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•Theoretical and experimental research on non-Gaussianity and non-stationarity excitation.•High-cycle fatigue of flexible structures excited close to natural ...frequencies.•Non-stationarity excitation significantly impacts the fatigue life.•Non-stationary vs non-Gaussian excitation were found much more important.
In vibration fatigue, flexible structures operate at or close to their natural frequencies. Therefore, it is common to consider the input excitation as well as the stress/strain response of the structure to be Gaussian and stationary. In reality, a non-Gaussian and non-stationary excitation is frequently observed, resulting in a possibly non-Gaussian and non-stationary response. The importance of this non-Gaussianity (typically observed via the kurtosis) has resulted in significant research on the relevance of the Gaussian assumption in fatigue life. For dynamic structures the prior research was mainly theoretically and numerically focused. This work researches the importance of non-Gaussianity and non-stationarity theoretically, numerically and experimentally. Y-shaped specimens were used in this research. The excitation close to the natural frequency is random and in all the researched cases with the same power spectral density (PSD). While the PSD was kept the same, the rate of non-Gaussianity and the non-stationarity were changed. The results show that when the excitation is stationary and non-Gaussian, the fatigue life is not significantly impacted, and that standard frequency-counting methods are applicable. However, for the case of a non-stationary, non-Gaussian excitation, the fatigue life was found to be significantly impacted and the Gaussian theoretical approach is questionable.
The damping capacity is one of major concerns for structures under dynamic loads, which significantly affects the safety and service life. Ultra-high performance concrete (UHPC) is a new generation ...of cementitious material characterized by superior mechanical strength and durability, exhibiting enormous potential in application to innovative structures. However, the weak steel fiber-matrix interface within UHPC leads to insufficient damping performance and poses threats to UHPC structures under extreme dynamic loading conditions. In this paper, various interfacial modulation approaches were investigated to improve static and dynamic damping properties of UHPC, including physical shape and chemical modification of steel fibers, macrofibers and microfibers hybridization. Results show that the interfacial modulation can significantly enhance damping ratio, loss factor and energy dissipation ratio of UHPC, where the chemical modification of steel fibers endows the highest damping ratio. The loss factor and energy dissipation ratio of UHPC reach 0.579 and 0.091 after interfacial modulation, which improved by more than 110% and 100%, respectively. The working mechanisms behind variations of damping performance are attributed to the toughening in interfacial bond and load transfer, leading to improvement in energy dissipating ability of interface. Furthermore, a comprehensive comparison between UHPC and existing vibration damping materials was conducted through multi-criteria analysis from the perspectives of damping performance, mechanical strength, processability and economic benefit, and UHPC with chemical modification of steel fibers exhibits the best overall performance. The findings contribute to inspiring a novel structural vibration control strategy through UHPC with enhanced static and dynamic damping properties for resisting extreme loads.
This paper proposes a novel method for calibrating error coefficients of the quartz accelerometer faster on the dynamic centrifuge, which can generate a continuous dynamic acceleration excitation. ...Firstly, working principle and structure of the dynamic centrifuge are analyzed, the error sources and the uncertainty of the dynamic centrifuge are expounded, and relevant coordinate systems are established. Then, according to the characteristics of the input specific forces and the propagating methods of the error sources, the accurate input specific forces acted on the input reference axes of accelerometer are obtained. Based on the calibration error model of the accelerometer, the corresponding dynamic calibration method is proposed. Finally, the second-order and high-order coefficients of the acceleration error model are calibrated, and the calibration precision of the coefficients is analyzed. Compared with traditional calibration methods, the method proposed in this paper not only greatly enhances the calibration efficiency by reducing the installing positions of the accelerometer, but also separates the centrifuge errors from the outputs of the accelerometer, which improves the accuracy and efficiency of the calibration method simultaneously.
•A new building system with suspended floor slabs is proposed.•Equations of motion and analytical solutions are provided for the proposed system.•The system response is optimized for the desired ...acceleration and displacement of the slab and main frame.•A performance comparison is made against a conventional composite system.•Enhanced performance of buildings can be achieved if the proposed system is used in medium to high-rise buildings.
