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• Strength–ductility synergy is achieved by lithiation in sulfur cathodes. • Brittle-to-ductile transition in amorphous lithiated sulfur is governed by cavitation nucleation mechanism. • Atomic chain bridging behind the crack tip enhances crack resistance at lower lithium concentrations. As an eminent candidate for next-generation cathode materials, sulfur undergoes severe volume changes during electrochemical cycling, resulting in material fractures and performance degradation. Although the electrochemical characteristics of lithiated sulfur have been rigorously studied, their mechanical properties, particularly fracture mechanisms, remain insufficiently understood. To address this gap, we conducted comprehensive atomistic simulations to investigate the fracture behavior of amorphous lithiated sulfur (a-Li x S). Our findings reveal that as lithium concentration increases, the fracture mechanism transits from brittle governed by nanoscale cavitation instability to ductile dominated by plastic shear bands launched at the crack tip. This transition is contingent upon local hydrostatic stress exceeding the cavitation strength, elucidated through atomic-level bonding dynamics including rupture, reformation and rotation. At low lithium concentrations, atomic chain bridging posterior to the crack tip is simultaneously identified to enhance crack resistance. The lithiation-induced fracture toughness enhancement is further quantified via domain J -integral analysis. Notably, a unique strength–ductility synergy is identified for a-Li x S, attributed to cooperation between lithiation induced heterogeneous bonding environments and atomic chain bridging. This study provides valuable atomic-scale insights into the fracture mechanisms of sulfur cathodes, thereby informing the design of high-energy and durable electrode materials for future applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• Increasing the relative vacancy concentration, temperature, and hydrostatic stress decreases the activation free energy barrier and critical radius of the nucleated voids. • Synergistic effects of multiple dislocation pile-ups at dislocation boundary facilitate vacancy condensation to form voids. • The simulation results predicted a size of ∼35 nm for the incipient voids, which is consistent with the experimental value of ∼50 nm. Void nucleation is of great significance in understanding ductile fracture. Recent experiments have shown that voids are nucleated via vacancy condensation and dislocation boundaries are the main nucleation sites. However, it is unclear what role is played exactly by dislocation boundaries in promoting void nucleation. Here we propose a new mechanism for dislocation boundary-induced void nucleation and develop a corresponding model based on the classical nucleation theory and vacancy diffusion theory. The model suggests that void nucleation is mainly influenced by hydrostatic stress, temperature, and relative vacancy concentration, whose contributions are systematically studied. It is also suggested that the vacancy formation energy and the interaction energy of hydrostatic stress and vacancy, which are absent in the previous models and introduced in ours, exhibit a clear tendency to lower the activation free energy barrier. Analysis of the nucleation kinetic suggests that the growth rate of void depends on the vacancy diffusion coefficient and vacancy concentration; the higher the values of these parameters, the faster the growth rate of the void. The kinetic feasibility of the newly proposed mechanism is examined using three-dimensional discrete dislocation dynamics simulations. The results predict that the size of incipient voids nucleated at the dislocation boundary is ∼35 nm, which is consistent with the experimental characterization value of ∼50 nm. Finally, when the relaxation of the dislocation boundary is considered, the synergistic effect is weakened. [ABSTRACT FROM AUTHOR]

• A novel hydrogen diffusion model coupled with CPFE framework was proposed. • The GB energy and HS jointly determine the distribution of hydrogen. • Hydrogen diffusion along GB is affected by the concentration within the grains. • Alloys with medium-sized grain microstructures exhibit good hydrogen tolerance. The influence of grain boundaries (GBs) on hydrogen transportation is a crucial factor in understanding the mechanisms of hydrogen embrittlement (HE) in polycrystalline alloys. Crystal plasticity models have the advantages in simulating the effect of grain microstructure on hydrogen diffusion in polycrystals, but they have not yet been coupled with hydrogen diffusion models that can take into account GB characters. In this study, a hydrogen diffusion model that considers the effect of GBs on the transportation of hydrogen was developed to study the local hydrogen diffusion behavior in bi-crystalline and polycrystalline alloys. The simulation results showed the capability of the model in describing the influence of hydrogen on the material deformation. In addition, this study found that the hydrostatic stress determined the hydrogen concentration in the far-GB region, whereas the GB energy played a control role in and near the GB regions. Notably, hydrogen exhibits a preference for localization near high-angle grain boundaries (HAGBs) and triple junctions. In alloys with medium grain size, hydrogen typically had low average concentration with a more uniform distribution. Furthermore, the hydrogen-assisted crack growth mechanism was analyzed as the continuous coalescence of primary and secondary cracks along GBs. This work presents a hydrogen diffusion model that considers GB effects, and the proposed model has advantages in predicting the deformation and fracture of polycrystalline alloys in hydrogen environments. [ABSTRACT FROM AUTHOR]

