Статьи на перевод PVDF_P(VDF-TrFE) / 2012-Dodds,J-Thesis (Development of Piezoelectric Zinc Oxide Nanoparticle-Poly(Vinylidene Fluoride))

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allow the film to generate maximum voltage corresponding to the highest rate of change of applied strain. Each film was subjected to five cycles of loading. It should be mentioned that some of these tests caused some films to fracture or plastically deform; the results corresponding to these failed tests were not included in the analysis. Nevertheless, this series of tests has revealed some of the inherent challenges with using thin films for sensing, particularly due to their fragile nature. This issue was rectified later (when these films were used as actuators) by fabricating more robust, thicker films.

Analysis of the film was performed by directly comparing the displacement input to the voltage output as simultaneously recorded by the oscilloscope. As with the free vibration sensing, a sensitivity ratio between dynamic strain (from displacement input) and output voltage was obtained for each test on each specimen.

4.4.2 Load Frame Sensing Results

Figure 17 shows a representative result for a 20 wt.% PVDF-TrFE/ZnO thin film sensor subjected to multiple cycles of loading. Here, the film’s voltage time history is overlaid with the applied dynamic strain time history (i.e., calculated using the applied sinusoidal position time history executed by the load frame). Figure 17 plots the generated voltage as a function of dynamic strain and demonstrates that the voltage corresponds linearly with the applied dynamic strain. It can be seen that the sensor possesses excellent linearity between dynamic strain and voltage response. Other films tested also exhibited similar performance.

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Figure 17: Load Frame: Voltage Response vs. Dynamic Strain

Load frame testing results in plots comparing voltage output and dynamic strain input, such as shown above for a 20 wt.% ZnO sample. The equation obtained from the straight-line

relation here is V = 1.014* D, with an r value of 0.997. This demonstrates the excellent

linearity of the sensor.

4.5 Sensor Performance Assessment and Comparison

PVDF-TrFE/ZnO sensors have good sensitivity, especially as compared with commercially available PVDF sensors, as demonstrated with the hammer impact testing, free vibration testing, and load frame testing. Sensor linearity is adequately demonstrated by free vibration and load frame testing. However, sensor resolution has some limits with respect to distance and input energy, as demonstrated with the hammer distance testing. Increasing relative ZnO content in PVDF-TrFE/ZnO films appears to increase sensitivity throughout these sensing tests.

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Chapter 5: Actuation and Guided Wave Testing

5.1 Introduction

The PVDF-TrFE/ZnO film of the previous sections is adequate for sensing applications but requires a different construction for potential use as an actuator. This chapter first presents the methodology behind guided waves and some experimental examples thereof and then proceeds to discuss the design and validation of PVDFTrFE/ZnO actuators. A patterned electrode was prepared before spin coating a thicker PVDF-TrFE/ZnO film to form the actuator. A pitch-catch test setup was used to confirm guided wave generation and to validate damage detection.

5.2 Piezoelectric Guided Wave Methodology

The principles of guided waves follow the principles of acoustic engineering on thin, typically metal, substrates. To begin this process, a piezoelectric material acting as an actuator receives an AC electronic excitation from a function generator. The piezoelectric material responds mechanically to electrical excitations due to the converse piezoelectric effect. Because the piezoelectric material is physically attached to the structure, the generated strains in the piezoelectric transducer subsequently induce mechanical vibrations in the structure [81]. In a thin structure, these vibrations mostly cancel out, but certain frequencies induce a common mode of vibration in the structure that travels outward in all directions (or primarily in one direction if the transducer is so designed) [82]. The resulting wave that propagates along the structural surface is referred to as a guided wave.

