Spectral Moments Research Articles

Coupled critical plane-pseudo excitation method for multiaxial fatigue analysis of structures under random vibration

This study proposes a coupled critical plane–pseudo excitation method for assessing multiaxial fatigue damage in mechanical structures subjected to random loads. Within the modal coordinate framework, we derive an expression for the spectral moment of the structural stress response using the pseudo excitation method, which facilitates the decoupling of spatial characteristics from frequency domain properties. For the input power spectral density, represented as an integer power function, we employ eigenvalue decomposition and the residue theorem to derive an analytical expression for the spectral moment of the modal displacement response. By constructing equivalent stress modes based on the maximum shear stress and normal stress criteria, we obtain an expression for the equivalent stress variance. Intelligent optimization algorithms are utilized to determine the critical plane position and the corresponding equivalent stress spectral moments, followed by the application of the Dirlik method for fatigue assessment. A numerical example involving random vibration multiaxial fatigue analysis of an L-shaped thin-walled plate demonstrates the effectiveness of the proposed coupled critical plane–pseudo excitation method in comparison to the traditional critical plane method that relies on the stress covariance matrix for calculating equivalent variance. The results confirm the accuracy and efficiency of our approach, and we further explore the impact of different forms of the excitation power spectral density and the damping ratios on the computational outcomes.

Comparative Analysis of Linear and Nonlinear sEMG Methods for Detecting Muscle Fatigue During Dynamic Biceps Curls

Muscle fatigue, a key concern in sports science, rehabilitation, and occupational health, influences performance, injury risk, and provides insights into muscle functionality and endurance. Surface electromyography (sEMG) has emerged as a vital tool for non-invasively tracking muscle electrical activity and gauging health. As its application for muscle fatigue assessment grows, identifying the most accurate analytical methods is essential. Current sEMG analyses employ both linear and nonlinear metrics to measure fatigue onset and progression, yet research is ongoing to determine which method is most effective in the context of dynamic contractions. The study was aimed to evaluate the efficacy of established linear and nonlinear methods in measuring muscle fatigue caused by dynamic contractions through surface electromyography (sEMG) signals. A group of twelve healthy individuals completed biceps curls at a consistent pace of one repetition per four seconds, which constituted 75% of their 10-repetition maximum. Concurrently, sEMG signals were captured from the biceps brachii muscle at 1000 Hz. To assess the sEMG signals during the initial, middle, and final sets of 10 repetitions, three linear metrics—mean frequency, median frequency, and spectral moment ratio (SMR)—along with two nonlinear approaches, namely sample entropy and detrended fluctuation analysis (DFA), were utilized. The study's outcomes indicated notable shifts in the SMR values and the two DFA-derived scaling exponents across the exercise sets. These results indicated that SMR, sample entropy, and DFA are effective in gauging muscle fatigue, with sample entropy and DFA demonstrating heightened sensitivity to the fatigue levels when compared to the linear metrics.

The use of 1D carbon nanotube interacting with caffeine molecules for applications in solar cells: structure optimisation, Raman analysis and optoelectronic properties

This study proposes a promising candidate for organic solar cells through the creation of a novel nano-hybrid system composed of caffeine molecules encapsulated within a semiconducting single-walled carbon nanotube known as NT14 (14,0). The stability and optoelectronic properties of this hybrid system, designated as CA@NT14, were thoroughly investigated. We determined the optimal diameter for the NT14 nanotube using molecular mechanics, Density Functional Theory, spectral moment’s method, and bond polarizability model, finding it to be approximately 1.18 nm. The encapsulation’s impact on the Raman-active modes, specifically the radial breathing mode and tangential mode, was analyzed through Raman spectra calculations, revealing structural stability and charge transfer from CA to NT14. Notably, the NT14’s Fermi level position increased upon encapsulation, indicating charge transfer and a type-II band alignment. The study further examined the bandgap characteristics of the hybrid system, finding that the encapsulation of CA within semiconducting NT14 nanotubes minimally impacts the nanotube’s bandgap, thus retaining its excellent visible light absorption properties. Conversely, encapsulating CA within metallic nanotubes significantly reduces their bandgap, limiting their light absorption range. The nonresonant Raman responses of NT14 pre- and post-encapsulation corroborated the charge transfer between CA and NT14. Future research will focus on computing the electronic transport characteristics, transmission spectra, I-V characteristics, and yield of CA@NT14 using DFT and Non-Equilibrium Green’s Function ( formalism. An experimental study will be conducted to validate these theoretical findings. These results indicate the potential of CA@NT14 hybrids as effective active layers in organic solar cells, highlighting their significance in advancing optoelectronic applications.

