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The uniaxial cyclic plasticity of cast AZ91 magnesium (Mg) alloy was investigated by conducting a series of cyclic straining and stressing tests at room temperature, and a unique cyclic plasticity (especially for ratchetting) and its physical nature were revealed. The experimental results demonstrate that the cast AZ91 Mg alloy behaviors tension-compression symmetry, because the dislocation slipping and twinning occur during both the tensile and compressive deformations; although the cast AZ91 alloy presents a certain pseudo-elastic behavior during unloading due to the detwinning, there is no obvious S-shaped asymmetric hysteresis loop like that of wrought Mg alloy in the cyclic tensile-compressive tests, and an obvious cyclic hardening is observed; moreover, the ratchetting of the cast AZ91 alloy presented in the cyclic stressing tests depends remarkably on the prescribed mean stress and stress amplitude, but slightly changes with the stress rate, and the evolution of responding peak/valley strain greatly differs from that of wrought Mg alloys and stainless steels. This work provides rich experimental data for establishing the constitutive model of cast Mg alloys. [ABSTRACT FROM AUTHOR]

The uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium (Mg) alloy is investigated by uniaxial stress-controlled cyclic tests at room temperature and with addressing the roles of different plastic deformation mechanisms. Different stress levels are prescribed to reflect the cyclic plasticity of the alloy controlled by diverse deformation mechanisms (i.e., dislocation slipping, deformation twinning and detwinning ones), and then the influences of stress level and stress rate on the ratchetting and fatigue life are discussed. The experimental results demonstrate that different evolution characteristics of whole-life ratchetting and fatigue life presented during cyclic tests with various mean stresses, stress amplitudes and stress rates are determined by the dominated plastic deformation mechanisms. It's worth noting that the ratchetting can occur in the compressive direction even in the cyclic tests with a positive (tensile) mean stress, and the fatigue life increases first and then decreases with the increase of mean stress on account of the interaction between dislocation slipping and twinning/detwinning mechanisms. Comparing the fatigue lives obtained in the asymmetric stress-controlled and symmetrical strain-controlled cycle tests, it is seen that the ratchetting deformation causes an additional damage, and then leads to a shortening of fatigue life. [ABSTRACT FROM AUTHOR]

The uniaxial ratchetting behavior and cyclic softening/hardening characteristics of Fe‐0.4C‐7Mn‐3.2Al steel are revealed by cyclic loading experiments. Electron backscattered diffraction (EBSD) and X‐ray diffraction (XRD) analyses have been carried out for assessing retained austenite (RA) transformation during cyclic deformation. Fe‐0.4C‐7Mn‐3.2Al steel exhibits a gradual transition from cyclic stabilization to slight cyclic hardening with increasing strain amplitude. RA transformation is one of the reasons for cyclic hardening. The ratchetting of Fe‐0.4C‐7Mn‐3.2Al steel shows two different characteristics under asymmetrical cyclic stressing. When the stress amplitude is below a certain value, the ratchetting is similar to that of typical cyclic stabilized materials in three stages of deceleration, stabilization and acceleration. However, when the stress amplitude reaches a certain level, the ratchetting evolution accelerates from the beginning to the fracture. The results show that the interaction between transformation‐induced plasticity and ratchetting strain can lead to accelerated accumulation of plastic strain. Highlights: The uniaxial ratchetting behavior and cyclic softening/hardening characteristics of a medium manganese steel are revealed.The transformation of retained austenite (RA) during cyclic deformation is analyzed.The interaction between transformation induced plasticity and ratchetting can lead to accelerated accumulation of plastic strain. [ABSTRACT FROM AUTHOR]

The whole-life ratchetting and fatigue failure, as well as the ratchetting-fatigue interaction, of polyamide-6 (PA6) were investigated by performing a set of uniaxial stress-controlled cyclic tests at room temperature and with different stress levels. The effects of mean stress, stress amplitude and stress ratio on the evolution of ratchetting strain and the fatigue life of PA6 are discussed. Simultaneously, the evolution of damage variable, which is defined as a function of equivalent modulus, is summarized to reflect the interaction of ratchetting and fatigue damage. The experimental results show that the uniaxial whole-life ratchetting and fatigue life of PA6 are sensitive to the mean stress, stress amplitude and stress ratio. The evolution of damage variable and its dependence on the stress level are similar to that of whole-life ratchetting, and present a tri-phased feature with respect to the damage rate. By comparing the fatigue lives obtained in the asymmetrical and symmetrical tests, a detrimental effect of ratchetting deformation on the fatigue life of PA6 is found. [ABSTRACT FROM AUTHOR]

Multiaxial whole-life transformation ratchetting and fatigue failure of superelastic NiTi SMA micro-tubes are observed in the non-proportionally multiaxial stress-controlled cyclic tests at the human-body temperature (310 K) under five different multiaxial loading paths. The effects of the axial peak stress and stress amplitude on the transformation ratchetting and fatigue life are addressed. It is found that the super-elastic NiTi SMA micro-tubes sustain more significant path-dependent fatigue failure in the multiaxial cyclic tests with lower axial peak stress and stress amplitude, while the fatigue lives of the NiTi micro-tubes under different loading paths are gradually converged with the increasing axial peak stress and stress amplitude. Based on the experimental findings, a fatigue failure model is proposed to predict the multiaxial fatigue life of the super-elastic NiTi SMA micro-tubes by considering the initial/saturated dissipation energy and the non-proportionality of the loading paths. It is shown that the predicted multiaxial fatigue lives agree with the experimental ones fairly well. [ABSTRACT FROM AUTHOR]

• The rate-dependent multiaxial fatigue life of PA6 is investigated by conducting a series of stress-controlled multiaxial fatigue tests involving the ratchetting. • A semi-empirical life-prediction model is established base on critical plane approach, in which the effects of loading path, stress level, and stress rate are considered. • A hybrid life-prediction model combing the critical plane approach and neural network is further developed for obtaining better accuracy and applicability. • Two physical constraints are designed to embed the known physics and domain knowledge into the hybrid model in this work to steer the training of neural network. • The hybrid model with physical constraints can produce both accurate and physically consistent predictions. The rate-dependent multiaxial fatigue life of PA6 is investigated by conducting a series of stress-controlled multiaxial fatigue tests involving the ratchetting. Based on the obtained experimental results and the critical plane approach, a semi-empirical life-prediction model is proposed, in which the effects of loading path, stress level, and stress rate are considered. To avoid the simplifications included in the semi-empirical model, a hybrid life-prediction model combing the critical plane approach and neural network is further developed for obtaining better accuracy and applicability. In addition, to compensate the insufficiency of data samples, two physical constraints (i.e., restricting the space of model parameters and generating domain knowledge-based data samples) are designed. The results demonstrate that: the hybrid model presents a better prediction accuracy than that of semi-empirical one; furthermore, by embedding the designed constraints into the hybrid model to steer the training of NN, the physically consistent solution is obtained and the extrapolation performance of modified hybrid model is significantly improved. [ABSTRACT FROM AUTHOR]

