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It is important but still challenging to detect structural damage with limited instrumentation in the spatially large civil infrastructure. In this paper, a damage detection algorithm is proposed employing a global–local approach with limited sensors for detecting structural damage of spatially large civil engineering structures. The proposed algorithm is based on measuring frequency response functions (FRFs) at convenient yet selective locations on the structure with very limited sensors to localize the region of damage in the structure using the proposed global diagnostic approach. Once the local region of damage is established, local diagnostics are performed in the isolated small region of the structure using high fidelity sensing, still, a limited number of sensors in the small localized region (segment) of the structure, to precisely locate the damage. The principal component analysis combined with fractal dimension theory is employed for local diagnostics. Numerical studies have been conducted on a 6 m simply supported beam and a 30-storey framed structure idealized as a shear building. Experimental verification of the proposed global–local damage diagnostic approach is also carried out. Studies presented in this paper indicate that the global–local approach for damage diagnosis of spatially large structures is very promising for precisely locating the damage with very limited sensors. [ABSTRACT FROM AUTHOR]

This paper presents frequency response function (FRF) expansion techniques that minimize the difference between the analytical and predicted FRF matrices to satisfy the FRF constraints. The measured FRF relationships at a small number of locations were used as constraints. The expansion method is useful for estimating the rotational FRFs that are difficult to measure or apply using frequency-based substructuring (FBS) techniques. The validity of the proposed method, including the effects of external noise, was confirmed using numerical examples. An FBS algorithm was also derived by incorporating the FRFs of each substructure, and the compatibility conditions were transformed into FRFs with pseudomasses at the joint nodes. A discrepancy between the FRFs of the synthesized system estimated using the proposed method and the analysis results of the entire system was observed in the numerical example, and the reasons for the discrepancy were investigated and discussed. It is estimated that the proposed FBS method can be enhanced by combining it with other dynamic substructuring techniques and supplementing additional information, such as measured FRFs. [ABSTRACT FROM AUTHOR]

Optimal sensor placement (OSP) is one of the essential factors affecting the accuracy of health management, particularly in health monitoring driven by mode information. A novel OSP method based on active learning is proposed to effectively capture modal shapes for Structural Health Monitoring (SHM). First, the optimal Latin Hypercube Sampling is carried out to generate initial sensor positions, and the corresponding amplitudes of modal shapes at these positions are obtained by a frequency response function. Subsequently, data-driven models are built to be treated as virtual sensors to reconstruct the integrated modal shapes of the structure, and the accuracies of the results are calculated. Then, considering the distribution of the input sensor position, an improved reliability-based expectation improvement function (IREIF2) is applied to find the optimal sensor positions by optimizing the parameters of the probability density function in IREIF2. Finally, the position and response of the optimal sensor are used to update the data-driven models for more accurate modal shape reconstruction, and the accuracies are calculated to determine whether the OSP process continues. Once the accuracies meet the desired criteria, the optimal sensor positions are also obtained. The superiority of the proposed method is verified by the comparisons with other OSP methods, and different case studies are also used to prove the proposed method can realize OSP for SHM. [ABSTRACT FROM AUTHOR]

Turbulence excitation is an unavoidable form of excitation in flutter flight tests, and it is also a necessary and effective excitation method in high-speed flights, dives, other high-risk test, and high-frequency modal information mining. However, because the turbulence excitation signals are unmeasurable in the time domain, although the turbulence response contains rich and valuable flutter test information, the randomness and low quality of the data often cause difficulties in modal analysis. Therefore, an improved frequency domain decomposition algorithm for turbulence response processing is proposed in this paper. First, due to the irrelevancy of the atmospheric excitation, the power spectral density function matrix of the multi-channel turbulence response is subjected to singular value decomposition. There is a mathematical relationship between the maximum singular value curve and the system frequency response function. Second, the modal assurance criteria are used to calculate the maximum singular value of a single-degree-of-freedom system. Finally, an orthogonal polynomial method is applied to fit the maximum singular value curve, and the system identification is performed directly in the frequency domain. The simulated data and a certain type of aircraft flutter flight test data are used to verify the proposed method, and the results confirm the effectiveness and engineering applicability of the method developed in this work. [ABSTRACT FROM AUTHOR]

Due to increasing demands regarding higher manufacturing accuracy and the machining of high-strength materials, the stiffness and load-carrying capacity of main spindles must be enhanced. One approach is to replace the conventionally used angular contact ball bearings, so called spindle bearings, with tapered roller bearings of the same size, which have a higher stiffness and load carrying capacity due to their larger rolling contact. In case the speed requirement is below a speed parameter n · d m = 0.9 × 10 6 mm/min, the poorer speed parameter of tapered roller bearings can be compensated by a sufficiently high lubrication quantity. However, the stiffness increase by using this bearing type in main spindles has not yet been quantified in field tests. Therefore, the aim of this paper is to demonstrate the stiffness advantage of a spindle with an elastically arranged tapered roller bearing compared to a conventional spindle with angular contact ball bearings by machining tests. First, the radial load displacement characteristic of the newly developed tapered roller bearing spindle is tested in advance on an isolated test rig. In machine cutting tests, full slots are milled in an aluminum block. During the tests, the axial and radial displacements of the spindle shaft relative to the housing are measured and evaluated in radial and axial direction. These tests are repeated and compared with the conventional spindle. By varying the process parameters such as cutting depth, feed rate or rotational speed, the increased stiffness of the tapered roller bearing spindle compared to the conventional spindle is illustrated based on the measured displacements. Moreover, frequency response measurements at different speeds underpin the improved damping of the tapered roller bearing spindle. Assuming that the speed requirements are not too high and the tapered roller bearing is sufficiently lubricated, the use of this type of bearing can significantly increase the stiffness and thus the manufacturing precision in the long term for main spindles. [ABSTRACT FROM AUTHOR]

Damage location vector (DLV) method is a model-based structural health monitoring approach that needs the frequency response–function response of the structure. A review of the literature indicates that although the DLV method accurately identifies the damage location in the single-damage structures, it does not work properly for the multi-damage. Accordingly, the aim of this research is to advance the DLV approach to increase its accuracy to detect the damage locations and severities in the multi-damage structures. In this regard, experimental and numerical studies were performed on the two-fixed ends steel beam having multiple damages with different intensities. During laboratory tests, the vibration response of steel beam specimens with multiple defects stimulated by hammer impact was measured. Different sensor locations were considered in the tests. A finite-element model of the steel beam was developed to calculate the dynamic response of undamaged beam under impact loading. Based on the fundamentals of hypothesis testing and data fusion, a threshold was derived to advance the DLV approach to detect the multiple damages. Moreover, the effect of sensor position on the performance of the DLV approach was investigated. The proposed method was also applied to a long-span box-shaped bridge to investigate its accuracy and efficiency for detecting damages in realistic complex structures. Moreover, the results obtained from the advanced DLV method were compared with other conventional methods, considering the effect of noise and different damage scenarios. The findings reveal that the advanced DLV approach proposed in this study accurately detects the defect locations and severities in the structures having multiple damages. [ABSTRACT FROM AUTHOR]

