Search

Polycrystalline materials are characterized by complex microstructures, which present significant challenges in accurately predicting their overall properties. Among the critical factors complicating the prediction of mechanical properties is the heterogeneous nature of plastic deformation, manifested as localized slip bands in these materials. Microscopic slip within slip bands has been shown to govern macro-scale material properties, such as strength and toughness. Yet, our understanding of the spatial heterogeneity associated with slip remains surprisingly limited.While ductility in polycrystalline materials stems from slip accumulation, localized slip is also implicated in crack initiation, propagation, and ultimate material failure. Recent experimental investigations have revealed that the observed trade-off between strength and toughness is attributed to the localized slip occurring within slip bands at the microscopic level. Over the past few decades, significant advancements in microscopy and characterization techniques have enabled researchers to capture slip bands and associated microstructural phenomena in polycrystals. However, these techniques are often expensive, challenging to interpret, and time-consuming to explore the multifaceted localization behavior under different conditions. Despite the breakthroughs in experimental capabilities, our computational capacity to model the development and implications of slip bands in polycrystalline solids have remained limited.Motivated by recent advancements in slip band characterization and the growing demand for a numerical method to complement the experiments, this dissertation aims to develop a crystal plasticity-based computational framework to explicitly model slip bands and heterogeneous micromechanical fields associated with them. Slip bands are treated as discrete microstructural entities, enabling the computation of associated stress and strain fields. The proposed model is validated against a wide range of experimental results and subsequently applied to multiple microstructure settings to elucidate micro- and macro-scale phenomena observed in various polycrystalline materials. These phenomena include neighbor-affected slip band localization, the role of slip bands in triggering subsequent slip or twin bands or orientation deviation zones in the neighboring grain, and the link between macroscopic strength and microscopic slip in polycrystalline materials, to name a few. Explicit slip band modeling is, furthermore, combined with machine learning and data-driven approaches to design materials with superior, targeted properties, such as reduced anisotropy or strain delocalization in titanium alloys.Integration of the framework developed here with novel experimental techniques holds immense potential to significantly advance our capabilities in predicting material properties. This transformative combination will bridge existing gaps in our understanding and enable more efficient and effective materials development, ultimately leading to improved material performance.

Abstract In traditional finite element failure analyses of geotechnical structures, the micro grain rotations cannot be modelled and numerical solutions are mesh dependent. In this study, a user element including rotational degree of freedom has been developed based on micropolar theory (Cosserat theory), then an enhanced non-associated sand model is calibrated with laboratory data and used to model the plane strain tests. The simulated results demonstrate the polarized model is able to model reasonably the sand behavior as well as the grain rotations in the localized region. What’s more, with this enhanced model, the mesh independent numerical solutions in terms of mechanical responses, shear bands thickness and orientations have been obtained. Article highlights In failure analysis of geostructures, significant rotations of soil grains have been observed to occur in the strain localized regions, but the current commercial Finite Element tools cannot model the micro rotations. Therefore, a user defined element must be developed to include the rotational degree of freedom. The micropolar approach is proven to be effective to model the grain rations in present paper. More suitable than other classical soil or sand constitutive models, the selected non-associated sand model in present paper is capable of describing well the contraction and shear dilatancy behaviors of sand. Then the model has been enhanced by means of micropolar technique, in this way, the reasonable strain localization phenomena in laboratory tests could be predicted well. There are always the mesh dependent problems for traditional simulations of the strain localization phenomena in finite element analysis. It can be found in present paper that the mesh independent numerical solutions are obtained by means of micropolar technique. Furthermore, the micropolar approach can obviously improve convergence difficulties in finite element analyses.

Deformation behavior of 1Al containing Mg alloy has been investigated in the present study. After annealing, the Mg-1Al alloy shows a typical basal texture. When compared to the pure Mg having a similar texture and grain size, the Mg-1Al alloy shows much higher strength and larger elongation. Slip trace analyses of the tensile strained specimens show that non-basal slips such as pyramidal I and II slips can be easily activated at an early stage of deformation in the Mg-1Al alloy and the grains in the Mg-1Al alloy are seen to accommodate a larger degree of deformation than those in the pure Mg at a given strain. With increasing tensile strain, however, there is a strain localization along the initially formed slip lines of non-basal slips, forming surface steps without activating multiple slip lines.

Laser dynamic flexible forming (LDFF) is a novel high velocity forming (HVF) technology, in which the foil metal is loaded by laser shock wave. Strain localization is readily to occur around the bulge edge, which will result in the ultimate dynamic failure. In this work, the microstructures before and after dynamic fracture are characterized by transmission electron microscopy (TEM) to investigate the dynamic failure mechanism. The plastic deformation regions of copper foil are composed of shock compression, strain localization and bulge. Microstructure refinement was observed in three different plastic deformation regions, particularly, dynamic recrystallization (DRX) occurs in the strain localization and bulge regions. In bulge region, extremely thin secondary twins in the twin/matrix (T/M) lamellae are formed. The microstructure features in the strain localization region show that superplastic flow of material exists until fracture, which may be due to DRX and subsequent grain boundary sliding (GBS) of the recrystallized grains. The grain coarsening in strain localization region may degrade the material flowing ability which results in the dynamic fracture.

