Strain Localization Mechanisms Research Articles

Image-based crystal plasticity analysis of strain localization mechanism in polycrystalline <i>α</i>Ti

Image-based crystal plasticity analysis of strain localization mechanism in polycrystalline <i>α</i>Ti

Numerical Simulation and Experimental Study of Plastic Strain Localization under the Dynamic Loading of Specimens in Conditions Close to a Pure Shear

Mechanisms of plastic strain localization under the dynamic loading of specially shaped specimens made of the AMg6 aluminum alloy and intended for tests under conditions close to a pure shear on a split Hopkinson–Kolsky pressure bar are studied theoretically and experimentally. The mechanisms of plastic flow instability are related to collective effects in the ensemble of microdefects in spatially localized regions visualized in situ using a CEDIP Silver 450M high-speed infrared camera. The calculation corresponding to the experimental loading scheme is implemented using wide range constitutive equations reflecting the dependence of structural relaxation mechanisms—manifestation of the collective behavior of microdefects—on the evolution of the localized flow shear instability. Microstructure analysis of deformed specimens included the study of the spatial relief (porosity) scaling by the data of a NewView-5010 microscope-interferometer in regions of plastic strain localization. An increase in the structural scaling exponent (Hurst exponent) reflected the degree of the multiscale correlated behavior of defects and the porosity induced by them in regions of localized plasticity. Infrared scanning of the strain localization region and numerical simulation followed by an estimate of the defect structure corroborated the hypothesis that effects of thermal softening do not play a decisive role in the process of plastic shear localization in the tested material under the considered loading regimes. A new, one of the possible ones, mechanism of plastic strain localization under dynamic loading is justified. It is caused by the multiscale collective behavior of mesodefects—structure-scaling transitions—and establishes the stadiality of the localized shear evolution.

Synextensional Granitoids and Detachment Systems Within Cycladic Metamorphic Core Complexes (Aegean Sea, Greece): Toward a Regional Tectonomagmatic Model

AbstractWithin deforming continental regions where metamorphic core complexes (MCCs) and synextensional granitoids are closely associated, deciphering the link between detachment faulting and magmatism often remains complex as (1) the rheological weakness of magma may stimulate mechanisms of strain localization, and conversely, (2) tectonic processes may open/close drains where magmas can intrude. Here we tackle this issue by focusing on the Cyclades with the comparison of five granitoid‐cored MCCs (Tinos, Mykonos, Ikaria, Naxos, and Serifos) and their flanking detachment systems. In this region, granitoids were emplaced into the middle/upper crust over a relatively short time period (15–9 Ma), while metamorphic domes were largely exhumed after more than 10 Myr of extension. None of those intrusions thereby proves to be a real candidate for the genesis of MCCs but would rather be a consequence of a warmer regime during lithospheric thinning. However, all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated. Indeed, late evolution stages of MCCs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred, interfering with the sequential development of detachments. During their tectonically controlled emplacement, magmatic products intruded already formed detachments at depth, locally inhibiting their activity, associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities. The newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions, followed by cataclasis along detachments.

Strain Localization in Mild (Low Carbon) Steel Observed by Acoustic Emission - ESPI Coupling during Tensile Test

Two strain localization modes: the Piobert-Luders band propagation and the development of necking, were investigated in uniaxial tensile tests for a low alloyed and low carbon steel. These two macroscopic localization phenomena were simultaneously monitored by speckle interferometry (ESPI) and acoustic emission (AE). The coupling of these two experimental techniques gives complementary information about strain localization features and mechanisms. For Luders bands, it was found that the acoustic activity heard during the travel of the Piobert-Luders band varies in closely correlated to the tensile force fluctuations, the relations between strain rate, band velocity, band width and plastic strain were investigated. Although the strain rate in the wake of the wave front is not always zero, the acoustic activity remains concentrated in the wave front itself. For necking, the acoustic activity is found to decrease regularly through the homogeneous plasticity stage and the diffuse necking stage and then increases when the localized necking starts, while ESPI patterns show a gradual strain concentration.

Special Features of Strain Localization and Nanodipoles of Partial Disclinations in the Region of Elastic Distortions

In the zones of strain localization in the region of elastic distortions and nanodipoles of partial disclinations representing the defects of elastically deformed medium, a theoretical analysis of the elastically stressed state and the energy of these defects, including the cases of their transformation into more complex ensembles of interrelated disclinations, is performed. Using the analytical results, the mechanisms of strain localization are discussed in the stages of nucleation and propagation of the bands of elastic and plastic strain localization formed in these zones (including the cases of nanocrystalline structure formation).