The use of digital image correlation for modal analysis is becoming an appealing option thanks to its non-contact and full-field measurement process. However, frequency response function (FRF) estimation can be challenging due to the limited number of time domain data and heavy measurement noise. Thereby, the present work aims to propose a method which improves the estimation accuracy of point-wise FRFs. Firstly, a Gaussian-process-based spatial-frequency model is proposed, which makes use of the intrinsic properties of the FRF and the local spatial information of field measurements. Then, a Bayesian solution is developed, which is enforced by a stable and efficient numerical procedure. Finally, the effectiveness of the proposed method is verified by making a comparison with the spectral estimator through the use of simulated data, and it is further validated based on an experimental application. [ABSTRACT FROM AUTHOR]

The vibrational characteristics of the Hybrid III and NOCSAE headforms are not well understood. It is hypothesized that they may perform differently in certain loading environments due to their structural differences; their frequency responses may differ depending on the impact characteristics. Short-duration impacts excite a wider range of headform frequencies than longer-duration (padded) impacts. While headforms generally perform similarly during padded head impacts where resonant frequencies are avoided, excitation of resonant frequencies during short-duration impacts can result in differences in kinematic measurements between headforms for the matched impacts. This study aimed to identify the natural frequencies of each headform through experimental modal analysis techniques. An impulse hammer was used to excite various locations on both the Hybrid III and NOCSAE headforms. The resulting frequency response functions were analyzed to determine the first natural frequencies. The average first natural frequency of the NOCSAE headform was 812 Hz. The Hybrid III headform did not exhibit any natural frequencies below 1000 Hz. Comparisons of our results with previous studies of the human head suggest that the NOCSAE headform's vibrational response aligns more closely with that of the human head, as it exhibits lower natural frequencies. This insight is particularly relevant for assessing head injury risk in short-duration impact scenarios, where resonant frequencies can influence the injury outcome. [ABSTRACT FROM AUTHOR]

Resonance frequencies of a mechanical system are the most important parameters that determine the dynamic characteristics of the system. When the frequencies of the harmonic forces under the influence of the system coincide with the resonance (natural) frequencies of the system, a resonance situation occurs and the mechanical system vibrates with high amplitudes. This can cause serious damage to the system by causing effects such as break, fatigue, impact, noise on the bearings, connections and elements in the system. It is possible to eliminate some resonance frequencies of mechanical systems in an examined frequency range using pole-zero cancellation methods by shifting a resonance frequency of the system to an anti-resonance frequency or an anti-resonance frequency to a resonance frequency. Thus, a solution can be obtained for some vibration problems. In this study, a new method using the Sherman-Morrison formula is presented to eliminate some of the resonance frequencies for some FRFs (Frequency Response Functions) of mechanical systems with pole-zero cancellation. In order to eliminate a selected resonance frequency of the system in any FRF determined using the presented method, a mass (kg) and a grounded spring (N/m) modification are made and the required modification values for this are calculated by solving the obtained non-linear equation set numerically. With the proposed method, it is aimed to contribute to the literature by presenting an alternative approach for the solution of vibration problems by cancelling some selected resonance frequencies. The main highlight of the method is there is no need of a matrix inversion for calculating the required modification values. This provides a very fast solution. In addition, the proposed method uses directly the FRFs of the active coordinates (response, excitation and modification) only, which makes the method very useful for practical engineering applications. [ABSTRACT FROM AUTHOR]

Accurate information on vehicle parameters is essential for vehicle design, vehicle-bridge interaction analysis, and road maintenance. Currently, most vehicle parameters identification (VPI) methods are deterministic and rely on strict experimental conditions. This paper proposes a simple method based on Bayesian framework to identify uncertain vehicle parameters, which only requires the vibration response of the vehicle under arbitrary excitation. First, the dynamics of the vehicle is analyzed and the link between excitation and response is established using the vehicle frequency response function. Subsequently, the likelihood function is formulated based on the difference function between the measured responses and the expected responses of updated model. Notably, this method introduces a creative probabilistic form of the frequency-domain response assurance criterion. Furthermore, a numerical simulation of the vehicle driving over bumps is performed to assess the influence factors on VPI, such as noise pollution and the excitation correlation between the front and rear wheel. At last, the parameters of a van are identified through field test and the reliability of the results is demonstrated. [ABSTRACT FROM AUTHOR]

Damages to various building structures often occur over their service life and can occasionally lead to severe structural failures, threatening the lives of its residents. In recent years, special attention has been paid to investigating various damages in buildings at the early stage to avoid failures and thereby minimize maintenance. Structural health monitoring can be used as a tool for damage quantification using vibration measurements. The application of various sensors for measuring accelerations, velocity and displacement in civil infrastructure monitoring has a long history in vibration-based approaches. These types of sensors reveal dynamic characteristics which are global in nature and ineffective in case of minor damage identification. In a practical application, the available damage detection approaches are not fully capable of quickly sensing and accurately identifying the realistic damage in structures. Research on damage identification from strain data is an interesting topic in recent days. Some work on the cross-correlation approach is now a centre of attraction and strictly confined to bridge or symmetric structures. The present paper uses strain data to validate the cross-correlation approach for detecting damage to building structures. The effectiveness of the methodology has been illustrated firstly on a simply supported beam, then on a 5-storey steel frame and a 6-storey scaled-down reinforced concrete shear building and lastly on a frame structure with moving load as a special case. The results show that this approach has the potential to identify damages in different kinds of civil infrastructure. [ABSTRACT FROM AUTHOR]

Multi-segment shell typical of underwater vehicles is currently important underwater equipment. Stealth performance is a key indicator to improve the working capacity of underwater vehicles. Low-noise design is an important means to improve stealthy performance. Accurate and effective identification of its structure Frequency Response Function (FRF) is the premise of low-noise design. However, the multi-section shell joint interface has many connection points, complex transmission path and typical nonlinear characteristics, which challenges the acquisition of its structural FRF. The Receptance Coupling Substructure Analysis (RCSA) is used to segment the multi-stage shell, which can realize the integration of finite element method and experimental method, and provide an effective way to obtain the FRF of multi-segment shell. At the same time, the multi-segment shell is a typical thin-wall structure, and its actual buckling process under impact is a nonlinear (large deformation) process. Therefore, the nonlinear joint interface is constructed based on linear connection, and the RCSA for nonlinear joint interface is studied. On the basis of accurately obtaining the FRF, it is of theoretical and application significance to study the low noise design of shell. [ABSTRACT FROM AUTHOR]

The substructure method based on the frequency response function is an effective method to solve the frequency response function of locally variable structure, which is especially suitable for the design of vibration absorber parameters in the inertial navigation vibration reduction system. In this paper, one inertial navigation vibration reduction system was taken as an example, and its random dynamic response model was established by using the substructure method based on the frequency response function. Then the influence of the variation of the stiffness and damping parameters of the vibration absorber on the random vibration response of the system was analyzed. The results showed that: (1) it is difficult to effectively reduce the vibration response of inertial navigation vibration reduction system by simply changing the axial stiffness of the vibration absorber, but probably increasing the response in other directions; (2) the vibration response of the inertial navigation vibration reduction system will decrease with the increase of damping, which indicates that the response of the inertial vibration reduction system mainly includes medium and low frequency. The numerical example shows that the proposed method is suitable for the optimization of vibration absorber parameters in the inertial navigation system.