This paper investigates the deformation and fracture propagation of sandstone specimen under uniaxial compression using the distributed fiber optic strain sensing (DFOSS) technology. It shows that the DFOSS-based circumferential strains are in agreement with the data monitored with the traditional strain gage. The DFOSS successfully scans the full-field view of axial and circumferential strains on the specimen surface. The spatiotemporal strain measurement based on DFOSS manifests crack closure and elastoplastic deformation, detects initialization of microcrack nucleation, and identifies strain localization within the specimen. The DFOSS well observes the effects of rock heterogeneity on rock deformation. The advantage of DFOSS-based strain acquisition includes the high spatiotemporal resolution of signals and the ability of full-surface strain scanning. The introduction to the DFOSS technology yields a better understanding of the rock damage process under uniaxial compression.

Stress-oftening is one of the significant features experienced by cohesive-frictional granular materials subjected to deviatoric loading. This paper focuses on mesoscopic evolutions of the dense granular assembly during a typical drained biaxial test conducted by DEM, and proposes mesoscopically-based framework to interpret both hardening and softening mechanisms. In this context, force chains play a fundamental role as they form the strong contact phase in granular materials. Their geometrical and mechanical characteristics, as well as the surrounding structures, are defined and analyzed in terms of force chain bending evolution, local dilatancy, rotation and non-coaxiality between the principal stress and the geometrical orientation of force chains. By distinguishing two zones inside and outside shear band, force chain rotations are shown to be of opposite sign, which may contribute to the observed macroscopic softening as one of the origin of the structural softening.

Metallic nanolaminates are a class of nanocrystalline materials composed of alternating layers of two or more dissimilar metals. These materials offer several advantages over traditional single phase nanocrystalline metals; their lamellar architecture and often immiscible constituent phases provide improved thermal stability and resistance to grain growth, while the nanocrystalline grain size and high interfacial density impart ultra-high strength and hardness. Cu-Nb nanolaminates synthesized via thin film deposition techniques have demonstrated extraordinary mechanical properties including strengths in excess of 2 GPa and significant plasticity prior to failure. Yet the limited volumes and film thicknesses (

The objective of this study is to quantitatively evaluate the soil structure behavior when under shear stress to understand the mechanism of shear zone formation using a micro-focus X-ray computed tomography (CT) scanner to visualize the internal samples without causing disturbance. A new image-analysis method was proposed to systematically evaluate the particle length and direction by fitting the particle as an ellipsoid. Subsequently, a direct shear experiment was conducted on soil materials, and shear band was scanned using a micro-focus X-ray CT scanner. After validating the proposed method, the soil structure was evaluated in the shear zone via image analysis on the CT images. Furthermore, the strain inside the specimen was evaluated using digital image correlation. The results showed that a partial change in the particle direction occurred when the volume expansion inside the shear zone exceeded the peak. In addition, the width of the shear zone was ~7.1 times the median grain size of the sand used, however, the region exhibiting a change in the direction of the particles was narrow and confined to the vicinity of the shear plane.

Unknown characteristics of the strain localization process are determined by the study of the structure of localized strain bands and adjacent matrix metal. Interference of oncoming waves (compression and unloading) reflected from the sample faces are shown to induce oscillations of the sample in the standing wave mode at the nodes of which the localized strain bands (precursors of damage) arise. Strain localization is accompanied by the formation of a flow of particles (interstitial and substitution atoms, fine particles of the doping phase, and impurities), which migrate from the matrix to the zone of spall damage. The main property of a standing wave, namely, capability of preventing energy exchange through the nodal points, provides a long-term sample deformation after the shock wave front passed. The distance passed by the particles under impulse load exceeds the drift of particles under quasi-static load by several orders of magnitude.

Optimizing the mechanical properties of composites through microstructural design has been a long-standing issue in materials science. In this study, we reinforced a typical polymer, i.e., polyethylene-terephthalate-woven fabric, with a type of Fe-based metallic glassy fiber (MGF) with an extremely large Young’s moduli. The MGF-reinforced fabrics, with three different fiber bundle orientations (0°, 45°, and 90°), were investigated by in situ electron-microscopy mechanical testing techniques in conjunction with a digital image correlation (DIC) technique. The fabrics exhibited a pronounced anisotropic mechanical response, and the associated characteristics were verified to depend on the fiber bundle orientation relative to the external load. Furthermore, localized strains near the intersections of the fiber bundles were found to be much higher than the global strain. It is confirmed that the restriction from warp to weft is the dominant factor influencing strain localization during deformation. Our results are enlightening for understanding the fracture mechanisms of composites.

