Narrow Results

The traditional dynamic analysis method is less efficient in the global and local vibrations analysis of long-span bridge structures. Therefore, to meet the need for an efficient solution of the refined analysis model of the train–track–bridge coupled system (TTBCS), a new hybrid dynamic model of the TTBCS based on the transfer matrix method (TMM) is proposed in this paper. It can solve the local high-frequency vibration response of the track structure and the global and local vibrations responses of the bridge structure simultaneously, accurately, and efficiently. First, according to the periodic characteristics of track and bridge structure, the periodic repeating parts are divided into cellular structures. For the bridge subsystem, fine cells are established to achieve the accurate solution of local vibration, and the rest of the super element cells are established by model condensation technology, which can significantly reduce the number of cells and save the transmission time. The state vector transfer model of the track and bridge subsystem is established based on the TMM, and the coupling calculation is realized by combining rail bridge force. The train system adopts the model of 10degrees of freedom and realizes the coupling with the rail system through the wheel–rail interaction force. With the movement of train load, the track and bridge cells established by the hybrid dynamic model approach (HMA) dynamically update the arrangement information, which not only realizes the calculation of ultra-long track based on a fixed number of track cells, but also moves the fine cell models of bridge with the change of load position. These dynamic update measures reduce both the number of cells and transfer solutions, save the transfer time, and further improve the calculation efficiency. Taking CRH-2 EMU passing through a 3-span simply supported steel truss bridge as an example, the results and time-consuming of the direct stiffness method, TMM, and HMA are compared, and the accuracy and efficiency of the hybrid model are proved.

As a kind of special terrain, the landslide disaster generated by liquefiable layer slopes under earthquake has become a major engineering challenge due to its large scale and long slip distance. In order to study the seismic response and damage mode of liquefiable layer slope, this paper follows the research line of “geological generalization, physical modeling, and result analysis”, and takes the liquefiable layer slope in the upper reaches of the Yellow River Class secondary terrace as the object of study, generalizes the physical model of the slope, and carries out shaking table test. Based on the PGA amplification coefficient, Fourier analysis and HHT time–frequency characterization of the model slope, it was found that the PGA amplification coefficient increases gradually along the slope height, reaches the maximum value at the top of the liquefiable layer and then decreases gradually, which indicates that the liquefiable sand layer has an obvious energy dissipation effect, and on the horizontal direction of model slope is a tendency to the surface effect; the seismic waves at the discontinuous interface change drastically, and the Hilbert time–frequency spectrum transforms from multiple peaks to a single peak; with the increase of the intensity, the intrinsic frequency of the overall model decreases, and the high-frequency component within the liquefiable sand layer decreases from 5–15Hz to 0–5Hz, indicating that the liquefiable layer has a filtering effect; the damage process of the liquefiable layer slope is the tensile crack at the top of the slope — seismic subsidence at the top of the slope — the shear yielding at the angle of the slope — shear surface penetration at the slope face — overall slope instability and flow-slip damage. The research results will provide a reference for the study of the disaster mechanism of the liquefiable layer slope.

Theoretical studies on the vibration of microcantilever beams in fluids, which are commonly used in micro- and nanoelectromechanical systems (MEMS/NEMS). When microcantilever beams are subjected to photo-thermal excitation, they show that the material properties such as the dynamic response and the one-dimensional temperature field will show significant differences from the macroscopic properties when the size appearance of the microbeam decreases to the scale below a dozen micrometers. In this paper, by correcting the scale constants of the beams, the photothermal vibration model of the microbeam is established using the physical neutral plane theory. The one-dimensional heat transfer equations and scale-corrected temperature field of the microcantilever beam under laser excitation are derived, which are solved by Galerkin’s method, based on the theory of thermoelasticity, the hydrodynamic model of the beams vibrating in incompressible liquids proposed by Sader et al. and the theory of Euler–Bernoulli beams. The equations governing the vibration of micro-cantilever beams corrected for scale effects in different fluids when subjected to photothermal excitation are obtained. The results show that the temperature field, resonant frequency, and quality factor of the microbeam will have a significant upward drift when the size of the microbeam is close to the scale parameter, and the scale effect has a non-negligible influence on the macroscopic performance parameters; the upward drift is gradually weakening when the thickness to scale ratio gradually increases. Finally, the property of the beam is almost the same as that of the theory when the thickness of the beam is 10 times the scale constant. The correction of the theory by the scale effect is insignificant.

