Effect of interfacial adhesion on the ultrasonic interaction with adhesive joints: A theoretical study using spring-type interfaces

The effect of interfacial properties on the reflection and transmission characteristics of ultrasonic waves at adhesively bonded joints is theoretically investigated. An adhesive joint is modeled as a double-interface model, namely, a homogeneous layer coupled to adherends by two spring-type interfaces with different interfacial stiffnesses. For the normal incidence of a one-dimensional longitudinal wave, theoretical results are obtained and validated by finite element simulation. When the thickness of the adhesive layer is sufficiently small compared to the wavelength, the amplitude reflection and transmission coefficients show monotonic dependence on frequency, which can be explained by the theoretical relation of the double-interface model to a single spring-type interface model. The reflection and transmission behavior is invariant if the values of the two interfacial stiffnesses are interchanged. For a relatively thick adhesive layer, on the other hand, the reflection coefficient shows local minima at multiple frequencies. As one interfacial stiffness decreases, the local minimum frequencies decrease and the local minima increase. If the values of the two interfacial stiffnesses are interchanged, the reflection coefficient remains invariant but the reflection waveform shows different features. The obtained reflection and transmission characteristics are discussed in light of the characterization of the interfacial adhesion.Naoki Mori, Naoki Matsuda, and Takayuki Kusaka, "Effect of interfacial adhesion on the ultrasonic interaction with adhesive joints: A theoretical study using spring-type interfaces", The Journal of the Acoustical Society of America 145, 3541-3550 (2019) https://doi.org/10.1121/1.511185

Reflection and transmission characteristics of Lamb waves at an adhesive lap joint of plates

The reflection and transmission characteristics of Lamb waves at an adhesive single lap joint of plates are examined theoretically by the hybrid finite element method. The adhesive joint is modeled by a linear spring-type interface, which is characterized by normal and tangential stiffnesses. For the incidence of the lowest-order antisymmetric (A0) Lamb mode in a low frequency range, it is shown that the reflection and transmission coefficients of the A0 mode take local maxima and minima at multiple frequencies. This behavior results from the interference of waves originating from the lowest-order antisymmetric guided wave mode in the overlap region. The peak frequencies of the transmission coefficient increase monotonically with increasing tangential stiffness, but are almost invariant with the normal stiffness of the adhesive joint. Furthermore, time-domain numerical simulation using the finite element method is carried out to discuss the theoretical results. As a result, for the A0 mode incidence, the reflection and transmission waveforms of the A0 mode from the lap joint are found to show long-oscillation tails. The spectral analysis for the obtained waveforms shows that these tails are necessary to identify the frequencies at which the reflection and transmission coefficients take local maxima and minima.Naoki Mori and Takayuki Kusaka, "Reflection and transmission characteristics of Lamb waves at an adhesive lap joint of plates", The Journal of the Acoustical Society of America 145, 3075-3085 (2019) https://doi.org/10.1121/1.510909

Harmonic generation at a nonlinear imperfect joint of plates by the S0 Lamb wave incidence

Nonlinear interaction of Lamb waves with an imperfect joint of plates for the incidence of the lowest-order symmetric (S0) Lamb wave is investigated by perturbation analysis and time-domain numerical simulation. The imperfect joint is modeled as a nonlinear spring-type interface, which expresses interfacial stresses as functions of the displacement discontinuities. In the perturbation analysis, under the assumption of weak nonlinearity, the second-harmonic generation at the joint is examined in the frequency domain by the thin-plate approximation using extensional waves. As a result, the amplitude of the second-harmonic extensional wave is shown to be in good agreement with the result of the S0 mode in a low-frequency range. However, it is found that the thin-plate approximation does not reproduce the amplification of the second-harmonic S0 mode, which occurs due to the resonance of the joint. Furthermore, the time-domain analysis is performed by the elastodynamic finite integration technique (EFIT). When the amplitude of the incident wave is relatively large, the fundamental wave and the second harmonic exhibit different behavior from the results by the perturbation analysis. Specifically, if the incident amplitude is increased, the peak frequency of the second-harmonic amplitude becomes low. The transient behavior of the nonlinear interaction is also examined and discussed based on the results for the weak nonlinearity

Second-harmonic generation of the lowest-order antisymmetric Lamb wave at a closed parallel crack

The second-harmonic generation of the fundamental antisymmetric Lamb wave at a closed parallel crack in an elastic plate is studied by numerical analysis. The closed crack is modeled as a spring-type interface with quadratic nonlinearity. Based on a perturbation method, the problem of nonlinear Lamb wave scattering is decomposed into two linearized problems, i.e., for the linear reflection/transmission of the incident Lamb wave at the crack and for the generation/radiation of the second-harmonic Lamb waves due to the contact nonlinearity. The reduced problems are solved by the finite element method in the frequency domain. Numerical results demonstrate significant effects of the crack resonance on the linear and nonlinear Lamb wave scattering responses, which appear as sharp peaks/dips in the reflection/transmission spectra and enhanced second-harmonic amplitudes at some frequencies. Two simple frequency selection rules are established which explain the enhanced generation of the second-harmonic Lamb waves. The time-domain analysis is also carried out to supplement the frequency-domain analysis, which confirms that the incident Lamb wave interacts with the crack at some specific frequencies in its bandwidth in a selective manner and enhances the generation of the second-harmonic components

Atiyah-Patodi-Singer index on a lattice

We propose a non-perturbative formulation of the Atiyah-Patodi-Singer(APS) index in lattice gauge theory, in which the index is given by the $\eta$ invariant of the domain-wall Dirac operator. Our definition of the index is always an integer with a finite lattice spacing. To verify this proposal, using the eigenmode set of the free domain-wall fermion, we perturbatively show in the continuum limit that the curvature term in the APS theorem appears as the contribution from the massive bulk extended modes, while the boundary $\eta$ invariant comes entirely from the massless edge-localized modes.Comment: 14 pages, appendices added, details of key equations added, typos corrected, to appear in PTE

Tangential bond stiffness evaluation of adhesive lap joints by spectral interference of the low-frequency A0 lamb wave

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.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

Abnormal Sound Detection in Pipes Using a Wireless Microphone and Machine Learning

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.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

Review Article Aberration in Signal Transduction Pathway in Human T-Cell Leukemia Virus Type I-Infected T Cells

Adult T-cell leukemia (ATL) is a fatal T-lymphoproliferative disorder, and its development is associated with infection by human T-cell leukemia virus type I (HTLV-I). The molecular mechanism of leukemogenesis has not yet been elucidated. However, several studies have suggested that aberrations in signal transduction in virus-infected T cells are involved in the development of the disease. For instance, NF-ﾎｺB/Rel, AP-1, and Jak/STAT signaling pathways are transiently activated in normal T cells by growth-signals, whereas they are constitutively deployed in HTLV-I-infected T-cell lines. The HTLV-I viral transactivator Tax has oncogenic properties, and is a key molecule in ATL development. Tax activates several transcription factors, including NF-ﾎｺB/Rel and AP-1 in HTLV-I-transformed T-cell lines, and induces the expression of STAT proteins. Unlike HTLV-I-transformed T-cell lines, primary ATL cells express low levels of Tax protein. Nevertheless, the NF-ﾎｺB/Rel, AP-1, and Jak/STAT signaling pathways in primary ATL cells are also constitutively activated. Thus, the aberration in signal transduction pathway may be a common key factor to prolonged survival and proliferation of HTLV-Iinfected T cells in vitro and in vivo, but the mechanisms seem to be different. Aberration in signal transduction could be targeted for the therapeutic control of HTLV-I-associated disease