Funding Information: Ivan Lomakin and Xavier Sauvage are grateful to the Embassy of France in Russia for supporting their collaboration through Mechnikov’s Scholarship. This work was partially supported by Saint Petersburg State University, Russia via Lot 2017 applied (id: 26130576 ). This work was partially supported by Academy of Finland through grant 317464 . XRD results were obtained by the “Center for X-ray Diffraction Methods” of the Research park of Saint Petersburg University. Anvils for HPT tool were produced in collaboration with center “Applied aerodynamics” of the Research park of Saint Petersburg University. Authors acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Nanomicroscopy Center (Aalto-NMC). Funding Information: Ivan Lomakin and Xavier Sauvage are grateful to the Embassy of France in Russia for supporting their collaboration through Mechnikov's Scholarship. This work was partially supported by Saint Petersburg State University, Russia via Lot 2017 applied (id: 26130576). This work was partially supported by Academy of Finland through grant 317464. XRD results were obtained by the ?Center for X-ray Diffraction Methods? of the Research park of Saint Petersburg University. Anvils for HPT tool were produced in collaboration with center ?Applied aerodynamics? of the Research park of Saint Petersburg University. Authors acknowledge the provision of facilities and technical support by Aalto University at OtaNano - Nanomicroscopy Center (Aalto-NMC). Publisher Copyright: © 2021 The Author(s) The influence of the structural state on deformation mechanisms under large strains was investigated in a Cu – 2 wt.% Be alloy. Torque evolutions during straining exhibit serrations typical of structural relaxations in the precipitation-hardened alloy, but not in the solid solution state. Microstructure characterization revealed that homogeneous deformation of the solid solution led to strong grain size reduction (20 nm), while localization occurred in the precipitated state with numerous shear bands. The role of CuBe precipitates on hindering dislocation glide and the resulting shear localization under large strain is discussed.

Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate.

Rolling contact fatigue cracks and thermally induced defects are common problems in the railway industry especially as demands for increasing loads, speeds, and safety continue to rise. Often, the two types of defects are found together in the field, however, whether one causes the other to occur is not completely agreed upon. The effect of thermal damage, in the form of a martensite spot on pearlitic steel test bars, on the fatigue life in uniaxial low cycle fatigue experiments was investigated by the authors. However, the focus of the current work was to characterize the damage evolution from the low cycle fatigue (LCF) tests and correlate the crack initiation and propagation with the initial thermal damage. Residual stress measurements, digital image correlation, and X-ray tomography were used to characterize the effects of the thermal damage before, during, and after fatigue testing, respectively. It was found that the thermal damage causes strain accumulation and crack initiation at the interface between the two materials. The strain evolution was visualized using digital image correlation (DIC), clearly showing the strain concentrations at the top and bottom of the white etching layers (WEL), where the residual stresses are also most tensile. X-ray tomography confirmed the planar crack growth from the martensite spot.

Carbon anodes participate in chemical reactions to reduce alumina in the Hall–Héroult process, of which coke aggregates make up a major part. The failure analysis of coke aggregates not only leads to a better understanding of the deformation mechanisms of anode paste under compressive loading but also can identify potential causes of structural defects in carbon anodes, such as horizontal cracks. The coke aggregates are composed of particles with different size distributions and shapes, which may strongly affect the failure behavior of the anode during compaction. In this paper, the effects of particle size distributions and shapes on the mechanical behavior and the failure of coke aggregates are investigated using the discrete element method modeling technique. The numerical results reveal that, although the mechanical behavior of coke mixtures is generally dependent on larger particles, the presence of fine particles in the coke aggregates reduces fluctuations in the stress–strain diagram. In addition, the rolling resistance model is employed as a parameter representing the effect of particle shape. It is shown that the rolling resistance model can be an alternative to the overlapped spheres model, which has a higher computational cost than the rolling resistance model. The second-order work criterion is used to evaluate the stability of the coke aggregates, the results of which indicate that the addition of fine particles as well as increasing the rolling resistance between the particles increases the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke mixture and by increasing the rolling resistance between the particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area prone to horizontal crack generations, the results of this study could lead to the production of carbon anodes with fewer structural defects.

Strain localization analysis for orthotropic-associated plasticity in cohesive–frictional materials is addressed in this work. Specifically, the localization condition is derived from Maxwell’s kinematics, the plastic flow rule and the boundedness of stress rates. The analysis is applicable to strong and regularized discontinuity settings. Expanding on previous works, the quadratic orthotropic Hoffman and Tsai–Wu models are investigated and compared to pressure insensitive and sensitive models such as von Mises, Hill and Drucker–Prager. Analytical localization angles are obtained in uniaxial tension and compression under plane stress and plane strain conditions. These are only dependent on the plastic potential adopted, ensuing, a geometrical interpretation in the stress space is offered. The analytical results are then validated by independent numerical simulations. The B-bar finite element is used to deal with the limiting incompressibility in the purely isochoric plastic flow. For a strip under vertical stretching in plane stress and plane strain as well as Prandtl’s problem of indentation by a flat rigid die in plane strain, numerical results are presented for both isotropic and orthotropic plasticity models with or without tilting angle between the material axes and the applied loading. The influence of frictional behavior is studied. In all the investigated cases, the numerical results provide compelling support to the analytical prognosis.

An in-depth study of the failure of granular materials, which is known as a mechanism to generate defects, can reveal the facts regarding the origin of the imperfections, such as cracks in the carbon anodes. The initiation and propagation of the cracks in the carbon anode, especially the horizontal cracks below the stub-holes, reduce the anode efficiency during the electrolysis process. The failure analysis of coke aggregates can be employed to determine the appropriate recipe and operating conditions in order to avoid the formation of cracks in the carbon anodes. In this paper, it will be shown that a particular failure mode can be responsible for the crack generation in the carbon anodes. The second-order work criterion is employed to analyze the failure of the coke aggregate specimens and the relationships between the second-order work, the kinetic energy, and the instability of the granular material are investigated. In addition, the coke aggregates are modeled by exploiting the discrete element method (DEM) to reveal the micro-mechanical behavior of the dry coke aggregates during the compaction process. The optimal number of particles required for the failure analysis in the DEM simulations is determined. The effects of the confining pressure and strain rate as two important compaction process parameters on the failure are studied. The results reveal that increasing the confining pressure enhances the probability of the diffusing mode of the failure in the specimen. On the other hand, the increase of strain rate augments the chance of the strain localization mode of the failure in the specimen.