A tail probability density distribution model of stress amplitude under band-limited stochastic vibration loadings

Resonance of engineering structures under stochastic vibration loadings is a primary cause of fatigue failure. Considering that cyclic loadings below the fatigue limit of the material contribute little to fatigue damage, the portion of the stress amplitude distribution that exceeds the fatigue limit is in focus in this paper, and a tail probability density distribution model of stress amplitude under band-limited stochastic vibration loadings is proposed. First, the response stress power spectral density functions under band-limited loadings are simulated by using the normal distribution and the Weibull distribution, respectively, and then the frequency domain signals are converted into time domain signals by adopting the inverse fast Fourier transform method. A two-parameter Weibull model of tail probability density distribution of stress amplitude is proposed, and the relationship between model parameters and power spectral moment parameters is given. Finally, the comparative analysis of the model accuracy and the vibration fatigue test of aluminum alloy notch specimen are carried out. It is demonstrated that the fatigue life predicted by the presented model is highly coincident with the results of tests and the time domain method.

Synthesis, structural characterization, thermal analysis and antimicrobial assay of Co (II) complexes derived from benzaldehyde N4-substituted thiosemicarbazone

Microwave assisted Co (II) complexes of N4 -Substituted thiosemicarbazone were synthesized (TSC-La-f) and characterized by UV-visible spectra, IR spectra, 1H and 13C NMR spectra, elemental analysis, TGA, magnetic measurements and molar conductance. On the basis of spectral data, magnetic moment and thermal analysis square planar geometry assigned to Co(II) complexes. The electrolytic nature of the complexes was confirmed by their moderately high molar conductance values. All the metal complexes were screened for antimicrobial assay. Among all the complexes (TSC-Le) showed more activity than other Co (II) complexes. From this study it was concluded that complexes are more potent bactericide and fungicide than their corresponding ligands. Keywords: Thiosemicarbazones, benzaldehyde, Co(II) complexes, antimicrobial activity.

A framework for rapid fatigue hotspot localization and damage assessment of plate with multiple holes based on the fatigue damage response spectrum method

AbstractThis paper proposes a novel framework for the random vibration fatigue assessment of thin‐walled plate with multiple holes based on the fatigue damage response spectrum method. Compared with other frequency‐domain evaluation methods, this framework fully exploits the dynamic characteristics of complex structures, decoupling the external excitation characteristics from the spatial characteristics of the structural response. This approach significantly enhances evaluation efficiency by avoiding the complex calculations associated with stress response spectral moments. The proposed method is employed to evaluate the contribution of each mode to the overall damage, additionally, the stress mode shapes are used to identify and refine the mesh around fatigue hotspots. Modal damage contribution factors are proposed to identify the key modes. By leveraging both structural dimension reduction and modal reduction techniques, the proposed framework can swiftly and accurately locate fatigue hotspots within complex structures and conduct precise fatigue assessments using the Absolute Sum ‐ Square Root of the Sum of Squares hybrid method. Finite element simulation analysis is conducted on a single‐lap structure containing numerous circular openings, validating the accuracy and efficiency of the proposed stochastic vibration fatigue assessment.

