Piezoelectric materials are a type of elastic material that undergoes deformation when subjected to physical external forces. They follow Hooke's law in terms of mechanical effects and are polarized crystals that convert mechanical deformation into electrical signals. When the pressure direction on the piezoelectric material changes, the internal charge also changes accordingly. Within the elastic limit of the piezoelectric material, the voltage strain is directly proportional to the material stress. It is generally believed that the piezoelectric material does not generate heat exchange during the piezoelectric effect [5].
According to the physical characteristics of piezoelectric materials, they can be divided into three types: piezoelectric single crystal materials, piezoelectric ceramic materials, and piezoelectric polymer materials. Piezoelectric single crystal materials, such as quartz crystals, have stable performance and high mechanical strength. However, their piezoelectric coefficients are generally lower than those of piezoelectric ceramics, and they are also expensive. Piezoelectric polymer materials, such as Polyvinylidene fluoride (PVDF), are widely used piezoelectric polymer materials. The advantage of PVDF is that it can be used to manufacture thin and flexible transducers of any shape on the surface of a structure. It has stable conductivity, mechanical toughness, and higher plasticity compared to piezoelectric ceramics. On the other hand, PVDF is limited in application due to its small piezoelectric coefficient and electromechanical coupling coefficient, relatively high dielectric coefficient and mechanical losses, and poor temperature and aging characteristics. Piezoelectric ceramics refer to polycrystalline piezoelectric materials, which are produced by solid-phase reactions and sintering of raw materials, resulting in randomly arranged small grains. Lead zirconate titanate (Pb(Zr1-xTiO3), PZT) piezoelectric ceramics are artificially synthesized polycrystalline piezoelectric materials. Compared with piezoelectric single crystals, their preparation is relatively simple[6]. The shape and polarization direction can be controlled, and their performance can be improved by doping, making the preparation of PZT sensors relatively simple. PZT sensors made from piezoelectric ceramics have the following characteristics: 1) significant dielectric, piezoelectric, and ferroelectric properties; 2) low structural impedance; 3) high sensitivity; 4) good wideband response; 5) good stability; 6) ability to work in low voltage and high-frequency environments. Moreover, PZT sensors are small in size and light in weight, making them suitable for the production of sensors that convert mechanical energy into electrical energy, enabling online monitoring and offline detection of structural damage. In the ultrasonic guided wave detection of rail structures, the excitation and sensing elements require high piezoelectric performance, and long-term working capability needs to be considered in structural health monitoring. Therefore, PZT material is chosen as the piezoelectric sensor material.
As shown in Fig. 1, piezoelectric ceramics are considered as bidirectional transducers that can convert between electrical signals and mechanical deformations. Both the positive and negative piezoelectric effects have high energy conversion efficiency and low cost[7]. Using piezoelectric ceramics, waveguide transducers can be manufactured to generate and receive ultrasonic guided waves.
Guided Wave Testing (GWT), also known as Ultrasonic Guided Wave Testing (UGWT) or Ultrasonic Long Range Testing (ULRT), utilizes guided waves as mechanical waves that are immune to electromagnetic interference. It is commonly applied in long-distance damage detection scenarios. Currently, guided wave testing is mostly focused on elongated structures, as they benefit from boundary constraints, allowing for long propagation distances with minimal energy loss. Guided waves are widely used in the inspection of metal pipelines, metal plates, and bars, and are now gradually being applied in rail inspection as well.
Figure 2 demonstrates the propagation process of ultrasonic guided waves in a medium. The excitation signal is converted from electrical to acoustic signals by the waveguide transducer and transmitted into the tested metal component. After passing through a defect, both forward propagating transmitted waves and reflected waves are formed. The ultrasonic guided wave signal propagates forward in the form of wave packets until it reaches the end of the metal structure, where it is received by a piezoelectric ultrasonic receiver transducer that converts the acoustic signal back into an electrical signal. By analyzing the acquired electrical signal, one can perform defect type analysis and obtain defect location information.
As a non-destructive evaluation method, ultrasonic guided wave testing propagates through a medium via coupling. Due to changes in the medium and the influence of boundary constraints, both longitudinal and transverse waves undergo multiple and complex reflections and refractions between the boundaries of the medium. The reflected waves, transmitted waves, and longitudinal and transverse waves that propagate in these media mutually convert, forming ultrasonic guided waves. The propagation velocity of the guided waves is related to the material properties of the medium and is influenced by elastic modulus, Poisson's ratio, and density. When the guided wave encounters damage, due to the different propagation media, the guided wave experiences reflection and transmission attenuation, ultimately resulting in a sudden change in the signal.