A Novel Sensor Design for Generation and Detection of Shear-Horizontal Waves Based on Piezoelectric Fibres

Transcription

1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic A Novel Sensor Design for Generation and Detection of Shear-Horizontal Waves Based on Piezoelectric Fibres Frank SCHUBERT 1, Bernd KOEHLER 1, Uwe LIESKE 1 1 Fraunhofer Institute for Ceramic Technologies and Systems, Branch Materials Diagnostics (IKTS-MD); Dresden, Germany; Phone: , Fax: ; frank.schubert@ikts-md.fraunhofer.de, bernd.koehler@ikts-md.fraunhofer.de, uwe.lieske@ikts-md.fraunhofer.de Abstract In the present work a new concept for a shear horizontal ultrasonic wave transducer based on piezoelectric fibre patch (PFP) technology is presented. In this type of sensor two separate fibre layers with opposite polarization are tilted by ±45 relative to the electrode fingers. Numerical 3-D simulations based on the Elastodynamic Finite Integration Technique (EFIT) show that with this new PFP design SH mode generation and detection is much more effective than with the hitherto existing PFP concepts. Due to its small thickness and flexibility the new PFP transducer is perfectly suited for SHM applications with embedded or surface-mounted sensors. Keywords: Shear horizontal waves, piezo-fibre patch (PFP), structural health monitoring (SHM), elastodynamic finite integration technique (EFIT) 1. Introduction The use of shear horizontal (SH) waves in nondestructive evaluation (NDE) and structural health monitoring (SHM) has significant advantages compared to the application of other wave modes. SH waves show no mode conversion while propagating parallel to the testing surface which leads to clear and well interpretable signals. Moreover, the leaky wave emission into fluid media is suppressed so that the loss of energy is small and the operating distance is correspondingly long. In plate-like structures the lowest-order SH mode (SH0) is free of dispersion and is therefore perfectly suited for long-range applications. With electromagnetic transducers (e.g. EMATs) SH waves can be directly generated and detected inside the material. However EMATs are relatively large and restricted to conductive materials. Conventional piezoelectric transducers have to be coupled by glue or highly viscous fluids in order to transmit shear forces. Moreover, they need a large backside seismic mass for the effective insertion of the shear forces. Both types of transducers are therefore not suitable for SHM applications. Piezoelectric Fibre Patches (PFPs) are well known from their adaptronic applications. However, they can also be used for the generation and detection of symmetric (S) and antisymmetric (A) Lamb waves in the context of SHM with guided elastic waves (see [1] for instance and Fig. 1). In order to demonstrate the main excitation mechanism for elastic waves a single pair of electrodes connected to the PZT fibres is displayed in Fig. 2. This configuration represents a one-dimensional capacitor. If a voltage U is applied to the electrodes an electrical field E is generated and an elastic stress = e E is produced in the plate-like region between the two electrode fingers. e := e 22 is the relevant piezoelectric constant and amounts to 15.8 As/m² for PZT-5A for instance. Due to the piezoelectric forces generated at the surface of the plate-like region (red arrows in Fig. 2) a bipolar elastic pulse is excited from a single pair of electrodes. Various snapshots of wave propagation in a PFP due to this bipolar excitation are given in Fig. 3. They were calculated by using the elastodynamic finite integration technique (EFIT, [2]).

2 Figure 1. Principle of Piezo Fibre Patches (PFPs) for adaptronic and SHM applications. PZT fibres are encapsulated in an epoxy matrix and joined with electrode fingers of alternating polarity. (Source: NMW GmbH Wuerzburg, Germany and Fraunhofer ISC, Wuerzburg, Germany). F = ee y d U F = -ee Figure 2. Principle of bipolar wave excitation in a PFP caused by a single pair of electrodes. The PZT fibres are lying in the direction of the y-axis. If a transducer with the lateral dimensions as used in Fig. 3 would be coupled to a plate the excitation mechanism would lead to a strong S0 wave in y-direction since compressive and tensile forces are dominant. All other wave modes would be rather weak and could be neglected. If instead of a single pair of electrodes various pairs with alternating polarity are used as shown in Fig. 1 (picture on the right) the piezoelectric forces at the inner electrode fingers cancel out each other and only the forces at the outer electrodes remain.

