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Subjected to external loads, granular materials experience severe deformation in a narrow zone before their failure. This phenomenon, which is called strain localisation or shear band, is of vital importance in assessing the stability of the geotechnical structure, studying the stress-strain behaviour of soil and rock materials, and analysing the interaction of soil and structure. The present study is aimed to investigate the effect of various factors on the pattern and inclination of shear band in a general threedimensional condition of stress using the Discrete Element Method (DEM). Several tests were simulated using a developed version of the TRUBAL program called GRANULE. The GRANULE code was further developed to add the capability of carrying out simulations with different intermediate principal stresses and modelling specimens containing non-spherical particles. The shear band was detected by tracking the motion of the particles and plotting the rotation distribution of particles within the sample. The results prove that the shear band inclination and its pattern, are greatly affected by intermediate principal stress, particle shape, and confining stress. Moreover, it was observed that the change in the b value plays a key role in the alteration of the 3D configuration of the shear band. [ABSTRACT FROM AUTHOR]

When modelling the phenomenon of strain localisation in strain-softening soils with the finite element (FE) method, nonlocal approaches have been commonly employed to avoid mesh dependency and numerical instability. This paper first presents the FE formulation of a critical state model for highly overconsolidated clays incorporating a nonlocal method. The performance of the nonlocal strain regularisation is subsequently assessed through a series of coupled hydro-mechanical (HM) analyses of undrained and drained triaxial compression tests on London clay. The mechanism behind the evolution of strain localisation in triaxial tests is investigated and a comparison with equivalent plane strain analyses is discussed. Finally, a comprehensive sensitivity study is presented, investigating the influence of the two nonlocal parameters, in the adopted nonlocal algorithm, on the predicted stress–strain responses. A key outcome is the derived linear relationship between the two parameters, which enables a unique stress–strain response to be achieved in either axisymmetric or plane strain analyses with multiple combinations of the two parameters. Such a modelling capability is essential in applications of the proposed nonlocal strain regularisation in large scale boundary value problems in which restrictions on element size exist. [ABSTRACT FROM AUTHOR]

The paper addresses the numerical simulation of strain localisation in stiff clays that exhibit softening behaviour. An elastoplastic constitutive model developed to incorporate key features of stiff clay behaviour is described first. A non-local formulation is then introduced for the regularisation of the analysis of localisation. A series of analyses were conducted to explore relevant aspects of the numerical simulation of localisation. A 3D analysis was also performed to assess the suitability of the approach presented for 3D applications. Finally, application to the simulation of a laboratory test on Beaucaire marl results in an excellent reproduction of experimental observations. [ABSTRACT FROM AUTHOR]

An innovative flexible membrane algorithm was developed and applied to the 3-D DEM modelling of axisymmetric triaxial compression tests on sands. This proposed flexible membrane was made of simply packed single spheres. The vertical and side extensions of the membrane were bounded by a series of boundary walls. These measures equipped the membrane with enough flexibility to contain local deformations of the cylindrical specimen which was comprised of un-bonded particles. With the application of the flexible membrane, the specimens failed with bulging shapes which were typical of axisymmetric triaxial tests. A comprehensive parametric sensitivity study indicated that the normal stiffness and particle size of the membrane spheres had limited influence on the test results, while the shear stiffness of the membrane spheres substantially enhanced the residual strength of the specimen. To examine the effectiveness of the flexible membrane algorithm, the numerical specimen was characterised on the macroscopic, mesoscopic and microscopic scales. The stress and strain response of the specimen well matched the laboratory test, and the volumetric strain presented apparent contraction and expansion stage. More importantly, the characterisation on the local void ratio clearly indicated the flexible membrane enabled full development of strain localisation inside the specimen. [ABSTRACT FROM AUTHOR]

A benchmark is conducted to characterise the Callovo-Oxfordian claystone behaviour for nuclear waste repository. The objective is to model gallery drilling and Excavation Damaged Zone (EDZ) around galleries. The proposed model is a cross-anisotropic elasto-viscoplastic model whose parameters are calibrated from laboratory tests. Gallery drilling is modelled numerically and shear fractures are reproduced with shear bands. The modelling highlights the contributions of shear banding, viscosity, and anisotropy for the EDZ behaviour. The EDZ and the gallery convergence can be correctly represented only if fractures are reproduced. Modelling shear banding is crucial and leads to a better description of the EDZ. [ABSTRACT FROM AUTHOR]

