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Naoki Mori, Daichi Wakabayashi, Takahiro Hayashi, Tangential bond stiffness evaluation of adhesive lap joints by spectral interference of the low-frequency A0 lamb wave, International Journal of Adhesion and Adhesives, Volume 113, 2022, 103071, https://doi.org/10.1016/j.ijadhadh.2021.103071., Transmission spectra of the lowest-order antisymmetric (A0) Lamb wave for adhesively bonded single lap joints are experimentally investigated in a frequency range below 0.4 MHz. Based on the theoretical results of the wave interference, a tangential bond stiffness evaluation method is proposed for the lap joints. Aluminum alloy plates bonded with epoxy adhesive on different conditions were prepared as bonded specimens. An air-coupled ultrasonic transducer was used to generate the A0 mode in the specimens, and the signals of the transmitted waves across the joints were measured with a pin-type piezoelectric transducer. Spectral analysis was performed for the measured waveforms, and the transmission coefficient was calculated for each specimen as a function of frequency. The experimental results showed that the transmission coefficient has local maxima and minima at multiple frequencies, called peak and notch frequencies, respectively. The sets of the peak and notch frequencies were different among the bonded specimens. Based on the peak and notch frequencies, the tangential bond stiffness was estimated for each bonded specimen. As a result, the bond stiffnesses obtained for the specimens with the same nominal bond thickness and adherend pretreatment were almost equal even if the bond lengths were different. Furthermore, the evaluation results showed that pretreatments such as sanding and contamination of adherend surfaces affect the bond stiffness due to the change in the interfacial stiffness.

This is the Accepted Manuscript version of an article accepted for publication in Japanese Journal of Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at; https://doi.org/10.35848/1347-4065/ac48cc., Controlling sound fields is a key technology for noise removal, acoustic lenses, energy harvesting, etc. This study investigated the control of sound field by a periodic layered structure. At first, we formulated the wave propagation in a periodic layered structure and proved that the wave fields constructed by the periodic boundary conditions are limited to plane wave modes with discretely different propagation directions. Numerical calculations clarified that the desired plane wave mode can be obtained in the transmitted wave through an intermediate thin-plate stacked region in a periodic layered structure, in which Lamb waves travel in each plate at different phase velocities and create phase difference at the exit of the intermediate thin-plate region. Further numerical investigations revealed that tuning frequency and the length of the thin-plate region provides wave field more dominantly with a single wanted plane wave mode.

Naoki Mori, Takahiro Hayashi, Ultrasonic interference and critical attenuation in metal-plastic bilayer laminates, Journal of Sound and Vibration, Volume 547, 2023, 117531, https://doi.org/10.1016/j.jsv.2022.117531., The resonance of ultrasonic waves in a metal-plastic bilayer laminate placed in an infinite medium is theoretically investigated, and critical attenuation is shown to occur under specific conditions. The bilayer laminate is subjected to normal wave incidence from the side of the plastic layer. Based on a linear viscoelastic model, the amplitude of the reflection spectrum for the laminate is formulated to obtain the exact results for wave resonance and critical attenuation by numerical calculations. Furthermore, the approximate formulae for the conditions of the resonance and critical attenuation are derived explicitly by Taylor expansions with respect to the loss factor of the plastic. As a result, the exact solution shows that the resonance frequencies of the plastic layer slightly increase with increasing loss factor. Their variation with the loss factor agrees well with the second-order approximation result but is insignificant in the cases of common plastics with relatively low loss factors. The exact results also show that the amplitude at each order resonance depends on the loss factor and has a minimum at a critical loss factor. The obtained critical loss factor decreases with increasing resonance order and is well reproduced by the zeroth-order approximation. Experiments are performed on two types of metal-plastic bonded laminates, demonstrating that the measured reflection spectra are in good agreement with the theoretical predictions. In particular, the amplitude of the measured reflection spectrum decreases significantly at a resonance frequency, which is theoretically predicted to satisfy the critical attenuation condition.

Kota Notani, Takahiro Hayashi, Naoki Mori, Abnormal Sound Detection in Pipes Using a Wireless Microphone and Machine Learning, MATERIALS TRANSACTIONS, 2022, Volume 63, Issue 12, Pages 1622-1630, 2022, https://doi.org/10.2320/matertrans.MT-I2022001., Abnormal sound detection using a one-class support vector machine (OCSVM) and a principal component analysis (PCA) is proposed aiming to stable and objective inspection without skilled plant inspectors. For measurement of acoustic signals, we developed a compact microphone unit that can work in sound detection, signal transmission, and power supply, wirelessly. Six signal parameters were extracted as features from filtered and segmented acoustic signals. Using the features standardized and reduced in dimensionality by PCA, an anomaly detection model using OCSVM is built to detect abnormal sounds. The proposed method is verified by acoustic diagnosis of sound waves leaking from pipeworks with running water. Diagnostic accuracies were evaluated for artificial abnormal sounds with different types of burst waves output from a piezoelectric element attached to the pipe and Pencil Lead Break sound in water flowing background noise. Burst wave changes could be detected in almost all patterns, and the diagnostic accuracy was 100% for the Pencil Lead Break sound.