3 Figure 3. Results of 3-D EFIT simulation for a bipolar excitation from one pair of electrodes of a free PFP. Left row: time snapshots of the u y displacement component across the complete PFP; second row: u y displacement component along the y-axis at x = 2.5 cm. The duration of the pulse excitation was 2 µs.

4 2. Conventional piezo fibre patch concept So far the excitation of SH modes with conventional PFPs as shown in Fig. 3 is very ineffective and only based on edge effects as a by-product of S- and A-mode excitation. This is demonstrated in Fig. 4, where the results of a 3-D EFIT simulation of wave propagation generated by a conventional PFP in a mm³ aluminum plate are shown. Figure 4. Numerical 3-D EFIT simulation of guided wave propagation in a mm³ aluminum plate generated by a conventional PFP with a size of mm² coupled to the surface of the plate. The piezo fibres are oriented in 0 and 180 direction (x axis) while the electrode fingers are pointing to the ±90 direction (y axis). Top row: Time snapshots of the absolute value of the particle velocity vector, v, across the top surface of the plate for times t 1 = µs, t 2 = µs, t 3 = µs; bottom row (from left to right): v x, v y and v z component of v at time t 3 = µs. The PFP in Fig. 4 has a size of mm² and is coupled to the top surface of the plate. The piezo fibres are oriented in 0 and 180 direction (x-axis) while the electrode fingers are pointing to the ±90 direction (y-axis). The thickness of the PFP in y-direction corresponds to half of the SH wavelength in aluminum (15.2 mm at f C = 100 khz) in order to produce strong SH waves. It should be noted that in this case the PFP is not optimized to effective S0 wave excitation (in contrast to the example in Fig.3). Nevertheless one can see from the wave front snapshots in Fig. 4 that beside the SH waves also significant S0 and A0 waves are generated with a specific directivity. The SH waves have their maxima in two main lobes in immediate vicinity of the ±90 direction (bottom left picture in Fig. 4). However they also show a directional null in this ±90 direction but also in 0 and 180 direction (x-axis). This is caused by the conventional PFP mechanism as shown in Fig. 5 (top picture).

5 Figure 5. Excitation principle for conventional PFP concept (top picture) and for the new concept (bottom picture). In the latter case two separate fibre layers with opposite polarization are tilted by ±45 relative to the electrode fingers. The conventional PFP consists of one single fibre layer with fibres pointing in ±x direction (blue lines) and of perpendicular electrodes (black lines) with alternating polarity. Therefore all mechanical forces generated at the inner electrode fingers cancel out each other (green arrows). Only the forces at the outer fingers remain and produce Lamb waves (red arrows). It is obvious that this kind of excitation directly leads to an SH zero point in ±y-direction. 3. Novel piezo fibre patch concept In order to avoid the ineffective SH wave excitation of conventional PFP transducers we propose a new design where two separate fibre layers with opposite polarization are tilted by ±45 relative to the electrode fingers (Fig. 5, bottom picture). Again all forces generated at the inner electrode fingers cancel out each other (green arrows). In contrast to the conventional PFP shear forces at the edges of the active area remain (red arrows). They should lead to a significantly better SH wave excitation without zero points in ±y direction. In order to verify this assumption another 3-D EFIT simulation based on the novel sensor design was performed. The size of the sensor is identical to the simulation shown in Fig. 4. The results in Fig. 6 demonstrate that the new excitation mechanism leads to a strongly focused SH wave in ±90 direction without any directional null in this direction. There are weaker SH waves in ±x direction since the length of the PFP in x-direction is not optimized for SH wave excitation (in contrast to the vertical dimension). Moreover, only a very weak A0 excitation can be observed. Significant S0 contributions are visible in two main lobes in the vicinity of the ±y direction.

