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The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over other numerical models available in the literature are that it can simulate the whole process of progressive failure leading to spread. The finite-element results are consistent with previous conceptual models proposed from field observations. The parametric study shows that, depending upon geometry and soil properties, toe erosion could cause three types of shear band formation: () only a horizontal shear band without any global failure; () global failure of only one block of soil; () global failure of multiple blocks of soil in the form of horsts and grabens. [ABSTRACT FROM AUTHOR]

This paper presents a multiscale investigation on the interplay among inherent anisotropy, fabric evolution and strain localisation in granular soils, based on a hierarchical multiscale framework with rigorous coupling of the finite-element method (FEM) and discrete-element method (DEM). DEM assemblies with elongated particles are generated to simulate inherent anisotropy and are embedded to the Gauss points of the FEM mesh to derive the required constitutive relation. Specimens prepared with different bedding plane angles are subjected to biaxial shear under either smooth or rough loading platens. Key factors and physical mechanisms contributing towards the occurrence and development of strain localisation are examined. The competing evolutions of two sources of anisotropy, one related to particle orientations and the other related to contact normals, are found to underpin the development of the shear band. A single band pattern is observed under smooth boundary conditions, and its orientation relative to the bedding plane depends critically on the relative dominance between the two anisotropies. Under rough boundary conditions, the non-coaxial material response and the boundary constraint jointly lead to cross-shaped double shear bands. The multiscale simulations indicate that the DEM assemblies inside the shear band(s) undergo extensive shearing, fabric evolution and particle rotation, and may reach the critical state, while those located outside the shear band(s) experience mild loading followed by unloading. The particle-orientation-based fabric anisotropy needs significantly larger shear and dilation for mobilisation than the contact-normal based one. The asynchrony in evolution of the two fabric anisotropies can cause non-coaxial responses for initially coaxial packings, which directly triggers strain localisation. [ABSTRACT FROM AUTHOR]

The drilling of galleries induces damage propagation in the surrounding medium and creates, around them, the excavation damaged zone (EDZ). The prediction of the extension and fracture structure of this zone remains a major issue, especially in the context of underground nuclear waste storage. Experimental studies on geomaterials indicate that localised deformation in shear band mode usually appears prior to fractures. Thus, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. In the classical finite element framework, strain localisation suffers a mesh-dependency problem. Therefore, an enhanced model with a regularisation method is required to correctly model the strain localisation behaviour. Among the existing methods, we choose the coupled local second gradient model. We extend it to unsaturated conditions and we include the solid grain compressibility. Furthermore, air ventilation inside underground galleries engenders a rock-atmosphere interaction that could influence the damaged zone. This interaction has to be investigated in order to predict the damaged zone behaviour. Finally, a hydro-mechanical modelling of a gallery excavation in claystone is presented and leads to a fairly good representation of the EDZ. The main objectives of this study are to model the fractures by considering shear strain localisation bands, and to investigate if an isotropic model accurately reproduces the in situ measurements. The numerical results provide information about the damaged zone extension, structure and behaviour that are in very good agreement with in situ measurements and observations. For instance, the strain localisation bands that develop in chevron pattern during the excavation and rock desaturation, due to air ventilation, are observed close to the gallery. [ABSTRACT FROM AUTHOR]

The paper describes a three-phase single-point material point method formulation of coupled flow (water and air) for hydro-mechanical analysis of geotechnical problems involving unsaturated soils. The governing balance and dynamic momentum equations are discretised and adapted to material point method characteristics: an Eulerian computational mesh and a Lagrangian analysis of material points. General mathematical expressions for the terms of the set of governing equations are given. A suction-dependent elastoplastic Mohr-Coulomb model, expressed in terms of net stress and suction variables is implemented. The instability of a slope subjected to rain infiltration, inspired from a real case, is solved and discussed. The model shows the development of the initial failure surface in a region of deviatoric strain localisation, the evolution of stress and suction states in some characteristic locations, the progressive large strain deformation of the slope and the dynamics of the motion characterised by the history of displacement, velocity and acceleration of the unstable mass. [ABSTRACT FROM AUTHOR]

