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Proceedings of the 27th Symposium on Ultrasonic Electronics 2006, 2006-11-15
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Integrated High Temperature Longitudinal, Shear, and Plate Acoustic
Wave Transducers
Kobayashi, Makiko; Jen, Cheng-Kuei; Ono, Yuu; Wu, Kuo-Ting
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Integrated High Temperature Longitudinal,
Shear, and Plate Acoustic Wave Transducers
Makiko Kobayashi†1, Cheng-Kuei Jen1
, Yuu Ono1 and Kuo-Ting Wu2
(1National Research Council Canada, IMI), (2McGill Univ., ECE)
1. Introduction
Due to the simplicity, speed, economy and capability to probe the interior of an opaque material, ultrasonic techniques are often used to perform nondestructive testing and characterization (NDT&C) of materials [1]. Many NDT&C are also mandatory to be performed at elevated temperatures, thus high temperature (HT) ultrasonic transducers (UTs) are in demand [2-3]. For applications of smart structures and materials there is also a critical need for integrated in-situ sensors for local and global damage detection and assessment. The purpose of this investigation is to demonstrate the fabrication of integrated HT longitudinal (L), shear (S) and surface (or plate) acoustic wave transducers directly onto desired structures or materials for NDT&C applications.
2. Shear and Longitudinal WaveProbes
The mode conversion from L to S waves due to reflection at a solid-air interface was reported [4,5]. It means that the L wave UT together with L-S mode conversion caused by the reflection at a solid-air interface can be effectively used as a S wave probe as shown in Fig.1. In Fig. 1, Li waves
generated by an L wave UT reach a solid-air interface and reflected as Lr and Sr waves. The
equations governing the reflection and mode conversion with respect to the L wave incident angle θ can be found in [6].
Li Lr Sr Air Solid ϕ θ θ Interface Li Lr Sr Air Solid ϕ θ θ Interface
Fig. 1 Reflection and mode conversion with an incidence of L wave at interface. 0 30 60 90 0.0 0.2 0.4 0.6 0.8 1.0 E n erg y R ef lect io n C o ef fi ci en t
Incident Angle, θ (degree) Rsl
Rll
Fig. 2 Energy reflection coefficient vs. incident angleθ.
In this study, a mild steel with the L wave velocity of 5900 m/s and S wave velocity of 3200 m/s was used as the substrate. Figure 2, where Rll
and Rsl are energy reflection coefficients of the L
and S waves, respectively, shows the calculated energy reflection coefficient for the mild steel substrate. It indicates that the maximum energy
conversion rate from the Li wave to the Sr wave is
97.5 % at θ = 67.2°, and the reduction of the energy conversion rate is within 1% in the θ range between 60.8° and 72.9°.
(a) (b)
Fig. 3 (a) Schematic diagram of an integrated S wave probe with the L wave UT located in a plane parallel to the direction of mode converted S wave where θ = 61.5°. (b) Ultrasonic signal in time domain of the S wave probe at 350 °C.
We have developed a paint-on technique to fabricate integrated L wave thick bismuth titanate (BIT)/lead zirconate titanate (PZT) composite film (> 40 µm) HTUTs [3]. The details of the fabrication techniques and film characteristics can be seen in [3]. Here a simple way is used to fabricate the S wave probe. First, let the L UT be fabricated in a plane parallel to the mode converted S wave direction on the substrate as shown in Fig. 3(a). By considering this criterion, the θ +ϕ needs to be 90°. From the Snell’s law, we can obtain θ = 61.5°. At this angle, the conversion rate is 96.7% that is only 0.8% smaller than the maximum conversion rate at 67.2°, based on the result in Fig. 2. Figure 3(b) shows the ultrasonic signal in time domain of the received Sr wave in the pulse-echo mode at 350 °C.
The Sn represents nth round trip of the S wave echoes traversing back and forth between the L UT and the probing end in Fig. 3(a). The center frequency of the S1 echo was 6.7 MHz and the 6 dB bandwidth was 3.8 MHz. The SNR of S1 echo was about 30 dB. The SNR is defined as the ratio of the amplitude of the S1 echo over that of the undesired signals between the Sn echoes in Fig. 3(b). The signal strength of the S1 echo at 350 °C was 5 dB smaller than that at room temperature.
