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The influence of a keyhole defect on local deformation behaviour in additive manufactured Ti-6Al-4V was investigated by comparing it to a representative bulk region without a defect. High resolution digital image correlation (HRDIC) was used to measure the differences in strain localisation at the microstructural length-scale. A nanoscale speckle pattern was used to allow small changes in strain to be detected and resolved within individual lamella and at pre-existing crack locations around the defect. Strain localisation was observed around the defect and formed well below the macroscopic yield stress. In contrast, minimal deformation was found in the bulk at this stress level. Following further deformation into the plastic regime, the strain localisation around the keyhole became more heterogenous with a distinct strain field. A large amount of strain localisation and slip was observed either side of the defect normal to the loading direction compared to relatively little in the regions close to the defect in line with the loading direction. This HRDIC observation was consistent with finite element analysis of the expected strain fields around the defect both below and above the yield point. Furthermore, micro-cracks were observed in αp/αp and αp/βt interfaces in both regions with the more pronounced strain fields around the defect leading to an increased number of long micro-cracks than in the bulk. The formation mechanisms of micro-cracks have been discussed, emphasising the role of localised strain caused by the defect.

Granular materials are made up of smaller particles, manifestation of microstructure results in a macroscopic response of granular material. Understanding the overall mechanical behaviour from microscopic parameters is one of the main challenges in many engineering fields including civil engineering. When modelling this kind of material by Discrete Element Model (DEM) using idealized circular grains, the effects of appropriate sample size and microscopic parameter changes have been a crucial subject. Previous research has primarily relied on the case of purely frictional granular materials. In this paper, we use DEM to investigate the appropriate sample size and the relationship between microscopic parameters and the macroscopic responses of cohesive-frictional granular assemblies by performing a series of biaxial tests. Our findings indicate that a minimum number of particles is required to balance between mechanical behaviour and computing time. In addition, through extensive parametric studies, the paper explores the impact of factors such as interparticle bonds, intergranular friction coefficients, and initial void index on the overall shear behaviour of granular assemblies. Also, the result reveals a strong correlation between shear band formation and the break field of cohesive contact (static variable) and the translations and rotations of grains (kinematic variable).

This paper discusses continuum models for simulating earthquake sequences on faults governed by rate-and-state dependent friction. Through detailed numerical analysis of a conventional strike-slip fault, new observations regarding the use of various continuum earthquake models are presented. We update a recently proposed plasticity-based model using a consistently linearized formulation, show its agreement with discrete fault models for fault thicknesses of hundreds of meters, and demonstrate mesh objectivity for slip-related variables. To obtain a fully regularized fault width description with an internal length scale, we study the performance and mesh convergence of a plasticity-based model complemented by a Kelvin viscosity term and the phase-field approach to cohesive fracture. The Kelvin viscoplasticity-based model can introduce an internal length scale and a mesh-objective response. However, on grid sizes down to meters, this only holds for very high Kelvin viscosities that inhibit seismic slip rates, which renders this approach impractical for simulating earthquake sequences. On the other hand, our phase-field implementation for earthquake sequences provides a numerically robust framework that agrees with a discrete reference solution, is mesh objective, and reaches seismic slip rates. The model, unsurprisingly, requires highly refined grids around the fault zones to reproduce results close to a discrete model. Following this line, the effect of an internal length scale parameter on the phase-field predictions and mesh convergence are discussed. [ABSTRACT FROM AUTHOR]

Granular materials are made up of smaller particles, manifestation of microstructure results in a macroscopic response of granular material. Understanding the overall mechanical behaviour from microscopic parameters is one of the main challenges in many engineering fields including civil engineering. When modelling this kind of material by Discrete Element Model (DEM) using idealized circular grains, the effects of appropriate sample size and microscopic parameter changes have been a crucial subject. Previous research has primarily relied on the case of purely frictional granular materials. In this paper, we use DEM to investigate the appropriate sample size and the relationship between microscopic parameters and the macroscopic responses of cohesive-frictional granular assemblies by performing a series of biaxial tests. Our findings indicate that a minimum number of particles is required to balance between mechanical behaviour and computing time. In addition, through extensive parametric studies, the paper explores the impact of factors such as interparticle bonds, intergranular friction coefficients, and initial void index on the overall shear behaviour of granular assemblies. Also, the result reveals a strong correlation between shear band formation and the break field of cohesive contact (static variable) and the translations and rotations of grains (kinematic variable). [ABSTRACT FROM AUTHOR]

