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• A grain size dependent and thermo-mechanically coupled phase field model is newly proposed. • The effects of grain size, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the elastocaloric effect of NiTi SMAs are revealed. • The COP mat of the optimal microstructure engineering scheme is up to 35.4, which is about 200% higher than that of uniform coarse-grained NiTi SMAs. • The microstructure engineering scheme with geometric gradient shows the lowest input stress of 195 MPa. By newly introducing a grain boundary energy, a grain size (GS) dependent and thermo-mechanically coupled phase field model was proposed to investigate the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs), and discuss the effects of GS, texture, gradient-nanograin and bimodal grain structures as well as geometric gradient on the eCE through simulations. The simulated results show that with decreasing the GS, the gradual reduction of the degree of martensite transformation (MT) and the variation of transformation mode in the nano-polycrystalline NiTi SMA system lead to the decreasing absorption of transformation latent heat, and the gradual weakness and even disappearance of local instability during MT decreases the hysteresis, resulting in the increase of the coefficient of performance (COP mat); the deformation dependence of COP mat and the grain orientation dependence of transformation instability lead to a dependence of COP mat on texture, which can be enhanced with decreasing the GS. It is further found that the microstructure engineering (from the perspectives of GS, texture, gradient-nanograin and bimodal grain structures) can significantly improve the COP mat , and the introduction of geometric gradient can effectively reduce the MT start stress. Several microstructure and geometric schemes with considerable adiabatic temperature change (Δ T ad) and high COP mat as well as relatively low MT start stress are reported, which can provide an important theoretical guidance for improving the eCE of NiTi SMAs and exploiting excellent SMA-based elastocaloric materials. The proposed phase field model and the characterization method for texture can be used as a reference to simulate the eCE and anisotropic MT of other SMAs. I. Microstructure engineering schemes: polycrystalline systems with different gradient-nanograin structures and bimodal grain structure; II. Simulated microstructure evolution; III. Improved elastocaloric effect [Display omitted] [ABSTRACT FROM AUTHOR]

• A multi-layered model of functionally graded (FG) coating-substrate structures is established. • The thermo-mechanical coupled contact shakedown map of FG structures is obtained. • Temperature-dependent yield stress and friction coefficient are considered. • The influences of friction heat on the shakedown map of FG structures are discussed. The sliding contact shakedown map composed of elastic shakedown limit and elastic limit is significant to the further optimization of functionally graded (FG) coating parameters. The friction heat induced by the relative sliding velocity of contact pairs can cause thermal stress and changes in contact parameters. Nevertheless, the influence of thermo-mechanically coupled effect on the sliding contact shakedown map of FG coating-substrate structures keeps unrevealed. Therefore, a mathematical optimization model is proposed to draw the shakedown map considering the temperature-dependent friction coefficient and yield stress. Based on the thermo-elasto-perfectly plastic material model and multilayered approximation of FG coating, the required thermo-elastic stress field with the temperature-dependent friction coefficient is obtained by employing the conjugate gradient method and discrete convolution-fast Fourier transform algorithm. The results show that the friction heat can cause the decreases of elastic shakedown limit and elastic limit, and increasing the sliding velocity can enhance the decreases. Moreover, the friction heat can elevate the residual tensile stress near the surface and weaken the residual compressive stress at a large depth from the surface. Meanwhile, the temperature-dependence of friction coefficient can cause the increases of elastic shakedown limit and elastic limit, but the introduction of temperature-dependent yield stress brings no obvious change to the shakedown curves due to the slight thermally-induced change of yield stress. Additionally, decreasing the distribution gradient index can not only improve elastic limit and elastic shakedown limit at large friction coefficients, but also reduce the distribution of residual tensile stress near the surface. [Display omitted] [ABSTRACT FROM AUTHOR]

