Search results for "CREEP"
showing 10 items of 167 documents
Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxation measurements
2011
We investigated the relaxation of the irreversible magnetization in a series of 200?nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)n/(PrBCO)m] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T?plastic vortex creep at high T, generated by the …
Origin of the fast magnetization relaxation at low temperatures in HTS with strong pinning
2010
The temperature T variation of the normalized magnetization relaxation rate S in high-temperature superconductors (HTS) with strong vortex pinning exhibits a maximum in the low-T range. This was reported for various HTS, and the origin of the faster relaxation at low T appearing in standard magnetization relaxation measurements was usually related to specific pinning properties of the investigated specimens. Since the observed behaviour seems to be characteristic to all HTS with enhanced pinning (generated by random and/or correlated disorder), we show that the S(T) maximum can be explained in terms of classic collective vortex creep. The influence of thermo-magnetic instabilities in the lo…
On the determination of vortex creep parameters in superconductors using standard magnetization relaxation data
2011
The relaxation of the irreversible magnetic moment m(t) in YBa2Cu3O7 (YBCO) films was investigated as a function of temperature T and the external magnetic field H along the c axis applied in zero-field cooling conditions, for the determination of vortex creep parameters. The data analysis was performed using the T and current density dependence of the normalized vortex creep activation energy, or by the fit of the m(t) data with the well known interpolation formula in the framework of the general vortex creep equation. It was found that (i) even for specimens with strong static pinning the characteristic pinning energy remains small in the low-T range, where the vortex creep appearing in s…
Vortex-creep activation energy in YBa2Cu3O7/PrBa2Cu3O7 superlattices
2010
Abstract YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U 0 was determined by analyzing the in-plane resistive transition of 200 μm wide bridges with the external magnetic field B oriented along the c axis. It was found that U 0 is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U 0 with the current I in the specimen: U 0 exhibits a plateau in the low- I region, then decreases signific…
Evolutionary history of treecreeper vocalisations (Aves: Certhia)☆
2008
AbstractSince the vocalisations of passerine birds are in general a good means to separate taxa when external morphological differences are few, song and call recordings of 33 treecreeper (Certhia) taxa were sonagraphed and their parameters analysed. The vocalisations show low intra-individual and intra-population variation. Phylogenetic evolutionary units at the population level were delimited by time, frequency and syntax parameters by means of principal-component and discriminant analyses. Traits of territorial song were traced on a phylogenetic tree based on cytochrome b sequences, and a mean acoustic character difference was calculated. All presently recognised nine species could be di…
Nonstationary flow surface theory for modeling the viscoplastic behaviors of soils
2016
Abstract This paper presents a three-dimensional elastic viscoplastic model that can describe the time-dependent behaviors of soft clays. The constitutive model is formulated based on the nonstationary flow surface theory and incorporates new developments, including (i) an improved definition of the nonstationary flow surface that is capable of capturing the stress–strain behaviors under different loading paths, (ii) a unique stress–strain—viscoplastic-strain-rate equation that is able to explicitly describe the nonstationary flow surface, and (iii) a final stable state concept that identifies the final equilibrium state at the end of creep and stress relaxation, which is also used to simpl…
Fractional viscoelastic behaviour under stochastic temperature process
2018
Abstract This paper deals with the mechanical behaviour of a linear viscoelastic material modelled by a fractional Maxwell model and subject to a Gaussian stochastic temperature process. Two methods are introduced to evaluate the response in terms of strain of a material under a deterministic stress and subjected to a varying temperature. In the first approach the response is determined making the material parameters change at each time step, due to the temperature variation. The second method, takes advantage of the Time–Temperature Superposition Principle to lighten the calculations. In this regard, a stochastic characterisation for the Time–Temperature Superposition Principle method is p…
Recyclable and Light-Adaptive Vitrimer-Based Nacre-Mimetic Nanocomposites.
2021
Nacre's natural design consists of a perfect hierarchical assembly that resembles a brick-and-mortar structure with synergistic stiffness and toughness. The field of bioinspired materials often provides attractive architecture and engineering pathways which allow to explore outstanding property areas. However, the study of nacre-mimetic materials should not be limited to the design of its architecture but ought to include the understanding, operation, and improvement of internal interactions between their components. Here, we introduce a vitrimer prepolymer system that, once integrated into the nacre-mimetic nanocomposites, cures and cross-links with the presence of Lewis acid catalyst and …
Creep Modeling with Time-Dependent Damping Parameters in Piezoelectric Actuators
2019
This paper develops a creep model based on the Kelvin-Voigt model with time varying damping parameters. In the piezoelectric actuators, the creep phenomenon is an important issue in precise positioning applications as well as the hysteresis property. It is well-known that the creep effect can be represented by a series connection of a number of Kelvin-Voigt elements as a viscoelastic model. In the motion for the continuous stepwise positioning, however, the creep shape is different for each response. Since the phenomenon can be captured as temporal creep relaxation, time-dependent damping parameters are introduced to improve the reproducibility of the creep for the various motion. On the ot…
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.