The acoustic black hole (ABH) phenomenon in thin-walled structures with a tailored power-law-profiled thickness allows for a gradual change of the phase velocity of flexural waves and energy focalization. However, ideal ABH structures are difficult to realize and suffer from potential structural problems for practical applications. It is therefore important to explore alternative configurations that can eventually alleviate the structural deficiency of the ideal ABH structures, while maintaining similar ability for wave manipulation. In this study, the so-called imperfect two-dimensional ABH indentation with different tailored power-law-profiled is proposed and investigated. It is shown that the new indentation profile also enables a drastic increase in the energy density around the tapered area. However, the energy focalization phenomena and the process are shown to be different from those of conventional ABH structure. With the new indentation profile, the stringent power-law thickness variation in ideal ABH structures can be relaxed, resulting in energy focalization similar to a lens. Different from an ideal ABH structure, the energy focalization point is offset from, and downstream of indentation center, depending on the structural geometry. Additional insight on energy focalization in the indentation is quantitatively analyzed by numerical simulations using structural power flow. Finally, the phenomenon of flexural wave focalization is verified by experiments using laser ultrasonic scanning technique.

In this work, the technique of the delay-and-sum method has been extended to the localization of multi-damage on large complex structures. Through a case study by locating multiple damages on a composite aircraft panel, the exact extensions of the method are clearly presented. A multi-damage detection method is first designed on the basis of the propagating characteristics of Lamb waves as the preprocessing of damage localization. In light of the high signal complexity induced by the structural characteristics, an adaptive valid data extraction strategy is presented, improving the adaptability of the delay-and-sum method for complicated structures in terms of the damage localization accuracy. Since the localization accuracy of the delay-and-sum method can be significantly influenced by the uncertainty of preset velocity, it is further assessed by a quantitative localization accuracy evaluation method, revealing some fundamental features of the method in a comprehensive way. In addition, a boundary processing approach is designed to extend the delay-and-sum method for the multi-damage case. Experimental results in different damage situations verify that multi-damage can be located accurately by the proposed extended method.

In the synchronized switching damping (SSD) techniques, the voltage on the piezoelectric element is switched synchronously with the vibration to be controlled using an inductive shunt circuit (SSDI). The inherent capacitance and the inductance in the shunt circuit comprise an electrically resonant circuit. In this study, a negative capacitance is used in the shunt circuit instead of an inductance in the traditional SSD technique. The voltage on the piezoelectric element can be effectively inverted although the equivalent circuit is capacitive and no resonance occurs. In order to investigate the principle of the new SSD method based on a negative capacitance (SSDNC), the variation of the voltage on the piezoelectric element and the current in the circuit are analyzed. Furthermore, the damping effect using the SSDNC is deduced, and the energy balance and stability of the new system are investigated analytically. The method is applied to the single-mode control and two-mode control of a composite beam, and its control performance was confirmed by the experimental results. For the first mode in single-mode control, the SSDNC is much more effective than SSDI. In other cases, the SSDNC is also more effective than the SSDI, although not significantly.

Large deformation of soft materials is harnessed to provide functions in the nascent field of soft machines. In this work, PEDOT:PSS noncovalent functionalized graphene–polyurethane dielectric elastomer composites (PEDOT:PSS–RGO@PU) have been synthesized as promising candidate materials for micro-actuator electromechanical applications. PEDOT:PSS conducting polymer chains not only reinforce the interaction between the polyurethane matrix and graphene sheets, but also prevent graphene sheets from aggregating and connecting, which are beneficial to forming microcapacitors in the matrix and suppressing the leakage current. The PEDOT:PSS–RGO@PU composite exhibits ultra high permittivity (350 at 1 kHz), low dielectric loss (∼0.2 at 1 kHz), low loss modulus (200 MPa), and low loss tangent (∼0.4), all being essential to create a high performance electric-induced strain material. The maximum thickness strain of 164% is significantly higher than reported values for polyurethane elastomers and nanocomposites.

In this paper, an improved semi-active control method is proposed and applied to the vibration control of a composite beam. This method is an improved version of the previously developed SSDV (synchronized switch damping on voltage) approach. In SSDV, a voltage source is connected to the shunting branch, in series with the inductor, which can magnify the inverted voltage and hence improve the control performance. Optimization of the voltage source is an important issue in all SSDV techniques. In the proposed approach, called adaptive SSDV, the voltage coefficient that controls the damping effectiveness is adjusted adaptively. An improved switching algorithm, which prevents the switch from over-frequently switching on-and-off and accordingly improves the control performance, is also proposed. Compared with previous SSDV techniques, this adaptive SSDV is the most stable, is independent of the excitation level, and is more flexible because the voltage coefficient is adjusted adaptively to achieve optimal control performances. The adaptive SSDV has been applied to the vibration control of a composite beam and its control results were compared with those of previously developed SSDV techniques. The effectiveness of the new switching algorithm was also verified by comparing it with the conventional switch. An experimental setup of a semi-active control system for the cantilever composite beam was established, and control experiments were carried out using different SSDV methods.