Achieving a sustainable built environment that is less vulnerable to seismic hazard has been placed on the forefront of research pertaining to structure performance under earthquake loading. This resulted in a recent shift in the philosophy outlining structural performance during large magnitude earthquakes from preventing collapse to providing highly resilient structures that are easier to repair after a major event. The change in performance philosophy requires the development of new and innovative structural systems that are capable of performing as desired. Research and development of such structural systems has been gaining attention in recent years and have shown significant potential. This study extends previously conducted research on suspended structural systems and floor isolation systems by introducing the analytical solution of buildings with suspended floor slabs with replaceable energy dissipation elements are numerically analyzed under dynamic excitations. The proposed systems comprise of a steel frame with floor slabs suspended from the beams using steel cables. There are two models considered in this study, first in which the slabs are suspended using fixed hooks attached to the beams, thereby coupling the motion of the slab to its immediate neighbor bottom and top floor. In the second model, the slabs are suspended from the beams using rollers such that the motion of the slab is only coupled to the floor immediately below it. Controlled energy dissipation in the structure is realized through the use of steel links that are installed between the bottom face of the suspended slab and the floor beam underneath the slab. The orientation and quantity of the steel links as well as the slab to be suspended in a typical frame is determined such that an optimal performance of the system is achieved. Suspending selected floor slabs and tuning the links for energy dissipation can significantly reduce the damage to the main structural frame and nonstructural components of the system, in addition to reduction in floor response. Moreover, post-tensioned cables are installed adjacent to the links to provide self-centering capability to the slab and reduce its residual drift. The optimal performance of the two models is evaluated for various size frames and found to out-perform a conventional frame in case of large structures. The study concludes with a few important design implications regarding the proposed system.
Seismic analysis of structure is employed to make the structure enable to resist the seismic forces and perform against the factors causing the failure of the structure under dynamic excitation. ...Among various response factors, the base shear and time period of buildings are predominant factors used in the analysis and design of the structure.The prime objective of the paper is to present an analytical study on non-linear seismic analysis of moment resisting framed buildings (as per Indian code IS1893 – 2016) to evaluate the base shear of different configurations of buildings according to different mode combination methods. The obtained results have been presented the comparative analysis of different combination methods. The paper also presents the evaluated results in the form of the time period values of the different buildings depending upon variation in its configuration. As a result, the responses of multistoried moment-resisting framed buildings have been evaluated for various models of considered buildings based on different mode combination methods, and the results of obtained responses have been analyzed in a comparative manner to understand the behaviour of buildings under various methods and configuration conditions. The work presented in the paper can support to develop better understanding of structural response and efficient designing of structures.
The complete elimination of flaws in complex parts raises the overall costs, especially under stringent safety or service targets, while advanced knowledge might grade the risk of running defected ...parts with savings.
Introducing the novel concept of defect tolerance, this work proposes to exploit experiment-based full-field measurements from optical techniques, in conjunction with fatigue spectral methods and dynamic excitation signatures, for the definition of a macro-scale defect risk index mapping, retaining the most advanced knowledge available about task-related loading, cumulated damage and inherent structural dynamics of a real object in assembling conditions. The acceptance threshold on the risk index map can discriminate among dangerous permutations of defect locations. Meaningful examples are given on an aluminium thin plate, as a lightweight structure with complex structural dynamics.
•The defect tolerance concept and task-oriented acceptance threshold are introduced.•Defected parts’ risk grading is based on optical full-field FRF testing & NDT.•The defined Risk Index retains the whole experiment-based structural dynamics.•Defect risk grading is enhanced thanks to experiment-based fatigue spectral methods.•Dynamic excitation signatures have deep impact on the defect tolerance mapping.