Experimental observations are performed to investigate the non-proportional multiaxial whole-life transformation ratchetting and fatigue failure of superelastic NiTi SMA micro-tubes in stress-controlled loadings at human-body temperature (310 K). The effects of axial mean stress and stress hold on the whole-life transformation ratchetting and fatigue life are investigated with uniaxial, torsional and five different multiaxial loading paths. The results show that the stress holds on the upper or lower transformation plateaus will both promote forward and reverse transformation, and lead to shorter fatigue life. The multiaxial fatigue lives of NiTi shape memory alloy depend significantly on loading paths and applied stress levels. [ABSTRACT FROM AUTHOR]

The internal length is the governing parameter in strain gradient theories which among other things have been used successfully to interpret size effects at the microscale. Physically, the internal length is supposed to be related with the microstructure of the material and evolves during the deformation. Based on Taylor hardening law, we propose a power-law relationship to describe the evolution of the variable internal length with strain. Then, the classical Fleck–Hutchinson strain gradient theory is extended with a strain-dependent internal length, and the generalized Fleck–Hutchinson theory is confirmed here, by comparing our model predictions to recent experimental data on tension and torsion of thin wires with varying diameter and grain size. Our work suggests that the internal length is a configuration-dependent parameter, closely related to dislocation characteristics and grain size, as well as sample geometry when this affects either the underlying microstructure or the ductility of the material. [ABSTRACT FROM AUTHOR]

The uniaxial ratcheting was experimentally observed on the polycarbonate (PC) polymers with different molecular weights (i.e., PC-7030PJ and PC-7020PJ) at room temperature. The effects of mean stress, stress amplitude, stress rate and peak hold-time on the ratcheting were observed. The results show that obvious ratcheting deformation occurs in the two prescribed polycarbonate polymers subjected to the stress-controlled cyclic loading; and the ratcheting strain accumulates cyclically in the direction of non-zero mean stress. The ratcheting greatly depends on the mean stress and stress amplitude, and the ratcheting strain increases more rapidly as the mean stress and stress amplitude increase. The ratcheting of two prescribed polycarbonate polymers is also significantly time-dependent, the ratcheting strains observed in the load cases with longer peak hold-time and at lower stress rate are larger than that with shorter peak hold-time and at higher stress rate. More importantly, a comparison of the ratcheting of two prescribed polycarbonate polymers shows that, at room temperature, the ratcheting of the polycarbonate polymer with a larger molecular weight (i.e., PC-7030PJ) is more remarkable than that with a smaller one (i.e., PC-7020PJ). [ABSTRACT FROM AUTHOR]

A series of uniaxial stress-controlled cyclic tests were carried out at different temperatures to investigate the uniaxial ratchetting of polycarbonate (PC) and its time-dependent behavior. The cyclic tests were conducted with a constant peak stress, various valley stresses and at four prescribed temperatures (i.e., 0, 30, 60 and 90°C). The effects of applied valley stress and ambient temperature on the uniaxial ratchetting of the PC are discussed. From the experimental observations, it is concluded that the ratchetting of the PC depends greatly on the test temperature; both the ratchetting strain and ratchetting strain rate increase with increase of test temperature. The effect of different valley stresses on the ratchetting is also dependent on the test temperatures, and fatigue rupture occurs in the load cases with negative valley stresses and at higher temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The uniaxial ratchetting of extruded AZ31 magnesium alloy was investigated in the cyclic tests with different mean stresses and at room temperature. The prescribed mean stresses and corresponding stress amplitudes were chosen to make the different plastic deformation mechanisms such as dislocation slipping, twinning and detwinning occur individually or simultaneously during the cyclic deformation of the AZ31 magnesium alloy with a hexagonal close packed crystal structure, and then the effect of mean stress on the uniaxial ratchetting of the magnesium alloy was discussed. It is seen that ratchetting occurs in the AZ31 magnesium alloy during the stress-controlled cyclic loading, but different evolution features are observed in the uniaxial ratchetting tests with different mean stresses. Different ratchetting behaviors are determined by the different deformation mechanisms of extruded AZ31 magnesium alloy occurring in the tensile and compressive parts of cyclic loading. A sigmoidal stress–strain hysteresis loop of the magnesium alloy presented in the cyclic tension–compression tests with some suitable mean stresses and stress amplitudes is caused by the twinning/detwinning deformation mechanism, which should be reasonably considered in constructing a cyclic constitutive model to describe the uniaxial ratchetting of magnesium alloys. [Copyright &y& Elsevier]

Highlights: [•] A cyclic finite plastic constitutive model is constructed. [•] A logarithmic stress rate is employed in the proposed model. [•] The Ohno–Abdel–Karim kinematic hardening rule is extended for finite plasticity. [•] The proposed model is implemented into a finite element code, ABAQUS. [•] Cyclic deformations of materials in the range of finite strain are predicted well. [ABSTRACT FROM AUTHOR]

Highlights: [•] Multiaxial transformation ratchetting of NiTi shape memory alloy is observed. [•] Multiaxial stress state promotes the transformation ratchetting. [•] Multiaxial transformation ratchetting depends remarkably on the loading path. [•] Axial mean stress influences the multiaxial transformation ratchetting greatly. [•] Dissipation energy decreases progressively with the number of cycles. [ABSTRACT FROM AUTHOR]