Early detection of combustion instabilities in liquid rocket engines: Assessment of statistical, recurrence, and fractal analyses for sub- and supercritical pressure conditions

Safe shutdown of a liquid rocket engine in ground testing prior to the onset of damaging combustion instabilities through reliable detection of instability precursors would translate to time and cost savings in engine development programmes. Methods derived from statistical, recurrence, and fractal analysis have been successfully applied in the literature to detect precursors in unsteady pressure signals from canonical combustion experiments, gas-turbine combustion experiments, and sub-scale rocket combustion experiments operated at low pressures. In the present work, several such methods were applied to data from two cryogenic oxygen-natural gas rocket experiments operated at higher pressures than previously reported; both sub- and supercritical with respect to oxygen. The goal was to identify methods that can discern limit-cycle instabilities from intermittently unstable operation and are sufficiently responsive to be applied as emergency shut-down criteria in engine tests. Among the methods applied were the standard deviation, variance of the auto-correlation, the second spectral moment, the ratio between determinism and recurrence rate, the Hurst-exponent, and the multifractal range. The second spectral moment, the Hurst-exponent, and a measure derived from the multifractal spectrum all have short detection delays for instability onset and short-lived could be discerned from self-sustaining instabilities with an appropriate choice of threshold value. They also have moderate computation cost which makes them of interest for potential real-time implementation. The Hurst-exponent has the additional advantage of a common threshold value for all test cases addressed, demonstrating its potential for broader application independent of combustion device or operating conditions.

Correlation and Spectral Density Functions in Mode-Stirred Reverberation – II. Spectral Moments, Sampling, Noise, EMI, and Understirring

Correlation and Spectral Density Functions in Mode-Stirred Reverberation – II. Spectral Moments, Sampling, Noise, EMI, and Understirring

A probability density function of stress amplitude applicable to power spectral density curves of structural random vibration response

The power spectral density curves of the random vibration response of a plate specimen with different spectral width coefficients were obtained by selecting different frequency ranges, and the probability distributions of the stress amplitudes were obtained by applying Monte Carlo method and rainflow counting method. Each probability distribution was divided into three segments by the corresponding zero-order spectral moment value, and each segment was fitted using either a Rayleigh function or an exponential function. Finally, a probability density function of stress amplitude was proposed by combining two Rayleigh functions and an exponential function. The proposed probability density function can be used to obtain the stress amplitude probability distribution for the power spectral density curves of structural random vibration response with any bandwidth, and the accuracy of the proposed function was verified based on the response data of a specimen of 7075-T651 aluminum alloy material under the action of six typical power spectral curves.

An acoustic exploration of sibilant contrasts and sibilant merger in Mixean Basque

Abstract This exploratory study investigates sibilants in Mixean Low Navarrese, an endangered variety of Basque. This variety has been described with ten different contrastive sibilants: /s̻, s̺, ʃ, t͡s̻, t͡s̺, t͡ʃ, z̻, z̺, ʒ, d͡z̺/. The objective of the paper is to (a) provide a detailed description of the acoustics of Mixean sibilants, and (b) elucidate whether ten categories can be proposed based only on acoustical data, or whether fewer categories should be considered. The study is based on free-conversation data of ten subjects (three females, seven males) aged between 80 and 85 years. We analyze metrics reflecting the place of articulation (spectral moments, and especially the center of gravity (CoG)), including also the temporal dynamics of CoG (using the discrete cosine transform of CoG measurements of nine intervals of each phone). We also explore the acoustic correlates of the contrasts between (a) voiced and voiceless sounds and (b) fricative and affricate sounds. The results show that only seven categories can be proposed based on acoustic measurements. The lamino-alveolar series reliably contrasts with the rest, but the distinction does not hold between the apico-alveolar and the postalveolar series. We found minimal differences in the analysis of dynamic data, and none in the static analysis.

Differential spectral characteristics of the Spanish fricative /s/ in the articulation of individuals with dysarthria and apraxia of speech

