Ratcheting Deformation Research Articles

Temperature-dependent damage of magnesium alloy with ratchetting–fatigue interaction effects: Experiments and mesomechanical theory

Fatigue failure is a significant concern for magnesium (Mg) alloy components. However, fatigue damage mechanisms of Mg alloys, particularly in the case of ratchetting–fatigue interaction at elevated temperatures, are still not well understood. In this paper, we combine experiments and theoretical analysis to investigate the high-temperature damage mechanisms of the extruded AZ31 Mg alloy, specifically focusing on the effects of ratchetting–fatigue interaction. We reveal a distinct demonstration of damage, namely the formation of microvoids in the alloy due to significant ratchetting deformation, defined as ratchetting damage. Notably, this ratchetting damage is more prevalent at higher temperatures. Considering the mesomechanics-based energy mechanisms associated with grain boundaries (or twin boundaries), a mesomechanical damage model is established to capture the ratchetting damage under elevated temperatures and large ratchetting deformations. This model can reasonably simulate the intricate process of damage evolution and predict the critical condition of microvoid or microcrack formation. This work has the potential to serve as a theoretical tool for the safety design of structures made from Mg alloys under complex mechanical and thermal conditions.

Influence of build direction on the ratchetting-fatigue interaction of heat-treated additively manufactured 316L stainless steel

This study has investigated the cyclic deformation and ratchetting-fatigue interaction of 316L stainless steel fabricated by laser powder bed fusion (LPBF) and subjected to a 900 °C/2 h post-heat treatment, considering both the vertical and horizontal orientations. Strain- and stress-controlled low cycle fatigue (LCF) tests are conducted to explore the cyclic deformation and the effects of the mean stress and stress amplitude on the fatigue life and ratchetting deformation. A detailed analysis of the cyclic deformation mechanism is conducted through X-ray microtomography and transmission electron microscope (TEM) observations. The results show that post-heat treated LPBF 316L steel exhibits cyclic hardening followed by a non-saturated cyclic softening stage. This behavior is attributed to the evolution of dislocation density and dislocation patterns, and the formation of surface cracks. The ratchetting strain and its rate are sensitive to the mean stress and stress amplitude. The fatigue lives of vertically built specimens are slightly higher under strain-controlled loading conditions, but significantly lower under stress-controlled loading conditions than those of horizontally built ones. Furthermore, ratchetting deformation can promote fatigue damage, resulting in a reduction in fatigue life.

Experimental study on temperature-dependent ratchetting-fatigue interaction of extruded AZ31 magnesium alloy

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.

A micromechanical cyclic constitutive model for ratchetting deformation of polymeric composites with imperfect interface

AbstractIn this study, a micromechanical cyclic constitutive model for accurately describing the ratchetting deformation of polymeric composites is proposed. The model is based on the mean‐field homogenization method and incorporates the nonlinear viscoelastic‐viscoplastic (VE‐VP) cyclic constitutive model of the polymeric matrix and the imperfect interface. First, the nonlinear VE‐VP model was linearized using the incremental affine method and then was introduced into the homogenization process. The imperfect interface was represented using a linear spring model. A modified Mori‐Tanaka model was developed to incorporate the nonlinear VE‐VP model and imperfect interface into the micromechanical constitutive model. To implement the proposed micromechanical model, a numerical algorithm was developed. The model is capable of capturing the influence of the imperfect interface on the ratchetting deformation of polymeric composites. Furthermore, it is applicable to simulate various loading histories, including monotonic tension, stress relaxation, creep, and uniaxial ratchetting loads. The proposed model was validated by comparing its simulations with those obtained from the full‐field finite element method (FEM) model. The proposed model can predict the ratchetting deformation of polymeric composites and promote the practical application of these material in structures.Highlights A micromechanical cyclic constitutive model is developed to describe the ratchetting deformation of polymeric composites with imperfect interface. Nonlinear viscoelastic‐viscoplastic deformation of polymeric matrix is considered. A modified Mori‐Tanaka method is employed to incorporate the nonlinear constitutive model and imperfect interface into the process of homogenization. Good agreement between simulations of the proposed model and the full‐field FEM model under various loading histories.

