Application of piezoelectric ceramics in structural health monitoring

PZT can not only be used to make various piezoelectric products, but in recent years PZT is gradually applied to structural damage detection. According to the positive and inverse piezoelectric effects of piezoelectric materials, PZT piezoelectric ceramics can be used as both driving and sensing elements. PZT piezoelectric ceramic hemisphere can be pasted in places where cracks or stress concentrations are likely to occur on components. The mechanical impedance or frequency response has high sensitivity to damage, which is making it the main indicator for studying damage identification.

In recent years, more and more researches on piezoelectric impedance technology have been used in structural health diagnosis. In 1995, Sun and others successfully used piezoelectric impedance technology for structural health diagnosis of assembled scaffolds, which was considered to be the beginning of the application of piezoelectric impedance technology in the field of structural health diagnosis. The advantage of piezoelectric impedance technology that it is sensitive to small damages to the structure, which is conducive to detect the initial failure of the structure. Moreover, the piezoelectric material PZT (lead zirconate titanate piezoelectric ceramic) often is used in piezoelectric impedance technology has a small size and a structure Simple and reliable. In addition, PZT is only sensitive to changes in the local area around it, which helps to isolate the overall mass loading of the structure,it changes in structural stiffness and boundary conditions, and the impact of structural damage near PZT piezo on the measurement results. Therefore, this technique is suitable for tracking monitoring links that have strict requirements on structural integrity or have a great impact on the life of the structure and damage is not easy to detect . This article will introduce the basic principles of piezoelectric impedance technology for structural health diagnosis.

Introduction to Piezoelectric Materials

Piezoelectric material is a special dielectric material with a piezoelectric effect and an inverse piezoelectric effect. The piezoelectric effect is a characteristic of certain piezo crystals discovered by the French brothers P.Curie and J.Curie in 1880. When a mechanical force (or pressure is released) is applied to the piezoelectric body in its polarization direction, the piezoelectric body will generate a charge and discharge phenomenon. This phenomenon is called the positive piezoelectric effect,on the contrary, a piezoelectric body is applied to the piezoelectric body. An electric field with the same (or opposite) polarization direction causes two effects: the inverse piezoelectric effect and the electrostrictive effect. The reverse piezoelectric effect, that is, the dielectric is mechanically deformed under the action of an external electric field, and the magnitude of the strain is proportional to the magnitude of the applied electric field, and the direction is related to the direction of the electric field. The electrostrictive effect, that is, the dielectric field F,which causes strain due to induced polarization. The strain is proportional to the square of the electric field and has nothing to do with the direction of the electric field. The inverse piezoelectric effect and the electrostrictive effect are essentially the results of polarization of the dielectric crystal under the action of an external electric field, which causes the crystal lattice to be distorted and manifests as a mechanical strain on a macro scale. Piezoceramics are called piezoelectric ceramics by mixing ingredients, sintering at high temperature, and randomly gathering the solid particles between the particles. PZT piezo can be used as a sensing element and a driving element, and can be embedded with other materials to form a composite material, so it has a wide range of application prospects, such as aircraft control on aircraft wings and vibration control systems. Active control of vibration and noise, structural health monitoring in equipment, etc.

The main features of PZT application in smart material structures are:

① Can be used as both a driver and a sensor;
② When used as a driver, its excitation power is small;
③ The response speed is faster, which is 1,000 times that of the shape memory alloy;
④ The size can be made small and thin, and can be installed on the surface of the structure or buried in the structure;
⑤ The combination is flexible. It can be used in the form of relatively large pieces, or it can be used in small pieces.

