Design of mode selective actuators for Lamb wave excitation in composite plates

Transcription

1 CEAS Aeronaut J DOI /s ORIGINAL PAPER Design of mode selective actuators for Lamb wave excitation in composite plates Daniel Schmidt Michael Sinapius Peter Wierach Received: 12 April 2012 / Revised: 20 November 2012 / Accepted: 13 December 2012 Ó Deutsches Zentrum für Luft- und Raumfahrt e.v Abstract Structural Health Monitoring based on Lamb waves, a type of ultrasonic guided waves, is a promising method for in-service inspection of composite structures. In this study, mode selective actuators are developed to excite a particular Lamb wave mode in quasi-isotropic CFRP (carbon fibre-reinforced polymer) plates. Different manufacturing technologies based on monolithic piezoceramics and piezocomposites are described. The actuators are based on interdigital transducer design in order to control the frequency as well as the wavelength of the desired mode within the excitation. To determine the wavelength of the desired Lamb wave mode, experimental dispersion diagrams of the CFRP plates are measured using air-coupled ultrasonic scanning technique. The dispersion diagrams show the A 0 and S 0 mode in a frequency range of khz. A mode selective actuator is designed to amplify the A 0 mode and to attenuate the S 0 mode at 40 khz. Within experimental tests the actuator and circular piezoceramic sensors are applied on a CFRP plate in order to determine the mode selectivity. These tests are accompanied by 2D finite element simulations. On the basis of simulations and experimental test the influence of different D. Schmidt (&) P. Wierach German Aerospace Center (DLR e. V.), Institute of Composite Structures and Adaptive Systems, Lilienthalplatz 7, Braunschweig, Germany daniel.schmidt@dlr.de P. Wierach peter.wierach@dlr.de M. Sinapius Technical University Braunschweig, Institut für Adaptronik und Funktionsintegration, Langer Kamp 8, Braunschweig, Germany m.sinapius@tu-braunschweig.de parameters such as number and width of electrode segments, excitation signals and apodization is investigated. The results show that a mode selectivity of the A 0 mode in CFRP plates can be achieved by the designed actuator. Keywords Structural Health Monitoring (SHM) Lamb waves Interdigital transducer Mode selective actuators and sensors CFRP plates 1 Introduction Structural Health Monitoring (SHM) based on ultrasonic guided waves, so-called Lamb waves, is a promising method for in-service inspection of aerospace structures without time-consuming scanning like conventional ultrasonic techniques. The implementation of SHM systems into aerospace applications enhances reliability, safety and maintenance performance as well as economic aspects. Lamb waves are able to propagate over long distances in plate-like structures with low attenuation and are highly sensitive to a variety of structural damages. Lamb waves are excited and received using a network of actuators and sensors which are permanently attached on the structure. By analysing the sensor signals different kinds of structural defects can be in principle detected and located [1, 2]. However, the presence of at least two Lamb wave modes (symmetric, S 0, S 1, S 2,, and anti-symmetric, A 0, A 1, A 2,, modes) at any given frequency, their dispersive characteristic and their interference at structural discontinuities produce complex wave propagation fields and sensor signals which are difficult to evaluate. In order to reduce the complexity of the Lamb wave propagation field, the German Aerospace Center develops mode selective actuators which are able to generate a particular Lamb

2 D. Schmidt et al. wave mode in CFRP plates (carbon fibre-reinforced plastic). By focusing on a frequency range where only the lowest order of symmetric (S 0 ) and anti-symmetric (A 0 ) modes exist, a first reduction of the complex wave propagation field can be achieved. Within this frequency range, mode selective actuators are able to attenuate the S 0 or the A 0 mode. The investigations presented in the paper are concentrating to attenuate the S 0 and to amplify the A 0 mode at 40 khz in CFRP plates. This paper first presents the principle design of modes selective actuators based on interdigital transducers (IDT). In this context different manufacturing technologies based on monolithic piezoceramics and piezocomposites will be proposed. Compared to monolithic piezoceramics, the piezocomposite technology provides a higher reliability of actuator and sensor which is essential for industrial applications of SHM systems. The third section presents the experimental determination of dispersion diagrams of CFRP plates using an air-coupled ultrasonic technique. The dispersion diagram is required in order to determine the wavelengths of different Lamb wave modes and to design mode selective actuators. In the forth section, a 2D finite element model for Lamb wave propagation in layered CFRP structures is developed. On the basis of the finite element model, the influence of different parameters such as number and width of electrode segments as well as excitation signals is investigated. Out of these investigations a mode selective actuator is designed and experimentally validated on a CFRP plate in the fifth section. Finally, a conclusion of the paper is proposed in the sixth section. 