Numerical simulation and experimental validation of ratchetting deformation of short fiber-reinforced polymer composites

This paper proposes a micromechanical model based on the finite element method (FEM) to analyze the ratchetting deformation of short glass fiber-reinforced polymer composites (SFRPCs). To reasonably consider the complex mechanical characteristics of the polymeric matrix under asymmetrical stress-controlled cyclic loading, a damage-coupled nonlinear viscoelastic-viscoplastic (VE-VP) cyclic constitutive model was constructed. The cyclic constitutive model of the matrix was introduced into the FEM model utilizing a user-defined material, and the corresponding experimental validation of the matrix was conducted. Subsequently, the cyclic constitutive model was introduced into the analysis of the micro-FEM model of the composites. The random orientation was considered to generate fibers in the FEM. Based on the analysis of the microscale representative volume element (RVE) model, the effects of different fiber content and cyclic peak stress on the ratchetting of SFRPCs were studied in detail. After validation, the effects of the fiber length and fiber stiffness on the ratchetting deformation of SFRPCs are discussed based on the developed micro-FEM model. The results show that the increase in the fiber length and fiber stiffness reduced the ratchetting deformation of SFRPCs within a specific content range. The model can assess the ratchetting deformation and damage evolution within the composites to provide more accurate predictions of the ratchetting deformation of SFRPCs and guide the expanded application of SFRPCs in structures.

Microstructure-crystallographic texture and substructure evolution in unpeened and laser shock peened HSLA steel upon ratcheting deformation

ABSTRACT Uniaxial ratcheting behaviours associated with microstructural development of unpeened and laser shock peened ASTM A 588 Grade D High Strength Low Alloy (HSLA) steel were studied in this investigation. The mechanism of plastic deformation and crystallographic texture evolution during ratcheting was also studied. For this, the primary experimental works involved were stress-controlled ratcheting fatigue tests (on unpeened/laser-peened specimens) at room temperature. Followed by this, microstructure and texture evolution on the surfaces and/or cross sections of the deformed specimens, using electron back scattered diffraction (EBSD) were studied in detail. Additionally, the substructural evolution of some selected samples was also studied using transmission electron microscopy (TEM). The results indicated that the average grain size of all the unpeened ratcheted specimens was marginally reduced after fatigue tests. The laser-peened specimen, however, showed a marginal increase in average grain size after ratcheting. Continuous dynamic recovery and recrystallisation (CDRR) during ratcheting was thought to be the cause of the observed reduction in average grain size in unpeened specimens, whereas continuous dynamic recrystallisation (CDRX) was believed to be the controlling factor for marginal accretion of grain size in laser-peened specimens. The plastic deformation of investigated steel was qualitatively explained by the observed dislocation patterns and their evolution.

Geometric discontinuity effect on creep-fatigue behaviors in a nickel-based superalloy hole structure considering ratcheting deformation

Multi-axial stress states are common in the most high-temperature components. In this paper, a ratcheting modified strain energy density exhaustion (RM-SEDE) model is developed to predict creep-fatigue life for hole structures. Afterwards, the numerical procedures are perfected to provide in-depth understandings in cyclic deformation, damage evolution and life prediction under multi-axial stress states. Some abnormal results contrary to uniaxial common sense, in terms of creep-fatigue life distributions and geometrically necessary dislocation accumulations, are well resolved based on the RM-SEDE. Among them, not only life prediction accuracy is improved, but also deterioration mechanism of ratcheting on creep-fatigue damage is explained.

