Amplitude Of Cycle Research Articles

An experimental investigation into soil–structure interactions of integral bridge abutments due to cyclic translations

Integral bridges are built without expansion joints to avoid many of the persistent issues with joint degradation and maintenance problems regularly experienced by traditional bridges. In the absence of expansion joints, the cyclic thermal displacements of the integral bridge would be transferred unmitigated by the joints to the abutment resulting in accentuated soil–structure interactions. Previous studies have mainly focussed on the rotational mode of integral abutment movement while the translational mode of movement and backfill deformations have received much less attention. This paper presents the results of laboratory tests conducted on a semiscale (1:2) physical model, which was designed using dimensional analysis to achieve field stress and strain similarity with a full-scale 1.92 m high-prototype integral abutment. The comparative effects from five tests covering low to high cyclic translation amplitude on backfill surface settlement and heave, and lateral earth pressures behind the abutment were investigated over assumed 120 seasonal cycles. Asymptotic settlement depth of the backfill was obtained in each test. Full passive failure surface was observed when the cyclic translation amplitude of the test exceeded the threshold amplitude. The findings also suggest that the settlement depth seemed to be linked to the development of passive failure surface in the following sense—the asymptotic settlement depth in those tests which developed full passive failure surface would attain the same (ultimate) asymptotic depth; conversely, when the passive failure surface had not or only partially developed in that test, the asymptotic settlement depth was dependent on the cyclic translation amplitude, but it was less than the ultimate observed for full passive failure surface. Lateral earth pressure only stabilized to an asymptotic state for the test with the lowest cyclic translation amplitude during the 120 cycles. Shakedown limit state was observed for this case. At larger cyclic translation amplitudes, the lateral earth pressure did not stabilize but continued to evolve with the elapsed cycles, and shakedown limit state was not observed for these cases.

Experimental investigation and micromechanical modeling of load partitioning behavior of duplex stainless steel 2205 during cyclic hardening

This paper aims to understand the load partitioning behavior between austenite and ferrite of duplex stainless steel 2205 under cyclic loading by experiments, including a series of low cycle fatigue tests and electron back scattered diffraction observation. Meanwhile, a crystal plasticity finite element model was developed to predict the cyclic micromechanical response of two constituent phases. The results show that more slip bands were found within the austenite. The cyclic plasticity of 2205 started within the austenite, and gradually translated into the ferrite with the elevation of applied cyclic amplitude. The plastic deformation within the austenite was inclined to form at the phase boundaries, while it promoted at grain boundaries in the ferrite. The micromechanical elasto-viscoplastic parameters of two constituent phases were calibrated accurately. The proposed crystal plasticity finite element model has a great ability of predicting not only the time- and amplitude- macroscopic cyclic hardening behavior but also the micro-deformation within two constituent phases, even the cyclic hardening of 2205 with different phase ratios can also be reproduced accurately. The prediction error on cyclic stress amplitude of 2205 during cyclic hardening can be controlled within 3.2%.

Estimation of limit cycle amplitude after onset threshold of thermoacoustic Stirling engine

This study estimates the amplitude saturation process of spontaneous pressure oscillations in the thermoacoustic Stirling engine, which harnesses a stack-screen regenerator, after the engine onset. To observe spontaneous initiations leading to finite amplitudes of sound in the engine, the engine is studied experimentally to see how the acoustic pressure fluctuates in the time domain. To describe this, the nonlinear effects of the flow resistance of the stacked-screen regenerators, empirically proposed as an effective radius reff whose size decreases with increasing Reynolds number, are incorporated into the framework of Rott’s linear thermoacoustic theory for depicting the evolution of oscillations from initiation to a steady state. The numerical calculations are based on the coupling between pressure oscillations and the imaginary part of a complex angular frequency which is obtained numerically from the characteristic equation derived from the transfer matrices modeling the engine. For the scenarios of steady oscillations, the measured results are elucidated with calculations by consciously adjusting r0 which is involved in reff and estimates the threshold of thermoacoustic instability. By plotting with the adjusted r0, the measured stability limits expressly agree with the stability curve calculated from Rott’s theory. This work improves Rott’s frequency-domain theory to the temporal description of the thermoacoustic Stirling engine, from start-up to limit cycle oscillation.

