Blades play a critical role in the wind turbine system. Therefore, their structural health monitoring is very important. Blades are damaged by sudden changes in wind load, cracks due to collision of foreign objects, and disasters, such as lightning strikes, hail, and typhoons. Moreover, blades are expensive to maintain. Defects or damages to wind turbine blades reduce the life span and power generation efficiency of the wind turbine and increase safety risks and maintenance costs. Therefore, it is very important to detect blade damage to prevent problems in the wind turbine. Ultrasonic inspection is suitable for blades made of composite materials. Piezoelectric ceramic, which is a typical piezoelectric element, has relatively high sensitivity compared to other sensors. However, it suffers from brittle fractures and thus difficult to apply to curved structures. To overcome the limitations of piezoelectric ceramics, a piezoelectric flexible line sensor that can be applied to curved surfaces was manufactured using the dice-and-fill method for a [Pb(Li_0.25Nb_0.75)]_0.06 [Pb(Mg_0.33Nb_0.67)]_0.06 [Pb(Zr_0.50Ti_0.50)]_0.88O₃ with 0.7 wt% MnO₂ (PZTNMML) ceramic disc. Instead of a typical ultrasonic inspection method with limited surface contact, a laser capable of producing ultrasonic excitation of ultrasonic waves over a large area from a long distance was used. The possibility of detecting a defect on the wind turbine blade using a piezoelectric flexible line sensor and laser ultrasound was confirmed in this study.

Carbon nanotube/polymer-based composites have led to studies that enable the realization of low-cost, high-sensitivity piezoresistive strain sensors. This study investigated the characteristics of piezoresistive multi-walled carbon nanotube (MWCNT)/epoxy composite strain sensors subjected to tensile and compressive loads in one direction at relatively small amounts of strain. A patterned sensor was designed to overcome the disadvantage of the load direction sensitivity differences in the existing sensors. The dispersion state of the MWCNTs in the epoxy polymer matrix with the proposed dispersion process was verified by scanning electron microscopy. An MWCNT/epoxy patterned strain sensor and a patch-type strain sensor were directly attached to an acrylic cantilever beam on the opposite side of a commercial metallic strain gauge. The proposed patterned sensor had gauge factors of 2.52 in the tension direction and 2.47 in the compression direction. The measured gauge factor difference for the patterned sensor was less than that for the conventional patch-type sensor. Moreover, the free-vibration frequency response characteristics were compared with those of metal strain gauges to verify the proposed patch-type sensor. The designed drive circuit compensated for the disadvantages due to the high drive voltage, and it was confirmed that the proposed sensor had higher sensitivity than the metallic strain gauge. In addition, the hysteresis of the temperature characteristics of the proposed sensor is presented to show its temperature range. It was verified that the patterned sensor developed through various studies could be applied as a strain sensor for structural health monitoring.

In this study, we investigate the piezoelectric characteristics of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ (PNN-PZT) with MnO₂ additive (0, 0.25, 0.5, 1, 2, and 3 mol%). We focus on the fabrication of a piezoelectric ceramic for use as both actuator and sensor for ultrasound transducers. The actuator and sensor properties of a piezoelectric ceramic depend on the piezoelectric strain coefficient d and piezoelectric voltage coefficient g, related as g = d/ε^T. To increase g, the dielectric constant ε^T must be decreased. PNN-PZT with MnO₂ doping is synthesized using the conventional solid-state reaction method. The electrical properties are determined based on the resonant frequencies and vibration modes measured by using an impedance analyzer. The MnO₂ addition initially improves the tetragonality of the PNN-PZT ceramic, which then saturates at a MnO₂ content of 1 mol%. Therefore, the dielectric constant and piezoelectric coefficient d₃₃ steadily decrease, while the mechanical properties (Q_m, Young’s modulus), tanδ, electromechanical coupling coefficient k, and piezoelectric voltage coefficient g were improved at 0.5–1 mol% MnO₂ content.