The safety of the high-speed train traveling through the stationary thunderstorm downburst wind was studied. First, a thunderstorm wind test device was used to simulate the stationary thunderstorm downburst wind. Based on the rigid model pressure measurement tests, the aerodynamic forces of the train traveling along different paths through the stationary thunderstorm downburst wind were measured. The influence of the radial distance of the crossing path on the aerodynamic force coefficients of the train was investigated. On this basis, an unsteady aerodynamic model of the high-speed train crossing through the stationary thunderstorm downburst wind was established, and dynamic response analysis was carried out using the SIMPACK multibody dynamics simulation software to explore further the safety of the high-speed train crossing through the stationary thunderstorm downburst wind. The research showed that the stationary thunderstorm downburst wind field has significant spatial variation characteristics compared with the atmospheric boundary layer wind field. When the train passes through the thunderstorm downburst wind, the radial wind speed and wind yaw angle experienced by the train constantly change, and the change curve shows a symmetrical distribution. The aerodynamic force of the train will undergo sudden loading and unloading processes, and the lateral force coefficient of the train on different paths shows a “pulse-type” variation. Moreover, the lateral force coefficient increases with the increase of wind yaw angle. Under the influence of the thunderstorm downburst wind, the variation trend of the aerodynamic force coefficients of the train is consistent with that under crosswind. However, there are significant differences in the numerical values. Therefore, it is impossible to simply use the formula for calculating the aerodynamic force coefficients of the train under crosswinds to predict the aerodynamic force coefficients of the train under the thunderstorm downburst wind. While passing through the thunderstorm downburst wind, the overturning coefficient index plays a decisive role in the safety of train operations. Train rollover is the main form of train safety accidents, while derailment accidents are not easy. The numerical results obtained in this study are significant for evaluating the operational safety while moving trains traversing the stationary thunderstorm downburst wind.

In this paper, the dynamics of an axially translating functionally graded cantilever beam (FGCB) with time-varying length are studied under environmental temperature variations. Firstly, the governing partial differential equation for the FGCB under different temperatures is established based on the Euler–Bernoulli beam theory. Secondly, the Galerkin discretization and the assumed mode method (AMM) are employed to derive the vibration equations for each mode. Then, the coupling effects of the axial translation motion and the bending deformation on the vibration response of the FGCB are analyzed through numerical simulation. The results showed that different initial lengths, velocities and accelerations can influence the dynamic response greatly. Moreover, the increase in temperature will increase the response amplitude and decrease the frequency of FGCB, but the increase in functionally gradient parameter will decrease both the amplitude and the frequency of the FGCB. Finally, the wavelet transform (WT) is utilized to perform a time–frequency analysis. It is found that the time-dependent frequency obtained by using WT is consistent with the first-order static frequency obtained theoretically. The variation of frequency with time can be obtained quickly and accurately by using WT. The results of this research are of great significance for the application of composite axially translating beams in practical engineering.

The main goal of this research is to analyze the natural frequencies and dynamic response, and to utilize nonlinear control techniques for vibration control of a smart nanocomposite sector plate, while taking into consideration the effects of agglomeration and internal pores. The proposed composite configuration includes a porous core layer reinforced with agglomerated GPLs, as well as two layers of piezoelectric sensors and actuators. Both complete and partial agglomeration states are considered based on the Eshelby–Mori–Tanaka approach to predict the effective properties of the nanocomposite. The mechanical properties of the porous core are characterized using an open-cell metal foam with interconnected pores, notable for their low density and high surface area. The governing equations of motion are derived using Hamilton’s principle, which is based on the First-order Shear Deformation Theory (FSDT) plate theory and the finite element method. The nonlinear fuzzy PID controller is designed as a combination of a fuzzy PI component and a nonlinear PD component. The gains of the PD controller are dynamically adjusted using nonlinear gains to optimize its performance. In addition, a comprehensive analysis has been conducted to examine the effects of geometric dimensions, distribution of reinforcement, weight fractions of nanofillers, parameters of agglomeration, porosity coefficient, porosity pattern, and boundary conditions on the natural frequencies and dynamic response of smart porous nanocomposites. Numerical simulations demonstrate the efficacy of the proposed controller in significantly reducing vibration amplitudes compared to velocity feedback. The velocity feedback controller decreases deflection from 24.39$\mu \mathrm{\text{m}}$ to 8.37$\mu \mathrm{\text{m}}$, whereas the proposed controller achieves 3.66$\mu$m, representing a 56.27% improvement in performance.

