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A physically‐based constitutive modeling framework is proposed for the cyclic deformation and softening of Ni‐based superalloys. The constitutive model accounts for underlying mechanisms of grain size strengthening, dislocation strengthening, solid solution strengthening, and precipitate strengthening, commonly observed in these alloys. A constitutive model is proposed for the shearing of precipitates on their interaction with glide dislocations during cyclic deformation. This is the primary contributor to the experimentally observed cyclic softening behavior in Ni‐based superalloys. Model predictions of the cyclic stress–strain response and the cyclic softening (quantified in terms of the peak stress) are compared with the experimental counterparts for a range of strain amplitudes under fully‐reversed cyclic loading of Inconel 718 (IN 718). Further, model predictions of precipitate size are also compared with the experimentally measured transmission electron micrographs of precipitate sizes at the end of deformation. Concurrence in the cyclic stress–strain response, peak stress, and precipitate size provides validation for our constitutive modeling framework. Highlight: Development of a physically‐based constitutive modeling framework for cyclic deformation.Microstructure‐based strengthening mechanisms are considered in the model.Model incorporated the physical mechanisms of cyclic softening due to precipitate shearing. [ABSTRACT FROM AUTHOR]

Due to its good conductive properties, unalloyed (pure) aluminum, such as EN AW-1050A H24, finds new fields of application in electromobility. To optimize components, the cyclic material behavior must be understood and described precisely as a foundation of a proper fatigue life estimation. Various cyclic tests were performed to not only derive the cyclic parameters to describe the material but also to find the most suitable procedure to deal with the challenges faced during the experiments. The main point of interest is the comparison between a surface-mounted clip-on extensometer and an optical system both used for strain control in cyclic tests. For the analysis of the anisotropic behavior of EN AW-1050A H24, un-notched flat specimens were extracted from sheet metal lengthways and crossways in respect to the rolling direction. While the cyclic material behavior for specimens of both directions of extraction is characterized by cyclic softening in general, the specimens extracted crossways show a strain-amplitude-dependent cyclic softening with strong strain localization especially at the contact points of the knives of the clip-on extensometer leading to an increased quantity of invalid experiments as well as sudden fractures. In the study, it was possible to show the benefits of a contactless optical strain control system when dealing with very soft metallic materials such as EN AW-1050A H24. [ABSTRACT FROM AUTHOR]

Seismic events remain a significant threat, causing loss of life and extensive damage in vulnerable regions. Soil liquefaction, a complex phenomenon where soil particles lose confinement, poses a substantial risk. The existing conventional simplified procedures, and some current machine learning techniques, for liquefaction assessment reveal limitations and disadvantages. Utilizing the publicly available liquefaction case history database, this study aimed to produce a rule-based liquefaction triggering classification model using rough set-based machine learning, which is an interpretable machine learning tool. Following a series of procedures, a set of 32 rules in the form of IF-THEN statements were chosen as the best rule set. While some rules showed the expected outputs, there are several rules that presented attribute threshold values for triggering liquefaction. Rules that govern fine-grained soils emerged and challenged some of the common understandings of soil liquefaction. Additionally, this study also offered a clear flowchart for utilizing the rule-based model, demonstrated through practical examples using a borehole log. Results from the state-of-practice simplified procedures for liquefaction triggering align well with the proposed rule-based model. Recommendations for further evaluations of some rules and the expansion of the liquefaction database are warranted. [ABSTRACT FROM AUTHOR]

Strain-controlled low cycle fatigue (LCF) tests were conducted on conventionally grained (CG) and ultrafine-grained (UFG) Al-Mg-Si alloys treated under various aging conditions. In the cyclic stress response (CSR) curves, CG peak-aged (PA) alloys showed initial cyclic hardening and subsequent saturation, whereas CG over-aged (OA) alloys displayed cyclic softening behavior close to saturation. The UFG materials exhibited continuous cyclic softening except for UFG 3; it originates from the microstructural stability of the UFG materials processed by severe plastic deformation (SPD). Using a strain-based criterion, the LCF behavior and life of the CG and UFG materials were analyzed and evaluated; the results are discussed in terms of strengthening mechanisms and microstructural evolution. In the CG materials, the LCF life changed markedly owing to differences in deformation inhomogeneity depending on the precipitate state. However, the UFG materials displayed a decreasing LCF life as cyclic softening induced by dynamic recovery became more severe; additionally, a relationship between the microstructural stability of the UFG materials and the cyclic strain hardening exponent n′ was suggested. [ABSTRACT FROM AUTHOR]

Lateral spreading of an interlayered deposit adjacent to a meandering stream channel in Wufeng, Taiwan, during the 1999 Chi-Chi Earthquake is evaluated using two-dimensional (2D) nonlinear dynamic analyses (NDAs) with geostatistical modeling of the subsurface to assess their ability to approximate the observed magnitude and spatial extent of ground deformations, as well as identify the key factors and mechanisms that most contributed to the overall system response. In-situ data from borings and cone penetration tests (CPTs) depict thinly stratified overbank deposits of low-plasticity silty sands, silts, and clays, interlayered with laterally discontinuous channel-deposited sands. The three-dimensional (3D) subsurface is simulated using transition probability-based indicator geostatistics, conditioned on available CPT data and geological inferences. The NDAs are performed using the PM4Sand and PM4Silt constitutive models, within the FLAC finite difference program. Sensitivity analyses are performed to understand the influence of uncertainties in the stratigraphy, channel conditions, soil properties, input ground motions, constitutive model calibration protocols, and numerical boundary conditions, as well as the performance of alternate channel transects. Most analysis cases generally matched the maximum displacements observed near the channel but overestimated the extent of displacements away from the channel. The most favorable results were largely influenced by nonstationary stratigraphic trends and cyclic softening of fine-grained soils, in addition to the liquefaction of coarse-grained soils. This case history demonstrates the capabilities and limitations of current subsurface and NDA modeling procedures for predicting ground deformation patterns.