A three-dimensional non-isothermal and crystal plasticity based phase field model was proposed to simulate the cyclic deformation of nano-polycrystalline NiTi shape memory alloys (SMAs) reflecting their one-way shape memory effect (OWSME) and stress-assisted two-way effect (SATWSME), and then the microscopic mechanism of the cyclic degradation of shape memory effect (SME) was systematically revealed. The proposed model was numerically implemented in a two-dimensional plane strain nano-polycrystalline NiTi SMA system, where each grain was divided into a grain-interior (GI) phase and grain-boundary (GB) phase, and the GI phase was set to be embedded in the GB phase. For the GI phase, a crystal plasticity based phase field model was newly established by considering four inelastic deformation mechanisms, i.e., martensitic transformation (MT), martensitic reorientation (MR), austenitic plasticity caused by dislocation slipping and martensitic plasticity caused by dislocation slipping and deformation twinning, simultaneously; for the GB phase, a visco-plastic model was used to describe its plastic deformation. The simulated results show that: during the cyclic deformation of nano-polycrystalline NiTi SMAs involving the OWSME or SATWSME, the local internal stress caused by the MR or MT can lead to the accumulation of plastic deformation, which further increases the fraction of the martensite variants with favorable orientations by inducing a specific internal stress field after cooling and then results in the accumulation of residual martensite phase after heating; the accumulation of the irrecoverable strain mainly from the martensitic twinning and the decrease of recoverable strain cause the cyclic degradation of OWSME, which is further dependent on the loading rate and loading level; the SATWSME of the SMAs can exhibit a cyclic strengthening during the cyclic loading with a relatively low constant stress, while a cyclic degradation with a relatively high stress, which results from the competition between the transformation strain and the irrecoverable strain mainly caused by the austenitic plasticity that increases gradually during the cyclic deformation involving the SATWSME. These new findings are very helpful to clarify the physical nature of the cyclic degradation of SME and then alleviate the functional fatigue of NiTi SMAs by modulating the material design. • A multi-mechanism phase field model with the ability to simulate the SME cycling of NiTi SMAs is newly proposed. • The microscopic mechanism of the cyclic degradation of SME in NiTi SMAs is systematically revealed. • The accumulated irrecoverable strain mainly caused by martensitic twinning causes the cyclic degradation of OWSME. • The microscopic mechanism of the cyclic strengthening and cyclic degradation of SATWSME is clarified. [ABSTRACT FROM AUTHOR]

The fire-resistance design of welded steel bridges depends on the temperature-dependent mechanical properties at elevated temperatures. This study investigates experimentally the temperature-induced degradation of the mechanical properties of a bridge-steel weldment. A butt-welded joint, containing the base metal (BM) Q345qD steel and weld metal (WM), was welded via submerged arc welding. Monotonic tension tests of BM and WM cylindrical specimens were carried out under various temperatures in the range of 20–700 °C, and their stress-strain relationships, failure modes, yield strength, elastic modulus, and ultimate strength at elevated temperatures were obtained and analyzed. The reduction factors of yield strength, elastic modulus and ultimate strength are compared with those recommended in current design codes. It is shown that elevated temperatures can decrease significantly the material performance of the Q345qD weldment, and the degradation of the mechanical properties of the BM and WM differs with increasing temperature. The test data indicate that the reductions in yield strength, elastic modulus, and ultimate strength at 700 °C from those at 20 °C are 78%, 55%, and 84%, respectively, for the BM and 75%, 53%, and 79%, respectively, for WM. The predictive equations for characterizing the material properties and stress-strain relationships of the BM and WM at elevated temperatures are proposed, thereby providing essential data for evaluating the fire response of steel bridges. • The temperature-induced degradation of the mechanical properties of a bridge-steel weldment is investigated experimentally. • The reduction factors of yield strength, elastic modulus and ultimate strength are compared with those in design codes. • The predictive equations for characterizing the material properties and stress-strain relationships are proposed. [ABSTRACT FROM AUTHOR]