We explore the links between strain localisation and microstructural evolution in ice Ih deformed by dislocation creep. Using digital image correlation (DIC), we monitored the evolution of the strain field in two coarse-grained columnar ice samples deformed by creep (uniaxial compression at constant load) at −7 °C and 0.5 MPa up to 9.5% bulk shortening. After a brief transient (<0.2% bulk strain), in which strain localises nearby grain boundaries, viscoplastic strain concentrates in a few narrow intracrystalline shear bands that eventually extend over multiple grains. A comparison of pre- and post-deformation crystal orientation maps shows that strain localisation in shear bands is mainly accommodated by basal slip without producing significant dislocation-related substructures. Severe dynamic recrystallisation develops locally at grain boundaries that act as barriers to dislocation motion, particularly where basal shear transfer is ineffective. These observations are compared to full-field simulations reproducing the initial microstructure and experimental setup, which predict the stress and strain rate fields for deformation entirely accommodated by dislocation slip on the known slip systems in ice. The present data indicate that during the deformation of coarse-grained ice Ih at high homologous temperatures: (1) recrystallisation does not drive strain localisation but accommodates strain incompatibility, and (2) large strains can be accommodated by unimpeded basal slip with no formation of dislocation substructures. Observation (2) implies that intragranular orientation gradients are unreliable gauges of viscoplastic strain intensity at the grain scale and that the proportion of dislocation types in subgrains does not measure the relative contribution of different slip systems to deformation. Finally, we discuss the implications of this study for the interpretation and modelling of deformation by dislocation creep in rocks. • Transient strain localisation at most grain boundaries occurs at strains <0.2%. • At higher strains, deformation occurs on a network of intracrystalline shear bands. • Shear bands associated with dislocation slip can leave no microstructural record. • Intragranular orientation gradients do not gauge strain intensity. • Recrystallisation mainly accommodates grain-scale strain incompatibility. [ABSTRACT FROM AUTHOR]

The Anita Peridotite, in southwestern New Zealand, is a ∼1 × 20 km ultramafic massif that was rapidly extruded from beneath a Cretaceous arc within the 4 km wide mylonitic Anita Shear Zone. The peridotitic body contains a spectacular array of textures that preserve evidence for changing temperature, stress, and deformation mechanisms during the exhumation process. Olivine and orthopyroxene microstructures and lattice-preferred orientations (LPO) record a three-phase deformation history. Dislocation glide on the C- and E-type slip systems is recorded by coarse pre-mylonitised olivine grains, and occurred under hydrous conditions at T ∼650 °C, stress ∼200–700 MPa and strain rate ∼10 −15 s −1 , probably within hydrated sub-arc mantle lithosphere. Rare protomylonite pods record deformation by dislocation creep in porphyroclasts and dislocation-accommodated grain boundary sliding in the matrix on {0kl}[100] in olivine and (100)[001] in orthopyroxene, under conditions of T ∼730–770 °C, stress ∼52–700 MPa and strain rate ∼10 −15 s −1 . The massif, however, is dominated by mylonite and ultramylonite that wrap the protomylonite pods, comprising mostly fine-grained olivine neoblasts that lack internal distortions and have uniform LPOs. These textures indicate deformation occurred by grain-size sensitive (GSS) creep at T ∼650 °C, stress ∼69–137 MPa and strain rate ∼10 −15 s −1 , and thus during conditions of cooling and decreasing stress. GSS creep became more dominant with time, as the proportion of randomly-oriented neoblasts increased and formed interlinked networks that accommodated much of the strain. Grain boundary pinning allowed GSS creep to be maintained in polyphase regions, following mixing of olivine and orthopyroxene, which may have occurred by grain boundary transport in a fluid phase during a “creep cavitation” process. The results indicate that the Anita Peridotite recrystallised and underwent rheological weakening at a constant strain rate, with strain distributed across the entire section. This widespread deformation caused rapid exhumation of the peridotite from the lithospheric mantle into the overlying arc crust. The massif therefore records multiple overprinting phases of deformation under mantle and crustal conditions associated with the rapid exhumation of a large orogenic peridotite. [ABSTRACT FROM AUTHOR]