Study on Hopf bifurcation types and dominant parameters of grid‐connected voltage source converter considering non‐linear saturation elements

AbstractSubsynchronous oscillation events of grid‐connected voltage source converter (VSC) pose a potential risk to the security and stability of power systems. It is particularly challenging to reveal the subsynchronous oscillation mechanism precisely because the grid‐connected VSC consists of non‐linear components such as saturation. The describing function and the generalized Nyquist criterion are used in this paper to investigate the bifurcation types and dominant parameters of grid‐connected VSC. First, the condition of a supercritical Hopf bifurcation is investigated, which generates subsynchronous oscillations. Then, the dominant parameters of supercritical Hopf bifurcation are deduced, including the grid strength, the active power reference, or the proportional coefficient of phase‐locked loop. Finally, it is examined how the saturation's limit value impacts the limit cycle's amplitude and stability. The findings demonstrate that a system's stability depends on whether its closed‐loop frequency characteristic curve intersects the describing function's negative reciprocal curve. When the two curves intersect, the supercritical Hopf bifurcation appears, and changing the limit value can affect the limit cycle's amplitude. However, if the two curves do not intersect, altering the limit value cannot cause the supercritical Hopf bifurcation, and the subsynchronous oscillation will not happen. Variations in grid strength, active power reference, or proportional coefficient of phase‐locked loop will significantly affect the Nyquist curve, determining if supercritical Hopf bifurcation appears.

Experimental study on the behavior of single pile foundation under vertical cyclic load in coral sand

The bearing behaviors of piles in coral sand are critical for project safety due to the unique physical and mechanical properties of coral sand, such as high void ratio, high compressibility, high friction angle and easy to break. A series of model experiments were carried out in this study to evaluate the vertical behavior of single pile in coral sand foundation under cyclic dynamic load with the consideration of the influence of the amplitude of cyclic load, the static biased load and the number of cyclic. Load-settlement relationships, settlement-cyclic number relationships, pile tip and pile side resistances have been thoroughly investigated. Results showed that the load in coral sand foundation will transfer to the pile tip more obviously compared with the pile foundation in quartz sand. Therefore, the pile side resistance will be reduced while the corresponding pile tip resistance will be increased. Particularly, the weakening of side resistance mainly occurs in the first 500 cycles. The development of cyclic cumulative settlement is affected by the combination of the magnitude of static biased load and cyclic amplitude. The existence of static biased load is conducive to restrain the cyclic cumulative settlement when the amplitude of cyclic load is less than static load.

Numerical model of cumulative damage evolution of offshore wind turbine grouted connection under cyclic axial load

GCs in OWTs are exposed to fatigue loads that can lead to damage of the grout before ultimate limit state is reached. A uniaxial and fatigue compression test of high strength grout material were carried out to obtain the compressive strength, failure mode, fatigue life and load-displacement relationship of test cubes. A uniaxial and fatigue tests were carried out on GCs specimens to study the damage evolution of the GCs. The fatigue test on GC was done by applying six incremental amplitude cyclic load ranges of 30%–80% of ultimate load on the GC. The scalar damage degradation parameter (SDEG) from the result of the numerical simulation was used to estimate a damage index which measured the cumulative damage evolution. The cumulative damage evolutions were influenced by the cyclic load amplitude. The larger the load amplitude, the higher the damage index accumulated. The numerical simulation results compared with the DNV code method showed that the DNV code predicted lower damage values for fatigue loads ratio of up to 0.725 and predicted higher damage values beyond the 0.725 loads ratio for the GC. The proposed numerical model predicted the laboratory test results of the grout cube and the grouted connections.

Predicting the 25th and 26th solar cycles using the long short-term memory method

Abstract Solar activities directly or indirectly affect space missions, geophysical environment, space climate, and human activities. We used the long short-term memory (LSTM) deep learning method to predict the amplitude and peak time of solar cycles (SCs) 25 and 26 by using the monthly relative sunspot number data taken from the National Astronomical Observatory of Japan (NAOJ). The dataset is divided into eight schemes of two to nine slices for training, showing that the five-slice LSTM model with root mean square error of 11.38 is the optimal model. According to the prediction, SC 25 will be about 21$\%$ stronger than SC 24, with a peak of 135.2 occurring in 2024 April. SC 26 will be similar to SC 25 and reach its peak of 135.0 in 2035 January. Our analysis results indicate that the sunspot data from NAOJ is highly credible and comparable.

