1,721,022 research outputs found
Modalità e protocolli di prova in vitro per stent periferici
No full textVI Giornata di studio sui biomateriali e loro applicazioni in biomeccanic
On the effectiveness of standards application to threshold setting in vibration condition monitoring in industrial machinery
No full textOne of the goals of Industry 4.0 is to minimize unplanned maintenance emergencies and to leverage the efficiency of condition-based, just-in-time labor and repairs. Achieving the benefits of predictive maintenance via an Industry 4.0-style plant requires not only a data capture ecosystem, but also reference values to inform the monitoring system. Effectively estimating the values of monitoring thresholds is not an easy task, especially in those cases where the manufacturer does not provide data, or the machine is not new but the monitoring system has just been installed, therefore a measurement history is not yet available. The purpose of the present work is assess, through the analysis of two separate test cases, whether the monitoring thresholds suggested by different internationally recognized standards may serve as effective starting values in a vibration-based condition monitoring system. While general agreement on the order of magnitude of the thresholds is present in all cases, the differences between the standards allow for conclusions to be drawn on the most convenient format and processing technique to handle the data and to assess the most/least conservative standards. This paper constitutes, to the authors knowledge, the only systematic study in which the recommendations of all available international standards for vibration monitoring of non-rotating parts are applied to the same test cases and compared
Theory of critical distances: A discussion on concepts and applications
Full text linkTheory of Critical Distances (TCD) collects several methods adopted in failure prediction of components provided with stress concentration features. The idea of evaluating stress effect in a zone rather than in a single point was proposed decades ago but, only thanks to relatively recent works, TCD concepts showed to be a successful extension of Linear Elastic Fracture Mechanics (LEFM), able to assess strength and fatigue life. The increasing computational power has made Finite Element Method (FEM) widespread, hence stress fields can be easily extracted and used as input data for fatigue post-processing and durability analyses. In this scenario, TCD reveals as a powerful tool which, thanks to the introduction of a single material parameter (critical distance, (Formula presented.)), integrates classical fracture models by considering the presence of microscale phenomena acting in fracture process. In this sense, TCD behaves as a link between continuum mechanics and LEFM. Modalities and reasons for this connection to occur are interesting points of further investigations. Literature on TCD and its theoretical-experimental background is quite extended, nevertheless few industrial applications are available in literature to the best of authors’ knowledge. In this paper, an overview of concepts and applications related to TCD are reported highlighting the relevance of theoretical arguments in actual applications
A proposal of a unique formula for computing compliance in bolted joints
Full text linkThe connection system between mechanical parts with the greatest advantages in terms of production is the threaded connection. This type of connection has considerable stiffness but also high weight. Often the search for the reduction of the masses clashes with the limits dictated by production needs. A considerable effort has been made in making screws with higher performance materials and therefore guaranteeing greater tightening forces with smaller cross sections, but there have not been as many notable developments on the method of determining the compliance of tightened elements. The classical theory identifies three different conditions for calculating deformability, which are sometimes not easy to interpret and implement. The use of numerical techniques such as finite elements allows designers to be very precise, but requires a great deal. To facilitate the work of the designers and provide them with a more manageable tool to better understand the type of threaded connection to be designed, the present work proposes an analytical formulation that allows a quick assessment of the compliance value of the clamped elements, regardless of the geometric relationships and materials. To achieve this, starting from a literature analysis, a parametric finite element model was developed and, based on the results obtained, a formula is proposed that covers all the possible scenarios for determining compliance. The results were compared with the classical theory in order to verify the correctness and applicability of the formulation. At the moment the formulation is valid for screws whereas for the bolts with nut unified formula is under investigation
Vibration energy harvesting via piezoelectric bimorph plates: An analytical model
No full textVibration energy harvesting using piezoelectric cantilevers has been widely studied during the past decade. As an alternative to piezoelectric cantilevers, plate-like energy harvesters can be much more effective in marine, aerospace, and automotive applications. This work presents an exact two-dimensional model that can be used for analyzing thin, moderately thick, and thick piezoelectric bimorph plate vibration harvesters. The model allows to consider for the substrate layer both homogenous materials, and those with varying properties along the thickness direction. For the latter case, porous materials with various porosity distributions are herein considered, and the piezoelectric layers are assumed to be wired in both parallel and series configurations. Rayleigh damping assumptions are also used to model the structural damping of the harvesting system. Using Hamilton’s principle and Gauss’s law, the energy harvesting model is established based on the first-order and the third-order shear deformation theories. Applying an analytical procedure to the electromechanical governing equations, closed-form steady-state response expressions, which relate the voltage output and the vibration response of the harvester to harmonic input force, are derived. Finally, the proposed model is validated, and the power generation performance of the plate harvester is discussed through conducting extensive parametric studies, covering the effect of design parameters, such as the applied electric load, porosity characteristics, electrical configuration, and geometrical parameters