Uncertainty propagation of the frequency response function is crucial for vibration problems such as model calibration, and the probability assessment is another indispensable item in structural optimization. Considering only one of them will inevitably lead to a lack of uncertain knowledge of practical engineering structures. However, it is still challenging to evaluate the system response bounds and probability characteristics simultaneously and maximally reduce the computational cost. This paper focuses on the frequency response function involving correlated uncertainties and proposes a novel uncertainty propagation and probability assessment method. First, a convex model was established to quantify the correlated uncertainty parameters, and then, the Chebyshev polynomial function was developed as the surrogate model to efficiently quantify the uncertainty propagation from uncertainty parameters to system response. Subsequently, the novel normalized and coordinated transformation combined the uncertainty propagation method, making the uncertainty system response easy to assess. Note that the response value at the interpolation point can be employed as the input of probability assessment. One can also estimate the probability characteristics at different frequency positions during the construction of the surrogate model. Finally, two numerical examples were presented to demonstrate the effectiveness and cheaper computation by a discrete system and a continuous system, respectively. Results indicate that the proposed method can be conveniently and accurately applied to assess the bounds and probability characteristics of frequency response function involving correlated uncertainties. [ABSTRACT FROM AUTHOR]

The spindle tool is an important module of the machine tool. Its dynamic characteristics directly affect the machining performance, but it could also be affected by thermal deformation and bearing preload. However, it is difficult to detect the change in the bearing preload through sensory instruments. Therefore, this study aimed to establish a digital thermal–mechanical model to investigate the thermal-induced effects on the spindle tool system. The technologies involved include the following: Run-in experiments of the milling spindle at different speeds, the establishment of the thermal–mechanical model, identification of the thermal parameters, and prediction of the thermal-induced preload of bearings in the spindle. The speed-dependent thermal parameters were identified from thermal analysis through comparisons with transient temperature history, which were further used to model the thermal effects on the bearing preload and dynamic compliance of the milling spindle under different operating speeds. Current results of thermal–mechanical analysis also indicate that the internal temperature of the bearing can reach 40 °C, and the thermal elongation of the spindle tool is about 27 µm. At the steady state temperature of 15,000 rpm, the bearing preload is reduced by 40%, which yields a decrease in the bearing rigidity by approximately 16%. This, in turn, increases the dynamic compliance of the spindle tool by 22%. Comparisons of the experimental measurements and modeling data show that the variation in bearing preload substantially affects the modal frequency and stiffness of the spindle. These findings demonstrated that the proposed digital spindle model accurately mirrors real spindle characteristics, offering a foundation for monitoring performance changes and refining design, especially in bearing configuration and cooling systems. [ABSTRACT FROM AUTHOR]

The paper presents a method for determining the dynamic response of systems containing viscoelastic damping elements with uncertain design parameters. A viscoelastic material is characterized using classical and fractional rheological models. The assumption is made that the lower and upper bounds of the uncertain parameters are known and represented as interval values, which are then subjected to interval arithmetic operations. The equations of motion are transformed into the frequency domain using Laplace transformation. To evaluate the uncertain dynamic response, the frequency response function is determined by transforming the equations of motion into a system of linear interval equations. Nevertheless, direct interval arithmetic often leads to significant overestimation. To address this issue, this paper employs the element-by-element technique along with a specific transformation to minimize redundancy. The system of interval equations obtained is solved iteratively using the fixed-point iteration method. As demonstrated in the examples, this method, which combines the iterative solving of interval equations with the proposed technique of equation formulation, enables a solution to be found rapidly and significantly reduces overestimation. Notably, this approach has been applied to systems containing viscoelastic elements for the first time. Additionally, the proposed notation accommodates both parallel and series configurations of damping elements and springs within rheological models. [ABSTRACT FROM AUTHOR]

Because robotic milling has become an important means for machining significant large parts, obtaining the structural frequency response function (FRF) of a milling robot is an important basis for machining process optimization. However, because of its articulated serial structure, a milling robot has an enormous number of operating postures, and its dynamics are affected by the motion state. To accurately obtain the FRF in the operating state of a milling robot, this paper proposes a method based on the structural modification concept. Unlike the traditional excitation method, the proposed method uses robot joint motion excitation instead of hammering excitation to realize automation. To address the problem of the lack of information brought by motion excitation, which leads to inaccurate FRF amplitudes, this paper derives the milling robot regularization theory based on the sensitivity of structural modification, establishes the modal regularization factor, and calibrates the FRF amplitude. Compared to the commonly used manual hammering experiments, the proposed method has high accuracy and reliability when the milling robot is in different postures. Because the measurement can be performed directly and automatically in the operation state, and the problem of inaccurate amplitudes is solved, the proposed method provides a basis for optimizing the machining posture of a milling robot and improving machining efficiency. [ABSTRACT FROM AUTHOR]

The damping of spruce wood is analysed at different moisture content levels for the first three vibration modes of tangentially and radially vibrating samples. Two methods were used to determine the damping. The first was the vibration envelope fitting as an improved version of the well-known logarithmic decrement, and the second was the newer and recently increasingly used wavelet transform. Both methods showed that the damping of spruce wood first decreases and then increases with moisture content, with the damping in the first vibration mode being about 9% higher in the radial direction than in the tangential direction. In the second and third vibration modes, the damping in the tangential direction was higher than in the radial direction by about 10% and 8.8%, respectively. The measured damping factors from the envelope fitting had, on average, 15.9% higher values than those from the wavelet transform. It can be concluded from the results that the wavelet transform is more accurate for determining the damping factor, as it enables the decoupling of multi-degree of freedom systems if mode coupling is present. [ABSTRACT FROM AUTHOR]