PurposeThis study examines whether there are differences in the speech of speakers with dysarthria, speakers with apraxia and healthy speakers in spectral acoustic measures during production of the central-peninsular Spanish alveolar sibilant fricative /s/. MethodTo this end, production of the sibilant was analyzed in 20 subjects with dysarthria, 8 with apraxia of speech and 28 healthy speakers. Participants produced 12 sV(C) words. The variables compared across groups were the fricative's spectral amplitude difference (AmpD) and spectral moments in the temporal midpoint of fricative execution. ResultsThe results indicate that individuals with dysarthria can be distinguished from healthy speakers in terms of the spectral characteristics AmpD, standard deviation (SD), center of gravity (CoG) and skewness, the last two in context with unrounded vowel, while no differences in kurtosis were detected. Participants with AoS group differ significantly from healthy speaker group in AmpD, SD and CoG and Kurtosis, the first one followed unrounded vowel and the latter two followed by rounded vowels. In addition, speakers with apraxia of speech group returned significant differences with respect to speakers with dysarthria group in AmpD, CoG and skewness. ConclusionsThe differences found between the groups in the measures studied as a function of the type of vowel context could provide insights into the distinctive manifestations of motor speech disorders, contributing to the differential diagnosis between apraxia and dysarthria in motor control processes.

Language Dependency of /s/ Production: Native Dutch Versus Non-Native English.

With forensic recordings being collected in multiple languages increasingly often, this study investigates the language dependency of the voiceless alveolar fricative /s/ in speakers of native (L1) Dutch and non-native (L2) English. Due to phonetic similarity between the languages, Dutch learners of English may exhibit language-independent /s/ acoustics, making it an interesting feature for multilingual forensic speaker comparisons (FSCs). However, the findings show that out of the four spectral moments, center of gravity, standard deviation (SD), skewness, and kurtosis, only SD remained stable across the languages; the other measurements were language-dependent. The results were largely independent of the /s/ tokens' contexts, although an interaction between language and context was found for skewness and kurtosis: With a labial right phonetic neighbor, language dependency was largely reduced. The findings have implications for FSCs: as /s/ is language-dependent in speakers of L1 Dutch and L2 English, it shows limited potential for cross-linguistic speaker comparisons in forensic casework.

Microscopic self-dynamics in liquid Ne-D_{2} mixtures: Quantum features and itinerant oscillators reexamined.

In this paper, we report the results of a centroid molecular dynamics (CMD) study of the canonical velocity autocorrelation functions (VACFs) in liquid Ne-D_{2} mixtures at a temperature of T=30K and in the full D_{2}-concentration range (0%≤x_{D_{2}}≤100%). This binary system was selected because of its moderate, although sizable, quantum effects which, as far as its equilibrium properties are concerned, are fully described by the path integral Monte Carlo (PIMC) simulations that have been also implemented. A comprehensive test of the VACF spectral moments carried out using three physical quantities (namely, mean kinetic energy, Einstein frequency, and mean-squared force) obtained from PIMC was performed revealing the potentialities, as well as the limitations, of the CMD approach to the single-particle dynamics in these low-T liquid mixtures. Additional physical information was extracted from the canonical VACFs by fitting their spectra via two distinct methods: the Levesque-Verlet model (LV, very flexible but highly heuristic) and the itinerant oscillator model (IO, based on the physical ground of a single particle rattling inside a short-lived diffusing pseudocage). Both provided good fits of the CMD outputs, with LV being always more adequate than IO in the case of the Ne VACFs, while, as for the D_{2} VACFs, the LV superiority is evident only at high x_{D_{2}} values. However, a peculiar and systematic effect was found after analyzing the IO-fitted parameters: the estimated pseudocage masses turned out to be at least one order of magnitude lower than the corresponding values inferred from the PIMC simulations. This outcome concerns both the Ne and the D_{2} rattling molecule and, as we also discovered, had already been observed (but promptly forgotten) in purely classical simulations of liquid Ar. The possible physical origins of this finding have been finally discussed in some detail, also in connection with the result of the more recent exponential expansion theory (EET), which manages to shed more light on the concept of single particles rattling inside short-lived pseudocages, ultimately demonstrating its untenability.