A scaled model of a concrete gravity dam was designed, and nine tests were conducted to investigate the dynamic response characteristics of the dam subjected to underwater explosions. The pressure time histories in the water, and the velocity and acceleration time histories of the dam during various explosions were recorded. The acceleration signal was processed by the Fast Fourier Transform (FFT) to perform time–frequency conversion. The energy migration law of the acceleration signal along the dam elevation was explored based on the theory of wavelet packets. Furthermore, the effects of charge weight, explosion distance, and explosion depth on the dynamic response of the dam were comprehensively investigated. The results show that the distribution of peak acceleration along the dam elevation closely relates to the explosion distance, but the global response of the dam tends to weaken as the explosion distance increases. Increasing the charge weight causes an enhancement in the dam response. Compared with an explosion suspended in water, the bottom explosion causes a weaker response to the dam. With the increase of the dam elevation, the energy of the acceleration signal gradually shifts from low frequency to high frequency. The bubble curtain has a significant mitigation effect against underwater explosions.

Orifices designed into an arch dam for flood discharge significantly reduce the ability of the dam to resist an explosion, especially when an explosive charge is detonated close to an orifice, and the dam may suffer considerable damage in consequence. We created a fully coupled model to represent the dynamic interactions between reservoir water, explosive, arch dam and abutments. Shock wave propagation and damage initiation and propagation were compared between arch dams with and without orifices. The effects of orifices on damage modes and the spatial distribution of damage were investigated. Damage characteristics of the top and middle orifices subjected to explosive loads were also analyzed. Results showed that the top and middle orifices designed into an arch dam have an important effect on the impact resistance of arch dams, especially when an explosive is detonated at an orifice inlet. At a certain mass of explosive, complete failure of the dam wall at the adjacent sluice piers of the top orifices can occur, and penetration failure may also occur at the middle orifice.

Steel–concrete hybrid towers (SCHTs) have been adopted as a popular supporting structure for wind turbines with the increasing hub height and rotor diameter. The excessive transverse vibration of the supporting tower adversely compromises the turbine’s operation and leads to structural deteriorations. The tuned mass damper (TMD) has been validated as an efficient device integrated with high-rise structures in vibration suppressions. This study aims to provide a reliable method for determining the key parameters of the TMD to obtain the optimal performance in vibration suppressions. This study begins with a rigorous theoretical analysis to investigate the dynamic responses of the SCHT. The dynamic analysis serves as a basis for developing a dynamics model of the SCHT integrated with a TMD, which provides a programmable method to analyze the effect of the TMD on the dynamic responses of the SCHT. Then, an optimization model based on the genetic algorithm (GA) is established to obtain the optimal parameters of the TMD. Finally, a numerical test comprising 12 loading conditions is conducted to analyze the vibration mitigation effect of the TMD. The validation process also provides discussions about the influence of the wind turbine operation states and the fitness function adopted in the GA on the efficiency of the TMD. Those findings demonstrate the effectiveness and limitations of the TMD for mitigating vibrations of the SCHT. It thereby contributes to the understanding and improvement of vibration control strategies for SCHT structures and can guide future design and optimization efforts.

This paper investigates the dynamic response of steel box structures with corner connection under internal explosion using a validated numerical model. The main focus of present study is to explore the design strategies of corner connection. Several attempts have been made by comparing with the one without core connection, examining the effect of layout position, and analyzing the strain uniformity. The results show that the corner connection could significantly improve the blast performance in terms of shock wave convergence and permanent deformation. The design of layout position is critical to ensure the beneficial effect of corner connection. The layout in quadrant I performs best in reducing the bulging deformation of box structure. However, quadrants III and IV would even deteriorate the structural deformation. Strain uniformity analysis confirms the potential matching correlation between the wall surface and corner connection for avoiding vulnerability to suffer fracture at connection region. Finally, a modified dimensionless number is proposed to consider the effect of core connection on the deformation response of box structure. Exponential fitting function with high correlation coefficient is achieved for the relationship between the deflection–thickness ratio and modified dimensionless number.

The analysis of the dynamic response of fluid-saturated porous media under stress waves is of great practical value in the fields of geotechnical engineering, geophysics, earthquake engineering and marine engineering. In this research, a new fully explicit finite element algorithm for solving the coupled soil-pore fluid dynamic problem is developed based on the u–p formulation. The proposed method is a staggered algorithm that is realized with the central difference method used to solve the displacement u and the improved Euler method used to solve the fluid pressure p, it has a second-order theoretical accuracy since both the central difference method and the improved Euler method are second-order methods. The correctness and efficiency of the proposed method are investigated through two numerical examples. The computational results of the one-dimensional soil column model shows that the numerical solution of the proposed algorithm agree well with the theoretical analytical solution. The differences of the computational accuracy and efficiency between the proposed method and the other existing methods are also discussed in the numerical examples, and the results show that the proposed explicit method is higher in calculation efficiency than the implicit algorithm, and its accuracy is higher than that of the existing first-order explicit method.