Rate-dependent ratcheting characteristics of high density polyethylene subjected to cyclic pulsating compression

Monotonic and pulsating cyclic compression tests were performed systematically based on high density polyethylene (HDPE). The compressive creep behaviors were further implemented to compare with the ratcheting deformation under the same peak stress level. Monotonic compression experiments reveal that HDPE shows obvious four-stage characteristics which are sensitive to temperature, but independent of the stress rate when the temperature is greater than 80 °C. Interestingly, the strain range during the stage III always starts at 7.4% and ends at 62.6% under various temperatures and stress rates. Moreover, the compressive ratcheting behavior of HDPE significantly depends on temperature T, peak stress σp, stress rate σ˙, stress amplitude Δσ and stress ratio R, especially when T ≥ 80 °C, σp ≥15 MPa, σ˙ ≤0.1 MPa/s and R ≥ 0.8. Additionally, decreasing the stress rate or increasing the temperature will decrease the compressive modulus of HDPE. An approximate linear relationship between the creep strain rate and the ratcheting strain rate under different temperatures and stress ratios was observed. The compressive deformation characteristics of HDPE were compared with that of some other gasket materials, a universal phenomenon is found that the compressive ratcheting evolution with time can be characterized by creep approximately under the same loading peak stress, stress rate and temperature for various gasket materials when the stress ratio is greater than zero, which indicates that the ratcheting deformation for gaskets under cyclic loads may be estimated by the corresponding static creep strain in practical engineering.

Dynamic strain aging-mediated temperature dependence of ratcheting behavior in a 316LN austenitic stainless steel

Tensile tests and ratcheting tests were performed on a 316LN austenitic stainless steel at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, to study the influence of dynamic strain aging (DSA) on deformation behavior and microstructural evolution. The DSA temperature range in present work was determined to be ≥ 400 °C in both uniaxial tests and ratcheting tests. Especially in the ratcheting tests, a sudden drop in ratcheting strain accumulation as well as in steady ratcheting strain rate was evident when the temperature raising from 300 ° to 400 °C. While within the DSA temperature region, the ratcheting behavior was insensitive to the temperature. Microstructural examinations revealed dislocation cells and tangles in the sample ratcheted at 300 °C (free of DSA), while complex dislocation structure in the samples ratcheted at ≥ 400 °C (with DSA) that was consisted mainly by high-density dislocation walls, planar slip bands, and Lomer-Cottrell locks. This clearly indicates a change in predominant ratcheting deformation mechanism from cross-slip at DSA-free temperature region to planar-slip at DSA region. It was further demonstrated that, within the temperature range from 400 °C to 600 °C, the DSA-induced hardening effect balanced with the thermally-activated softening effect and this balance led to temperature-insensitive ratcheting behavior as experimentally observed. Finally, the DSA-affected dislocation structure was quantitatively evaluated in the ratcheted samples, with an aim in correlating to the ratcheting behavior.

Effect of hydrogen on low-cycle fatigue behavior of HRB400 steel under asymmetric cyclic loading

This paper focuses on the study of ductility changes, ratcheting, and low-cycle fatigue behavior of materials after hydrogen charging. Hot-rolled ribbed bar 400 (HRB400) steel was electrochemically charged with H2SO4 solution, and the samples subjected to different hydrogen charging times were subjected to quasi-static tensile and low-cycle fatigue tests under asymmetric cyclic loading. The tests indicated that hydrogen charging changed the initial yield stress of the material, but the ultimate strength remained almost unchanged; hydrogen charging significantly changed the cyclic hysteresis behavior of the material, which increased the elastic range of the stable hysteresis curve; hydrogen charging changed the pattern of ratcheting deformation. The pattern of ratcheting deformation in the later stage was very different from that of the sample without hydrogen charging; hydrogen charging reduced the ductility of the material and resulted in a significant decrease in fatigue life. The fracturing of the hydrogen-charged samples exhibited apparent quasi-cleavage and intergranular fracture characteristics. The non-hydrogen-charged samples had two failure modes (fatigue failure and increased ratcheting deformation resulting in necking instability), whereas the hydrogen-charged samples had only one failure mode of fatigue failure. Based on the experimental analysis, a fatigue life equation that simultaneously considers the effect of hydrogen embrittlement index, mean stress, and stress amplitude is proposed. The equation was used to predict the low-cycle fatigue life of hydrogen-charged specimens under asymmetric cyclic loading, and the error between the results and the measured values was in the two-factor band.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Ratcheting characterization and its effect on low cycle fatigue behavior of DP600 steel sheet under cyclic shear path