2 Mode selective actuator design and manufacturing process The generation of a particular Lamb wave mode can be achieved by controlling the frequency as well as the wavelength (k) of the desired mode within the excitation. An appropriate technical solution is to use a piezoelectric substrate with applied interdigitated electrode pattern, socalled interdigital transducers [3, 4]. Such transducers are widespread in telecommunication systems as surface acoustic wave filters (SAW) for frequency selection [5]. The electrode configuration is made of two comb-like electrodes with opposite polarity. The electrode distance corresponds to the half-wavelength of the desired Lamb wave mode being selected for health monitoring of the plate-like structure at a suitable frequency. The bandwidth of the frequency response function of the excited Lamb wave can be controlled by the number of electrodes. Furthermore, the frequency response function can be modified by apodization which is well known from the theory of surface acoustic wave filters. Apodization means that the overlaps (l O ) of each electrode pair are varying along the length of the transducer (see Fig. 1). By suitable dimensioning of the overlaps the transducer can be in principle designed to a specific frequency response. These possibilities of modifying the frequency response function can be utilized to enhance the effectiveness of the actuator regarding mode selectivity. As actuators and sensors, piezopolymers, such as polyvinylidene fluoride (PVDF) or piezoceramics (PZT) are commonly used for Lamb wave-based SHM. Piezopolymers are flexible and can be applied on curved structures. They are lightweight as well as cheaper and easier to manufacture than piezoceramics [6]. But piezopolymers posses a much lower modulus of elasticity and actuation force in comparison to piezoceramics. Therefore, piezopolymers are less suitable for actuator applications. Due to the limited temperature range of -40 to?110 C, piezopolymers are inappropriate for aerospace applications. Hence, the presented work is focused on piezoceramicbased SHM. Starting point for the manufacturing of interdigital transducer is a monolithic piezoceramic plate with a typical thickness of mm. This piezoceramic plate is already provided with uniform electrodes on the upper and lower surface and polarized by the manufacturer. In a first step the piezoceramic plate is additionally metallised with gold by a sputtering process. This procedure ensures wraparound electrodes and thus the electrical connection from the upper side of the final actuator. In a second step the electrode structure is made by a laser ablation process. The laser parameters are adjusted to remove only the metallised layer and avoid mechanical damages of the piezoceramic. Due to the polarisation direction and the electrical field, which is generated through the thickness of the piezoceramic, the transducer is working in the piezoelectric d 31 - effect. In this case a positive electrical field causes in-plane Piezoceramic λ Fig. 1 Schematic design of an IDT with apodization Positive Electrodes Negative Electrodes Electrical Field l O l E

3 Design of mode selective actuators for Lamb wave excitation in composite plates Electrical Contacts Electrical Isolation and Mechanical Precompression Flexible Electrode Piezoceramic Plate Fig. 2 Mode selective Lamb wave actuator based on interdigital transducer design with apodization contraction of the piezoceramic material which is used for Lamb wave excitation. A typical example of such interdigital transducer is shown in Fig. 2. The main problems of piezoceramics are their inherent brittleness and the failures of electrical contacts which lead to insufficient reliability. A promising alternative to conventional piezoceramics is to utilize the piezocomposite technology to increase the reliability of brittle piezoceramics. Piezocomposites consist of piezoceramic materials embedded in a ductile polymer. Further components like electrodes, electrical contacts or insulators are also embedded into the composite. Within the embedding process the polymer is typically cured in a temperature range of C. Due to the different coefficients of thermal expansion of the polymer and the piezoceramic material as well as to the shrinking of the polymer during curing, the piezoceramic material is provided with a mechanical pre-compression. This pre-compression protects the brittle piezoceramic material and allows bending loads on the piezocomposite which is essential for the application on curved structures. Further advantages of piezocomposites are reliable electrical contacts, electrical insulation as well as high durability under variable environments. In recent years, different configurations of this piezocomposite type have been designed and manufactured [7]. The principle design of piezocomposites is shown in Fig. 3. One possible solution of manufacturing mode selective actuators based on the piezocomposite technology is the embedding of the piezoceramic with interdigitated electrode pattern into the polymer. Another solution is to arrange individual piezoceramic elements in a distance of halfwavelength within the piezocomposite (see Fig. 4). The piezoceramic elements work in the piezoelectric d 31 -effect. 