Low cycle fatigue (LCF) and ratcheting experiments were carried out on annealed low-carbon steel at room temperature within a laboratory environment, utilising stress and strain control modes. The annealed low-carbon steel consistently demonstrates a cyclic softening response over its LCF lifespan, across all tested strain amplitudes. Notably, it was observed that ratcheting strain rises while ratcheting life declines with both rising mean stress and stress amplitude. Annealed low-carbon steel, being entirely ferritic and lacking precipitation or substitutional solid solution strengthening or hard phase strengthening, exhibits a restricted ability to withstand or alleviate the accumulation of ratcheting strain, particularly under very low mean stress conditions. In both LCF and ratcheting, significant substructure formation was detected. Nevertheless, there was no discernible difference in substructure formation between LCF and ratcheting when employing electron channelling contrast imaging techniques. The existing mean stress-based fatigue life prediction model has successfully forecasted ratcheting and LCF life within the 102–105 cycles range. A novel approach utilising modulus is introduced to characterise the cyclic hardening/softening behaviour of alloys in stress and strain-controlled experiments. The cyclic hardening model based on modulus effectively captures the responses observed in cyclic hardening/softening during LCF and ratcheting experiments.

Due to its good conductive properties, unalloyed (pure) aluminum, such as EN AW-1050A H24, finds new fields of application in electromobility. To optimize components, the cyclic material behavior must be understood and described precisely as a foundation of a proper fatigue life estimation. Various cyclic tests were performed to not only derive the cyclic parameters to describe the material but also to find the most suitable procedure to deal with the challenges faced during the experiments. The main point of interest is the comparison between a surface-mounted clip-on extensometer and an optical system both used for strain control in cyclic tests. For the analysis of the anisotropic behavior of EN AW-1050A H24, un-notched flat specimens were extracted from sheet metal lengthways and crossways in respect to the rolling direction. While the cyclic material behavior for specimens of both directions of extraction is characterized by cyclic softening in general, the specimens extracted crossways show a strain-amplitude-dependent cyclic softening with strong strain localization especially at the contact points of the knives of the clip-on extensometer leading to an increased quantity of invalid experiments as well as sudden fractures. In the study, it was possible to show the benefits of a contactless optical strain control system when dealing with very soft metallic materials such as EN AW-1050A H24.

Seismic events remain a significant threat, causing loss of life and extensive damage in vulnerable regions. Soil liquefaction, a complex phenomenon where soil particles lose confinement, poses a substantial risk. The existing conventional simplified procedures, and some current machine learning techniques, for liquefaction assessment reveal limitations and disadvantages. Utilizing the publicly available liquefaction case history database, this study aimed to produce a rule-based liquefaction triggering classification model using rough set-based machine learning, which is an interpretable machine learning tool. Following a series of procedures, a set of 32 rules in the form of IF-THEN statements were chosen as the best rule set. While some rules showed the expected outputs, there are several rules that presented attribute threshold values for triggering liquefaction. Rules that govern fine-grained soils emerged and challenged some of the common understandings of soil liquefaction. Additionally, this study also offered a clear flowchart for utilizing the rule-based model, demonstrated through practical examples using a borehole log. Results from the state-of-practice simplified procedures for liquefaction triggering align well with the proposed rule-based model. Recommendations for further evaluations of some rules and the expansion of the liquefaction database are warranted.

Strain-controlled low cycle fatigue (LCF) tests were conducted on conventionally grained (CG) and ultrafine-grained (UFG) Al-Mg-Si alloys treated under various aging conditions. In the cyclic stress response (CSR) curves, CG peak-aged (PA) alloys showed initial cyclic hardening and subsequent saturation, whereas CG over-aged (OA) alloys displayed cyclic softening behavior close to saturation. The UFG materials exhibited continuous cyclic softening except for UFG 3; it originates from the microstructural stability of the UFG materials processed by severe plastic deformation (SPD). Using a strain-based criterion, the LCF behavior and life of the CG and UFG materials were analyzed and evaluated; the results are discussed in terms of strengthening mechanisms and microstructural evolution. In the CG materials, the LCF life changed markedly owing to differences in deformation inhomogeneity depending on the precipitate state. However, the UFG materials displayed a decreasing LCF life as cyclic softening induced by dynamic recovery became more severe; additionally, a relationship between the microstructural stability of the UFG materials and the cyclic strain hardening exponent n′ was suggested.

A series of pure-shear cyclic tests were performed on VHB™4910 dielectric elastomer at various temperatures, and the investigations were focused on the influence of temperature variations on the cyclic deformation of the elastomer and the influences of loading level and loading rate on the temperature-dependent cyclic deformation. The results demonstrate that the temperature-dependent deformation of VHB™4910 dielectric elastomer primarily manifests its viscoelasticity, while the hyper-elastic part shows comparatively weaker temperature-dependence. At different temperatures, the elastomer exhibits a cyclic softening during the strain-controlled cyclic loading. Both the peak stress and residual strain reduce with increasing the temperature and reach a stable state more rapidly at higher temperatures. A more pronounced temperature-dependence is exhibited in the stress response of the elastomer at a high loading rate; while a lower rate-dependence is observed at a high temperature. The ratchetting of the elastomer presented in the stress-controlled tests becomes more prominent as the temperature increases, but the rate-dependence of ratchetting is weaker at higher temperatures. Moreover, the temperature-dependence of ratchetting is more significant at a high-stress level as compared to that at a low-stress level.