In this article, we introduce the Deformation Infrared Calorimetry (DIRC) technique for resolving spatial distributions of heat and work in samples subjected to uniaxial loading under isothermal conditions. Heat and work distributions are computed from synchronized temperature and strain fields obtained by infrared thermography (IRT) and digital image correlation (DIC). The DIRC data acquisition and processing are described in the first part of the article. Then, we show the relevance and usage of DIRC by employing it to characterize the cyclic evolution of the thermomechanical response of a superelastic NiTi wire termed functional fatigue. Particularly, we evaluated the evolution of the heat, work, and internal energy changes upon the repeated propagation of stress-induced martensitic transformations (SIMTs) shear-bands, producing a highly heterogeneous deformation scenario. Further postprocessing of DIRC allowed us to associate the functional fatigue of the material with the evolution of the production/recovery of martensite and the energy storage and dissipation during the propagation of SIMT shear-bands.

The mechanical behavior and the influence of compaction banding on the hydraulic properties in soft porous rocks were studied. The tested rock was Calcarenite Tuffeau de Maastricht. In the frame of experimental investigations, triaxial and oedometric tests were conducted under dry and drained conditions. The results demonstrated that the rock is forming discrete compaction bands under high confining stresses and steep angle shear bands under low confining stresses. Permeability measurements during the oedometric and triaxial compression tests under drained conditions demonstrated that the axial permeability decreases with increasing axial strain. The maximum permeability decrease was three orders of magnitude for 40% of axial strain.

The mechanical response of nuclear structural materials and their lifetime are strongly affected by radiation effects. This influence is of concern, especially in body centered cubic materials, which exhibiting a well-defined ductile to brittle transition. The ductile to brittle transition temperature itself is dose-dependent and may rise to or above the room temperature. In the current work, irradiation effect is modeled to predict the dose-dependent changes of the effective dislocation mobility, represented by the Defect Induced Apparent Temperature shift ( Δ DIAT). Mainly dislocation based crystal plasticity material model is used rather than a phenomenological approach. This material model accounts for both thermally activated dislocation mobility and dislocation mobility in an athermal regime of body centered cubic materials. The defect-induced evolution of Δ DIAT in turn analyzed and their relations with the fracture response are highlighted and discussed.

International audience; The impact of the Portevin-Le Chatelier (PLC) effect on plasticity and fracture ahead of a severe notch is investigated by DIC field measurements for C-Mn steel. The PLC effect causes an intermittent activity of highly localized strain rate bands. This first high temperature investigation of the effect of a notch on the PLC localization bands is successfully reproduced numerically in terms of strain localization in 3D FE simulations. This thereby validates quantitatively the McCormick-type model under these complex conditions. It is found that, when the PLC effect is at play at elevated temperature (175 • C), a flat to slant crack transition is observed. In contrast, at room temperature, when only Lüders bands are active, the crack remains mostly flat, i.e. normal to the loading direction of the SENT samples. This behaviour may be related to the early loss of symmetry and intermittency of the plastic zone that is found for PLC even affecting the initial Lüders bands at elevated temperature. During the slant fracture at high temperature, only half the fracture energy is absorbed compared to flat fracture at room temperature. The McCormick-type model simulations predict slant strain rate bands through the sample thickness, that are consistent with the slant fracture found at elevated temperature. Accordingly, no slant bands are found for simulations outside the PLC domain. Both experiments and simulations show PLC strain localization bands that are flip-flopping up and down during crack propagation. By combining the McCormick-type model with a Rousselier porous plasticity model, the flat fracture of the sample at room temperature and the associated macroscopic curve are reproduced successfully, but the toughness at elevated temperature is overestimated.

Stress corrosion cracking (SCC) of oxygen-free phosphorous-alloyed copper was investigated in sulphide- and chloride-containing deoxygenated water at 90 degrees C with sulphide concentrations of 0.001 and 0.00001 M. Several intergranular defects were found in the specimen exposed to the high sulphide environment. Similar defects were not found in the low sulphide environment, where only slight corrosion on grain boundaries and slip lines occurred. Hydrogen content measurements show an increase in hydrogen uptake of the plastically deformed specimens, which is dependent on the sulphide concentration and on plastic deformation of copper. However, the highest hydrogen content was measured in friction stir welds, welded in air without shielding gas, and tested in the high sulphide environment. The embedded oxide particles in the weld metal act as local hydrogen trapping sites and selectively react with the sulphide solution. A relatively thick air-formed oxide film covers the copper canisters when deposited, which transforms into a sulphide film in the repository conditions. Thus, some of the coupon specimens were pre-oxidised. The conversion of the pre-existing Cu2O film into Cu2S film occurs quickly and the transformation is almost 100% efficient. The structure and properties of the Cu2S films, susceptibility of copper to sulphide-induced SCC and hydrogen uptake of copper in reducing, anoxic repository conditions are discussed.