The paper reports new observations of strain localisation following dynamic impact. Two dimensional physical model tests have been performed to investigate the response of dry granular soils to the impact of a free falling steel plate. The tests have been performed to study the mechanics of the densification of soils during dynamic compaction, which is a widely used process to improve the performance of deep soil deposits by repeated dropping of large weights onto the ground surface. High speed photography and digital image correlation techniques have enabled the deformation patterns, soil strains and strain localisations to be observed. Tests have been performed on sand with a range of densities and a sand-silt mixture. The results have shown that the soil deformations are comprised of a conventional bearing capacity mechanism at the surface and a series of compaction bands that propagate downwards beneath the impacting plate. Similar patterns are observed in all tests, however as the compressibility of the soil increases the contribution from the bearing capacity mechanism decreases. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microstructurally small fatigue crack growth (FCG) rate is studied by DIC. • FCG rate retardation is associated with shear strain localisations. • Grain boundaries control interaction between strain localization and FCG. • Shear strain localisations lead to formation of grain sub-structures. • Intense cross slip facilitates the overcoming of the strain localisation zone. Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant. [ABSTRACT FROM AUTHOR]

Since the pioneering works of John Ramsay in the 1970's and 1980's, the analysis of exceptional exposures of small-scale shear zones (i.e. 10-3 – 10−1 m thick) in granitoid rocks provided invaluable insights into the processes controlling strain localisation in the middle and lower continental crust. Indeed, recent advancement in field, microstructural and petrological analyses of such small-scale shear zone have shed new light on the metamorphic, tectonic and fluid conditions promoting shear zone nucleation and development in granitoid rocks. In this paper we provide an overview of these new insights, comparing and integrating the results obtained from field, and microstructural and petrological analyses of small-scale shear zones in granitoid plutons and meta-granitoids from the Alps. A review of the deformation temperature shows that the granitoid shear zones development occurs between 350 and 600 °C, with most of them localising in a restricted temperature window between 450 and 500 °C. At these conditions, the magmatic assemblage is metastable and subjected to a series of metamorphic reactions. Furthermore, the development of shear zone does not occur under-closed system conditions. Introducing or expelling fluids and mass (i.e. metasomatism) during deformation has mineralogical consequences that control the rheology and the way shear zone evolves. Among the main mineralogical and microstructural changes, the breakdown of magmatic feldspar(s) into fine-grained aggregates steers both the rheology and fabric evolution of shear zones in granitoid rocks, triggering further mechano-chemical feedback mechanisms. Future research should consider the occurrence of feedback processes between deformation, metamorphic and metasomatic processes to understand and quantify the evolution with time and strain of shear zone geometry and rheology, as well as of the development of larger-scale shear zone networks. • Small-scale shear zone are widespread features of meta-granitoid units and plutons. • They develop at the transition between greenschist and amphibolite facies. • A set of retrograde metamorphic reactions aid strain softening and localisation. • Grain-size-sensitive and phase mixing processes steer shear zone rheology. • Future research should aim at the quantification of deformation-fluids feedbacks. [ABSTRACT FROM AUTHOR]

In this study, the strain localisation under cyclic loading in an advanced powder metallurgy Ni-based superalloy at 450 °C was characterised by SEM-DIC, and the related deformation substructures were characterised by electron channelling contrast imaging under controlled diffraction conditions (cECCI). The results indicate strain mainly accumulates in slip/strain bands due to dislocation slip on the {111} plane, and the strain saturates at 1000 cycles with no evident increase in strain value within strain bands or strain band density from 1000 cycles to 10,000 cycles. Analysis of the strain components in the local coordinates associated with each strain band shows the strain band mainly consists of shear strain ϵ xy along the strain band and transverse strain ϵ yy normal to the strain band. The strain bands can transmit through, be deflected, or be blocked at grain boundaries depending on the grain boundary characteristics and the neighbouring grain orientation. Comparing the strain bands obtained by SEM-DIC and deformation substructures obtained by cECCI shows that the strain bands originate from underlying continuous slip bands which show the sheared γ′ within the slip bands and dislocation pairs looping around the γ′ precipitates, but the deformation localised in the discontinuous slip bands is not well characterised by SEM-DIC at this lateral resolution of strain. [ABSTRACT FROM AUTHOR]

