Página 1 dos resultados de 1419 itens digitais encontrados em 0.023 segundos

## Failure by Simultaneous Grain Growth, Strain Localization, and Interface Debonding in Metal Films on Polymer Substrates

Vlassak, Joost J.; Lu, Nanshu; Wang, Xi; Suo, Zhigang
Fonte: Materials Research Society Publicador: Materials Research Society
Tipo: Artigo de Revista Científica
Português
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In a previous paper, we have demonstrated that a microcrystalline copper film well bonded to a polymer substrate can be stretched beyond 50% without cracking. The film eventually fails through the co-evolution of necking and debonding from the substrate. Here we report much lower strains to failure (around 10%) for polymer-supported nanocrystalline metal films, whose microstructure is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. We therefore propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth and film debonding from the substrate.; Engineering and Applied Sciences

## Earthquake Cycle Deformation in the Tibetan Plateau with a Weak Mid-Crustal Layer

DeVries, Phoebe M. R.; Meade, Brendan J.
Tipo: Artigo de Revista Científica
Português
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Geodetic observations of interseismic deformation across the Tibetan plateau contain information about both tectonic and earthquake cycle processes. Time-variations in surface velocities between large earthquakes are sensitive to the rheological structure of the subseismogenic crust, and, in particular, the viscosity of the middle and lower crust. Here we develop a semianalytic solution for time-dependent interseismic velocities resulting from viscoelastic stress relaxation in a localized midcrustal layer in response to forcing by a sequence of periodic earthquakes. Earthquake cycle models with a weak midcrustal layer exhibit substantially more near-fault preseismic strain localization than do classic two-layer models at short (<100 yr) Maxwell times. We apply both this three-layer model and the classic two-layer model to geodetic observations before and after the 1997 (M_W = 7.6) Manyi and 2001 (M_W = 7.8) Kokoxili strike-slip earthquakes in Tibet to estimate the viscosity of the crust below a 20 km thick seismogenic layer. For these events, interseismic stress relaxation in a weak ((viscosity leq10^{18.5} Pa⋅s)) and thin (height ≤20 km) midcrustal layer explains observations of both preseismic near-fault strain localization and rapid (>50 mm/yr) postseismic velocities in the years following the coseismic ruptures. We suggest that earthquake cycle models with a localized midcrustal layer can simultaneously explain both preseismic and postseismic geodetic observations with a single Maxwell viscosity...

## Stability and Localization of Rapid Shear in Fluid-Saturated Fault Gouge: 1. Linearized Stability Analysis

Rice, James Robert; Rudnicki, John W.; Platt, John Daniel
Tipo: Artigo de Revista Científica
Português
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Field observations of major earthquake fault zones show that shear deformation is often confined to principal slipping zones that may be of order 1–100 μm wide, located within a broader gouge layer of order 10–100 mm wide. This paper examines the possibility that the extreme strain localization observed may be due to the coupling of shear heating, thermal pressurization, and diffusion. In the absence of a stabilizing mechanism shear deformation in a continuum analysis will collapse to an infinitesimally thin zone. Two possible stabilizing mechanisms, studied in this paper, are rate-strengthening friction and dilatancy. For rate-strengthening friction alone, a linear stability analysis shows that uniform shear of a gouge layer is unstable for perturbations exceeding a critical wavelength. Using this critical wavelength we predict a width for the localized zone as a function of the gouge properties. Taking representative parameters for fault gouge at typical centroidal depths of crustal seismogenic zones, we predict localized zones of order 5–40 μm wide, roughly consistent with field and experimental observations. For dilatancy alone, linearized strain rate perturbations with a sufficiently large wavelength will undergo transient exponential growth before decaying back to uniform shear. The total perturbation strain accumulated during this transient strain rate localization is shown to be largely controlled by a single dimensionless parameter E...

