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Fatigue life is a critical property of structural materials for ensuring public safety. While the reversibility of deformation mechanisms under cyclic loading can greatly enhance fatigue life, the evolution of this reversibility during continuous fatigue has not received much attention, despite its significant impact. This study used in-situ neutron diffraction to analyze the evolution of multiple deformation mechanisms activated during the early stage of low-cycle fatigue at ±1 % strain amplitudes in fully austenitic Fe-17Mn-0.5C steel. In the steel with near-zero Δ G γ → ϵ controlled by carbon concentration, the leading partial dislocations move in { 111 } γ grains during tension and in { 200 } γ grains during compression, creating stacking faults. Under cyclic loading, these mechanisms exhibit reversible stacking faults behavior due to the activation of trailing partial dislocations, ultimately leading to the reversibility of both twinning-induced plasticity and transformation-induced plasticity. As the number of fatigue cycles increases, stacking faults become more prominent, while the contribution from twinning faults diminishes. The pronounced stacking faults cause work hardening at the intersections of newly formed ε-martensite plates and twinned areas, thus accelerating phase transformations that require higher critical resolved shear stresses. However, the negative Δ G γ → ϵ and increased stability of ε-martensite during fatigue cycles result in a quasi-reversible martensitic transformation behavior with an increase in residual ε-martensite. This work provides insights into enhancing a fundamental understanding of fatigue mechanisms by demonstrating the unusual quasi-reversibility characteristics of high-Mn steel and highlighting its potential as a fatigue-resistant structural material. [Display omitted] • In-situ neutron diffraction was used on Fe-17Mn-0.5C steel under low-cycle fatigue. • Reversible stacking faults cause reversible TWIP and TRIP effects. • Stacking faults increase with cycles; twinning faults decrease. • Work hardening from stacking faults accelerates phase transformations requiring higher stress. • Negative Δ G γ → ϵ and stable ε-martensite lead to quasi-reversible phase transformation. [ABSTRACT FROM AUTHOR]

During the service, turbine blades and vanes of aero-engine and gas-powered turbines sustain surface-initiated damage owing to the synergistic effect of cyclic stresses, high temperature, and oxidizing environment. As a result, oxidation-fatigue interaction curtails the life of these components. This research investigates the oxidation-fatigue damage mechanisms of CM 247 DS LC, at 850 °C for strain amplitudes (Δε/2) of 0.5–0.8 %. It is observed that the micro-mechanism of fatigue-oxidation interactions at 850 °C is a complex phenomenon involving multiple competing mechanisms acting simultaneously. Both the distinguishing damage features and oxygen ingress affect the strain localization and are attributed to the divergence in fatigue behavior. The strain is more uniform at Δε/2 of 0.5 % in as-received (H0) samples, accounting for the highest fatigue life. Compared to the cyclic heat-treated (H500) samples, the strain localizations during low cycle fatigue deformation are more effectively alleviated in H0 samples, resulting in the highest fatigue life. [ABSTRACT FROM AUTHOR]

In this paper, full-stage low cycle fatigue failure mechanisms of nickel-based single crystal superalloy with [001], [011] and [111] orientation at 530 °C were systematically studied by employing acoustic emission (AE), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). [001], [011] and [111] specimens show cyclic stability, slight cyclic hardening, and obvious cyclic hardening, respectively. Cyclic stability response of the [001] orientation is related to short dislocation multiplication in early stage and dislocations cutting the γ′ during crack propagation, finally failure occurs in the form of extended steps. Moreover, long curving and immovable dislocations accumulate and tangle in the γ-channel during crack initiation in the [011] specimen and cross-slip motion of dislocations during crack propagation, resulting in slight cyclic hardening, and ultimately cleavage-like fracture occurs. With respect to the cyclic hardening in the [111] specimen, short dislocations pile up at the γ/γ′ interface in the early stage, the subsequent formation of dislocation networks and parallel dislocations further reduce the probability of dislocation interactions among different slip systems, ultimately fracturing along the {111} crystallographic plane. Revealing the differences in full-stage failure mechanisms among the three orientations lays a solid theoretical foundation for applications. [ABSTRACT FROM AUTHOR]

The past studies in the low cycle fatigue (LCF) behavior of a γ′ strengthened nickel-based superalloy SU-263 were restricted to a maximum temperature of 923 K, though their service conditions far exceed this limit. In the present work, the LCF behavior of SU-263 is investigated at 1023 K with strain amplitude varying from ± 0.3% to ± 0.8% at 1023 K. During fatigue, the alloy displayed initial cyclic hardening followed by softening. The fatigue life decreased drastically with increasing strain amplitude. The alloy depicted gradual initial hardening at low strain amplitudes and sharp initial hardening at higher strain amplitudes, along with extensive softening until fracture. Significant increases in slip band density, stacking faults, dislocation network formation, and dislocation-dislocation and dislocation-precipitate interactions were identified as the deformation mechanisms responsible for cyclic hardening. In contrast, dislocation annihilation and shearing of γ′ precipitates were found to control cyclic softening. Dislocation-precipitate interactions were associated with looping at low strain amplitudes, while precipitate shearing occurred at high strain amplitudes. The alloy exhibited a tendency bilinear strain-life behavior in the Coffin-Manson plot with the corresponding shift in the fracture mode at ± 0.5% strain amplitude. At strain amplitudes below ± 0.5%, the alloy showed a mixed mode of failure, predominantly transgranular in nature, while at high strain amplitudes, the failure becomes predominantly intergranular. [Display omitted] • SU-263 exhibited a bi-linear Coffin-Manson behavior at 1023 K. • Transition from transgranular to intergranular mode of failure resulted in dual slope. • Increasing strain amplitude, shifted γ′ - dislocation interaction from looping to shear. • The width of the stacking fault increased with increasing strain amplitude. [ABSTRACT FROM AUTHOR]

Thermo-mechanical creep-fatigue (TMCF) behaviors of a TiAl alloy were investigated by conducting various creep, creep-fatigue, and thermo-mechanical tests and performing detailed microstructural analysis. A new creep model was proposed to unify primary strain hardening and tertiary accelerating damage effects. The TMCF failure of TiAl alloys is accompanied by the crack initiation from surface oxides/carbides and mixed transgranular and intergranular cracking. It revealed that introducing thermo-mechanical cycles significantly reduces the creep damage rate during the creep dwell stage and the fatigue process, i.e., the creep-delaying effect, which is attributed to the non-proportional strengthening effect induced by thermal cycles. A life model based on the average minimum creep strain rate was proposed to predict the creep-dominated TMCF life. This model incorporates the complex creep-fatigue interactions on creep damage and demonstrated good agreement with experimental results under varying loading conditions. The present work provides new insights into understanding the creep-dominant thermo-mechanical damage evolution of TiAl alloys. [ABSTRACT FROM AUTHOR]