Driving Mechanisms in Low-Order Modeling of Longitudinal Combustion Instability

Several test cases in the literature have shown that both transverse and longitudinal high-frequency combustion instability can be driven by the injector dynamics. In these cases, pressure oscillations result in fluctuations in propellant mass flow rate, which yields pulsing heat release. This fundamental mechanism is the focus of the present work, with the aim of including this effect in a quasi-1D nonlinear model of Euler equations suited to studies of longitudinal combustion instability. In particular, the injection dynamics is represented through a simplified formulation, which is the core of the proposed response function. The analysis also addresses the influence of combustion efficiency on the main characteristics of the resulting limit cycle (frequency and amplitude). The obtained model is tested comparing the quasi-1D simulations against the experimental data of the continuously variable resonance combustor available in the literature, considering three different geometrical configurations, with different lengths of the oxidizer post. The proposed formulation is capable of reasonably reproducing the unstable behavior, as well as providing a simple model that explains the mechanism that leads to a low average combustion efficiency during unstable operation.

Effect of strain amplitude and confining pressure on the velocity and attenuation of <i>P</i> and <i>S</i> waves in dry and water-saturated sandstone: an experimental study

In rock physics, much attention has been paid to the study of the processes of strain of natural materials at small strains. Experiments using high-precision measurements have allowed new knowledge at micro/nano level to be acquired. The microplasticity of solids is studied in materials science, but there is also data regarding some rocks. The property of microplasticity of natural materials is still little studied. The study was carried out on rock samples. The effect of strain amplitude and confining pressure on the velocity and attenuation of P and S waves in dry and water-saturated sandstone has been studied. The method of reflected waves was used in the frequency range of 0.5–1.4 MHz at four strain amplitudes (0.5–1.67)·10−6 Amplitude cycling causes an open and closed hysteresis effect for wave velocity and attenuation. This has been observed for both dry and water-saturated sandstone. The hysteresis loop overlaps in both states. The amplitude changes in the velocity of P-wave in dry sandstone is 1.12 %, and the attenuation of P-wave in dry sandstone is 5.43 %. As for S-wave, its maximum attenuation in dry sandstone reaches 8.81 %. The behavior of a wave velocity and attenuation can be explained by the combined effect of viscoelasticity and microplasticity. Elastoplastic transition strongly depends on the details of the microstructure, its defectiveness, and other parameters. The characteristics of the complications of wave parameters can be the signs of the internal structure of the subject.

Research on Panel Flutter Considering the Effect of Convective Active Cooling

The aeroelastic characteristics of the panel under the action of coolant are obviously different from the flutter characteristics of the traditional panel. In order to solve this problem, the dynamics model of the panel flutter was established in this paper based on von Karman’s large deformation theory and the Kirchhoff–Love hypothesis. The panel dynamics equations were discretized into constant differential equations with finite degrees of freedom by Galerkin’s method, and solved by the fourth Runge–Kutta method in the time domain. The nonlinear modified piston theory was used to predict the unsteady aerodynamic loads, and the accuracy of the flutter analysis model was verified. On this basis, the effects of the head-panel pressure of coolant, the pressure drop ratio, the coolant injection direction, and the inertial resistance and viscous resistance on panel stability and flight stability were investigated, respectively. The results showed that reducing the pressure drop ratio, and reducing or increasing the head-panel pressure (valuing away from the freestream pressure) can improve the critical dynamic pressure when bifurcation occurs. At M∞=5.0, the pressure drop ratio causes a 22.1% increment in the critical dynamic pressure. The influence of the coolant injection direction on the panel bifurcation is mainly influenced by the head-panel pressure. The inertial resistance slows down the convergence process of the panel response, increases the limit cycle amplitude, and reduces the critical dynamic pressure of the panel, while the viscous resistance plays the opposite role. Based on these conclusions, this paper finally proposes the suppression method of panel fluttering from head-panel pressure, inertial resistance, viscous resistance, etc.