The dynamic behavior of the large-pitch screw during turning affects the stability of the cutting process, which in turn impacts the machining quality of the large-pitch screw. The large-pitch screw turning system among the machine tool, cutting tool, and the workpiece is taken as the present research object, and the frequency response function modeling of the large-pitch screw turning process system is carried out. The concept of generalized modal field and generalized stiffness field of large-pitch screw turning process system is introduced. Considering the dynamic change of the whole process system with the change of tool position, the dynamic characteristic information of the processing system is obtained and analyzed and ultimately reflects the inherent properties of the large-pitch screw turning process system and the ability to resist deformation. The cutting stability prediction model based on support vector machines (SVM) is established, and the average prediction error is 5.04%. The artificial bee colony algorithm is used to optimize the cutting parameters, and finally, the optimization method of large-pitch thread cutting stability based on SVM is proposed. This method can reduce the cutting vibration and effectively improve the cutting stability. [ABSTRACT FROM AUTHOR]

Purpose: In many engineering structures, undesirable harmonics can occur due to changes in operating conditions. The effects of these unwanted harmonics can be eliminated by assigning the zeros of point or cross Frequency Response Functions (FRFs) of the structure to the frequencies of the unwanted harmonics. In this study, a method is proposed for assigning the zeros of point or cross FRFs to particular frequencies. Methods: The method is based on Sherman–Morrison (SM) formula known in matrix theory. The inverse structural modification problem was investigated by making spring modifications on a structure to assign zeros. Results: The required modifications are calculated by solving the set of equations containing nonlinear combinations of the spring coefficients. Conclusion: There is no need for a physical or modal model of the structure because FRFs are used directly in the method. Therefore, it is quite practical in applications. The accuracy and efficiency of the presented method are illustrated by different numerical simulations and successful results are obtained. [ABSTRACT FROM AUTHOR]

The dynamic substructuring design describes the dynamic responses in the modal, frequency and physical domains using coupling methods at joint nodes. This paper presents dynamic structuring methods to modify existing substructuring methods by merging the concept of interfacial forces in the satisfaction of compatibility conditions at joint nodes. The algorithms correspond to a type of model reduction approaches that use a few modes of substructures with the assumption of pseudo-masses at the joint nodes of the substructures. It was verified through numerical examples that the pseudo-masses rarely affected the dynamic responses of the synthesized structure. A frequency response function-based substructuring method was derived by applying interface forces at joint nodes to frequency response function (FRF). The proposed dynamic substructuring methods are expressed in explicit forms for synthesizing substructures without any numerical schemes. The validity of the proposed method was illustrated using two numerical examples. The limitations of this study are evaluated using numerical examples.

The use of digital image correlation for modal analysis is becoming an appealing option thanks to its non-contact and full-field measurement process. However, frequency response function (FRF) estimation can be challenging due to the limited number of time domain data and heavy measurement noise. Thereby, the present work aims to propose a method which improves the estimation accuracy of point-wise FRFs. Firstly, a Gaussian-process-based spatial-frequency model is proposed, which makes use of the intrinsic properties of the FRF and the local spatial information of field measurements. Then, a Bayesian solution is developed, which is enforced by a stable and efficient numerical procedure. Finally, the effectiveness of the proposed method is verified by making a comparison with the spectral estimator through the use of simulated data, and it is further validated based on an experimental application.

Purpose: This study presents a nonlinear transformation-based data augmentation approach to diagnose delaminations such as interlaminar, through-hole and buckle, in relatively complex composites. The approach uses tailored vibration signals of relatively simplified composites and employs a convolutional neural network (CNN). Methods: The present study involves modeling several composite structures with and without interlaminar, through-hole and buckle delaminations using finite-element method (FEM)-based simulation. A vibration experiment is also conducted for the case of interlaminar delamination. In our method, it is assumed that all vibration signals of simplified composites and healthy condition of complex real composites are obtained that paves the way to investigate the delamination conditions of real composites. Then, the present approach adopts the nonlinear transformation method mapping the signals of complex healthy composites and simplified healthy composites. The difference between the transformed signals of complex real composites and the reference signals of simplified composites is utilized for the virtual spectrogram computed by the short-time Fourier Transform (STFT). These virtual spectrograms fed into a CNN-based diagnosis system to diagnose the types and locations of delaminations. Results: For interlaminar cases, the simulation and experimental vibration signals are diagnosed with 100 and 92.5%, respectively. Through-hole and buckle cases are both diagnosed with an accuracy of 100%. Using nonlinear transformation to classify vibration signals prior to diagnosis has been shown to be highly advantageous for delamination diagnosis. Conclusion: The results allow the diagnosis of three cases of complex composites with simplified composites using the delamination diagnosis system. [ABSTRACT FROM AUTHOR]

In many mechanical systems, it is very common to bring together different structural elements or subsystems produced by different producers and create a whole coupled system. It is often not possible to manufacture an entire mechanical system in one place. Although the dynamic properties of each of the subsystems produced by different manufacturers are known, it is a matter that should be known and studied how the dynamic behavior of the new system will be after the creation of a new system by combining these subsystems. In this study, a method for structural coupling of mechanical systems is presented in order to contribute to the solution of structural dynamic problems. It is based on Sherman–Morrison formula known for solving mathematical inverse problems of modified matrices. The method is very useful and practical for real mechanical engineering applications due to only the frequency response functions belong to the coupling coordinates of the subsystems are used. The main highlight of the presented method is there is no need a matrix inversion for calculations. [ABSTRACT FROM AUTHOR]

The paper examines the possibility of using a nonlinear Quasi-Zero-Stiffness (QZS) spring in a vehicle suspension. The response of a Single Degree of Freedom (SDOF) model to harmonic base excitations is obtained which may be unstable and unbounded depending on the excitation level and the damping ratio. This is followed by obtaining the response of the SDOF model to harmonic base excitation with an amplitude that is varying according to a road profile spectrum. Such dependency of the excitation amplitude to the frequency changes the qualitative behaviour of the nonlinear system and the responses would be always bounded. The QZS spring also improves the isolation of the system. Transient responses of a quarter car model with a QZS suspension to a road hump and random road profile are investigated. The maximum acceleration of the vehicle with the QZS suspension passing over the speed hump is considerably lower than a vehicle with a conventional linear suspension. Wb weighted RMS of acceleration (BS 6841-1987) is also lower by as much as 14% for a vehicle with the QZS suspension travelling at 30 km/h on a class E road compared to its linear counterpart. [ABSTRACT FROM AUTHOR]