• Dynamic experiments of U75VG rail flash-butt welded joint at different regions were carried out. • A new dynamic constitutive model of the U75VG rail flash-butt welded joint was proposed. • The influences of driving speed on the transient contact pressure, equivalent stress, and equivalent plastic strain of U75VG rail flash-butt welded joint were investigated. Rail welded joint is one of weak links in high-speed railway, and its dynamic performance has important effects on the safety, stability, and comfort of the high-speed train. In this work, the dynamic mechanical responses at different regions of U75VG rail flash-butt welded joint were investigated. The results show that the base metal (BM), welding zone (WZ), and heat-affected zone (HAZ) of U75VG rail flash-butt welded joint has an obvious strain rate effect, i.e., the material strength increases with increasing the loading strain rate. The strength of WZ is similar to that of BM, but its toughness is much lower than that of BM. The toughness of HAZ is the same as that of WZ, but its strength is much lower than that of BM and WZ. Based on the experimental results, an improved dynamic constitutive model was established to reasonably simulate the stress–strain curves of different regions from quasi-static loading to dynamic loading. Finite element (FE) analysis of transient rolling contact between rail welded joint and wheel under different conditions was performed. Compared with the results from quasi-static wheel-rail contact simulation, the equivalent plastic strain at each region of rail welded joint increases significantly and presents non-uniform distribution after considering the dynamic effect. When the wheel contacts the junction between different regions, the impact will cause a significant stress concentration at the rail welded joints. The maximum wheel-rail load by the implicit dynamic analysis is about 4 ∼ 12 times that obtained by the quasi-static analysis. Moreover, the maximum contact pressure, equivalent stress and equivalent plastic strain of each area of rail welded joints gradually increase with the increase in axle load and driving speed. [ABSTRACT FROM AUTHOR]

• The crystal plasticity model well describes the tensile behavior of the interstitial high entropy alloy at different temperatures. • The temperature effects on the yield stress and strain hardening ability are further revealed by constitutive modeling. • The strength-ductility map is predicted to explore performance optimization by adjusting the temperature and grain size. Previous tensile tests showed that the typical coarse-grained (CG) interstitial high entropy alloy (iHEA) with the nominal composition Fe 49.5 Mn 30 Co 10 Cr 10 C 0.5 (at.%) has higher yield stress and better strain hardening at cryogenic conditions compared to that at room temperature. However, the yield stress of the fine-grained (FG) iHEA is little influenced by decreasing temperature, while the strain hardening is significantly enhanced. The fundamental reasons for these observations need to be further investigated. Thus, a micromechanism-based crystal plasticity model is developed to investigate the temperature effect on the tensile behavior of the iHEA. A thermodynamic model is established to calculate the stacking fault energy of the iHEA at different temperatures. The developed constitutive model is verified by comparing the simulated the stress-strain curves and martensite volume fraction of the CG and FG iHEAs at different temperatures with the corresponding experimental results. Moreover, the contributions of different strengthening mechanisms, such as dislocations, grain boundaries, nano-precipitation, and lattice friction, are quantified to reveal the effect of temperature on the iHEA's yield stress. Furthermore, the grain size effects on deformation twinning and martensite phase transformation are considered. The developed constitutive model is further applied to predict the stress-strain curves of the iHEA with various grain sizes at different temperatures. The present study thus provides a useful modeling tool for understanding the underlying mechanisms of the strength and plasticity in the iHEA, paving a way for optimizing the mechanical properties of advanced alloys at various temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• A polycrystal model containing > 100 grains with dislocation-penetrable grain boundaries was constructed within a discrete dislocation dynamics framework. • The effects of dislocation source parameters and grain size on the yield stress of ultrafine-grained polycrystals were systematically studied. • In the initial, grain boundary dominated regime, the change of dislocation density is proportional to plastic strain and independent of the initial dislocation density. • The combined effects of source length, grain size, and initial dislocation density on flow stress can be captured by a single equation. In this study, the effects of grain size and dislocation source properties on the yield stress of ultrafine-grained (UFG) polycrystals were examined using three-dimensional multiscale discrete dislocation dynamics (DDD). A polycrystal model containing multiple grains with randomly distributed orientations was constructed within a multiscale DDD framework. Grain boundaries (GBs) were assumed to be penetrable by dislocations, with two dislocation-GB interaction mechanisms, i.e., dislocation absorption at GBs and dislocation emission from GBs, being considered. The simulation investigated the dislocation source effect and demonstrated a non-monotonic dependency of flow stress on dislocation source length, where the lowest flow stress corresponds to a Frank-Read (FR) source length of d /4 where d is the grain size. When the length of a FR source in the polycrystalline sample exceeds this value, the simulated yield stress increases owing to the constraining effect of grain boundaries on dislocation movement. The grain size dependence of the yield stress shows deviations from the classical Hall–Petch relationship as the exponent in the Hall–Petch type relation ranges from about 0.91 to about 0.98, depending on the initial dislocation density in the samples. Detailed analysis indicates that the grain size dependence of the yield stress is mainly controlled by the effect of grain boundary constraints on dislocation activation. A secondary effect arises from grain size dependent dislocation accumulation and the resulting Taylor hardening. The activation and operation of FR sources were quantitatively examined to further understand the origins of source length and grain size effects. A theoretical model is proposed to account simultaneously for the effects of source length, grain size, and initial dislocation density on the yield stress of polycrystals in the UFG regime. [ABSTRACT FROM AUTHOR]

• A multi-layered model of functionally graded coating-substrate structure is proposed. • The contact elastic shakedown map of functionally graded structure is obtained. • The factors affecting the contact shakedown are discussed. The elastic shakedown analysis of rolling/sliding contact with friction heat is essential for the safety evaluation of tribological components with a functionally graded (FG) coating. However, the corresponding works are still lacking due to the complexity of thermo-elastic contact stress calculation and residual stress optimization induced by the non-homogeneity of material properties. An effective multi-layered model for the thermo-elastic rolling/sliding contact of two similar elastic cylinders with the FG coating involving arbitrarily distributed thermo-elastic properties is established, and the corresponding thermo-elastic stress field is obtained subsequently by using the conjugate gradient method and fast Fourier transform algorithm. Then, a specific bisection iteration procedure is applied to evaluate the elastic shakedown limit (ESL), and the effects of friction coefficient, rolling speed, distribution gradient indexes of various thermo-mechanical properties, and FG coating thickness on the ESL are discussed, respectively. The results show that the increases of friction coefficient and rolling speed can cause the decrease of ESL, the ESL can be improved significantly by adjusting the distribution gradients of shear modulus and yield stress appropriately, and the relations between the ESL and various heat conduction parameters show a strong dependence on the sliding velocity. [ABSTRACT FROM AUTHOR]

• Hybrid polymer composites are reconstructed by the random digital models. • Systematic experimental and numerical studies for thermal conductivity are conducted. • A prediction model is proposed based on the extensive numerical results. • An interesting experiment is performed under the guidance of the prediction model. Polymer composites have a wide range of applications in frontier science and technology, such as flexible electronic packaging and thermally actuated soft robotics. However, most of the theoretical and numerical researches on the thermal properties of polymer composites have been focusing on the composites with homogenous mono-fillers. In this paper, a comprehensive study on the effective thermal conductivity of the composites containing both particulate and fibrous fillers is performed through experimental, numerical and theoretical approaches. Firstly, the PDMS composites with aluminum nitride particles and/or multiwalled carbon nanotubes are prepared. Then, a series of random digital models are constructed by the simulated annealing method and monte carlo method to characterize the microstructure of such composites. And then, the lattice Boltzmann method is used to calculate the effective thermal conductivity of the random microstructure. The present numerical results are fully verified by experiments. Finally, a phenomenological prediction model is developed by the nonlinear regression method based on the systematic numerical simulations. To show the versatility of the present model, an interesting experiment on thermal management materials is designed and performed. By virtue of this model, quantitatively controlling the temperature of electronic devices may be fulfilled. [ABSTRACT FROM AUTHOR]

• LSP induces more prolonged elastic-plastic transition in tensile deformation of Ti-6Al-4 V alloy. • Accurate modeling is obtained using a deformation-mechanism-based constitutive model. • Residual stress distinctly weakens the initial yielding due to the premature yielding of substrate. • Grain refinement enhances both the yielding and strain hardening. Systematical microstructure characterization, mechanical testing and constitutive modeling were carried out to quantify the effects of residual stress and grain refinement on tensile properties of Ti-6Al-4 V alloy treated by laser shock peening (LSP). Microstructure characterization showed that grain size is refined to nanoscale in the outmost surface. Meanwhile, LSP treatment introduced a maximum compressive residual stress of 600 MPa in the surface region. The macroscopic tensile test indicated a more prolonged elastic-plastic transition in the LSP-treated Ti-6Al-4 V alloy than the as-received one. As a result, the 0.2% offset yield strength of the LSP-treated Ti-6Al-4 V alloy was reduced by about 52 MPa, while the flow strength at the subsequent strain hardening stage was enhanced. To quantitatively evaluate the individual influence of gradient microstructure and residual stress on the tensile response, we established a deformation-mechanism-based and size-dependent constitutive model. Implementation of the constitutive model demonstrated that the surface grain refinement enhances both the initial yielding and strain hardening of LSP-treated Ti-6Al-4 V alloy. In contrast, the residual stress has a significant weakening effect on the initial yielding but has little influence on its strain hardening behavior. The established microstructure and deformation mechanism-based model can help guide the LSP processing to further improve the mechanical performance of Ti-6Al-4 V alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

