves travel more slowly
than compression waves so that with more careful positioning (for
example at the bottom of the sample in the photograph) and by
using a receiver that is sensitive both to compression and shear-waves
it is possible to collect an electrical signal containing waves
representing both waves of type 2 (compression) and 4 (crack-tip
shear). Clearly, these two types of waves will arrive at the receiver
at different times and will be separated in time in the electrical
signal. If the time separation can be measured then the distance
to the crack-tip can be calculated provided the compression-wave
speed and Poisson’s ratio or the shear-wave speed is known.
With suitable signal processing this method could form the basis
of a crack-imaging system. We would welcome the opportunity to
develop such an instrument on behalf of a client.
How do shear-waves come from compression-waves?
The second figure should help to clarify the situation. It is
a photograph of ultrasonic waves travelling from top to bottom
parallel to the free surface of a glass block and grazing the
surface. Think of it as a reflection where the angle of incidence
is 90o. The free surface must be free of stress: if it were not
it would deform until it was free of stress so, where the compression-waves
graze the surface, there must be a surface strain which superimposes
an additional set of local stresses on those due to the compression-waves,
resulting in zero net surface stresses (surface normal component
of stress must be zero and so must two orthogonal shear components
of stress). Note, the surface strains and additional surface stresses
must be travelling at the speed of the compression-waves.
The surface shear stresses so created travel
at the speed of compression waves not at the speed of shear-waves
- so these are supersonic sources of shear-waves in the glass.
It is the supersonic nature of the sources that results in the
shear-waves being generated at an angle to the compression waves—it
is the same reason that shock waves emerge from the tip of a supersonic
aircraft with a defined cone angle.
In a nutshell, mode-conversion of compression-waves
is due to the need to have zero surface stress (all tensor components
zero). It is worth noting that shear-waves with particle displacements
parallel to the surface (so called SH waves) do not generate compression-waves
at all upon reflection and follow simple reflection rules; but
shear-waves with particle motion perpendicular to the surface
(SV-waves) have a critical angle below which both compression-waves
and SV-waves are generated upon reflection but above which only
SV-waves are reflected.
In conclusion, mode-conversion from crack
tips offer a way to detect the presence of cracks in materials
and it should be possible to build an instrument to image cracks
in this way.
Figure 1—Plane ultrasonic compression waves in glass are scattered from a horizontal slot to represent a crack. Note cylindrical shear waves generated at the slot-tip.
Figure 2—Ultrasonic compression
waves in glass travel (top to bottom) parallel to a free surface
and generate shear waves by mode-conversion.
Lorenz Wegener joined Cambridge Ultrasonics
in December 2002 and Jay Sen Kuan worked over the summer in 2003
as a summer-student. Lorenz joined after completing a research
fellowship at the Cavendish Laboratory, Cambridge University.
Having studied for his MA in Natural Sciences (Physics) and PhD
in Physics, both at Cambridge, Lorenz also spent a year in the
USA working at Bell Laboratories (Lucent). In the Cavendish Lorenz
worked mainly on the theory of condensed matter - on the relationship
between polarons and colossal magnetoresistance; an effect that
could one day result in faster and higher density hard disk drives.
However, he has also done work on the atmosphere of Jupiter and
on cross-talk in optical fibres. Lorenz’s responsibility
is to write software and to be involved in brainstorming.
Jay Sen Kuan is now a final-year student
in the Engineering department at Cambridge University. He has
worked on a short project to investigate the use of class-D amplifiers
for ultrasonic applications. In his final year project Jay Sen
plans to work on a novel class-D audio amplifier based upon a
DSP—a closely related project. A class-D amplifier (as opposed
to more common classes A, B and AB) makes use of digital pulse
width modulation (PWM) signals. The power amplifier stage is relatively
simple and straightforward; software algorithms can be used to
provide filters (tone controls) and negative feedback to control
distortion. Jay Sen will use a DSP used by Cambridge Ultrasonics
and we are please to announce that Texas Instruments has kindly
donated a DSP development system to Jaysen to help him with his
project.
Most ultrasonic imaging methods make use
of focusing in some form or another. Ultrasonic C-scan uses a
single focused transducer with slow, mechanical scanning to build
up an image at a focal plane that is parallel to the scanning
plane (itself generally parallel to a surface of the sample).
It usually takes several seconds or minutes to create an image.
In medical diagnostic applications an array
of transducers (typically a linear array 1-D) is used to focus
and steer an ultrasonic beam electronically and hence rapidly.
In these transmit-focusing applications there is a real focus
of waves and the size of the focus determines image resolution
and detail. The image plane is perpendicular to the line of the
transducers and so generally perpendicular to the surface of the
sample. This kind of equipment can create images at the rate of
10 or more frames per second, like video, so that moving images
are possible, which is an advantage for medical applications.
However, it is possible to synthesize the
effect of focusing without having a real focus of waves—the
focusing is all done virtually, in software; the technique is
known as synthetic aperture focusing (SAFT). The method is not
linked to data capture so it can be done off-line; it is as fast
(or slow) as the processor permits.
Cambridge Ultrasonics has had an interest
in SAFT for several years and has recently created a Matlab program
for synthetic aperture focusing, which we are interested to apply
to clients’ applications. Matlab is not the fastest language
for executables but it is good for prototyping.
A few years ago we worked on a 2-D array with application to cardiac
imaging in real-time. It was intended to make use of both transmit-focusing
and SAFT on the received signals. The quantities of data were
potentially huge but we thought-up a scheme to minimize the amount
of data for processing.
One advantage of SAFT is that the image plane can be at whatever
orientation the user would like.
SAFT is also used to enhance radar and sonar images. A form has
even been used on images from arrays of radio-telescopes. SAFT
can be used on images from the smallest size to the largest.
Unlike the car...size does not matter as far as the usefulness of SAFT technique is concerned.
Cambridge Ultrasonics provides a virtually
unique consultancy service, focusing on the field of novel ultrasonic
systems. We are a source of ideas and innovation for our clients,
applying our knowledge and experience in physics, mathematics,
electronics, signal processing and software development to make
novel ultrasonic systems. We work with our clients’ R&D
departments to help them find solutions to their problems.
Over the last fifteen years our clients
or collaborators have included large companies like BMW, NASA,
Hewlett Packard, Siemens, BP, Shell, Alcatel and Marconi, pharmaceutical
businesses, oil service businesses, research institutes, universities
and British government agencies. Most of our clients return to
make use of our services.
In the field of inspecting concrete and
heterogeneous materials we have an international reputation for
excellence. We are working with European partners to commercialize
our knowledge in the field and to create new inspection equipment
to help improve the safety and longevity of the stock of concrete
structures.
Our staff are all graduates and commonly
have PhD degrees with several years of post-graduate experience.
We have close links with Cambridge University, particularly the
Physics and Engineering Departments.
Cambridge Ultrasonics is located in a quiet, rural location a few miles from Cambridge. It is a family-run business, offering the benefits of continuity and a dependable, conscientious service.
Cambridge Ultrasonics helps its clients
in the following ways: