Free Vibration Tests Research Articles

Experimental study on vibration control performance of TMD-STF damper

A tuned mass damper (TMD) using shear thickening fluid (STF) as an energy dissipation medium is designed and manufactured. In addition, a TMD incorporating silicone oil (TMD-Si49) is fabricated to compare and analyze the vibration control performance of TMD-STF. Free vibration, forced vibration, and random excitation tests are conducted on a single degree of freedom (SDOF) structure model equipped with TMD-STF and TMD-Si49, respectively. The displacement and acceleration response of the SDOF model are recorded to analyze the vibration control performance of TMD-STF and TMD-Si49 under different load conditions. The results of the free vibration test show that TMD-STF affects the coupling damping ratio of the test model, exhibiting time-varying damping characteristics, but does not impact the coupling frequency ratio. In the forced vibration test, TMD-STF effectively expands the effective frequency range of vibration control performance by approximately 17.0% compared to TMD-Si49. The random excitation test demonstrates that TMD-STF exhibits significantly improved control of acceleration response rather than displacement control. The research findings on TMD-STF provide novel insights for vibration control in engineering structures.

Vibration analysis of bio-inspired sandwich composite beam under rotating environment based on the Internet of Things technology

Sandwich composite materials with corrugated cores are mostly used in industries like aircraft, marine construction, automobiles, and construction. Sandwich composite constructions are made for their lightweight, high strength, and high stiffness, and also the sandwich core composite structures are useful for dynamic studies. A novel design inspired by a woodpecker beak microstructure creates fillet edge and champer edge designs. This novel design motivates us to achieve the energy absorbing results starting with free vibration studies followed by rotating environment vibration studies. ASTM E1876 standards, a non-destructive approach, are used to determine the material properties of carbon fiber-polylactic acid (CF-PLA) and glass fiber reinforced polymer (GFRP), along with alternative dynamic method (ADM) for core property evaluation. The hand lay-up technique is used for fabrication and evaluated for free vibration testing with appropriate boundary conditions. Internet of Things technology has been used for analyzing vibrations of sandwich structures in rotating environment setup with 250, 500, 750, and 1000 rpm. It was observed that the fillet edge pattern has a more efficient natural frequency than the champer edge pattern. Furthermore, the parametric evaluations were performed in various ply configurations in numerical simulations for free vibration and the rotating Environment. Also, the effective results are seen in fillet core bioinspired sandwich structure that achieves efficient stiffness in both free vibration and rotational environments, despite various boundary conditions and parametric circumstances. Therefore, the fillet edge 3D printed core design with particular GFRP factsheet orientation brings out the novelty of this research.

Modal identification of existing concrete half-joint bridges through free vibration tests

Vibration-based tests are widely used for the assessment of existing structures. They allow the estimation of modal parameters, a possible deviation of which may be symptomatic of the onset of damage in the structure. Specifically, in the case of bridges, free vibration testing is a rapid, low-cost method that requires the installation of a limited number of sensors on the structure. This contribution presents the results of the dynamic identification for a series of existing concrete half-joint bridges based on free vibrations recorded after the transit of heavy trucks. The bridge deck modal characteristics (frequencies, damping ratios and mode shapes) are obtained via a recently developed identification framework based on the variation mode decomposition. These experimental estimates are also analyzed and used to calibrate a finite element model of the bridge deck, based on in-situ material characterization tests. Negligible fluctuations both in frequency and mode shapes are observed between experimental and numerical findings, which suggests that the analyzed overpasses do not present any relevant damage or cracking process under the service loads.

INVERSE ANALYSIS OF THE AMPLITUDE-DEPENDENT DAMPING PROPERTIES OF EPOXY/GLASS FIBER LAMINATES

In this paper, we present the experimental results obtained and corresponding inverse analysis employed to determine the amplitude-dependent damping properties of laminates made of glass fiber reinforced plastic (GFRP). Free damped vibration tests of cantilever beams with different symmetric stacking sequences were performed. Logarithmic decrement analysis was used to evaluate the effective amplitude-dependent loss factor of the samples. Inverse analysis was used to find the single-ply dynamic properties based on the laminated beam theory and the complex moduli approach. It was found that the loss factors of GFRP laminates almost linearly depend on the maximum strain amplitude. The maximum value of the loss factor of unidirectional ply was realized in the transverse direction to the fibers and reached ~ 0.023 for strain amplitudes up to 0.1% in the frequency range of 20-60 Hz.

Study on the mounted effect of the damper on the cable based on an Improved Separation of Variable Method

This paper investigates the influence of dampers on cable vibration control. The mode shape of the cable changes after the installation of dampers, which changes the dynamic characteristics of the system. Thus, an Improved Separation of Variable Method is proposed to model the vibration behavior of cable–damper systems, including the amplitude and phase of cable vibration. The analysis based on this method reveals that the damper affects the entire cable vibration through the phase difference, which is closely related to the amplitude of vibration. The nonlinear ordinary differential equations obtained in the previous step are solved, and the theoretical solutions of the amplitude and phase are obtained. To validate the theoretical solution, a free vibration test of cable of 19.56 m is conducted. Finally, a parametric analysis of the cable–damper model was conducted based on the verified theoretical framework. The results indicate that the amplitude decay rate of the cable’s vibration initially increases and then decreases with an increasing damper coefficient, and the frequency change depends on the vibration mode and damper position. Furthermore, after the damper is installed, the motion of the cable exhibits a traveling wave effect, which results in a damper clamping effect on the cable due to the phase difference of the traveling wave. The application of parallel negative stiffness can reduce the clamping effect of lower modes of the system; however, its effectiveness is not significant for higher modes.

Investigation of vibration, flexural, tribological and flame retardant properties on rubber crumb-reinforced polymer composites

This research aims to fabricate polymer matrix composites with improved mechanical, flame retardant, and damping properties by incorporating waste rubber crumbs as the reinforcement. The damping characteristics of the polymer matrix composites reinforced with different weight percentages of rubber crumb such as 0, 2.5%, 5%, and 7.5% were analyzed using a free vibration test. The experimental results confirmed that the addition of rubber crumb significantly improved the damping characteristics of the material. In addition, the rubber crumb-reinforced composites showed improved mechanical and fire-retardant properties. The flexural strength improved exceptionally with the increase in rubber reinforcements up to 7.5%, whereas the tensile strength increased up to 2.5% weight and reduced with the further increase in rubber. Enhanced flame-retardant properties were witnessed in the polymer composites reinforced with rubber crumb than that of the plain Epoxy - Glass fibre composites. Thermogravimetric analysis (TGA) were also used to ascertain the thermal properties. The scanning electron microscope was employed to analyze the surface and fractography of the polymeric composites. The overall results showed the contemporary effect of rubber crumbs on polymer composites.

Geometric nonlinear stiffness and frequency of the traditional spring-suspended free-vibration testing device

Free vibration tests on bridge deck sectional models in wind tunnels have been widely adopted to study the wind-resistant performances of bridges. The traditional spring-suspended free-vibration testing device is suitable for small torsional amplitude vibration tests. When large torsional displacement occurs, a geometry nonlinear will be introduced, and the stiffness and frequency of the traditional device will change. However, quantitative research on this subject is inadequate at present. Based on proper simplifications and assumptions, new analytical formulas are derived that describe how the torsional and vertical stiffnesses of the traditional device vary as the torsional and vertical displacement change for different spring lengths, initial strains, joint spacings, and stiffnesses. The amplitude-dependent torsional frequency is also quantified based on the potential energy function. It is found that the torsional stiffness reduction rate Rα and frequency reduction rate RfAα nonlinearly increase with the torsional displacement α and amplitude Aα, respectively. In the parameter ranges considered, R10° is basically within 4%–6%, R20° is basically within 15%–28%; Rf10° is basically within 1%–2%, Rf20° is basically within 6%–11%. From the view of geometry nonlinear, the maximum torsional amplitude of the traditional device is suggested to be less than 4°. Although Rα can be effectively reduced by increasing the initial strain and length of the upper and lower springs, it cannot be completely eliminated whatever increasing the above parameters. The vertical stiffness reduction rate can be ignored.

Experimental and numerical investigations on seismic responses of wind turbine structures with amplifying damping transfer system

Wind turbine (WT) with a long period is a dynamic sensitive but lightly damped structure, which may be prone to earthquakes, especially for the near-fault pulse-like records containing large amplitude of velocity pulse with a period close to WT's period. An efficient and convenient solution to enhance the damping effect of WT, amplifying damping transfer system (ADTS), is applied by transferring the upper rotation of the wind turbine to its bottom with an amplified damping mechanism. The feasibility of the ADTS is experimentally validated through free vibration tests, sinusoidal excitation tests, and shaking table tests for both non-pulse-like earthquakes and pulse-like earthquakes. Test results of free vibration validate that more than 6.2% extra damping ratio is provided by the ADTS to the tested WT model; while the supplemental stiffness of ADTS shifts the frequency of the tested WT model from 2.16 Hz to 2.61 Hz. Sinusoidal tests show ADTS reduces the displacement (acceleration) response of the tested WT by about 73.88% (58.16%) when subjected to resonant frequency. It is also observed that the ADTS is effective in controlling the seismic responses of non-pulse-like earthquakes and pulse-like earthquakes, providing at least 30% of the reduction in displacement and acceleration. Simulations with high accuracy indicate that the pulse signal usually concentrates the majority of the input energy of near-fault pulse-like earthquakes; while the ADTS can absorb most of the input energy (over 90%), protecting the WT away from undergoing excessive vibration.

Dynamic performance of the Mosque-Cathedral of Córdoba under different earthquake scenarios: The Abd al-Rahman I sector

The Mosque-Cathedral of Córdoba is one of the most important monuments in Spain. Owing to its authenticity, its integrity and its influence on later architecture, it was declared as a World Heritage Site by the UNESCO in 1984. The monument is located in Andalusia, in southern Spain. This region presents a moderate seismic hazard, and it is affected by two kinds of seismic sources: a) far away earthquakes of large/very large magnitude; b) close earthquakes of moderate magnitude. Particularly, the Mosque-Cathedral had to endure the well-known 1755 Lisbon's earthquake (Mw = 8.5, at 488 km) and the 1504 Carmona earthquake (Mw = 6.8, at 82 km). This research has analysed its dynamic behaviour under several earthquake scenarios: code-base response spectrum, previous historic earthquakes and fault maximum magnitude earthquakes. However, after the seismic hazard analysis and due to the lack of faults registered surrounding the building, no worst-case scenario considering the maximum expected magnitude has been found. Owing to the complexity of the building, this work has focused on the Abd al-Rahman I sector, which is the most aged part, dating from the VIII century. A refined 3D numerical model has been developed and calibrated through free ambient vibration tests. The distribution of the maximum amplitudes (internal displacements and absolute accelerations) under the different scenarios has been comparatively presented. Also, the seismic performance has been assessed considering the drifts and the damage expected. The results showed that the damage would be concentrated in the contact between the arcades and the courtyard wall. Also, the lower part of the North-South closing wall would be damaged. According to the seismic performance, the building is expected to present moderate damage. In contrast, the analysis of the drifts, according to the codes, suggest that the building would resist each damage limit. Also, it was shown that the far fault earthquake scenario is more demanding than the close fault earthquake scenario. Next, it should be noted that the behaviour of the building for a seismic action of 475-year return period is expected to be similar to the far-fault earthquake scenario. The most demanding earthquake scenario has been the 975-year return period one.

Mechanical and vibrational study of raw and surface‐treated ramie fiber hybridized carbon fiber epoxy composites

AbstractHybrid polymer composites' lightweight, biodegradability, damping, and cost‐effectiveness have made them popular in recent decades. This study evaluated the raw and alkali‐treated hybrid composite's free vibration studies and mechanical properties by changing the ramie fiber layers (core) from two to four and keeping the carbon fiber layers (top and bottom) constant. Thus, to enhance the mechanical properties, the ramie fiber was treated prior with 2, 3, and 4 wt% of sodium hydroxide (NaOH). These composites were characterized chemically, mechanically, morphologically, and vibrationally via Fourier transform infrared spectroscopy, flexural and tensile tests, field emission scanning electron microscope, and free vibration setup, respectively. The tensile and flexural samples were loaded under quasistatic conditions and tested at 1 and 2 mm/min to evaluate tensile and flexural properties, respectively, and a free vibration test built the natural frequencies and reliable damping factor using clamped‐clamped and clamped‐free boundary conditions. Two layers of carbon fiber (exterior layer) and two layers of ramie fiber (core layer) at 3 wt% NaOH produced maximum tensile strength (523 MPa) and flexural modulus (68,869 MPa) among natural and hybrid composite combinations. This study shows that hybridization and alkaline treatment improve composite material mechanical properties in a specific ratio.Highlights Mechanical and vibration analysis of carbon/ramie hybrid composites was studied. Ramie fiber's surface treatment was carried out in NaOH at 2, 3, and 4 wt%. Surface treatment uplifts mechanical and vibration properties than untreated. Ramie fiber's chemical consequences were analyzed through Fourier transform infrared spectroscopy and field emission scanning electron microscope tests. Surface treatment at 3 wt% builds higher mechanical and vibrational properties.

Automatic modal identification of bridges based on free vibrations and advanced signal decomposition techniques

Free vibration tests are attractive because they can be performed by means of a network consisting of few sensors temporarily installed on the structure in such a way to limit duration and cost of the experimental campaign. Additionally, free vibration tests are usually performed by introducing an initial perturbation that can induce a structural response significantly higher than the ambient excitation. This, in turn, allows to reduce the noise-to-signal ratio in the final measurements and/or to consider less stringent requirements about the technical specifications of the sensors. Since free vibration tests can provide accurate estimates of the modal parameters while being rather cheap and easy to implement, they have been performed in many applications, such as the experimental dynamic characterization of base-isolated buildings, masonry towers, ancient tie-rods, and bridges. An efficient and automatic computational framework is thus presented for the modal identification of bridges based on their free vibrations. The novel procedure proposed in the current work combines advanced signal decomposition techniques and a robust approach for damping identification. Two advanced signal decomposition techniques are considered, namely the Variational Mode Decomposition and the Empirical Fourier Decomposition. Experimental applications are then illustrated for roadway and railway bridges.

Simulation-Based Model-Updating Method for Linear Dynamic Structural Systems

The dynamic characteristics of buildings and their behavior under various dynamic loads play a crucial role in civil engineering applications, particularly for earthquake-resistant structural design. Employing a precise mathematical model of the structural system makes it possible to accurately predict the actual structural performance under dynamic loads, such as winds and earthquakes. Given this perspective, finite element model-updating approaches in structural systems have gained significant attention in recent decades. This paper proposes a simulation-based model-updating technique that utilizes measured free vibration responses to the correct structural parameters of multi-degree-of-freedom systems. A five-degree-of-freedom building model is subjected to shaking table tests to demonstrate the effectiveness of the proposed method. The experimental data for this method consists of the dynamic behavior of the system under the seismic excitation of the El Centro 1940 earthquake and the results of the free vibration tests. The MATLAB/Simulink parameter estimation tool is employed to establish a correlation between the analytical model and the measured dynamic response from the building model. Compared to the measured structural responses, the updated analytical model, which incorporates the proposed simulation-based model-updating technique, demonstrates high accuracy in predicting the responses through effective corrections of stiffness and damping coefficients.

A Neural Network Approach to Estimate the Frequency of a Cantilever Beam with Random Multiple Damages.

Natural frequency is an important parameter in the structural health monitoring (SHM) system. Any changes in this parameter indicate structural alteration due to damage. This study provides a neural network (NN) solution as an alternative to the finite element (FE) method to measure the natural frequencies of a cantilever beam with random multiple damage. It is based on a statistical dataset of a free vibration test obtained from the APDL (Ansys parametric design language) simulation using a MATLAB (matrix laboratory) script. The script can generate an unlimited number of possible damage combinations for any given parameters with the help of the Monte Carlo (MC) technique. MC helps to generate a random number of damages in random locations at each simulation. Damage conditions are controlled by three parameters including damage severity and damage size (in terms of the mean and standard deviation of damage). Moreover, the method proposes a curve-fitting equation to validate the predicted natural frequency for the first three modes obtained from the neural network model. Both methods are in good agreement with each other, having minimal errors in the range of 0.2-3% for each mode. The frequency result shows that the beam frequency is 8.6486 Hz if the area reduction is 10%, whereas it comes down to 7.2338 Hz if there is a 30% area reduction. A two-level factorial test shows that damage severity is the most impactful factor compared to the damage sizes on the frequency shift event. This indicates that damage alters the composition of the beam and has an impact on its frequency change with the assumed damage parameters. Therefore, the proposed NN model can estimate the frequency shift for various damage scenarios. It can be utilized in the vibration-based damage identification process to predict the frequency changes of the damaged beam without any computational burden.

