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Advances in Ultrasonic transducer Development H.H.J. Huynen, F.H. Dijkstra, T. Bouma, Röntgen Technische Dienst bv, Rotterdam, The Netherlands H. Wüstenberg Bundesanstalt für Materialforschung und -Prüfung, Berlin, Germany Contact

Summary:

RTD Ultrasonic transducer development is focused on finding solutions for common and new NDT applications. With techniques like TOFD, continuous scanning at high temperature and Rotoscan, there is a request for more sensitivity, signal to noise ratio and flexibility. In this paper, the RTD approach to these requests will be presented. For applications, like TOFD, where a high bandwidth is appreciated, Piezo Composites offer a significant improvement, compared to conventional piezoelectric ceramics such as Lead-zirconate-titanate (PZT) and lead-metaniobate (LM). In other applications, where a bandwith of 50% (-6dB) or less is appreciated, conventional PZT ceramics still form a good solution.
Continuous scanning at high temperatures up to 400°C could decrease shutdown time of installations. At this moment, new developments in ultrasonic transducer design and ultrasonic equipment can offer solutions. Compensation of temperature-dependent wedge delay, new wedge materials, piezo-electric element design and an optional active cooling system could give accurate thickness measurements at temperatures up to 400°C. Phased Arrays could be useful in mechanised inspection of pipeline girth welds. Together with BAM (Berlin), RTD optimises Phased Arrays, through simulation and testing in a simulated field-application. Results with Phased Arrays are compared to an existing system using conventional transducers.

1. Introduction

Ultrasonic detection techniques strongly depend on the transducer types used. RTD optimises or develops transducers to suit these detection techniques. For example: transducers were optimised for the existing Long Range Ultrasonics (LoRUS) technique for corrosion detection developed by RTD some time ago. TOFD is an other technique where a high sensitivity and bandwidth is appreciated, new crystal materials as Piezo Composites could offer an improvement.
New developments are carried out in transducers for continuous scanning at temperatures up to 400°C and in phased arrays. These types of transducers have to prove themselves in the field. Sample applications for high temperature scanning are inspection of Hot Pipework and Heaters. Sample applications for Phased Arrays are inspections where many transducers are needed, like pipeline girth weld inspections.
In the following sections, developments and advances in piezo-composites, high temperature transducers and phased arrays are described.

2. Piezo Composites compared to conventional piezoelectric ceramics

Piezo Composites as a crystal material in ultrasonic transducers promise significant advantages, but how do Piezo Composites really compare to conventional crystal materials? Therefore, a comparison was made between conventional transducers built up with Lead-zirconate-titanate (K350) and Lead-metaniobate (K81) crystals on one hand, and transducers with piezo composite (Z=11MRayls) elements on the other hand.

Fig 1: Transducer configuration

The following transducer type was used:

Wave type:	Compression
Probe type:	dual crystal (TRL) with PMMA wedge
Angle:	45° in austenitic steel
Crystal:	Frequency 2MHz, dimension: 10x18mm
Tuning:	The transmitter is tuned to 50 Ohms
Focus Soundpath	~25mm

The results, together with some characteristics of the piezo-electric elements used, are shown in the following table.

Piezo-electric element	Coupling factor	Acoustic Impedance (MRayls)	Matching-Layer (7,8 MRayls)	Backing (MRayls)	Sensitivity (Attenuation dB)	Centre-frequency (MHz)	Bandwidth (at -6dB)
PZT	~0.5	~30	None	8.2	20	1.95	41%
PZT			Yes	8.2	26	1.84	52%
PZT			Yes	3	30	1.82	45%
LM	~0.3	~21	None	8.2	12	1.98	59%
LM			Yes	8.2	16	1.77	73%
LM			Yes	3	22	1.80	62%
LM			None	12	10	1.90	77%
Comp.	~0.6	~11	None	8.2	26	1.46	114%
Comp.			None	3	30	1.74	56%
Table 1: Comparison of several crystal, matching layer, backing configurations

Fig 2: PZT, Matching layer and 3MRayls backing.

Fig 3: Piezo Composite and 3MRayls backing.

The results show that for transducers with low and medium bandwidth (up to 50%), which are used in most of the applications, which size defects by using an echo amplitude method (like DGS), the higher sensitivity that the Piezo Composite transducer would offer due to the higher coupling factor and better matching to the wedge, is also achieved by using a PZT ceramic with a matching-layer. In this case the use of Piezo Composite would hardly offer any advantages.