To study the dynamic response of curved polymer sandwich panels under contact explosion loads, three deformation stages of curved polymer sandwich panels were analyzed. A single degree of freedom rigid plastic dynamic model was established. The bending moment and membrane force effects of the panel and sandwich on deformation were considered in the model. The dynamic response of curved polymer sandwich panels with different panel and sandwich thicknesses under contact explosion loads was calculated through numerical simulation. The mid-span displacement of the panel calculated based on the dynamic model is in good agreement with the numerical calculation results. On this basis, the deformation and velocity changes of the panels were further analyzed, and the force and motion of the panels and sandwich at each stage in the dynamic model were verified. Research has shown that increasing the thickness of panels and sandwich panels can enhance the blast resistance of curved polymer sandwich panels, and increasing the front thickness has a more significant effect on improving the blast resistance of sandwich panels, which can provide a reference for future structural design and engineering applications.

This paper examines the free and forced vibration characteristics of metal foam nanocomposite shallow arches with two Titanium alloy layers under a moving load. The middle core is made of a six-layer porous aluminum reinforced with graphene platelets in which different dispersion distributions are taken into account. The basic governing equations in the sandwich arch under moving load are established based on the first-order shear deformation model, Halpin–Tsai modified rule and Hamilton’s principle. The equations of motion are extracted for the sandwich arch with simply-supported boundaries by applying the Fourier expansions. An eigenvalue solution is applied in the free vibration problem and the Newmark time marching scheme is used for the forced vibration problem. The computed results from this investigation are compared with those reported data in the literature. Then, novel numerical examples are presented in detail to show the influences of important effective parameters on the free and forced vibrations of shallow sandwich arches.

The use of permeable piles as an effective drainage method in liquefiable sites has become widely accepted. In this study, the seismic response of both the liquefiable soil and the pile was simulated using FLAC3D software to validate the anti-liquefaction performance of the permeable pile. A group of permeable piles designed according to the China foundation code were numerically modeled with various opening ratios (i.e. area of openings/total surficial area). The numerical results showed that the permeable pile is able to enhance liquefaction resistance by dissipating excess pore water through the drainage holes. The bending moments and axial force of the permeable pile decrease but the ultimate bearing capacity increases in the process of drainage. It is found that the excess pore water pressure ratio (EPWPR) of soil around permeable pile under seismic loading reduces rapidly with increasing opening ratio, but the excess pore water pressure tends to keep nearly a stable level once the opening ratio is beyond a critical value of 0.5%. As a result, the critical value of the opening ratio may be considered as the optimum parameter to design the permeable pile against liquefaction.

The prosthetic foot is an essential component of transtibial prosthesis whose mechanical properties affect the balance and gait function. The objective of this study is to examine and compare the biomechanical factors of prosthetic gait and balance for low-cost Ranger foot and somewhat comparatively expensive dynamic response (DR) foot most commonly used in developing countries. Thirty-six subjects with unilateral traumatic transtibial amputation were analyzed for balance and gait attributes using the HUMAC^Ⓡ Balance and Tilt system and 3D motion analysis system with force platform and high-precision optoelectronic cameras, respectively. The paired $t$-test was used to compare the means for static and dynamic balance variables, stride characteristics, and physiological cost index with the $p$-value set as 0.05 for statistical significance. The results revealed that the prosthetic foot design affects the overall balance performance and gait attributes including the physiological energy cost. The DR foot performed better than the Ranger foot in terms of stride characteristics and energy consumption with a significant difference ($p<0.05$) except for the mean walking velocity ($p=0.055$). However, with no articulation at the ankle joint, the Ranger foot improved balance scores with increased stability score ($p=0.0003$), minimizing the excursion of CoP and reducing path length and average velocity. Therefore, the use of Ranger and low-profile DR multiflex feet should be encouraged in the transitory and definitive transtibial prosthetic rehabilitations, respectively, considering the dual advantages of balance and gait attributes.