Mechanical behavior of a multifunctional titanium alloy Gum Metal was investigated by conducting tensile tests at various strain rates and applying digital image correlation (DIC) technique. Stress–strain curves confirmed low Young’s modulus and high strength of the alloy. The determined values of yield strength had a tendency to increase, whereas the elongation to the specimen rupture tended to decrease with increasing strain rate. True stress versus strain curves were analyzed using selected lengths of virtual extensometer (VE) placed in the strain localization area. When the initial length of the VE was the same as the gauge length, work hardening was observed macroscopically at lower strain rates, and a softening was seen at higher strain rates. However, the softening effect was not observed at the shorter VE lengths. Evolution of the Hencky strain and rate of deformation tensor component fields were analyzed for various strain rates at selected stages of Gum Metal loading. The DIC analysis demonstrated that for lower strain rates the deformation is macroscopically uniform up to the higher average Hencky strains, whereas for higher strain rates the strain localization occurs at the lower average Hencky strains of the deformation process and takes place in the smaller area. It was also found that for all strain rates applied, the maximal values of Hencky strain immediately before rupture of Gum Metal samples were similar for each of the applied strain rates, and the maximal local values of deformation rate were two orders higher when compared to applied average strain rate values.

This work presents some novel results obtained by using the strain injection techniques for modeling crack propagation in challenging 3D benchmark tests. The techniques were already tested and validated by static and dynamic simulations in 2D ADDIN EN.CITE ADDIN EN.CITE.DATA [ HYPERLINK \l "_ENREF_1" \o "Dias, 2012 #1526" 1-4], so the main goal of this paper is to verify if the most important advantages of the method, low computational cost and independence of the results on the finite element mesh, are kept in 3D. The methodology, implemented in the finite element framework, consists essentially in injecting those elements that are going to capture the cracks with some enhanced strain modes for improving the performance of the elements for modeling propagating material failure.

International audience; A 3D multi-scale approach is presented to investigate the mechanical behavior of a macroscopic specimen consisting of a granular assembly, as a boundary value problem. The core of this approach is a multi-scale coupling, wherein the finite element method is used to solve a boundary value problem and a micromechanically based model is employed as constitutive relationship used at a representative volume element scale. This approach provides a convenient way to link the macroscopic observations with intrinsic microscopic mechanisms. The plane strain triaxial loading condition is selected to simulate the occurrence of strain localization. A series of tests are performed, wherein distinct failure patterns are observed and analyzed. A system of shear band naturally appears in a homogeneous setting specimen. By defining the shear band area, microstructural mechanisms are separately investigated inside and outside the shear band. The normalized second-order work introduced as an indicator of instability occurrence is analyzed not only on the macroscale but also on the micro scale.

Ultrafine grained austenitic stainless steel, with two different types of grain size distributions, was studied for tensile deformation behavior. Tensile deformed specimens were analyzed by electron backscatter diffraction using scanning electron microscope. It was found from this study that uni-modal grained stainless steel (SS) having a larger fraction of submicron grains exhibited early fracture, which is attributed to the development of extensive strain localization. On the other hand, the microstructure of SS having bimodal grain size distribution showed a good combination of strength and ductility. EBSD analysis of the deformed region of these two samples revealed the presence of a distinct transition zone between undeformed or less deformed and extensively sheared regions. Multiple micro shear bands were found to be associated with the transition zone of unimodal type microstructure. The micro shear bands seen in the transition zone of unimodal SS led to the development of strain-induced martensite (SIM), which, in turn, is helpful in delaying the strain localization. However, in bimodal grained SS, the larger fraction of micron size grains undergoes a shape change, with a rotation towards [112] orientation, which results in the formation of a larger fraction of SIM having [112] orientation. The propensity for development of high SIM was found to prevent strain localization in bimodal grained SS.

The work is devoted to the experimental study of mechanical behavior regularities of the structural steel 40Cr under postcritical deformation of specimens in conditions of strain localization formation in the form of neck during extension. The results of testing specimens cut from the initial samples with a neck are given. Various levels of preliminary postcritical deformation of initial samples are implemented. Using a noncontact 3D video system for recording displacement and strain fields of Vic 3D Correlated Solutions, based on the digital images correlation technique, the displacement and strain fields in the gauge length of both the original specimens with the neck and after the groove were recorded in two different schemes to eliminate geometrical heterogeneity. On the base of obtained results, the stiffness and strength of 40Cr steel were evaluated in a necked specimen at various stages of postcritical deformation. It is shown that the material in the peripheral areas of the gauge length of the sample is in a strengthened state, which does not depend on the degree of previously achieved postcritical deformation, and the strength of the material in the neck-forming zone on the initial specimens is thereby increased.

