at locations with complex geometry such as weld toes. ... For example, the structural elements in a steel frame building are normally covered by fire-proof ...
Piezoelectric Paint Sensor for Nondestructive Structural Condition Monitoring
Yunfeng Zhang Department of Civil & Environmental Engineering, Lehigh University 13 E. Packer Avenue, Bethlehem, PA 18015, USA ABSTRACT This article presents a novel piezoelectric paint-based structural condition monitoring method. The piezoelectric paint concerned is composed of tiny piezoelectric particles mixed within polymer matrix. A special fabrication technique has been developed to apply large area of piezoelectric paint directly onto the surface of host structures in an efficient manner. Because of the electro-mechanical coupling properties of piezoelectric paints, the vibration responses of host structures can be monitored by measuring the output voltage signals from the piezoelectric paint sensor. Additionally, a novel surface crack detection technique has been conceived and validated experimentally, in which cracks of the host structure is detected by observing the measured signals from a piezoelectric paint sensor with multielectrode configuration. Piezoelectric paint has a high potential for replacing conventional coating materials by integrating structural health prognostic and diagnostic functions with the functionalities of conventional coating materials such as corrosion protection and decoration. Most appealing is that piezoelectric paints would serve multiple functions – it can simultaneously measure dynamic strain caused by structural vibration while monitoring cracks at the sensor location. This promising structural health monitoring technique is believed to be useful in many important applications including nondestructive structural condition prognostic and diagnostic of aircraft structures, naval ships and large-scale civil structures. INTRODUCTION Piezoelectric materials include ferroelectric ceramics, polymers (e.g., PVDF), and composites. Piezoelectric materials, because of their electro-mechanical coupling properties, have the ability to sense changes in their environment. Because of its unique characteristics, piezoelectric sensors have recently gained an increasing popularity in the field of structural health monitoring. Piezoelectric sensors can be used as self-powered sensors because the electric charge generated by piezoelectric materials in response to mechanical loads can be measured directly without the need for external power excitation. For applications where power consumption is a significant constraint, piezoelectric sensors can be very valuable. However, most piezoelectric ceramic materials are typically brittle; prefabricated piezoelectric sensors do not fit surfaces with complex geometry, and these sensors do not work well where strain is large as the sensor may crack. To overcome these problems, polymer-based piezoelectric paints have been studied by a few researchers [1-5]. Piezoelectric composite materials consisting of ferroelectric ceramics and polymer have received much interest as transducer elements because of their desirable material properties that may not be attainable in a single-phase material. For example, toughness, flexibility, lightness, and ease of processing are typical features of polymers; however, their piezoelectric activity is usually low. On the other hand, ceramics have a strong piezoelectric response, but they are heavy, brittle, and rigid. The combination of polymer and ferroelectric ceramic to form piezoelectric composites offers the unique blending of the high electro-active properties of ferroelectric ceramics and the mechanical flexibility and formability of organic synthetic polymers. Piezoelectric composites can be classified according to the connectivity of piezoelectric ceramics and matrix phases. The piezoelectric paint concerned belongs to piezoelectric 0-3 composite materials. The “0-3” means that the ceramic particles are randomly dispersed in a polymer matrix. Conceivably, 0-3 composites can be more easily fabricated in complex shapes than other forms of composites. Although various approaches have been tried for producing 0-3 piezoelectric composites [6-7], most of them have limitations such as high temperature firing to cure, vapor deposition of electrodes, complex fabrication process and therefore high cost of making even a small piece of sample, and long poling time. For example, high temperature curing makes the process unsuitable for use on temperature sensitive materials such as plastics and FRP composites. The special equipment and fabrication conditions required for making piezoelectric composites with the conventional methods might explain the fact that piezoelectric paint is still not available as a low-cost sensor for the structural health monitoring of civil infrastructures, in which thousands of sensors are typically required for reliable evaluation of structural health conditions. To overcome the technical hurdles associated with traditional fabrication methods, a novel in-situ fabrication technique for piezoelectric paint sensors has recently been developed by Zhang [5] so that large areas of piezoelectric paint can be efficiently applied onto the surface of host structures.
