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Alzheimer’s disease (AD) is an important brain disease associated with aging. It involves various functional and structural changes which alter the EEG characteristics. Although numerous studies have found changes in delta, theta, alpha, and beta power, fewer studies have looked at the changes in the resting state EEG gamma activity characteristics in AD. This study aimed to investigate the alterations in the frequency and power values of AD patients’ resting-state EEG gamma oscillations compared with healthy elderly and young subjects. We performed Fast Fourier Transform (FFT) on the resting state EEG data from 179 participants, including 59 early stage AD patients, 60 healthy elderly, and 60 healthy young subjects. We averaged FFT performed epochs to investigate the power values in the gamma frequency range (28–48 Hz). We then sorted the peaks of power values in the gamma frequency range, and the average of the identified highest three values was named as the gamma dominant peak frequency. The gamma dominant peak frequency of AD patients ( M eyes−opened = 33.4 Hz, M eyes−closed = 32.7 Hz) was lower than healthy elderly ( M eyes−opened = 35.5 Hz, M eyes−closed = 35.0 Hz) and healthy young subjects ( M eyes−opened = 37.2 Hz, M eyes−closed = 37.0 Hz). These results could be related to AD progression and therefore critical for the recent findings regarding the 40 Hz gamma entrainment because it seems they entrain the gamma frequency of AD towards that of healthy young.

Current researches of acoustic emission (AE) mainly put the focus on fault diagnosis of traditional isotropic materials machining process or analysis of fiber-reinforced composite (FRC) tensile or bending strength, while there are merely studies on AE research of FRC grinding process. Numerous kinds of damage occur during FRC grinding process owing to their complicated structure. The main purpose of this paper is to extract proper index to estimate AE signals of FRC grinding and to recognize damage patterns, thus realizing FRC processing on-line detection. AE signals are obtained by single-grain grinding experiments of quartz fiber–reinforced silicon dioxide matrix composite (SiO 2 /SiO 2 ). The AE signal features are discussed, and the outstanding character of frequency components is proposed. The frequency of each damage pattern is analyzed and verified. AE effective voltage value (EVV) and event number percentage (ENP) of peak frequency (PF) of AE signals with processing parameters are researched. The results show that for the same kind of FRCs, frequency components of AE signals are only affected by damage patterns rather than processing parameters or grinding directions, thus being a proper estimate index. There are four main frequency bands during SiO 2 /SiO 2 grinding. The frequency 6.4–9.8 KHz corresponds to fiber fracture, 14.8–17.9 KHz is fiber debonding, 23.6–26.4 KHz is debris rubbing with workpiece and tool, and 34–35.5 KHz is matrix crack. EVV has a similar changing trend to grinding force with machining parameters. AE ENP of PF that the maximum peak amplitude (PA) corresponds to could quantitatively confirm the main damage modes under each processing condition.

In this paper, a new method is proposed to identify the frequency and amplitude of weak signals by using a non-smooth system. The variable scale limit system of smooth and discontinuous (SD) oscillator without considering the phase is adopted as the identification system. By using the non-smooth stochastic subharmonic-like Melnikov method, an analytical expression of chaotic threshold under Gaussian white noise is given. Based on the phase diagram and Poincaré section diagram, the influence of noise intensity on the recognition system is studied. According to the non-smooth variable scale-convex-peak frequency identification method, the frequency of the signal to be detected can be accurately identified. Using the numerical simulation, the amplitude of the signal to be measured is identified according to the amplitude bifurcation diagram of the reference signal. The optimal value range of the parameters of the identification system is determined. Through an example of early fault of wheelset bearing of high-speed train, the frequency and amplitude of the early weak fault signal can be identified and the fault location can be determined, which verifies the effectiveness of the above method. The results show that the non-smooth system can identify the frequency and amplitude of the weak signal submerged in strong noise, and it has a wider application and higher accuracy than the continuous system.

