Lap joints are commonly used in civil infrastructure and mechanical assembly structures. Detecting the connection status of bolts, particularly in the early stages of use, is critical for ensuring structural integrity. A shear horizontal (SH) guided wave-based bolt looseness detection method, excited by a magnetostrictive patch transducer (MPT), is presented in this paper. A normalised bolt-looseness index I nor BL was proposed based on the wave energy transmission theory by analysing the signals received at the transducer for the transmitted wave passing through the actual contact area and the transducer for the direct incoming wave. Numerical and experimental studies were conducted on a lap joint with four bolts. The relationship between the I nor BL and bolt loads was obtained, verifying the effectiveness of this method for detecting the extent of the bolt looseness under various bolt tightness states. The results also show that SH0 waves, excited by an MPT, have better detection capability than Lamb waves, when the surface of the lap joint is subjected to liquid medium such as water and professional paint for bridges.

A single acoustic metamaterial with multifunctional use is highly needed for various applications. Herein, a bifunctional acoustic metamaterial for beam switching between the focusing beam and bottle beam is demonstrated, which consists of a groove structure for binary phases and a partitioned piezoelectric transducer (p-PZT) for incident wavefront modulation. The p-PZT is divided into inner and outer regions for selective excitation, and the focusing beam and bottle beam can be switched in real-time by applying different exciting signals on the p-PZT. Theoretical calculation, numerical simulation, and experimental measurement are conducted to verify the effectiveness of the proposed bifunctional acoustic metamaterial for beam switching. Furthermore, the focal plane of both the focusing beam and bottle beam can be linearly tuned by the operating frequency. This work may find potential applications in medical ultrasonic therapy, sound printing, and biological particle manipulation.

Sound transmission efficiency and beaming capability are the most important considerations in the design of single-slit lens toward high-performance acoustic imaging and detection. Here, we demonstrated numerically and experimentally enhanced sound focusing through a single subwavelength slit surrounded by grooves on both surfaces. A microscopic coupled-wave model was employed to optimize the slit-groove structure for both transmission enhancement and beam focusing. The simulated and measured results were in good agreement and readily verified the effectiveness of the proposed method in enhancing sound focusing with a single-slit lens. This work paves an alternative avenue for the design of single-slit lens, which may find potential applications in the broad field of acoustics, such as tumor boundary treatment, edge detection, and subwavelength acousto-optic modulation.

This paper presents a far-field enhanced-resolution underwater ultrasonic imaging on the basis of a single-slit structured planar focusing lens. The iterative angular spectrum approach was employed to optimize the planar focusing lens from a designated focal pattern, characterized by focal length and full width at half maxima. Numerical simulations and experimental investigations were conducted on different target objects. Compared to the conventional method, both results showed that the performance of far-field ultrasonic imaging was comprehensively enhanced with the proposed planar focusing lens, in terms of resolving capability, working distance, operational bandwidth, and robustness. The single-slit structured planar focusing lens is cost effective and easy to fabricate, which would greatly benefit ultrasonic imaging and drive new applications of ultrasound in various scenarios.