PVDF transducer for SAFT imaging of concrete structures.

PVDF transducer for SAFT imaging of concrete structures Sanat Wagle 1,Kamal Raj Chapagain 1, Werner Bjerke 1, Frank Melandsø 2 and Terje Melandsø 1 1 Elop As, Nordvikvegen 50, 2316, Hamar, Norway. More info about this article: http://www.ndt.net/?id=22249 Abstract E-mail: sanat.wagle@elop.no 2 Department of Physics and Technology, UiT The Arctic University of Norway, 9037 Tromsø, Norway. A prototype of an ultrasonic roller scanning system was developed for concrete inspection using a piezoelectric PVDF array transducer operating in separate transmission and receiver modus. The ultrasonic roller was equipped with a multi-layer and multi-element transducer made by stacking pre-polarized PVDF films in a folded sequence using an adhesive. A large concrete area was scanned with relatively short inspection time and good image quality using a customized electronic platform and our own adaptation of the SAFT algorithm. The broadband characteristics obtained from the PVDF transducer made it possible to use a wide range of adjustable operational frequencies. This is highly beneficial for concrete imaging where typically aggregates with a large variation in sizes will limit the image quality and enforce a tradeoff between penetration depths and resolution. Keywords: Ultrasonic measurement, Concrete structure, PVDF, SAFT image, Multi-layer transducers 1 Introduction Polyvinylidene fluoride (PVDF) and its copolymer (PVDF-TrFE) are widely used to make ultrasonic transducer because of their piezoelectric properties. These materials are known to yield both higher loss and lower piezoelectric coupling than comparable piezoelectric ceramics when used in transducers [1-6]. However, it is possible to increase transducer efficiency, sensitivity and bandwidth by using two or more PVDF layers with embedded electrodes [2, 5-7]. A multilayer construction also opens up for configuring or polarizing the transducer in many different ways such as folded, barker coded and switchable barker coded [2, 5-7]. NDT methods using ultrasound are useful for the monitoring infrastructures like bridges, railways and roads during regular operations to prevent failure and increase safety [8, 9]. Ultrasound inspection methods should ideally provide efficient ways for inspection, assessment of damage, and for surveying the detailed condition of concrete structures. However, large concrete structures pose many challenges for inspection due to aggregates with a wide size distribution and inhomogeneous nature. Hence, high pulse-echo sensitivity and broad bandwidth are desired for the transducer design in order to mitigate acoustic scattering and wave attenuation. A broader bandwidth is also known to improve the image resolution in the axial direction [5]. [ID248] 1

Elop is developing lower frequency ( 500 khz) multi-layer and multi-element novel ultrasonic scanning systems based on PVDF transducers. These transducers are assembled in two separate rollers and used in transmission (Tx) and receiver (Rx) modus to analyze defects in the concrete structures. Advantages of the used multi-layer transducer are for example, an increased signal-to-noise ratio (SNR) and a lower resonance frequency. The roller system allows for lager area inspection in less time compared to the other systems available in the market. A Tx and Rx modus configuration will also increase electrical shielding and thereby reducing the capacitive currents set up during pulse excitation. The used configuration is also known to reduce the required dynamic range for the front-end electronics [2,3]. Moreover, the system uses the SAFT (Synthetic Aperture Focusing Technique) algorithm for image reconstruction, which will increase image resolution and signal to noise ratio, and therefore improve the image quality. Some of the desired features for the scanner system would be inspection speeds from 10 to 100 cm/s, area coverage of 1 m 2 in 10 s, use of dry coupling materials, adjustable operational frequency for enhanced penetration or resolution, and in situ evaluation and 3-D real time video with high resolution. A prototype of the proposed ultrasonic scanner with all the element and connector is shown in Fig. 1(a). In this paper, we have focused on the multi-layer and multi-element transducers used in the roller assemble, and explored their ability to image defects inside the concrete structures. (a) (b) Fig. 1: (a) Current product with all elements and connectors to the display unit, (b) Eight layer four elements array transducer with PCB and cables in casing. 2 Experimental Procedure 2.1 Transducer design To investigate the benefits of combining a multi-layer and multi-element design, several prototypes were made by stacking pre-polarized PVDF films in a folded sequence using an adhesive. In this configuration, the multilayers are connected electrically in parallel and acoustically in series. As a result, this configuration improves the electrical impedance match between the transducer and front-end electronics. The transducer will emit energy to both the sides in the longitudinal direction, and in order [ID248] 2