Nonlinear energy sinks (NES) have been shown to have broadband absorption for multiple modes. Furthermore, linear dynamic absorbers (LDA) often only operate effectively within located frequency ranges and are unable to achieve broadband absorption. Therefore, this manuscript first proposed using a hybrid LDA and NES to design the circular ring absorption system. Compared with LDA, broadband absorption could be achieved through configuring and tuning linear stiffness. Both the linear and nonlinear stiffnesses are introduced by adding vertical and horizontal springs. The linear and nonlinear stiffness effects on the circular ring absorption system are investigated simultaneously. The dynamic model of the circular ring absorption system with hybrid LDA and NES is constructed, and the approximate analytical solution is obtained using the harmonic balance analysis. The expressions of frequency response and force transmissibility are given, and the analytical results are verified numerically. By comparing the amplitude-frequency response curves of the circular ring with LDA only, NES only, and a hybrid LDA and NES, it is confirmed that a hybrid LDA and NES can better control the forced vibration of the circular ring absorption system. Hybrid LDA and NES were shown to suppress main resonance peaks of the circular ring absorption system, making the device resistant to misalignments. In a linear system, this can only be achieved by resonance matching, so a hybrid LDA and NES have the same effect as adding multiple linear absorbers. Some properties of damping, nonlinear stiffness, and mass ratios in a hybrid LDA and NES are investigated. These parameters determine the specific shape of the force transmissibility curve of the ring absorption system. It is found that a proper increase in the mass ratio of the hybrid LDA and NES can obtain a better vibration reduction effect; adjusting the damping ratio and nonlinear stiffness ratio of the hybrid LDA and NES can improve the vibration reduction effect to a certain extent. Based on the analysis of the parameters effects of hybrid LDA and NES on vibration control, the parameters of hybrid LDA and NES are compared. The analysis of the results demonstrates that by selecting reasonable damping, nonlinear stiffness, and mass ratios, the frequency band of the circular ring absorption system can be broadened. Finally, an experiment is performed to validate the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

Purpose: Owing to the thin-walled structure of blade, machining chatter occurs easily in blade machining processes. Moreover, material removal in machining has a non-negligible effect on the modality of the blade and can intensify the occurrence of machining chatter. Methods: To solve this problem, we introduce a method of applying tension on the blade to reduce the peak value of the frequency response function and improve the machining stability. In this study, the mode shape diagram and natural frequency of the initial thin-walled blade are analysed using finite element software. Subsequently, the influence of material removal on the frequency response function of the blade is studied. Additionally, the machining stability of a thin-walled blade is predicted by introducing a prediction model of stability by considering the effect of material removal on the modal parameters. Furthermore, the response displacement of the blade under the application of tensile force is investigated, and the stability lobe diagram is plotted based on frequency response function analysis. Results and conclusions: The results show that applying tensile force on thin-walled blades is an effective method to reduce the system response displacement and improve the machining stability. It is well known that the machining stability limits of a blade can be improved with tensile force. To validate the prediction results, a device for applying tensile force to thin-walled blades is designed, and modal measurement and blade machining experiments are conducted. [ABSTRACT FROM AUTHOR]

Abstract Composite materials have attracted researchers in vibration and noise control applications due to their significant dynamic characteristics such as high strength and high damping level. In this paper, a Glass Fiber Reinforced Composite material (GFRC) is presented as a vibration isolation system to control vibration levels in industry. In addition, the impact of integration of a mechatronic control system to improve the machining process and increase the control of vibration nature. A prototype of an industrial cam–follower machine is motorized, and the Frequency Response Function (FRF) is recorded using a B&K data acquisition analyzer at five rotational speeds. The transmitted vibrations to the machine foundation are estimated without any isolation system. Then, two optimized GFRC plates of optimum stacking sequences are used as an isolation system to reduce the transmitted vibration. The displacement transmissibility is calculated theoretically and experimentally. The results show that the use of GFRC plates as an isolator reduces the vibration level of the system by 98.46% and 98.5% for [90/90/90/0/0]s and [90/ ± 45/ ± 35/90/ ± 35]s GFRC configurations respectively.

Finite element model updating provides an important supplement for finite element modelling. However, some studies have shown that if the tested structure involves local nonlinearities due to damages, material properties and large deformation et al., it is difficult to achieve an accurate modified model using conventional model updating methods that are based on the assumption of linear structures. To address this issue, a two-stage model updating method separating the effects of local nonlinearities is proposed in this paper. Firstly, the underlying linear frequency response function is obtained by using the conditioned reverse path method. Then, combined with the Sherman-Morrison-Woodbury formula and the model updating objective function established by the frequency response function similarity metric, then structural model updating and damage detection are carried out as the second stage. Three numerical examples are given to illustrate the effectiveness of the proposed method. This method can not only accurately identify the location and quantify the extent of structural damages, but also has the advantages of not based on sensitivity, not depending on the selection of frequency points, not repeatedly calling the initial model et al. The proposed method has high computational efficiency and avoids the numerical problems often encountered by conventional frequency response function-based model updating methods. [ABSTRACT FROM AUTHOR]

Composite materials have attracted researchers in vibration and noise control applications due to their significant dynamic characteristics such as high strength and high damping level. In this paper, a Glass Fiber Reinforced Composite material (GFRC) is presented as a vibration isolation system to control vibration levels in industry. In addition, the impact of integration of a mechatronic control system to improve the machining process and increase the control of vibration nature. A prototype of an industrial cam–follower machine is motorized, and the Frequency Response Function (FRF) is recorded using a B&K data acquisition analyzer at five rotational speeds. The transmitted vibrations to the machine foundation are estimated without any isolation system. Then, two optimized GFRC plates of optimum stacking sequences are used as an isolation system to reduce the transmitted vibration. The displacement transmissibility is calculated theoretically and experimentally. The results show that the use of GFRC plates as an isolator reduces the vibration level of the system by 98.46% and 98.5% for [90/90/90/0/0]s and [90/ ± 45/ ± 35/90/ ± 35]s GFRC configurations respectively.Article Highlights: Composite materials play a significant role in the isolation and control of vibration levels of the industrial cam–follower machine for various rotational speeds. In the absence of an isolation system, high levels of vibration energy were transmitted to the foundation of the machine with a response of 22.7*103 [(m/s2)/v]. With integrating a mechatronic control system with the GFRC isolation system, the isolation of transmitted vibration increases by 65% and 62.5% when using composite isolators of configurations [90/90/90/0/0]s and [90/ ± 45/ ± 35/90/ ± 35]s respectively. [ABSTRACT FROM AUTHOR]

Anomaly detection and section estimation methods using multiplexed fiber Bragg gratings (FBGs) have been developed to identify propellant supply anomalies rapidly and autonomously in spacecraft propulsion systems. FBGs were used to measure the external strain of the pipe due to internal pressure fluctuations in the pipe during the pressure response caused by a water hammer. The measured strain was substituted into the constructed conversion formula to obtain the differential pressure through the FBGs. As a result, compared with the differential pressure measured by a conventional pressure sensor at the same position, the error was 2% for the maximum pressure of the water hammer and 1% for the natural frequency, indicating high applicability as a differential pressure sensor in piping with FBG simply wrapped around the outside of the pipe. Since the peak intensity and frequency of the measured values in the frequency domain change during flow anomaly and bubble localization, the frequency response function (FRF) between response data of the section to be detected was calculated, and the least-squares error of the FRF was used as the degree of anomaly. As a result, flow anomalies and bubble localization became detectable and sectional estimation was possible. • Multiplexed FBGs enable highly accurate anomaly detection for spacecraft propulsion systems. • Simple and accurate pressure dynamic response measurement method realized by FBGs. • Anomaly evaluation and detection by frequency response function between normal and target data. • Estimation of anomaly occurrence section by round robin comparison of anomaly degree. [ABSTRACT FROM AUTHOR]