In metal additive manufacturing (MAM), microstructural properties such as texture, residual stresses, and dislocation density have emerged as key factors ruling the resulting mechanical performances. In this study, cylindrical AISI 316L specimens, fabricated with laser powder bed fusion (LPBF), were tested under cyclic elastoplastic (EP) deformation using a constant strain amplitude to highlight the evolution of residual stresses (RS), dislocation density and texture with increasing number of EP cycles, N , across the hardening-softening (H–S) transition stage, in the attempt to find correlations between relevant microstructural parameters and macroscopic properties. The structural and microstructural analysis is carried out through whole powder pattern modeling (WPPM) of neutron diffraction (ND) data and Electron Back-Scattering Diffraction (EBSD) analysis. The H–S transition is found to occur within 7–9 cycles, with RS fading out already after 5 cycles. Across the H–S transition, the trend of the maximum tensile stress correlates closely with the trend of WPPM-calculated total dislocation density, suggesting a major role of dislocations' characteristics in the evolution of macroscopic mechanical properties. EBSD analysis reveals the rearrangement of geometrically necessary dislocations (GND) into cellular structures, and moderate grain refinement, which are deemed to be responsible for the quick fading of RS in the very early stage of EP loading. ND-based texture analysis reveals a (220) preferential orientation retained throughout the EP tests but with orientation density functions (ODFs) changing non-monotonically with N , suggesting preliminary partial randomization of grains around the deformation axis followed by the recovery of crystallographic anisotropy and more localized ODFs. [Display omitted] • AISI 316L Stainless Steel (SS) samples were fabricated via Laser Powder Bed Fusion (LPBF). • Cyclic elastoplastic (EP) response and microstructural evolution of SS were correlated using Neutron diffraction and EBSD. • Residual stress quickly relaxed through microstructural shifts caused by cyclic EP deformation. • Dislocations formed cellular structures and were reduced in density following the EP deformation. • The hardening-softening behaviour correlates with dislocation density and arrangement, and texture development. [ABSTRACT FROM AUTHOR]

The total axial strain controlled low-cycle fatigue (LCF) behaviour of a wrought nickel-base superalloy Superni 263 has been evaluated at 298 K, 673 K, and 923 K employing the strain amplitudes in the range ± 0.25% to ± 0.80%. All the tests were performed using a triangular wave form at a constant strain rate of 10−3 s−1. The Superni 263 alloy was subjected to a solutionizing treatment at 1373 K/1.5 h followed by water quenching. An ageing treatment of 1073 K/8 h was given to the solutionized samples. The microstructure of the alloy is composed of γʹ in the intragranular regions and discrete M 23 C 6 precipitates on the grain boundaries. Few undissolved (Ti, Mo)C and TiN precipitates were also noticed in the matrix. The strain amplitude and temperature played a significant role in the evolution of macroscopic cyclic stress response and the development of deformation substructure. The alloy displayed serrated flow in the plastic portions of stress-strain hysteresis loops in the tests conducted at 673 K and 923 K, predominantly at strain amplitudes higher than ±0.40%. Several manifestations depicting the occurrence of dynamic strain ageing (DSA) were derived by analysing both the macroscopic and micro-mechanistic information generated. The comparative evaluation of monotonic and cyclic stress-strain curves was conducted and the factors responsible for rapid cyclic hardening during DSA were identified. The fatigue life decreased drastically with increasing temperature, particularly at low strain amplitudes. The strain-life data could be represented well by using Basquin and Coffin-Manson relationships. The slope in Coffin-Manson relationship revealed more negative values under the influence of DSA. The crack initiation and propagation modes at all the test conditions remained transgranular without the indications of interference from creep and oxidation effects. The deformation substructure at elevated temperatures was characterized by the occurrence of co-planar slip, stacking faults, micro twinning and high dislocation density in between the slip bands. [ABSTRACT FROM AUTHOR]

Turbine blades of three different service conditions made by a directionally solidified superalloy were investigated to reveal the reduction in the mechanical properties caused by the microstructural degradation in service. The tensile and low cycle fatigue experiments were carried out using the small coupons sampled from the turbine blades. The microstructural degradation reduces the tensile strength and the fatigue life of the samples from the turbine blades. A quantitative relationship between the microstructural degradation and the residual life reduction was proposed by defining a microstructural damage factor based on the strengthening mechanism of the Ni-based superalloy. The fatigue fracture behaviour was studied with the help of the scanning electron microscopy. The results showed that the eutectic microstructure and carbides are the critical positions where the cracks initiates and propagates. In addition, scanning electron microscopy and Vickers hardness measurements were adopted to characterize the microstructural variation at different positions of the turbine blades. The Vickers hardness has an obvious variation at different positions of the turbine blades after different service exposure conditions, which is closely related to the microstructural degradation including the coarsening, rafting and dissolution of the γ ' precipitates. [ABSTRACT FROM AUTHOR]

Abstract Bainitic steel is applied in many cyclic loading projects due to its excellent comprehensive mechanical properties. In this study, the cyclic hardening/softening behavior and microstructure evolution of high-carbon nano-bainitic steel were investigated to analyze the cyclic deformation mechanism. Results showed that the optimal tempering temperature was determined to be at low temperature. During cyclic deformation, the retained austenite is transformed into martensite according to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) orientation relationships, and deformation twins are generated in the retained austenite. The cyclic deformation of high-carbon steels at a total strain amplitude of 0.6% manifests no obvious cyclic softening behavior after low-temperature tempering, mainly due to the high density of dislocation tangle around martensite and deformation twins induced by cyclic deformation. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the fatigue tests of powder metallurgy (PM) FGH96 were performed at 650 °C with four different types of dwell to study its cyclic mechanical behavior and creep-fatigue interactive deformation behavior. As shown in test results, PM FGH96 shows obvious feature of cyclic softening. Load level and dwell time are found to be important factors affecting the velocity and degree of cyclic softening. Some regular patterns are clearly displayed in the curves of creep deformation and stress relaxation, which is speculated to affect the characteristics of FGH96's sensitivity to dwell. Based on the test data and analysis, a practical cyclic plasticity-creep model was established by combining Chaboche model and simplified θ -projection model, and its capability of describing the deformation of creep-fatigue interaction behavior has been proved. The investigation of deformation and cyclic properties will be useful for instructing the establishment of creep-fatigue interactive life predicting model in the future. [ABSTRACT FROM AUTHOR]

Abstract Uniaxial tensile tests and low cycle fatigue tests were performed on 304 stainless steel treated with different torsional pre-strains. The relationship between torsional pre-strain and mechanical properties is established. Furthermore, the changes in the amount of martensite phase transitions are measured during low cycle fatigue tests and after specimen fracture are observed. This is mainly because martensitic transformation is a frequent phenomenon during plastic deformation, which is closely related to the mechanical properties. In this study, it is found that torsional pre-strain increases yield strength and reduces elongation. The ductility of 304 stainless steel decreases with the increase of torsional pre-strain, while its fatigue life does not. As the amplitude of torsional pre-strain increases, the fatigue life of the material changes in s-type. In particular, the fatigue life increases when the torsional pre-strain is between 5% and 8%. From the analysis of the microcosmic distribution of the martensite, it is found that the fatigue life is affected by the combined action of martensite transformation and ductility. The test results suggest that it should skip the relatively dangerous small strain area when improving mechanical properties of 304 stainless steel by way of changing shear pre-strain. [ABSTRACT FROM AUTHOR]

Abstract Combined microscopy and morphology were employed to investigate the effect of ratcheting-fatigue characteristic of GH4169 superalloy. All tests were implemented on a closed-loop servo hydraulic testing machine at 650 ℃. The cyclic deformation behaviors and damage mechanisms of nickel-based superalloy GH4169 were investigated by means of a series of symmetric and asymmetric stress-controlled tests under various stress amplitudes and mean stress levels at 650 ℃. The results exhibit a distinct reduction in fatigue life with the increasing of mean stress and stress amplitude. A continuous cyclic softening until final failure is observed for all the tests. Moreover, the ratcheting effect begins to operate exclusively beyond the threshold value corresponding to a stress amplitude of 600 MPa joint with a tensile mean stress of 50 MPa. A transformation in terms of cracks propagation mode from transgranular to transgranular-intergranular mixed mode has been illustrated. Meanwhile, a higher stress amplitude enhances a more intense slip trace. In addition to the damage caused by high temperature oxidation, the interaction between slip band and grain boundary is the underlying dominant factor on intergranular damage. [ABSTRACT FROM AUTHOR]

Abstract Low Cycle Fatigue crack growth tests have been performed at 250 °C in order to study fatigue crack growth under large scale yielding conditions in a material widely used at high temperature by the automotive industry for cylinder head applications. The studied material was a cast aluminum alloy AlSi7Cu3Mg (close to A319) produced by Lost Foam Casting. Two different microstructures were investigated: one containing large natural pores and another where pores have been removed by Hot Isostatic Pressing (HIP). Fatigue Crack Growth Rates (FCGR) have been measured by in situ surface optical microscopy for different loading conditions all inducing generalized plasticity and compared to assess the influence of pores on the FCGR. In situ observations coupled to post mortem analysis revealed strong crack interactions with both pores and large hard particles on specimen surfaces and in the bulk. FCGR ranging between 10 − 6 and 10 − 4 m/cycle appear to be mainly sensitive to applied strain amplitudes. Although pores promoted secondary crack initiations and crack coalescences, they seemed to have a limited effect on steady-state FCGR which has been analytically modeled using energy densities. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Abstract The fatigue behavior and damage mechanisms of Inconel 718 are investigated and discussed under thermomechanical fatigue (TMF) and isothermal fatigue (IF) test conditions. The experimental results show that cyclic temperature accelerates fatigue degradation compared to isothermal equivalents, and the fatigue life under in-phase (IP) TMF mode is shorter than that under out-of-phase (OP) mode. The S-N data can be fitted by Coffin-Manson equation under LCF condition, and TMF lifetime can be predicted by the model based on hysteresis loop energy with a comprehensive consideration of mechanical and thermal strains. [ABSTRACT FROM AUTHOR]

Low cycle fatigue behaviors of a 3Re Ni-base single crystal superalloy with the [001] and [011] orientations have been investigated. At 760 °C, both the [001] and [011] specimens fracture along crystallographic planes. However, the results also indicate a large difference in fatigue deformation behaviors of these two oriented specimens, which is reflected in different deformation mechanisms. For the cyclic stability behavior of the [001] specimens, uneven cross-slip and tangled dislocations and continuous stacking faults cause local stress concentration in the matrix and have an influence on work hardening, while the stress concentration is reduced by dislocation pairs and stacking faults shearing γ′ phases. With respect to the [011] specimens, high density matrix dislocations, parallel dislocations and secondary cracks increase resistance to dislocation movement, resulting in cyclic hardening. We discovered continuous stacking faults that have been rarely reported in the low cycle fatigue. Also, these findings might bring about substantial insights that fatigue fracture and deformation mechanisms are responsible for low cycle fatigue anisotropy at intermediate temperature. [ABSTRACT FROM AUTHOR]

A series of uniaxial strain-controlled fatigue tests were performed at 650 °C to explore the high-temperature low cycle fatigue (LCF) behavior and corresponding microstructure evolution of a novel 9Cr–3W–3Co martensitic G115 steel. The fatigue test results showed that the steel exhibited a cyclic softening behavior and the degree of softening S increased with the total strain amplitude, which was related to the decrease of dislocation density and subgrain development. Further, statistical analysis of the subgrain size after LCF tests at different strain amplitudes showed that the development of subgrain or cell depended on both the plastic strain level and cycles, among which the effect of plastic strain level is dominant. The variation of degree of softening Δ S was used to make comparison between G115 steel and P92 steels to evaluate the effects of the strain amplitudes on the respective S . The G115 steel experienced the minimum relative increment compared with P92 steel. This resulted from the strengthening by fresh and fine Cu-rich precipitates. The number density of Cu-rich particles increased with the total strain amplitude and that weakened the effect of strain amplitude on degree of softening. SEM was employed to investigate the fracture morphology; representative images of the G115 steel after the LCF tests showing three regions: fatigue crack initiation sites, stable crack growth region, and rupture region. Larger the strain amplitude was, more the crack initiation sites formed, which corroborates the significant decrease in fatigue life with increase in the total strain amplitude. In addition, a modified plastic strain energy approach based non-Masing behavior of G115 steel is proposed to predict the fatigue life. [ABSTRACT FROM AUTHOR]

