Lead zirconate titanate/polyurethane(PZT/PU) composite for acoustic emission sensors W.K Sakamoto,P.Marin-Franch, D.Tunicliffe and D.K Das-Gupta Universidade Estadual Paulista-UNESP/Ilha Splteira, Sao Paulo-Brazil University of ales Bangor-UWB, School of Informatics BAE SYSTEM, Advanced Technology Centre-Sowerby

Abstract  Piezoelectric composite, made from Ferroelectric ceramic lead zirconate titanate(PZT) and vegetable based polyurethane(PU) polymer was doped with a semiconductor filer, graphite.  Resulting composite(PZT/PU):49/1/50 vol.% composition poled at lower field and shorter time due to the increased conductivity of the polymer phase following the introduction of graphite.  The PZT/C/PU composite showed higher pyroelectric coefficient in comparison with the undoped PZT/PU composite with 50/50-vol.% composition.  PZT/C/PU composite has shown the ability to detect both extensional and flexural modes of simulated acoustic emission at a distance up to 8.0m from the source, thus indicating that it may be used for detection of structural damages.

1. Introduction  Transducer senses a dynamics stress waves propagating through a structure which was generated by the release of energy due to a failure mechanism.  The elastic waves produced by an AE source is converted to a voltage signal by a resonant transducer and parameters such as peak amplitude or energy can be recorded.  These parameters are related to load, strain and temperature.  AE sensor can facilitate the continuous and non-destructive monitoring of the structure health.  Ferroelectric ceramic/polymer composites can combine mechanical strength and flexibility of polymer with the high piezo and pyroelectric activities of ceramic  The difficulty with ceramic/polymer composite is in achieving an efficient polarization of ceramic dipoles, because most of the applied voltage drops across the polymer phase.

1. Introduction  By adding a small amount of semiconductor filer a resistance in parallel with that of polymer is introduced, thus the resistance of the polymer phase and the poling process becomes effective.  The present paper reports some results of dielectric and electroactive characteristation of both PZT/PU and PZT/C/PU composite to detect structural damages.

2. Sample Preparation  diameter from 3um to 10um and fine-grained graphite were used as ferroelectric ceramic and semiconductor filer.  Vegetable-based polyurethane was added to both powders and mixed manually.  The mixture was pressed at 20Mpa at room temperature between two greased alumimium foil.  Aluminium electrodes with 1.0cm of diameter were vacuum evaporated onto both surface of the sample.  The PZT/C/PU composites 49/1/50-vol.%,in the thickness range of 200um to 350um were poled at 5*10 6 V/m DC field for 30 minute at 373K in silicone oil bath.  The PZT/PU(50/50vol.%)composites in the thickness range of 150um to 250um were poled at same temperature at 1*10 7 V/m for 1 hour.

3. Measurements  The pyroelectric currents were measured using a direct method in which a linear heating rate of 1 deg/min was applied to the polatised samples  Piezo d 33 Tester was used to measure the d 33 coefficient  The composite was stretched in the thickness direction and the electrical potential from piezoelectricity is compared with the value obtained from the standard ceramic sample.  The electromechanical coupling factor k t was obtained from the measurement of the complex impedance around the peak of the composite acting as a free resonator.  Using two simulated acoustic emission sources, ball bearing drop and pencil lead break, AE tests were carried out.  PZT/PU and PZT/C/PU were surface mounted on a 56*56cm fibreglass reinforced board(FRB)

4. Results and Discussion 1  In the direct method of measuring pyroelectric coefficient,a polarised sample is heated in a chamber at a reduce pressure(3*10 -2 torr) at a constant rate(1.0 deg/min) with its electrodes shorted and the current is monitored with an electrometer.  Pyroelectric coefficient (1) Ip: pyroelectric current, A: sample electrode area dT/dt: constant heating rate

4. Results and Discussion 2  Figure 1 shows the nature of the reversible pyroelectric current in the range of temperature of 300K to 353K in both PZT/C/PU and PZT/PU composites poled as described earlier.  The values of p(T):5.6pC/m 2 K for PZT/PU and 10.7pC/m 2 for PZT/C/PU at 303K.  The increased value of p(T) for PZT/C/PU composite can be attributed to the efficiency of poling process in the composite doped with small amount of graphite(1.0vol.%)  The semiconductor filler can create a continuous electric flux path in the polymer phase, thus reducing the voltage drop across the polymer phase. Fig 1-Reversible pyrorlrctric current for PZT/C/PU(poled at 373K in E=5MV/m for 30min)and PZT/PU(poled at 373K in E=10MV/m for 1 h) composite

4. Results and Discussion 3  The efficiency of poling process in graphite doped composite was observed also with the d 33 values of the piezoelectric coefficient.  For both composites poled as described earlier(E=10 7 V/m for PZT/PU and E=5*10 6 V/m for PZT/C/PU) the d 33 value obtained was 13.0pC/N  The electromechanical copuling factor k t was obtained by fitting the experimentally measured impedance using the equation. (2)  C 0 :capacitance, Ψ :mechanical loss, f 0 : resonance frequency R max is the amount of the real part of the impedance in the resonance frequency from the base line.  Ψ :0.13, f 0 :6.0MHz, R max :3, C 0 :106 pF  Used in equation 2 gave k t =0.04

4. Results and Discussion 4  Figure 2 shows the fitting of the real impedance of the composite.  To characterise the composite as AE detector, the FRB panel was exited using two different simulated source.  Ball bearing drop produces relatively large amplitude and low frequency stress waves while the pencil lead break produces stress waves with low amplitude and higher frequency. Figure 2-Experimental and theortical real impedance of PZT/PU 50/50 composite around resonant peak

4. Results and Discussion 5  Fig 3 shows the response of the both sensors to increasing energy impacts.  Taking a noise level of 2.0mV, it can be predicted that the lowest energy level detectable for PZT/PU and PZT/C/PU are 4.0*10 -6 and 3.0* Figure 3-Response of the sensors to a ball-bearing drop test

4. Results and Discussion 5  The sensors were also compared for their ability to detect AE at different distance.  Ball bearing drops of fixed height(5cm) were used.  Figure 4 shows that the sensor response follow an inverse law to distance.  The maximum distance in which the 0.25mJ ball bearing impact can be detected are 615cm and 821 for undoped and graphite- doped composite. Figure 4-Response of the sensor to a different distance AE sources

4. Results and Discussion 6  The pencil lead break experiment can show the ability of the sensor to detect the composite of a plate wave.  Figure 5 shows the time response of the PZT/C/PU sensor in which the extensional and flexural modes can be clearly observed. Figure 5-Response of the PZT/C/PU(49/1/50 vol.%) composite to pencil lead break experiment

4. Results and Discussion 7  The fast Fourier Transform(FFT) of the data of figure 5 is shown in figure 6.  A peak occuring at 5kHz due to the flexural mode and the peak at 75kHz may be attributed to the extensional mode. Figure-6 FFT of the response of the PZT/C/PU composite to a lead break

4. Results and Discussion 8  Adding small amount of semiconductor filler made PZT/C/PU composite better sensor for AE detection because the poling process became more efficient.  The sensitivity of the graphite-doped sensor was increased by a factor of 25% in comparison with the undoped sensor.  Further work is in progress on AE detection to obtain the response of the embedded sensor.