We pay a revisit to some classical geomechanics problems using a novel computational multiscale modelling approach. The multiscale approach employs a hierarchical coupling of the finite element method (FEM) and the discrete element method. It solves a boundary value problem at the continuum scale by FEM and derives the material point response from the discrete element method simulation attached to each Gauss point of the FEM mesh. The multiscale modelling framework not only helps successfully bypass phenomenological constitutive assumptions as required in conventional modelling approaches but also facilitates effective cross-scale interpretation and understanding of soil behaviour. We examine the classical retaining wall and footing problems by this method and demonstrate that the simulating results can be well validated and verified by their analytical solutions. Furthermore, the study sheds novel multiscale insights into these classical problems and offers a new tool for geotechnical engineers to design and analyse geotechnical applications based directly upon particle-level information of soils. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation (CPO) development in naturally deformed rocks. We use a polymineralic (Qtz + Pl + Kfs + Bt + Ms ± Grt ± Tur) mylonite from the Zanskar Shear Zone, a section of the South Tibetan Detachment (NW Himalaya), to demonstrate how quartz CPO intensity decreases from quartz-dominated domains to micaceous domains, independently of whether or not quartz grains are pinned by mica grains. We then use a bimineralic (Qtz + Ms) mylonite from the Main Central Thrust (NW Himalaya) to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz CPO. Our results draw distinctions between CPO weakening due to: (i) second phase drag, leading to ineffective recovery in quartz; and (ii) increased transmission and localisation of strain between interconnected mica grains. In the latter case, well-connected micaceous layers take up most of the strain, weakening the rock and preventing straining of the stronger quartz matrix. Our findings suggest that rock weakening in quartz-rich crustal rocks is influenced not only by the presence of mica-rich layers but also the degree of mica grain connectivity, which allows for more effective strain localization through the entire rock mass. [ABSTRACT FROM AUTHOR]

Deep geological repository is the preferred solution in many countries to manage radioactive wastes, such as in France where the Callovo-Oxfordian (COx) claystone is the candidate host rock. In such clay rock formation, the drilling of storage gallery creates an Excavation Damaged Zone (EDZ) with altered flow properties in the short term, while corrosion processes release large amounts of gas in the long term. Assessing the evolution of gas pressures in the near-field and predicting the effect of the EDZ on gas transport remains a major issue. This paper presents a second gradient two-phase flow hydro-mechanical (H2M) model tackling the multi-physics couplings related to gas transfers and fractures development. The EDZ is reproduced by shear strain localisation bands using a microstructure enriched model with a second gradient approach. The gas migration is captured by a biphasic fluid transfer model. The impact of fracturing on the flow properties is addressed by relating the permeability and the water retention curve to mechanical strains. Using this tool, numerical modelling of a drift in the COx claystone is performed with the aim of emphasising the influence of the HM couplings on gas migrations at nuclear waste disposal scale. [ABSTRACT FROM AUTHOR]

The determination of work-hardening for ductile materials at large strains is difficult to perform in the framework of usual tensile tests because of the geometrical instability and necking in the specimen at relatively low strains. In this study, we propose a combination of experimental and numerical techniques to overcome this difficulty. Extruded aluminium alloys are used as a case since they exhibit marked plastic anisotropy. In the experiments, the minimum diameters of the axisymmetric tensile specimen in two normal directions are measured at high frequency by a laser gauge in the necking area together with the corresponding force, and the true stress–strain curve is found. The anisotropy of the material is determined from its crystallographic texture using the crystal plasticity theory. This data is used to represent the specimen by a 3D finite element model with phenomenological anisotropic plasticity. The experimental true stress–strain curve is then used as a target curve in an optimisation procedure for calibrating the hardening parameters of the material model. As a result, the equivalent stress–strain curve of the material up to fracture is obtained. [ABSTRACT FROM AUTHOR]