Engineering Method for Calculating the Limiting Amplitudes of Stress Cycles for Structural Steels

In large-sized metal structures, various stress concentrators are often present, which affects the operation of the material. These are intermittent bonds, holes, welded joints, material defects, etc. As a result of overloads under the action of an external cyclic load on structures in the area of stress raisers, the cycle asymmetry, the level of maximum stresses and deformations increase. In this case, the determination of the limit values of the stress cycles can be performed using a diagram of the limit stress amplitudes. The paper presents an engineering method for calculating the limiting stress amplitudes and constructing Hay’s diagrams. It is based on the use of mathematical models of classical linear and structural-mechanical fracture mechanics. Analytically and by calculation, the validity of the method is shown, which consists in determining the endurance limits and limiting stress amplitudes under high-cycle loading in a wide range of variation of the cycle asymmetry coefficient for ferrite-pearlite steels with a yield strength of up to 400 MPa. Thus, a generalized calculation method has been developed for determining the endurance limits for high values of cycle asymmetry and cycle stresses. The error of the method is estimated in the area of low-cycle load. The influence of constant and average load in a cycle on the endurance limit has been investigated both for high-cycle and low-cycle loading. The proposed approach allows to construct Smith and Hay’s fatigue diagrams for the tensile region, taking into account the structural characteristics of the material and the error allowed for engineering calculations.

Solar cycle 25 amplitude prediction based on sunspot number increase rate

We propose a prediction of the amplitude of the 25th cycle of solar activity based on the analysis of data on 24 previous solar cycles, which relate to the statistical relationship between the rate of increase in the number of sunspots in the phase of the growth curve and the amplitude of the cycle. It turned out that the forecasting result depends on several initial assumptions, in particular, which section of the growth curve is taken into account for forecasting, as well as whether all 24 cycles are considered, or only the odd ones. The assumption of a monotonic or non-monotonic growth curve also plays an important role. Regarding the latter, the cluster analysis showed that until the 35th month of the cycle, there are no visible signs of non-monotonicity of the growth curve, similar to the one that was in the 24th cycle. We conclude that the maximum smoothed number of sunspots in the 25th cycle Wmax (25), most likely, should be equal to 185 ± 18 units in the new system, which corresponds to the average power of the solar cycle, with the implementation of the Hnievyshev-Ohl rule. With such parameters of this cycle, there are no signs of approaching the deep minimum of the age cycle in the middle 21st century. This does not exclude the fact that this deep age minimum can occur suddenly and sharply immediately after the 25th cycle, as was the case, for example, in the Dalton minimum. A possible sign of the upcoming age cycle may be the abnormally long decline phase of the current 25th cycle, about 10 years, as it was in the 4th cycle.

Experimental behavior of circular concrete-filled steel tabular columns under near-fault cyclic loadings

This study investigated the behavior of concrete-filled steel tabular (CFST) columns under near-fault cyclic loading experimentally. Ten CFST columns were tested: one under monotonic loading, one under conventional cyclic loading, and other eight under near-fault cyclic loadings. The results indicated that slow local buckling occurred at plastic hinges after 4%-drift cycles, resulted in slow strength degradation and pinching. These became more pronounced as the cycle amplitude increased. Load-carrying and deformation capacities marginally decreased by the effect of cyclic loading. Secant stiffness sharply dropped by the pulses of 4%-drift amplitude. The earlier arrival of the pulses caused higher drops of secant stiffness. Near-fault pulses imposed a considerable large amount of energy to columns, which resulted in slow local buckling, slow strength degradation, but significant stiffness degradation. Shifting the pulse cycles to the cycle five of a conventional cyclic loading history was proposed for testing structural components under near-fault earthquakes.