In this work, a series of uniaxial cyclic tests of VHB 4910 dielectric elastomer were carried out to investigate the uniaxial ratchetting and its dependence on the stress level, stress rate, peak/valley stress hold and loading history. The results show that in the uniaxial stress-controlled cyclic tests, VHB 4910 produces obvious ratchetting, and the higher the applied stress level and the lower the stress rate, the more remarkable the ratchetting is. Compared with the experimental results of pure-shear cyclic deformation, the apparent modulus of VHB 4910 during uniaxial cyclic deformation is lower and the dissipation is smaller. In addition, the uniaxial ratchetting of VHB 4910 is also affected by the peak/valley stress hold and loading history. The peak stress hold will promote the generation of ratchetting, but the valley stress hold will hinder the increase of ratchetting strain. The cyclic loading history with a higher mean stress will inhibit the development of ratchetting in the subsequent cyclic loading case with lower mean stress. • The pure mechanical cyclic tests are carried out on the VHB 4910 DE in the uniaxial mode. • The dependences of cyclic deformation characteristics on the strain level and rate are explored. • The effects of stress level, stress rate, stress hold time and loading history on the uniaxial ratchetting are addressed. • Compared with the pure mechanical experimental results in the uniaxial and pure-shear mode. [ABSTRACT FROM AUTHOR]

Based on the experimental observations, a cyclic nonlinear viscoelastic constitutive model was proposed to describe the uniaxial time-dependent ratcheting of polyetherimide (PEI) polymer under tension-compression and tension-tension cyclic loading. The model was constructed by extending the nonlinear viscoelastic Schapery model (Schapery, Polym. Eng. Sci., 9, 295 (1969)). The extension emphasized the changes of parameter functions used in the original model, which enabled the model to describe the ratcheting of polymer material. Comparing the simulations with corresponding experimental results, the capability of the extended model to predict the uniaxial time-dependent ratcheting of PEI was verified. It is shown that the extended model can reasonably describe the uniaxial time-dependent ratcheting of the polymer under the tension-compression and tension-tension cyclic loading with different peak-holdings, stress rates, and stress levels. POLYM. ENG. SCI., 52:1874-1881, 2012. © 2012 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Abstract: Dislocation patterns and their evolutions in 316L stainless steel during the multiaxial ratchetting deformation were observed by transmission electron microscopy (TEM). The microscopic observations indicate that the dislocation evolution presented during the multiaxial ratchetting with four kinds of multiaxial loading paths is similar to that in the uniaxial case [G. Z. Kang et al., Mater Sci Eng A 527 (2010) 5952]. That is, dislocation networks and dislocation tangles are formed quickly by the multiple-slip and cross-slip of dislocation activated by applied multiaxial stress; and then polarized patterns such as dislocation walls and elongated incipient dislocation cells are formed at the last stage of multiaxial ratchetting. The dislocation patterns evolve more quickly from the modes at low dislocation density to the ones at high density during the multiaxial ratchetting than that in the uniaxial case, and some traces of multiple-slip are observed in the multiaxial ones. The dislocation evolution during the multiaxial ratchetting deformation is summarized by comparing the observed dislocation patterns with those presented in the multiaxial strain-controlled and symmetrical stress-controlled cyclic tests. The multiaxial ratchetting of 316L stainless steel can be microscopically and qualitatively explained by the observed evolution of dislocation patterns. [Copyright &y& Elsevier]

Abstract: The whole-life transformation ratchetting and fatigue failure (including functional fatigue and structural fatigue) of super-elastic NiTi shape memory alloy were observed by uniaxial stress-controlled cyclic loading tests and at room temperature. The effects of peak stress, mean stress and stress amplitude were discussed, and then the interaction of transformation ratchetting and fatigue was then investigated. It is concluded that the whole-life transformation ratchetting of super-elastic NiTi shape memory alloy depends greatly on the stress levels and loading modes, and the dissipated energy in each cycle progressively decreases with the increasing number of cycles, which represents a functional degradation. The results also show that the structural fatigue life of the NiTi alloy presented under the stress-controlled cyclic loading is dependent on the peak stress, mean stress and stress amplitude, and the transformation ratchetting shortens the fatigue life apparently. The fatigue life decreases with the increasing dissipated energy, and a balance between dissipated energy and structural fatigue life should be reasonably considered in the design of damping devices of the NiTi alloy. [Copyright &y& Elsevier]

Abstract: Dislocation patterns and their evolution in 316L stainless steel subjected to uniaxial stress-controlled cyclic loading with occurrence of ratchetting deformation were observed by transmission electron microscopy (TEM). The microscopic observations show that the dislocation patterns change from low density patterns such as dislocation lines and pile-ups to those with higher dislocation density such as dislocation tangles, veins, walls, and cells, when the macroscopic ratchetting strain progressively increases with the number of cycles. Although one or two kinds of dislocation patterns mentioned above are prevailing in most of the grains at certain stage of ratchetting deformation, other patterns can be also observed in some grains at the same time. The features of dislocation evolution presented during the uniaxial ratchetting deformation are summarized by comparing with the dislocation patterns observed during monotonic tension and symmetrical uniaxial strain-controlled cyclic loading. The uniaxial ratchetting of 316L stainless steel can be qualitatively explained by the observed dislocation patterns and their variation with the number of cycles. [ABSTRACT FROM AUTHOR]

Abstract: Based on the experimental results, a uniaxial stress-based fatigue failure model was first proposed to predict the fatigue life by addressing uniaxial ratchetting–fatigue interaction. In the uniaxial model, the fatigue and ratchetting damages were reflected by axial stress amplitude and maximum axial stress, respectively. A parameter FP was defined by introducing a weight factor of ratchetting damage and correlated with the life. Then, the model is extended to multiaxial version by adding a factor describing the effect of multiaxial loading path. The proposed model provides good prediction to the uniaxial and multiaxial fatigue lives of the materials with different cyclic softening/hardening features. [Copyright &y& Elsevier]