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There are many types of guided waves, but the ones most relevant to SHM applications are Lamb waves. Lamb waves, also called plate waves, propagate on thin metallic structures by bouncing back and forth off the edges of the substrate, per Snell’s law, at the interface between the structure and (generally) the air [83]. Lamb waves propagate through the whole thickness of the structure and are thus able to detect defects that would otherwise be invisible to the naked eye. Additionally, Lamb waves have better resolution than more traditional bulk-wave testing methods on thin structures [84]. Lamb waves can excite thin structures in either asymmetric (also called “a” or flexural waves) or symmetric (also called “s” or extensional waves) modes. Both of these modes can be excited for effective SHM results, but exciting a single mode at a time is most efficient [85]. In fact, one way to detect the presence of damage is to note the switch from an “a” wave input (actuator pitch) to an “s” wave output (sensor catch), or vice versa [86]. An infinite number of modes exist, all of which are optimal at different combinations of frequency of excitation and structural thickness. Lower frequencies and lower modes simplify SHM assessments, because it is easier to excite a single mode at lower frequencies [87]. More details, including Lamb wave equations, can be found in Viktorov [88].

Lamb waves have the advantage of being able to travel long distances with limited attenuation, allowing for a small number of transducers to completely assess a structure [83]. An excellent overview of guided wave technologies, materials, techniques, and current applications has been presented by Raghavan et al. [31]. More specifically, Gu et al. [89] have investigated the different wave modes in a 0.187 inch (4.75 mm) thick steel plate and has charted their phase and group velocities. Lamb waves have even

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been demonstrated to be powerful enough to be used for SHM in high-density polyethylene plates, despite the high attenuation inherent in such materials [90].

A mode selection tradeoff exists between lower wave modes with low dispersion (and thus a farther propagation distance) versus higher wave modes with the ability to detect damage at a finer resolution (i.e., limited by the wavelength of the signal). The material, thickness, and geometry of any particular substrate have specific phase and group velocities and characteristics for guided waves traveling in them at different frequencies. To maximize one mode while minimizing others, the peak velocities for each mode must be selected in that particular material’s group velocity curves [89]. The frequencies corresponding to these maxima will be the optimal frequencies of excitation for the electrode with certain spacing between its electrode fingers. The phase velocity can also be determined from the group velocity chart to help with expected wave speed calculations. Bellan et al. [91] have performed this process to calculate the finger spacing optimal for guided wave propagation in FRPs.

Piezoelectric sensors are capable of detecting the guided waves travelling through a material. Sensors on a surface are excited mechanically by these waves and, because they are piezoelectric, respond by generating charge. The voltage signal generated by the sensors can be measured, recorded, and analyzed to give an indication of the structural state, as is discussed in Section 5.5.

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Figure 18: Guided Wave Propagation

This schematic demonstrates the simplified propagation of a guided wave through a thin

substrate from an actuator to a sensor for SHM.

Guided waves can be classified as traveling either through an undamaged or a damaged surface. Naturally, some initial undamaged baseline must be provided in order for guided wave sensor outputs to be meaningful. If only one mode has been initially excited in a structure, the appearance of additional modes can indicate that the Lamb waves have encountered damage such as a crack, boundary, or delamination [92]. The overall scheme of the pitch-catch setup with the guided wave sent through a material is illustrated in Figure 18.

Some additional factors to consider for pitch-catch SHM are the distance between actuator and sensor, the magnitude of actuation excitation, and nature of the applied voltage to the actuator. The maximum distance between the actuator and the sensor is closely related to the maximum actuation voltage provided. Field-based SHM applications are sometimes limited by the current that an amplifier can provide. In order to provide a sine wave-based excitation, the amplifier needs to provide a current that fits the requirements based on Equation 5.1:

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where i is the current required, f is the maximum frequency of excitation, C is the capacitance of the actuator, and V is the maximum voltage of excitation. Based on the desired frequency of excitation, the geometric dimensions and material properties of the actuator (which determines C), and the current limit (i) of the selected amplifier, the maximum voltage of excitation can be determined.

Time-domain analysis examines the differences between the signals in the undamaged structure versus results after damage has occurred. This analysis provides a qualitative but not quantitative sense of structural damage. The time to first impulse and the maximum voltage response can both be measured, but these are not perfect damage indicators. In the time domain, Lamb waves reflecting off structural damage and structure boundaries make even these simple analyses and damage identification difficult.