Cyclic failure characteristics of silty sands with the presence of initial shear stress

Liquefaction of silty sands remains an outstanding issue since it continues to lead to catastrophic consequences in recent earthquake events. The cyclic characteristics, such as cyclic failure pattern and cyclic resistance, are the fundamental aspects of liquefaction analysis. The difficulty and uncertainty of characterization and evaluation of cyclic behavior and liquefaction resistance of silty sand mainly come from the complicated interactions of various influencing factors, such as packing density, confining pressure, initial shear stress, cyclic loading amplitude, soil properties, and soil fabric. This study presents a series of laboratory testing results to identify the cyclic failure patterns of silty sands considering different soil states, fines contents, initial static shear stress, etc. It is found that the failure patterns are related to the states of soils and the cyclic loading characteristics, i.e., the combination of initial shear stress and cyclic loading amplitude for silty sands with similar soil fabric, and that the cyclic resistance of silty sands is also a function of soil states and initial static shear stress for similar soil fabric. Critical state soil mechanics is implemented to characterize the cyclic failure patterns and cyclic resistance. Implications of the present study to the existing liquefaction assessment methods are also discussed.

Dynamics of coupled thermoacoustic modes: Noise and frequency spacing effects

Practical combustion systems consist of multiple thermoacoustic modes that are linearly stable or unstable at a wide range of frequencies. When coupled, the modal interactions are a strong function of the frequency spacing between them – having very significant implications on interpretation of linear stability or instability of modes from experimental data. Frequency spacing between the modes particularly influences the stability and existence of potential limit cycle solutions. In this paper, we extend earlier deterministic studies to include the effects of noise, studying how deterministic dynamics change with the addition of noise and the simultaneous impact of frequency spacing (i.e., closely, or widely spaced). Specifically, this paper characterizes three key effects of noise, which are (1) distribution of limit cycle amplitudes (“diffusion”), (2) shift in the average limit cycle amplitudes (“noise induced drift”), and (3) introduction of new bifurcations that are not present in the deterministic system. Key conclusions from this work are that in highly noisy practical environments, a deterministic/ low noise analysis will not be sufficient to explain even critical qualitative features of the system – such as existence of stable limit cycles or their amplitudes. Moreover, the nature of noise-induced effects on nonlinear modal interactions can be fundamentally different for degenerate, closely spaced, and widely spaced modes.

Dynamic analysis of a soft capsule robot self-propelling in the small intestine via finite element method

To reduce potential trauma to the intestine caused by the rigid shell while also optimising its progression efficiency, an elastomer coating was applied to a self-propelled capsule robot for small-bowel endoscopy. The robot is self-propelled by its periodically excited inner mass interacting with the main body of the capsule in the presence of intestinal resistance. This work explored the dynamic responses of the capsule with different elastomer coatings (i.e., different elastic moduli and thicknesses) in the lumen of the small intestine through a three-dimensional finite element analysis. The driving parameters of the robot, including the amplitude, frequency and duty cycle of a square-wave excitation, were further tested to reveal the dynamics of this soft robot. By analysing numerical results, the proposed finite element model can provide quantitative predictions on the contact pressure, resistance force and robot-intestine dynamics under different elastomer coatings. It was found that the softer the elastomer coating is, the lesser the contact pressure between the robot and the intestine, thus implying lesser trauma. The findings of this work can provide design guidelines and an evaluation means for robotic engineers who are developing soft medical robots for bowel examinations as well as clinical practitioners working on capsule endoscopy.

Investigation of CFRP-Countersunk Bolted Assembly Fatigue Damage under Three-Point Bending via Experimental and Numerical Analysis

In this research, the fatigue damage behavior under three-point bending of a composite joint incorporating a single countersunk fastener is investigated. Firstly, a self-developed fatigue test system was set up to test the fatigue characteristics of CFRP-countersunk bolted assembly under the displacement amplitude cycles of 103 to 106 to study the formation and expansion rule of damage and cracks. It found two typical damage processes, both of which involve some formal interface damage between fiber and matrix. Based on the experiment, a finite element fatigue damage analysis on this assembly was carried out according to the Hashin failure criterion. The simulation result shows an identical fatigue damage location and fatigue life with the experimental phenomenon. Moreover, it predicted the final fatigue life of the specimen under 10 hz cyclic loading with 1 mm displacement and 10 Nm bolt preloading. This research provides guidance for the engineering fatigue issues of single-bolted joint composite connection structures and provides a reference for the corresponding technical specifications formulation.