Pre-deformation influenced the microstructure of extruded magnesium alloys such that the mechanical properties and fatigue resistance of these alloys could be enhanced. In this study, free-end torsion was conducted to change the distribution of dislocation, extension twins and texture components in the extruded magnesium alloy AZ31 rod. Results showed that the extension twins at the center were induced not by shear strain but by the Swift effect. The compressive principal stress on the cylindrical surface greatly contributed to the formation of extension twins at the edge. The softening effect of de-twinning, which exceeded the impeditive effect of dislocations, significantly decreased the yield strength of magnesium alloy rods. Strain-controlled low-cycle-fatigue experiments were conducted on specimens subjected to different pre-torsion deformation conditions. The fatigue resistance of magnesium alloy rods was found to be enhanced by pre-torsion deformation, which was mainly related to the reduction of cyclic mean stress and cyclic stress amplitude, as well as to the slow strain hardening during the cyclic tensile-loading stage. [ABSTRACT FROM AUTHOR]

The cyclic deformation behavior of a fully processed, non-oriented electrical steel sheet is investigated in dependence on loading condition and control mode. Therefore, strain- and stress-controlled fatigue tests are performed to determine cyclic deformation curves in the low- and high-cycle fatigue regimes. With symmetric strain control, continuous cyclic hardening occurs, while in asymmetric stress control with non-zero mean stress, pronounced ratcheting takes place. Depending on the loading condition, the material can exhibit a transitional period with cyclic softening. This effect is present at intermediate amplitudes in the yield point region. It emerges as the dislocation movement is initially inhibited, while at higher amplitudes, cyclic hardening occurs from the very beginning. The evolving dislocation structures are characterized by the predominantly planar slip behavior of the alloy. [ABSTRACT FROM AUTHOR]

In this work, laser direct energy deposition technology was used to produce hybrid manufactured TC17 titanium alloy with wrought substrate. The cyclic deformation behavior in the total strain range of 1.5%, 2.0%, 2.5%, and 3.0% at ambient temperature and dislocation morphology was investigated. The experimental results show that the three specimens represent cyclic softening when the total strain range is large than 1.5%. All the hybrid manufactured specimens are fractured in the deposition zone and the size of the deformated α phase decreases with increased strain. Slip transfer between α phase and β matrix and a small amount of dislocation tanglement in β matrix was observed in the specimen with strain of 1.5% and cyclic hardening. The dislocation density in β phase increased abruptly when the strain range is 2.5%, which promotes cyclic softening. The heterogeneous deformation between the high and low density dislocation zones formed in the α phase of the cyclic softening samples is the cause of the deformation and fracture of the α phase. The tension-compression asymmetry under forward and reverse loading revealed the Bauschinger effect in specimens with total strain exceeding 1.5%. Dislocation tanglement promoted cyclic softening by increasing the local back stress, at the same time, the back stress helped easier dislocation movement during reverse loading. Meanwhile, dislocation rearrangement and annihilation reduced the friction stress and further accelerate cyclic softening. • LCF properties of hybrid titanium alloys is revealed for the first time. • Critical strain of cyclic softening/hardening of hybrid specimens is obtained. • Dislocation motion mechanism and related internal stress analysis are clarified. [ABSTRACT FROM AUTHOR]

The effects of Mg addition on the cyclic deformation and fatigue life behaviors of Al-Cu 224 cast alloys were investigated under isothermal low cycle fatigue (LCF) at 300 °C and out-of-phase thermo-mechanical fatigue (OP-TMF) in the temperature range 60–300 °C. The results show that all tested alloys experienced cyclic softening during both LCF and OP-TMF, whereas the softening ratio of Mg-containing alloys was lower than that of the base alloy free of Mg because of the lower coarsening rate of θ'-Al 2 Cu precipitates. Under both LCF and TMF, near-surface porosity and brittle intermetallic particles were considered sources of crack initiation. TMF lives were inferior to LCF lives under the same strain amplitude because of the combined damage effect of the cyclic mechanical and thermal loadings. Under LCF, the addition of Mg enhanced the fatigue performance because of the co-existing θʹ and θʺ precipitates and the higher thermal resistance of θʹ; under TMF, it led to a slight reduction in fatigue performance. The hysteresis energy model was successfully applied to predict the LCF and TMF lifetimes. The predicted results agree well with the experimentally measured LCF and TMF lifetimes. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) behavior of modified 9Cr–1Mo steel is studied at 300 °C, in the domain of dynamic strain aging (DSA), over a wide range of total strain amplitudes (∆ε t /2) from ± 0.25% to ± 0.50%, at two different strain rates of 10 −2 s −1 & 10 −3 s −1 . Inverse effect of strain rate is observed on LCF behavior that is increase in the cyclic stress response, reduction in fatigue life and decrease in plastic strain amplitude with decrease in strain rate. Irrespective of strain rate and strain amplitude there is cyclic softening till failure. High dislocation density and bowing of dislocations is observed at low strain rate of 10 −3 s −1 . The observed cyclic softening, irrespective of strain amplitude and strain rate, is attributed mainly to recovery of dislocations and formation of cell structure. The increase in the rate of fatigue crack growth at the strain rate of 10 −3 s −1 is associated with DSA. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850 °C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed. [ABSTRACT FROM AUTHOR]

Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF cycling over a wide range of temperature from 573 to 973 K. DSA was found to be pronounced at 823–873 K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage-II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case. [ABSTRACT FROM AUTHOR]

Strain-controlled low cycle fatigue tests were performed on 60 kg/m 90-UTS plain C-Mn rail steel with fully pearlitic microstructure. Analysis of hysteresis loop shape and properties revealed that the steel deviated from the ideal Masing type material behaviour to a little extent. To evaluate the fatigue damage in rail steel, energy-based analytical approach proposed by Morrow based on Masing hypothesis was employed. The Morrow energy model resulted in reasonably good approximation of both average plastic strain energy and fatigue life in comparison to experimentally observed values. In addition to that the plastic strain energy density (Δ W p ) was correlated to fatigue life (2 N f ) through a simple power law and very well represented by Coffin-Mansion type relationship similar to conventional strain-life (Δε p – 2 N f ) relationship. Finally, the concept of fatigue toughness, an energy parameter, was introduced to life prediction in rail steel which was proved to be a suitable and reliable alternative to strain-life approach in fatigue damage evaluation with high degree of accuracy. [ABSTRACT FROM AUTHOR]