Abstract: Volcanic provinces are structurally active regions – undergoing continual deformation along faults. Within such fault structures, volcanic ash gouge, containing both crystalline and glassy material, may act as a potential fault plane lubricant. Here, we investigate the frictional properties of volcanic ash gouges with varying glass fractions using a rotary shear apparatus at a range of slip rates (1.3–1300 mm/s) and axial stresses (0.5–2.5 MPa). We show that the frictional behaviour of volcanic ash is in agreement with Byerlee's friction law at low slip velocities, irrespective of glass content. The results reveal a common non-linear reduction of the friction coefficient with slip velocity and yield a frictional law for fault zones containing volcanic ash gouge. Textural analysis reveals that strain localisation and the development of shear bands are more prominent at higher slip velocities (>10 mm/s). The textures observed here are similar to those recorded in ash gouge at the surface of extrusive spines at Mount St. Helens (USA). We use the rate-weakening component of the frictional law to estimate shear-stress-resistance reductions associated with episodic seismogenic slip events that accompany magma ascent pulses. We conclude that the internal structure of volcanic ash gouge may act as a kinematic marker of exogenic dome growth. [Copyright &y& Elsevier]

Abstract: The aim of this study was to investigate mechanisms of progressive shear surface development using a series of specialised triaxial cell tests. Intact and remoulded samples of Gault Clay from the Ventnor Undercliff on the Isle of Wight in southern England were subjected to pore pressure reinflation (PPR) testing in a triaxial cell, in which failure is generated by increasing pore pressure under a constant total stress state. In addition, a novel very long term (>500days) creep test was undertaken, in which the sample eventually failed at a constant stress state below the failure envelope. The experiments showed that undisturbed samples of the Gault Clay failed in a brittle manner, generating a linear trend when plotted using the Saito technique. On the other hand, remoulded samples showed ductile behaviour, as indicated by a non-linear Saito trend. A number of otherwise identical PPR tests were conducted in which the rate of increase in pore water pressure was varied. These tests showed that strain rate generated at any point in the PPR tests depended on both the effective stress and the rate of change of effective stress. The latter is important because a change in stress generates a change in strain. Thus, whilst tests at different rates of change of effective stress are similar when plotted in q–p′ space and in strain–p′ space, they are markedly different in strain rate–p′ space. The long term creep test failed when the stress state had been constant for over 80days. This mechanism was reminiscent of creep rupture, occurring below the failure envelope defined in the conventional experiments. We conclude that first time failure in the Gault Clay is a progressive mechanism dominated by the development of micro-cracking, which leads to strain localisation and the development of one or more shear surfaces at failure. Whilst this mechanism may usually occur in response to a change in stress, the study indicates that failure can develop progressively. In the remoulded Gault Clay shear strains cannot localise along a singular shear surface. The results provide new insight into the mechanisms of landslide movement operating within the Ventnor landslide complex and indicate that present movements are likely to be occurring on a pre-existing shear surface. The lab tests suggest that this material is unlikely to undergo catastrophic failure. [Copyright &y& Elsevier]

Abstract: Analogue and numerical models have shown that the strength of the lithospheric mantle controls the mode of lithosphere deformation. In extension, the presence or absence of a high strength brittle mantle respectively leads to localized or distributed rifting. However, first order geophysical data question the existence of such a brittle mantle. Here we use 2-D finite-element large strain modelling to quantify the impact of a ductile localizing mantle – instead of brittle – in triggering continental rifting. As a novelty, the mantle rheology considers the effect of grain boundary sliding during strain-induced grain size reduction, which may promote a significant strength drop and subsequent strain localization at low mantle temperature (<700–800°C). Our results reveal that such ductile localizing mantle implies varying modes of continental rifting that mainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust. A medium to strong lower crust implies coupling between the upper crust and ductile localizing mantle, yielding to narrow continental rifting. In contrast, a weak lower crust implies decoupling between the upper crust and ductile localizing mantle, giving rise to a switch from distributed faulting at incipient strain to localized faulting at large strain. Ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting, although a critical amount of weakening is required. Such ductile localizing mantle provides a relevant geological and mechanical alternative to the brittle mantle. It moreover provides a wider variety of modes of upper crustal faulting that are commonly observed in nature. [Copyright &y& Elsevier]