• A deformation mechanism-based constitutive model accounting for internal deformation heterogeneities is developed for heterogeneous materials. • The established model quantitatively predicts the tensile response of laminates with various microstructures, including the true stress-strain curves and back stress. • The effects of interface affected zone on strain hardening are essential for understanding the improved strength-ductility combination of laminates. Deformation mechanism-based constitutive modeling can serve as a flexible tool to optimize the microstructure of hetero-structured metals for achieving better mechanical performance. In this work, a dislocation density-based constitutive model, which considers geometrically necessary dislocations (GNDs) and the back stress induced by internal heterogeneous deformation, is developed for describing the deformation of hetero-structured laminate materials. The heterogeneous laminates are considered to consist of alternating coarse-grained and nano-grained layers separated by extended interfaces (interface affected zones, IAZs). The respective deformation mechanisms of each phase are established based on experimental information. The evolution laws of back stress and GNDs for the coarse-grained layers, nano-grained layers and IAZs are formulated within a unified framework based on dislocation pile up theory. The simulated tensile responses are in quantitative agreement with experimental data for heterogeneous copper/bronze laminates with different interface spacings and different volume fractions of the nanostructured layers. The model also allows to represent the back stress and strain-hardening rate accurately. It is found that smaller interface spacing, implying a higher volume fraction of the IAZ, leads to a higher density of GNDs and more significant back stress. The IAZ is demonstrated to be the key to understanding the strength-ductility combination of laminates. The developed model can be readily applied to other kinds of metallic materials with architectured microstructures. [ABSTRACT FROM AUTHOR]

• Incremental secant linearization method is developed to address unmatched time scales in linearizing elasto-viscoplastic constitutive laws, and implemented into homogenization approaches to predict deformations of heterogeneous materials. • Newly developed secant linearization method can play a key role in considering the interaction between matrix and inclusion. • The homogenization approaches for elasto-plastic and elasto-viscoplastic composites can be unified with the help of newly developed secant linearization method. Unmatched time scales of elastic and viscoplastic responses are not reasonably considered in linearizing the constitutive laws of constituent phases in elasto-viscoplastic microscopic heterogeneous materials, which makes the interaction among the constituent phases very difficult to be described by homogenization approaches. To address this issue, a new incremental secant linearization method is developed by solving linearized equations of stress, strain and time increments obtained from the Hooke's law and the Taylor's expansion of stress increment function. Subsequently, the new linearization method is implemented into the Mori-Tanaka's (M−T) and self-consistent (SC) homogenization approaches. Finally, the stress–strain responses of elasto-viscoplastic microscopic heterogeneous materials (including the composites and polycrystalline materials) under different loading conditions are predicted by the incremental secant linearization-based M−T and SC approaches, and the predicted results are compared with the results obtained by other approaches, such as finite element, fast Fourier transform and generalized affine linearization methods. The comparison shows that the new secant linearization takes an important role in the accurate and effective simulations of the stress–strain responses of elasto-viscoplastic microscopic heterogeneous materials, and the predictions are independent of loading step size if the step size is not too large. Meanwhile, the homogenization approaches of elasto-viscoplastic and elasto-plastic microscopic heterogeneous materials are expected to be unified since the new secant linearization method provides the same mathematical structure for the linearized elasto-viscoplastic constitutive model as that for the elasto-plastic one. [ABSTRACT FROM AUTHOR]

The viscoelastic-viscoplastic deformation of Multi Jet Fusion-printed polyamide 12 (MJF PA12) was investigated with experimental and numerical approaches. Multi-loading–unloading–recovery tests were conducted to distinguish between the viscoelastic and viscoplastic deformations. The influence of the void defects on deformation of PA12 was investigated through the micro-computed tomography (μCT) and field emission scanning electron microscope. A finite-strain viscoelastic-viscoplastic constitutive model based on the logarithmic stress rate for the matrix of MJF PA12 was developed within the thermodynamic framework as well as an introduction of the accumulated plastic deformation–induced damage into the proposed model. A representative volume element was modeled based on the μCT results of MJF PA12. The simulated results, such as stress–strain and strain–time curves, agreed with the experimental results. Moreover, the model revealed the mechanism that the low tensile ductility of MJF PA12 is caused by the increase in strain localization and narrowing of the shear band. • The viscoelastic-viscoplastic deformation of MJF PA12 was experimentally investigated. • A finite-strain viscoelastic-viscoplastic constitutive model for the matrix of MJF PA12 was developed. • The model reasonably captured the deformation of PA12 as compared to the experiments. • The model was capable of revealing the mechanism behind the low tensile ductility of PA12. [ABSTRACT FROM AUTHOR]