to reduce ringing in the received signal caused be wave bouncing, the rear side of transducer is normally coupled to a backing material. However, the choice of backing material and its form involves several trade-offs. A thick and lossy backing that matches well in acoustic impedance with the transducer will reduce transducer ringing and improves bandwidth. At the same time, it will take away half of the energy which is undesirable for application involving low SNR signals [10]. In this work, a customized acoustic backing was adhered to the transducer on one side. This design imposes only a minor reduction in the transducer efficiency, and at the same time, manage to maintain important broadband features of the PVDF transducer. The other side of the transducer was coupled to the cylinder wall of the roller. An eight-layer transducer array was used as shown in Fig. 1(b), including four active elements, acoustic backing, PCB and cables. The individual elements were characterized by two methods to understand their acoustic behaviour. 2.2 Impedance measurement The first characterization method uses an impedance analyser (TREWACC system TE 1000) in the frequency ranges from 0.5 MHz up to 2 MHz. For these measurements, one electrode of the electrically parallel configured transducer was connected to analyser, while the other electrode was grounded. The impedance analyser was self-calibrated for open, short, and 50 Ω loads. (a) (c) (b) (d) Fig. 2(a) & (b) Admittance and phase measurements for single, four and eight layer transducer, (c) & (d) Similar measurements for eight layer transducer with and without backing. 2.3 Ultrasonic measurement The second method uses an ultrasonic measurement setup which consists of an arbitrary wave generator (Agilent 81150A) to drive the transducers, and a 1 mm needle hydrophone attached to an ultrasonic scanning system. This system was used to measure frequency behaviour and ultrasonic pressure fields [ID248] 3

generated by the transducer elements. All hydrophone measurements were performed in double distilled water at room temperature. Measurements were done with a second derivative of Gaussian pulse (Ricker pulse) with different central frequencies. The amplitude of the ultrasonic field in a plane parallel to the transducer surface was scanned and plotted. 2.4 NDT measurement Finally, the transducer units are prepared and attached to the PCB and casing, and then assembled in the cylindrical roller scanner. The scanner uses a separate Tx and Rx wheel with elastomer rings as coupling to the inspected material. The material properties of all layers in the acoustic path of the roller assembly (backing, transducer film, front layer, roller wheel and elastomers) are optimized to yield minimal acoustic reflections between the layers. The axial distance between the Tx and Rx rollers in the rolling direction was kept at 12 cm. 2.5 Electronics Platform The electronics platform used for the ultrasonic scanning consists of a high voltage pulser unit together with a receiving unit that is capable of handling 16 individual Tx and Rx channels. The receiving unit consists of a multi-channel variable gain amplifier (VGA), including a low noise amplifier (LNA), antialiasing filter (AAF) and analog-to-digital converter (ADC). A bipolar square pulse with a center frequency of 150 khz was used to excite the Tx transducers. This will excite a pulse propagation into the material structure and the acoustical reflections caused by this pulse at boundaries (such as defects) are then picked up by each Rx transducers and sent into the trans-impedance amplifier (used as a preamplifier. This signal was then fed to the Rx channels for digitizing and further processing in software. The roller was moved with a constant scanning step on structure surface, and A-scan data were acquired by rolling. Then, these acoustic data were used to create B, C and D-scan images after post-processing by software. The scanner is equipped with a rotational encoder that keeps track of positional information with reference to a starting point. Averaging is enabled in the platform for reducing random noise. 2.6 SAFT Image An adapted version of the SAFT algorithm was used for image construction. Synthetic aperture offers a different way of focusing, where a sequence of pulses from each transmitter element and their positions are applied. This can be used to create a focused transmitter and receiver beam synthetically (a synthetic aperture) from all the pulses and receiver positions [11,12]. SAFT is based on subsequent focusing of the data measured on an aperture to every point of the reconstructed area through superposition of the recorded time series. SAFT on a fixed array has the potential to increase resolution and improve the signal to noise ratio (SNR), whereas SAFT on a moving array utilizes the movement to synthesize a larger array. [ID248] 4