PZT structure
PZT piezo ceramic is a continuous solid solution of Pbzro3 and PbTio3 and has an ABO3 perovskite structure. Found in the early 1950s, PZT is an important piezoelectric ferroelectric material with important technical application value. Piezoelectric ceramics are crystalline dielectric materials that do not have a center of symmetry. A crystalline dielectric that does not have a center of symmetry has no 432-point group crystal with extremely low inverse piezoelectric effect due to the extremely high symmetry. The deformation of the symmetric dielectric of the crystal caused by the inverse piezoelectric effect. Under the action of the electric field, the dielectric is polarized. Because there is no ionic bond between the leftmost side ion and the rightmost positive ion (and other (chemical bonds), so during the polarization process, a large relative displacement can occur between them, which shows a large inverse piezoelectric effect on a macro scale. Expressed as: S = dE, which is proportional to the magnitude of the electric field. That is, for piezoelectric materials, electrical and mechanical quantities are coupled to each other. The energy stored in the medium consists of two parts, one is strain energy and the other is electromagnetic energy. According to modern structural dynamics theory, when damage and defects occur in the equipment and structure, such as cracks, loose bolts, etc., its rigidity and mechanical impedance characteristics will change, and the natural frequency and mode of the structure will also change. Therefore, the degree of damage can be given quantitatively based on changes in mechanical impedance. However, the change of mechanical dynamic impedance with frequency is difficult to measure with conventional methods. Utilizing the self-driving and self-sensing characteristics of piezoelectric elements, PZT piezo ceramics can act as both a driving element and a sensing element to excite the structure to obtain the dynamic response of the structure, thereby establishing a bridge between mechanical characteristics and electrical information, and mechanical dynamic impedance information. Changes can be reflected by simple measured electrical information. When a certain external voltage is applied to the surface of the piezoelectric ceramic sheet, a lateral surface force is generated on the surface of the beam. These surface forces will drive the beam to generate different vibrations (when the upper and lower PZTs are subjected to the same voltage, they will cause longitudinal vibration of the beam; when the reverse voltage is applied, they will cause bending vibration of the beam. In turn, the vibration causes deformation of the beam, and the deformation characteristics can be reflected in the form of electrical signals through the sensing characteristics of the piezoelectric ceramic sheet. Therefore, the dynamic admittance characteristics of the piezoelectric ceramic sheet pasted on the structure can reflect the damage status of the structure. The frequency-dependent admittance (inverse impedance) is obtained from the piezoelectric coupling effect and the interaction of PZT with the structure. The capacitive admittance of a free PZT piezoelectric cylinder transducer is the baseline of the admittance as a function of frequency. The second item contains the impedance information of the PZT material itself and the impedance information of the external structure. Considering that the piezoelectric system has been determined after the piezoelectric ceramic sheet is attached to the external structure, the impedance AZ of the PZT material itself is constant, and the impedance value of the external structure is the only parameter that affects the second term, thereby controlling all of the piezoelectric system. Changes in admittance Y. When the parameters and performance of PZT are kept constant, the structural impedance Z uniquely determines the value of the second term. Any change in piezoelectric sodium conductivity corresponds to structural damage and defects, so that the value of piezoelectric sodium conductivity can be used to structure damage is identified.

Implementation of PZT for structural health monitoring

Because of the piezoelectric effect and the inverse piezoelectric effect of the piezoelectric element, the piezoelectric element has a dual function of driving and sensing. Using this feature, it is possible to achieve online and real-time health monitoring of the structure. Part of the PZT material is connected to the power source that generates the excitation signal through a wire. The voltage or charge is used to drive the power supply to apply an excitation signal (voltage or charge) to the PZT. Because the PZT material has the inverse piezoelectric effect, that is, it will deform under the action of an electric field. The PZT material is embedded (or pasted) on the base material, so its own deformation will be transmitted to the base material, with the base material deforming or moving together. At this time, the PZT is equivalent to a driver and generates deformation by receiving the excitation signal. At the same time, some PZT material piezoceramic tubes are arranged on the base material and are not connected to the power supply. When the base material deforms or moves, this deformation or movement will be transmitted to the PZT material. Due to the piezoelectric effect of the PZT material, an electric charge is generated inside the PZT material, and the magnitude of the electric charge changes with the size of the deformation or movement. At this time, the PZT is equivalent to a sensor. Then use the measuring device to measure and collect the output signal of this PZT sensor in real time, and it can reflect the deformation or movement of the base material in real time and online, so as to realize the real-time and online health monitoring of the structure.

Compare the data collected in real time with the vibration data when the structure is normal, and see if the PZT output signal changes (such as cracks or looseness of the structure, etc., in theory, it will cause the PZT output in the structure to change. If it changes, it is considered that structure has a failure. When a failure occurs, the signal can be transmitted to the controller in time to deal with the structure failure in a timely manner to achieve online, real-time monitoring, failure diagnosis and failure processing of the structure.

PZT can act as both a driving element and a sensing element to excite the structure to obtain the dynamic response of the structure. The principle of positive and inverse piezoelectric effect is used to analyze the dynamic response relationship between the piezoelectric ceramic sheet and the external structure. When the external structure changes, the corresponding piezoelectric impedance also changes. By measuring the admittance change of the piezoelectric ceramics, the state of the structure can be predicted in real time. PZT is suitable for both macro damage and minor damage, and which has a good development prospect in the structural health monitoring of buildings in the future.