The advantages of piezoelectric paint for use as a sensor in structural health monitoring applications include: (i) it is a selfpowered sensor; for applications where power consumption is a significant constraint, this can be very valuable; one application that uses this salient feature of the piezoelectric paint sensor is to use it as a power source for other electronics devices, such as wireless transmitters; (ii) With the proposed fabrication method, the piezoelectric paint is directly deposited onto structural surfaces and thus conforms to curved surfaces and adheres well to the host structure; (iii) by choosing appropriate polymer materials for the matrix phase, the properties of piezoelectric paints can be tuned to optimum for a particular application; for example, with proper polymer materials, the paint can be made flexible and tough which is necessary for the monitoring of FRP structures undergoing large deformation; (iv) the ease of processing of the piezoelectric paint can be utilized to form complex sensor patterns. Additionally, piezoelectric paint can be used as an intelligent coating material which has a high potential for replacing conventional coating materials by integrating structural health prognostic and diagnostic functions with the functionalities of conventional coating materials such as corrosion protection and decoration. Such intelligent coatings would serve multiple functions – simultaneously measure dynamic strain caused by structural vibration while monitoring cracks at the sensor location. Additionally, novel surface crack detection technique, which is based on piezoelectric paint, has been conceived and validated experimentally by Zhang [5]. Figure 1 shows the schematics of piezoelectric paint sensor array and its potential applications.
Piezoelectric Paint – a promising material for real-time structural condition prognostics and diagnostics
Lead Wire
Electrodes
Piezoelectric Paint
Host Structure
Naval Ships
Mechanical Structures
Piezoelectric Paint Sensor Array
Civil Infrastructures
Aircraft Structures Sensor Electronics
Piezoelectric Paint Sensor: potential applications Monitoring structural vibration response
Monitoring structural response to impact, blast load, and ultrasonic signals
Detection of cracks and overstress condition
Built-in sensor array for in situ structural condition prognosis and diagnosis
Data Acquisition System
Host structure w/ curved surface Piezoelectric paint sensor array
Data Processing & Interpretation
Piezoelectric Paint Sensor System
Figure 1. Piezoelectric Paint Sensor and Its Applications PREPARATION OF PIEZOELECTRIC PAINT The piezoelectric paint concerned is composed of three major components: piezoelectric ceramic particles (i.e., filler), a polymer binder which will facilitate the suspension of filler during application and bind the filler together after curing, and chemical additives to enhance the paint mixing, deposition and curing properties. Selection of proper materials for the polymer matrix phase is a critical step in paint formulation because the polymer binder material has a significant effect on the final properties of piezoelectric paint. The polymer matrix material chosen for this study was a 2K epoxy product – resin AboCast 8109-3 with a hardener AboCure 8109-3 from Abatron, Inc. The epoxy has a high dielectric strength of 16-kV/mm and a blend viscosity of 16-20 poises at 21 °C. The mixture ratio for the epoxy Part “A” (resin) and Part “B” (hardener) is specified as 10:1 by the supplier and was followed in this study. The lead-zirconate-titanate (PZT) particles used for the piezoelectric paint are PZT-5A powders supplied by Morgan Electro Ceramics. PZT-5A was selected because of its high sensitivity and a high stability with time and temperature that means it is useful under a wide variety of environments [8]. Chemical additives were used to improve the handling properties of paint mixtures during paint spraying
process. Following mixing and stirring, the wet paint was sprayed directly onto one side of a thin aluminum sheet. Before the paint deposition, the surface of the aluminum sheet was pre-treated using a pretreatment solution from Carpenter Chemicals, which is described as a one-step, room-temperature organic phosphating system [9]. The sample mean value of the paint (i.e., coating) thickness is approximately equal to 0.28 mm, obtained by using a digital microscope. The paint was then cured for two days at room temperature before subsequent poling and electroding steps. Shown in Figure 2-(a) is an aluminum sheet with the piezoelectric paint coating bent into a curved shape. Figure 2-(b) illustrates the microstructure of the piezoelectric paint coating which was observed using a 100x digital microscope.