Abstract: The transformation ratchetting of super-elastic NiTi shape memory alloy was observed by the uniaxial stress-controlled cyclic tests [Kang, G.Z., Kan, Q.H., Qian, L.M., Liu, Y.J, 2009a. Ratchetting deformation of super-elastic and shape memory NiTi Alloys. Mech. Mater. 41, 139–153]. It is concluded that the NiTi alloy presents apparent ratchetting behaviour, and the ratchetting is collectively caused by the cyclic accumulation of residual induced-martensite and the transformation-induced plastic deformation (i.e., namely transformation ratchetting). Based on the experimental results, a cyclic constitutive model was constructed in the framework of generalized plasticity [Lubliner, J., Auricchio, F., 1996. Generalized plasticity and shape memory alloys. Int. J. Solids Struct. 33, 991–1003] to describe the transformation ratchetting of super-elastic NiTi alloy. The proposed model simultaneously accounts for the evolutions of residual induced-martensite and transformation-induced plastic strain during the stress-controlled cyclic loading by introducing an internal variable zc , i.e., cumulated induced-martensite volume fraction. The dependence of transformation ratchetting on the applied stress levels and the phase transformation hardening behaviour of the NiTi alloy are also considered in the developed model. The anisotropic phase transformation behaviours of the alloy presented in the tension and compression cases are described by employing a Drucker–Prager-typed transformation surface. It is shown that the simulated results of transformation ratchetting obtained by the proposed model are in good agreement with the corresponding experiments, since the typical features of transformation ratchetting are reasonably captured by the proposed model. [Copyright &y& Elsevier]

Abstract: The uniaxial time-dependent ratchetting of polyester resin and glass fiber reinforced polyester resin matrix composites was observed by the stress-controlled cyclic tension–compression with non-zero tensile mean stress and tension–tension tests at room temperature. After the ratchetting of the polyester resin had been observed by the cyclic tests with different loading conditions including some time-related factors, such as stress rate and peak stress hold, the ratchetting evolutions of the continuous and short glass fiber reinforced resin matrix composites were also investigated by the stress-controlled cyclic tests, respectively. It is concluded that: both the polyester resin and its composites present apparent ratchetting deformation, i.e., the ratchetting strain accumulates progressively in the tensile direction during the cyclic tension–compression with non-zero tensile mean stress and tension–tension tests; the ratchetting depends on the applied stress amplitude, mean stress, stress rate and peak stress hold, and the time-dependent ratchetting is obvious even for the continuous glass fiber reinforced resin matrix composites with high fiber volume fraction (such as 40% and 50%); the time-dependent ratchetting of the polyester resin and its composites mainly stems from the viscosity of the polyester resin, while the addition of glass fiber into the resin matrix improves the resistance of the composites to the ratchetting deformation and lowers the time-dependence of the ratchetting simultaneously. [Copyright &y& Elsevier]

Abstract: Uniaxial ratcheting and fatigue failure of tempered 42CrMo steel were observed by the tests under the uniaxial stress-controlled cyclic loading with non-zero mean stress [G.Z. Kang, Y.J. Liu, Mater. Sci. Eng. A 472 (2008) 258–268]. Based on the obtained experimental results, the evolution features of whole-life ratcheting behavior and low-cycle fatigue (LCF) damage of the material were discussed first. Then, in the framework of unified visco-plasticity and continuum damage mechanics, a damage-coupled visco-plastic cyclic constitutive model was proposed to simulate the whole-life ratcheting and predict the fatigue failure life of the material presented in the uniaxial stress cycling with non-zero mean stress. In the proposed model, the damage was divided into two parts, i.e., elastic damage and plastic damage, which were described by the evolution equations with the same form but different constants, since the maximum applied stresses in most of loading cases were lower than the nominal yielding strength of the material. The ratcheting of the material was still described by employing a nonlinear kinematic hardening rule based on the Abdel-Karim–Ohno combined kinematic hardening model [M. Abdel Karim, N. Ohno, Int. J. Plast. 16 (2000) 225–240] but extended by considering the effect of damage. The maximum strain criterion combined with an elastic damage threshold was employed to determine the failure life of the material caused by two different failure modes, i.e., fatigue failure (caused by low-cycle fatigue due to plastic shakedown) and ductile failure (caused by large ratcheting strain). The simulated whole-life ratcheting behavior and predicted failure life of tempered 42CrMo steel are in a fairly good agreement with the experimental ones. [Copyright &y& Elsevier]

Based on the existed experimental results of 304 stainless steel, the evolution of fatigue damage during the stress-controlled cyclic loading was discussed first. Then, a damage-coupled visco-plastic cyclic constitutive model was proposed in the framework of unified visco-plasticity and continuum damage mechanics to simulate the whole-life ratcheting and predict the fatigue failure life of the material presented during the uniaxial stress-controlled cyclic loading with non-zero mean stress. In the proposed model, the whole life ratcheting was described by employing a non-linear kinematic hardening rule, i.e., the Armstrong-Frederick model combined with the Ohno-Wang model I, and considering the effect of fatigue damage. The damage threshold was employed to determine the failure life of the material. The simulated whole-life ratcheting and predicted failure lives are in a fairly good agreement with the experimental ones of 304 stainless steel. [ABSTRACT FROM AUTHOR]

Abstract: Ratchetting is a kind of special cyclic deformation behavior of the materials and structures subjected to a cyclic stressing with non-zero mean stress, and should be emphasized in the safety assessment and life estimation of engineering structures. In the last two decades, the ratchetting had been extensively studied by experiments and simulations. In this paper, the recent progresses in phenomenon observation and constitutive modeling for the ratchetting behavior of metals, polymers and composites are reviewed first. The finite element implementation and engineering application of the developed models in recent years and other corresponding topics are also commented. Some suggestions for further studies are proposed as a conclusion of the review. It should be noted that the referred literature here is mainly published after 1997. [Copyright &y& Elsevier]

Abstract: Based on the experimental results of uniaxial ratchetting–fatigue interaction obtained at room temperature for the materials presenting different cyclic softening/hardening features [Kang GZ, Liu YJ, Li Z. Experimental study on ratchetting–fatigue interaction of SS304 stainless steel in uniaxial cyclic stressing. Mater Sci Eng A 2006;435–436:396; Kang GZ, Liu YJ. Uniaxial ratchetting and low-cycle fatigue failure of the steel with cyclic stabilizing or softening feature. Mater Sci Eng A, in press], two kinds of stress-based fatigue failure models were proposed to predict the fatigue life of the materials by addressing the ratchetting–fatigue interaction occurred. In the primary stress-based failure model (namely, SBF model), the effect of ratchetting strain produced in the asymmetrical cyclic stressing on fatigue life was reflected by an item dependent upon the stress ratio R. Comparison with the experimental results shows that the SBF model provides a good prediction for the cyclic stable material, annealed 42CrMo steel, but over-estimation for the materials presenting cyclic softening feature, i.e., SS304 stainless steel and tempered 42CrMo steel, since the effect of cyclic softening/hardening feature on the ratchetting–fatigue interaction is not concluded in the primary model. To overcome such shortcoming, the primary model was then extended by introducing a new variable FP to address the effect of cyclic softening feature. In the modified stress-based failure model (namely, MSBF model), the variable FP was determined by applied maximum stress and stress ratio as well as the cyclic softening feature of the material. It is shown that the MSBF model gives fairly good predictions for both SS304 stainless steel and tempered 42CrMo steel by addressing the effect of cyclic softening feature on the fatigue life reasonably. [Copyright &y& Elsevier]