International audience; The slip localization behavior of the polycrystalline nickel base superalloy Inconel 718 during monotonic tensile loading at room temperature, is investigated for the first time in relation to the 3D microstructure. Multi-modal data merging tools are used to recombine high resolution digital image correlation (HR-DIC) data with 3D electron back-scatter diffraction tomography (3D EBSD), over a wide region of interest. This procedure enables reconstruction of the slip band planes in the 3D microstructure. Statistical analyses conducted over 500 individual slip bands reveal strong correlations between their location and specific microstructure configurations. In particular, over half of the slip bands emanate from triple junction lines (3D lines defined by the junction of three crystals). Moreover, the most intense and longest slip bands, which would become critical fatigue crack nucleation sites during cyclic loading, are located close and parallel to particular annealing twin boundaries and are simultaneously connected to triple junction lines. Crystal plasticity finite elements calculations are performed on the experimental microstructure to identify the slip activity that results in the formation of high intensity slip bands (localized plasticity) or zones of high lattice rotation (non-localized plasticity) in these particular microstructure regions.

This paper investigates the mechanical behavior and strain localization of methane hydrate bearing sediments (MHBS) at different temperatures and water pressures via the distinct element method (DEM). A thermo-hydro-mechanical contact model of MHBS has been employed in the DEM to simulate biaxial tests and mechanical behavior of MHBS is studied with different temperatures and water pressures. Three MHBS specimens are simulated at different temperature-pressure conditions to provide an insight on the evolution of macro and micro variables during strain localization of MHBS, as well as the geometric characteristics of shear band. The results show that MHBS appears to be strain-softening with dilatancy and the existence of MH will increase the cohesion but decrease the friction angle of MHBS. As temperature increases or water pressure decreases, the cohesion and friction angle of MHBS decrease with increasing dilatancy. The strain localization is closely related to bond breakage as well as localization of other micro and macro variables such as contact force chains, principal stress fields, particle velocity fields and average pure rotation rate distributions. As temperature and water pressure change, three different types of shear band can form. The thickness and inclination angle of these shear bands increase as the temperature increases or water pressure decreases.

International audience; The recent development of the high resolution and discontinuity-tolerant digital image correlation technique enables the extraction of discontinuities within a displacement field. The technique provides quantitative analysis of discontinuities arising from slip, shear bands, cracks, and grain boundary sliding in a variety of material systems, including polycrystalline metallic materials. The discontinuity-tolerant digital image correlation method can be implemented to retrieve not only quantitative discontinuity analysis but also the local strain and rotation fields that operate near these discontinuities. The present implementation includes high-resolution digital image correlation (HR-DIC) measurements collected in a scanning electron microscope for analysis of both the plastic and elastic fields that develop during deformation of polycrystalline metallic materials. The combination of the discontinuity-tolerant DIC technique with the computation of internal gradients enables extraction of non-localized strain and rotation fields during plastic deformation of a nickel-based superalloy. Therefore the lattice rotation/expansion and plastic localization that occur during deformation can be determined in a single experiment. This method is validated using synthetic images with preset deformation, and experimental measurements using the electron back scatter diffraction (EBSD) technique.

Digital image correlation, commonly referred to as DIC, enables full-field measurements of displacements and strains from a surface of interest. While DIC offers major advantages over commonly established methods, such as strain gauges or extensometers, some questions remain in guaranteeing data precision of the DIC measurements. This paper reports on improvements in the precision of DIC measurements of large deformations in copper tensile specimens by optimized patterning and lighting conditions, as well as quantifying the improvements in the data. In the context of detection of strain localization at welds in copper canisters for spent nuclear fuel disposal, the trade-off between spatial resolution and strain resolution is critical. To quantify the improvements in measurement conditions, we propose a method for estimation of spatial resolution and noise level of a full-field strain measurement, which does not require imposing a displacement field with variations in strain or displacement of well-defined length scale.

International audience; We develop a crystal plasticity model to investigate the coupled actions of crystal orientation, grain neighborhood and grain-to-grain elasto-plastic interactions on dynamic strain aging (DSA) and the onset and development of associated plastic strain localization in Al-Li alloys. Considering simple model multilayered microstructures with preferred orientations representative of rolled alloys, the aim is to identify grain orientation couples that can limit dynamic strain aging induced strain localization without compromising the flow stress and strain hardening properties. To this end, a slip system-based formulation of dynamic strain aging is implemented in a crystal plasticity finite element framework. The model validity is first checked with the simulation of a tensile specimen loaded at quasi-static applied strain rate. The introduction of dynamic strain aging allows predicting complex propagation of intense plastic localization bands. We further investigate the influence of crystal orientations on early strain localization in Al-Cu-Li-Mg alloys, by performing simulations representative of the early stage of a Kahn Tear test for single crystals and layered polycrystals. Using experimentally reported crystal orientations for rolled microstructures, the simulation results show that in both single and multilayered crystals, there is a strong influence of dynamic strain aging on localization patterns, as well as a significant orientation dependence. In multilayered crystals, the nature of strain localization can be remarkably modified when stand-alone crystals of a certain orientation are coupled with other orientations: strain localization may intensify or fade away depending on the coupling with neighboring orientations.