A basic form of damage interpretation involves finding the difference between the voltage signal from the pristine state and the voltage signal from the damaged state. This method has been successfully used to observe structural damage in structures [31, 93]. It has been used to observe the changing thicknesses of composite structures, using PZT disc actuators and sensors for this pitch-catch structural characterization [93]. Clarke et al. [94] have also used a difference-based damage detection strategy when assessing damage in a corrugated steel door on a shipping container and Croxford et al. [95] have evaluated the potential of this subtraction method in a pitch-catch SHM setup with a special emphasis on temperature compensation.

There are many examples of guided waves being used in real-life structural applications. Bingham et al. [96] have used guided waves to detect mines on ship’s hulls. Gu et al. [30] have used time-domain based methods to detect delamination in

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composite beams using PZT materials for pitch-catch SHM. Alleyne [97] has studied guided wave propagation across welds in steel plates. Bad welds have been identified by the presence of additional modes in the measured guided wave signals. Others have been able to embed PZT sensors and actuators in concrete [98]. Concrete cracks in t- caps in bridges have been detected with such pitch-catch methods. Pipes have also been inspected for damages using the simple pitch-catch approach [99]. Piezoelectric materials and associated pitch-catch techniques have even been tested for possible use on space structures [8]. PVDF/PZT composites constructed with electrodes designed for a specific mode have been used as a permanently embedded sensor to detect delamination between the layers in multi-layered adhesively-bonded aluminum plates [100].

5.3 Piezopolymer Actuators

Polymers that are piezoelectric, or piezopolymers, require a specific type of electrode construction in order for them to function effectively as actuators. A series of parallel finger-like electrodes are often deposited on the surface of the piezopolymer film. Generally, these will be referred to as fingered electrodes. More specifically, there are two classes of transducers that employ fingered electrodes, including inter-digital transducers (IDTs), which have every other electrode finger electrically connected, and comb transducers, which have adjacent electrode fingers electrically connected, as can be seen in Figure 19. When an alternating voltage is applied to these fingers, they excite the piezopolymer in a direction parallel to the surface of the structure and thus propagate waves along the structural surface. Fingered electrode construction is required because

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Figure 19: Fingered Electrodes: Comb and IDT

This illustration demonstrates the difference between comb and IDT electrodes. Comb electrodes have one available electrical connection, whereas IDT electrodes have two.

PVDF is not sensitive enough to act as an actuator without some frequency guidance from electrodes. Fingered electrodes can help to amplify the thin film’s piezoelectric properties by exciting surface waves at an optimal frequency, directly related to the distance between fingers. There are many examples of researchers investigating PVDF used with fingered electrodes, including where Hay et al. [84] have mechanically bonded PVDF comb electrode actuators and sensors to pipes to successfully detect water loading in a pipeline. Another example is when Gu et al. [89] have constructed monolithic PVDF IDT electrode actuators and sensors using photolithography to investigate mode propagation in steel plates and damages (e.g., various-sized voids) in carbon FRP.

Fingered electrodes have interesting properties when used for piezopolymer actuators, which are different between IDTs and comb electrodes. For all fingered electrodes, adjacent electrodes on one side of the material act like a fringing field capacitor, as can be seen in Figure 20 [101]. Because having just two fingers creates the same capacitance as the stray capacitance in wires attached to the electrodes,

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Figure 20: Fringing Field Effect

The fringing field effect is demonstrated by the changing dimensions and therefore changing electron flow from the left figure to the right figure.

repeating this fingered pattern is necessary to create the higher capacitance needed for any SHM application. Both fingered electrodes require a top reference electrode in order to allow poling and to allow finger voltages to change relative to a reference. Comb electrodes should, for optimal design, have fingers spaced one wavelength apart, whereas IDT electrodes should have fingers spaced half a wavelength apart [102]. The optimal IDT spacing based on guided wave characteristics is discussed in Section 5.4.1.

In addition to finger spacing, the electrode thickness and number of electrode fingers should be considered when designing a fingered electrode. Electrodes of a certain thickness are required to maintain conductivity across the film. It has been experimentally determined that for this thesis’s film construction, an electrode thickness of at least 50 nm is required. The number of fingers would be ideally infinite in order to generate more piezoelectric actuating power but is practically limited to less than 100 for this thesis due to real-life geometrical constraints (i.e., transducers have difficulty measuring cracks in the structure they are directly on top of).