Gradient structured (GS) metals can simultaneously achieve high strength and high ductility, which is the pursuit of structural materials. Abundant studies have pointed out that significant kinematic hardening is key to their strength-ductility combination. Unfortunately, few constitutive models were established to simulate and analyze the characteristic kinematic hardening behavior of GS metals. Thus, a systematic and deep understanding of the relationship between graded microstructure and the resulting macroscopic response needs further effort. In this work, we develop a strain gradient plasticity model that considers plasticity heterogeneities from the grain to the sample scale. A back stress model, which accounts for the dependency of dislocation pileups on grain size, is established to describe the cyclic deformation properties of GS materials. The established model unifies the concepts of geometrically necessary dislocations accommodating plasticity heterogeneities, grain size-dependent back stress, and reversible dislocation motion during reverse loading into a strain gradient plasticity framework. A finite element implementation of the model quantitatively predicts the uniaxial tensile and tensile-compressive responses of a GS copper bar as well as of homogeneously grained reference samples. The simulation indicates that the enhanced kinematic hardening of GS copper results mainly from fine grains in the GS layer and contributes to the considerable ductility of the GS material. The strain gradient cyclic plasticity model enables future investigation of the cyclic/fatigue behavior of GS materials, which is vital for the engineering application. • A deformation-mechanism-based model considering the internal plasticity heterogeneities is established for GS materials. • The established model unified the grain scale GNDs, the resulting back stress, and reversible dislocations into a constitutive framework. • GS material exhibits enhanced kinematic hardening which results mainly from fine grains in the gradient layer. [ABSTRACT FROM AUTHOR]

[Display omitted] • MD is used to study nanoindentation on CoCrNiFeMn high-entropy alloy (HEA). • There are no obvious load-drop phenomena in the nanoindentation of the HEA.. • The twin boundary provides a slip path for dislocations. To study the effects of twin boundary and high-entropy on elastic–plastic behavior of high-entropy alloys (HEAs), molecular dynamics (MD) was employed to simulate the nanoindentation on single-crystal CoCrNiFeMn HEA (sc-HEA), twinned CoCrNiFeMn HEA (tw-HEA) bicrystal and twinned Ni (tw-Ni) bicrystal. The deformation behaviors of the three samples were then compared with each other. Simulations revealed that the load-drop phenomenon during the indentation in the HEAs is not so apparent as that in the Ni. Microstructure characterization showed that a dense dislocation network was localized below the indentation pit of the HEAs. These phenomena are related to the damping spreading behavior of dislocations underneath the indenter. Through the analysis of the plastic zone underneath the indentation, it is found that the twin boundary inhibits dislocation penetration, and moreover, provides a slipping path for dislocations. The radial distribution of dislocation density proves that dislocations in the indentation of the HEAs are more concentrated than that in the traditional metals. Understanding the sluggish dislocation behavior and twin boundary effect help understand the deformation mechanisms underlying the mechanical response of HEAs. [ABSTRACT FROM AUTHOR]

