Search results for "Creep"
showing 10 items of 167 documents
Effects of forest patch size on physiological stress and immunocompetence in an area-sensitive passerine, the Eurasian treecreeper ( Certhia familiar…
2004
We manipulated the primary brood size of Eurasian treecreepers (Certhia familiaris) breeding in different sized forest patches (0.5-12.8 ha) in moderately fragmented landscapes. We examined the effects of brood size manipulation (reduced, control, enlarged) and forest patch size on physiological stress (heterophil-lymphocyte ratios; H/L), body condition and cell-mediated immunocompetence (phytohaemagglutinin test). Nestlings' H/L ratios were negatively related to forest patch area in control and enlarged broods, whereas no effects were found in reduced broods. The effects of forest patch area were strongest in enlarged broods, which had, in general, twofold higher H/L ratios than control an…
The finite element implementation of 3D fractional viscoelastic constitutive models
2018
Abstract The aim of this paper is to present the implementation of 3D fractional viscoelastic constitutive theory presented in Alotta et al., 2016 [1]. Fractional viscoelastic models exactly reproduce the time dependent behaviour of real viscoelastic materials which exhibit a long “fading memory”. From an implementation point of view, this feature implies storing the stress/strain history throughout the simulations which may require a large amount of memory. We propose here a number of strategies to effectively limit the memory required. The form of the constitutive equations are summarized and the finite element implementation in a Newton-Raphson integration scheme is described in detail. …
Fractional model of concrete hereditary viscoelastic behaviour
2016
The evaluation of creep effects in concrete structures is addressed in the literature using different predictive models, supplied by specific codes, and applying the concepts of linear viscoelastic theory with ageing. The expressions used in the literature are mainly based on exponential laws, which are introduced in the integral expression of the Boltzmann principle; this approach leads to the need of finding approximated numerical solutions of the viscoelastic response. In this study, the hereditary fractional viscoelastic model is applied to concrete elements, underlining the convenience of using creep or relaxation functions expressed by power laws. The full reciprocal character of cree…
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…
The Rehabilitation of Segmental Bridges Subjected to Time-dependent Deflections Through Additional Prestressing
2022
Segmental bridges built by cantilevering with in-situ casting in the ‘60s-‘80s of the XX Century present a central hinge on the main span or a central simply supported span between the two cantilevers (drop-in span). Due to time-dependent phenomena like creep of concrete and to unexpected loss of prestressing, several bridges of this typology show an abnormal deflection at hinges causing discomfort of traffic as well as insufficiencies of checks at Service and Ultimate Limit States. Moreover, these bridges are today at the end of their nominal service life, impacting significantly on Italian road network. A methodology for retrofitting these bridges by partial recovering of deflection and o…
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…