The effect of hydrostatic pressure on the shear deformation behavior of 10 Cu symmetric tilt interfaces has been investigated in the athermal deformation regime (10 K) using molecular dynamics simulations. Cu bicrystals were deformed under shear with a superimposed hydrostatic stress, until initial yield. For most interfaces, the shear strength increased with increasing compressive hydrostatic pressure. However, for the Σ 9 { 221 } and Σ 5 { 210 } interfaces, this trend was reversed. Neither the sign nor the magnitude of the pressure-induced elevation in shear strength was found to correlate with interface structure or particular deformation mechanism(s). The findings suggest that in general, metrics of interfacial excess volume and dilatation cannot reliably predict the pressure-sensitivity of shear strength; however, for interfaces containing the E structural unit, the annihilation of free volume appears to correlate well with the observed pressure-sensitivity. • Molecular Dynamics simulations were used to compute the shear strength of 10 Cu bicrystals containing symmetric tilt interfaces under applied hydrostatic pressure. • Computed values of the pressure-sensitivity of interfacial shear strength suggest the presence of other effects besides pressure-induced changes in lattice stiffness. • Notably, some interfaces exhibited anomalous weakening under applied compression. • Observed effects can be attributed, in some interfaces, to the presence of high free volume structural units. [ABSTRACT FROM AUTHOR]

The paper is devoted to the determination of shakedown limit states of porous ductile materials under cyclically repeated loads by considering the hollow sphere model with von Mises matrix. We adopt Melan's statical approach based on time-independent residual stress fields. First of all, we determine the exact solution for the pure hydrostatic loading. It turns out that the collapse occurs by fatigue. Next, suitable trial stress fields are built with additional terms to capture the shear effects. This theoretical fatigue criterion of porous materials agreed with numerical simulations and experimental results provided in literature according to which, in ductile metals, the strain to fracture under cyclic loading is considerably lower than the one reached monotonically. Finally, the built criterion is compared to numerical simulations derived by both shakedown direct method and incremental simulations. [ABSTRACT FROM AUTHOR]

In this paper, hydrostatic strength properties of nanoporous materials are investigated by addressing the limit state of a hollow sphere undergoing isotropic loading conditions. Void-size effects are modelled by treating the cavity boundary as a coherent-imperfect homogeneous interface. The hollow sphere is assumed to be comprised of a rigid-ideal-plastic material obeying to a general isotropic yield criterion. The latter is defined by considering a simplified form of the yield function proposed by Bigoni and Piccolroaz in [Int J Solids Struct; 41: 2855–2878], resulting able to account for a broad class of pressure-sensitive materials whose plastic response is also affected by the stress Lode angle. The corresponding support function is consistently derived and discussed. The exact solution of the limit-state problem is fully determined, providing a closed-form description of stress, strain-rate and velocity fields, as well as the macroscopic hydrostatic strength of nanoporous media. Proposed approach allows to consistently generalise available analytical solutions for porous and nanoporous materials, by accounting for a general plastic response of the solid matrix and for void-size effects. Finally, present exact solution, as well as the identification of the support function for the adopted general strength criterion, open towards novel kinematic limit-analysis approaches for describing macroscale strength properties of nanoporous materials under arbitrary triaxial loadings. [ABSTRACT FROM AUTHOR]

A model of a polycrystalline material is studied, where each grain consists of several zones with different plastic properties. The size and configuration of the zones as well as their properties are mimicking the situation inside the crystals of artificially aged Al alloys with precipitate free zones (PFZs). The properties of the different zones are conjectured based on micromechanical models and experimental observations of the AA6000 series of Al alloys. A periodic patch of such a composite material, subjected to plane-strain tension, is modelled using the finite element method. The material behaviour is described by a single crystal plasticity model. The results of the simulations show a number of characteristic features emerging in composite material systems, including sliding and distortion of grain boundaries and shear band formation at grain boundary triple points. The assumptions made about the properties of the different zones in the crystals are evaluated. The distributions of plastic strain and deviatoric and hydrostatic stresses in the composite crystalline systems are discussed. [ABSTRACT FROM AUTHOR]