As is well known, numerically handling, by means of finite element codes, localisation problems involving softening materials is still quite delicate. As soon as strain localisation occurs, mesh dependence and serious problems of convergence take place. This paper deals with this type of problem in the case where localisation occurs in an ideal naturally cemented granular specimen tested under plane strain conditions. Different versions (a local elasto-plastic, a local viscoplastic and a non-local viscoplastic) of the same strain softening model are taken into consideration and the relative numerical results are critically discussed and compared. The snap-back problem is numerically taken into account and it has been demonstrated to be affected not only by the softening parameters but also by the viscous nucleus definition. To highlight the relationship between viscosity and non-locality, the results of a numerical parametric analysis are finally discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Providing experimental results for longitudinally-restrained RC beams. [•] Effect of longitudinal and transverse steel bars on the arching action is studied. [•] Effect of end support rotation on the behaviour of restrained RC beams is studied. [•] Investigating effect of strain penetration on the response of restrained RC beams. [Copyright &y& Elsevier]

This paper describes the results of triaxial compression tests carried out at effective cell pressures ranging from 12.5 to 100 kPa to investigate the influence of fabric structure on the yield and failure of intact Reigate silver sand. In some of the tests, a digital image-based technique was used to determine the instant of onset of strain localisation, and the distribution of strain localisations within the specimen as overall deformation progressed. Comparative tests on intact and reconstituted specimens showed that fabric structure in the intact material allows the mobilisation of stress ratios close to peak before the onset of dilation, and increases the shear modulus at a given effective cell pressure and strain. Localisation was found to start at or after the onset of dilation, with a tendency to delay at increasing effective cell pressure. More localised deformation was observed at low effective cell pressures. Consistency between the critical state strengths of intact and reconstituted specimens is demonstrated, provided that the effect of shear band geometry is taken into account in stress analysis. [ABSTRACT FROM AUTHOR]

Abstract: We review the geodynamic evolution of the Aegean–Anatolia region and discuss strain localisation there over geological times. From Late Eocene to Present, crustal deformation in the Aegean backarc has localised progressively during slab retreat. Extension started with the formation of the Rhodope Metamorphic Core Complex (Eocene) and migrated to the Cyclades and the northern Menderes Massif (Oligocene and Miocene), accommodated by crustal-scale detachments and a first series of core complexes (MCCs). Extension then localised in Western Turkey, the Corinth Rift and the external Hellenic arc after Messinian times, while the North Anatolian Fault penetrated the Aegean Sea. Through time the direction and style of extension have not changed significantly except in terms of localisation. The contributions of progressive slab retreat and tearing, basal drag, extrusion tectonics and tectonic inheritance are discussed and we favour a model (1) where slab retreat is the main driving engine, (2) successive slab tearing episodes are the main causes of this stepwise strain localisation and (3) the inherited heterogeneity of the crust is a major factor for localising detachments. The continental crust has an inherited strong heterogeneity and crustal-scale contacts such as major thrust planes act as weak zones or as zones of contrast of resistance and viscosity that can localise later deformation. The dynamics of slabs at depth and the asthenospheric flow due to slab retreat also have influence strain localisation in the upper plate. Successive slab ruptures from the Middle Miocene to the Late Miocene have isolated a narrow strip of lithosphere, still attached to the African lithosphere below Crete. The formation of the North Anatolian Fault is partly a consequence of this evolution. The extrusion of Anatolia and the Aegean extension are partly driven from below (asthenospheric flow) and from above (extrusion of a lid of rigid crust). [Copyright &y& Elsevier]