## Stability and Localization of Rapid Shear in Fluid-Saturated Fault Gouge: 2. Localized Zone Width and Strength Evolution

Platt, John Daniel; Rudnicki, John W.; Rice, James Robert
Tipo: Artigo de Revista Científica
Português
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47.169014%
Field and laboratory observations indicate that at seismic slip rates most shearing is confined to a very narrow zone, just a few tens to hundreds of microns wide, and sometimes as small as a few microns. Rice et al. (2014) analyzed the stability of uniform shear in a fluid-saturated gouge material. They considered two distinct mechanisms to limit localization to a finite thickness zone, rate-strengthening friction, and dilatancy. In this paper we use numerical simulations to extend beyond the linearized perturbation context in Rice et al. (2014), and study the behavior after the loss of stability. Neglecting dilatancy we find that straining localizes to a width that is almost independent of the gouge layer width, suggesting that the localized zone width is set by the physical properties of the gouge material. Choosing parameters thought to be representative of a crustal depth of 7 km, this predicts that deformation should be confined to a zone between 4 and 44 μm wide. Next, considering dilatancy alone we again find a localized zone thickness that is independent of gouge layer thickness. For dilatancy alone we predict localized zone thicknesses between 1 and 2 μm wide for a depth of 7 km. Finally, we study the impact of localization on the shear strength and temperature evolution of the gouge material. Strain rate localization focuses frictional heating into a narrower zone...

## Crustal Thickening, Partial Melting, and Strain Localization: Insights from the Leo Pargil dome, NW India

Jessup, M.; Cottle, J.; Langille, J.; Lederer, G.; Ahmat, T.
Tipo: Sonstiges
Português
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Abstract HKT-ISTP 2013 AO

## On the interplay between strain rate and strain rate sensitivity on flow localization in the dynamic expansion of ductile rings

Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article Formato: application/pdf
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In this work a stability analysis on flow localization in the dynamic expansion of ductile rings is con ducted. Within a 1 D theoretical framework, the boundary value problem of a radially expanding thin ring is posed. Based on a previous work, the equations governing the stretching process of the expanding ring are derived and solved using a linear perturbation method. Then, three different perfectly plastic material constitutive behaviours are analysed: the rate independent material, the rate dependent mate rial showing constant logarithmic rate sensitivity and the rate dependent material showing non constant and non monotonic logarithmic rate sensitivity. The latter allows to investigate the interaction between inertia and strain rate sensitivity on necking formation. The main feature of this work is rationally dem onstrate that under certain loading conditions and material behaviours: (1) decreasing rate sensitivity may not lead to more unstable material, (2) increasing loading rate may not lead to more stable material. This finding reveals that the relation between rate sensitivity and loading rate controls the unstable flow growth. Additionally a finite element model of the ring expansion problem is built in ABAQUS/Explicit. The stability analysis properly reflects the results obtained from the numerical simulations. Both proce dures...

## Finite element analysis of AISI 304 steel sheets subjected to dynamic tension: The effects of martensitic transformation and plastic strain development on flow localization

Rodríguez-Martínez, José Antonio; Rittel, D.; Zaera, Ramón; Osovski, S.
Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article Formato: application/pdf; text/plain
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The paper presents a finite element study of the dynamic necking formation and energy absorption in AISI 304 steel sheets. The analysis emphasizes the effects of strain induced martensitic transformation (SIMT) and plastic strain development on flow localization and sample ductility. The material behavior is described by a constitutive model proposed by the authors which includes the SIMT at high strain rates. The process of martensitic transformation is alternatively switched on and off in the simulations in order to highlight its effect on the necking inception. Two different initial conditions have been applied: specimen at rest which is representative of a regular dynamic tensile test, and specimen with a prescribed initial velocity field in the gauge which minimizes longitudinal plastic wave propagation in the tensile specimen. Plastic waves are found to be responsible for a shift in the neck location, may also mask the actual constitutive performance of the material, hiding the expected increase in ductility and energy absorption linked to the improved strain hardening effect of martensitic transformation. On the contrary, initializing the velocity field leads to a symmetric necking pattern of the kind described in theoretical works...

## INVESTIGATION OF FULL SCALE HORIZONTAL PIPE-SOIL INTERACTION AND LARGE STRAIN BEHAVIOUR OF SAND

BURNETT, ALEXANDER
Fonte: Quens University Publicador: Quens University
Português
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Oil and gas pipelines buried off the east coast of Canada are in a complex and dynamic environment with many inherent risks, including scour events by large icebergs in shallow water, which present a significant hazard to their safety and structural integrity. A thorough understanding of pipe soil interaction under large deformations imposed either by iceberg scour or other means, is critical to the design of oil and gas pipelines and to the current understanding of pipeline-soil interaction. Iceberg keel gouging events are a large strain numerical problem and experimental data evaluating shear strain behaviour of the test material, strain localization of the soil shear bands, and material flow into the void left behind the displaced pipe are needed in order to model the pipe-soil interaction behaviour involved in these processes. A full scale laboratory test configuration was developed to performed experimental work on horizontal pipe-soil interaction, modeling a large deformation event similar to iceberg scour. Lateral pipe displacement tests were conducted with industry grade pipelines up to 0.61 m in diameter, tested at two burial depths in both loose and dense synthetic olivine sand. Transparent sidewalls were incorporated into the test apparatus to enable the use of digital image correlation (DIC) to determine soil displacements. DIC provides a new source of high quality experimental data which can capture the complex large strain soil behaviour in full scale pipe-soil interaction tests. DIC was used to observe the shear strain behaviour of the test soil...

## Strain localization analysis for single crystals and polycrystals: Towards microstructure-ductility linkage

FRANZ, Gérald; ABED-MERAIM, Farid; BERVEILLER, Marcel
Fonte: PERGAMON-ELSEVIER SCIENCE LTD Publicador: PERGAMON-ELSEVIER SCIENCE LTD
Português
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In this paper, we performed a strain localization analysis for single crystals and polycrystals, with the specific aim of establishing a link between the microstructure-related parameters and ductility. To this end, advanced large-strain elastic plastic single crystal constitutive modeling is adopted, accounting for the key physical mechanisms that are relevant at the microscale, such as dislocation storage and annihilation. The self-consistent scale-transition scheme is then used to derive the overall constitutive response of polycrystalline aggregates, including the essential microstructural aspects (e.g., initial and induced textures, dislocation density evolution, and softening mechanisms). The resulting constitutive equations for single crystals and polycrystals are coupled with two strain localization criteria: bifurcation theory, which is also related to the loss of ellipticity in the associated boundary value problem, and the strong ellipticity condition, which is presented in full detail along with mathematical links allowing for hierarchical classification in terms of conservativeness. The application of the proposed coupling to single crystals and polycrystals allows the effect of physical microstructural parameters on material ductility to be investigated. Consistent results are found for both single crystals and polycrystals. In addition...

## Formability prediction of thin metal sheets using various localization criteria

ALTMEYER, Guillaume; ABED-MERAIM, Farid; BALAN, Tudor
Português
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The aim of this paper is to give an overview of the theoretical basis of the most significant and commonly used localization criteria reformulated into a unified framework, and to apply these criteria to different materials in order to determine their formability domains. After giving a general material description based on a phenomenological approach, theoretical foundations of localization criteria are presented together with their advantages and drawbacks. These criteria rely on diverse theories: maximum load principle, bifurcation analysis, Marciniak-Kuczynski analysis, and stability analysis by a linear perturbation method. They are then applied to a brass and a Dual Phase steel and the predicted Forming Limit Diagrams (FLD) are discussed.

## Overview of the theoretical relations between necking and strain localization criteria.

ALTMEYER, Guillaume; ABED-MERAIM, Farid; BALAN, Tudor
Fonte: Arts et Métiers ParisTech Publicador: Arts et Métiers ParisTech
Português
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Many criteria have been developed during last decades to predict diffuse or localized necking and shear banding. The lack of confrontation of these models with each other on relevant applications makes their choice difficult for the designer. It is proposed to reformulate these plastic instability criteria in an unified framework, to compare their theoretical bases to establish links between them and then to highlighten their limitations. In the case of diffuse necking, a comparison is made between the criteria based on bifurcation analysis and on those based on maximum force principle for elastic-plastic materials. In the case of localized modes, it is shown that the predictions of the Marciniak – Kuczynski approach, based on a multizone model, tend to those of the loss of ellipticity criterion when the initial defect size tends to zero (no initial defect introduced). In the case of elasto-viscoplastic behavior, an approach based on a linear stability analysis is mentioned.