A Weighted Adaptive Range-Averaging Technique to Improve the Precision Consistency of Polarimetric Variable Fields

Abstract The two main metrics for the performance evaluation of radar-variable estimators are the bias and standard deviation (SD) of estimates. Depending on the estimator properties, the bias may increase as the signal-to-noise ratio (SNR) decreases. The standard deviation, however, always rises as the SNR becomes smaller. For instance, if estimates are computed from 16 samples (typically used for WSR-88D surveillance scans) using a rectangular data window and the maximum unambiguous velocity is ∼9 m s−1, the standard deviation of reflectivity estimates increases 1.6 times as the SNR drops from 20 to 2 dB. But for estimates of differential reflectivity, differential phase, and copolar correlation coefficient, SDs increase ∼6.7, ∼6, and ∼54 times, respectively. Hence, this effect impacts the polarimetric variables substantially more than the spectral moments. Additionally, the polarimetric variable SD is also sensitive to the correlation between signals in horizontal and vertical channels leading to reduced data quality in the regions where the correlation coefficient is low. Such increases in the variability of estimates are observable in the fields of dual polarization variables as an increased spatial inhomogeneity (or noisiness) in the areas where radar echoes exhibit low-to-moderate SNRs and/or decreased correlation coefficient. These effects can obscure the visual identification of weather features as well as adversely impact algorithms. Herein, a novel method that applies variable smoothing in the range where the smoothing intensity depends on the SDs of estimates is presented. It applies little or no range averaging in the regions where data SDs are deemed adequate while using more aggressive smoothing in areas where data appear noisy. Significance Statement The noisiness in the fields of polarimetric variables is an issue that has plagued dual-polarization weather radars since their inception. This is because standard deviations of polarimetric variable estimates increase significantly more with the reduction in SNR than those of spectral moment estimates. A typical mitigation approach that indiscriminately averages a fixed number of estimates in the range may lead to unnecessary loss of range resolution in regions where data appearance is satisfactory. Further, such an approach can be inadequate in regions with a high variability of estimates leading to insufficient enhancement of weather feature visibility. In this study, a method that mitigates these issues is proposed.

Line shapes of isotropic and depolarized CILS spectra of neon: Interaction potential and pair polarizability

The present work proposes three interaction potentials between two gaseous neon atoms and applies them to the study of models representing the anisotropy and trace of the induced pair polarizability tensor. The proposed empirical interatomic potentials for the gaseous Ne-Ne interaction are developed by fitting the Improved Lennard-Jones (ILJ), Modified Tang-Toennies (MTT) and Barker et al. (BFW) potentials simultaneously to the spectroscopic and transport properties over a wide temperature range. The validity of the proposed interatomic potentials is verified by comparing the calculated and experimental second pressure virial coefficient over a wide temperature range. In addition to this, the results show satisfactory accordance between the experimental and calculated vibrational energy levels using the deduced pair potential parameters compared to those reported for neon gas. These comparisons exhibit acceptable accuracy when the present work potential parameters are inserted to reproduce the transport and thermo-physical properties, besides the spectral moments and the spectral line-shape intensities for neon gas. The isotropic and depolarized collision-induced light scattering (CILS) spectra are produced and analyzed as functions of the potential considered. They exhibit sensitivity to both the attractive part of the potential and the short-range values of the polarizability anisotropy or mean at intermediate and high frequencies. Mean and anisotropy are deduced directly from spectral line shapes using a new inversion method.

Clinical Focus: The Development and Description of a Palette of Transmasculine Voices.

The study of gender and speech has historically excluded studies of transmasculine individuals. Consequently, generalizations about speech and gender are based on cisgender individuals. This lack of representation hinders clinical training and clinical service delivery, particularly by speech-language pathologists providing gender-affirming communication services. This letter describes a new corpus of the speech of American English-speaking transmasculine men, transmasculine nonbinary people, and cisgender men that is open and available to clinicians and researchers. Twenty masculine-presenting native English speakers from the Upper Midwestern United States (including cisgender men, transmasculine men, and transmasculine nonbinary people) were recorded, producing three sets of speech materials: Consensus Auditory-Perceptual Evaluation of Voice sentences, the Rainbow Passage, and a novel set of sentences developed for this project. Acoustic measures vowels (overall formant frequency scaling, vowel-space dispersion, fundamental frequency, breathiness), consonants (voice onset time of word-initial voiceless stops, spectral moments of word-initial /s/), and the entire sentence (rate of speech) that were made. The acoustic measures reveal a wide range for all dependent measures and low correlations among the measures. Results show that many of the voices depart considerably from the norms for men's speech in published studies. This new corpus can be used to illustrate different ways of sounding masculine by speech-language pathologists performing gender-affirming communication services and by higher education teachers as examples of diverse ways of sounding masculine.