It is well known that the characteristic length scale in ultra-fine grained and nanocrystalline metals has a significant effect on the mechanical behaviour. The inhibited ability to accommodate imposed strain with conventional dislocation mechanism has led to the activation of unconventional deformation mechanisms. For one, grain coarsening at shear bands has been observed to occur within metals with sub-micron grain size upon cyclic deformation. Such grain coarsening is often linked to the observed cyclic softening behaviour. The purpose of this study was to investigate the relationship between strain localisation associated with shear banding and the observed deformation-induced grain coarsening in ultra-fine grained metals. The investigation was carried out using ultra-fine grained, oxygen-free high conductivity copper processed by accumulative roll-bonding. A close relationship between strain localisation and deformation-induced grain coarsening was revealed. As strain localisation is not only found at shear bands, but also at other places whereby heterogeneous microstructure or geometric discontinuity is present, hence the present study bears a general significance. Such strain localisation sites may also include a hard constituent embedded in a relatively ductile matrix, micro-crack tips and artificial notches. The stress concentration at these sites provides a high input of strain energy for grain boundary motion leading to grain coarsening. Furthermore, when the grain size is very small, the stress gradient leading away from the stress concentration sites is also believed to increase the driving force for grain boundary migration within the affected regions. [ABSTRACT FROM AUTHOR]

The aim of this work is to analyse the strain fields that take place in several asphalt mixture specimens subjected to compression tests. The grid technique is used for this purpose. The main features of this strain measurement technique are first described. The obtained results are then discussed. They show that very strong heterogeneities take place in the strain fields. They are due to the very different mechanical properties of the constituents. Various testing conditions are also investigated because they directly influence the quality of the measurements and the ability of the grid technique to detect small strain amplitudes with a good spatial resolution, this last feature being crucial here because of the very nature of the material under test. [ABSTRACT FROM AUTHOR]

Abstract: The lifetimes of thermal barrier coatings (TBC) are known to show considerable scatter. Experimental original data have revealed the strong correlation between spallation patterns and local strain field. This strain field was shown to be a function of the substrate single crystal orientation. Thus, this study addresses the strain variation in the single crystal substrate and its influence on TBC system life. This task was achieved using both finite element analysis (FEA) to take into account crystal plasticity in the substrate single crystal and a recent TBC life model [1]. [Copyright &y& Elsevier]

In this paper, an analytical model based on the concept of strain localisation is proposed for the analysis and prediction of the response of quasi-brittle materials in uniaxial compression tests, such as mortar, plain concrete with different compression strengths, as well as fibre-reinforced concrete. The proposed approach, referred to as Overlapping Crack Model, relies only on a pair of material constitutive laws, in close analogy with the Cohesive Crack Model: a stress-strain relationship describing the pre-peak behaviour of the material and a stress-interpenetration relationship for the description of the post-peak response. In the paper it will be shown how the stress-interpenetration relationship can be deduced from experimental data and how it depends on the compression strength and on the crushing energy of the tested materials. A wide comparison between the stress-displacement curves predicted by the proposed model and those experimentally found in the literature will show the effectiveness of the present approach to capture both stable softening or sharp snap-back post-peak branches by varying the slenderness or the size-scale of thetested samples. [ABSTRACT FROM AUTHOR]

The article presents a study which explores a photographic and photoelastic study of a granular material. It states that a glass ballotini reveals stress conditions information when viewed using polarised light. The study uses procedures that indicated some possibilities for experimental soil mechanics. Results show that the use of graphics and standard image processing software offers comparison of features of the incremental strain and stress fields.

The numerical challenges that arise in modelling shear bands in soft sensitive (SS) clays have not yet been fully resolved. Convincing and well-accepted solutions have yet to be found. This paper presents some novel information related to the shear band phenomenon in SS clays. In this study, the hypothesis is that the generation and dissipation of excess pore pressure from shear bands could regularise the strain softening and result in a mesh independent shear band thickness. The generation and dissipation of excess pressure is modelled by a coupled consolidation process. The simulation aims at modelling two counteracting mechanisms in the SS clay. First, the shear band narrows because of strain softening. Second, the internal pore water pressure drainage reduces the rate of strain softening. This counteracting mechanism provides an inherent regularisation technique for SS clays. This study presents some numerical results involving these two counteracting mechanisms. This study also shows that an inherent internal parameter applicable for SS clays can be defined by the ratio between soil permeability and the applied strain rate. In the case of SS clays, the range of this parameter varies from 0 to 0.0002 mm. [ABSTRACT FROM AUTHOR]