The study of the dynamic response characteristics of “W”-type deep canyon terrain to double-span concrete arch bridges under earthquake action holds great practical significance. In this research, a bridge in Sichuan Province is taken as the object of study. The boundary-integral equation method and peak frequency shift method are combined to apply an embedded linear time–history analysis algorithm to the finite element spatial dynamic calculation model of the entire bridge, resulting in an improved model. By comparing these two methods with model test results, the seismic response characteristics of the middle part of a “W” concrete arch bridge under different foundation depths and seismic intensities are examined. The boundary integral equation method was utilized to calculate ground motion response at any point on site, revealing a significant amplifying effect of increased seismic wave intensity on acceleration response at the top of the arch bridge. When input seismic wave intensity increased from 0.1 g to 0.3 g, maximum acceleration at buried depths of 3 m and 8 m in the middle of the arch bridge foundation increased by 102.63% and 79.16%, respectively, indicating that shallow buried depth structures are more sensitive to seismic wave intensity. Furthermore, using peak frequency shift rules for analyzing seismic wave propagation characteristics in “W”-type deep canyon topography confirms the sensitivity of shallow buried depth structures to seismic wave intensity and reveals the mechanism through which topography influences seismic wave propagation. This study provides a helpful method for understanding the propagation law and energy distribution characteristics of seismic waves in complex terrain. It was observed that the displacement at the top of the arch bridge increased significantly with an increase in seismic intensity. When subjected to 0.1 g, 0.2 g, and 0.3 g EI-Centro seismic waves, the maximum displacement at the top of the arch bridge model with a foundation buried depth of 3 m was 8 mm, 32 mm, and 142 mm, respectively. For arch bridge models with an 8-m foundation buried depth, these displacements were measured at 6.2 mm, 21 mm, and 68 mm, respectively. The results from model tests verified that increasing the depth of foundation burial effectively reduces the displacement at the top of the structure. Furthermore, by combining a boundary-integral equation method and peak-frequency shift method, this study accurately predicted significant influences on W-shaped double deep canyon topography from seismic response, and successfully captured stress concentration and seismic wave amplification/focusing effects on arch foot structures. The calculated results from both methods align well with model test data which confirm their effectiveness and complementarity when analyzing seismic responses under complex terrain conditions for bridge structures.

This paper investigates the problem of real-time parameter identification for ship maneuvering parameters and wave peak frequency in an ocean environment. Based on the idea of Euler discretion, a combined model of ship maneuvering and wave peak frequency (ship–wave) is made a discretion, and a discrete-time auto-regressive moving-average model with exogenous input (ARMAX) is derived for parameter identification. Based on the ideas of stochastic gradient identification and multi-innovation theory, a multi-innovation stochastic gradient (MI-SG) algorithm is derived for parameter identification of the ship–wave discretion model. Maximum likelihood theory is introduced to propose a maximum likelihood-based multi-innovation stochastic gradient (ML-MI-SG) algorithm. Compared to the MI-SG algorithm, the ML-MI-SG algorithm shows improvements in both parameter identification accuracy and identification convergence speed. Simulation results verify the effectiveness of the proposed algorithm.

Sandstone oil–gas reservoirs in the Junggar Basin, China have great development potential. However, their ultra-deep formation depth leads to high crustal stress and high breakdown pressure. Therefore, in this research, we studied the cyclic hydraulic fracturing of tight sandstone with different combinations of “high-pressure duration + low-pressure duration” under high-stress conditions. Through laboratory experiments, the pump pressure curves, hydraulic fracture morphology, acoustic emission counts, and peak frequency of the samples were obtained. The results showed that: (1) Compared with conventional hydraulic fracturing, the breakdown pressure of cyclic hydraulic fracturing was reduced by more than 30%, the minimum threshold of cyclic pump pressure required for sample breakdown was between 60%Pb and 70%Pb, and cyclic hydraulic fracturing more easily formed complex and diverse hydraulic fractures. (2) In cyclic hydraulic fracturing, under the same upper limit of cyclic pump pressure, the shorter the high-pressure duration, the fewer the cycles required for sample breakdown. (3) Under the same “high-pressure duration + low-pressure duration” condition, the lower the upper limit of the cyclic pump pressure, and the greater the number of cycles required for sample breakdown. (4) The AE cumulative counts curves fluctuated greatly during cyclic hydraulic fracturing, rising in an obvious step-wise manner and the AE peak frequency was banded and mainly divided into three parts: low frequency, medium frequency, and high frequency.