Abstract: The cyclic deformation, especially the time-dependent ratcheting behaviors of SiC particulate reinforced 6061Al alloy composites in uniaxial cyclic straining and cyclic stressing at room and high temperatures (573K) were analyzed by finite element code ABAQUS, respectively, using the single particulate model of the composites and employing axi-symmetrical 2-D 6-node elements. The ratcheting behaviors of un-reinforced matrix metals occurred in uniaxial cyclic stressing with non-zero mean stress were addressed by a new visco-plastic constitutive model with nonlinear kinematic hardening rule developed by Kang [G.Z. Kang, Mech. Mater. 36 (2004) 299–312] for cyclic stable materials. In the calculation, the particulates were assumed as elastic during the whole loading process. Comparing with the experimental results of T6-treated SiCp/6061Al composites, it is shown that the finite element model employed is reasonable to simulate the uniaxial time-dependent cyclic deformations of the composites at room and high temperatures, and some microscopic deformation features and the effect of particulate shape on the overall response of the composites are also revealed by the numerical simulation. Some significant conclusions are obtained, which are useful to establish a constitutive model to describe the ratcheting of the composites. [Copyright &y& Elsevier]

Abstract: The cyclic deformation behaviours of SiC particulate reinforced 6061Al alloy composites and aligned short Al2O3 fiber reinforced pure aluminum composites were analyzed under uniaxial cyclic straining and cyclic stressing by finite element code ABAQUS using the single particulate and fiber models of the composites and employing axi-symmetrical 2D six- and eight-node elements, respectively. The ratchetting and mean stress relaxation behaviours occurred under uniaxial cyclic stressing and cyclic straining with non-zero mean stress and mean strain were addressed, respectively, by using a new type of constitutive model with non-linear kinematic hardening rule similar to that developed by Abdel-Karim and Ohno [M. Abdel-Karim, N. Ohno, Int. J. Plasticity 16 (2000) 225] for un-reinforced matrix metals. The particulates and short fibers were assumed as elastic during the whole loading process. Based on the corresponding experimental results of T6-treated SiCp/6061Al composites, the reasonability of the finite element model to simulate the cyclic deformations of the composites was verified first. Then, the cyclic deformation of aligned δ-Al2O3f/Al composites was numerically simulated by the similar finite element model, where the stress transfer between matrix and short fiber occurred under cyclic loading and the effects of fiber volume fraction and fiber aspect ratio on the ratchetting and mean stress relaxation of the composites were discussed in details. Some significant conclusions are obtained, which are useful to establish a constitutive model to describe the cyclic deformation of such composites. [Copyright &y& Elsevier]

A visco-plastic constitutive model was proposed to simulate the uniaxial/multiaxial ratcheting of cyclically stable materials (such as U71Mn rail steel), and its finite element implementation was also achieved. The kinematic hardening rule used in the proposed model is similar to that developed by Abdel-Karim and Ohno [Int. J. Plast. 16 (2000) 225], except for a modification to the dynamic recovery term. The proposed model is verified by simulating the uniaxial/multiaxial ratcheting of U71Mn rail steel at room temperature. In the finite element implementation of the proposed model, based on radial return method and backward Euler integration, a new implicit stress integration algorithm is proposed by combining the successive substitution method developed by Kobayashi and Ohno [Int. J. Numer. Meth. Engng. 53 (2002) 2217] with Newton–Raphson solving method of non-linear scalar equation. Simultaneously, a new expression of consistent tangent modulus is also derived for rate-dependent plasticity. Numerical examples are given to verify the advantage of the implementation and the capability of the model in simulating ratcheting and cyclic stress relaxation. [Copyright &y& Elsevier]

The ratcheting and strain cyclic characteristics of U71Mn rail steel were experimentally researched under uniaxial and non-proportionally multiaxial cyclic loading at room temperature. The effects of cyclic strain, stress and their histories on strain cyclic characteristics and ratcheting were studied, respectively. It is shown that: U71Mn rail steel exhibits a cyclic stabilization and non-memorization for previous loading history under strain cycling; however, the ratcheting of the material depends greatly not only on the current values of mean stress and stress amplitude, but also on their histories; the non-proportionality of multiaxial loading path only causes a negligible additional hardening for the material. Based on the Ohno–Wang non-linear kinematic hardening model [Int. J. Plast. 9 (1993) 375, 391], the uniaxial and multiaxial ratcheting behaviours of the material were simulated by a visco-plastic constitutive model. The simulated results are in good consistence with the experimental ones. [Copyright &y& Elsevier]

• A deep learning-based novel method is established for multiaxial fatigue life prediction. • Six series of existing fatigue data of different materials are analyzed respectively. • The main loading conditions concerned in the existing fatigue researches are satisfactorily tackled. • The proposed method is efficient and can be well applicable for different materials and various loading conditions. It is well-known that conventional multiaxial fatigue life prediction models are generally limited to specific materials and loading conditions. To remove this limitation, a novel attempt is proposed in this work based on the deep learning (i.e., an improvement of artificial neural network in machine learning approaches, which is powerful to learn representations of data with multiple levels of abstraction). To comprehensively evaluate the prediction capability of proposed deep learning-based method, six series of existing fatigue data of different materials are, respectively, analyzed, in which the main loading conditions concerned in the low-cycle and high-cycle fatigue researches are included, such as loading modes (stress-controlled/strain-controlled modes), loading levels (stress/strain amplitude and mean stress/strain), and loading paths (uniaxial/multiaxial and proportional/non-proportional paths), as well as for low-cycle and high-cycle fatigue regimes. Comparison of the predicted and experimental results shows that: all the loading conditions mentioned above can be handled satisfactorily by the proposed deep learning-based method; excellent prediction accuracy is achieved, and the predicted lives in each study case fall almost within the scatter band of 1.5 times. In addition, four groups of specifically designed data are used to evaluate the extrapolation capability of the proposed method, and the results show that the extrapolation capability gets weaker if the distinctions between the loading paths involved in the training dataset and test one increase. [ABSTRACT FROM AUTHOR]