The presence of defects produced by additive manufactured (AM) processes in structural Ti alloys such as Ti-6Al-4V is known to have serious implications on the deformation and fatigue behaviour of engineering components. However, there is little understanding about the localised plastic deformation patterns that develop around AM defects, and the associated local conditions that could lead to the nucleation of micro-cracks under creep loading conditions. In this work, the effects of the morphology and volume fraction of AM defects and temperature on the strain localisation behaviour around such defects in Ti-6Al-4V will be addressed. To that purpose, a novel rate-dependent crystal plasticity formulation is proposed to describe the mechanical behaviour of the alloy's predominant α ′ (HCP)-phase. Representative volume elements (RVEs) of the AM produced microstructures are digitally reconstructed from EBSD orientation maps obtained on planes perpendicular and transversal to the microstructure's AM growth direction. Calibration of the single crystal model for the α ′ -phase is carried out from macroscopic uniaxial tensile data from polycrystalline AM specimens at different strain rates and temperatures and published creep data. Furthermore, RVEs containing AM defects of different morphologies and volume fractions are relied upon to investigate the strain localisation behaviour around the defects under uniaxial loading at ambient and high temperatures. It is found that the extent of the localised accumulated plastic strain around defects depends greatly on whether the voids surface are smooth or have sharp corners, with the latter being associated with more severe localisation patterns. Moreover, a numerical investigation into the crack initiation behaviour of AM Ti-6Al-4V under uniaxial creep loading at 450 ° C revealed that the development of the local conditions suitable for the nucleation of creep damage/micro-cracks is accelerated in the presence of typical AM defects, and the extent of that acceleration depends strongly on their morphology. An AM defect shape parameter is introduced to quantify the way their morphology affects the time for creep crack initiation/damage. • A novel crystal plasticity model for α ′ - phase AM Ti-6Al-4V was proposed. • The severity of the predicted localised deformation patterns around AM defects was studied. • Gradients of plastic deformation in grains found at AM defects' surface were determined. • Two micro-crack indicators were explored to predict the onset of creep damage at both 450 and 600°C. • An AM defect shape parameter was introduced to study the influence of defect morphology on failure. [ABSTRACT FROM AUTHOR]

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]

Continuum Damage Mechanics provides a framework for the description of the mechanical response of concrete and rocks which encompasses distributed micro-cracking, macro-crack initiation, and then its propagation. In order to achieve a consistent setting, an internal length needs to be introduced to circumvent the difficulties inherent to strain softening and to avoid failure without dissipation of energy. Upon inserting this internal length, structural size effect is captured too. This paper reviews some the progresses achieved by the author since the introduction of the nonlocal damage model in 1987. Among them, the early proposals exhibited a proper description of the inception of failure but a poor one for complete failure since it is not straightforward to model a discrete cracking with a continuum approach. Candidate solutions, e.g. by considering a variable internal length are outlined. Then, the coupled effects between material damage and material permeability are considered. Is is recalled that the permeability of the material should be indexed on the damage growth in the regime of distributed cracking. Upon macro-cracking, there is a change of regime and it is the crack opening that controls the fluid flow in the cracked material. Both regimes may be captured with a continuum damage approach, however.

In the present study we elucidate the nature of local strain statistics evolution during tensile deformation in polycrystalline materials. A rate-independent formulation was implemented within a crystal plasticity framework by the means of representative volume element (RVE) analysis. Local elastic strain, as well as stress, were found to obey a normal distribution, whereas the statistics of local plastic strain conforms to a lognormal distribution. In line with experimental observations, the plastic strain becomes progressively localised and the local regions of large strains make significant contribution to the overall average strain increase. The results reveal the nature of strain inhomogeneity at the microscale and emphasize the fact that in metallic materials the elastic strain accumulation represents an additive process, whereas plastic deformation is a multiplicative process.