The cyclic loading experiments on HP‐172B sealant bonded tubular butt‐joints were carried out with the strain‐controlled mode, and the effects of strain amplitude and mean strain on the cyclic relaxation stress and cyclic softening variable of the bonded butt‐joint were investigated. The experimental results show that the stress–strain response curve of the butt‐joint presents a corrugated shape under cyclic loading. The cyclic stress relaxation and cyclic softening of the bonded butt‐joint occur. The increase of mean strain and strain amplitude intensifies the cyclic stress relaxation and cyclic softening behaviors. According to the experimental observation, a nonlinear viscoelastic model is proposed to predict the mechanical behavior of butt‐joints under cyclic loading, which considers the cyclic stress relaxation and cyclic softening of the bonded butt‐joint. The comparisons between the model calculation and the experimental data show that the calculated stress–strain curve is in good agreement with the experimental data. Highlights: The cyclic loading experiments on adhesively bonded joints were carried out.The stress–strain curve presents a corrugated shape under cyclic loading.A viscoelastic model was proposed according to the experimental observation.The model can describe the mechanical behaviors of adhesively bonded joints. [ABSTRACT FROM AUTHOR]

The seismic performance of Balboa Boulevard during the 1994 MW 6.7 Northridge earthquake was examined through nonlinear deformation analyses (NDAs) using advanced tools to (1) investigate the failure mechanism leading to ground deformations at this site; (2) evaluate the accuracy of the adopted analysis methods, engineering procedures, and state-of-the-art tools to reasonably estimate horizontal ground displacements; and (3) identify key factors and parameters contributing to earthquake-induced ground deformations at this site. One-dimensional (1D) liquefaction vulnerability indexes (LVIs) and permanent displacements using Newmark sliding block analyses were also estimated and compared against ground deformations observed after the earthquake. The geotechnical characterization of Balboa Boulevard was assessed based on field and laboratory data obtained from two investigation campaigns. Transitional probability geostatistics were used to develop stratigraphic models that capture the heterogeneity and the spatial variability patterns of sand-like and clay-like soils present at this site. The stratigraphic models were implemented in the finite difference software FLAC and the behavior of sand-like and clay-like soils simulated using the PM4Sand and PM4Silt constitutive models, respectively. Sensitivity analyses were performed to address uncertainties associated with the spatial variability of soils, input ground motions, the proportion of sand-like and clay-like soils within the soil deposit, and the strength properties of these materials. Results from NDAs suggest that a compounded effect of both liquefaction of sand-like soils and cyclic softening of clay-like soils led to the excessive ground deformations at Balboa Boulevard. This study sheds light on the importance of using appropriate engineering procedures and numerical modeling protocols in the prediction of deformation patterns, the selection of key input parameters, as well as the applicability of LVIs in complex sites.

Low cycle fatigue (LCF) tests were carried out on Ti6Al4V alloy at room temperature for different strain amplitudes ranging from 0.5% to 2%. The microstructure and texture evolutions as a function of strain amplitude have been investigated in the present study. Cyclic softening behaviour was observed at all strain amplitudes, and it was found to increase with increasing the strain amplitude. The fatigue transition life was observed to be at 1.25% strain amplitude. Only (10 1 ¯ 2) type tensile twins were formed during the deformation, and this was found to be significant at higher strain amplitude of 2%. The grains along [ 10 1 ¯ 0 ] were appeared to be the twinning parent, and the twinned products were appeared along [0001]. The [ 10 1 ¯ 0 ] grains were further found to have substantial cyclic deformation than the other grains, such as [ 2 1 ¯ 1 ¯ 0 ] and [0001]. The dominant texture was found along [ 10 1 ¯ 0 ] during the LCF tests. However, the sample at higher strain amplitude of 2% had also prominent [0001] texture along with [ 10 1 ¯ 0 ] texture. Fractographic examination revealed a dimple fracture surface at lower strain amplitude (0.5%), whereas a quasi-cleavage type fracture was observed at higher strain amplitudes (1% & 2%). [ABSTRACT FROM AUTHOR]

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

The strain-controlled low cycle fatigue tests were carried out at 298 ​K for the newly developed micro-grain casting K4169 superalloy, and the cyclic stress response, deformation and fracture behaviors of the alloy were studied. The results show that the fatigue life of the alloy was increased by more than two times compared with other casting K4169 alloys. This can be partly attributed to grain refinement impeding crack initiation and propagation. When the strain amplitude was in plastic range (Δε t /2 ​≥ ​0.5%), the alloy displayed a short period of initial cyclic stability followed by cyclic softening to fracture. By observing dislocation configurations, only a few dislocations moved around grain boundaries at initial cycles, leading to short cyclic stability. Afterwards, dislocations in the slip bands cut γ" phase continuously leading to cyclic softening. When the strain amplitude was in elastic range (Δε t /2 ​≤ ​0.4%), the alloy exhibited elastic deformation and the peak stress kept stable. In addition, the crack initiation site changed from the MC carbides to the persistent slip bands on surface with increasing strain amplitudes. [ABSTRACT FROM AUTHOR]