Coating
Al substrate 250 µm 100 x (a)
(b) Figure 2. Piezoelectric paint coating on an aluminum sheet: (a) bent sample; (b) section view
The poling of ferroelectric materials to induce piezoelectricity is an important stage in the manufacture of transducers. The poling process switches the polar axis of crystallites to the symmetry-allowed directions nearest to that of the externally applied electrical field. Poling of composites having a polymer matrix with 0-3 connectivity is especially difficult because the electric field within the high-dielectric-constant grains is far smaller than in the low-dielectric-constant polymer matrix. Therefore, very large electric fields are required to pole these types of composites. However, large electric fields often cause dielectric breakdown of the samples. To improve poling of piezoelectric materials, corona discharge technique has been developed [10-11]. In the Corona poling method, electric charge from a corona point is sprayed onto the unelectroded surface of the sample, creating an electric field between the sample faces. Because of the absence of electrodes, there is no shortcircuiting of the sample at weak spots. This is particularly beneficial for poling of piezoelectric paint sensors because shortcircuited piezoelectric paint spot has to be manually removed from host structure after poling. Furthermore, Corona poling is believed to be suitable for large-area poling and mass production [11]. In this study, Corona poling technique was employed to activate the piezoelectric effect of intelligent coating. The poling was performed under a constant DC voltage of 30 kV at the corona point to pole the sample at an elevated temperature. The distance between the corona point and the coating surface was approximately 60 mm. After Corona poling, silver paint was then applied to the top side of the piezoelectric paint to form an electrode. The base metal was used as the other electrode of a complete circuit. Each pair of electrodes forms one piezoelectric paint sensor, as shown in Figure 1. VIBRATION RESPONSE OF PIEZOELECTRIC PAINT SENSOR After depositing and curing of the piezoelectric paint, poling and electroding of the cured paint, electric charges will be generated in each piezoelectric paint sensor in response to structural vibrations because of the electro-mechanical coupling properties of piezoelectric paints and unlike conventional sensors such as strain gages, this electric output can be measured directly as voltage signals without using any external power excitation. The measured voltage signal is proportional to the dynamic strain level of the host structure at the sensor location. The effectiveness of piezoelectric paint sensors for dynamic strain measurement was examined using a test setup shown in Figure 3. The Al sheet with piezoelectric paint sensors was mounted as a cantilever beam to a heavy steel block. An impact hammer was used to excite free vibrations of the Al sheet by hitting the tip of Al sheet and then letting the Al sheet vibrate freely. The output from the piezoelectric paint sensor was measured as a voltage signal using a SigLab 20-42 digital signal analyzer. Charge amplifier was not used in the test because the output voltage from the piezoelectric paint sensor was strong enough to drive the dynamic signal analyzer which has a very high input impedance. The cable connecting the piezoelectric paint sensor to the dynamic signal analyzer was electrically shielded and has a length of 64 inches. The sampling frequency for the impact hammer test was 12.8 kHz. The response of the piezoelectric paint sensor under impact load is shown in Figure 4. The first thing observed in the response signal is an electrical-magnetic noise signal with a frequency of 60-Hz. The cause for this noise signal can be explained by the fact that the vibration test was conducted in a room with many electro-mechanical
equipments such as welding machine and electrical furnaces. In this figure, it is seen that upon the hitting, the Al sheet first vibrated in its high modes and after several cycles, the high modes damped out and the lower mode response became the dominant vibration modes of the thin sheet. An accelerometer was also mounted on the thin sheet to provide a comparison regarding the frequency contents between the accelerometer measured signals with the piezoelectric paint sensor. It was found that the timing of major event occurrence and frequency contents of measured signals from these two different types of sensors were very close. The piezoelectric paint sensor was also observed to have a good repeatability in its output signal when subjected to similar impact forces. Additionally, the performance of piezoelectric paint sensors in monitoring the vibration response of a cantilever beam subjected to harmonic loading was also found satisfactory [4].
