1,720,967 research outputs found
The effects of the cold expansion degree on the fatigue crack growth rate in rail steel
No full textThis study investigates the effects of the cold-expansion degree on the fatigue strength of rail steels. Two degrees of cold-expansion, 2% and 4%, and two stress levels were investigated. Results indicate that cold-expansion postpones crack initiation and extends fatigue life, especially with higher degrees of cold-expansion and lower stress levels. Linear superposition principle coupled with the weight function method results in an overestimated fatigue life of cold-expanded specimens, suggesting that reliable predictions can be achieved if both the residual stresses redistribution due to propagating crack and the crack closure effects due to plastic wake generation are considered in the simulations
Optimization of a device for a fumigation process
No full textAn equipment was designed and realized that has allowed to perform the fumigating process optimization for the foodstuff. The equipment, easy to use, allow to exhaust completely the Magnesium of alluminium phosphide pellets and to reduce the phosphine production time from 5 days to 2 days. In addition it is particularly convenient during the fumigazion in cold periods
Flexural fatigue behaviour of pervious concrete: Lifetime evaluation and statistical model development
No full textFatigue crack growth rate dependency on cold expansion degree in railway steel
No full textThe increase of railway track occupancy due to high speed trains introduction in the morning and cross-border transportation of hazardous and non-hazardous materials during late hours, require particular attention for the fatigue design as well as the maintenance planning of railway superstructures. For this purpose, strategies finalized to increase the fatigue strength of railway structural parts assume great interest for railway engineers. Cold expansion technique, for example, is less expensive than other mechanical and thermal treatments, but in the literature there are no data concerning its use in the railway field, being almost all available studies focused on aluminum parts. In this paper, fatigue crack growth response of drilled specimens extracted from rails submitted or not to cold expansion are presented. Two degree of cold-expansion, 2% and 4%, are investigated in terms of fatigue crack growth rate, and the results compared to those obtained with not-expanded specimens. It is found that cold expansion postpones fatigue crack initiation, and that this effect is more pronounced for increasing degree of cold expansion. A preliminary predictive model was developed, based on the sum of the Stress Intensity Factors calculated with the weight functions and concerning the residual stress field induced by cold expansion, and the one generated by the external loads
An Optimized machine for fast grape cooling by cryogenic fluid
No full textA plant for the rapid cooling of a quantity of fresh grapes to be sent to a mash-destemmer machine
has been designed. It uses a cryogenic fluid: liquid CO2. The cooling time for the considered plant is
about 8 s. The machine works with a grape flow of approximately 2 ÷ 3 tons/h, with an adjustable
thermal jump (Tinlet – Toutlet) of 20 K. This machine is suitable in the cryomaceration process of the
grapes. The cryomaceration process can also be performed using traditional, large capacity
refrigeration units; they are very expensive, and very often damage the cooled product, due to the
friction generated by the passage in the pipes, causing the formation of dregs that reduce the quality
of the final product. This is avoided by the considered machine. It consists of: a vibrating table, that
allows the stratification of grapes producing only one layer of grapes cluster; infrared temperature
transducer that allow to determine the inlet grapes temperature; nozzles for the injection of liquid CO2;
insulated belt bucket elevators, that allow to sent the grapes to the mash-destemmer without thermal
exchanges; mash-destemmer machine acting in a CO2 inert environmental and at the maceration
temperature.
The process is managed by a PLC that, by setting the input CO2 quantity, check the process
temperature. The CO2 quantity used depends, naturally, on the process temperature jump.
In order to optimise the cooling process, a computational fluid dynamics (CFD) model of a tunnel for
grapes cryomaceration has been developed. The current study has focused on understanding the
main thermal phenomena, such as the conduction and convection heat transfer, aimed to the cooling
plant optimisation. Simulations were carried out by a CFD code of the ANSYS package, CFX11.
Because of symmetry, only half of the model was simulated. Initial analyses were carried out to test
the goodness of the mesh. The simulations were performed on several models to investigate the plant
efficiency with different grapes sizes and, moreover, the necessary CO2 flow, as a function of the
initial temperature of the grapes.