Inclined pile groups have been adopted in engineering practice (e.g. offshore wind turbines) to withstand strong horizontal environmental loads that are repetitive in nature. Field reactions including the additional stresses and excess pore water pressures are responsible for the cyclic degradation of foundation soil, which may induce the accumulative deformation and fundamental frequency shifting of the marine structure. In this study, a theoretical model incorporating an inclined pile group and a saturated half-space was established, wherein the pile and saturated soil were described by the E–B beam and Biot poroelasticity theories, respectively. The pile and soil were coupled through the receptance matrix, which was established at the pile–soil interaction points in the transform domain. The dynamic impedances of the inclined pile group are first obtained, and then the field responses, including stresses and excess pore water pressure in the saturated ground, are evaluated for different types of loads applied to the pile cap. The frequency response function and spatial distributions of the field responses were obtained for varying rake angle of pile and permeability of soil. It was found that the horizontal force induces the largest responses in the ground when compared to the vertical force and the overturning moment applied to the inclined pile group. It is beneficial to employ an inclined pile group to withstand the horizontal force and overturning moment because the foundation stiffness increases, and the additional stresses in the ground decrease with increasing rake angle. [ABSTRACT FROM AUTHOR]

Background: Rubber mounts are widely used to isolate vibrating components. Their complex stiffness characteristics, including dynamic stiffness and loss factors, are highly concerning in terms of vibration analysis and optimization. Rubber mounts show non-linear behavior with preload, leading to difficulty to predict their complex stiffness. Dynamic testing is generally necessary. Objective: An approach to identify the complex stiffness of preloaded rubber mounts in both vertical and horizontal directions simultaneously is developed. Methods: Tested Frequency Response Functions (FRF) of a mass suspended by rubber mounts are transformed to an FRF matrix of the mass center to decouple the Z Degrees of Freedom (DOF) and RZ DOF from other DOFs, which allows complex stiffness to be identified from the two decoupled DOFs. A software tool to implement automatically the FRF transformation and parameter identification is developed. An EPDM rubber mount is tested using the device and its complex stiffness is identified using the software to validate the proposed approach. Results: The driving-point FRFs of the mass center calculated from the identified complex stiffness are very close to the corresponding FRFs determined from the test data. The comparison between the Finite-Element Analysis (FEA) results of the surficial FRFs and the test results shows good consistency as well. Therefore, the proposed approach and its supporting algorithm are validated. Conclusion: the proposed approach allows for swift identification of high-accuracy complex stiffness of preloaded rubber mounts in both vertical and horizontal directions simultaneously. [ABSTRACT FROM AUTHOR]

Frequency response functions (FRFs) are one of the most useful methods for representing machine tool dynamics under force excitation. FRFs are usually obtained empirically through output measurements, and force excitations are controlled by an external device such as hammers or shakers. This study offers an operational identification method that utilizes the calculation of force applied during the process as an input for FRF identification. Force excitation is provided through the face milling of a thin-walled workpiece, and acceleration measurements are taken during the process. The FRF is calculated at a designated position by sampling workpiece-cutting tool contacts as individual tap tests and substituting a force calculation as input. Force coefficients need to be known for the force calculation. An experimental force coefficient identification method is proposed. In that case, a similar thin-walled workpiece at a point with known FRF and acceleration measurements is utilized. Results are confirmed with FRFs obtained in the same location for both FRF identification and force coefficient identification approaches. [ABSTRACT FROM AUTHOR]

Dynamic load of compressors is of great concern to designers of refrigeration products since pipes and sheet metals are easy to be excited, leading to noise or fatigue failure. The excitation is impossible to measure directly and difficult to calculate according to generation mechanism; thus, dynamic load identification which skips the mechanism is a more practicable way. However, the reported approaches have the problems of inaccuracy and clumsy. Therefore, an easy-to-implement approach is proposed, in which the accelerations and the frequency response functions from test are transformed to an arbitrarily given point based on rigid body dynamics, and then the equivalent six-degrees of freedom dynamic load of this point is calculated. This approach is validated using a motor with eccentric mass, whose centrifugal force is identified with the proposed approach and also calculated in analytical way. Comparison indicates that the error is within 5% except for the rotational speeds close to natural frequencies. Curve fitting or interpolation is recommended to correct the data in order to reduce the influence of natural frequencies. Cases studies with a rotary compressor and a scroll compressor respectively are presented. For the rotary compressor, the force magnitudes in X and Y directions are almost the same and pretty close to their quadratic polynomial fitting curves with zero intercept respectively, conforming to the characteristics of centrifugal force; the RZ component is larger than the RX and RY components as expected, but the latter two are not negligible; and there is a step change at 45 rps due to the removal of torque compensation. For the scroll compressor, the force magnitudes in X and Y directions are different, but very close to their quadratic polynomial fitting curves, respectively; the Z component has a jump in high-speed range; and there is no significant difference of dynamic force in the cooling and the heating modes. [ABSTRACT FROM AUTHOR]

The spindle tool is an important module of the machine tool. Its dynamic characteristics directly affect the machining performance, but it could also be affected by thermal deformation and bearing preload. However, it is difficult to detect the change in the bearing preload through sensory instruments. Therefore, this study aimed to establish a digital thermal–mechanical model to investigate the thermal-induced effects on the spindle tool system. The technologies involved include the following: Run-in experiments of the milling spindle at different speeds, the establishment of the thermal–mechanical model, identification of the thermal parameters, and prediction of the thermal-induced preload of bearings in the spindle. The speed-dependent thermal parameters were identified from thermal analysis through comparisons with transient temperature history, which were further used to model the thermal effects on the bearing preload and dynamic compliance of the milling spindle under different operating speeds. Current results of thermal–mechanical analysis also indicate that the internal temperature of the bearing can reach 40 °C, and the thermal elongation of the spindle tool is about 27 µm. At the steady state temperature of 15,000 rpm, the bearing preload is reduced by 40%, which yields a decrease in the bearing rigidity by approximately 16%. This, in turn, increases the dynamic compliance of the spindle tool by 22%. Comparisons of the experimental measurements and modeling data show that the variation in bearing preload substantially affects the modal frequency and stiffness of the spindle. These findings demonstrated that the proposed digital spindle model accurately mirrors real spindle characteristics, offering a foundation for monitoring performance changes and refining design, especially in bearing configuration and cooling systems.