In order to study the effects of ε-Al 3 Ni phase on elevated temperature properties, Al-12Si-0.9Cu-0.8Mg- x Ni alloys with Ni contents of 1.0%, 2.5% and 4.0%, respectively, were prepared by gravity casting and their microstructure, tensile and low cycle fatigue properties at 350 °C were investigated. The results show that the microstrucure mainly consists of α-Al, eutectic Si, ε-Al 3 Ni, δ-Al 3 CuNi, and Q-Al 5 Cu 2 Mg 8 Si 6 phase. Thermodynamic calculation indicates that the ε-Al 3 Ni weight fraction increases with Ni contents while other phase contents are kept constant, and this result is consistent with the microstructure and XRD analysis. With the increase of ε-Al 3 Ni content, the tensile strength at 350 °C increases from 94 MPa to 116 MPa, while the elongation decreases from 2.8% to 2.0%. The Al-Si alloy with 2.5% Ni exhibits the optimal low cycle fatigue property with a fatigue strength coefficient of 198.29 MPa and fatigue strength exponent of − 0.1295. The coarsening of ε-Al 3 Ni phase is obvious with a higher Ni content of 4.0%, and the debonding of coarse ε-Al 3 Ni phase accelerates the propagation of micro-cracks and degrades the fatigue properties. The ε-Al 3 Ni phase is beneficial to the mechanical properties of the alloys and thermally stable during fatigue test at 350 °C. [ABSTRACT FROM AUTHOR]

With the increase of exhaust temperature in automobiles resulting from the stricter environmental and fuel consumption regulations, high temperature fatigue failure of exhaust manifolds is being a significant issue for safety concern. In this research, low cycle fatigue behavior of a Nb-bearing cast austenitic steel with the "Chinese-script" type primary Nb(C,N), was investigated in the temperature range of 600–950 °C. It is indicated that the fatigue lifetime was predominantly dependent on the inelastic strain amplitude, in spite of fatigue temperature. However, the change of deformation substructure from dislocation cells, slip bands to subgrains was observed from 600 °C to 950 °C, which was induced by the increased thermal recovery. Correspondingly, the cyclic stress amplitude changed from cyclic hardening at 600 °C to cyclic softening and cyclic stress saturation at 800 °C and 950 °C, respectively. Besides, strain heterogeneity was also noted between the dendrite core and the interdendritic region, thus regarded as a promotion for crack initiation. The generated data and the understanding will be helpful for alloy design and fatigue lifetime prediction next. [ABSTRACT FROM AUTHOR]

The low-cycle fatigue test and the compression creep-fatigue test of the cast Al-9Si-CuMg alloy for cylinder heads were carried out at high temperature. The cyclic mechanical response characteristics of the material under two kinds of test loads were analyzed. The microstructural evolution process and fracture mechanism of the material in the process of failure were studied, and the damage modes of the material under two kinds of loads were summarized. The results showed that, the stress-strain response (hysteresis loop) of the cast Al-9Si-CuMg alloy exhibited cyclic stability under high temperature low-cycle fatigue loading at 200 ℃, and the crack occurred at the holes and defects. When the temperature was raised to 350 ℃, the voids and defects in the material would no longer be the crack source, and the cyclic stress-strain response of the material would also show the characteristics of continuous softening. When coupled with the effect of compressive creep, the yielding radius and elastic modulus of the cyclic stress-strain response were almost unchanged, but the yield center moved upward significantly. Microscopic observation indicated that, to some extent, the effect of compressive creep at high temperature could prevent the initiation and propagation of the large-sized crack within the material, and prevent the rapid development of damage under low-cycle fatigue loading. The eutectic silicon particles inside the material firstly cracked under the compressive creep-fatigue load and formed many tiny cracks or cavities. These tiny cracks extended in the aluminum matrix and connected with the neighboring cracks until they broke. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) tests were performed on Ti-6Al-4V alloy at room temperature. The fatigue hardening/softening of the Ti-alloy were determined. Both the back stress ( X ) and the friction stress ( σ F ) were evaluated from analysis of fatigue hysteresis loops and their cycle-dependent behaviors during fatigue tests were obtained experimentally. It is shown that the cyclic stress response or the fatigue hardening/softening of the Ti-alloy actually results from the competition effect of the back and the friction stress behaviors during fatigue cycling. A two-slope behavior in both the ε pa -2 N f and the X -2 N f plots was also obtained. The micro-mechanisms associated with the back and friction stress behaviors as well as the two-slope behavior in the ε pa -2 N f and X -2 N f plots were further discussed with the help of OM and TEM observations of fatigue failure specimens at representative strain amplitudes. In this study, the DIC technique was also introduced to characterize quantitatively the cyclic deformation heterogeneity of the Ti-alloy at the grain scale. [ABSTRACT FROM AUTHOR]

The aim of this investigation was to examine the role of accumulation of ratcheting strain on low cycle fatigue behavior of aluminum 7075-T6 alloy. For this purpose, first ratcheting behavior of the alloy was studied using asymmetric stress-controlled low cycle fatigue tests up to 2000 cycles. Post-ratcheting stain-controlled low cycle fatigue tests were done on ratcheted specimens up to failure. Further, stain-controlled low cycle fatigue tests without any previous ratcheting were also conducted up to failure in order to compare the behavior with and without previous ratcheting. A few ratcheted specimens were studied using transmission electron microscope to assess the dislocation features generated due to ratcheting. The results indicated that increase of either stress amplitude or mean stress increased the ratcheting strain accumulated in the material. Ladder-like dislocation structure prevailed in the specimens deformed at lower stress levels while dislocation tangles were predominantly present in specimens deformed at higher stresses. Analysis of post-ratcheting low cycle fatigue behavior showed that fatigue life of the alloy decreased with the increase of previously imposed ratcheting strain. Comparison of low cycle fatigue behavior of the investigated alloy with and without ratcheting indicated that ratcheting strain was detrimental to fatigue life of aluminum 7075-T6 alloy. [ABSTRACT FROM AUTHOR]

