Robust Structural Health Monitoring Transducers Based on LTCC/PZT Multilayer

Sylvia Gebhardt, Markus Flössel, Andreas Schönecker Fraunhofer Institute for Ceramic Technologies and Systems

Dresden, Germany Sylvia.Gebhardt@ikts.fraunhofer.de

Uwe Lieske, Thomas Klesse Fraunhofer Institute for Nondestructive Testing

Dresden, Germany

Abstract—Structural Health Monitoring (SHM) systems allow for permanent and online characterization of condition and health of structural components. We introduce novel SHM modules fully based on inorganic materials which show a distinguished improve of temperature and chemical stability compared to known piezoceramic-polymer-patches. Using ceramic multilayer technology lead zirconate titanate (PZT) discs are integrated into a package of low temperature cofired ceramic (LTCC) sheets enabling for the production of compact ultrasonic transducers with integrated electronic circuits and signal processing units.

Keywords-LTCC/PZT modules; multilayer; microsystems; structural health monitoring; piezoceramic; ultrasonic transducer; lamb waves

I. INTRODUCTION

Structural Health Monitoring (SHM) is an innovative technology allowing to permanently monitor and record the health and condition of structural components e.g. of aircraft, rotorcraft, pipelines, bridges, wind turbines and automobiles. Thereby a sensor network becomes integral part of the engineering construction detecting and controlling damages of the system.

The use of Lamb waves has been considered as promising technique for nondestructive testing of structural components. Lamb waves (also known as plate waves) are two-dimensional acoustic waves that can be generated in relatively thin solid plates having free boundaries. Lamb waves can be excited by ultrasonic transducers and propagate over considerably long distances [1].

Most SHM systems using transmission of Lamb waves are based on piezoceramic/polymer patches acting as ultrasonic transducer. They are flexible but show low temperature and chemical stability. Moreover integration of electronic control and signal processing units are not solved yet.

We recently introduced a new approach of SHM transducers made by ceramic multilayer technology [2, 3]. The basic idea of the concept is to laminate sintered piezoceramic components e.g. lead zirconate titanate (PZT) discs into a package of low temperature cofired ceramic (LTCC) green layers and to subsequently sinter the assembly. The so-called LTCC/PZT multilayer combines the advantages of microsystems- and piezo-technology. 3D electronic circuits can be integrated into the LTCC package and are connected to

signal processing units which are soldered on top of the device. Ultrasonic transducer function is provided by the piezoceramic disc. This approach gives rise to very compact and robust SHM devices.

II. DESIGN AND PREPARATION

The preparation of LTCC/PZT multilayer was already discussed in detail in [4]. Fig. 1 shows a schematic setup of the here considered SHM transducer layout.

Figure 1. Schematic layout of LTCC/PZT multilayer

We used Heraeus “HeraLock® Tape HL2000” green sheets as basic LTCC material. The setup consisted of 3 LTCC layers above and below the PZT disc, and 2.5 LTCC layers around the PZT disc. One LTCC layer consisted of two single LTCC sheets with 133 µm green layer thickness each. The green layer thickness of the middle LTCC layer was adjusted to obtain a

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thickness of about 0.54 mm after sintering which corresponds to the thickness of the PZT component. As such we used electroded PZT discs of PIC 255 (Ø = 9.97 mm, t = 0.54 mm) from PI Ceramic GmbH, Germany. The cavity of the LTCC middle layer was cut by laser machining. Internal electrodes and catch pads applied by screen printing technology as well as through connections by vias filled with electrode pastes were used for reliable termination of the PZT disc and connection to the surface of the LTCC/PZT multilayer. The assembly was thereafter laminated and isostatically pressed. Sintering was carried out using a sintering press “PEO 603” from ATV Technologie GmbH, Germany with binder burnout at Tburn = 450 °C for tburn = 2 h and firing at Tsint = 865 °C for tsint = 30 min under pressure of p = 0.05 MPa.

Nine identical SHM transducers were manufactured parallel in one (100 x 100) mm panel as shown in Fig.2a. In order to investigate design rules for effective emitting and receiving of acoustic waves, geometrical size of the single SHM transducers was varied. Therefore separation into single transducer samples was done by wafer dicing for square and laser machining for round shapes. Transducers with geometries: (20 x 20) mm, (25 x 25) mm, Ø 20 mm and Ø 25 mm were fabricated for electrical characterization and comparison to FEM simulation (Fig. 2b).