Corroded surfaces of offshore mooring chains typically show a very irregular shape with different pit geometries. Fatigue tests were performed in three‐point bending on specimens with hemispherical notches, representing an idealized geometry of a corrosion pit. Digital image correlation was used to detect crack initiation lives and locations. The aim of the study was to quantify mean stress relaxation (MSR) and its influence on fatigue initiation life. MSR data were obtained from strain‐controlled fatigue tests on R4‐grade offshore mooring chain steel. Interestingly, MSR was observed even at a stress amplitude below the cyclic yield stress. For initiation lives below 100,000 cycles, significant improvement in predictions was obtained if cyclic softening and MSR were accounted for. Using only the monotonic stress‐strain curve led to less accurate predictions. For longer lives, scatter in the base material S‐N data and residual stresses from machining of the notch made predictions less accurate. [ABSTRACT FROM AUTHOR]

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel. [ABSTRACT FROM AUTHOR]

Motion capture is the current gold standard for assessing movement of the human body, but laboratory settings do not always mimic the natural terrains and movements encountered by humans. To overcome such limitations, a smart sock that is equipped with stretch sensors is being developed to record movement data outside of the laboratory. For the smart sock stretch sensors to provide valuable feedback, the sensors should have durability of both materials and signal. To test the durability of the stretch sensors, the sensors were exposed to high-cycle fatigue testing with simultaneous capture of the capacitance. Following randomization, either the fatigued sensor or an unfatigued sensor was placed in the plantarflexion position on the smart sock, and participants were asked to complete the following static movements: dorsiflexion, inversion, eversion, and plantarflexion. Participants were then asked to complete gait trials. The sensor was then exchanged for either an unfatigued or fatigued plantarflexion sensor, depending upon which sensor the trials began with, and each trial was repeated by the participant using the opposite sensor. Results of the tests show that for both the static and dynamic movements, the capacitive output of the fatigued sensor was consistently higher than that of the unfatigued sensor suggesting that an upwards drift of the capacitance was occurring in the fatigued sensors. More research is needed to determine whether stretch sensors should be pre-stretched prior to data collection, and to also determine whether the drift stabilizes once the cyclic softening of the materials comprising the sensor has stabilized. [ABSTRACT FROM AUTHOR]

The tensile, creep, fatigue and creep-fatigue tests of the nickel-based superalloy GH4169 were carried out. According to the deformation characteristics of GH4169 alloy, the Ohno-Karim kinematic model (O-K model) can be used to describe the tensile behavior. The creep constitutive model presented in this paper can be used to predict the three-stage creep characteristics of the GH4169 alloy. The modified Ohno-Karim kinematic hardening model, combined with an isotropic hardening model, can well predict the cyclic softening behavior of the material under symmetric loads and the mean stress relaxation behavior under asymmetric loads. Based on the modified Ohno-Karim kinematic hardening model, isotropic hardening model and creep constitutive model, a non-unified constitutive model was established. The creep-fatigue behavior of the GH4169 alloy under symmetric and asymmetric loads is simulated by using the non-unified constitutive model. The simulation results are very close to the experimental results; however, the prediction results of the time-dependent relaxation load are relatively small. [ABSTRACT FROM AUTHOR]

• A computational strategy based on gradient damage is developed for low-cycle fatigue. • Mixed-hardening model is combined with elasto-plastic gradient damage. • Developed framework captures stress-strain hysteresis better than Chaboche model. • Low-cycle fatigue crack growth is accurately simulated by the developed framework. • The framework avoids computational burden of conformal meshing and remeshing. This work presents a novel computational strategy based on gradient enhanced damage and cyclic yielding with nonlinear mixed hardening behaviour for simulating low-cycle fatigue of ductile materials. A new yield function is introduced which accommodates the cyclically evolving state variables during mixed hardening and the damage induced during the application of repetitive loads. The resulting constitutive relations are derived in an incremental form. The non-local strains facilitate the computation of incremental damage in the material. A finite element scheme is developed from the governing physics through full coupling between damage and cyclic-elastoplastic deformations. In order to obtain an accurate computational response, a stress-return mapping scheme is formulated for the damage-dependent yield function. The developed numerical strategy is investigated for specimens made of various ductile materials exhibiting elasto-plastic behaviour under low-cycle fatigue loads. The stress-strain hysteresis and cyclic softening computed by the present numerical framework agree well with experimental data available in literature. The low-cycle fatigue crack-growth results are also accurately captured for various fatigue loading conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

• Mild hardening for the hydrogen-free TRIP steel is due to the martensitic transformation. • Hydrogen-induced TRIP steel shows cyclic softening throughout its life. • Compressive mean stress develops in the TRIP steel under tensile mean strain cycling. • The GND map shows higher dislocations near the grain boundaries for the hydrogen-induced TRIP. This study investigates the synergistic assessment of cyclic plasticity damage and hydrogen-induced cracking of transformation-induced plasticity (TRIP) steel subjected to asymmetric strain-controlled fatigue loading. The presence of hydrogen in TRIP steel results in cyclic softening throughout its life, suppressing the hardening behavior induced by martensitic transformation. The mild hardening observed in hydrogen-free specimens is attributed to the predominance of hardening induced by martensitic transformation over cyclic softening. The increase in mean strain leads to higher mean stress relaxation, suppressing the compressive stress developed during the deformation-induced martensitic transformation phenomenon. Geometrically necessary dislocation (GND) density map and fractographic images substantiate the hydrogen-enhanced decohesion failure mechanism, decreasing fatigue resistance for hydrogen-induced TRIP steel. The failure mechanisms of the fatigue damage due to the ingress of hydrogen atoms in the material are proposed for further insights. [ABSTRACT FROM AUTHOR]