In this study, the low cycle fatigue deformation of 316 LN stainless steel has been assessed and compared for relatively low and high temperatures. Low cycle fatigue tests were carried out on the steel at 298 K and 873 K employing strain amplitudes ranging from ±0.3% to ±1.0% at a strain rate of 3×10 −3 s −1 . At both the temperatures, the material exhibited initial hardening followed by gradual softening to the saturation stage and final failure. Comparison of hysteresis loops at saturation revealed that the material exhibited Masing behavior at lower strain amplitudes and non-Masing behavior at relatively higher strain amplitudes at 298 K whereas a reverse trend was observed at 873 K. Manifestation of Masing and non-Masing behavior was reflected in the dual slope cyclic stress- strain relationship. The change from Masing to non-Masing behavior and vice versa depending on the temperature and strain amplitude has been corroborated with the transmission electron microscopic and electron backscattered diffraction investigation. The dislocation substructure was found to depend on the temperature and strain amplitude. At 298 K, pile-up in the planar slip and tendency to develop cell structure was observed at lower and higher strain amplitudes, respectively. However, dislocation pinning resulting from DSA and well developed cell structure was observed at lower and higher strain amplitudes, respectively at 873 K. The fatigue life has been predicted based on the hysteresis energy approach considering both the Masing and non-Masing behavior and it was observed that the non-Masing analysis predicts the fatigue life more accurately than the Masing analysis. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) performance of IN718 superalloy under different heat treatment conditions are investigated with controlled strain amplitudes from 0.6% to 1.0% at 350 °C and 650 °C. Standard heat treatment (ST) and delta-phase aging treatment (DA) are applied to produce two types of precipitation microstructures. Under the same strain amplitude and temperature, the fatigue lives of ST specimens exceed that of DA specimens. And strikingly, ST specimens exhibit cyclic softening behavior in contrast to cyclic hardening behavior for DA specimens. The different stress responses can be rationalized by the dislocation interaction mechanisms with the precipitates. For the ST specimens, the dislocation shearing of γ′′ precipitates governs the microscopic deformation process, while the micrometer-sized δ phases are not shearable but act as the obstacles against dislocation slipping and lead to the cyclic hardening. Our TEM observations clearly show that recrystallization nucleation is prevalent in the DA specimens under 650 °C, which is interpreted by the high dislocation density induced by the δ phases. The results shed new lights on the fatigue microscopic deformation mechanisms of IN718 superalloy. [ABSTRACT FROM AUTHOR]

The low cycle fatigue (LCF) behavior of AZ31B magnesium alloy with fiber texture was investigated in two asymmetric loading manners with strain ratios of R ε =0 (named as T-T loading manner) and R ε =-∞(named as C-C loading manner), respectively. It was found that {10–12}<10–11> extension twinning-detwinning occurred in the samples in both the manners. Although the characteristics of twinning-detwinning alternations in the two manners exhibited little difference, the mechanical behaviors were quite different. Detwinning was induced during tensile unloading in the T-T manner and resulted in an evident Bauschginer effect, and cyclic softening in the tension was observed. The LCF life in this case was 68 cycles. In contrast, cyclic hardening occurred in both the tension and compression in the C-C manner with a life of 53 cycles. The twinning-detwinning contribution to the plastic strain-range was reduced especially in the C-C manner. The difference of twinning-detwinning contribution to the plastic strain-range between the two loading manners was responsible for the different LCF lives. [ABSTRACT FROM AUTHOR]

The purpose of this investigation is to analyze the mechanisms of cyclic deformation and the initiation and propagation of microcracks during low cycle fatigue in the bainitic steel 16CrMnV7-7. The slip systems and their associated Schmid Factor are analyzed in the bainitic ferrite laths and correlated with the short crack path using scanning electron microscopy observations (SEM) in combination with electron backscattered diffraction (EBSD) measurements. Moreover, the developed dislocation structure was analyzed and correlated with the formation and propagation of microcracks. The principal results show that microcracks initiate in lath boundaries and along slip systems with the highest Schmid Factor and low Taylor Factor. Besides, it was observed after experimental evidence that the parameters controlling crack propagation are associated with the crystallographic misorientation between bainite blocks and with the tilt/twist misorientation angle between slip planes of adjacent bainitic ferrite laths. [ABSTRACT FROM AUTHOR]

The effect of volume fraction of δ phase, which was changed through changing the heat treatment conditions, on the strain-controlled fatigue behavior of Nickel-based GH4169 alloys at room temperature was investigated. The δ phase had almost no effect on fatigue life at strain amplitude higher than 0.5%. However, the fatigue life increased with increasing volume fraction of δ phase at the strain amplitude of 0.4%. The alloys with three different volume fractions of δ phase exhibited dual-slope Coffin-Manson relationships, which could be attributed to the changes in the deformation mode from twinning to slip bands as well as the crack initiation mode from transgranular to intergranular with increasing strain amplitude. [ABSTRACT FROM AUTHOR]

Microstructure and dislocation structures in high alloyed heat resistant Sanicro 25 austenitic steel cyclically strained in a wide interval of constant strain amplitudes both at room temperature and at temperature of 700 °C were studied by means of transmission electron microscopy (TEM). The spatial arrangement of dislocations was determined using the technique of oriented foils. The character and the Burgers vectors of dislocations were determined. It was found that at room temperature strong planarity of dislocation slip prevails. Cyclic plastic deformation is localized into thin bands of high dislocation density which have different structure than ladder-like arrangement. No distinctive wall and channel dislocation configurations were observed but bands have character of alternating dislocation rich and dislocation poor areas. Pronounced plastic strain localization leads to cyclic softening. In high temperature cyclic straining enhanced cross-slip leads to substantial increase of dislocation density. Objects of nanometer size coherent with the matrix were found simultaneously with the evidence indicating pinning of dislocations. The high dislocation density, its homogeneous distribution and interaction of nanoclusters and gliding dislocations are the cause of exceptional cyclic hardening of this material at high temperatures. Due to this hardening, the saturated cyclic stress at 700 °C is remarkably high. [ABSTRACT FROM AUTHOR]

This work reports the low cycle fatigue behavior of a wrought 316LN stainless steel under different control modes at room temperature. Under symmetrical strain and stress cycling, the steel exhibits consistent loading-amplitude-dependent cyclic hardening/softening and fatigue life characteristics. Under asymmetrical stress cycling, the steel is significantly hardened due to mean stress, and the fatigue life at the same strain amplitude is significantly reduced due to ratcheting strain. With the increase of mean stress, though the ratcheting strain level is increased, the fatigue life is prolonged. The effect of mean-stress hardening and ratcheting strain on fatigue life is discussed in terms of strain amplitude and micro-crack initiation and propagation. The Smith-Walker-Topper (SWT) model and a newly proposed fatigue life model based on the Coffin-Manson equation were used to predict the fatigue life under mean stress, and the proposed model yields more robust predictions. [ABSTRACT FROM AUTHOR]

