An attractive feature of FRP structures is that Fiber-Optic Sensors (FOS) can be naturally embedded during the production process and provide important structural information immediately after ...construction. Thus, FOS became very useful also for Vibration-Based Monitoring (VBM) applications. In VBM, damage can be identified by detecting damage-related changes in the modal characteristics of a structure, such as natural frequencies and strain mode shapes. However, natural frequencies can exhibit a low sensitivity to certain types of damage, especially when compared to their sensitivity to temperature. Strain mode shapes though have been proved to be more sensitive to local damage and less to temperature than natural frequencies. In this work, an FRP footbridge is subjected to periodic annual VBM. The dynamic strains of the bridge are monitored with embedded Fiber-optic Bragg Gratings (FBG), a type of FOS that allows determining natural frequencies and strain mode shapes accurately from low-amplitude strain data. Vibration modes are identified from the dynamic strains and the influence of temperature and heavy loading is investigated. The identified modes are used for calibrating a finite element model. Possible damage scenarios on FRP structures are simulated and their influence on modal characteristics is investigated and compared to this of temperature.
This study conducts a fractional factorial design to evaluate the influence of test settings on the results of Experimental Modal Analysis (EMA) which is the method often used to determine natural ...frequencies and damping rate of structures. The structures in this work are simple beams cut from lightweight rail-way vehicle car bodies. The original specimens have very low structural damping, whereas others have been supplemented with Constrained Layer Damping (CLD) patches to significantly increase vibration energy dissipation.
It will be shown that test settings do influence Experimental Modal Analysis (EMA) results in ways that are seldom intuitive or foreseeable. The statistical significance of some factors, or combinations thereof, proves that their effects are not entirely attributable to random phenomena. The results show that a statistical analysis like the one presented in this paper should be conducted prior to extracting accurate damping data from EMA tests. The technique reduces the number of tests needed to assess the influence of settings. This technique, not only highlights the statistically significant factors, but also quantifies their influence, thus helping the researcher make an informed decision regarding test settings.
•The analysis of variance determines which factors do have a significant influence on EMA analysis.•Test settings depend on beam type, modal parameters, and support configuration.•Test settings have a strong influence on modal damping.•The fractional factorial design reduces the number of tests necessary.•The proposed statistical analysis should be conducted prior to extracting accurate damping data.
Transmissibility-based operational modal analysis (TOMA) is a recent research area that has seen its most significant developments over the last two decades. The advantage of this new approach is its ...independence from the excitation spectrum. All methods developed have been based on Response Transmissibility (RT) functions or Power Spectrum Density Transmissibility (PSDT) functions. The RT-based methods identify modal parameters from various load conditions and the PSDT-based methods use a single load condition by combining multiple reference outputs. This work proposes a unified concept for TOMA that relates the scalar RT and PSDT functions. This new concept shows that scalar PSDT functions combine n RT functions due to a single load. Two applications emerge from the unified concept: a) the matrix representation permits to identify modal parameters (natural frequencies, damping ratios and mode shapes) from PSDT functions in only one load condition, b) a procedure based on the joint diagonalization of matrices with PSDT functions from different load conditions allows the identification of RT functions. A numerical simulation and actual field data were used to assess the applications of unified concept. The results demonstrate the capacity of new concept to estimate modal parameters and RT functions of structures in operations conditions.
Vibration-based structural health monitoring of civil structures relies on the repeated identification of dynamic structural characteristics of the structure from output-only vibration data. Natural ...frequencies and displacement mode shapes are the most commonly employed dynamic characteristics; yet their sensitivity to local damage of moderate severity is rather low with respect to their sensitivity to other factors such as temperature, necessitating data normalization. Strain mode shapes offer a higher sensitivity to local damage, but their accurate identification in a dense grid is challenging given the very small dynamic strain levels that are encountered under ambient excitation. In this article, a method is presented for tackling this challenge. It consists of three stages. First, fiber-optic Bragg grating strain sensors are attached to the structure and interrogated with a tunable laser performing a wavelength sweep. In this way, the measured strain amplitudes have the required accuracy but synchronization errors are introduced between the different Bragg sensors. Second, a modal analysis is performed on the dynamic strain data using an accurate parametric system identification technique. This is followed by a synchronization step which compensates for the delays introduced by the wavelength sweep. Finally, the synchronized strain mode shapes are employed as damage-sensitive features, either directly or via a newly proposed quantity, the top-to-bottom strain ratio. The method is validated by progressive damage testing of a complex, prestressed concrete “roof” beam, reinforced with steel fibers. It is observed that the proposed method can identify both the presence and the location of the damage in a relatively early stage.