• Cyclic deformation behaviors of distinct heat-treated 42CrMo steels were studied. • Isothermal annealed and annealed 42CrMo steels roughly present cyclic stabilizing features. • Tempered and normalized steels exhibit cyclic softening behaviors. • The dramatic increases in dislocation density, substructures and grain refinement induce cyclic stabilizing. • Dislocation cells and slips through cutting precipitates promote cyclic softening and ratchetting. The aim of this paper is to investigate the cyclic deformation behaviors and microstructure evolution mechanisms of 42CrMo steels (which were machined mechanically from the M42 bolt) with various microstructures obtained under four different heat-treatments, i.e., isothermal annealing, annealing, high-temperature tempering and normalizing treatments. Cyclic stress–strain response curves were measured at the uniaxial constant stress- and strain-controlled cyclic loading with positive mean stress/strain. The tested results show that ratchetting can be induced under cyclic loading with positive non-zero mean stress, causing severer fatigue damage. Moreover, it is found that isothermal annealed and annealed 42CrMo steels roughly show cyclic stabilizing features, but tempered and normalized cases present remarkable cyclic softening characteristics. Compared to the formers, the latter two exhibit serious ratchetting and cyclic deformation evolution, which quickens fatigue fracture. Additionally, the microstructure analyses reveal that cyclic stabilizing behavior is mainly related to the dramatic increases in cross-slip dislocation density, substructures, grain refinement and plastic strain. And, cyclic softening behavior is highly correlated with the dislocation development from low-density patterns (i.e., dislocation lines and tangles) to high-density modes (i.e., dislocation cells), as well as dislocation slip through cutting the precipitates. Therefore, we can conclude that cyclic stabilizing/softening features play significant roles in cyclic deformation behavior i.e., ratchetting of 42CrMo steels based on the different microstructure evolution mechanisms, which should be carefully taken into account for optimizing fatigue properties by considering these factors. [ABSTRACT FROM AUTHOR]

Different micro-regions of weldment are expected to present various cyclic softening behaviours during creep–fatigue interaction (CFI), which has a significant influence on the remaining mechanical properties. The present work aims to investigate the cyclic softening and remaining creep behaviour of different micro-regions of P92 steel weldment using electron backscattered diffraction (EBSD) and nanoindentation technology. CFI tests were performed and interrupted at various pre-set fatigue cycles at elevated temperature of 650 °C and hold time of 180 s, and thereafter EBSD analyses, nanoindentation tests and high-temperature creep tests were conducted on these interrupted specimens. Experimental results reveal that the microstructure of the parent metal and fine austenite grain heat-affected zone, which is adjacent to the intercritical heat affected zone, are sensitive to CFI loading. Notably, the microstructural evolution in intercritical heat affected zone almost reaches saturated after fatigue cycles of 20%Nf. Nanoindentation results uncover that the intercritical heat affected zone exhibits the lowest elastic modulus and microhardness, and with increasing the number of fatigue cycles, the weakest region changes. Accordingly, creep fracture location transits and creep life was degraded. Finally, a fatigue damage parameter based on the microhardness was developed to capture the effects of CFI on remaining creep life.

This paper investigated the microstructure and failure reason of the TP347H/T91 dissimilar steel welded joints using Ni-based filler metal under ultra-supercritical condition. There was a large amount of carbide precipitation and δ-ferrite phase around the microstructure of T91 heat-affected zone. Carbon elements migrated from T91 to the weld and TP347H side during operation. Columnar grain growth was observed in the weld metal near the interface and extends across the interface to the TP347H side. With the extension of operation time, the microstructure of T91 gradually degenerated, and the deposition around the tissue increased. The creep cavity was formed due to the inconsistent deformation between the secondary particles and the matrix under the action of load. The average temperature of T91 was 6 °C higher than that of TP347H, and the peak stress was located in the fine-grain heat influence zone of T91. The cavities were easier to nucleate and expand in the T91 fine-grain heat-affected zone, which led to the formation of a type IV crack making welding joint happened early failure. [ABSTRACT FROM AUTHOR]

• The effect of heat treatment on the fatigue life of 3D printed SS 316 L is investigated. • Increased fatigue life of heat treated specimens is observed. • Heat treatment reduced the Young's modulus of the considered specimen. • Microhardness is increased after cycling while it decreases after heat treatment. • Peak stress vs cycle behavior is suitably predicted using Non-linear Chaboche model. The success of 3D printing relies on developing components with desired strength that heat treatment processes can further improve. While SS 316L is a widely used structural material for several industrial applications, the fatigue behaviour of its 3D-printed version is investigated herein. The low cycle fatigue (LCF) behaviour of heat-treated (HT) 3D-printed SS 316L specimens was carried out and compared with the without heat-treated (WHT) specimens thereof. The heat treatment considerably affected its LCF behaviour, which can be attributed to the microstructural changes post heat treatment. The cyclic softening is observed in both HT and WHT specimens. However, the degree of softening is lower (12.35 %) for HT compared to WHT specimens (25.30 %) and 62 % higher fatigue life for HT compared to WHT specimens. Further, the hardness values obtained are 176 and 169 HV for WHT and HT, respectively, while it is 238 and 223 HV for the same, before and after fatigue tests. Fractography revealed fewer pores and reduced fatigue striations in the HT specimens. Considering the Chaboche non-linear model, finite element modelling was employed to capture the specimens' fatigue behaviour. The proposed model is found to be suitable for predicting the cyclic behaviour of 3D-printed austenitic steels. [ABSTRACT FROM AUTHOR]