## Failure-Oriented Multi-scale Variational Formulation: Micro-structures with nucleation and evolution of softening bands

Sánchez, Pablo Javier; Blanco, P.J.; Huespe, Alfredo Edmundo; Feijóo, R.A.
Fonte: Elsevier Science Sa Publicador: Elsevier Science Sa
Tipo: info:eu-repo/semantics/article; info:ar-repo/semantics/artículo; info:eu-repo/semantics/publishedVersion Formato: application/pdf
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This contribution presents the theoretical foundations of a Formulation (FOMF) for modeling heterogeneous softening-based materials undergoing strain localization phenomena. The multi-scale model considers two coupled mechanical problems at different physical length scales, denoted as macro and micro scales, respectively. Every point, at the macro scale, is linked to a Representative Volume Element (RVE), and its constitutive response emerges from a consistent homogenization of the micro-mechanical problem. At the macroscopic level, the initially continuum medium admits the nucleation and evolution of cohesive cracks due to progressive strain localization phenomena taking place at the microscopic level and caused by shear bands, damage or any other possible failure mechanism. A cohesive crack is introduced in the macro model once a specific macroscopic failure criterion is fulfilled. The novelty of the present Failure-Oriented Multi-scale Formulation is based on a proper kinematical information transference from the macro-to-micro scales during the complete loading history, even in those points where macro cracks evolve. In fact, the proposed FOMF includes two multi-scale sub-models consistently coupled: (i) a Classical Multi-scale Model (ClaMM) valid for the stable macro-scale constitutive response.ii) A novel Cohesive Multi-scale Model (CohMM) valid...

## Strain localization in pyroxenite by reaction-enhanced softening in the shallow subcontinental lithospheric mantle

Hidas, Karoly; Garrido, Carlos J; Tommasi, Andrea; Padron-Navarta, Jose; Thielmann, M; Konc, Zoltan; Frets, Erwin; Marchesi, C
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica
Português
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We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the b

## Shear strain localization in elastodynamic rupture simulations

Daub, Eric G.; Manning, M. Lisa; Carlson, Jean M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We study strain localization as an enhanced velocity weakening mechanism on earthquake faults. Fault friction is modeled using Shear Transformation Zone (STZ) Theory, a microscopic physical model for non-affine rearrangements in granular fault gouge. STZ Theory is implemented in spring slider and dynamic rupture models of faults. We compare dynamic shear localization to deformation that is uniform throughout the gouge layer, and find that localized slip enhances the velocity weakening of the gouge. Localized elastodynamic ruptures have larger stress drops and higher peak slip rates than ruptures with homogeneous strain.; Comment: 6 pages, 3 figures

## Strain localization in a shear transformation zone model for amorphous solids

Manning, M. L.; Langer, J. S.; Carlson, J. M.
Tipo: Artigo de Revista Científica
Português
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We model a sheared disordered solid using the theory of Shear Transformation Zones (STZs). In this mean-field continuum model the density of zones is governed by an effective temperature that approaches a steady state value as energy is dissipated. We compare the STZ model to simulations by Shi, et al.(Phys. Rev. Lett. 98 185505 2007), finding that the model generates solutions that fit the data,exhibit strain localization, and capture important features of the localization process. We show that perturbations to the effective temperature grow due to an instability in the transient dynamics, but unstable systems do not always develop shear bands. Nonlinear energy dissipation processes interact with perturbation growth to determine whether a material exhibits strain localization. By estimating the effects of these interactions, we derive a criterion that determines which materials exhibit shear bands based on the initial conditions alone. We also show that the shear band width is not set by an inherent diffusion length scale but instead by a dynamical scale that depends on the imposed strain rate.; Comment: 8 figures, references added, typos corrected