Experimental study on earthquake‐induced pounding between steel structures

AbstractPounding between adjacent structures has been repeatedly observed during major earthquakes. The phenomenon may cause some local damage at the points of interactions, it may also lead to substantial destruction, permanent deformations or total collapse of colliding structures. The effects of earthquake‐induced structural pounding have been studied for more than three decades now. However, most of the studies concerned masonry as well as reinforced concrete structures and investigations on steel structures are very limited. The aim of the present paper is to show the results of the shaking table experimental investigation focused on earthquake‐induced pounding between three models of steel structures in a row. The models were constructed out of four steel columns made from rectangular box section with additional steel skew bracings so as to prevent transverse and torsional vibrations. To obtain different dynamic characteristics of the models, additional plates were added at the top of each tower. Two configurations were investigated. In the first one, two plates were mounted on the external structures and only one plate at the top of the middle tower. In the second configuration, only one plate was mounted at the top of external towers and two plates on the middle structure. Firstly, the free vibration tests were conducted. Then, the steel towers with different in‐between gap sizes were tested under different earthquakes. The results of the study confirm that interactions between insufficiently separated steel structures may significantly influence the structural behaviour during earthquakes. In most of the cases, pounding has resulted in the increase in the peak responses. In some cases, however, it has played a positive role leading to the decrease in the structural response under seismic excitation. Anyway, even if the response is reduced, interactions should be prevented at the design stage of steel structures since they may result in considerable damage at the points of impacts.

Damage identification using convolutional neural networks from instantaneous displacement measurements via image processing

This work investigates the effectiveness of using convolutional neural networks (CNNs) and instantaneous displacement measurements for damage identification in beams. The study involves subjecting laboratory beams to eight distinct damage scenarios and capturing the vertical positions of 60 points along the beam length during free-vibration tests using a high-speed camera. The data obtained was subsequently used to train a CNN in a supervised manner to estimate the level of damage at each point. Results showed that the CNN models were able to correctly localize and quantify the damage levels when trained on data from all damage scenarios. The soundness of the proposed methodology was demonstrated in a robustness assessment, where all eight damage scenarios were correctly identified even when two of them were excluded from the training dataset.

Passive Damping of Solar Array Vibrations Using Hyperelastic Shape Memory Alloy with Multilayered Viscous Lamina

A novel passive vibration-damping device is proposed and investigated for a large deployable solar array. One strategy for achieving high damping in a solar panel is using a yoke structure comprising a hyperelastic shape memory alloy and multiple viscous adhesive layers of acrylic tape. The effectiveness of the proposed system in achieving a high damping performance was demonstrated by conducting free vibration and low-level sine sweep tests using a solar array, and a 0.75-m-long flexible dummy structure was simulated. We also investigated the dependence of the damping performance of the proposed structure on the number of viscous lamina layers. Finally, the damping characteristics of the proposed system were assessed under predictable on-orbit temperature conditions.

The Reinforcement Effect of Nanoclay on the Damping Characteristics of Hybrid Laminated Composites

The Hybrid Nanocomposite (HNC) laminates are processed by reinforcing the chopped strand mat glass fiber and organically modified montmorillonite clay (OMT) in the polyester matrix. The different percentage by weight (1, 2, 3, and 5% ) of OMT are mixed by high speed shear mixer into the polyester matrix and then the HNC laminates are fabricated by introducing pre-mixed polyester-clay into the CSM by hand lay-up technique. A tensile test result reveals that nanoclay in polyester matrix significantly improves the tensile modulus and strength of HNC. The results also indicate that increasing the amount of nanoclay beyond 2% by weight reduces the mechanical properties but surpassing the basic glass fiber filled system. The reinforcing effect of clay enhances the flexural modulus and strength. Considerable improvement in impact energy absorption is seen from the impact test. Dynamic mechanical analysis shows that incorporation of nanoclay improves the storage modulus and Loss tangent of HNC. Free vibration tests and logarithmic decrement methods are used to predict the dynamic characteristics such as natural frequency, and damping factor for the first four modes of different clay concentrations. Dynamic results show that second phase nanoscale dispersion between the matrix and E-glass fiber significantly enhances the internal damping of hybrid composites.