Unraveling submicron-scale mechanical heterogeneity by three-dimensional X-ray microdiffraction

Shear banding is a ubiquitous phenomenon of severe plastic deformation, and damage accumulation in shear bands often results in the catastrophic failure of a material. Despite extensive studies, the microscopic mechanisms of strain localization and deformation damage in shear bands remain elusive due to their spatial-temporal complexities embedded in bulk materials. Here we conducted synchrotron-based X-ray microdiffraction (μXRD) experiments to map out the 3D lattice strain field with a submicron resolution around fatigue shear bands in a stainless steel. Both in situ and postmortem μXRD results revealed large lattice strain gradients at intersections of the primary and secondary shear bands. Such strain gradients resulted in severe mechanical heterogeneities across the fatigue shear bands, leading to reduced fatigue limits in the high-cycle regime. The ability to spatially quantify the localized strain gradients with submicron resolution through μXRD opens opportunities for understanding the microscopic mechanisms of damage and failure in bulk materials.

Deformation mechanism of strain localization in 2D numerical interface tests

Heterogeneity arises in soil subjected to interface shearing, with the strain gradually localizing into a band area. How the strain localization accumulates and develops to form the structure is crucial in explaining some significant constitutive behaviors of the soil–structural interface during shearing, for example, stress hardening, softening, and shear-dilatancy. Using DEM simulation, interface shear tests with a periodic boundary condition are performed to investigate the strain localization process in densely and loosely packed granular soils. Based on the velocity field given by grains’ translational and rotational velocities, several kinematic quantities are analyzed during the loading history to demonstrate the evolution of strain localization. Results suggest that tiny concentrations in the shear deformation have already been observed in the very early stage of the shear test. The degree of the strain localization, quantified by a proposed new indicator, α, steadily ascends during the stress-hardening regime, dramatically jumps prior to the stress peak, and stabilizes at the stress steady state. Loose specimen does not develop a steady pattern at the large strain, as the deformation pattern transforms between localized and diffused failure modes. During the stress steady state of both specimens, remarkable correlations are observed between α and the shear stress, as well as between α and the volumetric strain rate.

A Probabilistic Crack Band Model for Quasibrittle Fracture

This paper presents a new crack band model (CBM) for probabilistic analysis of quasibrittle fracture. The model is anchored by a probabilistic treatment of damage initiation, localization, and propagation. This model regularizes the energy dissipation of a single material element for the transition between damage initiation and localization. Meanwhile, the model also takes into account the probabilistic onset of damage localization inside the finite element (FE) for the case where the element size is larger than the crack band width. The random location of the localization band is related to the random material strength, whose statistics is described by a finite weakest link model. The present model is applied to simulate the probability distributions of the nominal strength of different quasibrittle structures. It is shown that for quasibrittle structures direct application of the conventional CBM for stochastic FE simulations would lead to mesh-sensitive results. To mitigate such mesh dependence, it is essential to incorporate the strain localization mechanism into the formulation of the sampling distribution functions of material constitutive parameters.

Numerical simulation of plastic strain localization and failure mode transition in metals under dynamic loading

The research is focused on simulation and experimental investigation of high-strain rate loading to study the mechanisms of plastic flow localization and failure mode transition in metals (AlMg6). The task of breaking down barriers by metal shell was examined. Model based on the statistical-thermodynamic description of deformation of solids with mesoscopic defects (microshears and micro-cracks) was used to simulate the behavior of metals under dynamic loading. Experimental studies were also carried out. The real-time lateral surface temperature was measured by high speed infrared camera CEDIP Silver 450M. The consistent result of “in-situ” experiment is the elevated temperature of strain localization area that doesn’t exceed 72°C. The results of numerical simulation are in a good agreement with the in-situ measured infrared experimental data. Experimental and theoretical (numerical) study of dynamic strain localization allowed us to establish the structure induced mechanism of plastic strain localization as possible mechanism of transition to the adiabatic shear failure. Evaluated temperature in the strain localization area (72°C) will not support conventionally used mechanisms of plastic strain localization as autocatalytic temperature control visco-plastic phenomena. Structural study of defect induced scaling properties revealed the correlated behavior of microshear ensemble that can be classified as structural transition providing the dynamic strain localization.

An analytical model for predicting the response of RC beams strengthened with strain localized steel plate

A new plating system based on strain localization mechanism has been recently proposed for strengthening RC structures to overcome the debonding problem. With this new technique, weaker zones are placed on a steel plate to accommodate large local strains, hence delaying or preventing interfacial debonding. This paper develops an analytical method to predict the performance of RC beams strengthened with this new technique. The results agree well with those from finite element analyses. A parametric study is also conducted to investigate effect of the geometric dimension and material properties of the weaker zone on performance of such strengthened RC beam.

