![]() Lippman and the Curie brothers predicted and confirmed the existence of inverse piezoelectric effect in theory and experiment, respectively, that is, the material with piezoelectric effect will produce corresponding deformation under a certain electric field, and the deformation of the material will be restored when the applied electric field is removed. They found that, when an external force (pressure or tension) is applied in a specific direction of some dielectric crystals, the surface of both ends of the crystal will generate positive and negative bound charges of equal amount of electricity, and the density of bound charges is proportional to the magnitude of the applied stress, which is called the “positive piezoelectric effect”. The working principle of a piezoelectric sensor depends on “piezoelectric effect” of piezoelectric materials, first discovered by the Curie brothers in 1880. ![]() We conclude by highlighting some challenges and opportunities for future developments. In particular, focuses are placed on the high-performance piezoelectric materials, covering organic piezoelectrics, inorganic piezoelectrics and piezoelectric composites, engineered by various strategies, and piezoelectric sensors operated in active and passive modes for SHM. In this review, we provide an overview of the currently available piezoelectric materials and sensors for SHM. Moreover, some reviews were focused on the monitoring techniques, such as impedance-based SHM, ultrasonic Lamb, or both. Numerous review articles have been published on the applications of piezoelectric SHM, for instance, bonded structures, polymer-matrix composites, aircraft applications, wind turbine blades, bridge applications, or other engineering structures. In the recent years, lots of work has been focused on the piezoelectric sensors, such as piezoelectric transducers, smart aggregates, direct deposition piezoelectric sensors on structure, flexible smart sensors, and so on. To meet the growing demands for high-performance piezoelectric sensors for SHM, there has been considerable research interest in this domain. Compared with other monitoring sensors or techniques, piezoelectric sensors have numerous advantages, such as small size, light weight, low cost, availability in a variety of formats, high sensitivity, and so on. Therefore, sensors based on piezoelectric effect could be used as multipurpose sensors to realize the SHM using a variety of methods, including electromechanical impedance technology, ultrasonic propagation monitoring, acoustic emission, and stress monitoring. Piezoelectric materials are capable of becoming electrically polarized upon the application of external stress or deform in response to electrical stimuli. Various kinds of sensors have been developed to realize the SHM, such as strain gages, accelerometers, fiber optical sensors, displacement sensors, piezoelectric sensors, and laser Doppler vibrometers. Sensors with high sensitivity, good reliability, and low cost are the cornerstone for the structural health monitoring (SHM). In addition, driven by the growing demands of internet of things, the SHM market is predicted to expand at a high compound annual growth rate of 14.6 in the following 5 years. It is estimated that the market of SHM reached up to USD 2 billion in 2022. SHM is of particular importance for complex engineering structures, which require costly maintenance, to significantly lower the maintenance cost and guarantee the safety and reliability thereof. Structural health monitoring (SHM) is a ubiquitous technology to evaluate the status and integrity of structures, and even predict their lifetime by constantly collecting and analyzing the data acquired from the sensors integrated in the structures.
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