This paper discusses strain localisation in granular media by presenting experimental, full-field analysis of mechanical tests on sand, both at a continuum level, as well as at the grain scale . At the continuum level, the development of structures of localised strain can be studied. Even at this scale, the characteristic size of the phenomena observed is in the order of a few grains. In the second part of this paper, therefore, the development of shear bands within specimen of different sands is studied at the level of the individual grains, measuring grains kinematics with x-ray tomography. The link between grain angularity and grain rotation within shear bands is shown, allowing a grain-scale explanation of the difference in macroscopic residual stresses for materials with different grain shapes. Finally, rarely described precursors of localisation, emerging well before the stress peak are observed and commented. [ABSTRACT FROM AUTHOR]

The effect of the in-service scattered damage in the pipe wall metal is evaluated, and the effect of hydrogen absorbed by metal on the variation of the structure and mechanical properties of the main gas pipeline after a long-term operation is found. The main regularities in the graded nature of the static deformation process and the effect of hydrogenation on the scattered damage and fracture accumulation are found. [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]

The surface strengthening mechanisms of the 15Kh2MFA(II) heat-resistant steel are analyzed after the laser shock-wave treatment in the air and epoxy resin. The regularities are established in the formation of the ordered surface relief of the steel after treatment. The optical and digital analysis of the surface is performed which allows determining the size of irregularities, taking into account the stochastic and cyclic nature of their formation. The effect of treatment of the 15Kh2MFA(II) heat-resistant steel on the regularities of its static failure is established. [ABSTRACT FROM AUTHOR]

SUMMARY A hierarchical multiscale framework is proposed to model the mechanical behaviour of granular media. The framework employs a rigorous hierarchical coupling between the FEM and the discrete element method (DEM). To solve a BVP, the FEM is used to discretise the macroscopic geometric domain into an FEM mesh. A DEM assembly with memory of its loading history is embedded at each Gauss integration point of the mesh to serve as the representative volume element (RVE). The DEM assembly receives the global deformation at its Gauss point from the FEM as input boundary conditions and is solved to derive the required constitutive relation at the specific material point to advance the FEM computation. The DEM computation employs simple physically based contact laws in conjunction with Coulomb's friction for interparticle contacts to capture the loading-history dependence and highly nonlinear dissipative response of a granular material. The hierarchical scheme helps to avoid the phenomenological assumptions on constitutive relation in conventional continuum modelling and retains the computational efficiency of FEM in solving large-scale BVPs. The hierarchical structure also makes it ideal for distributed parallel computing to fully unleash its predictive power. Importantly, the framework offers rich information on the particle level with direct link to the macroscopic material response, which helps to shed lights on cross-scale understanding of granular media. The developed framework is first benchmarked by a simulation of single-element drained test and is then applied to the predictions of strain localisation for sand subject to monotonic biaxial compression, as well as the liquefaction and cyclic mobility of sand in cyclic simple shear tests. It is demonstrated that the proposed method may reproduce interesting experimental observations that are otherwise difficult to be captured by conventional FEM or pure DEM simulations, such as the inception of shear band under smooth symmetric boundary conditions, non-coaxial granular response, large dilation and rotation at the edges of shear band and critical state reached within the shear band. Copyright © 2014 John Wiley & Sons, Ltd. [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: The Wainui Shear Zone is a ∼ 1.2 km wide steeply dipping ductile shear zone that is intimately associated with the western margin of a very extensive granite batholith in central New Zealand. Shear sense indicators and lineation orientations within the central high-strain portion of shear zone consistently show a reverse dip-slip east-side-up sense of motion. Shallower foliation in the margins of the shear zone is interpreted to pre-date the central high-strain zone. However, U–Pb and Ar–Ar data reveal that all ductile deformation occurred between ∼114 and 109 Ma (and probably between 114 and 111 Ma) requiring: (1) that deformation kinematics within the shear zone changed over a short time period, and (2) the shear fabrics formed immediately after emplacement of the voluminous granite batholith. The temporal and spatial link between the deformation and plutonism is hypothesised to be a result of thermal weakening of the cooler Paleozoic crust by Cretaceous granite intrusion, which promoted localisation of deformation onto the intrusive contact. Deformation was initially distributed over a wide zone but became focused into a central mylonite zone. Continental-scale shortening is recorded as a network of ductile shear zones that formed on thermal contrasts within the upper, middle and lower crust along the New Zealand Gondwana margin. [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]