6 Figure 6. Numerical 3-D EFIT simulation of guided wave propagation in a mm³ aluminum plate generated by a novel PFP transducer with a size of mm² coupled to the surface of the plate. The piezo fibres are oriented in ±45 direction (x-axis) while the electrode fingers are pointing to the ±90 direction (y-axis). Top row: Time snapshots of the absolute value of the particle velocity vector, v, across the top surface of the plate for times t 1 = µs, t 2 = µs, t 3 = µs; bottom row (from left to right): v x, v y and v z component of v at time t 3 = µs. Each PFP can be optimized with respect to the desired wave mode, its directivity and the center frequency of the excitation. In the present case our goal was to excite strong SH waves in ±y direction having a center frequency of 100 khz. Therefore the thickness or vertical size of the PFP was chosen to coincide with half of the wavelength of the SH wave in aluminum, i.e mm in this case. However the length of the PFP, i.e. its size in x-direction, can be chosen arbitrarily in order to stronger focus or defocus the perpendicular SH wave field. In Fig. 7 we increased the length of the PFP to 120 mm (instead of 60 mm) which leads to an increased focusing of the SH wave in ±y direction. However the main lobes of the S0 wave are also increased and their maxima are shifted towards the y-axis which may lead to disturbing effects in practice. Moreover the A0 contributions seem to be increased as well. From these findings it becomes clear that for practical applications the choice of the length of the PFP could be critical and should always represent a good compromise between strength and directivity of the desired wave mode and of the disturbing secondary wave modes.

7 Figure 7. Numerical 3-D EFIT simulation of guided wave propagation in a mm³ aluminum plate generated by a novel PFP transducer with a size of mm² coupled to the surface of the plate. The piezo fibres are oriented in ±45 direction (x-axis) while the electrode fingers are pointing to the ±90 direction (y-axis). Top row: Time snapshots of the absolute value of the particle velocity vector, v, across the top surface of the plate for times t 1 = µs, t 2 = µs, t 3 = µs; bottom row (from left to right): v x, v y and v z component of v at time t 3 = µs. Conclusions and Outlook In this paper it was demonstrated that a novel PFP concept with two separate fibre layers with opposite polarization tilted by ±45 relative to the electrode fingers can effectively produce strong directional SH waves without the characteristic directional null of conventional PFPs with one single fibre layer. The results were obtained for an isotropic aluminium plate but can be generalized to pipes, other shell structures and also to composite materials if necessary. However, the new transducer concept needs to be validated experimentally in the future. With the new PFP design, a novel so far missing type of sensor for SHM with guided waves will be available. Convenient applications can be found in monitoring of structures with permanent or temporal fluid contact (e.g. piping systems with fluid transport medium, underwater offshore structures or civil engineering structures exposed to rain or moving water). For reasons of symmetry the direction of the electrode fingers can be rotated by ±90 without changing the overall functionality of the new PFP. Nevertheless, an electrode configuration perpendicular to the desired beam direction is preferred because in this case a frequency adaptation of the excited (and detected) wave field is possible by selectively switching certain electrode fingers on and off. In this way the width of the active area can be changed and optimized to the desired frequency. Moreover a phase delayed and mode-selective excitation and detection is also possible based on various trace wave lengths.

8 References 1. M. Lehmann, A. Büter, B. Frankenstein, F. Schubert, B. Brunner, Monitoring system for delamination detection Qualification of structural health monitoring (SHM) systems, In: Proc. of Conf. on Damage in Composite Materials, CDCM 2006, Stuttgart, September 2006, also published in NDT.net - The e-journal of Nondestructive Testing - ISSN: , December 2006, Vol. 11 No F. Schubert, Numerical time-domain modeling of linear and nonlinear ultrasonic wave propagation using finite integration techniques Theory and applications, Ultrasonics 42, , 2004.