It is very important to know how the reservoir rock and its fluid properties are linked to seismic dynamic response. Literatures show that there are a variety of rock-physics models such as the most famous Biot-Gassmann equation aimed at the relationship between seismic velocity and liquid saturation. Most of these models make a fundamental assumption of one fluid phase or homogeneous phase within the pore volume. In this paper, we discuss possible seismic velocities change in a two immiscible pore fluids (i.e. water-gas) saturated reservoir with patchy saturation distribution. It is found that P-wave velocity of a reservoir rock with the same saturation but different pore fluid distribution exhibits noticeable variation and deviate overall from Gassmann's results. We use DEM theory to explain this phenomenon. It belongs to hybrid approach in rock-physics modeling and can handle complex pore-fluid-distribution cases. Based on the modeling study, we found that various fluid-distribution models may significantly affect the modulus and P-wave velocity. The seismic reflection time, amplitude and phase characteristics may change with the choice of pore-fluid-distribution models. Relevant rock mechanical experiments indicate the same trend of seismic responses. It also be proven by seismic reservoir monitoring experiment (time lapse study) that incorrect conclusion may be drawn about the strong seismic reflection in pure Utsira Sand if the microscopic pore-fluid-distribution effects are not taken into account.

The frequency dependency property of viscoelastic material leads to the dynamic analysis of compound structures which are complex and costly. Furthermore, using commercial finite element software, it is difficult to carry out the dynamic response analysis with this characteristic. Based on finite element iterative and modal strain energy (MSE) method, a mode superposition algorithm was proposed to solve the dynamic response of viscoelastic damping structure in this paper. Through iterative and MSE method, the modal frequency and loss factor for each mode can be obtained. Before calculating the next order modal frequency, the modal mode and corresponding load are extracted and the response of node was calculated at first in this algorithm. As a consequence, the node displacement response can be solved by summarizing the response results of each independent mode in the required frequency range. Numerical calculation of U shaped pipe with viscoelastic damping layer illustrates that the method is simple and practical. Moreover, the simulations with this method agree quite well with the experimental derived results. In the meantime, the damping layer parameters study shows that the position of the damping layer has an obvious effect of reducing the vibration response of U shaped pipe, but the width of the damping layer has little influence. The procedure proposed in this paper can be extended to analyze other more complex structures with viscoelastic material.

Foil-air bearings have presented their advantageous performance due to their different structures when compared to traditional air bearings. However, it is the nonlinear characteristic of this kind of bearing that has drawn studies on dynamic response of the rotor-bearing system, especially rotor stability. In this paper, an improved foil dynamic model with internal bending moment included has been proposed to determine the nominal stiffness of the foil structure. Based on that, the nominal stiffness of the foil structure has been investigated with different geometry parameters of the foil structure. By such means, the stability of the rotor-bearings system has been theoretically studied through an equation system in a common turbocharger structure. The results can be effectively used for designing and suitably selecting some geometry parameters of foil-air bearings to have a good rotor performance in this case.

The published work on analytical ("mathematical") and computer-aided, primarily finite-element-analysis (FEA) based, predictive modeling of the dynamic response of electronic systems to shocks and vibrations is reviewed. While understanding the physics of and the ability to predict the response of an electronic structure to dynamic loading has been always of significant importance in military, avionic, aeronautic, automotive and maritime electronics, during the last decade this problem has become especially important also in commercial, and, particularly, in portable electronics in connection with accelerated testing of various surface mount technology (SMT) systems on the board level. The emphasis of the review is on the nonlinear shock-excited vibrations of flexible printed circuit boards (PCBs) experiencing shock loading applied to their support contours during drop tests. At the end of the review we provide, as a suitable and useful illustration, the exact solution to a highly nonlinear problem of the dynamic response of a "flexible-and-heavy" PCB to an impact load applied to its support contour during drop testing.

In active magnetic bearing (AMB) system, the catcher bearings (CB) are indispensable to temporarily support the rotor from directly impacting the stators. In most cases, traditional CB cannot bear the ultra-high speed, vibrations and impacts after a rotor drop event. To address the shortcomings, a double-decker ball bearing (DDBB) with inner two face-to-face angular contact ball bearings are proposed to be used as CB in an AMB system, and the dynamic response of the rotor after a rotor drop event is experimentally analyzed. The results indicate that using a DDBB as a CB helps to reduce the following collision forces after a rotor drop. Larger ball initial contact angles and smaller pre-load force on the inner layer bearings, larger radial clearance of the outer layer bearing and choosing AISI 10AISI 1045 steel which has a larger density for the adapter ring can effectively reduce the maximum impact force after a rotor drop event.