Figure 2. (100 x 100) mm LTCC/PZT multilayer panel (a) for preparation of square and round SHM transducers separated by wafer dicing or laser

machining (b)

III. CHARACTERIZATION

All SHM transducers were polarized for 5 min at 2 kV/mm at room temperature. Permittivity and dielectric loss of all samples were measured (1 V, 1 kHz) both before and 24 h after polarization. Ferroelectric hysteresis curves were determined at 10 Hz with electric fields up to 2 kV/mm using a Sawyer-Tower circuit. For comparison of material properties single PZT plates of the same material have been characterized as reference samples.

For investigation of the emission and receiving characteristics SHM transducer samples were glued on a 600 mm x 300 mm aluminum plate with 2 mm thickness using Automix Rapid 03 (Delo) epoxy adhesive. Measurements using laser vibrometer were carried out to characterize the angular radiation pattern of square and round shaped transducers. The measurement setup is shown in Fig. 3. For excitation of ultrasonic waves a single pulse of U = 100 V amplitude (Fig. 4) was applied to all transducers.

600mm

300m

m

100mm100mm

Aluminum plate

LTCC

90° Scan

70mm 70mmScaning directionLaservibrometer

Scaning directionLaservibrometer

600mm

300m

m

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LTCC

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70mm 70mmScaning directionLaservibrometer

Scaning directionLaservibrometer

Figure 3. Measurement setup for transducer characterization

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100

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Figure 4. Excitation waveform

IV. FEM SIMULATION

FEM simulation was carried out using ANSYS Multiphysics. Boundary conditions were selected according to the actual measurement setup as depicted in Fig. 5. For symmetry reasons only the upper half of the plate was calculated.

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(b)

Aluminum plateAdhesiveLTCCPZT-Disc

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z

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Aluminum plateAdhesiveLTCCPZT-Disc

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540 µm

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Figure 5. FE mesh and cross section of FE model for SHM transducer

The material properties are listed in Table I. LTCC and adhesive were assumed to exhibit linear elastic and isotropic behavior due to small strains of ultrasonic waves.

TABLE I. MATERIAL PROPERTIES FOR FEM SIMULATION

PZT: PI Ceramic (PIC255) [7] �� 7.80E+03 kg/m3

e31 -7.10 N/Vm e33 13.70 N/Vm e15 33178 N/Vm cE

11 1.230E+11 N/m² cE

33 9.711E+10 N/m² cE

12 7.670E+10 N/m² cE

13 7.025E+10 N/m² cE

44 2.226E+10 N/m² cE

66 2.315E+10 N/m² �S

11/�o� 930 (unitless) �S

33/�o� 857 (unitless) Aluminum (AW 5754)

E 70E+09 Pa �� 2700 kg/m³ �� 0.34

LTCC: Heraeus (HeraLock® Tape HL2000) E 120E+09 Pa �� 2900 kg/m³ �� 0.24

Adhesive: Delo (Automix 03 Rapid) E 2E+09 Pa �� 1160 kg/m³ �� 0.34

V. RESULTS AND DISCUSSION

Results on dielectric and electromechanical data show that the surrounding LTCC multilayer package has a significant impact on PZT material properties. Table II gives an overview on relative permittivity and remnant polarization values for the SHM transducer. Dielectric properties as well as remnant

polarization are reduced compared to values measured on single PZT disc. This tendency has been also cited in [5] and can be explained by chemical reaction of the PZT with the surrounding LTCC [6] as well as mechanical clamping of the PZT disc in the LTCC multilayer setup. Measurements as well as FEM calculations to prove the latter are currently under investigation.

TABLE II. DIELECTRIC AND FERROELECTRIC MATERIAL PROPERTIES OF SHM TRANSDUCER COMPARED TO SINGLE PZT DISC

�33T/�0

@ 1 kHz tan �

@ 1 kHz Pr [µC/cm2]

@ 10 Hz Ec [kV/mm]

@ 10 Hz

Single PZT disc (PIC 255)

1609 0.019 31.1 1.56

LPM with integrated PZT disc

800 0.013 10.1 1.20

For characterization of ultrasonic transducer performance Lamb waves generated by transmission of ultrasonic waves into the aluminum plate were detected by laser vibrometer measurements. Fig. 6 shows the radiation pattern generated by square and round shaped SHM transducer after 50 µm travel time.