Continuum Damage Mechanics provides a framework for the description of the mechanical response of concrete and rocks which encompasses distributed micro-cracking, macro-crack initiation, and then its propagation. In order to achieve a consistent setting, an internal length needs to be introduced to circumvent the difficulties inherent to strain softening and to avoid failure without dissipation of energy. Upon inserting this internal length, structural size effect is captured too. This paper reviews some the progresses achieved by the author since the introduction of the nonlocal damage model in 1987. Among them, the early proposals exhibited a proper description of the inception of failure but a poor one for complete failure since it is not straightforward to model a discrete cracking with a continuumapproach. Candidate solutions, e.g. by considering a variable internal length are outlined. Then, the coupled effects between material damage and material permeability are considered. Is is recalled that the permeability of thematerial should be indexed on the damage growth in the regime of distributed cracking. Upon macro-cracking, there is a change of regime and it is the crack opening that controls the fluid flow in the cracked material. Both regimes may be captured with a continuum damage approach, however. [ABSTRACT FROM AUTHOR]

A severe, spurious dependence of numerical simulations on the mesh size and orientation can be observed in elasto-plastic models with a non-associated flow rule. This is due to the loss of ellipticity and may also cause a divergence in the incremental-iterative solution procedure. This paper first analyses the dependence of the shear band inclination in a biaxial test on the mesh size as well as on the mesh orientation. Next, a Cosserat continuum model, which has been employed successfully for strain-softening plasticity, is proposed to prevent loss of ellipticity. Now, numerical solutions result for shear band formation which are independent of the size and the orientation of the discretisation. [ABSTRACT FROM AUTHOR]

Shearing of magma during ascent can promote strain localisation near the conduit margins. Any mechanical and thermal discontinuities associated with such events may alter the chemical, physical and rheological stability of the magma and thus its propensity to erupt. Lava spines can record such processes, preserving a range of macroscopic and microscopic deformation textures, attributed to shearing and friction, as magma ascends through the viscous-brittle transition. Here, we use a multidisciplinary approach combining petrology, microstructures, crystallography, magnetics and experimentation to assess the evidence, role and extent of shearing across a marginal shear zone of the 1994–1995 lava spine at Unzen volcano, Japan. Our results show that crystals can effectively monitor stress conditions during magma ascent, with viscous remobilisation, crystal plasticity and comminution all systematically increasing towards the spine margin. Accompanying this, we find an increase in mineral destabilisation in the form of pargasitic amphibole breakdown displaying textural variations across the shear zone, from symplectitic to granular rims towards the spine margin. In addition, the compaction of pores, chemical and textural alteration of interstitial glass and magnetic variations all change systematically with shear intensity. The strong correlation between the degree of shearing, crystal deformation and disequilibrium features, together with distinct magnetic properties, implies a localised thermal input due to shear and frictional processes near the conduit margin during magma ascent. This was accompanied by late-stage or post-emplacement fluid- and gas-induced alteration of the gouge, as well as oxidation and glass devitrification. Understanding and recognising evidence for strain localisation during magma ascent may, therefore, be vital when assessing factors that regulate the style of volcanic eruptions, which may provide insights into the cryptic shifts from effusive to explosive activity as observed at many active lava domes. [ABSTRACT FROM AUTHOR]

To examine the impact of preexisting weaknesses on fracture coalescence during volcanic edifice deformation, we triaxially compressed Mount Etna basalt while acquiring in situ dynamic X‐ray microtomograms and calculated the internal strain tensor fields using image correlation. Contraction localization preceded dilation and shear strain localization into the protofault zone. This onset of strain localization preceded macroscopic yielding and coincided with increases in the magnitude and volume of rock experiencing dilation, and spatial clustering of the strain populations. The exploitation of weaknesses by propagating fractures enabled the dominant shear strain to switch senses as propagating fractures lengthened along 30–60° from σ1. Scanning electron microscopy images reveal pore‐emanated fractures, and fractures linking pores. These experiments provide evidence of internal contraction preceding dilation and shear, consistent with inferences from field and laboratory observations. The transition from contraction to dilation may provide a precursory signal of volcanic flank eruption. Plain language summary: Directly observing how rocks break at seismogenic depths in natural settings is at present impossible. Here we used X‐ray imaging techniques to view deforming, and then breaking, basaltic rocks at stress conditions equivalent to the flanks of the upper part of the Mount Etna volcano. We tracked how preexisting weaknesses, including pores and fractures produced during the fast cooling of the lava, controlled the growth and coalescence of new fractures and faults. At 50% of the stress at failure, shear and dilative strains began to concentrate in the volume that eventually developed the largest connected fracture network. The localization of contractive strains preceded this shear and dilation localization. These data sets provide observations of fracture growth within Etna basalt that previous studies could only infer, and thus constraints on how the volcanic edifice deforms under magmatic and tectonic stresses and eventually ruptures in flank eruptions. Key Points: Failure in Etna basalt occurred through contraction localization, and then shear and dilation localization, consistent with geophysical observationsShear and volumetric strain localized into a protofault zone preceding 50% failure stress, and macroscopic yieldingThe precursors to volcanic flank eruptions may include a transition from contraction to dilation [ABSTRACT FROM AUTHOR]