3 Dispersion diagram of CFRP plates The aim is to develop mode selective actuators for CFRP structures. Thus, the investigations are carried out on CFRP plates which exhibit a quasi-isotropic lay-up in order to Fig. 3 Schematic design of the piezocomposite [7] Fig. 4 Mode selective Lamb wave actuators based on piezocomposite technology reduce variations of phase velocity and wavelength in different in-plane directions. These plates have dimensions of 1, , mm and are made of 7 plies in a h i ð0=90þ f = þ 45= 45= 0=90 configuration. The design f s of mode selective actuators requires the determination of wavelengths of the desire Lamb wave modes and therefore the evaluation of the dispersion diagram of the CFRP plates. This is realized using air-coupled ultrasonic technique which measures the out-of-plane component of the Lamb wave propagation field by air-coupled ultrasonic sensors. The methodology combined with software analysis tools provides the measurement of specific Lamb wave modes regarding wavelength, phase and group velocity as well as signal attenuation. The measurement can be performed in different angles of the x y plane which is fundamental for the characterisation of anisotropic structures, such as CFRP. Figure 5 shows the experimental set-up for determination of dispersion diagram of the CFRP plates. In this setup an actuator is applied on the plate surface and excited using a rectangle burst signal with 3 positive pulses and a voltage up to 70 V. This actuator is a piezocomposites with an embedded circular piezoceramic disc (PIC 255, PI Ceramic GmbH) which has a diameter of 10 mm and a thickness of 0.2 mm. To receive the out-of-plane

4 D. Schmidt et al. Ultrasonic System Scanning Sensor CFRP Plate z y x Actuator Fig. 6 B-Scan starting at the actuator position along the x axis (black positive amplitudes, white negative amplitudes) Fig. 5 Experimental set-up for determination of dispersion diagram of the CFRP plates component of the Lamb wave propagation field, an aircoupled sensor is moved in a meander-shaped track by a portal scanner. The different measurements are performed at frequencies of 25, 50, 75, 100, 131, 226 and 405 khz. Within this frequency range only the lowest order of symmetric (S 0 ) and anti-symmetric (A 0 ) modes exist. The measurements up to 100 khz are realized using a high frequency broadband microphone. Whereas the measurements between 131 and 405 khz are realized by different narrowband ultrasonic sensors. Because of the air-coupling as well as the mode conversion into longitudinal waves the amplitudes of the sensor signal are very low. Therefore, ultra-low noise preamplifiers and a 12th order band pass filter on the receiver side are necessary. The further analogue signal processing, data conversion and scanner controls are provided by the ultrasonic system USPC 4000 AirTech (Ing.-Büro Dr. Hillger). During the scanning process the amplitude signal as a function of time is measured at each point in x- and y- direction of the scanning grid. On the basis of B-scans, which show the amplitude signal as a function of time along a path in x y plane, each Lamb wave mode can be identified by different gradients. This gradient represents the phase velocity and can be separately measured by analysing the zero-crossing of the amplitude signal (see Fig. 6). The experimentally determined phase velocities and wavelengths of the A 0 and S 0 mode by air-coupled ultrasonic technique are shown in Figs. 7 and 8. In addition, the dispersion diagram of the CFRP plates was measured using scanning laser vibrometry which was performed by the Otto von Guericke University Magdeburg [8]. The scanning laser vibrometry as well as the ultrasonic technique are appropriate for measurement the dispersion diagram with the same accuracy. Moreover, the experimental measurements are compared with the theoretical phase velocities Fig. 7 Phase velocity of A 0 and S 0 mode for a 2 mm CFRP plate, 0 direction Fig. 8 Wavelength of A 0 and S 0 mode for a 2 mm CFRP plate, 0 direction and wavelengths which are determined by the method of guided waves in multiple layers proposed in [9, 10]. 4 Finite element modelling A systematic investigation of different actuator parameters based on experimental methods can be extremely cost and