In this study, a thermodynamically consistent damage model is presented to predict failure in glassy polymers. This model is based on the Eindhoven Glassy Polymer (EGP) multimode model, and it considers the effects of plastic deformation and hydrostatic stress on damage evolution. The model is implemented as an ABAQUS user material (UMAT) subroutine. Three experiments are simulated to show the model's abilities. First, the model is used to predict failure in a T-fitting burst pressure test to investigate the model's capabilities in predicting damage evolution and failure. This test shows two failure locations one dominated by the plastic deformation and the other by the hydrostatic stress and provides the possibility to investigate the model capabilities in capturing these contributions to the failure. Through comparison with experimental results, it is demonstrated that the model can effectively take into account the influence of hydrostatic stress and plastic deformation on failure. Second, the D-split test of notched pipe rings is simulated to show the time-dependent behavior of the model. Third, the cyclic behavior of a polycarbonate specimen under compression is investigated. • Presenting a thermodynamically consistent constitutive damage model for thermoplastic polymers. • Coupling viscoelasticity–viscoplasticity and ductile damage. • The model takes into account contributions of plastic deformation and hydrostatic stress to failure. • Failure prediction of a T-fitting burst pressure test at different pressure rates. [ABSTRACT FROM AUTHOR]

• Plasticity of aluminium alloy 2024-T351 highly depends on deviatoric stress tensor. • Non-quadratic yield criteria may induce wrong stress states and deformations. • Non-associated flow rules may improve the non-quadratic yield criteria predictions. • A novel plasticity model is presented to solve issues still obeying the convexity. • More reasonable calibration of ductile fracture is obtained using a given approach. Accurate material behaviour and its response to loading are needed for a reliable calibration of ductile failure criteria. Non-quadratic yield functions are often necessary for such a description. Nevertheless, it leads to the prediction of stress states that are inconsistent with the expected behaviour and deformations that are in contradiction with the experiments when the associated flow rule is adopted. These problems may be solved by applying a less restrictive non-associated flow rule. Therefore, an isotropic non-associated non-quadratic phenomenological plasticity model was developed as a function of the second and third invariants of deviatoric stress tensor. The yield function that is convex was proposed to allow for different yield stresses in tension, compression and shear. For simplicity, the same function was used for the plastic potential. Each of these functions is described by seven material parameters. The scheme of the so-called next increment corrects error method was adopted for the implementation of the proposed model within finite elements. Above that, three additional plasticity models were calibrated for 2024-T351 aluminium alloy. It is shown how the non-associated flow rule resolves the above-mentioned problems. Moreover, the possible extensions of the proposed yield function are described to include the hydrostatic stress dependence. [ABSTRACT FROM AUTHOR]

• A coupled diffusion-crystal plasticity finite element model is used to study stress-assisted hydrogen diffusion towards notch tips. • The model is used to deconvolute the contributions of texture, grain morphology, and notch geometry on the diffusion of hydrogen. • It is shown that localized plasticity and hydrogen distribution near the notch are affected by the orientations of crystals. • It is shown that the peaks of hydrogen concentration are closer to the notch root in the "textured" model. • It is shown that by sharpening the notch tip, the effect of texture on hydrogen transport becomes less significant. Hydrogen embrittlement is an important degradation mechanism affecting the lifetime of engineering components. The diffusion of hydrogen atoms into the metal lattice can be affected by the localized stresses that develop around service-induced flaws. In addition, the magnitudes of flaw tip stresses are affected by the flaw geometry, as well as the texture and microstructure of the metal. Microstructural effects can be significant, particularly in metals such as zirconium with a high degree of elastic and plastic anisotropy. This study uses a coupled diffusion-crystal plasticity finite element approach to conduct a comprehensive study on the redistribution of hydrogen atoms in the vicinity of service-induced flaws in zirconium polycrystals. The effects of texture, grain size, and notch tip geometry on the concentration of hydrogen atoms in the lattice sites are investigated. The results indicate that material texture can significantly affect the distribution of hydrogen atoms as well as the location of maximum hydrostatic stress and maximum hydrogen concentration. The modeled highly textured microstructures have connected regions of peak hydrogen concentration. It is shown that as the notch tip becomes sharper, the effects of texture on the hydrogen localization near the notch become less significant. It is also shown that with changing grain orientations, it is possible to move the location of peak hydrogen concentration away from the notch tip. [ABSTRACT FROM AUTHOR]

