Xiuquan Li

Concrete is the most widely used structural material in modern civil engineering, and its internal integrity is closely related to public safety and service life. Ultrasonic nondestructive testing is essential for concrete structural health monitoring, where acoustic transmission, anti-interference capability, and signal quality strongly affect detection accuracy. To overcome the insufficient integration of acoustic impedance matching, electromagnetic shielding, and signal conditioning in existing embedded piezoelectric ultrasonic sensors, this study develops an integrated ultrasonic sensor based on a previously proposed acoustic matched smart aggregate, incorporating an electromagnetic shielding structure and an on-board preamplifier circuit. By jointly optimizing acoustic transmission, electromagnetic compatibility, and front-end signal conditioning, the sensor improves signal quality and robustness in concrete environments. Experiments demonstrate enhanced signal reception, stable anti-interference performance, and a significantly improved signal-to-noise ratio. Ultrasonic CT imaging further confirms its ability to identify internal defects in concrete, supporting its application in concrete inspection and structural health monitoring.

Air-coupled ultrasonic testing provides non-contact structural evaluation for scenarios in which liquid or gel couplants are impractical. However, its field deployment is limited by severe acoustic impedance mismatch, microvolt-scale received signals, electromagnetic interference, and cable-induced signal degradation. To address these coupled limitations, this study developed an integrated air-coupled ultrasonic sensing-head that combines a manufacturable hollow-glass-microsphere epoxy matching layer, a conformal conductive shielding layer with controlled grounding, and a closely coupled low-noise front-end preamplifier. The integrated architecture was evaluated through long-range weak-signal reception experiments and a preliminary wall-climbing robotic tile-debonding inspection. Under shielded conditions, integrated front-end preamplification improved the signal-to-noise ratio by 23.70 to 34.80 dB relative to the unamplified baseline over propagation distances of 400 to 2000 mm. It also outperformed downstream external preamplification by 10.43 to 12.92 dB. After tapping-based zoning of the inspection regions, the robotic experiment showed that the sensing-head could acquire non-contact through-transmission signals and distinguish region-level feature differences under mobile inspection conditions. These results support a compact sensing-head strategy that improves acoustic coupling, EMI immunity, and weak-signal preservation for air-coupled ultrasonic inspection.

Bolted joints are critical load-transfer components in bridges, wind turbines, aerospace systems, mechanical equipment, and offshore platforms, where preload loss can degrade stiffness, accelerate fatigue, and compromise safety. For structural health monitoring, early monitoring of preload reduction before marked loosening is essential, yet existing ultrasonic guided wave indicators remain affected by frequency dependence, non-monotonic responses, amplitude drift, and environmental disturbances. This study proposes an early-warning-oriented preload identification method that combines broadband excitation, multi-frequency narrowband reconstruction, spectral centroid extraction, optimized weighted fusion, and fixed SC-domain linear calibration from one reference loading group. Using a 20–250 kHz Chirp response, 14 narrowband signals from 50 to 180 kHz were reconstructed for an M20 single-bolt specimen tested over 50–90 N·m. The fused spectral centroid index exhibited a stable, monotonic, and approximately linear relationship with preload. When fixed weights and calibration coefficients were transferred to held-out repeated-loading groups, all Pearson correlation coefficients exceeded 0.99. Feature-level robustness tests showed that the arithmetic mean of the spectral centroid reduced temperature-induced Range% by 98.42–99.08% and RSD by 98.89–99.31% relative to energy-based features. This work provides an interpretable multi-frequency spectral descriptor and a calibration transfer framework for repeatable early warning of preload loss in a controlled single-bolt configuration.

To address the limitations of traditional acoustic experimental equipment, such as large volume, discrete modules, and complex operation, this paper proposes and implements a set of digital portable acoustic teaching systems. The hardware component is based on an FPGA, enabling a highly integrated design for signal source excitation and multi-channel synchronous acquisition. It supports the output of various signals, including pulses, sine waves, chirps, and arbitrary waveforms. The software component is developed based on the Qt framework, offering cross-platform compatibility and excellent graphical interaction capabilities. It supports signal configuration, data acquisition, real-time processing, result visualization, and historical playback, establishing a closed-loop experimental workflow of signal excitation–synchronous acquisition–real-time processing–data storage–result visualization. The system supports both local USB connection and remote TCP operation modes, accommodating scenarios such as real-time classroom experiments and cross-regional collaborative teaching. The verification results of three typical experiments, namely, multi-media sound velocity measurement, TDOA hydrophone positioning, and remote acoustic detection, demonstrate that the system performs well in terms of measurement accuracy, positioning stability, and the feasibility of remote detection. This study demonstrates the technical advantages and engineering adaptability of a digital teaching platform in acoustic experimental education. It provides a scalable system solution for cross-regional hybrid teaching models and practice-oriented education under the framework of emerging engineering disciplines. Future work will focus on expanding experimental scenarios, enhancing system intelligence, and improving multi-user collaboration capabilities, aiming to develop a more comprehensive and efficient platform to support acoustic teaching.

Carbonation depth is essential to determine the durability and predict the remaining service life of concrete structures. This study proposes a multi-frequency Rayleigh wave approximation method (MFRWA) to evaluate carbonation depth by exploiting the frequency-dependent penetration depths of ultrasonic Rayleigh waves. A series of numerical simulations are conducted to investigate the effective penetration depth of Rayleigh waves and the feasibility of the proposed MFRWA method on carbonation depth evaluation. Subsequently, the accelerated carbonation experiment is conducted to evaluate the carbonation depth using low-frequency and high-frequency Rayleigh waves, and the measured results from the Rayleigh wave method are compared with the ones from the phenolphthalein indicator and thermalgravimetric analysis (TGA) method. The results show that carbonation depth measured by Rayleigh wave method meets well with the one from TGA technique, demonstrating that the proposed method could provide a non-destructive and precise carbonation depth estimation. The proposed MFRWA method contributes a novel scheme for concrete carbonation evaluation and holds substantial potential in both laboratory and field applications.

The baseline-free damage detection method of Lamb waves has the potential to obtain damage information efficiently in plate structures through damage scattering signals. However, the missing detection of damage occurs occasionally due to the angular scattering characteristic of Lamb waves. To solve this problem, a novel baseline-free damage detection approach based on path scanning at the detection region edges using mobile piezoelectric transducers is proposed herein. Several sensing points carrying separated damage scattering signals were picked out from the scanning paths. By removing the direct and boundary reflected signals, the damage signals were extracted and exported to a delay-and-sum imaging method to locate the damage. Two experiments with and without mobile transducers were conducted to validate the proposed method on an aluminum plate with artificially fabricated crack-like damage. The results show that the proposed baseline-free approach can locate the crack-like damage with high accuracy and efficiency and avoid potential loss of damage information. The proposed baseline-free method provides a novel and practical damage detection approach when considering the angular-dependent scattering characteristic of Lamb waves and can enhance the credibility of results in damage detection.