SUMMARY This article presents the developments of an ongoing research aimed at modelling the influence of fissuring on the behaviour of clays. In particular, it recalls the main results of an extensive laboratory investigation on a fissured bentonite clay from the south of Italy and presents the data of a new investigation on the evolution with shearing of the strain fields developing within the clay, resulting from Digital Image Correlation (DIC). Element test results are analysed in the framework of continuum mechanics and linked to the clay fissuring features, once characterised using the Fissuring IDentity (F-ID) chart. This article compares the bentonite behaviour with that of other fissured clays of different F-IDs, highlighting the common behavioural features. Thereafter, the soil response at the macro level is related to the DIC-derived strain fields evolving within the clay with loading. For this purpose, DIC was successfully used to investigate the deformation processes active in the fissured clay and the sources of the localisation phenomena. DIC is shown to provide indications of the extent to which highly to medium fissured clays element test results can be of use to model the clay behaviour according to continuum mechanics. Copyright © 2012 John Wiley & Sons, Ltd. [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]

Abstract: The fracture toughness of steel 15Kh2MFA(ІІ) after the PTL was investigated. It was established that the crack start is a multilevel process, in which the defining role is played by the turning modes of deformation. Regardless of the PTL modes in air and in the aggressive medium (electrolytic hydrogen), the resistance to brittle failure of the steel investigated increases as compared to static fracture toughness of the material in the initial state. [Copyright &y& Elsevier]

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: Microstructural deformation mechanisms in adiabatic shear bands in Ti6Al4V are studied using traditional TEM and selected area diffraction, and more advanced microstructural characterisation techniques such as energy dispersive X-ray spectroscopy, high angle annular dark field STEM and conical dark field TEM. The shear bands under investigation are induced in Ti6Al4V samples by high strain rate compression of cylindrical and hat-shaped specimens in a split Hopkinson pressure bar setup. Samples from experiments interrupted at different levels of deformation are used to study the evolution of the microstructure in and nearby the shear bands. From the early stages of adiabatic shear band formation, TEM revealed strongly elongated equiaxed grains in the shear band. These band-like grains become narrower towards the centre of the band and start to fraction even further along their elongated direction to finally result in a nano-crystalline region in the core. In fully developed shear bands, twins and a needle-like martensite morphology are observed near the shear band. [Copyright &y& Elsevier]

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting S tage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands ( Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing. [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]

The article discusses the stability of a long granular slope within the framework of perfect elastoplasticity. It introduces a modified strategy for assessing the slope safety factor and the concept of the angle of natural repose has been bordered on through considering its dependency on the friction angle and dilatancy parameter. Thus, the problem of slope stability has been tackled along with the mechanical condition which shows the basic role of contractive materials.

Abstract: A new finite element code for the solution of the Stokes and heat transport equations is presented. It has purposely been designed to address geological flow problems in two and three dimensions at crustal and lithospheric scales. A variety of rheologies has been implemented including nonlinear thermally activated creep and brittle (or plastic) frictional models. A cloud of particles is used to track materials in the simulation domain which allows to record the integrated history of deformation; its density is variable and dynamically adapted. The code is built on the Arbitrary Lagrangian–Eulerian kinematical description: the computational grid deforms vertically and allows for a true free surface while the computational domain remains of constant width in the horizontal direction. The code can be run in sequential or parallel mode. The parallelisation is based on the MPI paradigm and the domain decomposition algorithm is presented. The solution to the large system of algebraic equations resulting from the finite element discretisation and linearisation of the set of coupled partial differential equations to be solved is obtained by means of an efficient (sequential or massively parallel) direct solver. Details of implementation concerning plasticity, cloud handing, nonlinear convergence, strain accumulation and pressure smoothing are given. The sequential 2D version of the code is used to run the numerical sandbox benchmark at normal and high resolutions. The 2D parallel version is used in the case of a thermo-mechanically coupled extension experiment in which the mantle is present. The 3D parallel version is used to run a crustal scale orogeny experiment. The overall performance of the code, as well as the respect of the incompressibility constraint, of the yield stress criterion and of the volume conservation are discussed, along with parallel scalability. Benchmark results of scalar field advection, the Rayleigh–Taylor experiment and the falling block experiment are presented in Appendix. [Copyright &y& Elsevier]

