Publications

Many damage identification methods use the information from mode shapes. In order to test the robustness of these methods, it is a common practice to introduce uncertainty on the mode shapes in the form of independent noise at each measured location. In doing so, the potential spatial correlation in the mode shapes uncertainty is not taken into account. A better approach consists in adding uncorrelated noise on the time domain responses at each sensor before doing the identification. The spatial correlation resulting from the identification can then be evaluated using the covariance matrices of the identified mode shapes. In this study, we apply this approach to the numerical example of a simply supported beam. Modal identification is performed using stochastic subspace based algorithms developed in the toolbox MACEC. The covariance matrices of the mode shapes shows that there is a strong spatial correlation in the mode shapes uncertainty. This result shows that adding independent noise directly on the mode shapes is not a very realistic approach to assess the impact of noise on damage identification methods. The approach used to characterize noise uncertainty on modeshapes identification is totally general and can be applied to any mode, structure or sensing technology.

A major challenge of infrastructure management is to predict the remaining capacity of degrading structures and safely prolong their lifetime. In reinforced concrete (RC) structures, concrete cracking has a significant effect on durability and stiffness properties. Structural integrity degradation is often assessed by estimating the global stiffness loss through vibration-based structural health monitoring. Yet, this is challenging as the modal characteristics might also be affected by environmental and support conditions. At the same time, the development of models that enable studying the modal characteristics of cracked concrete structures has received little attention so far. This paper proposes a novel, visual inspection-based method to predict the decrease in effective elastic moduli of existing concrete structures from observed longitudinal and transverse cracks which are typical for corrosion and load-induced damage in RC elements. Discrete and smeared finite element models are developed to establish a relation between the geometrical crack properties and the changes in the concrete’s smeared dynamic stiffness parameters, as defined within an orthotropic material model. It is found that the crack pattern has a significant influence, with transverse cracks generally reducing the stiffness parameters more than longitudinal cracks. Experimental data support the proposed relations’ ability to tune the parameters of the orthotropic material model based on crack properties from corroded or mechanically loaded RC beams. The proposed relations enhance the assessment of serviceability limit states in RC beams and offer a valuable tool to evaluate dynamic test data obtained from on-site monitoring.

Resonant metamaterial panels can achieve exceptional vibroacoustic attenuation by subwavelength local resonators attached to a host structure. While single-mode resonators provide improvements only in a narrow band, recent advancements propose multi-modal resonators for broadband attenuation. However, existing designs utilize only a limited number of modes, and effective methodologies are lacking to systematically design resonator layouts that achieve an appropriate amount of resonances across the target frequency range, with an adequate participating mass. In this study, we tackle this challenge by developing a dedicated topology optimization method for multi-modal resonator design in metamaterial panels. The method leverages effective thin plate modelling, i.e. a homogenized metamaterial representation through an effective mass density, which provides accurate and very efficient vibroacoustic predictions. The optimization objective is to maximize the broadband diffuse field sound transmission loss (STL) of the metamaterial panel while constraining mass. The optimization problem is solved by gradient-based mathematical programming, for which the necessary (adjoint) sensitivities of objective and constraints are derived. The efficacy of the proposed topology optimization approach is demonstrated by targeting the suppression of coincidence dips in orthotropic host plates. For narrowband coincidence dips, the optimized resonator exhibits maximized mass participation in one single mode of interest. For broadband coincidence dips, the method effectively generates multi-modal resonators with up to 6 resonances distributed across the target frequency range. It is finally demonstrated that the topology-optimized designs surpass the performance of simpler, parametrically optimized multi-modal resonator layouts, as well as of conventional treatments by a damping layer of rubber.

To investigate monitoring of concrete members by integrating fiber optic technology, a series of static and dynamic loading tests have been conducted on a posttensioned concrete girder with a span of 16.8m and a total depth of 0.8m. For strain measurements optical fiber strain sensors, based on Bragg gratings, have been used. In literature only few papers deal with changes of dynamic parameters in posttensioned concrete at increasing damage levels. The obtained test results demonstrate the feasibility of fiber Bragg grating strain sensors, also for dynamic monitoring. On the dynamic behavior of the tested girder, it is found that, although a drop in eigenfrequencies is noticed with increasing load, significant shifts are only measured when the reinforcement starts yielding. Also with respect to damping values, no clear indication of damage in an early stage is observed. Further results show that the curvature of the mode shapes changes with increasing load and is related to the formation of a plastic hinge. An interesting observation is the change in phase angle between the modal strains and the vertical acceleration, which already occurred at an early stage.

Abstract : This report presents the results of a 6-month technical effort to develop a realistic mission profile for each of six helicopter types in current and future Army inventories: observation, utility, utility/tactical assault, attack, crane, and transport. The profiles included in this report are the product of operational data gathered over two decades, starting with early NACA efforts and including extensive studies conducted by the Army Air Mobility R and D Laboratory and private enterprise.

No abstract is provided for this article.

No abstract is provided for this article.

The hybrid deterministic-statistical energy analysis method has proven to be a versatile framework for modeling built-up vibro-acoustic systems. The stiff system components are modeled deterministically, e.g., using the finite element method, while the wave fields in the flexible components are modeled as diffuse. In the present paper, the hybrid method is extended such that not only the ensemble mean and variance of the harmonic system response can be computed, but also of the band-averaged system response. This variance represents the uncertainty that is due to the assumption of a diffuse field in the flexible components of the hybrid system. The developments start with a cross-frequency generalization of the reciprocity relationship between the total energy in a diffuse field and the cross spectrum of the blocked reverberant loading at the boundaries of that field. By making extensive use of this generalization in a first-order perturbation analysis, explicit expressions are derived for the cross-frequency and band-averaged variance of the vibrational energies in the diffuse components and for the cross-frequency and band-averaged variance of the cross spectrum of the vibro-acoustic field response of the deterministic components. These expressions are extensively validated against detailed Monte Carlo analyses of coupled plate systems in which diffuse fields are simulated by randomly distributing small point masses across the flexible components, and good agreement is found.

No abstract is provided for this article.

Vibration serviceability has become an important issue in the design of modern slender footbridges with large spans. This paper presents the measurements and the numerical predictions of the footfall-induced vibrations of a pedestrian bridge. A series of experiments were carried out with different-sized groups crossing the bridge, varying in number from 10 up to 50 participants and recording the vertical and lateral accelerations at different locations on the bridge. Two types of tests were performed: free walking and synchronized walking by means of a metronome signal, recorded on a tape recorder carried by the group of students. The effect of the test type is analyzed and shows a magnitude in difference between the vertical accelerations caused by the free and the synchronized walking. The increasing trend of the acceleration levels with increasing group size is also clearly observed. A numerical prediction method is used to simulate the synchronized walking experiments based on an updated finite element model of the bridge and a single pedestrian load model. It is shown that the predicted acceleration level is sensitive to the assumptions made regarding the level of synchronization between the pedestrians and the magnitude of the dynamic load generated by each pedestrian. Taking into account these specific measurement conditions, a fair agreement is obtained between the predicted and the observed vertical acceleration levels at seven positions along the length of the footbridge.