This study examines the combined performance of Portland cement (PC), the binder, and fly ash (FA), the additive, towards improving the mechanical performance of the South Australian copper-gold underground mine cemented paste backfill (CPB) system. A series of unconfined compressive strength (UCS) tests were carried out on various mix designs to evaluate the effects of binder and/or additive contents, as well as curing time, on the CPB&rsquo, s strength, stiffness and toughness. Moreover, the failure patterns of the tested samples were investigated by means of the three-dimensional digital image correlation (DIC) technique. Making use of several virtual extensometers, the state of axial and lateral strain localization was also investigated in the pre- and post-peak regimes. The greater the PC content and/or the longer the curing period, the higher the developed strength, stiffness and toughness. The use of FA alongside PC led to further strength and stiffness improvements by way of inducing secondary pozzolanic reactions. Common strength criteria for CPBs were considered to assess the applicability of the tested mix designs, with regards to stope stability, 4% PC + 3% FA was found to satisfy the minimum 700 kPa threshold, and thus was deemed as the optimum choice. As opposed to external measurement devices, the DIC technique was found to provide strain measurements free from bedding errors. The developed field of axial and lateral strains indicated that strain localization initiates in the pre-peak regime at around 80% of the UCS. The greater the PC (or PC + FA) content, and more importantly the longer the curing period, the closer the axial stress level required to initiate localization to the UCS, thus emulating the failure mechanism of quasi-brittle materials such as rock and concrete. Finally, with an increase in curing time, the difference between strain values at the localized and non-localized zones became less significant in the pre-peak regime and more pronounced in the post-peak regime.

In situ monotonic tensile experiments of thermo-induced shape memory polyurethane (SMPU) at different loading rates were carried out by the digital image correlation (DIC) method and infrared camera FLIR&reg, A655sc in natural convection (NC) and forced convection (FC) conditions, respectively. The multiform strain localization of SMPU was observed by the DIC method, and the influence of thermo&ndash, mechanical coupling on the strain localization was analyzed by using the FLIR to measure the temperature field caused by the internal heat generation. The experimental results show that the strain localization mode strongly depends on the strain rate and convection condition, and the strain localization mode can be transformed by changing the convection condition from NC to FC. The competition mechanism between the strain hardening induced by the increasing loading rate and strain softening induced by the internal heat generation is indicated, the transition modes of strain localization are clarified, and the influences of thermo&ndash, mechanical coupling on shape memory effect are discussed.

Hypo-elastoplasticity is a flexible framework for modeling the mechanics of many hard materials under small elastic deformation and large plastic deformation. Under typical loading rates, most laboratory tests of these materials happen in the quasi-static limit, but there are few existing numerical methods tailor-made for this physical regime. In this work, we extend to three dimensions a recent projection method for simulating quasi-static hypo-elastoplastic materials. The method is based on a mathematical correspondence to the incompressible Navier-Stokes equations, where the projection method of Chorin (1968) is an established numerical technique. We develop and utilize a three-dimensional parallel geometric multigrid solver employed to solve a linear system for the quasi-static projection. Our method is tested through simulation of three-dimensional shear band nucleation and growth, a precursor to failure in many materials. As an example system, we employ a physical model of a bulk metallic glass based on the shear transformation zone theory, but the method can be applied to any elastoplasticity model. We consider several examples of three-dimensional shear banding, and examine shear band formation in physically realistic materials with heterogeneous initial conditions under both simple shear deformation and boundary conditions inspired by friction welding., Final version. Accepted for publication in Computer Physics Communications

Strain localization, a cross-scale issue degrading homogeneity and formability during hot working of TA15 titanium alloy, is complicated by lamellar microstructure for hierarchical feature, spatial morphology, and orientation relationship. In this work, by taking advantage of hat-shaped specimens to maintain deformation mode, the effect of heterogeneous lamellar microstructure on strain localization is accessible and visualized by cross-section comparison enlightened by cake cutting. Moreover, the evolution behavior and mechanism are also investigated by the two-scale crystal plasticity model. The mechanism of strain localization during hot working of lamellar microstructure is re-recognized as the structural softening governed by an abrupt phase interface weakening and continuous geometrical softening. The driving force of strain localization is the ‘collapse’ of the stress gradient between constituent phases intensified by the progression of compelled uniformed deformation. The key to microstructure-dependent strain localization is the influence of lamellar orientation on germination and colony interface on intensification. Colonies with lamellar orientation deviated 45 o from the direction of maximum shear stress are endowed with the intrinsic potential of strain localization. Large-size and strongly-orientated colony interfaces intensify strain localization. These findings provide new insights into microstructure-dependent strain localization supporting a more efficient design of processing routing to control localization powerfully.

Bendability is an important property of ultrahigh-strength steels since the typical applications of such materials include structures manufactured by air-bending. Conventional methods to evaluate bendability, such as the bending test according to the standard VDA-238 or the conventional tensile test do not provide sufficient information to evaluate bendability of ultrahigh-strength steels due to the average nature of the material response in these tests. In this study, the mechanical properties were determined using thin tensile specimens cut from the surface of the sheet and the evaluation of bendability was carried out using frictionless bending tests. The results of the experiments and FE-modelling presented in this paper reveal that the mechanical properties of the sheet surface have a significant impact on bendability. Novel ultrahigh-strength steel with better work-hardening capacity at the surface caused by a layer of relatively soft ferrite and lower bainite has good bendability, especially when the bend line is aligned transverse to the rolling direction. Microstructural investigations reveal that in a conventional steel with a relatively hard surface microstructure, the deformation localizes into shear bands that eventually lead to fracture, but similar shear banding was not present in the novel steel surface. This can be attributed to the better work-hardening capacity which delays the onset of shear localization and fracture.