Structural and temporal evolution of a reactivated brittle–ductile fault – Part I: Fault architecture, strain localization mechanisms and deformation history

Faults are by nature dynamic, as their architecture and composition evolve progressively in space and through time steered by the interplay between strain weakening and hardening mechanisms. This study combines structural analysis, geochemistry and chlorite geothermometry to investigate deformation and strain localization mechanisms of the Kvenklubben fault, a Paleozoic brittle–ductile thrust in northern Norway, with the goal to constrain their temporal variations and the consequences thereof on fault architecture development and rheological behavior. The fault evolved from an initially discrete brittle feature slipping mainly by seismogenic ruptures to a wide brittle–ductile phyllonite deforming by aseismic creep. The formation of mechanically weak phyllosilicates by decarbonation of footwall dolostones and carbonation of hanging wall metabasalts was the main weakening mechanism, whereas partitioning of fluid flow and fracture sealing following transient high pore pressure-driven embrittlement caused episodic and localized strain hardening. The interplay between strain weakening and hardening mechanisms caused the fault core to widen. We suggest that the ability for carbonate-hosted faults to slip by seismogenic rupture is also a function of the faults' structural-evolutionary stage, and that it decreases progressively with fault maturity. This study demonstrates the importance of calibrating the present-day fault anatomy against the dynamic character of faults, which evolve geometrically, compositionally and mechanically in space and through time.

Structural and temporal evolution of a reactivated brittle–ductile fault – Part II: Timing of fault initiation and reactivation by K–Ar dating of synkinematic illite/muscovite

Present-day exposures of ancient faults represent only the end result of the faults' often protracted and heterogeneous histories. Here we apply K–Ar dating of synkinematic illite/muscovite to constrain the timing of the complete temporal evolution of a complex, multiply-reactivated brittle–ductile fault, the Kvenklubben Fault in northern Norway. All obtained ages vary as a function of grain size. Geologically significant events are identified principally on the basis of detailed structural analysis presented in a companion paper (Torgersen and Viola, 2014). Faulting initiated at 531±11Ma, but most strain was accommodated during Caledonian compression at 445±9Ma. The fault was reactivated extensionally at 121±5Ma. C and O isotopic composition of carbonates and silicates in the fault rocks demonstrates that mineral authigenesis was linked to wall–rock disintegration through dolomite decarbonation and metabasalt carbonation. We suggest that the commonly observed case of age decreasing with grain size in K–Ar and 40Ar/39Ar dating of brittle fault rocks can be interpreted as a consequence of mixing between two end-member illite/muscovite generations: an authigenic and a protolithic, in which the finest authigenic grains constrain the timing of the last faulting increment. Integrating detailed structural analysis with age dating is the key towards a better understanding of fault architecture development and the temporal evolution of strain localization and deformation mechanisms.

The effect of microstructure heterogeneity on the microscale deformation of ultrafine-grained aluminum

Abstract

Debonding Inhibiting Mechanism of Strain Localization Plating System

AbstractRecently, a new plating system was proposed based on a strain localization mechanism for addressing the critical debonding problem in externally bonded reinforcing systems for reinforced concrete (RC) structures. The system delays or inhibits debonding by localizing the large strain of the externally bonded reinforcement in certain areas and leaving other places in a smaller strain condition to ensure that global debonding does not occur. This debonding avoidance strategy is in line with the concept of controlling a flood by channeling water away rather than blocking it. Two techniques have been used to achieve strain localization: (1) a nanotechnology called surface mechanical attrition treatment and (2) variation in the cross-sectional area of bonded reinforcement. To better understand this newly developed system, this paper presents a numerical study of RC beams strengthened with such plating systems. This study leads to a better understanding of the mechanism for inhibiting debonding and the d...

The Minimized Power Geometric model: An analytical mixing model for calculating polyphase rock viscosities consistent with experimental data

AbstractHere we introduce the Minimized Power Geometric (MPG) model which predicts the viscosity of any polyphase rocks deformed during ductile flow. The volumetric fractions and power law parameters of the constituting phases are the only model inputs required. The model is based on a minimization of the mechanical power dissipated in the rock during deformation. In contrast to existing mixing models based on minimization, we use the Lagrange multipliers method and constraints of strain rate and stress geometric averaging. This allows us to determine analytical expressions for the polyphase rock viscosity, its power law parameters, and the partitioning of strain rate and stress between the phases. The power law bulk behavior is a consequence of our model and not an assumption. Comparison of model results with 15 published experimental data sets on two‐phase aggregates shows that the MPG model reproduces accurately both experimental viscosities and creep parameters, even where large viscosity contrasts are present. In detail, the ratio between experimental and MPG‐predicted viscosities averages 1.6. Deviations from the experimental values are likely to be due to microstructural processes (strain localization and coeval other deformation mechanisms) that are neglected by the model. Existing models that are not based on geometric averaging show a poorer fit with the experimental data. As long as the limitations of the mixing models are kept in mind, the MPG model offers great potential for applications in structural geology and numerical modeling.

