Novel Approach to Make Low Cost, High Density PZT Phased Array and Its Application in Structural Health Monitoring
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1 Novel Approach to Make Low Cost, High Density PZT Phased Array and Its Application in Structural Health Monitoring B. XU, S. BUHLER, K. L1TIAU, S. ELROD, S. UCKUN, V. HAFIYCHUK and V. SMELYANSKIY ABSTRACT In this paper we present a cost effective method to fabricate flexible, high density piezoelectric tape, and expl'ore its potential application as a new type of Lamb wave phased array for Structural Health Monitoring (SHM) through simulating the Lamb wave propagation generated from single element and multielement arrays. The piezoelectric tape consists of many patterned and densely packed PZT (lead zirconate titanate) elements sandwiched between two flexible films/foils. The PZT elements can have different shapes and distribution densities and can be addressed individually and/or by groups. The piezoelectric tape phased array can be easily integrated into various mechanical structures and has good conformability to curved surfaces. INTRODUCTION Using Lamb waves generated from a piezoelectric phased array is one of the most promising methods for the structural health monitoring of very large area composite structures and has been received extensive studies [1]. Most of these studies using bulk piezoelectric PZT pieces with the lateral dimension of several millimeters. However, due to the brittleness and very little bending strength of piezoceramics, the phased array made from these "large" PZT pieces has very poor conformability to curved surfaces, and hence most of the experiments are conducted on a flat structure such as a flat aluminum plate [2]. Recently, we have developed a novel, cost-effective method to fabricate large area, flexible piezoelectric tapes based on the screen printing laser transfer technology invented at PARe [3]. Many (hundreds to thousands) patterned and tightly packed PZT elements can be sandwiched between two flexible foils/films such as one metal foil and one patterned metalized polymer film, through a batch Baomin Xu, Steven Buhler, Karl Littau, Scott Elrod and Serdar Uckun, Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA Vasyl Hafiychuk and Vadim Smelyanskiy, NASA Ames Research Center, Mountain View, CA 94035, USA
2 transfer process using excimer laser. The PZT elements can have different shapes and distribution densities, and can be addressed individually and/or by groups. The lateral dimension of the PZT elements can be as small as about 100µm. With different designs the piezoelectric tape can work as linear array and various 2-D arrays such as ring array, grid array, concentric circular array, etc. As a first step to explore the potential application of the flexible piezoelectric tape as a new type of Lamb wave phased array for SHM, in this paper we present the preliminary computer simulation results on Lamb wave propagations generated from single element and multi-element array with the lateral dimension of PZT elements at about 100µm scale, and the approach to fabricate a flexible, tightlypacked phased array with a large number of tiny PZT elements. COMPUTER SIMULATION OF LAMB WAVE PROPOGATION Our approach involves the use of large numbers of PZT sensors printed in close proximity, forming sensor arrays. We consider acoustic waves which are generated by PZT elements and travel in a solid structure between two parallel surfaces. When such a structure has a thickness on the order of an acoustic wavelength, that waves propagating between the surfaces are Lamb waves. As the same PZT elements can be used as transmitters and receivers to excite and detect Lamb waves, we will focus our attention on the physics of the actuator sensor system from the viewpoint of SHM. It is very well known that any elastic perturbation propagating in the plate structure can be expanded in its normal modes. The pronounced properties of symmetric (S0, S1, S2,.. ) and the antisymmetric wave modes (A0, A1, A2, ) are presented in Fig. 1. Shown in the x axis is the product of the frequency times the thickness (f d) of the structure, and in the y axis, the phase velocity v=ω /k. We can see that the main zero s modes are most suitable for SHM. Considering that for SHM methods, the wave number will be on the same order or greater than the 1/thickness, we can directly determine all characteristics of the waves from the frequency. For example, if f ~1MHz for a given thickness (i.e. ~1mm), we can determine that the wave number for the symmetrical mode will be approximately 1 mm -1 and for the anti-symmetrical mode approximately 2mm -1. For a given material structure geometry, we can tune the frequency of the system for best operation. Propagation from Single PZT Elements Computer simulations of Lamb wave propagation in a circular aluminum plate is shown in Fig. 2. The axially symmetric geometry of the assumed case is shown in Fig. 2a. For better understanding of the scales in Fig. 2b and 2c, the center of the circular plate is expanded to show how transducers are mounted to the structure. The radial propagation of symmetric (top plot in Fig. 2d) and anti-symmetric (bottom plot of Fig. 2d) Lamb waves are represented by different colors in the Figure. The plotted modes form the distribution which is obtained by superposition of the generated and reflected waves from the circular free edge boundaries. It should be noted that the value of parameters like displacement, strain and stress are orders of magnitude higher for the finite sample of small radius compared