5 Design of mode selective actuators for Lamb wave excitation in composite plates time-consuming. Simulations using the finite element method (FEM) can reduce the needed effort and allow for the variation of single parameters without unintentionally influencing other parameters. Furthermore, the FEM provides data to get a profound understanding of the excitation and propagation of different Lamb wave modes in CFRP plates. For Lamb wave investigations a model has been build and solved using the ANSYS Ò software. The model represents only the 2D cross section of the CFRP plate in order to reduce the calculation time. In accordance to Sect. 3 the CFRP plate consist of seven plies. Each ply is modelled separately with its corresponding material properties. In general, the FEM provides the opportunity to couple different physical problems in one solution process. This offers the possibility to analyse excitation, propagation and receiving of Lamb waves within one single model. Elements with quadratic displacement behaviour and structural capabilities are standard in different FE software. ANSYS Ò in particular also provides such elements with additional piezoelectric capabilities. Accordingly, the CFRP plate is meshed with the 2D-element PLANE183 and the actuators as well as the sensors are meshed with the coupled-field-element PLANE223. These higher order elements produce more precise results during comparative computation times. For modelling the dynamic problem of Lamb wave propagation, other important parameters that distinctly influence time cost and accuracy are the mesh density and the time steps size. During examination of the various parameters a wide frequency range is used and with this the time step size needs to be adapted accordingly. Higher frequencies do not only require smaller time steps sizes but also reduce the wavelength of the analysed Lamb waves. Therefore, the element size has to be adjusted. To evaluate the simulation parameters, numerical, analytical and experimental calculated dispersion diagrams of the CFRP plates have been compared. For the following investigations a time step size of 0.2 ls, an element size in x direction of 2 mm and in y direction of 0.2 mm are chosen. The actuators/sensors with a thickness of 0.2 mm are modelled using the material properties of the piezoceramic material PIC255 (PI Ceramic GmbH). The bonding layer between the actuators/sensors and the plate structure is not represented by the FE model. For excitation of Lamb waves, a rectangle burst signal with bipolar pulses is used. On each actuator electrode a voltage of 1 V (peak to peak) is applied. The sensor signals are analysed using the maximum of the absolute value. Within the simulation one parameter under investigation is the electrode width (l E )of an IDT (see Fig. 1). In this case the number of actuator electrodes is set to 7 and an excitation signal of 2 pulses is used. The distance between the electrodes is kept constant and aligned to the wavelength of 20 mm in order to amplify the A 0 mode at 40 khz. The sensor width is set to Fig. 9 Max. amplitude of A 0 and S 0 mode at different electrode widths, actuator with 7 electrodes, 40 khz, 2 mm thick CFRP plate the half-wavelength of 10 mm. Figure 9 shows that an increasing electrode width up to 8 mm causes near linear rising amplitudes of the A 0 mode. When the electrode width exceeds 8 mm, saturation of the signal amplitudes appears. These results are similar to those in the literature [9]. In order to maximise the amplitude of the desired mode without increasing the amplitude of the undesired mode, an electrode width of 80 % of the half-wavelength seems to be practicable with respect to manufacturing requirements. Although the maximum amplitude ratio of A 0 to S 0 mode is observed at 3 mm electrode width, an electrode width of 8 mm is chosen for further investigations because the amplitudes of the S 0 mode are comparatively low in both cases. Further investigations are concentrating on the correlation between the amount of electrodes and signal pulses. As before, the A 0 mode at 40 khz is primarily exited and the number of electrodes and pulses are varied from 1 to 10. The electrode width of the actuator as well as the sensor is set to 8 mm. It can be seen from Fig. 10 that signal saturation of the A 0 mode occurs when the number of pulses reaches half of the electrode number. This is due to the fact that the signal pulses consist of positive and negative half waves, while a single electrode only originate a single half wave at once. If the number of positive and negative half waves fits to the number of electrodes, highest amplitudes of the desired mode with a minimum length of the wave packet can be produced. While the primarily excited A 0 mode shows rising amplitudes for an increasing number of pulses and electrodes, the S 0 mode does not show this distinct behaviour (see Fig. 11). The influence of the number of pulses shows a similar effect as before, but the number of electrodes does not. The reason for this is the modification of the frequency response function of the S 0 mode caused by different numbers of electrodes. With an increasing number of electrodes, the location of zeros and maximums of the side lopes within the response function is varying.