Abstract Modal parameters (natural frequencies, mode shapes and modal damping) help to understand the dynamic behaviour of complex systems like machine tools. There are several approaches for finding ...the modal parameters. The Experimental Modal Analysis (EMA) has proven to be effective at standstill of a machine tool. The excitation, realized with impulse hammer or shaker, and excited responses at several locations are measured. Alternatively, the Operational Modal Analysis (OMA) can be deployed for finding the modal parameters during operation. Here, responses to excitation resulting from operation are only measured. The modal parameters are mathematically identified from the measured signals in both cases but with different methods. This paper discusses, to what extent both approaches (EMA and OMA) can lead to plausible identification of natural frequencies of a machine tool during milling. Concerning the EMA, attention is paid to capturing the excitation. Process forces can be assumed to be the most significant excitation. However, there are other excitation sources beside the process forces (e.g. drives, hydraulic and pneumatic aggregates), which are considered by this assumption to be a part of disturbances with consequence for the identification of the modal parameters. Regarding the OMA, attention is paid to the fact that the excitation is assumed to be broadband like the white noise. Unfortunately, this assumption does not match the characteristics of a real excitation. This paper contains the identification of natural frequencies of a machine tool during milling within both approaches. The achieved results are compared and discussed.
•Shake table tests were performed on a real-scale structure strengthened with CRM.•Strengthening was designed based on structural features and damage state.•Acceleration and displacement capacity ...were significantly enhanced.•Strengthening was applied to the outer wall surface to minimize inconvenience on occupants.•CRM requires fast installation, includes durable materials and is 30 mm thick.
The seismic vulnerability of unreinforced masonry structures threatens the life and health of people and the conservation of the built heritage in earthquake prone areas, highlighting the need of gaining a deeper understanding of their dynamic behaviour and of developing appropriate strengthening technologies. This work investigates the effectiveness of a low-impact technology consisting of composite reinforced mortar (CRM) applied to the outer surface of masonry walls for enhancing their seismic capacity, without the need to evacuate the building. Shake table tests were performed under natural accelerograms (applied in horizontal and vertical directions) on a real-scale structure, built with tuff blocks and consisting of a façade with a window, two side walls (one of which with a door near the corner) and an inclined timber roof. The mock-up was tested unstrengthened, then repaired and strengthened by applying a glass fibre reinforced polymer mesh embedded in a lime-based plaster to the outer surface of damaged walls. The experimental investigation showed the effects of CRM on damage accumulation, failure mode and on the increase of base acceleration and displacement capacities.
•A novel evaluation method for rolling energy loss of tracked vehcile road wheels.•The identified damping ratios of tire via the half power bandwidth method.•Damping ratios of road wheels correlates ...well with the rolling energy loss.•Deformation and stiffness of road wheels have postive correlation with the rolling resistance.
A novel evaluation method for rolling energy losses of road wheels of tracked vehicle is proposed, in which damping of road wheel surface is identified based on single vibration excitation and single point picking up frequency response function using a simplified modal experiment with an acceleration sensor. Three road wheels of tracked vehicle with different tread rubber are utilized as specimens during the modal experiments. The half power bandwidth method is employed to identify the viscous damping parameters. The damping parameters of road wheels are ranked by their values, and then these values of every road wheels are compared with their corresponding rolling energy losses to validate their correlative relationship. Moreover, the nonlinear deformation and stiffness of road wheels are investigated about their correlation with the rolling energy losses through model development and experimental validation. The results prove that the damping ratios of road wheels are correlated well with the rolling energy losses for all the three road wheels. The proposed evaluation method could effectively evaluate the rolling energy losses of road wheels, which suggests a simplified and economical alternative over the conventional rolling energy losses experimental method of road wheels.
