1,721,064 research outputs found
Confidence interval of the ‘single-moment’ fatigue damage calculated from an estimated power spectral density
No full textThis article develops a theoretical approach to account for the uncertainty of the fatigue damage caused by the sampling variability of a power spectral density estimated from a finite length stationary record. The moment generating function of the estimated spectral moments is obtained and used to approximate their probability distribution with that of a chi-square random variable with a newly chosen number of degrees of freedom. This approximate distribution allows the confidence interval of both the ‘true’ spectral moments and ‘single-moment’ damage to be derived. A numerical example is finally presented to demonstrate the correctness of the proposed solutions
Some analytical expressions to measure the accuracy of the "equivalent von Mises stress" in vibration multiaxial fatigue
No full textThe “equivalent von Mises stress” (EVMS) was first proposed in 1994 by Preumont and co-workers as a frequency domain reformulation of von Mises stress, for the fatigue analysis of vibrating structures under multiaxial random stresses. The EVMS criterion is a simple, but very powerful tool to estimate fatigue damage with time domain analysis of simulated stress histories, or frequency domain evaluation by spectral methods. Despite its simplicity, the EVMS criterion is based on some inherent assumptions, which may lead to inaccurate damage estimations in some particular conditions (e.g. materials with very different axial/bending and torsion S–N curves). This paper aims to derive some analytical expressions to measure the accuracy of EVMS criterion for various combinations of material fatigue properties and loading conditions (e.g. combined axial/bending and torsion loadings). These expressions constitute an original contribution, as similar analytical approaches have not been proposed in literature. The accuracy of EVMS approach is then tested with typical material fatigue properties from literature. The range of applicability of EVMS criterion is then be identified for specified intervals and combinations of S–N parameters
Fatigue analysis of random loadings. A frequency-domain approach
No full textService loadings in structures and mechanical components can be modelled as random processes. The durability assessment under such complex loadings is commonly approached in time-domain by using counting methods and damage accumulation rules. An alternative approach could be developed in frequency-domain, where the random loading is characterised by its power spectral density. This book aims to provide an overview on methods for fatigue analysis of random loadings, with particular focus on frequency-domain approach. Classical time-domain load characterisation, counting methods and linear damage rule are first reviewed. Then, frequency-domain spectral methods for analysis of stationary random loadings are discussed, with particular emphasis on Gaussian and non-Gaussian load analysis. Application examples are also developed, with both numerical simulations and experimental load measurements. A general comparison of spectral methods is finally presented. This book should help to shed some light on the frequency-domain fatigue analysis of random loadings and it should be especially useful for researcher working in the field of structural and durability assessment under service loadings
On thermal stress and fatigue life evaluation in work rolls of hot rolling mill
No full textThe paper proposes an approach for thermal stress evaluation and fatigue life assessment of work rolls in hot rolling mills. Temperature and thermal stress distributions are calculated with a simplified finite element approach, based on a plane model of work roll loaded on its boundary by rotating thermal actions for a typical hot rolling configuration. A transient of one hour is simulated and the calculated temperature field is next applied as thermal loading in elastic–plastic mechanical simulation, to get thermal stresses and elastic–plastic strains in work roll. Calculated cyclic stresses and
strains are finally used to estimate work roll service life, based on Universal Slopes equation that is modified to account for the multiaxial stress state. A comparison of results by three multiaxial fatigue criteria is provided. The obtained results show that, according to such multiaxial criteria, stress multiaxiality would have a great effect on fatigue life, despite it is generally neglected in approaches usually adopted in literature
Finite element analysis of optimized piezoelectric bimorphs for vibrational “energy harvesting”
No full textThe concept of “energy harvesting” is to design smart systems to capture the ambient energy and to convert it to usable electrical power to supply small electronics devices and sensors. The goal is to develop autonomous and self-powered devices that do not need any replacement of traditional electrochemical batteries. Piezoelectric devices are commonly used, due to their high conversion efficiency and easy of manufacture. The purpose of this paper is to numerically analyze the electromechanical response of piezoelectric bimorphs subjected to vibrations. The bimorph is made up of two layers of piezoelectric material glued on a stainless steel shim, which form a cantilever beam with a tip mass that has the capability to convert the mechanical bending strain within the piezoelectric layers into electric charges on its external surface The strategy here used to increase the average mechanical strain, and hence the generated power output, is to modify the geometry of rectangular piezoelectric beam, which is traditionally used in applications. Optimized configurations with trapezoidal shapes (direct and reversed), with either constant width or constant volume, have been proposed and numerically analyzed. A detailed 3D finite element model is used to evaluate and to compare the electromechanical response of the proposed optimized bimorphs, in terms of resonant frequency, harmonic transfer function, output voltage and power. The electromechanical vibration response has been studied with a modal analysis and a harmonic coupled simulation with imposed base acceleration. The obtained results confirm an increment in the electric performance of the proposed optimized bimorphs, with a net increase in specific volumetric power compared to the traditional rectangular configuration
A statistical definition of an equivalent stress amplitude for multiaxial random loadings
No full textAn innovative approach for CAE based analysis of complex fatigue loadings
No full textThis paper presents an approach, called "Projection-by-Projection" (PbP) criterion, which is suitable for the CAE-based fatigue analysis of complex structures subjected to random loadings. The method framework allows a time domain, as well as a frequency domain analysis of random multiaxial stress. A CAE-based design of a structural component is discussed as an example, to show the capabilities of PbP method
On the performance improvement of piezoelectric energy harvesters
No full textAmong the available solutions to harvest energy from vibrating and non-vibrating sources, piezoelectric bimorphs seem to be a suitable choice due to their capability to directly convert applied strain energy into usable electric power. This work reviews the techniques adopted to increase the specific power per unit of volume of piezoelectric scavengers. The design goal is to obtain a uniform stress distribution on the piezoelectric layers, in order to exploit the electro-mechanic energy conversion capability of the whole material. Two approaches are then followed to improve the performance: by designing an optimal shape of the device, or by imposing an optimal bending deflection. For both strategies, results from numerical models and experimental tests are compared
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