6 D. Schmidt et al. Fig. 10 Max. amplitude of the A 0 mode at different numbers of pulses and electrodes, electrode width of 8 mm, 40 khz, 2 mm thick CFRP plate Thus, the set-up of seven electrodes shows a zero location close to the actuation frequency of 40 khz, whereas the set-up of eight electrodes shows a maximum close to actuation frequency of 40 khz. 5 Experimental tests Based on the investigations on the finite element model in the forth section, a mode selective actuator is designed and manufactured. The actuator is designed to attenuate the S 0 mode and thus to amplify the A 0 mode at a frequency of 40 khz. At this frequency only the lowest order of symmetric and anti-symmetric modes exists which is a first reduction of the complex Lamb wave propagation field. Due to the dispersion diagram of the CFRP plate, the wavelength of the A 0 mode at 40 khz is approximately 20 mm. The actuator is manufactured as a piezocomposite and consists of five piezoceramic elements (PIC 255, PI Ceramic GmbH) which have respective dimensions of mm. The element width of 8 mm shows sufficient amplitudes of the A 0 mode, as shown in Fig. 9. The distance between the elements corresponds to the halfwavelength of 10 mm. The final actuator that is used for the experimental tests is shown in Fig. 4. The actuator is bonded in 0 -direction on the upper surface of the CFRP plate which is presented in Sect. 3. As bonding layer a cyanoacrylate adhesive (Sicomet Ò 50, Henkel AG & Co. KG) is used. Due to the low viscosity of the uncured adhesive, very thin and uniform bonding layers can be ensured. Each actuator element is excited by a rectangle burst signal with three bipolar pulses, whereby the signal of adjacent elements has a 180 phase difference. The voltage of the excitation signal is 15 V (peak to peak). In order to distinguish the S 0 mode from the A 0 mode a pair of circular piezoceramic sensors is collocated bonded on the upper and lower plate surface. In case of the symmetric S 0 mode both sensors show equal amplitude signals over time without a phase shift. But in case of the anti-symmetric A 0 mode the amplitude signals of the upper and lower sensor show a 180 phase shift. The sensors are piezocomposites with an embedded circular piezoceramic disc (PIC 255, PI Ceramic GmbH) which has a diameter of 10 mm and a thickness of 0.2 mm. The distance between the actuator and the sensors is set to 200 mm. The generation of excitation signals as well as the recording and analysing of sensor signals is done by the USPC 5000 Health-Monitoring-System (Ing.-Büro Dr. Hillger). The sensor signals are filtered using a 12th order band pass of khz. The amplitudes are measured building the absolute value of the signal of each Lamb wave mode. In a first setup only the first element of the actuator is driven. The sensor signals in Fig. 12 show that the A 0 and Fig. 11 Max. amplitude of the S 0 mode at different numbers of pulses and electrodes, electrode width of 8 mm, 40 khz, 2 mm thick CFRP plate Fig. 12 Experimental set-up and sensor signal by driving the first element of the actuator

7 Design of mode selective actuators for Lamb wave excitation in composite plates As a result, the experimental tests show a good correlation with the finite element model. The designed actuator can be sufficiently attenuated by the S 0 mode in a CFRP plate. Furthermore, an apodization modifies the frequency response of the actuators in such a way that the mode selectivity can be improved. 