In this paper, the average energy of the γ -photon ( E a , ph ) is calculated and the correlation between log E a , ph and the logarithm of the synchrotron peak frequency ( log ν p ) is discussed for a sample of 620 BL Lac objects. The results show that there exists a positive correlation between the γ -photon energy and the synchrotron peak frequency for BL Lac objects, which implies that the γ -ray emissions are related to the low-energy emissions of radio to X-ray bands, and the high energetic γ -ray emissions are only partially from the synchrotron self-Compton process (SSC) for BL Lac objects.

Based on the mathematical modeling of the geomechanical condition of a coal face, it is shown that destruction of the face zone occurs in the form of squeezing-out and spalling within an operational cycle. The acoustic noise in the longwall is analyzed, and the noise spectrum peak frequency is determined. The peak frequency is reflective of the size of the damage area in the face zone.

In this paper, the rotating instability (RI) in an axial compressor has been investigated numerically in order to examine the capability of URANS method to simulate its typical characteristics of RI broadband humps with multi-peak frequencies (MPFs) and further to uncover the underlying flow mechanism. A full-annulus modeling solution has been adopted to fully capture the wide range of different length-scale flow disturbances that circumferentially propagating around the compressor rotor. During the transient computing process, long-term monitoring up to 50 revolutions has been carried out to achieve a fine frequency resolution, and that would be essential for resolving the MPFs with small frequency interval. It is shown that the MPFs feature of RI has been successfully captured by the full-annulus URANS approach, and also its frequency range and spectral feature agree well with the experimental results. Further, with a circumferential mode decomposition on the MPFs characteristics of RI, it has been found that the MPFs result from the interaction between long- and short-scale flow disturbances which circumferentially propagate around the compressor rotor near the clearance region. Detailed examination on the numerical three dimensional flow field indicates that the short-scale disturbance is induced by the unsteady oscillation of tip clearance vortexes with inter-passage phase delay. The long-scale disturbance is caused by the mistuning of the wave number of the short-scale disturbance and the blade number within the whole annulus.

Impact damages have been a major concern in the design of laminated composite structures since the damages mainly occur within the materials in a very short time. In this article, impact induced damage mechanisms of woven composites were investigated by means of novel SHPB-AE coupled tests. A step-consisting new methodology was developed; first, damage mechanisms of woven composites were determined by peak frequency analysis via wavelet transform (WT); second, the extraction of a range of acoustic emission (AE) parameters was achieved from AE energy, duration, and amplitude on the basis of peak frequency analysis; finally, the quantification of damage mechanisms was accomplished through a normalized cumulative AE count. In order to confirm the features of damage, the microscopic characterization of damage mechanisms was performed for the materials. As a result, the determination of impact damage mechanisms in tested composite materials was possible by peak frequency analysis via WT along with waveform and intensity examinations. In addition, the mechanisms could be identified by confirming the progress-based distribution and band formation of peak frequencies. AE parameter ranges were also extracted in terms of amplitude, duration, and energy according to the damage mechanisms. The damage process of the woven composites under impact loading began in the matrix and immediately caused fiber breakage to occur. Subsequently, multiple damage mechanisms were shown, such as fiber-matrix debonding, fiber pull-out, severe deformation and rupture in Kevlar fiber tips. The proposed methodology for the novel SHPB-AE coupled test of woven composites therefore showed an effective way to figure out in situ information on the damage under impact loading.