A notable exception to percussion instruments not having harmonic overtones are drums belonging to the Indian musical family, particularly Tabla and Mridangam. A common aspect in both these ...instruments is the presence of an applied mass on the membrane to change its density distribution. While considerable work has been done to improve such membrane models for Tabla, relatively less has been done for Mridangam. It has been shown in past literature that the first few overtones are nearly and almost exclusively harmonic for such drums. Such a behaviour has been attributed to the aforementioned applied mass. While this inference has remained the same after improvements in membrane models of Tabla, the same is not true for Mridangam. The importance of this work is to show that while the applied mass does in fact lead to nearly harmonic overtones it doesn’t do so exclusively (i.e. there are non–harmonic overtones present as well). In this work, a new membrane model for Mridangam based on extensive measurements and testing is developed. The pseudospectral method and FEM are used to solve the vibration problem using this new membrane model. Since the results from simulation based on the current membrane model present new findings that were not captured by simpler models of the past, experimental modal analysis was done to support these findings and to ascertain the assumption of treating the multiple layers as a single layer of effective density. A reconciliation is provided to the apparent harmonic nature of these drums by studying the transient response of membrane under excitation by Chaapu stroke and comparing it to similar studies done in the literature. Such a study serves to illustrate that there are perhaps some other reasons as to why these non–harmonic overtones might be suppressed. It is important to note that the current work deals exclusively with the vibration of the Mridangam membrane and does not consider any other effects like that of the air-cavity or the shells, which may or may not affect the acoustic response of the actual instrument.
This article attempts an investigation on the free vibration behavior of the laminated composite angle ply shell experimentally and the obtained results are validated by laminated shell model based ...on modified higher order zigzag theory. The angle ply shell is modeled by placing six layers of 45° glass fiber laminae. The mechanical properties such as Poisson's ratio, Young's modulus and shear modulus are determined experimentally. The experimental procedure is performed using a vibration testing instrument by Bruel and Kjaer. The theoretical laminated shell is modeled by taking thickness coordinate to radius of curvature of shell ratio in the strain displacement relationship with shear-free conditions at free surfaces. For the theoretical computations of natural frequencies, a C
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finite element model with nine-noded quadrilateral shell element with seven degrees of freedom at each node has been chosen. Theoretical findings have been compared with experimental results and also with relevant published results in the existing literature. Moreover, extensive parametric studies have been performed to investigate the effect of geometric configurations and lamination schemes with different boundary properties on free vibration behavior of laminated shells, which will serve as benchmark solutions for future researchers.
Modal substructuring is an established subdomain of dynamic substructuring in which the handling of multi-point connections between subcomponents proves to be a problematic task. This is particularly ...the case with experimental models, where even small inconsistencies in the interface dynamics can greatly affect the accuracy of the substructuring process. An established solution to this problem is based on the projection of physical interface motion on the modal basis of one of the substructures. Although both the dimension and the origin of the projection basis prove to have a great impact on the accuracy of the substructuring prediction, the established procedures do not provide a general criterion for their appropriate selection. The aim of this paper is to propose an alternative weakening approach based on singular value decomposition (SVD) that enables the construction of a vector space and the establishment of a criterion for the selection of its dominant components. Considering two substructures involved in a certain step of a substructuring procedure, SVD is performed on the interface-related components of the substructures’ concatenated modal bases. The dominant left singular vectors, therefore, embody the interface dynamics of both substructures and provide a consistent orthonormal basis for the constraint-weakening process. The proposed empiric criterion for estimating the required vector-space dimension is based on the magnitude of singular values. The efficiency of the proposed methodology is presented in comparison to the established modal-constraint approach with three numerical examples and an experimental–analytical transmission simulator coupling procedure. It is shown that the proposed singular vector constraint approach in conjunction with the criterion to estimate the weakening basis size, provides a consistent and reliable substructuring procedure.
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•The problem of multi-point connections in modal substructuring is addressed.•Singular vector constraints are proposed as an extension of the modal constraints.•Dominant left singular vectors embody the mutual interface dynamics.•Higher consistency of the substructuring prediction can be achieved.
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