The relocation of Instrumentation Innovation from London to Cambridge and the adoption of a new trading name, Cambridge Ultrasonics, have not been widely publicised. One of the purposes of this newsletter is to let readers know of these changes. The change of location has been dramatic - previously located in the heart of polluted and congested London we have now moved close to Cambridge in what we consider to be a rural idyll, surrounded by fields, cows and clean air.

Cambridge Ultrasonics and the Institut für Massivbau, Darmstadt are looking for other organisations to join a project and develop a second generation of ultrasonic instruments for testing concrete. The research projects of the last few years have been successful but have been aimed more at proving the feasibility of the technology in the laboratory rather than developing it to the point where site inspections can be done. The absence of equipment able to work on-site is now limiting full evaluation of the technique.

The kind of partners that are sought are: owners of large concrete structures, agencies of government involved in health and safety, inspection companies and manufacturers of inspection equipment. There are already five potential participants in the project within Europe. Partners from USA and Asia would also be welcome.

Details from David Andrews on Tel: +44 (0)1954 231 494.

Our existing range of products and services is:

• Consultancy services on ultrasonic instrumentation.
• Visualisation experiments on transducers - consultancy service.
• Research transducers for concrete inspection - for sale.

We also expect to introduce finite element modelling services this year.

Cambridge Ultrasonics (Instrumentation Innovation) was approached in 1993 by the Institut für Massivbau, Darmstadt, one of the leading German research institutes on large concrete structures, and asked to design and build new pulse-echo equipment for testing concrete. The initiative has led to several important advances over the last few years in the technology of ultrasonic inspection of concrete. It is now possible to make inspections that were previously thought to be impossible. This breakthrough responds to the growing need to inspect large concrete structures such as: off-shore structures, bridges, nuclear power stations, tunnels and dams.

Ultrasound is an attractive probe for inspection purposes because:

• It is not an ionising radiation and is, therefore, intrinsically safe.
• Inspection systems have low power requirements so can be made rugged and portable.

The Darmstadt pulse-echo equipment works with reflected ultrasonic waves through a single test-surface - unlike conventional transmission equipment requiring access to two facing surfaces. A breakthrough in the technology is based upon using new transducers and signal processing.

Research transducers are now available

• With high sensitivity.
• Needing only modest drive powers.
• With wide frequency range - allowing advanced computer signal-processing to be used,
  · that can be arranged in arrays - allowing a wide range of signal-processing algorithms to be used.

The Darmstadt pulse-echo system is best suited for answering the question, “What lies under the surface of this concrete?” The equipment has worked well for the last two years and successfully completed many evaluation experiments. The Institut für Massivbau was sponsored by the German Institute of Standards, the German Concrete Association and several German construction companies.

The pulse-echo system has been able to detect a variety of targets at various ranges:

• 0 to 500 mm range - line targets as small as 20 mm diameter such as voids and reinforcement bars have been detected with a success rate in excess of 90% and range error of only +/-5%.
• 500 mm to 2 m - larger targets such as plane back-walls and honeycombing have been detected with a success rate in excess of 90%.
• Networks of fine cracks can be detected, for example, around reinforcement bars.
• Tendon ducts can be located to good precision.
• Works from a single surface in reflection.
  

It is anticipated that further improvements can be made in the performance but the present performance already satisfies many inspection requirements. It is also worth noting that this system could be used by existing inspection engineers with little or no re-training, allowing the new technology to be introduced quickly.

Figure Captions:

• Signals from the Darmstadt pulse-echo system showing (upper signal) the weak echoes from a stressing tendon duct at 114 mm cover (lying mainly outside the ultrasonic beam) and the back-wall echo at 504 mm. In the lower signal the transducer array was positioned precisely over the tendon, causing a strong echo, but obscuring the the back-wall.
• Dr Otto Kroggel and Mr Michael Ratman operate the Darmstadt pulse-echo system.
  