Highlights • The concentration of nano-voids in the BMG is introduced to the constitutive model as an internal variable whose evolution characterizes the nucleation and coalescence of nano-voids during the deformation. • Failure criterion applying to failure characteristics of BMGs under different stress states is established. • Deformation and failure behaviors of BMGs and evolution of nano-void in the shear bands can be well predicted by the developed constitutive model. Abstract The formation of micro-cracks caused by the nucleation and coalescence of nano-voids in the shear bands is regarded as a main failure mechanism of bulk metallic glasses (BMGs). Based on such a failure mechanism and the free volume theory, a 3-D constitutive model of BMGs is developed in this work within the framework of continuum mechanics; and the concentration of nano-voids in the BMG is introduced into the constitutive model as an internal variable whose evolution characterizes the nucleation and coalescence of nano-voids during the deformation. Moreover, to describe both the brittle and ductile failure modes presented during the tensile and compressive deformations of BMGs, respectively, a failure criterion is established by introducing a stress triaxiality, which can describe the failure characteristics of BMGs under different stress states. Finally, through the finite element implementation of the developed constitutive model, the effectiveness of the model is validated by comparing the simulated results with the experimental ones of BMGs; in addition, the nucleation and coalescence of nano-voids in the BMGs during the tensile and compressive deformations are also predicted, and the comparison of predicted results and experimental ones demonstrates the reasonability of the developed model. [ABSTRACT FROM AUTHOR]

• A nonlocal crystal plasticity model explicitly including the interactions between dislocations and grain boundaries is developed. • Finite element implementation of the model quantitatively captures the grain size effect. • The strengthening mechanisms in gradient grained material are systematically investigated by the crystal plasticity finite element simulation. • Small grains contribute significantly to the geometrically necessary dislocation-related hardening in gradient grained materials. Gradient grained metals whose microstructure is characterized by a spatially graded grain size distribution show a better strength-ductility combination than their homogeneous counterparts. Kinematic hardening associated with geometrically necessary dislocations (GNDs) is considered to be a dominant strengthening mechanism in gradient grained metals. However, the precise kinematics of GND accumulation and the nature of the back stress fields remain unclear, restricting the understanding of their deformation mechanisms. In this work, a nonlocal crystal plasticity model which explicitly accounts for the interaction between dislocations and grain boundaries is developed. The nonlocal feature is achieved by introducing a flux term to account for the spatial redistribution of dislocations due to their motion. In addition, back stress produced by the spatial variation of GND density introduces an explicit internal length scale into the model. The nonlocal nature of the model on the slip system level enables the direct investigation of strain gradient effects caused by internal deformation heterogeneities. Furthermore, the interaction between dislocations and grain boundaries leads to the formation of pileups near grain boundaries, which is key to studying the grain size effects in polycrystals. Finite element implementation of the model for polycrystals with different grain sizes quantitatively captures the grain size effect. Simulation results of gradient grained materials and their homogeneous counterparts demonstrate that smaller grains lead to higher GND density and enhanced back stress. Small grains significantly contribute to the GND-induced isotropic hardening and GND-induced kinematic hardening in gradient grained metals. This investigation helps to understand the underlying strengthening mechanisms of gradient grained metals, and the model can be readily applied to other kinds of heterogeneous materials. [ABSTRACT FROM AUTHOR]

The impact dynamic mechanical behavior of a magnesium alloy (extruded AZ31B) is first investigated by using the split-Hopkinson pressure bar (SHPB) apparatus. The dynamic tensile and compressive tests are performed at various strain rates ranging from 1250–2150 s −1 and from 1600–3500 s −1 , respectively. The loading direction is set perpendicular to the extrusion direction of the AZ31B alloy. Experimental observation shows that the strain-rate sensitivity of the AZ31B alloy present under dynamic tension is higher than that under compression. Based on crystal plasticity and the approach of strain-rate sensitivity control used in the Preston–Tonks–Wallace model, a new impact dynamic constitutive model is established to describe the dynamic stress-strain responses of the AZ31B alloy. Finally, comparing simulations with corresponding experiments, the simulations are found to agree well with the experiments, thus verifying the developed model. [ABSTRACT FROM AUTHOR]

• A fatigue life prediction model based on cGAN has been developed. • Strain amplitude, strain rate, temperature, dissolved oxygen levels, and stacking fault energy are considered as conditions. • The prediction error of fatigue life has been reduced from five times to not higher than triple. • Probabilistic estimation of environmental fatigue life has been achieved. The thermo-mechanical-chemical coupling effect presents significant challenges in accurately predicting the fatigue life of 316 austenitic stainless steel in high-temperature and high-pressure water environments (referred to hereafter as environmental fatigue). The complexity of environmental fatigue experiments results in limited and dispersed data, further making the life prediction difficult. Traditional fatigue life prediction models are often constrained by specific loading conditions and do not adequately account for the complex environmental influences. To address these issues, this paper proposes a novel environmental fatigue life prediction model of 316 stainless steel utilizing conditional Generative Adversarial Networks. The proposed model incorporates critical environmental factors, loading conditions and stacking fault energy, allowing direct prediction of environmental fatigue life. A comparative analysis on the predicted and experimental results reveals that the cGAN-based model significantly improves the prediction accuracy, reducing the fatigue life prediction error from a factor of 5 to within 3. To quantify the uncertainty in fatigue life prediction, the Monte Carlo Dropout method is employed to enable a probabilistic assessment of fatigue life. Furthermore, four environmental and loading conditions are established to evaluate the model's extrapolation capability. The results demonstrate that the probabilistic fatigue assessment effectively captures data distribution and achieves high prediction accuracy. [ABSTRACT FROM AUTHOR]