This study was performed to clarify the effect of Al addition on low-cycle fatigue (LCF) properties of high-Mn twinning induced plasticity (TWIP) steels, which were electrochemically charged by hydrogen. For this purpose, two alloys, Fe-17Mn-0.8C (0Al), and Fe-17Mn-0.8C-2Al (2Al) were used. It was clearly shown that internal hydrogen degraded the resistance to fatigue fracture in both steels. At high strain amplitude, Al addition effectively improved the resistance to hydrogen embrittlement (HE) by preventing intergranular cracking. In contrast, at low strain amplitude, LCF life was reduced more in 2Al than in 0Al, because large-size inclusions in 2Al promoted initiation of fatigue cracks. These results demonstrate that Al addition does not always provide beneficial effect on LCF behavior of TWIP steels in a hydrogen environment, but that the behavior depends strongly on the strain mode or amplitude. [ABSTRACT FROM AUTHOR]

The effect of temperature on low cycle fatigue behavior of a directionally solidified nickel-base superalloy has been investigated from 550 to 850 °C. The fatigue life reached maximum at about 700 °C. The fracture modes can be divided into three types: crystallographic plane facets fracture (shear fracture) at low temperature region (<600 °C), mixed fracture mode at middle temperature region (600–700 °C) and non-crystallographic fracture (I type fracture) at higher temperature region (>700 °C). The dominant reason of the fracture mode transition with temperature is the deformation of the γ′ precipitated phase at different temperatures. The precipitated phase is sheared by dislocations which are restricted to the primary {111} slip planes at 600 °C. As the temperature increases, the deformation of γ′ precipitated phase becomes anomaly and some secondary precipitates can be observed in the matrix. [ABSTRACT FROM AUTHOR]

In the paper, different morphologies of bainite were obtained through isothermal quenching at 320 °C and 395 °C in a medium-carbon carbide-free bainitic steel. The cyclic deformation mechanism was explored by using low cycle fatigue testing. The volume fraction of retained austenite was measured by X-ray diffraction and the space partitioning of the solute atoms was constructed by three-dimensional atom probe. Results showed that the fatigue life at 320 °C was always higher than that at 395 °C under low and high total strain amplitude. The cyclic softening at the early fatigue stage increased the plastic strain of the sample which was responsible for the reduction of the fatigue life at 395 °C. Strain-induced retained austenite to martensite contributed to initial cyclic hardening, but almost having no effect on the subsequent cyclic stable/softening behaviors. The finer bainitic ferrite sheaves obtained at 320 °C changed the small fatigue crack propagation direction and delayed the crack propagation rate, which was beneficial for the fatigue properties. In addition, the substitutional atoms did not redistribute between the retained austenite and bainitic ferrite before and after cyclic deformation. [ABSTRACT FROM AUTHOR]

Low cycle fatigue behavior of titanium alloy Timetal 834 has been studied at 300 °C, 450 °C and 600 °C under fully reversed constant total strain amplitudes (Δε t /2) ranging from 0.4% to 1.2%. The alloy exhibited initial cyclic softening followed by cyclic hardening at 450 °C for Δε t /2≥0.4% while at 300 °C and 600 °C, cyclic softening followed by cyclic stability was noticed in the cyclic stress response curves for Δε t /2>0.4%. Precipitate shearing was observed to be the micro-mechanism responsible for cyclic softening at all test temperatures. However, dislocation-dislocation and dislocation – solute interaction was observed to be the micro-mechanism responsible for cyclic hardening only at 450 °C. TEM observations revealed that the severity of planar slip is found to peak at 450 °C. [ABSTRACT FROM AUTHOR]

In order to understand the simultaneous operation of fatigue and creep mechanisms, strain rate ( ε ̇ ) effect on low cycle fatigue (LCF) behaviour of recently developed power plant grade wrought Ni-based superalloy IN740H was studied. Fatigue experiments were conducted in air, at 760 °C at ε ̇ values 10 −2 , 10 −3 and 10 −4 s −1 . The fatigue life significantly decreased at lower ε ̇ values. Bilinear Coffin-Manson slopes were obtained for all ε ̇ values. Through microstructural studies, it was ascertained that decrease in fatigue life at lower ε ̇ value can be attributed to ingress of oxygen by grain boundary diffusion and intergranular crack initiation and growth. [ABSTRACT FROM AUTHOR]

In this work, the dual phase titanium alloy Ti–5Al-7.5V was subjected to low-cycle fatigue at room temperature to reveal the deformation mechanisms of fatigue and crack initiation. Transmission electron microscope analysis showed that planar dislocation slip, most of them localized at the basal and prismatic planes of α phase, is the primary deformation mode of low-cycle fatigue. Multiple slips operate concurrently in the high Schmid factor planes within a single grain. Moreover, a limited number of observed microcracks were formed along the basal slip bands. Electron back-scattered diffraction analysis evidenced that numerous microcracks were formed along basal planes. And, the microcracks were confined in primary α grains without propagation to surrounding transformed β matrix, indicating that the alloy exhibits a high tolerance to microcrack propagation. The α grain aggregate oriented for basal slip is a fatigue-critical microstructure configuration, which provides a high Schmid factor path by linking adjacent grains and potentially lead to internal crack initiation with the formation of a field of facets in the cycling process. While the silicides were observed in contact with the microcracks and slip bands, no evidence of crack initiation from silicides was detected. Given that the microcracks were formed along the pre-existing slip bands, it remains an open question whether the silicides can act as the dislocation sources for planar slips and facilitate crack nucleation from these silicides. • Ti–5Al-7.5V alloy gives a good balance of high strength and ductility. • Deformation of low cycle fatigue was dominated by planar slip in α grains. • Near basal cracking can be attributed to the slip bands deviating from the basal plane through multiple cross slip mechanism. • Small fatigue crack growth favors for α grains. • The Si-containing inclusions were widely dispersed and were observed in contact with the microcracks and slip bands. [ABSTRACT FROM AUTHOR]