• A first approach to evaluate ratcheting performance of rail flash butt welds is presented. • The numerical study combined a dynamic wheel–rail weld rolling contact simulation, multiple quasi-static wheel–rail weld contact simulations and a cyclic loading simulation. • The weld region was divided into 23 zones due to various mechanical properties across the rail weld. • The region with the lowest hardness in the softened zone has the shortest RCF initiation life. • RCF is likely to initiate more deeply in the softened zone than the parent rail and the region around the bond line. The ratcheting performance of a new R400HT rail flash butt weld was numerically evaluated under a typical heavy haul in-service condition. Due to varying mechanical properties across the rail weld, the weld region was divided into 23 zones in the numerical study. A dynamic finite element simulation of wheel–rail weld rolling contact was first carried out to obtain the total vertical contact force and its variation along rolling distance. Multiple quasi-static wheel–rail weld contact simulations were then performed by applying the total vertical contact force to determine the non-Hertzian contact pressure distribution when the wheel was located at different positions across the weld in the rolling direction. Based on each normal contact pressure distribution, the Haines and Ollerton's strip theory and Carter's theory were employed to estimate the corresponding longitudinal surface tangential traction distribution. Finally, a cyclic loading simulation was conducted to evaluate the ratcheting performance of the rail weld in terms of RCF initiation life by repeatedly translating these normal contact pressure and longitudinal tangential traction distributions on the running surface. The results reveal that the subzone with the lowest hardness in the softened zone has the shortest RCF initiation life among the weld region, followed by the region around the bond line. The parent rail provides the longest RCF initiation life and therefore has the best resistance to RCF. Additionally, the existence of the softened zone can shorten the RCF initiation life of the parent rail and the bond line section, particularly the regions located adjacent to the softened zone. The possible location of RCF initiation in the softened zone of the rail head can reach deeper than those in the parent rail and the region around the bond line. The results can help us understand the failure of flash butt rail welds in heavy haul operations and the numerical method can be applied to study different rail welds and rail welds under different conditions such as in a curved track. [Display omitted] [ABSTRACT FROM AUTHOR]

Pore structures provide extra freedoms for the design of porous media, leading to desirable properties, such as high catalytic rate, energy storage efficiency, and specific strength. This unfortunately makes the porous media susceptible to failure. Deep understanding of the failure mechanism in microstructures is a key to customizing high-performance crack-resistant porous media. However, solving the fracture problem of the porous materials is computationally intractable due to the highly complicated configurations of microstructures. To bridge the structural configurations and fracture responses of random porous media, a unique generative deep learning model is developed. A two-step strategy is proposed to deconstruct the fracture process, which sequentially corresponds to elastic deformation and crack propagation. The geometry of microstructure is translated into a scalar of elastic field as an intermediate variable, and then, the crack path is predicted. The neural network precisely characterizes the strong interactions among pore structures, the multiscale behaviors of fracture, and the discontinuous essence of crack propagation. Crack paths in random porous media are accurately predicted by simply inputting the images of targets, without inputting any additional input physical information. The prediction model enjoys an outstanding performance with a prediction accuracy of 90.25% and possesses a robust generalization capability. The accuracy of the present model is a record so far, and the prediction is accomplished within a second. This study opens an avenue to high-throughput evaluation of the fracture behaviors of heterogeneous materials with complex geometries., Competing Interests: Competing interests statement:The authors declare no competing interest.

Cracking from a fine equiaxed zone (FQZ), often just tens of microns across, plagues the welding of 7000 series aluminum alloys. Using a multiscale correlative methodology, from the millimeter scale to the nanoscale, we shed light on the strengthening mechanisms and the resulting intergranular failure at the FQZ. We show that intergranular AlCuMg phases give rise to cracking by micro-void nucleation and subsequent link-up due to the plastic incompatibility between the hard phases and soft (low precipitate density) grain interiors in the FQZ. To mitigate this, we propose a hybrid welding strategy exploiting laser beam oscillation and a pulsed magnetic field. This achieves a wavy and interrupted FQZ along with a higher precipitate density, thereby considerably increasing tensile strength over conventionally hybrid welded butt joints, and even friction stir welds., (© 2022. The Author(s).)

Torsional whole-life transformation ratchetting is investigated under pure-torsional and non-proportional multiaxial loadings of NiTi SMA micro-tubes at human-body temperature (310 K), where three paths of torsional loadings and five paths of multiaxial ones are considered. It is observed that the evolution of the torsional whole-life transformation ratchetting depends strongly on the loading paths and stress levels, and the fatigue lives of pure-torsional loadings decrease faster than that of uniaxial and multiaxial ones with the increase of peak stress. Based on the experimental investigations, a life-prediction model which depends on the applied stress levels is proposed for NiTi SMA micro-tubes, where the martensite transformation and reorientation of NiTi SMAs are considered. The predicted fatigue lives under uniaxial, torsional and non-proportional multiaxial loadings are mostly located within the triple error band., (Copyright © 2019 Elsevier Ltd. All rights reserved.)