Advanced Transducer Development for Long Range Ultrasonic Inspection Systems
A G Haig
School of Engineering and Design, Brunel University, Uxbridge, UK
TWI, Cambridge, UK
P J Mudge and Dr T-H.Gan
TWI, Cambridge, UK
Professor W Balachandran
School of Engineering and Design, Brunel University, Uxbridge, UK
Paper presented at Fourth International Conference on emerging technologies in non-destructive testing, Stuttgart, Germany, 2-4 April 2007.
One new emerging NDT inspection technology is the use of long-range guided waves for defect detection in pipes. This technology provides rapid screening of the full cross section of large lengths of pipe from a single location and is able to detect cross-sectional wall loss greater than 1% pipe wall area, which is ideal for detecting corrosion. Current tools for guided waves require rigid transducers with a substantial mounting mechanism. As there may be as many as several hundred transducers in a tool, these devices can be bulky and heavy. The use of flexible transducers for long range pipe inspection has been investigated. A prototype tool capable of transmitting a single longitudinal wave mode and backward-going signal suppression has been developed. The results show accurate defect detection capability and the design demonstrates many practical advantages. The signal to noise ratio is equivalent to the previous tooling. The average sensitivity has been shown to be 1.6 times that of the existing transducers.
Over time corrosion and fatigue occur in metal pipe work, particularly if exposed to hostile environmental conditions. Defects can cause leaks or failures that can result in a mass loss of product, expensive reconstruction work, damage to the local and global environment and human harm. For example, the Pipeline and Hazardous Materials Safety Administration has reported that in the U.S.A. alone incidents of pipeline failure that have caused hospitalization or death have occurred 64 times a year on average over the last 20 years. 
It has become common practice to use NDT techniques to find defects in pipelines and pipework to prevent incidents occurring. There are a number of techniques available.
The conventional ultrasonic method uses high frequency ultrasound to measure the wall thickness, where a probe is passed over the outer surface in search of thinning. This method only examines the volume of metal directly under the test probe location. The Long Range Ultrasonics method uses an array of transducers around a pipe to transmit and receive low frequency ultrasound along the axis. Pulse-echo techniques can then be used to detect features and defects up to several hundred meters in each direction and provides 100% coverage without disrupting operation. There are some practical and technical challenges for improving Long Range Ultrasonics. Many of the issues involved are inherent of the transducers used in the equipment.
The application of a new type of transducer for long range ultrasonic inspection has been identified in the literature.  Experimental work has been carried out to build upon the work carried out in the literature and to explore the transducer's suitability for use with a portable inspection tool.
As pipes have a regular cross section, they are natural acoustic wave guides. At low ultrasonic frequencies there are numerous wave modes (equivalent to lamb waves) that can exist around the entire circumference and propagate along the length of the pipe. A portion of a lamb equivalent wave will be reflected when it encounters a change of acoustic impedance (a reduction in wall thickness or a change in material). A transducer arrangement can be used to isolate the transmission of a non-dispersive wave mode, which can then be used in a pulse-echo system for the detection of features.  Recent developments have shown that the manipulation of transducer arrays around a pipe circumference can be used to gain reliable information about the location and severity of detected defects for the purpose of integrity inspection and condition monitoring. 
It is common to use transducers with rigid monolithic shear piezoelectric elements in long range ultrasonic equipment. Shear transducers transmit the ultrasonic vibrations by applying shear stress on a surface, which can be used to transmit lateral shear or compression waves. As these transducers are rigid, they will not conform to the shape of the surface and are effectively limited to tangential contact, which makes them prone to poor acoustic coupling. Two key aspects that limit the test range are the signal to noise ratio and amplitude of the transmitted and received signals. This is determined by the transducer's sensitivity to useful wave modes and sensitivity to noise.
Macro Fibre Composite actuators (MFCs) were identified in the literature as a transducer that could potentially be used to improve long range ultrasonic inspection. Thien et al (2006)  have demonstrated that MFCs can be used as transducers for generating and receiving guided waves in metal pipes.
MFCs were originally developed for applying or measuring a momentary change in strain. However, as they can operate with an alternating signal up to the MHz range, they can be used to generate and receive sound.  The MFCs are constructed from a number of thin piezoceramic rods placed between two layers of interdigital electrodes, which allows the electric field to be applied along the length of the rods. The components and then housed between two layers of polyamide film.  This design is flexible, which means that it can conform to a curved surface and should provide good acoustic coupling. They are relatively low cost and low weight. They are known to be durable and perform reliably over their life cycle. 
In the literature, a ring of eight MFCs were used to transmit a symmetric guided wave as part of a guided wave pulse-echo system that was shown to be capable of correctly detecting defects.  This indicates that the apparent practical advantages can be used to create a pipe inspection tool. However, it is not known if such a tool could perform as well as the state of the art.  The interdigital electrode design, relatively large contact surface area and ability to conform to the surface were expected to result in a good amplitude performance. However, as the type of displacement generated across the contact area is non-uniform, it was not known if they could be used to generate a longitudinal wave efficiently or if they can be effectively used in an array arrangement to suppress the transmission and reception of both signals in the wrong direction and unwanted modes (as is the case with the state-of-the-art equipment).
3 Experimental set up
A number of MFCs produced by Smart Material Corp. with D33 type polarization and active area dimensions 14mm x 28mm were used as both transmitters and receivers.  A prototype tool was produced for fitting onto a 4" schedule 40 pipe. This tool consists of three rings of eight transducers. The eight transducers were equally spaced around the circumference. The centres of the rings were separated by 30mm and the transducers were aligned such that the displacement occurs axially. This should mean that they will generate symmetric longitudinal waves. The MFCs were mounted on 1mm thick PVC, which held the transducers and wiring in place.
Once the tool is wrapped around the pipe, an inflatable collar is placed around it and clamped in place. Pressure applied to the collar will load the transducers onto the surface ( Figure 1).
Fig.1. Prototype MFC tool on a 4" schedule 40 pipe
3.1 Pressure Requirement
The transmission of energy is facilitated by mechanical traction between the transducer and the pipe surface. The inflatable collar was used as a pneumatic system that applied a load at a normal to the surface to assure firm contact. A minimum pressure should be identified for the MFCs that will allow good coupling. An experiment to characterise the relationship between pneumatic load and signal amplitude was required.
In this experiment, the input pressure was increased from 10psi to 40psi with a step increase of 5psi. This test was carried out using the centre ring of the prototype in pulse-echo mode at four locations on a 4.1m long pipe. The peak to peak amplitude of the reflections from the pipe end were recorded and compared with the input pressure.
3.2 Tool Comparison
A commercial guided wave pipe inspection system called Teletest ® was used as a transmitter/receiver, as shown in Figure 2.  The 4" longitudinal Teletest tool uses three rings with sixteen transducers per ring. The prototype was designed such that it can be interchangeable with the tools that come with the Teletest system. The ring arrangement of the prototype is designed to mimic that of the Teletest? tool. A multiple ring arrangement allows the system to generate a single longitudinal wave mode and limit the inspection to a single direction.
Fig.2. Teletest ® 'Mini-test' tool on a 4" schedule 40 pipe
A test was conducted on a 6m long 4" schedule 40 pipe. A tool location was chosen at 2.4m from one end of the pipe and a saw cut equal to 9% cross-sectional wall loss was introduced at 0.5m from the same end. Both tools were tried at the tool location using a range of frequencies from 20kHz to 100kHz with 5kHz steps. Both sets of data were studied and the best results for L(0,2) mode isolation for each tool are compared in the results section. This test was carried out twice using both a single ring and three rings method.
4.1 Pressure Requirement
The pressure test was repeated four times. The data is plotted in Figure 3 and a trend line has been added.
