1,720,997 research outputs found
Comparison between acoustic emission sensors and piezoelectric patches for damage detection in concrete beams
Full text linkAcoustic emission (AE) sensing is an established technique to monitor crack nucleation and propagation in different types of structural elements and for different damage scenarios. Although effective for crack monitoring in civil infrastructure, off-the-shelf AE sensors can be expensive, discouraging their installation in large numbers for assessing the global structural performance or monitoring multiple assets in the infrastructure network. For this reason, this paper explores the use of piezoelectric (PZT) patches as an alternative low-cost solution to AE sensors. These patches are considerably cheaper, do not require external power supply and can be easily attached to existing structural elements or embedded into new structures. Moreover, thanks to the direct and inverse piezoelectric effect, the same set of patches can act both as sensors and actuators, meaning that they can be used to “passively” monitor crack propagation over time, as well as to locate and characterise damage in ageing structures at a certain time by using the patches as emitters and receivers of waves. In this framework, this paper proposes a comparison between traditional AE sensors and PZT patches. The comparison is performed on a set of three concrete beams subjected to four-point bending loading, each equipped with two grids of sensors, i.e. 8 AE sensors and 8 patches. The beams are tested under increasing monotonic loading, from zero-load uncracked condition up to collapse
Teaching students to control mechanical systems through an engaging laboratory activity
No full textThis paper describes a didactic activity in which students learn how to control a mechanical system composed of two pulleys and two springs powered by a DC motor. The control types studied in the course are P, PD, PI, and PID; these controls were applied for both velocity and position control, with and without ground springs, and students were instructed on system modeling and control implementation using Arduino in conjunction with Matlab and Simulink. The experimental results were finally compared with theoretical expectations. The students worked in groups of four to five people, and all of them successfully implemented the required control algorithms and submitted comprehensive final reports. Feedback from students was gathered anonymously through a questionnaire, revealing that students appreciated the activity as it provided clarity on theoretical concepts that might have been challenging if taught solely in a traditional classroom setting. The positive response from students has led to the decision to repeat this activity in subsequent years
Modal analysis of turbine blades by means of distributed optical fiber sensors
No full textThis paper investigates the possibility of identifying and monitoring the modal shapes of a turbine blade by means of continuous optical fiber sensors based on Optical Backscatter Reflectometry (OBR). The advantage of this approach would be the possibility of embedding the sensors in future carbon fiber blades, in order to make this modal analysis approach available also for the blade operating conditions, since no modifications in the blade fluid-structure interaction occur. The paper describes the proposed method and provides some experimental results obtained on a 3D printed model of an existing steam turbine blade
Optimization of continuous sensor placement for modal analysis: Application to an optical backscatter reflectometry strain sensor
No full textThe methods for experimental modal analysis are consolidated, as are the sensors used to perform this analysis. The characteristics of the most commonly used sensors are to be “discrete” and “independent”, that is, each sensor can measure the magnitude of interest in a single point and can be placed independently from the others on the structure. This kind of layout is widely used, for example, with accelerometers, but it presents a strong complexity if the structure is extended or characterized by high modal density in the frequency range of interest, or if the number of modes to be identified is high. Often the “discrete” sensors also have nonnegligible masses compared to the mass of the structure to be analyzed, thus introducing the well-known “mass effect”. Recently, new dynamic sensors have become available based on optical fibers such as the well-known Fiber Bragg grating sensors or the more recent Optical Backscatter Reflectometry (OBR) sensors. Both are sensors of the “continuous” type, or rather, they are a succession of sensors connected in series inside a single optical fiber. The purpose of this paper is to explore the use of OBR for modal analysis and introduce an original algorithm that allows the OBR sensor to be arranged in an optimized manner, to identify up to a certain number of modes in the most effective way and to respect the constraints imposed by the optical fiber itself, such as fiber length or maximum sensor curvature. The results are illustrated by means of the experimental results obtained on a structure characterized by a simple geometry
SHM Campaign on 138 Spans of Railway Viaducts by Means of OMA and Wireless Sensors Network