## Disruption of Thermally-Stable Nanoscale Grain Structures by Strain Localization

Khalajhedayati, Amirhossein; Rupert, Timothy J.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
47.181704%
Nanocrystalline metals with average grain sizes of only a few nanometers have recently been observed to fail through the formation of shear bands. Here, we investigate this phenomenon in nanocrystalline Ni which has had its grain structure stabilized by doping with W, with a specific focus on understanding how strain localization drives evolution of the nanoscale grain structure. Shear banding was initiated with both microcompression and nanoindentation experiments, followed by site-specific transmission electron microscopy to characterize the microstructure. Grain growth and texture formation were observed inside the shear bands, which had a wide variety of thicknesses. These evolved regions have well-defined edges, which rules out local temperature rise as a possible formation mechanism. No structural evolution was found in areas away from the shear bands, even in locations where significant plastic deformation had occurred, showing that plastic strain alone is not enough to cause evolution. Rather, intense strain localization is needed to induce mechanically-driven grain growth in a thermally-stable nanocrystalline alloy.; Comment: 6 figures

## Strain localization and percolation of stable structure in amorphous solids

Shi, Yunfeng; Falk, Michael L.
Tipo: Artigo de Revista Científica
Português
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Spontaneous strain localization occurs during mechanical tests of a model amorphous solid simulated using molecular dynamics. The degree of localization depends upon the extent of structural relaxation prior to mechanical testing. In the most rapidly quenched samples higher strain rates lead to increased localization, while the more gradually quenched samples exhibit the opposite strain rate dependence. This transition coincides with the k-core percolation of atoms with quasi-crystal-like short range order. The authors infer the existence of a related microstructural length scale.; Comment: 4 pages, 4 figures

## Strain localization in a nanocrystalline metal: Atomic mechanisms and the effect of testing conditions

Rupert, Timothy J.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Molecular dynamics simulations are used to investigate strain localization in a model nanocrystalline metal. The atomic mechanisms of such catastrophic failure are first studied for two grain sizes of interest. Detailed analysis shows that the formation of a strain path across the sample width is crucial, and can be achieved entirely through grain boundary deformation or through a combination of grain boundary sliding and grain boundary dislocation emission. Pronounced mechanically-induced grain growth is also found within the strain localization region. The effects of testing conditions on strain localization are also highlighted, to understand the conditions that promote shear banding and compare these observations to metallic glass behavior. We observed that, while strain localization occurs at low temperatures and slow strain rates, a shift to more uniform plastic flow is observed when either strain rate or temperature is increased. We also explore how external sample dimensions influence strain localization, but find no size effect for the grain sizes and samples sizes studied here.; Comment: Published in Journal of Applied Physics

## Grain fragmentation in sheared granular flow: weakening effects, energy dissipation, and strain localization

Lieou, Charles K. C.; Elbanna, Ahmed E.; Carlson, Jean M.
Tipo: Artigo de Revista Científica
Português
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We describe the shear flow of a disordered granular material in the presence of grain fracture using the shear-transformation-zone (STZ) theory of amorphous plasticity adapted to systems with a hard-core inter-particle interaction. To this end, we develop the equations of motion for this system within a statistical-thermodynamic framework analogous to that used in the analysis of molecular glasses. For hard-core systems, the amount of internal, configurational disorder is characterized by the compactivity $X = \partial V / \partial S_C$, where $V$ and $S_C$ are respectively the volume and configurational entropy. Grain breakage is described by a constitutive equation for the temporal evolution of a characteristic grain size $a$, based on fracture mechanics. We show that grain breakage is a weakening mechanism, significantly lowering the flow stress at large strain rates, if the material is rate-strengthening in character. We show in addition that if the granular material is sufficiently aged, spatial inhomogeneity in configurational disorder results in strain localization. We also show that grain splitting contributes significantly to comminution at small shear strains, while grain abrasion becomes dominant at large shear displacements.; Comment: 16 pages...

Jagla, E. A.