Under the background of dual carbon strategy and coal shortage,the existing demand response mechanism of evaluation and incentive is relatively extensive,causing challenges such as excessive ...redundancy of invitation and difficulty in mining high-quality users,so the accurate evaluation and dynamic incentive decision method of aggregators' demand response considering contribution degree is proposed. Firstly,the accurate evaluation system of demand response considering static and dynamic process is developed and the static evaluation follows the existing demand response evaluation system at the grid side,while the dynamic evaluation focuses on the consistency between the actual load and the target load in each period. Then,the user contribution index is described from the economic and load dimensions,and a two-part incentive settlement scheme with static and dynamic evaluation is designed. Based on that,the dynamic incentive decision model of aggregator's demand response with the contribution degree is got. Fin
Metal inserts are widely used particularly in bolted connections for joining composite members. In this work, a theoretical basis is provided for analyzing the damping behavior of a cantilever ...composite square hollow member embedded with metal inserts along its length. Analytic stress solutions around the boundaries of the insert and the member are introduced while deriving the expression of modal damping ratio. Kelvin–Voigt and amplitude-dependent damping models are considered for the formulation. Experiments are performed on the perforated composite tube reinforced with brass, copper, and steel annular inserts subjected to transverse base motion of varying amplitudes. The acceleration responses recorded from the tests are analyzed to evaluate the damping ratio of the fundamental mode. It is observed that the damping ratio evaluated from the experiments matches well with the proposed value. Furthermore, it is found that the damping ratio increases when the holes are reinforced with the inserts. The reason for the increase in damping stems from (i) frictional effect at the boundaries of the insert and the tube and (ii) material damping of the inserts. For example, the damping amplification of the tube with five steel inserts is noted to be around 3.0 and 2.1 times with respect to the unperforated and perforated (with holes) cases, respectively.
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•Dynamic modal analysis during reduced Francis turbine scale model tests is proposed.•It enables a fair identification of the eigenfrequencies of the test rig.•The hydroacoustic ...parameters of the draft tube cavitation flow can be then identified.•The proposed procedure might be applied for standard model tests in industrial context.
Francis turbines operating at off-design conditions experience the development of unfavourable cavitation flows in the draft tube at the runner outlet, which induce pressure pulsations and hydro-acoustic resonances in the worst cases. The assessment of hydropower plant units at off-design conditions is possible by means of one-dimensional numerical simulation, which however requires a proper modelling of the draft tube cavitation flow. The corresponding hydro-acoustic parameters can be identified for a wide number of operating points on the reduced scale model of the machine by modal analysis of the hydraulic test rig. This identification approach is efficient but can however be time-consuming for an industrial project. The paper aims at proposing and validating a faster procedure to identify the eigenfrequencies and the corresponding eigenmodes of a hydraulic test rig featuring a reduced scale model of a Francis turbine operating in off-design conditions. The test rig is excited by injecting a periodical discharge with a rotating valve whose frequency linearly increases from 0 to 7 Hz. Based on the response of the test rig, measured by pressure sensors placed along the pipes, the eigenfrequencies and the corresponding eigenmodes are identified for several operating conditions. The hydro-acoustic parameters are then identified by using a one-dimensional numerical model of the test rig. The results are in very good agreement with those obtained with the standard procedure, i.e. with a stepwise increase of the excitation frequency. This new approach represents an important gain of time and might be applied to assess hydropower plant stability in an industrial context.
A major trust of modal parameters identification (MPI) research in recent years has been based on using artificial and natural vibrations sources because vibration measurements can reflect the true ...dynamic behavior of a structure while analytical prediction methods, such as finite element models, are less accurate due to the numerous structural idealizations and uncertainties involved in the simulations. This paper presents a state-of-the-art review of the time-frequency techniques for modal parameters identification of civil structures from acquired dynamic signals as well as the factors that affect the estimation accuracy. Further, the latest signal processing techniques proposed since 2012 are also reviewed. These algorithms are worth being researched for MPI of large real-life structures because they provide good time-frequency resolution and noise-immunity. Keywords: modal parameters identification, time-frequency algorithms, wavelet transform, synchrosqueezing transform, civil structures, dynamic excitation sources.
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