Variability of Atmospheric CO2 Over the Arctic Ocean: Insights From the O‐Buoy Chemical Observing Network

AbstractAs the Arctic climate rapidly warms, there is a critical need for understanding variability and change in the Arctic carbon cycle, but sparse spatial coverage of observations has hindered progress. This work analyzes measurements of atmospheric CO2 in the Arctic from long‐term on‐ice measurements (the O‐Buoy Network), as well as coastal observatories from 2009 to 2016. The on‐ice measurements showed smaller seasonal amplitudes than coastal observatories, in contrast to the general observation of poleward increases of seasonal cycle amplitude. Average on‐ice measurements were also lower than their coastal counterparts during winter and spring, contradicting the expectation that CO2 increases poleward in boreal winter. We compared the observations to CO2 simulated in an updated version of GEOS‐Chem 3‐D chemical transport model, which includes new tracers of airmass history and CO2 sources and sinks. The model reproduced the observed features of the seasonal cycle and showed that terrestrial biosphere fluxes and synoptic transport explain most CO2 variability (both synoptic and interannual) over the Arctic Ocean surface. The polar airmass partially isolates the Arctic Ocean surface air from terrestrial CO2 exchange, which explains the reduced seasonal cycle amplitude and winter maxima. All Arctic coastal sites had similar CO2 interannual variability, particularly in summer, which was largely reproduced by the model. The interannual variability observed over sea ice, however, was distinct from the coastal sites and not reproduced by the model. Air‐sea CO2 exchange in and around sea ice, which was once thought to be negligible, may be an important driver of interannual variability over the Arctic Ocean.

Neural ordinary differential equation for sequential optimal design of fatigue test under accelerated life test analysis

The literature on fatigue analysis can be classified into parametric or analytic approaches that try to model the fatigue data with a specific distribution, such as the optimal sequential Accelerated Life Test (ALT), considering the fatigue life cycle and stress amplitude. To the best of our knowledge, no work incorporates the accelerated lifecycle distribution of fatigue data into the parametric approach, nor does it incorporate the complex relationships in the elastic–plastic behavior of the material in the ALT models using the sequential optimal test design. In this work, we define an optimization model that takes both parametric and analytic solutions into account and minimizes the negative log-likelihood of the ALT model when the fatigue lifecycle follows a lognormal distribution for several constraints extracted from physical attributes of elastic–plastic models and assumptions of the ALT approach, where model-based optimization is sequentially updated. We utilized a model-based deep neural network solution based on the differential equation of the analytical model to solve the optimization problem. Our simulation results based on empirical data prove the practical implementation of our approach and validate the result for obtaining the optimal test plan and estimated parameters of the ALT and elastic–plastic model.

Autonomic Nervous System Adaptation and Circadian Rhythm Disturbances of the Cardiovascular System in a Ground-Based Murine Model of Spaceflight.

Whether in real or simulated microgravity, Humans or animals, the kinetics of cardiovascular adaptation and its regulation by the autonomic nervous system (ANS) remain controversial. In this study, we used hindlimb unloading (HU) in 10 conscious mice. Blood pressure (BP), heart rate (HR), temperature, and locomotor activity were continuously monitored with radio-telemetry, during 3 days of control, 5 days of HU, and 2 days of recovery. Six additional mice were used to assess core temperature. ANS activity was indirectly determined by analyzing both heart rate variability (HRV) and baroreflex sensitivity (BRS). Our study showed that HU induced an initial bradycardia, accompanied by an increase in vagal activity markers of HRV and BRS, together with a decrease in water intake, indicating the early adaptation to fluid redistribution. During HU, BRS was reduced; temperature and BP circadian rhythms were altered, showing a loss in day/night differences, a decrease in cycle amplitude, a drop in core body temperature, and an increase in day BP suggestive of a rise in sympathetic activity. Reloading induced resting tachycardia and a decrease in BP, vagal activity, and BRS. In addition to cardiovascular deconditioning, HU induces disruption in day/night rhythmicity of locomotor activity, temperature, and BP.