On the other hand, in the case of broadband applications, like TOFD, where lead-metaniobate is used and a matching-layer would lead to bandwidth loss, we can see a substantial gain offered by using Piezo Composite crystals. The extra sensitivity gained can be about 16 dB.

Certainly, we have to take in mind that other characteristics, like temperature, flexibility or electrical tuning, can influence the choice as to when to use Piezo Composite elements or conventional crystals.

3. Transducers for continuous scanning at high temperatures

To perform inspections in service rather than during shutdown can save downtime and thus a lot of money. Therefore, it is needed to develop accurate thickness measurements at high temperatures for scanning critical areas. In this way, information is acquired as to when the object has to be repaired or replaced.

New developments in materials and ultrasonic equipment offer possibilities to manufacture ultrasonic transducers which can perform continuous scanning on surfaces with temperatures up to 400°C. Such new developments are:

• Ultrasonic equipment which can (in real-time) compensate for changes in wedge delay due to heating up of the wedge. This enables more accurate thickness measurements.
• Novel wedge materials. These materials can withstand extremely high temperatures and still have good ultrasonic properties. At the moment, several Poly-imides, composites and ceramics are tested at RTD on characteristics such as attenuation, sound velocity and wear resistance.
• Different combinations of crystals, backings and matching layers are tested to benefit from the maximum sensitivity and signal to noise achievable.
• Passive and Active Cooling: Passive cooling is achieved by new designs of the transducer housings. An other option would be an active cooling system that is easy to operate and transportable. An independent system can be built into the probes, which provides the necessary cooling.

Couplants for continuous scanning at temperatures above 350°C are difficult to find. Improvements in this area would be welcome.

Sample Applications: Monitoring of hot Pipe work and Heaters.

4. Phased arrays for pipeline girth welds

In NDT phased arrays are now becoming available. RTD has chosen an approach, where phased arrays are optimised for specific applications. Therefore we have established a partnership with BAM, Berlin to develop the necessary know-how and software. An example of an application where Phased Arrays could be useful is mechanised inspection of pipeline girth welds (RTD Rotoscan). For such inspections, typically up to 20 transducers are needed. Selection and manufacture of these transducers for new inspection job need careful attention and is time consuming. By using phased arrays, most of the transducers can be emulated by the phased array, and in this way increased flexibility and reduced preparation times are offered.

Important is the question: How should the phased array be designed? Which design criteria should be used? Together with BAM, RTD has started a project to design and optimise Phased Arrays, through simulation and testing in a Rotoscan application. Results from a Rotoscan system with Phased Arrays are compared to an existing Rotoscan system using conventional transducers.

The project will answer several questions:

• do beam profile/side lobes of a phased array compare to conventional single element transducers?
• what is the influence from the inevitably large footprint of the phased array in a pipeline girth weld inspection?
• How should the phased array(s) be designed to cover all functions of a pipeline girth weld inspection system such as RTD Rotoscan.

The results will show if phased arrays are an alternative for the transducer systems, which are nowadays used in pipeline girth weld inspection.

5. Conclusions

In broadband applications, like TOFD, Piezo Composites can offer significant advantages. In applications where a bandwidth of lower than 50% (-6dB) is appreciated, for example applications where sizing is done by the DGS method or other amplitude methods, transducers designed with conventional ceramics offer good results. Therefore it is suggested that for critical applications the choice of crystal material is considered on a case-by-case basis.

New designs and developments in transducers for scanning at high temperatures show promising results. New wedge materials show less attenuation and new crystal designs offer higher sensitivities. Together with real-time compensation of wedge delay, this offers accurate thickness measurements up to 400°C . Couplants which can be used above 350°C still are a problem.

Phased arrays for pipeline girth weld inspections can offer flexibility and a significant decrease in work preparation time if these can be used instead of the normally used transducer system of up to 20 transducers. A project of RTD together with BAM will lead to an optimised design as well as insight in the limits of phased arrays, where replacement of conventional transducer systems by phased arrays for pipeline girth weld inspection is concerned. RTD Ultrasonic transducer development is focused on finding solutions for common and new NDT applications. This special service is possible by the extensive field experience and research facilities that RTD has built up in more than 60 years.

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