Thermal ageing of Alloy 690 triggers an intergranular (IG) carbide precipitation and is known to promote an ordering reaction causing lattice contraction. It may affect the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor (PWR) components. Four conditions of Alloy 690 (solution annealed, cold-rolled and/or heat-treated) were aged between 350 and 550 °C for 10 000 h and characterized. Although no direct observation of ordering was made, variations in hardness and lattice parameter were attributed to the formation of short-range ordering (SRO) in all conditions with a peak level at 420 °C, consistent with the literature. Prior heat treatment induced ordering before thermal ageing. At higher temperatures, stress relaxation, recrystallization and α-Cr precipitation were observed in the cold-worked samples, while a disordering reaction was inferred in all samples based on a decrease in hardness. IG precipitation of M 23 C 6 carbides increased with increasing ageing temperature in all conditions, as well as diffusion-induced grain boundary migration (DIGM).

A fine dispersion of nanoscale gold surface markers was achieved on the surface of a Mg-alloy sample using a styrene-assisted remodeling process. The markers were used for a high resolution digital image correlation strain-field analysis, to achieve a better understanding of the effect of pre-existing twinning on strain localization during in-situ tensile deformation. A complex pattern of heterogeneous strain localization was found. The results show that twins do not deform uniformly, and that relative differences in Schmid factor may influence the early occurrence of strain localization. Additionally, strain localization was seen to occur preferentially at grain boundaries and twin boundaries due to the lack of efficient plastic mechanisms for strain transfer across such interfaces. The results demonstrate the ability to measure strain localization on a fine scale in Mg-alloys, and highlight the complex interaction between deformation mechanisms during plastic deformation.

Numerical analyses incorporating appropriate damage models provide an opportunity to predict the strength and deformation capacity of steel structures. This paper presents a practical calibration for the ductile damage model of S355 and high-strength steel S690Q, S700MC, S960Q based on tensile coupon test results. A combined linear and power expression is adopted to calibrate the post-necking damaged stress–strain relations of the investigated steels, upon which the undamaged stress–strain relations are estimated further. Damage initiation criterion is based on the Rice-Tracey model and damage evolution law is related to the calibrated damaged stress and the estimated undamaged stress. Fracture of the tensile coupons is modelled using a critical damage variable. Tensile coupon tests on the investigated steels are modelled in ABAQUS with the explicit solver. Results show that combining the proposed post-necking stress–strain relations and ductile damage model generates very good predictions for strain localization and final fracture of the tensile coupons. Numerical engineering stress–strain curves agree well with the experimental results. It also indicates that high-strength steels are more susceptible to damage than S355. The damage variable of S960Q is about 2 times as large as that of S355 from the onset of necking to the final fracture.

International audience; In most voided metallic materials, the failure process is often driven by the competition between the phenomena of void coalescence and plastic strain localization. This paper proposes a new numerical approach that allows an accurate description of such a competition. Within this strategy, the ductile solid is assumed to be made of an arrangement of periodic voided unit cells. Each unit cell, assumed to be representative of the voided material, may be regarded as a heterogeneous medium composed of two main phases: a central primary void surrounded by a metal matrix, which can itself be assumed to be voided. The mechanical behavior of the unit cell is then modeled by the periodic homogenization multiscale scheme. To predict the occurrence of void coalescence and macroscopic strain localization, the above multiscale scheme is coupled with several relevant criteria and indicators (among which the bifurcation approach and an energy-based coalescence criterion). The proposed approach is used for examining the occurrence of failure under two loading configurations: loadings under proportional stressing (classically used in unit cell computations to study the effect of stress state on void growth and coalescence), and loadings under proportional in-plane strain paths (traditionally used for predicting forming limit diagrams). It turns out from these numerical investigations that macroscopic strain localization acts as precursor to void coalescence when the unit cell is proportionally stressed. However, for loadings under proportional in-plane strain paths, only macroscopic strain localization may occur, while void coalescence is not possible. Meanwhile, the relations between the two configurations of loading are carefully explained within these two failure mechanisms. An interesting feature of the proposed numerical strategy is that it is flexible enough to be applied for a wide range of void shapes, void distributions, and matrix mechanical behavior. To illustrate the broad applicability potential of the approach, the effect of secondary voids on the occurrence of macroscopic strain localization is investigated. The results of this analysis reveal that the presence of secondary voids promotes the occurrence of macroscopic strain localization, especially for positive strain-path ratios.

Dynamic recrystallization (DRX) often takes place during hot deformation of metallic materials, which then exerts significant influence on the final microstructures and mechanical properties of the formed components. A considerable number of published papers related to DRX, however, suffer from non-negligible flaws originating from inappropriate experimental design. In this paper, the sources of these flaws are critically assessed, including the misinterpretation of DRX mechanisms, the strain localization on tested samples, the neglected post dynamic recrystallization and possible phase transformations which mask the real hot deformation microstructure. Solutions to eliminate or quantify these disturbing factors during DRX studies are suggested, yielding more accurate approaches to investigate DRX behaviour of metallic materials. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 mu m. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain. (C) 2020 The Authors. Published by Elsevier Ltd.