Fig.3. Pressure test results for four tests. The measured amplitudes have been normalised between 0-1 for comparison
4.2 Tool Comparison
The data collected were processed using the Teletest software, which accounts for attenuation and gives an indication of the severity of the defect. The prototype tool was loaded with 40psi of air pressure. Data for the single ring test showed similar results between the two tools, except that the prototype tool gave greater average amplitude of 1.6 times the original tool. The pipe reflection amplitude was compared with the noise level and was found to a similar ratio for both tools. The result of the three ring excitation test is shown in Figure 4 as a comparison between the A-scans where the best frequency found for each is used.
Fig.4. Detection of flaw using tools
The pressure test showed that the MFCs used could be coupled well to a steel pipe with as little as 30PSI applied to them. Although the MFCs had no solid housing, there was generally less ringing observed, which indicates good coupling. Both tools shared the issue of being sensitive to a 'switch on' spike shortly after transmission ( Figure 4). The three ring excitation test showed that the prototype tool was equally capable of detecting the defect as the state of the art, although the ring spacing chosen limited the output power to the point where the recorded amplitude was lower.
It has been shown that these transducers could be used in a portable or fixed long range ultrasonic inspection tool. There have also been a number of practical advantages observed. The prototype has a significantly lower radial height, which means it requires less space between a pipe and neighbouring objects for its placement. With the current transducers it is necessary to encase the piezoelectric element in a heavy housing. On large pipe sizes it maybe technically beneficial to use hundreds of transducers, but a high weight will cause a serious usability issue, particularly as pipeline inspection is often required in areas that pose very difficult working conditions, such as elevated or excavated pipework inspection. The prototype demonstrated that a significant reduction in weight can achieved with the use of MFCs.
A transducer that can be used for long range ultrasonic inspection has been identified in the literature.  A prototype portable MFC based tool was developed. Results have shown that this prototype can detect defects with 9% cross-sectional wall loss and is as capable of detecting defects as the state-of-the-art equipment. The MFCs are highly sensitive, which is a benefit that can be used once a more optimum transducer arrangement has been found. As they are conformable, they couple better to curved surfaces. The use of heavy housing is not necessary for these transducers to perform well.
Future work should investigate ring spacing for an MFC tool to optimize the output power for a useful frequency range. An investigation into housing materials is also required as a tool for field inspection will need to keep the transducers protected.
We are very grateful to Smart Material Corp. for their cooperative and collaborative efforts for this research.  .
- Mudge, P. and Catton, P. 2006, Monitoring Of Engineering Assets Using Ultrasonic Guided Waves, European Conference on Nondestructive Testing 2006, 25th-29th September, Berlin, Germany.
- NASA 2003, Method Of Fabricating A Piezoelectric Apparatus, United States Patent 6,629,341 B2.
- PHMSA 2007, Hazardous Liquid Filtered Incidents File, Natural Gas Transmission Incidents File, Natural Gas Distribution Incidents File, Pipeline and Hazardous Materials Safety Administration.
- Plant Integrity Ltd. 2007, Teletest Focus Long Range Ultrasonic Inspection System, Cambridge, UK.
- Rose, J. 2002, A Baseline And Vision Of Ultrasonic Guided Wave Inspection Potential. Journal Of Pressure Vessel Technology, Volume 124, Issue 3.
- Silk, M. And Bainton, K. 1979, The Propagation In Metal Tubing Of Ultrasonic Wave Modes Equivalent To Lamb Waves, Ultrasonics, Volume 17, Issue 01, Pages 11-19.
- Smart Material Corp. 2007, Macro Fibre Composite Datasheet, Dresden, Germany.
- Thien, B.A., Puckett, A.D., Park, G., Farrar, C.R. 2006, Detecting And Locating Cracks And Corrosion In Pipes Using Ultrasonic Guided Waves, Proceedings Of 3rd European Structural Health Monitoring Conference 2006, 5th-7th July, Granada, Spain.
- Wilkie, W., High, J. And Bockman, J. 2002, Reliability Testing Of NASA Piezocomposite Actuators, Proceedings Of Actuator 8th International Conference 2002, 10th-12th June, Bremen, Germany.
- Williams, R. B. And Inman, D. J. 2002, An Overview Of Composite Actuators With Piezoceramic Fibers, SPIE Proceedings, Volume 4753, Issue 2.