3 Figure 1. Lamb wave phase velocity dependence (dispersion relation) for aluminum plate structure Figure 2. General sketch of a) the panel with transducer, b) the distribution of the radial displacements in the vicinity of two mounted transducers c) for a single transducer and d) distribution of the radial displacements for symmetrical (top, f = 1MHz) and anti symmetrical modes (bottom, f = 2MHz) for aluminum plate with thickness = 1mm and radius R = 5cm (PZT 5H transducer with radius of 100μm and thickness of 40μm, dark red in computer simulation corresponds to e r =10-5, and dark blue is 10-7 ). to real structures without such boundaries. The matter is that the elastic perturbation stimulated by PZT actuator will reflect from the circular boundary and as a result by continues excitation amplitude of the all elastic fields will increase what makes it easier to detect signal and using them for SHM of small size samples. Let us analyze the relationship connecting the applied electric field E and strain e generated by this electric field through the d 31 piezoelectric constant for samples without boundaries. The oscillatory strain, generated as the transducer is excited by an oscillatory electric field with certain frequency f, will generate Lamb waves in a plate structure and the parameter of those waves will be determined by the dispersion relation. Propagating through the structure, the Lamb wave perturbations will generate the strain under the receiving sensor. The strain acting on the receiver must be greater than some minimum strain resolution of the PZT sensor. In this case, we can consider the generated and detected strains to be uniform across the relatively small PZT elements. We will detect damage or structural defects by comparing the input and output signals, and we will be able to localize damage using conventional time-of-flight and triangulation techniques.
4 Let us estimate a maximum detecting distance of the PZT elements for certain elastic materials with no damping. Specific computer simulation will be performed for the system consisting of composite materials (CFRP) with mounted PZT transducer and sensor. The applicability of a high-density PZT array sensor system can be estimated by analyzing the amplitude waves propagating through the elastic media. We will estimate the strains under the transducer and receiver, which are located a certain distance R apart. Let us analyze the relationship connecting applied electric field E and strain e generated by this electric field through d 31 piezoelectric constant. The oscillatory stress distribution through the PZT thickness is σ = d 31 YE, where E = V/h, V being the applied voltage and h the PZT thickness. The displacements and strains must be equal at the PZT-substrate boundary and this means that the maximum strain can be reached at maximum electric field E max and e = d 31 E max. Calculating for a PZT Young s modulus of Y = Pa, a piezoelectric constant d 31 = m/v, and E max = 10 6 V/m, we obtain that e = Here we consider that the strains at the interface of the transducer and composite are the same and that the PZT elements have been ideally mounted to the underlying structure. More precise calculations can be made based on formulas obtained in [4]. Then if we consider that the detection threshold strain for a PZT element is about 10-7, we have three orders of power reduction that should be detectable by the receiver. Considering the radius of the element 10-4 m and that the strength of the Lamb wave attenuates with an amplitude 1/r 1/2 we can conclude that the signal could be hypothetically detected even for R of the order of one meter. In this case the voltage at the sensor would be of the order 10-2 V. Usually for determining structural defects, we have to separate signals scattered by the defect from the main incident signal, with a typical ratio of not less than As a result, at the distances of the order of 10cm, transducers and receivers can signal to each other, and system of two small elements can be valid for SHM for such distances in the case where we lose less than 10-3 in power for incident signal. It should be noted that we did not take into account, for example, that the amplitude of propagating waves attenuates proportional to a squared frequency due to scattering and dissipation. In the real case, we also do not have ideal mounting of the PZT to the substrate, especially when we attach a tape structure with embedded PZT array elements. But our estimation shows that the typical propagation distances between adjacent single PZT elements of 10 cm should be realistic for SHM. In the case of high-density PZT phased arrays, however, the signal intensity and maximum propagation distance will be dramatically increased above 10 cm. Propagation from Multi-Element PZT Arrays The simulations were carried out for a three dimensional system by finite element modeling using COMSL and ABAQUS software packages. As an example, the snapshot plot of wave propagation through a composite CFRP structure is presented in Figure 3. We can see how two closely spaced PZT elements generate Lamb waves propagating through the structure. When the waves reach the free edges of the structure they reflect back and interact with the wave generated by the PZT elements. The quasistationary distribution of the elastic strain for the plate structure 10cm in size and 1mm thick is presented in Figure 4. Figures 4a and 4b present total geometry of the structure where tiny PZT sensors can be barely seen.