• Experimentally observed load ratio effect in three steels disagreed with expectation. • Improvement of the strip yield model for plasticity-induced crack closure (PICC). • Out-of-plane constraint factor in compression used as a material parameter. • Irreversibility of plastic deformation proposed to explain different PICC behaviour. • Cyclic softening/hardening and brittle microcracking proposed as mechanisms. The paper focuses on differences between crack closure obtained by numerical models and by experimental fatigue crack growth rates, namely for three different steels (bainitic steel, predominantly pearlitic steel and additively manufactured austenitic stainless steel). The experimental data revealed a load ratio effect different from that predicted by the most used plasticity-induced crack closure (PICC) models. The term "irreversibility of plastic deformation" was proposed in this work to be used as a material property to estimate how strong the PICC effect would be in a material. Two basic phenomena, which are usually omitted in other models, were considered in the explanation: (i) cyclic softening/hardening, (ii) brittle microcracking at the maximum load. The strip-yield model has the ability of conducting fast and accurate simulations under the variable-amplitude loading. It was demonstrated that the results of this model are practically independent of the material properties. This is caused by the consideration of only monotonic elastic–plastic material properties. To simulate real load ratio effects, the parameter β in the strip-yield model (constraint factor in compression) is proposed to be used as a variable. It enabled a generation of different ratios of monotonic and cyclic plastic zones, which in turn helped to reproduce the crack closure values observed experimentally. Discussion and explanations were provided regarding the material microstructure. The proposed approach considers unequal tensile and compressive yield stresses caused by the different irreversibility of plastic deformation, which explains the dissimilarities in the load ratio effect observed in the investigated materials. It can also improve the accuracy of residual fatigue life estimations. [ABSTRACT FROM AUTHOR]

The Kahramanmaraş Earthquakes that occurred on 6 February 2023 resulted in extensive structural failures, including damages caused by soil liquefaction. This study focused on investigating the excessive settlements observed in buildings along Ataturk Boulevard in the Golbasi district, with a primary emphasis on the toppling failure of the Kayı Apartment. Field exploration, laboratory testing, and bearing capacity analyses were conducted to understand the failure mechanism. Since the foundation soil of Kayı Apartment predominantly consisted of clay, soil liquefaction alone could not explain the observed failure. Instead, the cyclic softening of clayey soil underground shaking was identified as a significant factor contributing to the damage. As one of the rare case histories documenting a failure caused by cyclic softening, the results of the study also shed light on the excessive displacements observed in nearby buildings with fewer stories that managed to withstand the earthquake without experiencing complete bearing capacity failure. [ABSTRACT FROM AUTHOR]

Softening behavior of lath martensitic steels is related to the coarsening of laths and dislocation evolution during cyclic deformation. Involving the physical mechanism, we developed a dislocation-based model to study the cyclic plastic response for lath martensitic steels. For a block, we proposed an interfacial dislocation evolution model to physically present the interaction between mobile dislocations in the block and interfacial dislocations by considering the coarsening mechanism of the laths. Moreover, the evolution behavior of backstress caused by dislocation pile up at the block boundary has been considered. Then, a hierarchical model based on the elastic-viscoplastic self-consistent (EVPSC) theory is developed, which can realize the scale transition among representative volume element (RVE), prior austenite grains (PAGs) and blocks. According to the proposed model, the effective mechanical responses including the cyclic hysteretic loop and peak stress at different cycles for lath martensitic steel have been theoretically predicted and investigated. [ABSTRACT FROM AUTHOR]

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.

The tensile, creep, fatigue and creep-fatigue tests of the nickel-based superalloy GH4169 were carried out. According to the deformation characteristics of GH4169 alloy, the Ohno-Karim kinematic model (O-K model) can be used to describe the tensile behavior. The creep constitutive model presented in this paper can be used to predict the three-stage creep characteristics of the GH4169 alloy. The modified Ohno-Karim kinematic hardening model, combined with an isotropic hardening model, can well predict the cyclic softening behavior of the material under symmetric loads and the mean stress relaxation behavior under asymmetric loads. Based on the modified Ohno-Karim kinematic hardening model, isotropic hardening model and creep constitutive model, a non-unified constitutive model was established. The creep-fatigue behavior of the GH4169 alloy under symmetric and asymmetric loads is simulated by using the non-unified constitutive model. The simulation results are very close to the experimental results; however, the prediction results of the time-dependent relaxation load are relatively small.

Motion capture is the current gold standard for assessing movement of the human body, but laboratory settings do not always mimic the natural terrains and movements encountered by humans. To overcome such limitations, a smart sock that is equipped with stretch sensors is being developed to record movement data outside of the laboratory. For the smart sock stretch sensors to provide valuable feedback, the sensors should have durability of both materials and signal. To test the durability of the stretch sensors, the sensors were exposed to high-cycle fatigue testing with simultaneous capture of the capacitance. Following randomization, either the fatigued sensor or an unfatigued sensor was placed in the plantarflexion position on the smart sock, and participants were asked to complete the following static movements: dorsiflexion, inversion, eversion, and plantarflexion. Participants were then asked to complete gait trials. The sensor was then exchanged for either an unfatigued or fatigued plantarflexion sensor, depending upon which sensor the trials began with, and each trial was repeated by the participant using the opposite sensor. Results of the tests show that for both the static and dynamic movements, the capacitive output of the fatigued sensor was consistently higher than that of the unfatigued sensor suggesting that an upwards drift of the capacitance was occurring in the fatigued sensors. More research is needed to determine whether stretch sensors should be pre-stretched prior to data collection, and to also determine whether the drift stabilizes once the cyclic softening of the materials comprising the sensor has stabilized.