Design and modeling of a novel multi-beam piezoelectric smart structure for vibration energy harvesting
No full textVibration energy harvesting holds great potential to achieve long-lifetime autonomous operation of low-power electronic devices. This work is concerned with analytical and numerical modeling and analysis of a unimorph beam and a novel multi-beam energy harvesting systems. Several of the former device and two identical proof masses are utilized to design the multi-beam harvester, aiming to propose a scavenger with resonance frequency of less than 100 Hz. Accuracy of the models are verified, and extensive parametric studies are presented. Results demonstrate that the proposed multi-beam harvester is capable of generating several milliwatts of power under harmonic base acceleration of 0.4 g
Modelling Strategy and Parametric Study of Metal Gaskets for Automotive Applications
Full text linkThis paper is focused on finite element simulation of cylinder head gaskets. Finite element codes support several methodologies, each of which has its own strengths and weaknesses. One of the key points lies in the influence of the gasket geometry on its final behaviour. Such a contribution can come from the detailed modelling of the gasket or by defining a global non-linear behaviour in which material and geometry non-linearities are summarised. Two approaches were used to simulate the gasket behaviour. The first one consists in using a 2D approach, which allows to model through-thickness non-linear behaviour of gasket. The second one consists in using conventional 3D finite element modelling. The numerical methods have been discussed and compared in relation to the accordance with experimental data, amount of information supplied and computational time required. Finally, a parametric study shows how some geometric parameters influence the compressive load and the elastic recovery of a single-layer steel gasket
Numerical-experimental comparison of a parametric test-rig for crossing and veering phenomena
No full textOn wave propagation and free vibration of piezoelectric sandwich plates with perfect and porous functionally graded substrates
No full textThis paper aims to develop analytical solutions for wave propagation and free vibration of perfect and porous functionally graded (FG) plate structures integrated with piezoelectric layers. The effect of porosities, which occur in FG materials, is rarely reported in the literature of smart FG plates but included in the present modeling. The modified rule of mixture is therefore considered for variation of effective material properties within the FG substrate. Based on a four-variable higher-order theory, the electromechanical model of the system is established through the use of Hamilton’s principle, and Maxwell’s equation. This theory drops the need of any shear correction factor, and results in less governing equations compared to the conventional higher-order theories. Analytical solutions are applied to the obtained equations to extract the results for two investigations: (I) the plane wave propagation of infinite smart plates and (II) the free vibration of smart rectangular plates with different boundary conditions. After verifying the model, extensive numerical results are presented. Numerical results demonstrate that the wave characteristics of the system, including wave frequency and phase velocity along with the natural frequencies of its bounded counterpart, are highly influenced by the plate parameters such as power-law index, porosity, and piezoelectric characteristics
Deep transfer learning for machine diagnosis: From sound and music recognition to bearing fault detection
Full text linkToday’s deep learning strategies require ever‐increasing computational efforts and demand for very large amounts of labelled data. Providing such expensive resources for machine diagnosis is highly challenging. Transfer learning recently emerged as a valuable approach to address these issues. Thus, the knowledge learned by deep architectures in different scenarios can be reused for the purpose of machine diagnosis, minimizing data collecting efforts. Existing research provides evidence that networks pre‐trained for image recognition can classify machine vibrations in the time‐frequency domain by means of transfer learning. So far, however, there has been little discussion about the potentials included in networks pre‐trained for sound recognition, which are inherently suited for time‐frequency tasks. This work argues that deep architectures trained for music recognition and sound detection can perform machine diagnosis. The YAMNet convolutional network was designed to serve extremely efficient mobile applications for sound detection, and it was originally trained on millions of data extracted from YouTube clips. That framework is employed to detect bearing faults for the CWRU dataset. It is shown that transferring knowledge from sound and music recognition to bearing fault detection is successful. The maximum accuracy is achieved using a few hundred data for fine‐tuning the fault diagnosis model
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