The paper presents a method for determining the dynamic response of systems containing viscoelastic damping elements with uncertain design parameters. A viscoelastic material is characterized using classical and fractional rheological models. The assumption is made that the lower and upper bounds of the uncertain parameters are known and represented as interval values, which are then subjected to interval arithmetic operations. The equations of motion are transformed into the frequency domain using Laplace transformation. To evaluate the uncertain dynamic response, the frequency response function is determined by transforming the equations of motion into a system of linear interval equations. Nevertheless, direct interval arithmetic often leads to significant overestimation. To address this issue, this paper employs the element-by-element technique along with a specific transformation to minimize redundancy. The system of interval equations obtained is solved iteratively using the fixed-point iteration method. As demonstrated in the examples, this method, which combines the iterative solving of interval equations with the proposed technique of equation formulation, enables a solution to be found rapidly and significantly reduces overestimation. Notably, this approach has been applied to systems containing viscoelastic elements for the first time. Additionally, the proposed notation accommodates both parallel and series configurations of damping elements and springs within rheological models.

Measuring the Frequency Response Functions (FRF) at the tool-tip is essential for the identification of chatter-free machining conditions. The tool-tip FRF in CNC machines are usually measured by impulse hammer tests in idle conditions, and the measured FRF remain relatively unchanged under operational conditions. This method is not effective in robotic machining, because the robot's vibration response in idle and operational conditions are significantly different. The robot's vibration response is pose-dependent and nonlinear and therefore strongly dependent on the operational conditions. This paper presents new methods for measuring the TCP (tool-tip) FRF of machining robots under operational conditions. In-process FRF are measured by leveraging the milling forces as the excitation source, and two approaches are proposed to achieve broadband, uncorrelated, and sufficiently exciting forces: i) milling of porous materials to generate randomized cutting forces, and ii) milling of a homogeneous material with spindle speed sweep. In the latter approach, the periodic content of cutting forces is used for excitation while in the former approach excitation by the random content is considered. A table dynamometer is used to measure the excitation (milling) forces and accelerometers are used to measure the resulting vibrations. The measured in-process FRF are then used to develop the chatter stability lobes diagram of the process, which determine the chatter-free combinations of the cutting depth and spindle speed for milling. Chatter experiments are conducted to confirm that the stability diagrams are more accurate when the presented in-process FRF measurements are used instead of the FRF measured in idle conditions. [ABSTRACT FROM AUTHOR]

In this study, optimum parameters of a variant tuned mass damper (V‐TMD) wherein, damper is connected to the ground are obtained using equal eigenvalue (EEV) criterion. In EEV, TMD parameters are obtained by equating eigenvalues of two modes. By equating eigenvalues, the modal damping ratios (i.e., negative real part of the eigenvalue) become equal and maximum in two modes. This optimum condition is achieved by finding the roots of a sixth order polynomial. One of the novel findings is regarding the existence of two sets of optimum parameters, out of which, one set matches exactly with one of the earlier study. Regions in which, single and two sets of optimum parameters exist, are clearly brought out. Performance of V‐TMD is demonstrated for harmonic as well as earthquake excitation. It is noted that displacement as well as acceleration of the main system reduces due to V‐TMD. Another significant finding is that the real part of eigenvalues of V‐TMD is larger than that of classical tuned mass damper (C‐TMD), which leads to faster decay of earthquake response and lower peak response. The optimum parameters obtained in this study, pertain to linear SDOF and V‐TMD. [ABSTRACT FROM AUTHOR]

Ultrasonic welding (USW) is recognized as a sustainable and green manufacturing process due to its high energy efficiency, cleanliness, and excellent welding attributes. The performance of USW majorly depends on small end vibration amplitude (SEVA), achieved through a sonotrode, whose design has been the focus of many researchers. A high SEVA is essential for achieving excellent welding attributes, minimizing wastage, and ensuring a clean environment. In this study, a novel USW sonotrode was designed to enhance the output amplitude of the Langevin ultrasonic transducer (LUT). The sonotrode was evaluated for eigenfrequency characterization and harmonic excitation response through finite element analysis. The performance of this novel ultrasonic transducer was evaluated in terms of SEVA and various types of stresses at an operating frequency close to but higher than the LUT's axial modal frequency. Variations in axial displacement and Von Mises (VM), axial, tangential, radial, principal, and shear stresses were examined along the length of the tool for titanium, aluminum, and steel sonotrodes. The SEVA of the aluminum sonotrode was 136.72% and 5.87% higher than that of the titanium and steel sonotrodes, respectively. The equivalent VM stress in the steel sonotrode was 159.2% and 379.2% greater than their aluminum and titanium counterparts. Selected cases from the present study were compared with prior research, revealing a reasonable agreement. This work provides insights into the performance of sonotrodes in USW, offering potential paths for improving the effectiveness of this sustainable manufacturing process. [ABSTRACT FROM AUTHOR]

Mechanical impedance (MI) defines the ability of a system to vibrate as a consequence of force application. In the recent years, the correlation of this parameter with tire-road noise and other characteristics has gained certain attention. Nevertheless, the information about this topic is still insufficient. Usually, the force is set through an impulse hammer as a master and the acceleration is measured through an accelerometer as a response in order to measure the corresponding Frequency Response Function (FRF). The objectives of the study presented in this paper are i) to analyse the differences between the axial mechanical impedance (complex ratio of force and velocity referred to the same point, named driving-point impedance) and the non-axial mechanical impedance (complex ratio of the force at the point i and velocity at the point j, named transfer impedance); ii) to analyse the effect of adding crumb rubber (2% by mixture weight) and of the percentage of bitumen on the mechanical impedance for the bituminous samples. Therefore, laboratory tests on asphalt concrete specimens have been performed, using an instrumentation system composed of i) an impact hammer reporting the impact force value; ii) an impedance head measuring the direct impact force and the direct acceleration at the hitting point location; iii) a piezoelectric accelerometer measuring the transfer acceleration at a certain distance from the hitting point location. Results demonstrate that the ratio between the repeatability and the average is quite constant, while for heights higher than 10 cm, also MI tends to be independent on the height. A number of recommendations have been made based on the results of the present research.

Composite sandwich structures are gaining attention due to their inherent properties, such as lightweight, low density, and high strength. The forced vibration response of these structures was studied experimentally to investigate the effects of external loads on these structures. In this work, four composite sandwich structures were manufactured using carbon fiber, glass fibers, and foam and tested on a specially designed vibration test rig by hitting the specimen with an impact hammer. The response was recorded by an accelerometer attached to the specimens. The accelerometer signal was amplified, and the input and output signals were transferred to LABVIEW via a data acquisition card and were processed in MATLAB. The impact hammer acts as an external excitation source, and the frequency response function was found for each specimen under various edge boundary conditions. Bode plots were plotted for each test, and the peak frequency and the phase difference were compared. It was found that composite sandwich specimens made of carbon fiber skins and carbon fiber honeycomb core showed a higher frequency response among all specimens (400 Hz). Furthermore, it was found that the foam core layer reduces the phase difference between the input and output signals from (360degrees) to (180degrees) compared with other honeycomb cores. Therefore, the procedure outlined in this research can be applied to other structures to investigate their vibration response. In addition, this work could be beneficial for the diagnosis of structure stability using a forced vibration response procedure.