In this paper, a high‐capacity fabricated hollow section is proposed as a lightweight replacement for concrete‐filled composite members which are commonly used as beam‐columns in seismic zones. The lateral cyclic performance of the fabricated hollow beam‐column comprising of mild steel plates (grade 250) and high strength steel (HSS) tubes (grade 750) is investigated. A finite element model is developed and validated against available experimental data. To incorporate the cyclic hardening and softening of both steel materials, a combined plasticity model obtained from low‐cycle material tests is incorporated in the finite element model. A comprehensive sensitivity analysis is undertaken to propose an optimum slenderness design for the thin‐walled steel elements with ductile failure. While the high strength steel tube elements can significantly increase the axial and lateral capacity of the hybrid section, the cyclic failure mechanism is governed by mild steel plate elements, rather than a sudden failure in the high strength steel tubes. [ABSTRACT FROM AUTHOR]

In aero-engines, cyclic softening can cause premature fracture of components, which is regarded as a challenge for the security of titanium alloy parts in service. In this work, the cyclic deformation behavior of LDEDed and β-forging TC17 titanium alloys with different morphology of α phase was explored, cyclic softening mechanism was investigated by using transmission electron microscopy (TEM) observation of dislocation morphology. The experimental results demonstrate that cyclic softening occurs when the total strain range increases to 2.0 %. The LDEDed specimen has a smaller stress amplitude and a larger plastic strain during cyclic softening than wrought specimen. In the softened deformation microstructure of LDEDed specimen, the size of α p decreases with the increase of strain, but the size and shape of α p and α s in wrought specimen do not change much. The main dislocation forms that dominate the cyclic softening in the α phase and the β matrix are slip bands and dislocation tangles, respectively, and the slip band in the α phase cannot pass through the secondary phase in the β matrix for slip transfer. Under the same strain, the lower dislocation density in the LDEDed microstructure is due to the larger directional back stress produced by the forward loading, which causes the dislocation rearrangement and annihilation under the reverse loading. • Cyclic deformation behavior of LDEDed and forging TC17 is compared. • Critical strain of cyclic softening/hardening of TC17 titanium alloy is obtained. • Cyclic softening mechanism related to α phase morphology are clarified. [ABSTRACT FROM AUTHOR]

• Investigated the effect of SPIF on the LCF performance of AA6061-T6. • Base metal exhibit a saturated cyclic response for all strain amplitudes. • Saturated response at low strains but cyclic softening at high strains in SPIF parts. • LCF life of SPIF components relies on overlap amount, scallop height, and texture. • Scallops induce stress concentration and promote crack initiation at the valleys. The current study focuses on the influence of single point incremental forming (SPIF) process on the low-cycle fatigue (LCF) performance of AA6061-T6 sheet. The base material (BM) displays a saturated cyclic response for all strain amplitudes. The SPIF-formed specimens show similar behavior, especially at lower strain amplitudes; however, they exhibit significant cyclic softening at higher strain amplitudes. The LCF life of SPIF specimens (considered in rolling and transverse directions) for all strain amplitudes is slightly reduced due to overlapping/repeated sheet deformation and scallops formed during tool movement. Overlapping affects strain accumulation during SPIF, and scallops affect strain accumulation during cyclic loading and act as a stress concentration site. The texture alteration during the incremental forming also influences the ductility and the subsequent LCF life of the SPIF components. [ABSTRACT FROM AUTHOR]

Multiaxial stress states frequently occur in technical components and, due to the multitude of possible load situations and variations in behaviour of different materials, are to date not fully predictable. This is particularly the case when loads lie in the plastic range, when strain accumulation, hardening and softening play a decisive role for the material reaction. This study therefore aims at adding to the understanding of material behaviour under complex load conditions. Fatigue tests conducted under cyclic torsional angles (5°, 7.5°, 10° and 15°), with superimposed axial static compression loads (250 MPa and 350 MPa), were carried out using smooth specimens at room temperature. A high nitrogen alloyed austenitic stainless steel (nickel free), was employed to determine not only the number of cycles to failure but particularly to aid in the understanding of the mechanical material reaction to the multiaxial stresses as well as modes of crack formation and growth. Experimental test results indicate that strain hardening occurs under the compressive strain, while at the same time cyclic softening is observable in the torsional shear stresses. Furthermore, the cracks’ nature is unusual with multiple branching and presence of cracks perpendicular in direction to the surface cracks, indicative of the varying multiaxial stress states across the samples’ cross section as cross slip is activated in different directions. In addition, it is believed that the static compressive stress facilitated the Stage I (mode II) crack to change direction from the axial direction to a plane perpendicular to the specimen’s axis.