In-situ x-ray tomography has been used to follow deformation processes in 3D during two triaxial compression tests, one on a specimen of bio-cemented Ottawa 50-70 sand and the other on a specimen of the non-cemented sand. The global stress-strain responses show that the bio-cementation process increases the shear strength (peak deviator stress is approximately doubled), and causes the material to exhibit a linear behaviour up until peak, as well as increasing the dilatancy angle. The residual strength of the two samples is very close at large strain. Quantitative 3D digital image analysis (porosity, cement-density and strain field measurements), reveals that a dilatant shear band gradually develops pre-peak in the reference material. The cemented sample however undergoes an abrupt change of deformation mechanism at peak stress: from homogeneous deformation to localised dilatant shearing, which is associated with a local loss of cementation. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose a method for measuring damage in ductile materials, from its inception to rupture. In the first stage of damage, which occurs before localisation, the usual method for determining damage through the measurement of stiffness variation is used. A damageable elastic-plastic model of the modified Lemaitre/Chaboche type is identified from these tests. An original method is proposed for measuring damage following the initiation of strain localisation. This method is based on a full 3D image correlation analysis using four cameras. The principle of the method consists in identifying the damage through tensile experiments on thin, flat-notched specimens subjected to tensile loading. Speckles are applied on both faces of each specimen in order to follow the strain fields on the two faces at the same time. These two strain fields are digitised simultaneously by two synchronised sets of two digital cameras. This paper shows how this method enables one to identify strain localisation and deduce the evolution of damage directly. Here, the method is developed for 15-5 PH stainless steel. [ABSTRACT FROM AUTHOR]

Abstract: In order to shed light on the cause for onset of strain localisation in plagioclase-rich rocks we have performed detailed microstructural analyses on a sheared anorthosite–leucogabbro using optical microscopy, electron backscatter diffraction (EBSD) and chemical analyses. The analysed sample is from an Archaean unit, SW Greenland, deformed at lower to mid crustal conditions (T = 675–700 °C and moderate pressure). The initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation. Recrystallised plagioclase grains (average size 80 μm) are dominantly found in (i) clusters, (ii) lenses and (iii) continuous bands subparallel to shear zone boundaries. Recrystallised grains in clusters and lenses display inherited crystallographic orientations. Their bulk crystallographic preferred orientation (CPO) is random; however, crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems. Recrystallised grains in continuous bands show a CPO with a single dominant active slip system, (001)<110>, aligned with the structural (XYZ) framework. For these parent and daughter grains, misorientation axes are random and the dominant slip system is different. Grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast. We infer that the cause for strain localisation is recrystallisation and development of a CPO in continuous recrystallised bands. Microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation-accommodated grain boundary sliding (DisGBS) in continuous bands. This inferred shift in dominant deformation mechanism would lower the strength of the shear zone. [ABSTRACT FROM AUTHOR]

A detailed study of spatial variation of void ratio and shear band thickness measurements is presented in this paper. A prismatic sand specimen that initially measured 57·4 mm wide × 120·5 mm long × 182·1 mm high was sheared under plane-strain (biaxial) loading conditions. The specimen was prepared at a relative density of 79% using F-75 Ottawa sand. X-ray computed tomography was used to scan the specimen before shearing and after the onset of the shear band. The specimen failed through a single shear band with an inclination angle of 65·6° measured from the direction of the minor principal stress. Computer algorithms were developed to calibrate CT images and quantify void ratio (e) variation within the specimen. CT data analysis revealed cross-sectional spatial variation in void ratio where density is higher in regions close to the edges of the specimen due to membrane effects and confining pressure with no significant variation in void ratio in the axial direction (top to bottom) before shearing. The shear band was easily identified from the CT images, and analysis of void ratio showed a noticeable jump in void ratio profile across the shear band. A detailed statistical summary of the thickness of the shear band and variation of void ratio across the shear band is presented and discussed. [ABSTRACT FROM AUTHOR]

Laboratory tests on samples of sand typically show instabilities in response related to shear band nucleation and evolution. Influences of constitutive characteristics (for instance, plastically driven elastic anisotropy), sensitivity to changes in constitutive parameters, and effects of testing conditions (for instance, related to stress conditions or to membrane penetration or stretching) on different types of material instability occurring in the post-localisation behaviour of sand specimens are investigated. Comparison between model prediction and available experiments shows that certain constitutive and testing features, which do not much influence the global response of a sample, may nevertheless strongly affect the critical and post-critical behaviours. The results obtained show that the proposed approach is general enough to permit the investigation of effects of experimental features that have not previously been studied. [ABSTRACT FROM AUTHOR]