The material point method, which combines features of finite-element and particle discretisation methods, has been extended to solve coupled flow-deformation problems in granular media. The method is applied to simulate the construction and failure of the Aznalcóllar dam. The brittle foundation clay is characterised by means of a strain-softening Mohr-Coulomb elasto-plastic model. The rupture process of the foundation is associated with a progressive failure mechanism. The model predicts the development of a localised shearing band that initiates at the toe of the dam at some intermediate stage of construction, and propagates downstream and upstream. The shape and position of the failure surface reproduce actual field observations. [ABSTRACT FROM AUTHOR]

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]

Particle crushing, shear banding, interface abrasion and migration of crushing products all have the potential to influence the behaviour of displacement piles in sands. This paper considers these particulate processes, reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests. The findings offer new insights into the mechanics of displacement piles in sands. [ABSTRACT FROM AUTHOR]

This paper presents a study of the micromechanics of granular materials as affected by the direct shear test scale using the discrete element method (DEM). Parametric studies were conducted to investigate the effects of specimen length and height scales (in relation to the particle size) on the bulk material shear strength and shear banding behaviour in the direct shear test. A mesh-free strain calculation method previously developed by the authors was used to capture and visualise the evolution of strain localisation inside the direct shear box. Simulation results show that the maximum shear strength measured at the model boundaries increases with decreasing specimen length scale and increasing specimen height scale. Micromechanics-based analysis indicates that the local and global aspects of fabric change and failure are the major mechanisms responsible for the specimen scale effect. Global failure along the primary shear band prevails when the specimen length scale and length to height aspect ratio are small, while progressive failure becomes more likely when the specimen length scale and aspect ratio become larger. Further quantifications of the specimen scale effects on the macroscopic behaviour of granular materials rely on the fundamental understanding of quantitative relationships between material fabric, anisotropy and bulk strength. [ABSTRACT FROM AUTHOR]

Precise knowledge of the post-peak constitutive response occurring within shear bands in sands is of keen interest in geomechanics, particularly for accurate modelling of progressive failure phenomena. There is mounting evidence that the displacement field within shear bands in sands is non-uniform and distinguished by distinct meso-scale features: namely, particle force chains. Experimental validation of such features will help elucidate the precise nature of the deformation field within shear bands. This paper presents experimental evidence of the kinematic signatures of force chain activity within shear bands in sands. The meso-scale kinematics are quantified from digital-image-correlation-based, grain-scale displacement analyses performed on digital images of specimens undergoing plane strain compression. As in previous work, the data reveal distinct, systematic patterns in the kinematics along the length of the shear band, which serve as indirect evidence of force chain build-up and collapse. Herein, local volume changes are shown to integrate into this pattern. Temporal changes in these patterns with the progress of deformation are also tracked. It is argued that the changes observed in the kinematics from softening to critical state provide a physical, meso-scale explanation for the progress of global stress-strain response through the post-peak regime. [ABSTRACT FROM AUTHOR]

The paper presents a quantitative assessment of particle orientation, contacts and spatial variation of void ratio in a sand specimen loaded under plane-strain conditions. The specimen has a prismatic shape, and measures 57 mm wide × 121 mm long × 180 mm high. It was prepared using a dry natural silica sand known as F-75 Ottawa sand at a dense packing, and was sheared until failure. The specimen failed along a well-defined single shear band. It was then stabilised by slowly percolating a low-viscosity epoxy. Cylindrical subsamples 8 mm in diameter and 50 mm high were cored from the stabilised specimen at regions of interest for further investigation. A synchrotron X-ray microtomography technique was used to acquire high-resolution three-dimensional images of the cores. The scans have a voxel size of 15·06 m, which is high enough to visualise individual sand particles. Post-scanning analyses included mapping the spatial distribution of void ratio and particle contacts inside and outside the shear band. A good correlation was found between void ratio and particle contacts. Statistical distributions of particle orientation within and outside the shear band were calculated: they show that most particles inside the shear band orient in the - plane towards the -axis, which is perpendicular to the shear band with angle in the range 70-90°. Particles outside the shear band have no preferred orientation in the - plane, and form chains with preferred angles of 30-60° from the direction of the applied major principal stress. [ABSTRACT FROM AUTHOR]

