Vibration-based damage identification is a well-known method to support health monitoring of civil engineering structures.Damage in such structures can be identified by measuring changes of the natural frequencies, damping factors or modal displacements of the structure.However, this approach suffers from the low sensitivity of these natural frequencies and modal displacements to certain types of damage.Modal strains and curvatures can be more sensitive to local damage, but direct monitoring of these quantities with sufficient spatial resolution is not possible with current measurement techniques due to the very small strain levels (submicrostrain) caused by ambient or operational excitation.To deal with this issue, we propose a novel mechanical transducer equipped with an optical fiber Bragg grating (FBG) sensor that enhances the sensitivity to strain with a factor larger than 30.The principle of operation of the transducer exploits a symmetric cantilever structure that enlarges the strain experienced by the FBG sensor compared to the strain applied to the transducer itself.We carried out dynamic and static tests to verify the ability of the strain-amplifying transducers to measure small-amplitude strain levels and to evidence the potential for carrying out FBG based modal strain measurements on concrete civil engineering structures.
The change of modal characteristics directly provides an indication of structural damage. Based on changes in frequencies and mode shapes of vibration, a damage identification technique is proposed in this paper for predicting damage location and severity. The method is applied at an element level with a conventional finite-element model. The element damage equations have been established through the eigenvalue equations that characterize the dynamic behavior. Several solution techniques are discussed and compared. The influence of simulated noise in the modal data is also presented. The method has been verified by a number of damage scenarios for simulated beams and has found the exact location and severity of damage. It is demonstrated that multiplying the damaged eigenvalue equations with the undamaged or damaged mode shapes provides more equations and guarantees the damage localization. The resulting equations, however, become more sensitive to the deviation of modal data and the direct solution often yields poor results. Numerical results show that the non-negative least-squares method can lead to satisfactory results in most cases. A regularization algorithm with error-based truncation is necessary to ensure the right solutions.
In this paper, the feasibility of structural health monitoring based on natural frequencies is investigated for a steel bowstring railway bridge in Leuven, Belgium. The data used in the study are obtained from an ongoing long-term monitoring campaign on the railway bridge and include acceleration measurements on the bridge deck and the arches. During the monitoring period, the railway bridge has been retrofitted, resulting in data for two distinct states of the structure. Particular attention is paid to removing the effects of environmental conditions, such as temperature, which affect the modal characteristics of the structure and therefore may lead to false-positive or false-negative damage detection. A comparison is made between standard linear regression and robust principal component analysis (PCA), two black-box modeling techniques which are adopted to remove natural frequency variations resulting from changes in the environmental conditions. In order to assess the success rate of these techniques, a receiver operating characteristic (ROC) curve analysis is performed, considering the actual retrofit as well as a number of more subtle structural changes, which are modeled using a detailed finite element model of the structure. The state transition can be observed for the actual retrofit as well as for smaller structural modifications that result in relatively small natural frequency shifts.
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No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
Locally resonant metamaterials can achieve unprecedented vibroacoustic performance by subwavelength distributions of small mechanical resonators on a host structure. Substantial broadband vibroacoustic attenuation can be achieved by multi-modal metamaterial panels, which exploit multiple translational and rotational resonator modes to manipulate the overall bending wave propagation. However, the multi-modal metamaterial concept has been studied only for idealized conditions, such as infinite panel extent and uniformly distributed resonators, limiting practical applicability. Efficient methodologies are still needed to study the behaviour of multi-modal metamaterial panels in real-world scenarios. In this work, this challenge is tackled by developing generalized effective medium models, i.e., homogenized material representations through equivalent macro-scale properties, tailored for finite-sized multi-modal metamaterial panels. For the special but important case of simply supported rectangular panels with uniformly distributed resonators, a dedicated analytical effective medium model is developed. For arbitrary boundary conditions and resonator distributions, effective medium finite elements are formulated. The diffuse sound transmission loss (STL) performance is efficiently predicted through Deterministic - Statistical Energy Analysis (Det-SEA), by coupling the effective medium model of the finite-sized metamaterial panel with a diffuse model of the surrounding sound fields. The proposed prediction approaches are validated against detailed FEM modelling, demonstrating that significant computational reductions are achieved while preserving accuracy. Results showcase that multi-modal metamaterial panels maintain broadband vibracoustic attenuation also when subjected to boundary effects and under partial metamaterial treatment.
No abstract is provided for this article.
Due to differences in stiffness, mass, damping, internal connections, external boundary conditions between a real structure and a FE model, differences will exist in the vibration characteristics including natural frequencies and mode shapes. To solve this discrepancy, FE updating has been widely used in civil engineering. The uncertain parameters in the FE model are adapted to minimize the differences. The Jalon Viaduct along the high-speed railway line between Madrid and Barcelona is selected as a case study. The viaduct is a continuous box girder supported on high piers. From ambient vibration, mode shapes have been derived in vertical, lateral and longitudinal direction. For the updating the natural frequencies and the three- dimensional mode shapes of the first five lateral and first five vertical modes of the viaduct are used. For this calibration, 14 updating parameters including 12 spring stiffnesses and 2 Young's moduli are selected. The fact that also longitudinal components of the mode shapes are considered helped the updating process. Due to the existence of some local minima of the objective function induced by interfering modes, a Multistart Solver is used to obtain the global solution. It is shown that the relative weight of some residuals has to be adjusted in order to have better results for the longitudinal components of the vertical modes. The updating results in a much better agreement between the measured and the predicted natural frequencies and mode shapes.
Strain mode shapes are attractive features for vibration-based monitoring as their sensitivity to local damage is known to be high. However, the assessment of their performance in situ has remained an open problem. This work presents the continuous dynamic strain monitoring of a steel railway bridge with FBGs for one year. The natural frequencies and strain mode shapes of ten modes are automatically identified from operational strain time histories, on an hourly basis. The influence of temperature on these modal characteristics is quantified and then investigated with a FE model of the bridge. A clear influence of temperature on the natural frequency of most modes is identified, especially during frost periods, contrary to the strain mode shapes, which are mostly insensitive to temperature changes, as confirmed also by the FE model. The FE model is employed to investigate also the influence of simulated local damage on the modal characteristics.
A 3-D dynamic analysis model for a coupled train-bridge system is established. The vehicle subsystem is modeled by multibody dynamics and each 4-axle coach is simulated as a 27-degree-of-freedom model. For the bridge subsystem a 3-D rail-ballast-beam finite element model is created, considering the elasticity and the continuity of the track system. The dynamic interaction between the bridge and the train is realized through the contact forces between the wheels and the track. Track irregularities and wheel hunting are included. The track irregularities are generated with the German high-speed track spectra, whereas the wheel hunting is simulated by a sinusoidal function with a random phase. The equations of motion of the coupled train-bridge system are derived by representing the bridge subsystem through its vibration modes. The proposed formulations are then applied to the Sesia viaduct located on the new Italian high-speed line between Torino and Milano. The dynamic responses of the bridge subjected to an Italian high-speed ETR500Y train are calculated and some results are compared with the measured data. The resonant train speed of the train-bridge system is discussed, and the running safety and the stability of the train vehicles on the bridge are evaluated. The results show that the Sesia viaduct is stiff enough in both vertical and lateral directions, fulfilling the design criteria for high-speed train passage. The numerical method proposed in this paper can be used to predict efficiently the dynamic behavior of coupled train-bridge systems with reasonable computational effort.
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