Conventional finite element analyses generally suffer from mesh dependency when considering softening effects. Non-local strain regularisation techniques have been developed to address this issue, which are generally complex to implement. In view of this, this paper introduces an more efficient procedure for implementation of a non-local method in Abaqus for Coupled Eulerian-Lagrangian (CEL) large deformation finite element undrained analyses. Results from a series of biaxial compression simulations demonstrate that the non-local method with a strain-softening Tresca model in CEL avoids mesh dependency. Owing to the stationary Eulerian element and the built-in mapping algorithm in CEL, high computational efficiency is achieved, adding no more than 12% to the computational cost. Guidance is provided on selection of internal length scales, element sizes and the search radius to ensure efficient non-local calculations, and it is shown that a softening scaling rule can also be used to allow use of practical mesh densities for some boundary value problems. Simulations of a number of classical geotechnical problems demonstrate the type of boundary value problems where the non-local method can effectively mitigate the mesh dependency, whilst also highlighting that the method fails to control the strain localisation that develops at soil-structure interfaces due to geometrical issues. [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]

A constitutive model for fibre-reinforced composites has been developed that takes imperfect fibre/matrix interfacial bonding into account. The model can predict the nonlinear material response of a composite in the large strain regime. Independent constitutive laws can be used for the constituents behaving elastic or elastic-plastic. A constitutive equation is derived for the composite moduli relating stresses to strains. The model is used to predict the development of the compressive failure; fibre kinking that is either formed by bifurcation or due to an initial fibre imperfection. A three-dimensional finite element model for kink band formation is used to validate the results obtained using the constitutive model with varying levels of interfacial bonding. [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]

Full range analysis of reinforced concrete (RC) members covering the post-crack and post-peak regimes is important for obtaining the deformation response and failure mode of structural members. When a RC member is subject to an increasing external load, the critical sections would exhibit cracking and/or softening. Due to stress relief effect in the proximity of crack opening and plastic hinging, unloading may occur at the adjacent regions. The variable stress states of discrete sections would lead to sectional variation of stiffness, which could not be accounted for by conventional structural analysis methods. In this paper, a nonlinear multilevel analysis method for RC frames whereby the frame members are divided into sub-elements and sectional analysis is utilised to evaluate stiffness degradation and strength deterioration is developed. At sectional level, the secant stiffness is determined from moment-curvature relation, where the curvature is evaluated based on both transverse displacements and section rotations of the frame member. Unloading and reloading behaviour of concrete and reinforcing steel is simulated. In implementing the multilevel analysis, secant iteration is performed in each step of displacement increment to obtain the convergent solution satisfying equilibrium. Numerical example of RC frame is presented to demonstrate the applicability and accuracy of the proposed nonlinear multilevel analysis method.

Damage by pitting corrosion of the surface areas of the local oil and gas pipelines was identified and numeri-cally analysed in this paper. Having used the methods of optical and scanning electron microscopy, the basic laws of structural and mechanical degradation were established. Based on the data of fractographic control, the shape of defects and substantiated morphological characteristics were examined. It was established that certain informative signs of the defects correspond to certain stages of operation. Having used methods of fractodiagnostics, the main reasons for the performance defects of main oil and gas pipelines were established, and the mechanisms of degradation as well as frac-ture of the materials they are made from were described. According to the findings, the cause of the decrease in the reli-ability of the main oil and gas pipeline networks is general and localised corrosion as well as technological defects in the pipeline metal. These studies are essential for the establishment of the complex of mechanical and corrosion-mechanical properties, particularly sensitive to degradation in the steel used for the pipe wall after a lengthy use of a pipe.

