Search results for " Creep"
showing 10 items of 38 documents
On the behavior of a three-dimensional fractional viscoelastic constitutive model
2016
In this paper a three-dimensional isotropic fractional viscoelastic model is examined. It is shown that if different time scales for the volumetric and deviatoric components are assumed, the Poisson ratio is time varying function; in particular viscoelastic Poisson ratio may be obtained both increasing and decreasing with time. Moreover, it is shown that, from a theoretical point of view, one-dimensional fractional constitutive laws for normal stress and strain components are not correct to fit uniaxial experimental test, unless the time scale of deviatoric and volumetric are equal. Finally, the model is proved to satisfy correspondence principles also for the viscoelastic Poisson’s ratio a…
EFFETTI DIPENDENTI DAL TEMPO NELLE SABBIE CARBONATICHE IN CONDIZIONI EDOMETRICHE
2012
Si illustrano i risultati di uno studio sperimentale sugli effetti dipendenti dal tempo di sabbie carbonatiche bioclastiche sottoposte a compressione monodimensionale. La sperimentazione è stata eseguita su materiale asciutto ricostituito in laboratorio. Per confronto una serie di prove è stata eseguita anche su sabbie di quarzo e di pomice. Il coefficiente di compressione secondaria C aumenta con la tensione verticale applicata v, se quest’ultima viene applicata interamente fin dall’inizio, anziché per incrementi successivi. In tutto il campo tensionale esplorato si osservano, tuttavia, oscillazioni che sono legate al crushing che è discontinuo, specie fino a valori di v di 15 MPa. I …
La variazione di schema statico nei ponti in cemento armato precompresso durante le fasi costruttive e l’influenza dei fenomeni lenti
2018
The construction of prestressed concrete bridges often presents different construction stages in which the structural static scheme changes many times, together with the characteristics of the construction elements. In fact, prefabrication and in situ assembly involve a multiple variation of the scheme with the introduction and removal of restraints, especially when temporary sup-ports are used. These transitional phases can strongly modify the structural behavior, in relation also to the deformations due to shrinkage and creep. In this study an evaluation of the effects of time-dependent phenomena is presented, for the case of complex sequences in which delayed restraints and removal of pr…
Fractional calculus application to visco-elastic solid
2009
It is widely known that fractional derivative is the best mathematical tool to describe visco-elastic constitutive law. In this paper it is shown that as soon as we assume the creep compliance function as power law type, as in the linearized version of the Nutting equation, then the fractional constitutive law appears in a natural way. Moreover, using Nutting equation for the creep function, the relaxation modulus is also of power law type whose coefficients (intensity and exponent) are strictly related to those of the creep compliance. It follows that by a simple creep test (or relaxation test) by means of a best fitting procedure we may easily evaluate the parameters of Nutting equation a…
Mechanical response of PLA-based biocomposites under creep conditions
2017
Over the last decades, polymer composites containing natural fibers are receiving growing attention both from academia and the industry, as possible alternatives of composites containing synthetic fibers [1]. Moreover, in order to reduce the environmental impacts related to the use of oil-derived polymer matrices, biodegradable ones obtained from renewable sources such as poly(lactic acid) (PLA) have been considered [2]. Several papers are available regarding the mechanical behavior of natural fibers reinforced PLA composites, but only few deal with their creep behavior [3]. In the present work, the tensile creep behaviour of PLA based composites, containing flax and jute plain weave woven …
CFD simulation of radially stirred baffled and unbaffled tanks
2019
Stirred tanks typically employed in process industries are provided with baffles. Although the presence of baffles is known to guarantee good mixing rates, unbaffled vessels may be compulsory in some applications as crystallization, bioremediation, biotechnology and ore industry. A better understanding of unbaffled stirred vessels flow dynamics may allow (i) a proper design to be performed and (i) conditions/processes where baffle presence can be avoided to be recognized. In the present study, the k-ω SST was used to simulate an unbaffled tank from early to fully turbulent regime (Re≈600-33,000). The unbaffled tank simulated has a diameter T=0.19m and is stirred by a standard six-bladed Rus…
Formation of dislocation patterns: Computer simulations
1996
Dislocations patterns have been extensively studied by means of TEM. In parallel, theoretical approaches have been developed by using two methods; reaction diffusion schemes and computer simulation models. This distinction is not rigid since some computer models include the former approach in their evolution equations. Independently from the difficulties each approach presents in formulating the collective behavior of dislocations, the aim of these studies is to exhibit simple dislocation patterns as persistent slip bands and/or cellular organization. In this context, computer simulations brought a methodology which undoubtedly is a complement to the existing approaches for dislocations. Ne…
<title>Indentation creep and stress relaxation in amorphous As-S-Se and As-S films</title>
2001
The indentation creep in as-deposited and photopolymerized As- S-Se and As-S films has been investigated. The results show pronounced relaxation of mechanical stresses in the films at room temperature. The relationship (sigma) equals B (epsilon) m between the deformation rate and the stress is observed and the values of the deformation rate sensitivity exponent m < 1 are found. The creep rate was found to be dependent on deposition conditions of layers, applied indentation load, structural state of the material and light irradiation during the creep test.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
Free energy and states of fractional-order hereditariness
2014
AbstractComplex materials, often encountered in recent engineering and material sciences applications, show no complete separations between solid and fluid phases. This aspect is reflected in the continuous relaxation time spectra recorded in cyclic load tests. As a consequence the material free energy cannot be defined in a unique manner yielding a significative lack of knowledge of the maximum recoverable work that can extracted from the material. The non-uniqueness of the free energy function is removed in the paper for power-laws relaxation/creep function by using a recently proposed mechanical analogue to fractional-order hereditariness.
How does shear zone nucleate? An example from the Suretta nappe (Swiss Eastern Alps)
2016
In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The…