For thin-walled metal parts, the effects of shear deformation must be considered. In this study, a device for cyclic shear tests was designed, and a DP600 sheet was used to study the fatigue behavior under a shear path without buckling. Ultimately, based on the results, the ratchet behavior, effect of ratchet deformation on fatigue, and law of ratchet inhibition were analyzed and discussed. The local strain concentration and high dislocation density caused by ratcheting loading led to local grain refinement during strain-controlled fatigue loading, improving the cyclic strength and fatigue life.

Uniaxial ratcheting behavior and molecular dynamics simulation evaluation of 316LN stainless steel

In this study, a series of uniaxial ratcheting experiments under different mean stresses with a constant stress amplitude were performed on a 316LN stainless steel at room temperature. Meanwhile, molecular dynamics (MD) simulation was carried out to reveal the microstructure evolution mechanism at the initial stage of ratcheting deformation at atomic scale. The results showed that the initial stage of ratcheting deformation could be divided into three stages: the stage of beginning (ratcheting strain growth rate (RSGR, dεr/dN) >0.01%), the stage of decreasing ratcheting rate (RSGR at 0.00001–0.01) and the stage of elastic/plastic shakedown (RSGR <0.00001) based on the changes of ratcheting strain rate. The shakedown ratcheting strain was linearly increased with increase of the mean stress in both MD simulation and experiment. And the ratchet deformation at different stages strongly depended on dislocation evolution. At the stage of beginning, the dislocation originated from coincident site lattice (CSL) boundaries and formed pile-up structures at random angle grain boundaries (RAGBs); while at the stage of decreasing ratcheting rate, the dislocation tangled at CSL grain boundaries or inside the grain, the dislocation density gradually increased and tended to be stable. When the given stress was insufficient to produce more plastic strain, the dislocation density balanced dynamically and the ratchet deformation entered the stage of elastic/plastic shakedown.

Influence of Dynamic Strain Aging on Ratcheting Deformation Behavior of SA333 Gr-6 Steel

Influence of Dynamic Strain Aging on Ratcheting Deformation Behavior of SA333 Gr-6 Steel

Low cycle fatigue behavior of zirconium–titanium–steel composite plate

The low cycle fatigue behavior of zirconium–titanium–steel composite plate under symmetrical and asymmetric stress control was studied. The effects of mean stress and stress amplitude on cyclic deformation, ratcheting effect and damage mechanism were discussed in detail. The results show that under symmetric stress control, the forward ratcheting deformation is observed. Under asymmetric stress control, the ratcheting strain increases rapidly with mean stress and stress amplitude increasing. Under high stress amplitude, the influence of mean stress is more significant. In addition, by studying the variation of strain energy density, it is found that the stress amplitude mainly promotes the fatigue damage, while the mean stress leads to the ratcheting damage. In addition, fracto- graphic observation shows that the crack initiates in the brittle metal compound at the interface, and the steel has higher resistance to crack propagation. Finally, the accuracy of life prediction model considering ratcheting effect is discussed in detail, and a high-precision life prediction model directly based on mean stress and stress amplitude is proposed.

Investigation on the Effect of Asymmetric Cyclic Loading on Ratcheting Deformation and Bulk Texture Development in HSLA Steel

Ratcheting deformation in association with bulk texture evolution in HSLA steel is studied in this investigation. The cyclic deformation behaviors and damage modes of the specimens are obtained at room temperature, employing asymmetric stress-controlled tests under various combinations of mean stress and stress amplitudes. Bulk texture measurements of the specimens are done adopting an X-ray diffraction analysis. The results exhibit an increase in strain accumulation from 0.23 to 30.61% associated with a sharp reduction in ratcheting life from 3320 to 880 cycles by the increment in mean stress/stress amplitude. A continuous cyclic softening until the final failure is observed for all the ratcheting tests. It is noticed that variation of mean stress is more detrimental than stress amplitude in controlling strain accumulation while the effect of stress amplitude is more pronounced than mean stress for the reduction of ratcheting life. The fracture surface morphology indicates typical striation formation in the crack propagation region along with overload failure. The bulk texture studies indicate that there is a transition between ‘ϒ’ fiber and ‘ζ’ fiber during all loading conditions. The texture intensity is increased from 5.71 to 11.68 with progressive ratcheting strain accumulation. Here also, influence of stress amplitude is more significant than the mean stress.