• The load amplitude–life curves of bolted joints were found to be incapable of meeting the engineering requirements. • The equivalent structural stresses of bolted joints under tensile–compressive loading were obtained. • Equivalent structural stress–life (S s – N) curve normalized for different F a – N curves was established. • A fatigue life prediction method for bolted joints was proposed and verified. In this study, load amplitude–life (F a – N) curves were obtained through tensile–compressive fatigue tests of bolted joints. It was observed that the correlation coefficient squared (R2) value of the F a – N curve with the same geometric and pre-tightening parameters was high, but the R2 value of the F a – N curve with all the parameters was low, indicating poor correlation and inability to meet the engineering requirements. Therefore, an equivalent structural stress model for the bolted joint was first developed based on a mechanical model with a strict mathematical definition to normalize these F a – N curves, which considered the bolted joint loads as the input conditions and integrated the geometric and pre-tightening parameters. Subsequently, a classical beam–shell equivalent finite element model of the bolted joint was constructed. The nodal loads in the bolted connection zone were coupled with the forces and moments of the beam element nodes through finite element simulation, and the equivalent structural stress (σ s) of the bolted joint was then obtained based on the equivalent structural stress model. Consequently, the equivalent structural stress–life (S s – N) curve and probabilistic stress–life (P – S s – N) curve normalized for different F a – N curves were obtained by fitting the data of σ s and N. Lastly, the accuracy of the fatigue life prediction method based on equivalent structural stress was verified by conducting the vibration fatigue test on the bolted joint structure of the subway antenna bracket. [ABSTRACT FROM AUTHOR]

• Hydrogen-assisted dislocation accumulation reduces the number of cycles to reach the first hardening peak. • Hydrogen-assisted twinning by reducing the SFE enhances the formation of the secondary hardening. • The hydrogen-assisted dislocation and twinning evolutions contribute to fatigue life degradation. This study investigates the cyclic plastic deformation and microstructural evolution of hydrogen-charged 316L stainless steel. The results of low-cycle fatigue tests indicate that the samples with and without pre-charged hydrogen exhibit initial cyclic hardening and sequential cyclic softening at a low strain amplitude (≤0.5 %). However, a secondary cyclic hardening at a high strain amplitude (≥0.7 %) is observed near the end stage in both uncharged and hydrogen-charged samples. Microstructural evolutions characterized by electron backscattered diffraction and scanning transmission electron microscopy reveal the evident hydrogen-assisted dislocation nucleation and motion, which significantly enhances the accumulation of dislocation in the first cyclic hardening stage, resulting in fewer cycles to reach the first peak of responding stress amplitude. Moreover, the observed hydrogen-assisted twinning accelerates the formation of the secondary peak of stress amplitude. The hydrogen-assisted dislocation and twinning evolutions contribute to a reduction in the fatigue life of the steel. [ABSTRACT FROM AUTHOR]

• Plastic deformation mechanisms at elevated temperature are revealed. • Whole-life ratchetting behavior at different temperatures is elucidated. • Temperature-dependence of fatigue life and fracture mode is discussed. The variations in the plastic deformation mechanisms of extruded AZ31 magnesium (Mg) alloy at elevated temperature and the temperature-dependence of its ratchetting-fatigue interaction were revealed by a series of uniaxial stress-control low-cycle fatigue experiments. The experimental results elucidate that: (1) The tension–compression asymmetry and the S-shaped inflection point in the stress–strain hysteresis loops are gradually attenuated with elevating the temperature, and the evolution of ratchetting is also remarkably affected by the elevated temperature due to the activation of abundant non-basal plane slipping; (2) The fatigue life of the alloy exhibits a pronounced temperature-dependence, and specifically, the fatigue life is greater at 100 °C compared to that at room temperature, but sharply decreases at 150 °C and 200 °C due to the significant ratchetting deformation at these two temperatures. Moreover, the dependence of fatigue life on the applied mean stress is also different at different temperatures; (3) Due to the significant ratchetting deformation in the alloy at elevated temperature, a transition from a brittle fatigue fracture mode to a ductile one also occurs as the temperature increases. This research provides valuable experimental data that serve as an important reference for the practical application of Mg alloys. [ABSTRACT FROM AUTHOR]

• Rate- and path-dependent functional fatigue of super-elastic NiTi SMA are investigated experimentally. • The effects of proportionally and non-proportionally multiaxial loading conditions on functional fatigue are discussed. • A new indicator is defined to correlate path-dependent functional fatigue. • Thermo-mechanically coupled effect is revealed. In this work, the rate-dependent and loading path-dependent cyclic deformation behaviors of super-elastic NiTi shape memory alloy (SMA) are investigated under multiaxial cyclic loadings with various loading frequencies. The evolutions of stress–strain response, temperature and dissipation energy with increasing the number of cycles are investigated systematically. It is found that the functional fatigue occurs in the process of cyclic deformation, and such a phenomenon is more significant under non-proportionally multiaxial loadings and with higher frequency. The temperature variation induced by the internal heat generation becomes more obvious with increasing the loading frequency. Moreover, it is revealed that the path-dependent functional fatigue has a positive correlation to a newly defined indicator for different loading paths, i.e., the degree of non-proportionality, and the rate-dependent functional fatigue can be attributed to the thermo-mechanically coupled nature of martensite transformation. The experimental results performed in this work and the revealing deformation mechanisms are helpful to establish a three-dimensional thermo-mechanically coupled cyclic constitutive model of NiTi SMAs. [ABSTRACT FROM AUTHOR]

A multi-mechanism macroscopic phenomenological constitutive model is established to characterize the uniaxial ratchetting of extruded AZ31 magnesium (Mg) alloy at room temperature in the framework of small deformation. Because the uniaxial ratchetting of the alloy depends greatly on the strong basal texture and different deformation mechanisms (i.e., dislocation slipping, twinning and detwinning) and presents apparent tension-compression asymmetry, two separated yield functions and different hardening rules are used in the proposed model regarding to different deformation mechanisms. That is, the traditional von Mises yield criterion and a flow rule in power-law form are used for the plasticity contributed by dislocation slipping, while the Cazacu–Barlat–Plunkett (CPB) yield criterion extended with a back stress tensor and an internal variable f (i.e., twin volume fraction) is employed to express the plastic deformation induced by twinning/detwinning. In addition, the modified Armstrong-Frederick (A-F) kinematic hardening models are adopted to reflect the strain hardening features contributed by different plastic mechanisms, and the interaction among different plastic mechanisms is considered in the developed constitutive model. The reproduced uniaxial ratchetting of extruded AZ31 Mg alloy by the proposed multi-mechanism constitutive model is in a good agreement with the corresponding experimental results obtained at room temperature, which validates the reasonability and capability of the proposed model. • A multi-mechanism model for uniaxial ratchetting of extruded AZ31 magnesium alloy is proposed. • Different yield surfaces and hardening rules regarding to different deformation mechanisms are employed. • The unique ratchetting behavior of extruded AZ31 magnesium alloy with different deformation mechanisms is reproduced. [ABSTRACT FROM AUTHOR]