6 Conclusion Fig. 13 Experimental set-up and sensor signal by driving all elements of the actuator with applied apodization Table 1 Overview of the signal amplitudes of A 0 and S 0 mode, 40 khz, 2 mm thick CFRP plate Number of excited actuator elements Finite element model A 0 mode amplitude (%) S 0 mode amplitude (%) Experimental test A 0 mode amplitude (%) 1-unapodized unapodized apodized S 0 mode amplitude (%) the S 0 mode are generated in an amplitude ratio of 100 to 11 %. For comparison, the amplitude ratio out of the finite element model is 100 to 15.1 %. In a second setup all elements of the actuator are driven. The resulting amplitude ratio is 100 % of the A 0 mode to 1.7 % of the S 0 mode. The finite element model shows an amplitude ratio at this configuration of 100 to 1.1 %. The amplitude ratio can be improved by applying an apodization. This is realized by controlling the voltage of the excitation signal of each actuator element using adjustable resistors. The apodization is set in such a way that the S 0 mode shows minimal amplitudes over the frequency bandwidth of the sensor signal. The aim of the apodization is to modify the frequency response functions of the S 0 mode as well as the A 0 mode so that a broadband reduction of the S 0 mode is achieved. Figure 13 shows the signal amplitudes of the apodized actuator. The signal amplitude of the S 0 mode is reduced to 0.2 % in contrast to the amplitude of the A 0 mode of 100 %. Table 1 summarizes the results of the finite element modelling and the experimental tests. Within the finite element modelling an apodization of the actuator is not realized. Structural Health Monitoring based on Lamb waves is a promising technology for in-service inspection of aerospace structures. Nevertheless, the Lamb wave propagation field is very complex due to the presence of at least two Lamb wave modes at any given frequency, their dispersive characteristic and their interference at structural discontinuities. To reduce the complex wave propagation field, the paper presents mode selective actuators which are able to excite a particular Lamb wave mode in CFRP plates. The principle design rules and different manufacturing processes for mode selective actuators are presented. Starting point for the design of actuators is the determination of the dispersion diagram of the CFRP plates in order to estimate the wavelengths of A 0 and S 0 mode at different frequencies. It could be demonstrated that the air-coupled ultrasonic technique is a suitable methodology for this task. Different parameters of an IDT such as number and width of electrode segments as well as excitation signals are investigated using a finite element model. Based on these investigations a mode selective actuator is manufactured and experimentally tested. This actuator is designed to amplify the A 0 mode at 40 khz in the CFRP plate. The results of the finite element modelling and the experimental tests confirm that mode selective actuators are able to excite a particular Lamb wave mode. Acknowledgments The DLR-Institute of Composite Structures and Adaptive Systems performs basic research on Structural Health Monitoring within a project of the German Research Foundation (DFG) in cooperation with several partners of the Helmut Schmidt University and the Otto-von-Guericke-University Magdeburg. The authors would like to thank the German Research Foundation as well as all project partners for their support. References 1. Giurgiutiu, V.: Structural Health Monitoring with Piezoelectric Wafer Active Sensors, Academic Press, Elsevier, (2008) 2. Rose, J.L.: Ultrasonic waves in solid media. Cambridge University Press, Cambridge (2004) 3. Monkhouse, R.S.C., Wilcox, P.W., Lowe, M.J.S., Dalton, R.P., Cawley, P.: The rapid monitoring of structures using interdigital Lamb wave transducers. Smart Mater. Struct. 9, (2000) 4. Veidt, M., Liu, T., Kitipornchai, S.: Modelling of Lamb waves in composite laminated plates excited by interdigital transducers. NDT&E International 35, (2002)

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