Although visualisation of ultrasound has been used for many years in ultrasonic research as a qualitative tool for following propagation effects it is still under-used in our opinion. Cambridge Ultrasonics commercialised the first solid-state schlieren system capable of visualising waves in air, water and glass in 1986. A number of systems have been sold to customers but we have recently withdrawn the product from sale (although we are continuing to support existing customers through servicing). As an alternative to outright purchase, we are offering our expertise and the use of our own visualisation equipment to perform investigations of transducers and propagation problems. For customers this has several financial benefits: there is no capital cost of the equipment, there is no need to train personnel and no need for darkroom space. Some well-known manufacturers of medical ultrasonic equipment have used our equipment in this way already.

The equipment can be linked to most other types of ultrasonic equipment due to its flexible triggering facilities so, for example, pulses from medical scanning transducers can be visualised - there have been one or two surprises!

For makers of medical scanners visualisation has two advantages: the fine spatial resolution (0.1 mm is possible) and real-time imaging. This makes visualisation much faster than conventional scanning with a hydrophone (perhaps 24 hours for a fixed setting of the scanner). Visualisation cannot yet replace hydrophone scanning for quantitative results but it does provide a useful check on performance since it's easier to see a beam anomaly in an image than to detect it in the pages of computer listings generated by a hydrophone scan.

Customers can send us their transducers (in confidence) for rapid assessment (alternatively - we can come to you). We are also happy to accept ultrasonic equipment to be used with transducers. We make a S-VHS PAL video recording of the results. Most transducers can be tested in a day and we are currently offering a discounted charge for the service of £500 for the first day. More complex jobs are priced individually.

The principle of visualisation is to use an optical effect to enhance the contrast to ultrasonic waves in a transparent material. Commonly used effects are: schlieren, shadow-graph and photo-elasticity. Schlieren and shadow-graph methods are closely related. Photo-elastic methods using linearly or circularly polarised light are used for visualising waves in solids in which three modes of stress waves can propagate: compression (scalar wave) and two orthogonal modes of shear (vector waves). For the purposes of medical ultrasound it is common to ignore the presence of shear waves and consider tissue as being only capable of supporting compression waves - water is the preferred medium.

It is possible to use schlieren to visualise both pulsed beams and continuous beams. If a continuous light source is used then only the ultrasonic beam profile can be seen and information about wave-fronts is lost. If a pulsed light source is used as a stroboscope then individual wave-fronts can be visualised. Stroboscopic visualisation gives considerably more information about the propagation and is generally to be preferred over continuous illumination. However, in order to resolve individual wave-fronts it is important that no wave-front should advance by more than a quarter of a wavelength whilst the light is pulsed on - otherwise the image will be smeared and wave-front detail will be lost. This criterion is equivalent to the condition that the duration of the flash should be less than a quarter of the period of the ultrasonic waves. For example, for ultrasound at 5 MHz frequency the light flash should be no more than 50 ns.

Figure Caption:

• Sketch showing the arrangement of light source, visualising medium (transparent), transducer (attached thereon) and camera, forming the principal components in a visualisation experiment.
• Ultrasonic waves visualised in water, travelling downwards, collide with an aluminium cylinder. Four snapshots are taken at 0, 13, 19 and 34 us. The arrow in the snapshot at 19 us shows a dislocation in the wave-train. Note the waves that have passed through the aluminium get ahead of waves that have travelled only in water. Causality can be used to identify linkages between the different wave-types.
• Sketch, taken from visualising experiments, to illustrate the scattering of ultrasound in concrete. The individual circles represent large aggregates in a model of a concrete sample; the roughly concentric circles represent the wave-fronts scattered from the aggregates and the horizontal lines represent waves impinging on the aggregates (see sequence to the left).

We have introduced a new range of transducers for concrete. Based upon our popular research transducers they are attached using fast-setting mortar to concrete surfaces - attaching takes just a few minutes. The transducers have high sensitivity and good bandwidth.

If your existing transducers cannot give a reading through thick high strength concrete why not use the Cambridge range of transducers? They are durable and water-resisting.