For the SAFT processing, we are using time-domain beamforming by back projection. This is done by back propagating the received signal via each grid point in the volume to be imaged. Back projection is also known as Delay-And-Sum (DAS) [12,13]. In order to improve the achievement, the processing was modified to include SAFT in both the along-track direction (denoted as the direction parallel to the rolling direction), and cross-track direction (perpendicular to the rolling direction). Due to the bistatic geometry and the fact that the imaging system operates in the nearfield with wide bandwidth and wide beamwidth, synthetic aperture imaging in the along-track direction will reduce side lobes and grating lobes in the cross-track direction. This is one of the many advantages of using SAFT in both alongtrack and cross-track directions. 3. Results and discussion The amplitude of the admittances and phase for the different transducer samples without backing, are shown in Fig. 2(a) and (b). From the figure, we see that as the number of layer increases the resonance frequency of transducer decreases. The resonance frequency of the transducers were around 12, 3.5 and 1.3 MHz for one, four and eight layer transducers, respectively. The amplitude of the admittances and phase for the eight-layer transducer elements with and without backing is shown in Fig. 2(c) and (d). The resonance frequency of transducer decreases from 1.3 to about 0.65 MHz with the backing attached. (a) (b) (c) Fig. 3 Acoustic field for eight layer transducer with backing at (a) 150 khz, (b) 300kHz, & (c) 600 khz The acoustic fields measured by a needle hydrophone setup of an eight-layer transducer with backing at different input frequencies are shown in Fig. 3 (a), (b) and (c). The distance between needle hydrophone tip and transducer surface was 40 mm for the measurement. In the figure, the highest pressure-amplitudes are observed around the center of the element. The intensity of the field around the center portion becomes enlarged with an increasing frequency for the input signal and showed a maximum around 600 khz. The resonance frequency obtained here with a needle hydrophone for the transducer with backing, is similar to what was obtained from the impedance analyzer measurements. Here, we should note that the transducers resonance frequency can be lowered further into the applicable range by using more layers and/or thicker films. [ID248] 5

(a) (b) (c) (d) (e) Fig. 4 Along track (x), cross track (y), and depth (z) SAFT images with electronic platform on 20 cm thick calibration block with tubular reflector of diameter 4 cm at 13 cm depth from top surface - selected cross sections (a) C scan at 20 cm from top surface, (b) C scan at 13 cm from top surface, (c) D scan at centre plane (d) B scan at centre plane and (e) B scan at 5 cm from centre plane. Fig. 5 Along- & cross-track SAFT images - selected cross sections from first field test on concrete bridge located at CC-Amfi, Hamar, Norway [ID248] 6

SAFT images of different cross section inside the calibration block with a cylindrical shaped defect are shown in Fig. 4. The calibration block contains a cylindrical through hole in 4 cm diameter parallel to the cross-track direction at a depth of 12.5 cm from the top surface. The scanning was carried out for 30 cm in the along-track direction with a scanning step of 0.25 cm, and 20 cm used in the cross-track direction with an eight element Tx and Rx roller. The back wall and cylindrical hole are clearly visible on the images planes obtained for different cross sections. Figure 5 shows the along-track and the cross-track SAFT images of selected cross sections from first field test with a fully equipped scanner on a 25 years old pedestrian concrete bridge located at the Olympic stadium in Hamar, Norway. The size of the aggregates is about 20-25 mm for the scanned concrete bridge. The scanning was carried out for 30 cm and 0.25 step size in the along-track direction, and with 20 cm on the cross-track direction. The eight-element Tx and Rx roller were operating at 150 khz central frequency. The thickness of the concrete bridge is about 23 cm which is clearly seen as a back-wall reflection. Other reflections are also seen in the scanned images which is believed to be caused by several scattering sources like aggregates, rough surfaces, material interfaces and reinforced bars. 4 Conclusion The present study shows that we are able to produce a broadband ultrasonic roller scanning system for concrete inspection using multilayer piezoelectric PVDF transducers using separate transmission and receiver elements. We are able to scan a large concrete area in a relatively short inspection time and obtain good image quality using the electronics platform and SAFT algorithm. The ultrasonic roller was equipped with a transducer containing eight multi-layer elements made by stacking pre-polarized PVDF films in folded sequences using adhesives. The electrical characterization of the multilayer transducers element shows a decrease in resonance frequency as the number of layers increased. Moreover, the multilayer configuration improves the electric impedance match between the transducer and front-end electronics, and increases the acoustical performance. The broadband characteristics of the PVDF transducers used in the ultrasonic roller, also made it possible to use a wide range of operational frequencies, which again yielded better performance (measured in terms of penetration and resolution), and larger flexibility with respect to various concrete types and inspection dimensions. Future work will be made on integration of acoustic and electronic units together with a display unit in a handheld device capable of displaying real-time 3D images of the internal state of the concrete structures. The proposed instrument will be user friendly, efficient and save significant scanning time. We can also increase the area of application to inspection of composite materials, thick and thin metals, and welded joints by increasing the resonance frequency of the ultrasonic roller with a different multilayer transducer design. [ID248] 7