Although embedded PZT disc was of the same size and shape in all SHM transducers angular radiation pattern of square and round transducer samples differ. This fact supports the assumption that the outer LTCC shape mainly determines radiation pattern of transmitted acoustic waves. Round SHM transducer (Fig. 6b) show uniform angular radiation pattern whereas square SHM transducer (Fig. 6a) exhibit preferred excitation in 0°, 90° and 45° direction which corresponds to ultrasonic waves whose wavelength is �/2 of SHM transducer size in the actual direction. The correlation of �/2 resonances to the transducer size was also preliminary observed in [8].

Figure 6. Radiation pattern of square (25 x 25) mm (a) and round Ø 20 mm (b) SHM transducer, snapshot at 50µs

The measured data are also proved by calculated results using FEM modeling as shown in Fig. 7. Simulated radiation pattern of round SHM transducer show uniform transmission of ultrasonic waves, again. The square SHM transducer exhibits a non uniform angular radiation pattern with raised signals in certain wave packages at 0° and 90° directions. Qualitatively there is a very good agreement between measured and

(a) (b)

calculated data. For quantitative predictions damping coefficients have to be adjusted to fit the measured results.

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Figure 7. Radiation pattern of round SHM transducer Ø 25 mm (upper), square SHM transducer (25 x 25) mm (lower) calculated by FEM simulation,

snapshot at 50µs

For the envisaged acoustic diagnostics of materials and structures using signature analysis a uniform transmission of ultrasonic waves is essential. Squared SHM transducer make the examination of signals difficult and require an enhanced computational effort for compensation of transmission and receiving anisotropy. Compensation might be possible for simple structures but fail as structure dimension and shape become more complex. Detected ultrasonic signals of round SHM transducers show less higher harmonics compared to square SHM transducer as can be seen from Fig. 8. That’s why fabrication of round SHM transducers should be favored although separation by laser machining is more laborious and time consuming compared to wafer sawed structures.

Figure 8. Measured receiving signal with anisotropic distortion for square SHM transducer (25 x 25) mm (lower) compared to uniform signal for round

SHM transducer � 20 mm (upper)

VI. CONCLUSION

LTCC/PZT multilayer offer the possibility of fabricating SHM transducer fully based on inorganic materials. This approach not only enables for a tremendous improvement of the robustness against environmental influences (e.g. chemical loads, humidity, working temperature) but also for integration of 3D electronic circuits, signal processing and electronic control. Typical application fields are seen in chemical and aircraft industries, where high temperature and chemical robustness are needed.

Performance of the so manufactured transducers is not only influenced by piezoceramic material properties but to a great extend by interaction with the surrounding LTCC material. Investigation on the emitting and receiving behavior showed that round transducer shapes allow for uniform signals and best transducer performance.

ACKNOWLEDGMENT

The authors gratefully acknowledge financial support by Allianz Industrie Forschung (AiF) and German Research Foundation DFG within SFB/Transregio 39 PT-PIESA.

REFERENCES

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[2] M. Flössel, U. Lieske, T. Klesse, and S. Gebhardt, “Ceramic Based Structural Health Monitoring (SHM) Modules for Rough Environment,” Proc. ACTUATOR 2012, Bremen, Germany, pp. 68-71, June 18-20, 2012, www.actuator.de.

[3] S. Gebhardt, D. Ernst, B. Bramlage, M. Flössel, A. Schönecker, “Integrated Piezoelectrics for Smart Microsystems – a Teamwork of Substrate and Piezo, ” Proc. CIMCTEC 2012, Montecatini Terme, Italy, June 10-14, 2012

[4] M. Flössel, S. Gebhardt, A. Schönecker, A. Michaelis, “Development of a Novel Sensor-Actuator-Module with Ceramic Multilayer Technology,” Journ. Ceram. Sci. Tech., vol. 01, pp. 55-58, 01/2010.

[5] M. Sobocinsky, R. Zwierz, J. Juuti, H. Jantunen, and L. Golonka, “Electrical and Electromechanical Characteristics of LTCC embedded Piezoelectric Bulk Actuators,” Advances in Applied Ceramics, vol. 109, pp. 135-138, 3/2010.

[6] M. Flössel, S. Gebhardt, A. Schönecker, A. Michaelis, “Investigation on LTCC/PZT interface in a novel sensor-actuator-module for metal die casting,” ISAF 2011, Vancouver, Canada, July 25-27, 2011

[7] PI Ceramic GmbH, Datasheet [8] M. Roellig, L. Schubert, U. Lieske, B. Boehme, B. Frankenstein, and N.

Meyendorf, “FEM assisted Development of a SHM-Piezo-Package for Damage Evaluation in Airplane Components,” Proc. of EuroSimE 2010, Bordeaux, France, April 26-28, 2010

Raised amplitude