Full text linkTunable in-plane topologically protected edge waves in continuum Kagome lattices
No full textIn this paper, we report the evidence of topologically protected edge waves (TPEWs) in continuum Kagome lattice. According to the bulk edge correspondence principle, such edge states are inherently linked with the topological characteristics of the material band structure and can, therefore, be predicted evaluating the associated topological invariant. Due to the non-trivial band structures shown in the context of quantum valley Hall effect, TPEWs are supported at the interface between two lattices characterized by different valley Chern numbers. The break of lattice symmetry is obtained here, in contrast with other similar works in continuum elastic structures, biasing in the stiffness properties of the unit cell, instead of manipulating mass at sublattice points. This opens new promising possibilities related to waveguide tunability and wave propagation control, exploiting the established techniques for stiffness modulation in elastic structures. A sensitivity analysis of robustness of the supported energy transport is provided, showing the amount of de-localized disorder the waveguide is immune to, and how performances are affected by perturbations in the nominal parameters of the lattice
Load estimation and vibration monitoring of scale model wind turbine blades through optical fiber sensors
No full textExperimental activities on wind turbine models allow to speed up their design as well as the development of robust and effective control algorithms. Intending to have large information on load distribution on the blades, this work presents an experimental setup of a model wind turbine blade instrumented with two different types of optical fiber sensors (FOS). The first one is a traditional chain of Fiber Bragg Grating sensors, able to measure a large number of strain values; the second is based on Optical Backscatter Reflectometry (OBR), able to provide a continuous strain measurement. The experimental setup is calibrated to estimate load distribution along the blade and the two technologies are compared. The results of the experimental campaign suggest that optical fiber sensors could represent an interesting solution for real-time continuous load monitoring purposes
Generalized plane wave expansion method for non-reciprocal discretely modulated waveguides
No full textIn this manuscript we investigate one-way wave propagation in spatio-temporal phononic wave-guides in which the modulation of material properties is provided piecewise in space and time. Non-reciprocal dispersion diagram is computed using a generalized plane wave expansion method, whose formulation, provided in the paper, is able to describe how a generic discrete or continuous unit cell is able to break the mirror symmetry in the momentum space, as already shown in other works by means of more expensive and less accurate computational tools. The proposed method is then used to verify a physical interpretation we provided for bandgap formation by means of wave superposition in spatio-temporal modulated waveguides
Acoustic scattering reduction of elliptical targets via pentamode near-cloaking based on transformation acoustics in elliptic coordinates
Full text linkCloaks for underwater applications designed for actual submarine acoustic stealth are still far from the technological advancement needed for being put in practice. Several challenges need to be overcome such as dealing with weight or non-axisymmetric shapes. In this paper, we introduce the use of elliptical coordinates to define quasi-symmetric transformations to retrieve the material properties of pentamode cloaks for elliptical shaped targets, along with a quantifiable approximation introduced by the rotation tensor being different from the identity. This is done analytically adopting transformation theory, in an attempt to generalize the usual approach for axisymmetric cloaks, with the aim of dealing with shapes closer to those of the actual cross section of a submarine. With respect to existing techniques for dealing with arbitrarily shaped pentamode cloaks, the introduced technique allows for a priori control on the principal directions of anisotropy and for enlarged design space in terms of possible combinations of material property distributions for the same geometry of the problem
L-PBF for the production of metallic phononic crystal: design and functional characterization
Full text linkVibration abatement often requires the adoption of peculiar solutions and/or foundations. This paper presents an innovative solution to this problem, consisting in a phononic meta-material realized via Laser Powder Bed Fusion (L-PBF) capable to prevent the propagation of vibrations within specific frequency ranges. The integration of this meta-material within existing supporting structures can, therefore, greatly reduce the needing of foundations capable to stop vibrations. After a description of the design procedure of the meta-material that shows how to satisfy the constraints imposed by L-PBF technology, the manufactured sample is described and analyzed to predict its band-gaps. Finally, the theoretical results are compared with experimental measurements. These results show a good agreement between expected and actual meta-material behavior
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