5 a b c Figure 3. Transient dynamics of the first principle strain field in a plate with two sensors for a) t= s, b) t= s and c) t= s. Parameters: plate of thickness 1 mm is made from CFRP with density 1580kg/m 3, Y=10 11 Pa, f=500 khz, parameters of the PZT element are the same as for Fig 2. a b Figure 4. First principle strain distribution for 3D simulation of the composite CFRP structure 1 mm thick with mounted PZT 5H element of radius 100 μm and 40 μm thick: a) Lamb waves generated by circular PZT element in the center of circular plate, b) waves generated by 2 square PZT actuators in square plate and c) small scale square plate singled out by square on the top of the plate (Red color corresponds to e=10-5 and blue to 10-7 ). c In Figure 4c we present expanded and stretched PZT elements to show the deformation state of the actuator and substrate. If the elements are excited with identical frequencies and have varying time delays τ m, the PZT elements will function as a phased array. This makes it possible to scan specific regions of the composite structure using guided waves. In this case, the PZT array plays a role similar to phased array radar, but on the surface of a structure. Phased array applications for SHM were originally proposed by V. Giurgiutiu [1], where he showed that the beamforming principles consists of two steps: (1) applying time delay τ m and weight w m to the propagating wave from the m-th element, and (2) summing up the output signals of the M PZT elements. Figure 5 shows the increasing improvement of both the amplitude and the resolution of an acoustic beam formed from a phased array. Resolutions of ~ 2 degrees can be obtained with 80 elements arrays. Larger and smaller arrays have improvement proportional to the number of PZT elements.
6 Relative Amplitud emitters 20-emitters 40-emitters 80-emitters Number of wavelengths off axis Figure 5. Lamb wave amplitude in the far field (100 waves from the source plane) for several sizes of phased arrays, with array pitch = 1/2 λ. PHASED ARRAY FABRICATION APPROACH The PZT elements inside the piezoelectric tape will have a lateral dimension of about 100 μm, a thickness of 40 to 60 μm, and the center-to-center distance of 200 to 500µm. These PZT elements need to be solidly bonded to a thin metal sheet with the thickness of about 25 to 100 μm such as stainless steel or aluminum foil to form a unimorph diaphragm structure. It is very difficult to use conventional ceramic processing to cost effectively fabricate such tightly-packed PZT array with so small geometries. Here, we describe the means of fabricating low-cost, high-density PZT elements in the form of a piezoelectric tape that can be used for phased array damage detection in composite structures. Of central importance is the ability to make piezoelectric elements that are sufficiently thin and have high electromechanical efficiency. PARC has developed a unique technology to fabricate large area, flexible piezoelectric tape in a cost-effective fashion using screen printing laser transfer methods. The sensor tapes can be easily mounted/bonded onto a curved surface. The piezoelectric tape can have thousands of small PZT elements between two flexible films/foils. The PZT elements can have different shapes and distribution densities, and can be addressed individually and/or by groups. Figure 6 shows a sample piezoelectric tape produced at PARC. Figure 7 illustrates the crosssection of the piezoelectric tape. Figure 8 shows the major steps to make the piezoelectric tape for SHM applications, based on the screen printing laser transfer process. First, the piezoelectric PZT elements are screen printed on a sapphire substrate with patterns according to the final device requirement and sintered at high temperatures for densification and grain growth. The use of a sapphire substrate allows sintering the PZT elements in the temperature range close to that for sintering bulk ceramics, such as 1100 to 1300 C, which results in highly densified elements to be formed. After sintering, electrodes are deposited on the top surface of the PZT elements with a shadow mask. The PZT elements are then bonded to the target substrate such as a metal foil. In order to avoid the mechanical damping of the bonding layer and to obtain electric contact between the PZT elements and the substrate, a very thin epoxy bond layer is used such that the electric contact is established through the asperity contact between the PZT elements and the substrate. After bonding, the PZT elements are exposed to an excimer laser through the sapphire substrate, which