Ratcheting deformation and fatigue of surface treated ZK60 magnesium alloy

Three surface treatment methods, laser shock penning (LSP), micro-arc oxidation (MAO) and a composition of LSP and MAO, are used for the preparation of modified layers at the surface of two orientation (ED and TD) specimens made by extruded ZK60 magnesium alloy. The ratcheting deformation behavior of different specimens under various stress-controlled loading was investigated. The results show that the ratcheting deformation under different loading conditions comes from the deformation mechanism of four modes: dislocation slip, twinning/detwinning transfer to dislocation slip, residual dislocation-retwinning collaboration, and residual twins-dislocation slip interaction. Considering the multi-mode ratcheting effect, a modified energy-based fatigue model is used to predict the fatigue life under various loading conditions.

Chemo-mechanical analysis of ratcheting deformation in silicon particle electrode under cyclic charging and discharging

Evidences have accumulated that high capacity silicon electrode will expand/shrink dramatically during cyclic charging/discharging operation. In this process, the electrode may undergo elastic-inelastic deformation, and cyclic asymmetric behavior of tension and compression can naturally result in the electrode ratcheting deformation. In this paper, a chemo-mechanical semi-analytical model is established to describe the ratcheting behavior of the silicon particle electrode based on concentration-dependent material properties. The results show that inelastic deformation can reduce the rapid increase of the stresses and further reduce the possibility of surface crack. However, the accumulated irreversible inelastic strain may become one of the important factors for mechanical degradation and even electrode failure. The influence of hydrostatic stress on electrode ratcheting behavior is discussed in detail. It can decrease the concentration gradient and stress level, thereby reducing the inelastic region and ratcheting deformation, which will prolong the predicted lifetime of the electrode. The diffusion coefficient, particle size and the charging rate have also been investigated. The results reveal that ratcheting deformation may be decreased by the high diffusivity, the small particle size and slow charging strategy, which will improve the mechanical stability and capacity of the electrode. Meanwhile, interparticle contact may worsen the ratcheting deformation. Increasing the electrode porosity appropriately can provide space for silicon particles to expand, which may reduce the ratcheting deformation. The present work gives a new explanation for ratcheting deformation in silicon particle electrode and provides a theoretical basis for guiding one in designing next-generation high capacity lithium-ion batteries.

Biaxial ratcheting analysis based on real behaviors of subsequent yield surface

To develop a plastic constitutive equation for general plastic deformation analysis, it is useful to confirm that the constitutive equation is viable for analyzing mechanical ratcheting in which a small strain accumulates along with cyclic loading. Many constitutive models and analyses on mechanical ratcheting have been proposed. However, most models do not consider distortion and rotation of the subsequent yield surface observed in the process of ratcheting deformations. In this study, behaviors of both the subsequent yield surface and the ratcheting deformations have been experimentally evaluated for the initially isotropic metals of SUS316FR stainless steel, 6-4 brass and S25C mild steel, which were subjected to biaxial mechanical ratcheting. Simulation analysis of biaxial ratcheting was conducted based on real behaviors of the subsequent yield surfaces such as swing rotation, translation, and distortion observed in ratcheting, and the two-surface hardening rule. The reliability of the proposed plastic constitutive model was verified by comparing the numerical results with the corresponding experimental results.

Experimental study on uniaxial ratchetting-fatigue interaction of extruded AZ31 magnesium alloy with different plastic deformation mechanisms

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.