The paper aims to highlight the advantages of using data supplied by digital image correlation (DIC) and infrared thermography (IRT) to study the thermomechanical behaviour of materials. It describes an experimental procedure for the determination of mechanical energy and heat sources involved locally during a heterogeneous tensile test. This procedure involves two complementary imaging techniques: DIC provides in-plane displacement fields, while IRT enables the temperature distribution at the specimen surface to be monitored. Numerous different application examples are successively proposed to underline the promising potential of this experimental approach. Kinematical assessments can reveal the extent of homogeneity of the deformation state for a given gauge length. They can also help to determine the relevance of the variables and/or material parameters introduced in the behavioural description at the length scale imposed by the spatial resolution of optical systems (typically 0.1 mm). Moreover, infrared and kinematical data can be used to derive heat source fields induced by the specimen loading and then to generate information on the dissipative or coupled nature of the deformation mechanisms. [ABSTRACT FROM AUTHOR]

Tests have been performed on an analogue two-dimensional granular material in a special laboratory apparatus that allows the application of general stress or strain conditions. Digital image correlation of pairs of consecutive photographs taken during the tests has enabled fields of displacement and hence strain to be determined. Thus direct observation of internal displacements and strains has been possible for a series of general strain increments with different orientations of principal strain and different imposed angles of dilation. This analysis has successfully provided clear evidence of evolving internal structures of deformation. The observed evolving structures consist of bands of localised deformation and ‘cells’ of low deformation between the bands. The orientations of the identified localised features and cells are seen to depend on the applied strain path. The characteristic features and dimensions of the bands have been approximately identified. [ABSTRACT FROM AUTHOR]

The rock mass can be unstable due to the coupling effect of stress and seepage field, adversely affected by the geological field conditions. In this regard, this study implemented a 3D digital image correlation (3D-DIC) technique to conduct triaxial compression tests on sandstone samples under different seepage pressures. We analysed the evolution of strength and permeability during the rock failure process and space-time evolution of the surface radial strain. The results show that the strength and brittleness of the rock decrease with increasing seepage pressure. Further, the mechanical properties of the rock sensitive to were highly dependent on the seepage pressure. We also established a function model of permeability and axial strain consistent with the test results. By analysing the radial-strain-field of the rock surface at specific time points, we observed a gradual process of strain localisation instead of a sudden occurrence before failure. We extracted the spatial coordinates of the first 5% radial strain and subsequently conducted the spatial relation analysis by average-nearest-neighbour-analysis (ANNA). In addition, the approximate failure surface was fitted using the first 5% radial strain points in space, and the distance between the radial strain points and the fitted failure surface in space was analysed. This study shows that points with large strain values gradually converged towards the location of the fracture surface during the gradual failure process of the rock. Finally, this study provides a new method for predicting the failure zones of rock. [ABSTRACT FROM AUTHOR]

Applications of advanced high strength steels in tailor welded blanks are a major focus to reduce vehicle weight and manufacturing cost, and to improve part performance. In the present work, formability of two types of laser welded steels in biaxial stretch forming has been studied. Laser welded blanks of the dual phase steel showed larger reduction of formability than those of high strength low alloy steel. This was due to formation of soft zones in the outer heat affected zone of dual phase steel where strain localisation and fracture occurred. Finite element simulations of dome testing incorporating local weld zone properties were able to accurately predict experimentally observed behaviour including deformation patterns and failure locations. In particular, the simulations have shown that under matching of the softened heat affected zone leads to reduced formability by localisation of strain; and that the fracture location changes from radial to circumferential plane when the flow curve of soft zone is evenly matched with the base metal. It was also concluded that the width of the soft zone has minor influence on formability during biaxial stretch forming. [ABSTRACT FROM AUTHOR]