The results of a three-dimensional (3D) analysis concerning the Senise landslide are presented. This landslide occurred after deep excavations had been carried out at the toe of the slope for constructing several buildings. The soils involved in the landslide were characterized by a pronounced strain-softening behaviour. The analysis is performed using a finite element approach in which an elasto-viscoplastic constitutive model is incorporated. The strain-softening behaviour of the soils is simulated reducing the strength parameters from peak to residual, with the accumulated deviatoric plastic strain. The results from this analysis account for the occurrence of a 3D progressive failure process in the slope. [ABSTRACT FROM AUTHOR]

Through-thickness crystallographic texture gradients may develop in extruded profiles and rolled sheets of aluminium alloys. These texture gradients are often modelled using the crystal plasticity theory in order to predict strain localisation more accurately. In this work, an experimental and numerical study was carried out for a flat extruded profile with texture gradients made of the aluminium alloy AA6063. Uniaxial tension and plane-strain tension specimens were produced from this profile in two orthogonal material directions and heat treated to different tempers to study the role of crystallographic texture and work-hardening on strain localisation. The microstructure, including the orientation, morphology and position of the grains, was obtained from EBSD scans. The plane-strain tension tests were simulated using two crystal plasticity finite element models: the first represents accurately the measured microstructure, whereas the second one only represents correctly the global texture but not the texture gradients and grain morphology, i.e., the grain orientations are assigned randomly. In addition, a coarser model was used to identify the material parameters based on data from the plane-strain tension tests. The simulation results showed that accurate modelling of the microstructure did not influence the simulation results significantly. The model with the same global texture but with random assignment of grain orientations gave similar predictions for both the global stress-strain behaviour and the local deformation patterns. [ABSTRACT FROM AUTHOR]

This paper extends the hierarchical multiscale approach developed earlier by the authors to model the coupled hydro-mechanical behaviour for saturated granular soils. Based on a hierarchical coupling of the finite element method (FEM) and the discrete element method (DEM), the approach employs the FEM to solve a boundary value problem while using the DEM to derive the required nonlinear material responses at each FEM Gauss integration point. It helps to bypass the phenomenological constitutive model required in the conventional FEM simulations and offers a natural pathway for scale bridging. The current extension features further key consideration of the coupled hydro-mechanical behaviour in a saturated granular soil due to the presence of pore fluid and its flow. By invoking Terzaghi’s effective stress principle, a u – p formulation is proposed for the multiscale framework to derive the effective stress from DEM solution of the representative volume element (RVE) embedded at each Gauss point which is then superimposed with the pore fluid pressure to obtain the total stress in solving the coupled governing equations for a fluid–solid mixture. This extension greatly enriches the predictive capacity of the multiscale approach for simulating saturated granular soils relevant to a wide variety of important civil/mining applications. The new approach is first benchmarked by closed-form solutions to classical 1D and 2D consolidation problems. It is then applied to simulate globally undrained biaxial compression tests on both dense and loose soils subjected to large deformation. We further discuss interesting fluid flow patterns and the failure modes (both localisation and diffuse liquefaction) observed from the simulations and provide detailed cross-scale analyses. [ABSTRACT FROM AUTHOR]

Numerical simulators have become indispensable in geomechanics to model increasingly more complex rock behaviours by harnessing the growing computational power available. Those tools aim at simulating more realistic scenarios while accounting for more physical processes. As such, geomechanical modelling remains particularly challenging because of the inherent multi-scale and multi-physics nature of all physical processes at play. In this context the standard approach of sequential coupling is reaching its limits when dealing with multiple physical processes and is maladapted for the modelling of material instabilities. We present in this study a novel numerical simulator, REDBACK, to model multi-physics Rock mEchanics with Dissipative feedBACKs in a tightly coupled manner. This tool provides both the prototyping flexibility to investigate more complex physics and non-linear feedbacks as well as the computational scalability to tackle three dimensional scenarios. We demonstrate the approach by modelling pore collapse and validating the approach against isotropic consolidation experiments on Bleurwiller sandstone as well as drained triaxial experiments on a diatomaceous mudstone. Through this exercise we identify the activation enthalpy dependency on confining pressure that is required to match the experiments. The results show that dilatancy corresponds to internal processes with negative activation volume, while contractancy corresponds to internal processes with positive activation volume. This calibration process demonstrates the importance of a physics-based approach in a multi-scale framework, where one can aim at extrapolating results outside the range of laboratory experiments based on the understanding of the underlying physical processes, when traditional engineering approaches are often limited to interpolation within the scope of sparse and expensive experiments. [ABSTRACT FROM AUTHOR]