Identification of nonlinear aerodynamic damping of cross-wind excited slender structures in atmospheric boundary layer flow

Due to the low natural frequency and low damping of slender structures, the crosswind aero-elastic effect become dominant around the vortex-resonance wind speed and can lead to undesired crosswind responses. For understanding and describing the aero-elastic effect of slender structures, aerodynamic damping is usually adopted. This study suggested an identification procedure for crosswind aerodynamic damping of slender structures by curve-fitting the probability density function (PDF) of the crosswind response. Firstly, the aerodynamic damping is described by a generalized Van der Pol (GVDP) model, and the analytical solution of response PDF, which is a function of the parameters of GVDP model, is derived by the equivalent nonlinear equation method. Next, the proposed identification method was validated by stochastic simulation of a crosswind-excited tall building. Finally, free-vibration wind tunnel test of a slender structure with slenderness ratio equal to 12 and 20 at open flat terrain was conducted and aerodynamic damping was then quantified by the present method. It is found that the present method provides a robust and accurate identification of crosswind aerodynamic damping of slender structures in atmospheric boundary flow.

The Role of Friction Stir Processing Parameters and Hybrid ZrC/WC Reinforcement Particles in Improving the Surface Composite Dissimilar Matrix’s Dynamic Behavior and Microstructure Refinement

This study examined the impact of hybrid reinforcement particles, specifically zirconium carbide (ZrC) and tungsten carbide (WC), as well as the parameters of friction stir processing (FSP), on the microstructure, mechanical properties, and dynamic behavior of aluminum alloys. The hybrid particles were integrated into the aluminum alloy using friction stir processing (FSP). The fabricated metal matrix composites (MMCs) were characterized using optical microscopy, scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). The results showed that the FSP parameters and reinforcement particles played an important role in improving the grain refinement of the MMCs. This study’s results suggest that the FSP samples’ UTS can be maximized using a tool rotation speed of 600 rpm and a traverse speed of 30 mm/min. The grain refinement in the composite surface was attributed to the dynamic recrystallization during the friction stir processing (FSP) process. The reinforcement particles also acted as grain growth restrictors, further refining the grain size. This resulted in a 34% increase in ultimate tensile strength compared to AA2024 alloys and a 12% increase compared to AA7075 alloys. The composite surface also exhibited enhanced dynamic properties, with an increase in impact energy of 26%. The free vibration test showed that the hybrid reinforcement particles significantly improved the strength and damping capacity of the aluminum alloys, resulting in a high resonant frequency. This is important for applications such as vibration damping and noise reduction.

Effect of Hybridization of Ramie Fibers on the Vibrational and Damping Responses of Ramie/Glass/Epoxy Resin Composite Laminates

Fiber reinforced polymer composites made with glass fibers are among the oldest and most popular kinds of composites in use today. Glass fiber reinforced composites' key benefits are their adaptability for specific material applications, which allows them to give a number of design advantages relating to strength, chemical stability, impact damage tolerance, heat insulation, and low cost. The focus of this research is to investigate the role of hybridized ramie fibers in the assessment of enhanced vibrational damping capabilities in fiber glass reinforced composites, as well as in the initial assessment to verify their acceptability for real-time applications. Composite molding employing the hand layup technique was used to fabricate hybrid epoxy composites with ramie to glass fiber weight ratios from 0 to 50%. A free vibration test was performed to determine the hybrid composite's vibration dampening capabilities as a function of the ramie fiber filler content. The results demonstrated that the damping ratio was reduced when the percentage of ramie fiber in the GFRP composite was raised from 10% weight to about 50% weight. But adding up to 40% wt of ramie fiber to the hybrid composite had the biggest effect on the damping ratio, natural frequency, storage modulus, and loss modulus. This means that using ramie fiber in hybrid composites will be cost-effective and good for the environment.