Abstract: Size dependence of the load–displacement and stress–strain curves is a phenomenon widely reported in the literature and readily observed in experiments. The present paper introduces two developments in the analysis of ductile rupture, one in the area of experimental technique and interpretation, and one in the area of numerical modelling. Firstly, the new experimental technique is described of multiple gauge length optical extensometry (laser scanning) that provides a uniquely powerful means of collecting the data needed for the study of size effects, from a single tensile test on a ductile specimen. The interpretation procedures are presented based on the considerations about the nature of deformation and energy expenditure within different material volumes in a tensile specimen. Consequently, the property of the specimen can be defined and identified that relates to the energy consumption per unit cross-sectional area required to rupture the specimen entirely (the essential work of rupture). Another energetic measure is also obtained from the test that relates to the plastic work dissipation per unit volume in pre-peak deformed material. Secondly, the observed size effects and specific energy determination procedures are discussed in the context of non-local coupled plasticity-damage modelling. Conventional local numerical models of inelastic deformation fail to predict the sequence of deformation responses observed in the tensile test, namely, uniform deformation followed by strain localisation, necking and rupture. It is shown here that coupled non-local damage-plasticity modelling is capable of capturing the hardening–softening and global–local plasticity transitions, and hence the trends observed in ductile fracture. The predictions of the coupled non-local damage-plasticity modelling for different deformation regimes are shown to agree qualitatively with the experimentally observed behaviour. The two aspects of the proposed approach, experimental and modelling, are presented here together since they are intimately linked at the fundamental level. The implications of the proposed approach are discussed for the deformation analysis of plastically deforming and damageable materials and structures at various scales, from macroscopic to miniature. [Copyright &y& Elsevier]

A method for characterisation of materials subjected to large strains beyond the levels when plastic instability occurs in standard tension tests is presented. Thin sheets of two types of hot-rolled steel are subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera and by digital speckle photography (DSP) in-plane pointwise displacement fields are obtained. By numerical differentiation and assuming plastic incompressibility the equivalent plastic strain is determined. The characterisation performed in this paper consists of estimating material parameters in two constitutive models. These models are a piecewise linear plasticity model and a parabolic hardening model. By using inverse modelling including finite element analyses (FEA) of the tension tests the material parameters are adjusted to achieve a minimum in a so-called objective function. The objective function is basically a least-square functional based on the difference between the experimental and FE-calculated displacement and strain fields. Due to the large deformations an adaptive meshing technique is used in order to avoid highly distorted elements. The DSP-technique provided measurements, where the uncertainty of the equivalent plastic strain varied between 0.0015 and 0.0056. The maximum obtained strain was approximately 0.8. The true stress–strain curves based on the estimated parameters are validated in the low strain region by comparison with curves from standard tension tests. [Copyright &y& Elsevier]

The evolution of fault populations is established for an area within the Late Jurassic Inner Moray Firth sub-basin of the North Sea. Sedimentation rates outstripped fault displacement rates resulting in the blanketing of fault scarps and the preservation of fault displacement histories. Displacement backstripping is used to establish the growth history of the fault system.Fault system evolution is characterised by early generation of the main fault pattern and progressive localisation of strain onto larger faults. This localisation is accompanied by the death of smaller faults and an associated change in the active fault population from power-law to scale-bound. Fault length populations evolve from a power-law frequency distribution containing all faults, to a power-law distribution with a marked non-power-law tail containing the largest faults. This change in population character is synchronous with the development of a fully-connected fault system extending across the mapped area and the accommodation of displacements almost exclusively on the largest faults. Strain localisation onto fewer and better connected faults represents the most efficient means of accommodating fault-related deformation and is considered to be a fundamental characteristic of the spatio-temporal evolution of fault systems. Progressive strain localisation requires complementary changes in the characteristics of associated earthquake populations. [Copyright &y& Elsevier]