In this study, mem-springs and mem-dashpots from a newly introduced family of mem-models are used as fundamental building blocks in hysteresis modeling. The usefulness of such assemblies of mem-models is investigated for both simulation and system identification. First, numerical simulations demonstrate the general capability of these models to describe strain ratcheting behaviors. Next, system identification is addressed by extending the concepts of mem-springs to include linear and nonlinear springs and those of mem-dashpots to include linear and nonlinear dashpots. A reconfigurable device made of steel and/or shape memory alloy wires and wire ropes provides a fitting test for the proposed mem-model-based family. A system identification procedure corroborated by physical insights is proposed and the results are validated using physics-based analysis. Multilayer feedforward neural networks are used for static nonlinear function approximation. The model class and system identification procedure proposed here are shown to extract similarities and dissimilarities among different configurations of the device by quantifying the spring and damping effects. [ABSTRACT FROM AUTHOR]

The present work aims to assess the influence of hydrogen on strain controlled low cycle fatigue (LCF) properties of P91 steel. Specimens were electrochemically charged using H 2 SO 4 solution, subsequently uniaxial tensile and LCF tests were performed at ambient temperature. An increase in strength and reduction in elongation are noticed for hydrogen charged samples relative to as received or uncharged specimens. Hydrogenated specimens depict drastic reduction in fatigue life as compared to the uncharged specimens. Irrespective of imposed strain amplitude, P91 steel show cyclic softening nature throughout its life. The peak stress amplitude and rate of cyclic softening for hydrogenated specimens are found to be more than the as received specimens at all strain amplitudes. The magnitude of proportional limit from master curve depicts that as received specimen exhibit near Masing behavior whereas hydrogenated specimens reveal non-Masing behavior. Local misorientation analyses carried out by electron back scattered diffraction technique are correlated with the evolution of local plastic strain and substructural development. The fracture morphology of tensile test transformed from dimple failure for uncharged specimen to quasi cleavage fracture for hydrogenated specimens. Finite element simulation considering Chaboche kinematic hardening rule is utilized to simulate the cyclic stress-strain behavior of as received and hydrogenated specimens. • Large reduction in cycles to failure for hydrogenated over as received P91 steel. • Hydrogen charged P91 steel depict higher rate of cyclic softening. • Misorientation maps show higher accumulation of fatigue damage in hydrogenated P91. • Stabilized cyclic stress response of P91 is simulated by kinematic hardening model. • Transgranular quasi cleavage fractures were evident in hydrogen charged specimens. [ABSTRACT FROM AUTHOR]

A series of pure-shear cyclic tests were performed on VHB™4910 dielectric elastomer at various temperatures, and the investigations were focused on the influence of temperature variations on the cyclic deformation of the elastomer and the influences of loading level and loading rate on the temperature-dependent cyclic deformation. The results demonstrate that the temperature-dependent deformation of VHB™4910 dielectric elastomer primarily manifests its viscoelasticity, while the hyper-elastic part shows comparatively weaker temperature-dependence. At different temperatures, the elastomer exhibits a cyclic softening during the strain-controlled cyclic loading. Both the peak stress and residual strain reduce with increasing the temperature and reach a stable state more rapidly at higher temperatures. A more pronounced temperature-dependence is exhibited in the stress response of the elastomer at a high loading rate; while a lower rate-dependence is observed at a high temperature. The ratchetting of the elastomer presented in the stress-controlled tests becomes more prominent as the temperature increases, but the rate-dependence of ratchetting is weaker at higher temperatures. Moreover, the temperature-dependence of ratchetting is more significant at a high-stress level as compared to that at a low-stress level. • Pure-shear cyclic deformation experiments were conducted on VHB™4910 DE at different temperatures. • Cyclic deformation of VHB™4910 DE presented a significant temperature-dependence at a high loading rate. • A pronounced rate-dependence of VHB™4910 DE's cyclic deformation was exhibited at a low temperature. • The temperature/rate dependence of cyclic deformation decreased with increasing the number of cycles and had an interactive effect. [ABSTRACT FROM AUTHOR]

Single pile foundation has been widely used for the most offshore wind turbines to resist the cyclic loading resulted from the wind and wave. The cyclic loading can result in the degradation of the shear strength and the stiffness of clays surrounding piles. This study proposed a practical framework for assessing the effect of cyclic softening of clays on the lateral response of piles. A 3D finite difference model was developed for the pile – soil interaction under cyclic loading and then verified by full-scale monotonic and cyclic lateral load tests conducted on the single pile in clay. The cyclic softening behavior of undrained clays was described by a nonlinear dynamic softening constitutive model developed by the authors. The effect of the loading type (i.e., one-way and two-way loading) and the loading amplitude on the cyclic lateral response of piles was investigated. Moreover, a prediction function was proposed and was demonstrated to be effective in estimating the evolution of cumulative peak lateral displacement of cyclically loaded single piles in clays. The proposed practical framework provides an alternative for predicting the cyclic response of single piles in clays based on the monotonic lateral load test of piles. • A nonlinear dynamic softening constitutive model was used to describe the cyclic softening behavior of undrained clays. • A 3D numerical model was developed for the lateral response of piles in clay under cyclic loading. • A prediction function was proposed to estimate the evolution of the cumulative peak lateral displacement of piles in clays. • The proposed practical framework provides an alternative for predicting the cyclic response of single piles in clays. [ABSTRACT FROM AUTHOR]

• Softening during interrupted low cycle fatigue at 650 °C was studied in 10%Cr steel. • Decrease in the friction stress is attributed to dislocation annihilation. • Gradual lath widening occurs due to disappearance of lath boundaries. • Additional precipitation on lath boundaries provides a more stable back stress. • Steady-state stage at 1–10 cycles is attributed to additional precipitation. The correlation between cyclic softening and microstructural evolution using the interrupted low cycle fatigue (LCF) tests with a low strain amplitude of ±0.2% at 650 °C in a 10% Cr steel was studied. Cyclic softening was analyzed in terms of the friction and back stresses caused by short-range and long-range obstacles, respectively. Softening was mainly attributed to a decrease in the friction stress due to the dislocation annihilation. Whereas the back stress remained more stable due to additional precipitation on the lath boundaries, despite the intense lath widening caused by the disappearance of lath boundaries. [ABSTRACT FROM AUTHOR]