• A unified theory for the nonlinear, time-dependent creep rupture process in CPNCs has been developed. • A two-scale homogenization approach with the small scale involving the polymer matrix containing nucleated micro-voids, and the large scale involving the randomly oriented CNTs with a damaged interphase and the damaged polymer matrix, is introduced. • The thermodynamic driving force and evolution equation for the progressive creep damage and rupture in CPNCs have been derived. • The effective stress in the damaged viscoplastic polymer matrix has been evaluated by a joint field-fluctuation and working-rate equivalence method. • The developed theory is validated with experiments for multi-walled CNT/polypropylene nanocomposites spanning over the whole three stages of primary creep, secondary creep, and tertiary creep. While the linear viscoelastic characteristics of polymer composites have been extensively studied, the nonlinear creep with progressive damage leading to the ultimate creep rupture in CNT-based viscoplastic nanocomposites still remains a challenging issue. In this paper, a unified approach is presented to study the whole lifetime of creep process including deformation, damage, and creep rupture. First, based on the concept of secant viscosity, a linear viscoelastic comparison composite is introduced to mimic the nonlinear viscoplastic nanocomposite, with the linear part further formulated in the Laplace transformed domain. Then, both progressive degradation of the interphase and creep damage of the polymer matrix are presented. A joint field-fluctuation method and work-rate equivalence is subsequently utilized to calculate the effective Mises stress and hydrostatic stress of the viscoplastic matrix. The principle of irreversible thermodynamics is then invoked to derive the thermodynamic driving force to characterize the evolution of creep damage and rupture process. After the theory is developed, it is demonstrated that the predicted effective creep strains of multi-walled CNT/polypropylene nanocomposites are calibrated by the experiments spanning over the whole three stages of primary creep, secondary creep, and tertiary creep. Both creep rupture time and rupture strain of the overall nanocomposite are found to increase with CNT volume concentration. It is concluded that population of CNT nanofillers can remarkably enhance the creep rupture resistance of CNT/polymer viscoplastic nanocomposites. [ABSTRACT FROM AUTHOR]

Abstract: The experimental and numerical work presented in this paper reveals that stress state has strong effects on both the plastic response and the ductile fracture behavior of an aluminum 5083 alloy. As a result, the hydrostatic stress and the third invariant of the stress deviator (which is related to the Lode angle) need to be incorporated in the material modeling. These findings challenge the classical J 2 plasticity theory and provide a blueprint for the establishment of the stress state dependent plasticity and ductile fracture models for aluminum structural reliability assessments. Further investigations are planned to advance, calibrate and validate the new plasticity and ductile fracture models. [Copyright &y& Elsevier]

In this communication, we present a reformulated finite-volume direct averaging micromechanics (FVDAM) framework for periodic multiphase materials with elastic-plastic phases. The original elastoplastic version is formulated using the secant stiffness matrix approach, which requires a computationally intensive solution procedure of the governing differential equations. For the first time, the reformulation makes use of the tangent plasticity matrix approach that incorporates linearities to the unit cell boundary value problem. The tangential FVDAM theory is implemented using a quasi-Newton-Raphson strategy that quantifies errors in the evaluation of surface-averaged stresses due to the linearization, hence allowing large load increments. The tangential formulation is vigorously and fully assessed vis-à-vis the secant approach for porous unit cells on several aspects, including the convergence of the algorithmic implementation with mesh and step sizes, solution accuracy, and computation times. The reformulation simplifies the solution to the governing differential equations relative to its predecessor, albeit at the cost of more computational efforts for the reforming and reinverting of the global tangent stiffness matrix (or the so-called tangent operator). Additionally, advantages of the FVDAM theory relative to the classical finite-element homogenization are highlighted. The tangent FVDAM is employed to demonstrate the plasticity-triggered architectural effects in the response of periodic porous materials under different loading modes. The common mechanisms responsible for differences in the elastic-plastic response are attributed to the effective and hydrostatic stress alteration in the matrix phase. The current work paves the ground for the incorporation of FVDAM into readily-available commercial computational tools through the user material subroutines that are based on tangent stiffness approaches, which will produce a paradigm shift for FVDAM to solve the challenging multiscale structural problem. • A tangent-matrix formulated finite-volume direct averaging micromechanics is developed for elasto-plastic porous materials. • The effectiveness and accuracy are compared between the tangent and secant matrix formulations. • The tangent-matrix based FVDAM is validated against the finite element and exact elastic-plastic solutions. • The effect of irregular pore distributions is tested for the effective and localized responses of porous materials. [ABSTRACT FROM AUTHOR]