On the effects of thermally weakened ductile shear zones on postseismic deformation

We present three‐dimensional (3‐D) numerical models of postseismic deformation following repeated earthquakes on a vertical strike‐slip fault. Our models use linear Maxwell, Burgers, and temperature‐dependent power law rheology for the lower crust and upper mantle. We consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate‐strengthening friction on a deep aseismic fault root. We find that the thermally activated shear zones have little effect on postseismic relaxation. In particular, the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. Stronger rheologies can give rise to an opposite polarity of vertical displacements, but the amplitude of the predicted transient deformation is generally negligible. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. We also investigate the discriminating power of models incorporating Burgers and power law rheology. These rheologies were proposed to explain postseismic transients following large (M7) earthquakes in the Mojave desert, Eastern California. Numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high‐quality geodetic observations for observation periods less than a decade. Longer observations, however, may potentially allow discrimination between the competing models, as illustrated by the model comparisons with available GPS and interferometric synthetic aperture radar data.

Is earthquake activity along the French Atlantic margin favoured by local rheological contrasts?

The seismological study of recent seismic crises near Oleron Island confirms the coexistence of an extensional deformation and a transtensive regime in the Atlantic margin of France, which is different from the general western European stress field corresponding to a strike-slip regime. We argue that the switch of the principal stress axes σ1/σ2 in a NW–SE vertical plane is linked with the existence of crustal heterogeneities. Events of magnitude larger than 5 sometimes occur along the Atlantic margin of France, such as the 7 September 1972 (ML=5.2) earthquake near Oleron island and the 30 September 2002 (ML=5.7) Hennebont event in Brittany. To test the mechanism of local strain localization, we model the deformation of the hypocentral area of the Hennebont earthquake using a 3D thermo-mechanical finite element code. We conclude that the occurrence of moderate earthquakes located in limited parts of the Hercynian shear zones (as the often reactivated swarms near Oleron) could be due to local reactivation of pre-existing faults. These sporadic seismic ruptures are favoured by stress concentration due to rheological heterogeneities.

Study on Cyclic Strain Localization and Fatigue Fracture Mechanism in High Manganese Twinning-Induced Plasticity Steels

The fatigue cracking mechanisms of two high Mn TWinning-Induced Plasticity (TWIP) steels are investigated in detail using electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Furthermore, the fracture surfaces of the fatigued steels have been studied by employing a field emission gun scanning electron microscope (FEG-SEM). The fine details of the fatigued surface topography are verified using an atomic force microscope (AFM). The results indicate that the fatigue crack embryos nucleate at an early stage of the fatigue life as a result of local straining at grain and annealing twin boundaries at sites, where persistent slip bands create dislocation piled-ups that impinge on boundaries. The EBSD measurements showed that unlike in monotonic straining, the formation of deformation twins is not observed under cyclic straining.

A spatial strain localization mechanism of zonal disintegration through numerical simulation

The zonal disintegration is studied from a viewpoint of the spatial strain localization, a prospective idea to tackle the problem. The three-dimensional loading and unloading models are modeled numerically based on a strain-softening heterogeneous constitutive model. In the loading model, after a cylindrical tunnel is excavated, the displacement-controlled loading is conducted in the direction of the tunnel axis. However, in the unloading model, after the model has reached a static equilibrium state, a cylindrical tunnel is excavated step by step. In loading and unloading conditions, numerical results reveal that the zonal disintegration phenomenon that annular regions with high shear strains are isolated or divided by those with lower shear strains is existent. Compared with results in the loading model, results in the unloading model are even consistent with the field observations. At a plane orthogonal to the tunnel axis, the regions with higher shear strains, far away from the tunnel surface, are formed by propagation of shear bands that do not originate from the plane.

Plastic strain localization and fracture mechanisms of metals subjected to dynamic loading

The mechanisms responsible for instability and localization of plastic shear during dynamic deformation of metals are investigated. The instability mechanisms are related to the collective effects in the ensemble of microshears in the regions of space localization. In-situ infrared scanning of the instability region and the analysis of the dislocation substructure confirm the suggestion about the key role of non-equilibrium transitions in the ensembles of defects for the localized plastic flow evolution. Equations that describe the relationships between the non-equilibrium transitions and the mechanisms of structural relaxation and plastic flow are used to model the plastic shear instability.