Figure 3. Vibration test setup for piezoelectric paint sensors
Figure 4. Response time history of piezoelectric paint sensor and accelerometer to impact loading PIEZOELECTRIC PAINT-BASED SURFACE CRACK DETECTION TECHNIQUE With advantages such as low cost and ease of implementation in large array size, piezoelectric paint sensor array might provide a promising technology for real-time structural condition diagnosis for civil infrastructures, where a great number of low-cost sensors are typically required for its large scale [12]. A unique opportunity exists for developing innovative intelligent structural systems integrated with the piezoelectric paint sensor technology. In particular, a piezoelectric paint-based surface crack detection technique has been conceived and validated experimentally by Zhang [5]. This surface crack detection scheme utilizes a piezoelectric paint sensor with a multiple-electrode configuration. To illustrate the basic idea, a piezoelectric paint sensor with two pairs of electrodes is used as an example here. The output from these two electrode pairs can go to two
separate channels in a readout device such as an oscilloscope. Excited vibrations in the monitored structure will generate an electric voltage in the two electrode pairs which are subsequently observed using a readout device. Two different scenarios can be identified for the voltage signals from the piezoelectric paint sensor: Scenario 1 – If there were no cracks crossing the piezoelectric paint sensor, then the signals in the two separate channels of the readout device would be identical to each other; Scenario 2 – If a crack did fully traverse the sensor, the measured signals at the two electrode pairs would clearly no longer be identical and this would manifest the occurrence of a crack (or cracks) at the sensor location. Figs. 2-(a) and 2–(b) show the experimental data corresponding to Scenarios 1 and 2, respectively. The damage index is defined as a normalized value of the signal difference from the two electrode pairs, Damage Index = [x1(t) – x2(t)] / {[RMS (x1(t)) + RMS (x2(t))]/2}
(1)
Where x1(t) and x2(t) are the measured voltage signals from each of the two electrode pairs associated with the same piezoelectric paint sensor, respectively; RMS(⋅) denotes the root-mean-square value of the measured voltage signal. An interesting comparison can be made between this piezoelectric paint-based crack detection method with conventional electric foil (or wire) method (widely used for fatigue crack monitoring), in which a loss of electrical connection is observed due to the fracture of the foil (or wire) after a crack occurs. The piezoelectric paint-based method, however, has several advantages over the electric foil method, which include: (i) the piezoelectric paint sensor can serve dual functions, that is, the sensor can simultaneously measure the vibration response of structures while monitoring the occurrence of cracks at the sensor location; (ii) with a specially formulated composition, the piezoelectric paint sensor can also be used to monitor the overstress condition in the host structure if the paint cracks at a pre-determined threshold strain level; (iii) The piezoelectric paint sensor is selfpowered which means it not only requires any external power supply but can be used as a power source for other electronic devices; (iv) Since the piezoelectric paint is directly sprayed onto the host structure to which the sensor is applied, it conforms to curved surfaces and adheres well to the host structure; for example, this feature is very appealing to the detection of cracks at locations with complex geometry such as weld toes. In many cases, fatigue cracks initiate at the surface and then propagate into the bulk of the material (Fisher et al., 1995). Therefore, it is believed that the proposed method provides a simple yet effective way to detect fatigue cracks, which are frequently found in structural connections such as bolt holes, welded connections, end of connection plates, cover plates, and etc. of bridges. Other ideal application for this method includes crack detection at hidden or inaccessible locations in a structure. For example, the structural elements in a steel frame building are normally covered by fire-proof coatings and nonstructural elements such as partition walls. After major hazardous events such as strong earthquakes, conventional practices require the removal of fire-proof coatings and partition walls for a visual inspection of the structural connections – a very time-consuming and costly process. However, if the piezoelectric paint sensor is applied at the time of construction, the proposed method is essentially non-destructive and involves only simple equipment for inspection in critical regions and the entire process could be done in a very short time. Unlike other inspection methods such as the ultrasonic-based NDE technique, the proposed method does not require high skills to interpret the results, as evidenced by the damage index histories shown in Figure 5. Additionally, the piezoelectric paint sensor can also be used for monitoring the cracks in aerospace and naval ship structures, such as fatigue cracks in aircraft fuselage or wings.
Figure 5. Damage index time history of multi-electroded piezoelectric paint sensor
CONCLUSIONS A novel technique for applying large areas of piezoelectric paint directly onto the surface of host structures has been developed and verified through a series of preliminary experiments. The in-situ fabrication procedure involves a spray paint technique and Corona poling technique which is deemed adaptable for on-site scanning poling applications. The effectiveness of piezoelectric paint sensor in dynamic strain measurement was verified through vibration tests, including the impact hammer tests described in this article and harmonic loading tests. The experimental data of this study clearly demonstrates that piezoelectric paint sensor, as an active sensor, is capable of measuring dynamic strains in vibrating structures without the use of an external excitation voltage. This paper also presents a novel surface crack detection method which utilizes a piezoelectric paint sensor with multi-electrode configurations and this piezoelectric paint sensor-based surface crack detection method has been validated experimentally. ACKNOWLEGEMENTS The author is grateful to the Pennsylvania Infrastructure Technology Alliance and Lehigh University for supporting this research. However, the results, opinions and conclusions expressed in this paper are solely those of the author and do not necessarily represent those of the sponsors. REFERENCES 1.
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