• An incremental strain gradient plasticity constitutive model is constructed. • The coupling effect of sample and grain sizes on the plastic hardening is considered. • A new iterative algorithm that can independently solve the gradient field and plastic strain increment is proposed. • The size-dependent torsional deformation of copper micro-wires is simulated reasonably. Experimental observations show that the torsional deformation of copper micro-wires exhibits obvious sample and grain size effects. In this work, based on the framework of the cyclic plastic J 2 flow rule, an incremental higher-order strain gradient constitutive model is established to describe the size effect observed in the torsional deformation of copper micro-wires. A novel kinematic hardening evolution rule is constructed in which the coupling effect of sample and grain sizes on the plastic hardening has been considered. In terms of numerical implementation, a finite element iterative algorithm which can independently solve the gradient field and plastic strain increment is proposed. Finally, the proposed model is implemented into the finite element software ABAQUS using a three-dimensional user-defined element and user-defined material subroutine. Simulated results show that the proposed model can capture the size-dependent torsional deformation of copper micro-wires since both the effects of sample and grain sizes are considered reasonably. This provides a good basis for the combination of strain gradient plasticity theory and cyclic plasticity constitutive model. [Display omitted] [ABSTRACT FROM AUTHOR]

• A gradient nano-grained polycrystal model with dislocation-penetrable grain boundaries was constructed within a discrete dislocation dynamics framework. • The mechanisms of synergetic strengthening in gradient nano-grained aluminum were systematically investigated from the view of dislocation dynamics. • The distribution of geometrically necessary dislocation is non-uniform along the gradient direction in the gradient nano-grained sample. • The gradient nano-grained sample has a stronger Bauschinger effect compared to its homogenous counterparts. Gradient nano-grained (GNG) metals have shown a better strength-ductility combination compared to their homogeneous counterparts. In this paper, the mechanical properties and the related deformation mechanisms of GNG aluminum were investigated using three-dimensional multiscale discrete dislocation dynamics (DDD). GNG polycrystalline models and uniform nano-grained (UNG) counterparts were constructed within a multiscale DDD framework. A dislocation-penetrable grain boundary model based on a coarse-graining approach was adopted to handle the interaction between dislocations and grain boundaries. The simulation results show that the yield stress and strain hardening of the GNG sample is larger than the value calculated by the rule of mixtures, indicating a synergetic strengthening induced by the gradient structure. The associated microstructure evolution demonstrates that dislocations initially activate and glide in the larger grains and then gradually propagate into the smaller grains in the GNG sample, this sequential yielding of layers with different grain sizes generates stress and strain gradients, which is accommodated by geometrically necessary dislocations (GNDs). Moreover, we found that the Bauschinger effect in GNG sample is stronger than those in component UNG samples, suggesting a significant back stress strengthening in the GNG sample during plastic deformation. Finally, a theoretical model is established which successfully describes the Bauschinger effect of GNG and corresponding UNG samples according to the features of dislocation evolution upon unloading. The present study provides insights into the outstanding mechanical property of GNG metals from the view of dislocation dynamics at the submicron scale and offers theoretical guidance for designing strong-and-ductile metals. [Display omitted] [ABSTRACT FROM AUTHOR]

The long-term performance of polymers under cyclic loading is important for safety assessments in engineering applications. The deformation process under the cyclic loading can be accelerated through use of temperature and stress. As for asymmetric cyclic loading, so called ratcheting, a time-temperature-stress (TTS) equivalence method in which all the parameters have clear physical meanings and can be determined experimentally, was proposed to predict the long-term cyclic loading behavior for polycarbonate using short-term data. Taking into consideration the effects of both the mean stress and the stress amplitude, the ratcheting compliance was defined and its evolution function was also provided. Next, the TTS equivalence method was validated using the long-term ratcheting test results for the polycarbonate. Time, temperature, and stress do show equivalent effects on long-term ratcheting of polycarbonate. Using the proposed method, time and cost can be dramatically saved for the assessment of the long-term cyclic loading performance of polycarbonate. [ABSTRACT FROM AUTHOR]

Highlights: [•] Compression–torsion ratchetting is dominated by stress amplitude and loading path. [•] HE steel with a higher carbon content gives a lower ratchetting strain and rate. [•] A modified cyclic plasticity model has the capacity to capture ratchetting features. [•] Method for calibrating material parameters from experimental results is presented. [Copyright &y& Elsevier]

• The evolution of dislocation patterns during cyclic loading is revealed by EBSD and ECCI. • A RVE from EBSD helps to provide accurate information of grains and orientations. • A crystal plasticity model is developed to predict cyclic shear behavior of TRIP steel. • Contribution of different mechanisms to mechanical properties is clarified under cyclic conditions. The transformation-induced plasticity (TRIP) steel has an excellent synergy between strength and ductility and is widely used in industry. Cyclic loading is often seen in its industrial application. Therefore, it is of great significance to study cyclic plastic behavior of the TRIP steel. The TRIP steel's cyclic shear behavior and microstructure evolutions were investigated by mechanical testing, electron backscatter diffraction (EBSD) characterization, and in-situ electron channeling contrast imaging (in-situ ECCI) observation. Then, a multiple-mechanism crystal plasticity constitutive model was developed, considering both dislocation slip and martensitic transformation mechanisms. Furthermore, an isotropic hardening law and a modified kinematic hardening rule were taken into account. The crystal plasticity model was implemented into DAMASK with the spectral method. A polycrystalline RVE with realistic grain morphology was constructed from the EBSD data. The simulations of the TRIP steel under cyclic shear loading showed that the samples under higher strain amplitude exhibit stronger cyclic shear hardening. The activation of the martensitic transformation mechanism promotes cyclic hardening. In grain level, the grains with Taylor factor (related to orientation) between 3.3 and 3.9 can activate more slip systems and have a better cyclic hardening ability. So, the Taylor factor is defined as an indicator describing the cyclic response of individual grains in polycrystalline materials. Considering the capability of the established modeling framework, it is of great significance to guide the TRIP steel to serve safely and help the microstructural design in grain scale. [Display omitted] [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]

• 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]

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]