CFX predictions were used to investigate the behaviour of nozzles which inject liquid CO2 in the
direction of product flow from a vibrating conveyor, so as to increase the thermal exchanges and to
optimise the system performances. CFD analysis has allowed to predict the necessary CO2 flow to
achieve the desired final temperature and to obtain an optimized machine for the fast grape cooling
A direct technique for the homogenization of periodic beam-like structures by transfer matrix eigen-analysis
No full textTo homogenize lattice beam-like structures, a direct approach based on the matrix eigen- and principal vectors of the state transfer matrix is proposed and discussed. The Timoshenko couple-stress beam is the equivalent continuum medium adopted in the homogenization process. The girders unit cell transmits two kinds of bending moments: the first is generated by the couple of the axial forces acting on the section nodes, the other one is due to the moments directly applied at the node sections by the adjacent cells. This latter moment is modelled as the resultant of couple-stress. The main advantage of the method consists in to operate directly on the sub-partitions of the unit cell stiffness matrix. Closed form solutions for the transmission principal vectors of the Pratt and X-braced girders are also attained and employed to calculate the stiffnesses of the related equivalent beams. Unit cells having more complex geometries are analysed numerically. As a result, the principal vector problem is always reduced to the inversion of a well-conditioned (3×3) matrix employing the direct approach. Hence, no ill-conditioning problems, affecting all the known transfer methods, are present in the proposed method. Finally, comparing the predictions of the homogenized models with the finite element (f.e.) results of a series of girder, a validation of the homogenization method is performed
Periodic beam-like structures homogenization by transfer matrix eigen-analysis: A direct approach
No full textThe paper deals with a direct approach to homogenize lattice beam-like structures via eigen- and princi-pal vectors of the state transfer matrix. Since the girders unit cells transmit two bending moments, one given by the axial forces, the other originated by nodal moments, the Timoshenko couple-stress beams is employed as substitute continuum. The main advantage of the method is the possibility of operating directly on the sub-partitions of the unit cell stiffness matrix. Closed form solutions for the Pratt and X-braced girders are achieved and used into the homogenization. Unit cells with more complex geometries are numerically addressed with direct approach, showing that the principal vector problem corresponds to the inversion of a well-conditioned matrix. Finally, a validation of the procedure is carried out comparing the predictions of the homogenized models with the outcomes of f.e. analyses performed on a series of girders
Homogenization of a Vierendeel girder with elastic joints into an equivalent polar beam
No full textIn this paper an homogenization procedure of a Vierendeel girders with elastic joints is shown. The method is based on the Stephen transfer matrix analysis and employs as substitute continuum a polar Timoshenko beam. The polar character of the equivalent beam arises quite naturally from the analysis of the pure bending eigenvector components of the girder force transfer matrix. Through the girder unit cell, two bending moments are transmitted: one is generated by the couple of axial forces acting
on each nodal section of the beam, the other is produced by the moments applied at the nodes of every ones by the adjacent cells and is modelled as the resultant of the micro-polar moments. Transfer force eigenvector analysis reveals that the unit cell bends maintaining the webs undeformed, a property that allows evaluating both the bending stiffnesses and the equivalent material micro-polar scale parameter by means of straightforward equations. The accuracy of the proposed method is verified by comparing the predictions of the homogenized model with literature data and with the results of the analysis of a series of girders carried out by the finite element method
Influence of normal load frequency on fretting fatigue behaviour by a critical plane-based approach
Full text linkIn the last decades, many catastrophic events due to the rupture of metallic components have been traced back to fretting fatigue. In order to avoid failures due to such a contact phenomenon, many researchers have focused on fretting fatigue behaviour of different materials. In the present work, an experimental campaign carried out on an Al 7075-T6 aluminium alloy is simulated by employing a multiaxial fatigue criterion proposed for fretting fatigue. The configuration simulated is a flat-to-flat bridge-shaped contact, characterised by cyclic normal load, with different frequencies, as well as different levels of fatigue loading applied to the specimen
Deep-subwavelength-optimized holey-structured metamaterial lens for nonlinear air-coupled ultrasonic imaging
Full text linkUltrasound non-destructive testing (NDT) is a common technique used for defect detection in different materials, from aluminium to carbon-fiber-reinforced polymers (CFRPs). In most cases, a liquid coupling medium/immersion of the inspected component is required to maximize impedance matching, limiting the size of the structure and materials. Air-coupled inspection methods have recently been developed for noncontact inspections to reduce contact issues in standard ultrasonic inspections. However, transmission of ultrasound in air is very inefficient because of the enormous impedance mismatch between solids and air, thus requiring a signal amplification system of high-sensitivity transducers. Hence, the captured signal amplitude may not be high enough to reveal any wave distortion due to defects or damage. This work presents a design of a holey-structured metamaterial lens with a feature size of λ/14 aiming at improvement of acousto-ultrasonic imaging using air-coupled transducers. The required effect is obtained by matching geometrical parameters of the proposed holey-structured metamaterials and the Fabry–Perot resonance modes of the structure. Transmission tests have been conducted on different fabricated metamaterial-based structures, to assess the frequency component filtering of the proposed method in both acoustic (f = 5 kHz, 20 kHz) and ultrasonic range (f = 30 kHz, 40 kHz). Results showed an improved sensitivity of damage imaging, with an increase in amplitude of the design frequencies of the lens by 11 dB. Air-coupled inspections were conducted on a stress-corrosion cracked aluminum plate and impacted CFRP plate using the holey-structured lens. Results showed an improvement in the damage-imaging resolution due to a wave-amplitude increase across the defective features, thus demonstrating its potential as an efficient and sensitive inspection tool for damage-detection improvement in geometrically complex components of different materials
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