The room temperature ratcheting response of equiatomic FCC CoCuFeMnNi high entropy alloy containing Cu-rich nanoclusters in HEA matrix at different mean stress and stress amplitude indicated the critical role of interaction among dislocations as well as dislocations and Cu-rich nanoclusters in determining the deformation response and damage accumulation contributing to final failure. Detailed microstructural investigation using EBSD and TEM capture the evolution of complex defect microstructure for different cyclic loading conditions. Perennial network of Cu-rich nanoclusters transforming into entangled Cu-rich nanofibers during ratcheting was observed in the three-dimensional elemental maps obtained using APT. This can be attributed to shearing of spherical clusters followed with enhanced short-range diffusion contributing to the decoration of dislocation structure formed by cyclic loading with copper atoms. Further, an increase in the concentration of copper in the solid solution matrix contributes to additional solid solution strengthening, compensating for cluster strengthening and leading to a lower steady-state creep rate with better ratcheting performance. Thus, synergistic operation of cluster and solid solution strengthening in improved ratcheting response of CoCuFeMnNi HEA showcases the unique ability of HEAs to offer excellent mechanical properties beyond superior strength-ductility combination. • Cyclic creep strain increases with increasing stress amplitude as well as mean stress. • EBSD and APT instigation for defect and damage evolution in ratcheting. • Increase in intragranular misorientation and GND density contributes to the failure. • Dislocation interaction with solute environment and Cu nanoclusters accelerates damage. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) processes are currently under consideration for marine based components, predominantly due to the numerous benefits that the techniques have to offer over more conventional manufacturing routes. However, there are multiple engineering challenges and questions associated with the introduction of AM based parts into safety critical applications related to the mechanical behaviour of such components. One of the main factors influencing the cyclic performance of a component is the surface finish. As-built AM parts typically exhibit a rough surface owing to partially melted powder being present at the surface and the layer-by-layer nature of the AM process, which together will likely hinder the fatigue response of the component. This behaviour is further influenced by the build orientation of the AM component, with alternative orientations providing a different surface profile alongside a contrasting microstructural morphology. Therefore, alternative finishing methods have been explored to maximise the fatigue performance of components whilst also considering cost and time. This research will explore the low cycle fatigue (LCF) behaviour of laser powder bed fused (LPBF) stainless steel 316L (SS316LN) built in two principal orientations (vertical (90°) and diagonal (45°)) and subsequently subjected to several post-manufacture finishing processes in order to identify the optimal finish for mechanical performance. The mechanical results are supported by microstructural, fractographic and advanced surface profilometry assessments, which have revealed that surface roughness can not be considered alone to be the controlling influence on LCF behaviour. An as-built surface finish will inherently provide a greater number of surface breaking stress raisers, however, a novel mass finishing polishing procedure has been found to produce a similar effective stress concentration factor compared to conventional longitudinal polishing, offering a more viable and less time consuming alternative. Several other key factors must also be considered when assessing the fatigue performance of LPBF built materials, including build direction and the resulting grain orientation, density of the additive structure and the material's sensitivity to the presence of notched features at the surface. Finally, the generated mechanical data has also been interpreted through empirical modelling, and the various data sets have been successfully correlated to enable longer fatigue life predictions. [ABSTRACT FROM AUTHOR]

In this paper, the fatigue behaviour and failure mechanisms of extruded AZ80 magnesium alloy samples were studied with different initial structures. The results show that the twinning-twinning behavior was inhibited to a certain extent by fine grain and precipitates, but the initiation of fatigue cracks was promoted. Studies of the fatigue life and fracture mechanism show that both residual twins and precipitated phases can cause fatigue crack initiation, but less fatigue damage is caused by the twin-induced cracks than by second phase-induced. [ABSTRACT FROM AUTHOR]

The ratcheting behavior in an extruded AZ31B magnesium alloy was investigated under uniaxial stress-controlled cyclic loading at room temperature. The results showed that basal < a > slip, prismatic < a > slip and {10–12} extension twinning/detwinning are the deformation mechanisms accommodating cyclic strain. The weight of the slips or the twinning/detwinning that relates to stress amplitude affects the valley strain and ratcheting strain. The increase of the weight of the dislocation slips results in an increment of strain hardening rate. When the stress amplitude is 105 MPa, the hardening rate is higher than that at the amplitudes of 45 MPa and 75 MPa, which leads to a ratcheting strain development deviating from the regular pattern. [ABSTRACT FROM AUTHOR]

The fatigue lives are reduced accompanying an additional cyclic hardening under strain controlled non-proportional cyclic loading in which principal directions of stress and strain are altered within a cycle. This study predicts non-proportional cyclic hardening and multiaxial fatigue life for several BCC and FCC metals under constant amplitude strain cycling. A novel procedure to determine non-proportional cyclic hardening form uniaxial tensile properties has discussed in this study. Standard plastic strain energy density based fatigue criteria with considering the non-proportional cyclic hardening effect successfully predicts multiaxial fatigue lives. The predictions of non-proportional cyclic hardening and multiaxial fatigue life through models are validated by experimental results of various BCC and FCC metals which are collected from literatures. [ABSTRACT FROM AUTHOR]

Total strain-controlled low cycle fatigue (LCF) tests of a nickel based superalloy were performed at 650 °C. Various hold times were introduced at the peak tensile strain to investigate the high-temperature creep-fatigue interaction (CFI) effects under the same temperature. A substantial decrease in fatigue life occurred as the total strain amplitude increased. Moreover, tensile strain holding further reduced fatigue life. The saturation phenomenon of holding effect was found when the holding period reached 120 s. Cyclic softening occurred during the LCF and CFI process and it was related to the total strain amplitude and the holding period. The relationship between life-time and total strain amplitude was obtained by combining Basquin equation and Coffin-Manson equation. The surface and fracture section of the fatigued specimens were observed via scanning electronic microscope (SEM) to determine the failure mechanism. [ABSTRACT FROM AUTHOR]

Carbon–manganese steel A48 (French standards) is used in steam generator pipes of the nuclear power plant where it is subjected to the cyclic thermal load. The Dynamic Strain Aging (DSA) influences the mechanical behavior of the steel in low cycle fatigue (LCF) at favorable temperature and strain rate. The peak stress of A48 steel experiences hardening–softening–hardening (HSH) evolution at 200 °C and 0.4% s −1 strain rate in fatigue loading. In this study, isotropic and kinematic hardening rules with DSA effect have been modified. The HSH evolution of cyclic stress associated with cumulative plastic deformation has also been estimated. [ABSTRACT FROM AUTHOR]

To determine the influence of peak- and over-aging on low-cycle fatigue (LCF) behavior of casting A319 alloy, the strain controlled low-cycle fatigue tests was studied at room temperature under different total strain amplitudes. At lower total strain amplitudes (0.2–0.3%), the A319 alloy presented initial cyclic hardening and then, steady stage and cyclic softening; while at higher total strain amplitudes (0.35–0.4%), it presented continuous hardening in the peak-aged specimens and constant softening in the over-aged specimens. At a given total strain amplitude, the hysteresis loops in the peak-aged specimens were relatively narrower than those in the over-aged specimens. The fatigue life of the peak-aged samples was higher than that of the over-aged samples at any given total strain amplitude. Fractographic morphology indicated that the size of the fatigue crack propagation zone at peak-aged condition was larger than that at over-aged treatment. The crack characteristic in the propagation region and the fast fracture zone also showed some differences between two aging conditions. [ABSTRACT FROM AUTHOR]