Dynamic response of asymmetric suspended structure subjected to random wind excitation

SummarySuspended structures, regarded as the expression of structural beauty, have attracted the focus of many architects and engineers. With the complexity of dynamic responses of the asymmetric suspended structures subjected to the random alongside wind excitation, an innovative method was proposed through combining the finite element method (FEM), the complex mode method (CMM), the pseudo excitation method (PEM), and the quadratic decomposition method (QDM) of frequency response function. The method could obtain unified closed‐form solutions of spectral moment and variance of node displacement and velocity and acceleration of asymmetric structures and its correctness was verified by existing numerical approaches. The proposed method is applicable to the random wind‐induced vibration response analysis of diverse linear complex structures. Notably, its efficacy lies in its circumvention of numerical integration, endowing it with high computational efficiency and accuracy. Extended comparative studies and discussion were also performed on effect of suspended span and comparisons of normal framed structure and symmetry structure. The results showed that the suspended span presented positive relation to horizontal displacement and inverse tendency to horizontal acceleration of the whole structure and vertical and horizontal acceleration of the columns near to suspended part should also be considered in engineering design.

Numerical Covariance Evaluation for Linear Structures Subject to Non-Stationary Random Inputs

Random vibration analysis is a mathematical tool that offers great advantages in predicting the mechanical response of structural systems subjected to external dynamic loads whose nature is intrinsically stochastic, as in cases of sea waves, wind pressure, and vibrations due to road asperity. Using random vibration analysis is possible, when the input is properly modeled as a stochastic process, to derive pieces of information about the structural response with a high quality (if compared with other tools), especially in terms of reliability prevision. Moreover, the random vibration approach is quite complex in cases of non-linearity cases, as well as for non-stationary inputs, as in cases of seismic events. For non-stationary inputs, the assessment of second-order spectral moments requires resolving the Lyapunov matrix differential equation. In this research, a numerical procedure is proposed, providing an expression of response in the state-space that, to our best knowledge, has not yet been presented in the literature, by using a formal justification in accordance with earthquake input modeled as a modulated white noise with evolutive parameters. The computational efforts are reduced by considering the symmetry feature of the covariance matrix. The adopted approach is applied to analyze a multi-story building, aiming to determine the reliability related to the maximum inter-story displacement surpassing a specified acceptable threshold. The building is presumed to experience seismic input characterized by a non-stationary process in both amplitude and frequency, utilizing a general Kanai–Tajimi earthquake input stationary model. The adopted case study is modeled in the form of a multi-degree-of-freedom plane shear frame system.

Spectral properties of Quebec French sibilants

Most acoustic work on sibilants has focused on English /s/ and /ʃ/, examining spectra of the middle portion of these consonants. Voiced sibilants, which are rarer cross-linguistically (Ohala, 1983), as well as sibilants in other languages, have received less attention; in particular, there has been no previous work on Quebec French. Previous cross-linguistic studies have suggested important differences in sibilant acoustics exist between languages (Gordon et al., 2002), including in the dynamics of spectral properties over the course of the segment (Reidy, 2016). This study adds to the literature on the typology of sibilant acoustics by examining the spectral dynamics of sibilants in Quebec French (/s, ʃ, z, ʒ/). ∼28 k word-initial, pre-vocalic tokens from more than 100 speakers are extracted from a large corpus of parliamentary speech (Milne, 2014). For each token, multitaper spectra (Reidy, 2013) over a 20 ms window are calculated at 17 equidistant points. A variety of acoustic measures, including segment duration and spectral moments, are reported. Preliminary results examining static measures reveal that the anterior sibilants have higher spectral speak (∼5700 Hz vs ∼4600 Hz for posterior ones), as expected; unlike in English, however, / s/ and /ʃ/ have similar average duration.

Spectral moment analysis of distortion errors in alveolar fricatives in Korean children*

Spectral moment analysis of distortion errors in alveolar fricatives in Korean children*