Abstract Thin-walled parts are typically difficult-to-cut components due to the complex dynamics in cutting process. The dynamics is variant for part during machining, but invariant for machine tool. The variation of the relative dynamics results in the difference of cutting stage division and cutting parameter selection. This paper develops a novel method for whole cutting process optimization based on the relative varying dynamic characteristic of machining system. A new strategy to distinguish cutting stages depending on the dominated dynamics during machining process is proposed, and a thickness-dependent model to predict the dynamics of part is developed. Optimal cutting parameters change with stages, which can be divided by the critical thickness of part. Based on the dynamics comparison between machine tool and thickness-varying part, the critical thicknesses are predicted by an iterative algorithm. The proposed method is validated by the machining of three benchmarks. Good agreements have been obtained between prediction and experimental results in terms of stages identification, meanwhile, the optimized parameters perform well during the whole cutting process.

The nonlinearities present in engineering structures are generally local in nature. Hence for system identification, it is required first to identify the spatial location and then the nonlinearities are characterized. Once the nonlinearity is characterized, the system identification involves identifying coefficient of nonlinear terms. In the present work, an improved version of traditional conditioned time and frequency domain reverse path method for nonlinear parameter estimation of multiple-degree-of-freedom structures is presented. Both numerical and experimental studies have been carried out to investigate the effectiveness of the proposed improved time and frequency domain parameter estimation techniques over the traditional ones. The comparisons between the time and frequency domain reverse path approaches are also carried out to highlight the strengths and weaknesses of each of these approaches. Numerical and experimental investigations reveal that both the time and frequency domain approaches proposed in this paper overcomes the major limitations of their traditional counterparts and are capable of estimating the nonlinear coefficients of multiple nonlinear attachments either grounded or attached between the masses. [ABSTRACT FROM AUTHOR]

Surrogate models are used to alleviate the computational burden in engineering tasks, which require the repeated evaluation of computationally demanding models of physical systems, such as the efficient propagation of uncertainties. For models that show a strongly non‐linear dependence on their input parameters, standard surrogate techniques, such as polynomial chaos expansion, are not sufficient to obtain an accurate representation of the original model response. It has been shown that for models with discontinuities or rational dependencies, for example, frequency response functions of dynamic systems, the use of a rational (Padé) approximation can significantly improve the approximation accuracy. In order to avoid overfitting issues in previously proposed standard least squares approaches, we introduce a sparse Bayesian learning approach to estimate the coefficients of the rational approximation. Therein the linearity in the numerator polynomial coefficients is exploited and the denominator polynomial coefficients as well as the problem hyperparameters are determined through type‐II‐maximum likelihood estimation. We apply a quasi‐Newton gradient‐descent algorithm to find the optimal denominator coefficients and derive the required gradients through application of ℂℝ$$ \mathit{\mathbb{CR}} $$‐calculus. The method is applied to the frequency response functions of an algebraic frame structure model as well as that of an orthotropic plate finite element model. [ABSTRACT FROM AUTHOR]

Microvibrations are one of the main factors contributing to platform jitter and a decline in pointing stability and precision. Among various disturbance sources, reaction wheel assembly (RWA) is one of the most significant ones and has drawn the interest of numerous scholars. How to evaluate the influence of the disturbances of RWA on the pointing accuracy of a spacecraft is an arduous task because it involves multiple disciplines. The acquisition of the frequency response function (FRF) from the disturbance source to the sensitive payload is one of the most crucial stages in integrated modeling. Direct measurement of the FRF in the six directions is challenging because of the restricted room for RWA installation in a spacecraft, particularly in small satellites. In this paper, a general method based on the Hv algorithm to obtain the FRF is presented. This method only needs the RWA, itself, as an excitation source. Then, in order to acquire the FRF, we use an optical remote-sensing satellite as the research object. The peak positions of FRF obtained by different RWAs are basically the same, while the amplitudes are slightly different, indicating that this method is effective. This method takes into consideration the coupling between the RWA and the spacecraft, making it possible to multiply the RWA disturbance measured at a fixed interface with the FRF, to determine the image motion of the sensitive payload. [ABSTRACT FROM AUTHOR]

The mechanized harvesting of coffee fruits is performed by machines that uses the mechanical vibrations principles and has been widely adopted due the operational cost reductions compared to manual way. Maturation stages, frequency and amplitude of vibration influence the fruits detachment during the mechanized harvesting by mechanical vibrations. In this context, the determination of the modal parameters of the coffee plant is essential, considering, the full and selective harvesting. Modal analysis can be performed experimentally by using a specific instrumentation and, from this technique, it is possible to determine the vibration modes, damping factors and natural frequencies of the system in order to identify its dynamical behavior, supporting in the design and set-up of harvesting machines. In this way, this study was developed with the objective of determining and evaluating the modal characteristics of the coffee fruit-peduncle-branch system, such as the natural frequency, ratio and damping coefficient, from impact modal analysis. Additionally, the influence of the maturation stage, branch configurations (with/without leaves and fruits) and plant position in relation to the modal parameters of the system were evaluated. For branches without leaves and with fruits in different stages of maturation, several scenarios were studied. The results obtained verify that branches with ripe fruits (25.51 Hz, 24.19 Hz and 16.00 Hz for upper, middle and lower position in the plant, respectively) generally showed higher mean natural frequencies than branches with unripe fruits (16.77 Hz, 15.30 Hz and 15.49 Hz for upper, middle and lower position in the plant, respectively). For the complete branches with leaves and fruits, no significant differences in the first natural frequency were found between the mean natural frequencies throughout the plant and for different maturation stages. This pattern was repeated for the second and third natural frequencies, except for the difference in the average natural frequency found for branches with ripe fruits in the middle third of the plant. This behavior may be an important finding for the adjustment of harvesting machines since there was no differentiation between the average values of the natural frequency throughout the plant.

In this study, elastic metamaterial sandwich plates with axially deformed Timoshenko beam cores, considering both the out-of-plane and in-plane deformations of the face plates, are designed and the vibration band-gap properties are explored. The beam cores act as local resonators that can bear axial force, bending moment and shearing force. The finite element method (FEM) and the spectral element method (SEM) are combined to create the finite/spectral element hybrid method (FE-SEHM) for establishing the dynamic model and calculating the frequency response functions (FRFs) of the elastic metamaterial sandwich plate with axially deformed beam cores. It is observed that the metamaterial sandwich plate possesses both the axial and transverse vibration band-gaps of the beams, and the two kinds of band-gaps are independent. Compared with the metamaterial sandwich plates with rod cores, those with axially deformed beam cores have more extensive application ranges for vibration reduction. [ABSTRACT FROM AUTHOR]