7 Figure 6. A sample piezoelectric tape produced at PARC Stainless steel Ground plane contact hole, not to scale PZT Air, void Top Tape Air, void Metalization layer patterned for interconnect Region of asperity contact Air, void Air, void Metallization on whole Spacer Interconnect contact hole, not to scale Figure 7. Layers of the piezoelectric tape illustrated in a cross-section schematic Figure 8. Flowchart to fabricate piezoelectric tape for SHM destroys the adhesion between the PZT and sapphire and facilitates the release of the PZT elements from the sapphire substrate. The used sapphire can be slightly polished for re-use. Electrodes are subsequently deposited on the second surface of the PZT elements, and the PZT elements are poled to generate the piezoelectric effect. Finally, a thin polymer film, such as a patterned metalized Mylar, will be bonded to the 2nd surface of the PZT elements. For the SHM application, the PZT elements are laser transferred to a thin sheet of metal foil such as stainless steel, which acts both as the bottom electrode and as one half of the unimorph that will launch a Lamb wave into the composite structure to which the assembly is bonded. In order to achieve maximum conversion into Lamb waves, the stainless steel sheet will be partially perforated around each
8 individual PZT element, as shown inside the Figure 8. Regions of the stainless steel outside the PZT elements will also be perforated in order to minimally damp the propagating Lamb wave. The top flexible sheet of Figure 6 is made of metalized Mylar or polyimide, and includes the patterned interconnects to electrically excite the individual PZT elements. In order to maintain signal integrity and optimize signal-to-noise at the frequencies of operation, it will be necessary to mount driver chips with both send and receive electronics immediately adjacent to each phased array element. One option for this is to use a polyimide flex circuit as the top sheet in the piezoelectric tape structure, and to mount the necessary electronics to it using conventional means. In order to address the many phased array units that would be mounted onto the surface of a large composite structure, it may be advantageous to use a high speed serial link to control each phased array unit, and to have the necessary electronics in the send/receive electronics of each unit to compose the necessary analog signals based on the transmitted digital data. SUMMARY High-density PZT phased arrays piezoelectric sensors offer a good set of capabilities that can be used for SHM systems. As discussed, the connection of PZT arrays will produce a sufficient signal which can be detected at the distances required for SHM. The PZT phased array approach addresses several system-level requirements that are not adequately addressed by current SHM technologies: Inspection of very large composite structures for structural integrity and defects such as cracks, delamination, debonding, and impact damage; Continuous inspection of composite cases or pressure vessels for structural integrity, defects, and impact damage without the use of embedded sensors that weaken the structure; Rapid inspection of composite structures (as an alternative or complement to traditional NDI/NDE methods). REFERENCES 1. Giurgiutiu, V Structural Health Monitoring: Fundamentals and Applications, Academic Press. 2. Lin, B. and V. Giurgiutiu Modeling and Testing of PZT and PVDF Piezoelectric Wafer Active Sensnors, Smart Mater. Struct., 15: Xu, B., D. White, J. Zesch, A. Rodkin, S. Buhler, J. Fitch, and K. Littau Thick Piezoelectric Films from Laser Transfer Process, Ceramic Transactions, 167: Lin X. and F. G. Yuan Diagnostic Lamb Waves in an Integrated Piezoelectric Sensor/Actuator Plate: Analytical and Experimental Studies. Smart Mater. Struct., 10:
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