The traditional delay-and-sum imaging algorithm usually requires sending an excitation pulse at each piezoceramic transducer and obtains a damage image by drawing only ellipses. A multi-delay-and-sum imaging algorithm is proposed for damage detection of thin-plate-like structures using sparse piezoceramic transducers. Compared with the traditional delay-and-sum imaging algorithm, the proposed algorithm sends only one excitation pulse for each detection. A reflection coefficient is employed in the proposed method to cancel the artifacts caused by the boundary reflection signals, and the reflection coefficient is determined by the distribution of piezoceramic transducers and strength of the reflection signals. An additional time compensation due to the excitation pulse is also made to reduce the error of damage locating. To increase the image pixel value of a damage, the damage image is obtained by drawing both ellipses and hyperbolas with transmitter–sensor pair signals. The experimental results obtained on an aluminum alloy plate demonstrate that the proposed multi-delay-and-sum imaging algorithm can identify a bonded mass damage efficiently and accurately.

To monitor bolt connection status and detect bolt looseness in engineering structures, a bolt pretightening force sensor was designed and fabricated with a strain gauge bonded on the top surface of a bolt head. The stress–strain states of the top surface of the bolt head under different pretightening forces were analyzed using the finite element method (FEM). It was found that the radial strain on the top surface of the bolt head and the pretightening force of the bolt have a good linear relationship. A bolt sensor integrated with the designed radial strain gauge on the top surface of the bolt head was designed and fabricated, and an experimental device was also set up. The experimental results showed a good linearity between the radial strain on the top surface of the bolt head and the bolt pretightening force. The FEM and experimental results demonstrate that the proposed method and the designed sensor can effectively measure the bolt pretightening force. The proposed method and sensor are a new way to monitor bolt connection status and detect bolt looseness. The proposed method is easy to implement and has good application prospects.

An algorithm to detect damage size and shape using a lead zirconate titanate (PZT) transducer array for a plate structure is developed in this paper. During the process of detection, the predetermined PZT transducers are chosen as actuators to generate Lamb waves, and others are chosen as sensors to detect the Lamb waves. When there is damage, the Lamb wave reflects at the boundary of the damage. The reflection point is on an ellipse with two foci at the actuator and sensor, and this ellipse is tangent to the boundary curve of the damage at the reflection point. The reflection point can be obtained by finding a special point on the ellipse, and this point has the shortest distance to the center of the curvature of the damage boundary curve. As a result, the damage boundary curve can be determined by finding various reflection points, and the envelope of the identified reflection points is employed to represent the shape of the damage. Experiments are conducted on an aluminum plate with 12 piezoceramic transducers to identify the sizes and shapes of bonded masses, which are considered as simulated damages. The experimental results indicate that the proposed method can obtain the size and shape of bonded masses with a relatively small error.

To detect small cracks in plate like structures, the high frequency characteristics of local dynamics were studied with the piezoelectric electro-mechanical impedance (EMI) method, and damages were monitored by the changes of the EMI. The finite element simulation model of EMI was established, and numerical analysis was conducted. The simulation results indicated that the peak frequency of the piezoelectric admittance signal is a certain order resonance frequency of the structure, and the piezoelectric impedance method could effectively detect the dynamic characteristics of the structure. The piezoelectric admittance simulation and experimental study of aluminum beams with different crack sizes were performed. Simulation and experimental results revealed that the peak admittance frequency decreases with the increase of crack size, and the higher resonance frequency is more sensitive to the small-scale damage. The proposed method has good repeatability and strong signal-to-noise ratio to monitor the occurrence and development of small-scale crack damage, and it has an important application prospect.

In this paper, an eddy current tuned mass damper (ECTMD) is utilized to control the vibration of a cantilever beam. The robustness of the ECTMD against frequency detuning is experimentally studied in cases of both free vibration and forced vibration. The natural frequency of the cantilever beam can be adjusted by changing the location of a lumped mass. For purposes of comparison with the ECTMD, the robustness of a tuned mass damper (TMD) is also studied. The experimental results in the free vibration case indicate that the ECTMD works well both in tuned and detuned situations, and the equivalent damping ratio of the cantilever beam equipped with the ECTMD is 2.08~5.91 times that of the TMD. However, the TMD only suppresses the free vibration effectively in the tuned situation. With forced vibration, the experimental results also demonstrate the robustness of the ECTMD in vibration suppression in detuned cases. On the other hand, the cantilever beam with TMD experiences 1.63~2.99 times the peak vibration of that of the ECTMD control.

During the propagation of ultrasonic waves in structures, there is usually energy loss due to ultrasound energy diffusion and dissipation. The aim of this research is to characterize the ultrasound energy diffusion that occurs due to small-size damage on an aluminum plate using piezoceramic transducers, for the future purpose of developing a damage detection algorithm. The ultrasonic energy diffusion coefficient is related to the damage distributed in the medium. Meanwhile, the ultrasonic energy dissipation coefficient is related to the inhomogeneity of the medium. Both are usually employed to describe the characteristics of ultrasound energy diffusion. The existence of multimodes of Lamb waves in metallic plate structures results in the asynchronous energy transport of different modes. The mode of Lamb waves has a great influence on ultrasound energy diffusion as a result, and thus has to be chosen appropriately. In order to study the characteristics of ultrasound energy diffusion in metallic plate structures, an experimental setup of an aluminum plate with a through-hole, whose diameter varies from 0.6 mm to 1.2 mm, is used as the test specimen with the help of piezoceramic transducers. The experimental results of two categories of damages at different locations reveal that the existence of damage changes the energy transport between the actuator and the sensor. Also, when there is only one dominate mode of Lamb wave excited in the structure, the ultrasound energy diffusion coefficient decreases approximately linearly with the diameter of the simulated damage. Meanwhile, the ultrasonic energy dissipation coefficient increases approximately linearly with the diameter of the simulated damage. However, when two or more modes of Lamb waves are excited, due to the existence of different group velocities between the different modes, the energy transport of the different modes is asynchronous, and the ultrasonic energy diffusion is not strictly linear with the size of the damage. Therefore, it is recommended that only one dominant mode of Lamb wave should be excited during the characterization process, in order to ensure that the linear relationship between the damage size and the characteristic parameters is maintained. In addition, the findings from this paper demonstrate the potential of developing future damage detection algorithms using the linear relationships between damage size and the ultrasound energy diffusion coefficient or ultrasonic energy dissipation coefficient when a single dominant mode is excited.