Damage by pitting corrosion of the surface areas of the local oil and gas pipelines was identified and numerically analysed in this paper. Having used the methods of optical and scanning electron microscopy, the basic laws of structural and mechanical degradation were established. Based on the data of fractographic control, the shape of defects and substantiated morphological characteristics were examined. It was established that certain informative signs of the defects correspond to certain stages of operation. Having used methods of fractodiagnostics, the main reasons for the performance defects of main oil and gas pipelines were established, and the mechanisms of degradation as well as fracture of the materials they are made from were described. According to the findings, the cause of the decrease in the reliability of the main oil and gas pipeline networks is general and localised corrosion as well as technological defects in the pipeline metal. These studies are essential for the establishment of the complex of mechanical and corrosion-mechanical properties, particularly sensitive to degradation in the steel used for the pipe wall after a lengthy use of a pipe. [ABSTRACT FROM PUBLISHER]

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 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.

Full range analysis of reinforced concrete (RC) members covering the post-crack and post-peak regimes is important for obtaining the deformation response and failure mode of structural members. When a RC member is subject to an increasing external load, the critical sections would exhibit cracking and/or softening. Due to stress relief effect in the proximity of crack opening and plastic hinging, unloading may occur at the adjacent regions. The variable stress states of discrete sections would lead to sectional variation of stiffness, which could not be accounted for by conventional structural analysis methods. In this paper, a nonlinear multilevel analysis method for RC frames whereby the frame members are divided into sub-elements and sectional analysis is utilised to evaluate stiffness degradation and strength deterioration is developed. At sectional level, the secant stiffness is determined from moment-curvature relation, where the curvature is evaluated based on both transverse displacements and section rotations of the frame member. Unloading and reloading behaviour of concrete and reinforcing steel is simulated. In implementing the multilevel analysis, secant iteration is performed in each step of displacement increment to obtain the convergent solution satisfying equilibrium. Numerical example of RC frame is presented to demonstrate the applicability and accuracy of the proposed nonlinear multilevel analysis method. [ABSTRACT FROM AUTHOR]

The strain rate dependency of carbon fibre reinforced plastic (CFRP) laminate manufactured in a resin transfer moulding (RTM) process was investigated up to strain rates of 10 2 s −1 . High-speed video imaging in combination with digital image correlation analysis (DIC) and high-speed infrared cameras were applied to evaluate the tests. Specimens were specifically designed for tests at high loading rates. Periodic strain localisations were observed. Thus, multiple fractures occurred at the highest strain rate. Considerable strain rate dependence could be identified. The stress strain curves, and in the case of 90° tension and compression also the strain to fracture exhibited a moderate rise with increasing strain rate. High rate shear tests at ±45° loading direction with large, localised deformations up to failure showed reduced hardening compared to quasi-static tests, probably caused by adiabatic heating. A local temperature rise of about 50 K was determined by high-speed infrared measurements. [ABSTRACT FROM AUTHOR]

Sedimentary geomaterials such as rocks frequently exhibit cross-anisotropic properties and their behaviour depends on the direction of loading with respect to their microstructure. As far as material rupture is concerned, localised deformation in shear band mode appears generally before cracks and material failure. The influence of cross-anisotropy on the shear strain localisation remains an important issue and is investigated in the present study. To do so, a constitutive elastoplastic cross-anisotropic model that includes anisotropy both on the elastic and plastic characteristics is defined. For the plastic part of the model, the anisotropy of a strength parameter is introduced with a microstructure fabric tensor. Then, the fractures are modelled with finite element methods by considering the development of shear strain localisation bands and an enriched model is used to properly reproduce the shear banding. The cross-anisotropy influence on shear banding is studied through numerical applications of small and large-scale geotechnical problems that engender fractures. The two considered applications are a plane-strain biaxial compression test and an underground gallery excavation. The numerical results provide information about the influence of cross-anisotropy on the appearance and development of shear bands. It has been noticed, among other observations, that the material strength vary with the loading direction and that the development and the shape of the excavation fractured zone that develops around a gallery is strongly influenced by the material anisotropy. [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]

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]

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]