Acknowledgment The authors would like to thank Roy Edgar Hansen, Principle Scientist at Norwegian Defense Research Establishment (FFI) and Aurotech Ultrasound for their help and support during the development of for the scanner. This work was supported by European Union through the project Horizon 2020 program, RFF Innlandet and Innovation Norway. References [1] L. F. Brown, Design Considerations for Piezoelectric Polymer Ultrasound Transducers, IEEE Trans. Ultrason., Ferroelect., Freq. Control, vol. 47, no. 6, pp. 1377-1396, November, 2000. [2] Q. Zhang, P. Lewin and P. Bloomfield, PVDF transducers-a performance comparison of singlelayer and multilayer structures, IEEE Trans. Ultrason., Ferroelect., Freq. Control, vol. 44, no. 5, pp. 1148-1156, September, 1997. [3] S. Robinson, R. Preston, M. Smith and C. Millar, PVDF Reference Hydrophone Development in the UK From Fabrication and Lamination to Use as Secondary Standards, IEEE Trans. Ultrason., Ferroelect., Freq. Control, vol. 47, no. 6, pp. 1336-1344, November, 2000. [4] R. G. Swartz and J. D. Plummer, On the Generation of High-Frequency Acoustic Energy with Polyvinylidene Fluoride, IEEE Trans. Sonics Ultrason., vol. 27, no. 6, pp. 295-303, November, 1980. [5] L. Xi, X. Li and H. Jiang, Variable-thickness multilayered polyvinylidene fluoride transducers with improved sensitivity and bandwidth for photoacoustic imaging, Appl. Phys. Lett.,vol. 101, pp. 173702 173704, October, 2012. [6] A. Decharat, S. Wagle and F. Melandsø, Evaluation of the Acoustical Properties of Adhesive free Dual Layer Piezoelectric PVDF Copolymer Transducer, IEEE International Ultrasonics Symposium, Prague, 2013, pp. 471 474. [7] S. Wagle, A. Decharat, and F. Melandsø, Adhesive-free dual layer piezoelectric PVDF copolymer transducers in sender and receiver sequence, IEEE International Ultrasonics Symposium,Chicago, pp. 2063 2066. [8] D Breysse, Nondestructive evaluation of concrete strength: An historical review and a new perspective by combining NDT methods, Constr. Build. Mater., vol. 33, pp.139-163, 2012. [9] O. Tsioulou, A. Lampropoulos and S. Paschalis, Combined Non-Destructive Testing (NDT) method for the evaluation of the mechanical characteristics of Ultra High Performance Fibre Reinforced Concrete (UHPFRC), Constr. Build. Mater., vol. 131, pp. 66-77, 2017. [10] B. A. J. Angelsen, Waves, Signals and Signal Processing in Medical Ultrasonics, Volume I, Trondheim: Department of Physiology and Biomedical Engineering, NTNU, 1996. [11] T. Stepinski, Synthetic Aperture Focusing Technique in Ultrasonic Inspection of Coarse Grained Materials," Uppsala University, 2007. [12] F. Lingvall, Time-domain Reconstruction Methods for Ultrasonic Array Imaging, Uppsala Univerisy, 2004. [13] M. Schickert, M. Krause and W. Müller, Ultrasonic Imaging of Concrete Elements Using Reconstruction by Synthetic Aperture Focusing Technique, J. Mater. in Civil Eng., vol. 15, no. 3, pp. 235-246, 2003. [ID248] 8