If one would like to generate and receive both L and S waves at the same time, then the S wave probe shown in Fig. 3(a) can be modified to achieve such a purpose. In fact, it simply makes a
20 40 60 80 S3 S2 Am pl it ud e ( a rb . un it ) Time Delay (µs) S1 L UT L i Lr Sr ϕ θ θ θ Probing End 38mm 25mm
50 100 150 200 A m plit ud e ( a r b . u n it ) Time Delay (µs) 3$ 3% 3$% 3$%3$% 50 100 150 200 A m plit ud e ( a r b . u n it ) Time Delay (µs) 3$ 3% 3$% 3$%3$%
slanted surface with an angle 45° from the intersection of the slanted plane and the line from the center of the L UT as shown in Fig. 4(a). The 45° angle plane will reflect the energy of the Li
wave into the Lr,45° wave normal to the probing end
as shown in Fig. 4(a). Therefore, in principle, the upper part of the L wave, generated from the L UT, can be used to produce the Sr wave and the lower
part to produce the Lr,45° wave. Figure 4(b) shows
ultrasonic signal in time domain in the pulse-echo mode at 350 °C, in which the Sr (S1) and Lr,45° (L1)
waves are observed simultaneously. The L1 represents the first round trip L wave echo traversing between the L UT and the probing end in Fig. 4(a). The center frequencies of the S1 and L1 echoes were 6.6 MHz and 6.9 MHz and the 6 dB bandwidths were 3.0 MHz and 3.8 MHz, respectively. During the top electrode fabrication of the probe, the area of the top electrode was adjusted so that the amplitude of the reflected Sr and Lr,45°
waves were nearly the same. The SNR of the L1 and S1 was about 20 dB. L UT 45° Li Lr Sr Lr,45° θ ϕ θ θ Probing End L UT 45° Li Lr Sr Lr,45° θ ϕ θ θ Probing End (a) 10 15 20 25 30 35 S1 Am p li tud e ( a rb . uni t) Time Delay (µs) L1 (b)
Fig. 4 (a) Schematic diagram of an integrated L and S wave probe with the L wave UT located in a plane parallel to the direction of mode converted S wave where θ = 61.5°. (b) Ultrasonic signal at 350°C obtained with the L and S wave probe in Fig.4(a)
3. Plate Acoustic Waves UTs
Plate or surface acoustic wave can be propagated in a distance of tens of cm on smooth metal substrates such as aluminum and steel and used for NDT&C of the substrate. In this study a 0.702 mm-thick stainless steel substrate is chosen. A 575µm-thick stainless steel mask with an interdigital transducer (IDT) electrode pattern (holes) was made using an electrical discharge machining method and the top aluminum electrode was deposited onto the BIT/PZT film by a vacuum evaporation method. Figure 5(a) shows the steel substrate together with thick BIT/PZT films and IDTs. The ultrasonic signals measured at 350°C in the pulse echo mode with a band pass filter between 0.8 MHz and 1.3 MHz is shown in Fig.5(b).
The measured plate acoustic wave velocity at room temperature was 2936 m/s which is smaller than the Rayleigh velocity 3030 m/s of the stainless steel substrate. Therefore the waves in Fig. 5(b) are
the 1st order anti-symmetrical mode of the plate wave. When the distance from the center of the IDT to the edge A is represented with A (=47mm) and that to the edge B is B (=125mm), the plate wave echoes shown in Fig. 5(b) are indicated with P2A,
P2B, P2A+2B, P4A+2B and P2A+4B. At 350°C the wave
velocity became 2754 m/s. In actual NDT applications, the edges A and B can be considered as a large crack. Therefore the integrated plate wave sensor can be used for large distance NDT. It is also observed that the wave attenuation in this frequency range for a distance of 30 cm is less than a few dB.
(a) (b)
Fig. 5(a) Actual device of an integrated plate acoustic wave operated in the pulse-echo mode. (b) Measured ultrasonic signals at 350°C
4. Conclusions
Integrated ultrasonic BIT/PZT around 90µm-thick piezoelectric films were fabricated onto steel substrates through a paint-on method. A S wave probe which uses the mode conversion from L to S waves and S&L wave probe simultaneously generating and receiving both L and S waves were demonstrated at 350°C. IDT has been also fabricated onto thick film UT on the stainless steel plate as a transducer for plate or surface wave generation and detection. The measurement results showed that plate acoustic waves of centre frequency of around 1MHz could be generated, propagated and received in a distance of more than 30 cm in pulse-echo mode at 350°C using our novel transducer. They can be used for long distance NDT&C and hence an excellent integrated sensor for diagnostics, prognostics health management of the aviation industry and other industries.
References
1. J. Krautkrämer and H.H. Krautkrämer :
Ultrasonic Testing of Materials (Springer-Verlag, Berlin, 1990).
2. T. Arakawa, K. Yoshikawa, S. Chiba, K. Muto, and Y Atsuta : Nondestr. Test. Eval. 7 (1992) 263.
3. M. Kobayashi and C.-K. Jen : Smart Mat. and Structures 13 (2004) 951.
4. B.A. Auld : Acoustic Fields and Waves in Solids
1&2 (John Wiley & Sons, New York, 1973). 5. M.O. Si-Chaib, H. Djelouah, and M.
Bocquet : NDT&E International 33 (2000) 91. 6. W.G. Mayer : Ultrasonics 3 (1965) 62.
IDT IDT
Stainless Steel Plate (0.702 mm Thick) 47 mm 90 mm 35 mm 34 mm Electrical Probe 172 mm Edge A Edge B 125 mm
