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High-speed railway S38C axles undergo surface induction hardening for durability, but are susceptible to fatigue cracks from foreign object impact. The neutron diffraction method was employed to measure the residual strain in S38C axles, obtaining microscopic lattice distortion data, for the gradient layer at a depth of 8 mm under the surface. The results showed that after induction-hardening, the microscopic lattice distortion had a gradient distribution, decreasing with the distance from the surface. However, in the case of impacting speed of 600 km/m, the average microscopic lattice distortion increased with the distance from the surface, reaching a maximum augmentation of 55 pct. These findings indicate a strong experimental basis, and improve our understanding of the relationship between macroscopic residual stress and decision-making, in regard to operation and maintenance. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) has a negative effect on the fatigue performance of titanium alloys. To reduce/eliminate this influence, the effect of an ultrasonic surface rolling process (USRP)-induced compressive residual stress and gradient nanostructure on the PEO coating growth and fatigue behavior of a PEO-coated Ti-6Al-4V alloy was investigated. The PEO process yielded a severe decrease in the fatigue life of the substrate. Owing to the USRP pretreatment, the low cycle fatigue life and high cycle fatigue life of the PEO-coated sample improved to similar values and considerably higher values, respectively, compared with those of the substrate. This is attributed to the synergistic effect of the gradient nanostructure and compressive residual stress, which retarded crack initiation and propagation, although the residual stress played the major role in the aforementioned improvement. Additionally, the nanostructure promoted nucleation and growth of the PEO coating, as surface nanocrystallization improves the chemical activity of the titanium alloy surface. The PEO treatment influenced the residual stress mainly at the top surface of the substrate, but had almost no influence on the stress distributed in the interior. • Micropores/microcracks in PEO coating are main reason for decreasing fatigue life. • PEO treatment only influences compressive residual stress at the topmost surface. • USRP pretreatment significantly improves the fatigue life of PEO-coated sample. • Surface nanocrystallization facilitates the growth of PEO coating. [ABSTRACT FROM AUTHOR]

• The residual stress was introduced into the DDD framework. • The gradient-grained sample with residual stresses exhibits a "first weakening and then strengthening" phenomenon. • The overall tensile strength increases as the compressive residual stress increases. • The compressive residual stress in the surface layer can suppress the activation and multiplication of dislocations in the sub-surface regions. A three-dimensional discrete dislocation dynamics (DDD) method was used to investigate the effect of residual stress on the stress-strain response of gradient nano-grained (GNG) metals. The GNG polycrystalline model was constructed, and different residual stress distributions were introduced in the DDD framework. The simulation shows that the distribution of residual stress has a significant influence on the tensile stress-strain curve. For a GNG sample containing both compressive and tensile residual stresses, the flow stress is firstly lower but ultimately higher than that of the sample without residual stress, which is caused by the combined effects of residual stress and gradient-grained structure. The evolution of dislocation microstructures indicates that the initial tensile residual stress in the sub-surface of the sample can promote the dislocation activation and thus lead to the decrease of overall yield stress. Whereas the compressive residual stress in the topmost surface of the sample can suppress the dislocation activation and multiplication in other regions, resulting in finally higher flow stress in comparison to that of the sample without residual stress. In addition, it is demonstrated that the reduction in the elastic modulus for GNG samples with residual stress observed in some experiments is due to the earlier plastic deformation that occurred in the region with initial tensile residual stress. The present study provides insights into the individual and combined effects of residual stress and gradient-grained structure in GNG metals. [Display omitted] [ABSTRACT FROM AUTHOR]

• Surface rolling deformed severity-dependent FF mechanism of titanium alloy is revealed. • The microstructural evolution of the samples with different surface rolling deformed severity before and after FF is revealed. • The difference between plain fatigue and FF of surface-strengthened Ti-6Al-4 V alloy was discussed. Surface plastic deformation methods are prevalently applied to improve the fretting fatigue (FF) property of titanium alloys. Understanding the intrinsic relation between surface plastic deformation severity and the FF property is of great significance to the application of these methods. Here, the influence of surface plastic deformation severity on the FF behavior of Ti-6Al-4V alloy is investigated. Two different levels of surface plastic deformation severity are prepared at the surface of Ti-6Al-4V alloy by employing ultrasonic surface rolling process. Uniaxial FF tests illustrate that both of the deformed samples have improved FF properties and although their fatigue life is comparable, the influences of their refined microstructures and compressive residual stresses are different. Pre- and post-mortem microstructural analyses reveals that the grains are coarsened or crystallized from amorphous material at the topmost surface under the multiaxial stresses to accommodate the plastic strain for the high surface deformation severity, while the grains are refined at the subsurface for the low surface deformation during FF loading. [ABSTRACT FROM AUTHOR]

• Fatigue damage of axle steel was accelerated by inappropriate USRP-treated parameters. • The optimal parameters of ultrasonic surface rolling for axle steel were determined by orthogonal testing. • The reduction of surface roughness, work hardening, and compressive residual stresses introduced by ultrasonic surface rolling process inhibited the crack initiation and propagation. The increasing complexity of railway services is placing higher demands on ensuring the fatigue resistance of critical components of high-speed trains. The ultrasonic surface rolling process (USRP) was employed to enhance the fatigue resistance of the EA4T axle steel used for high-speed trains in China. The optimal USRP parameters were first determined by orthogonal testing, with the rotational bending fatigue life as the evaluation index. Multidimensional experimental characterization and finite element simulation results demonstrated that the surface layer of the treated axle steel underwent violent plastic deformation, in which the grain size, hardness and geometrically necessary dislocation density showed distinct variation with depth from the surface. The surface roughness of the USRP-treated specimens was significantly less than for the untreated specimens, and the average compressive residual stress increased by a factor of 4.3. The 46.9 % increase in rotational bending fatigue strength (from 320 to 470 MPa) was attributed to the combined action of the large compressive residual stresses, the work-hardened surface layer, and less surface roughness. These results demonstrate that ultrasonic surface rolling is an effective surface hardening technique to improve the fatigue resistance of railway axles. [ABSTRACT FROM AUTHOR]

• Surface rolling deformed severity-dependent fatigue mechanism of titanium alloy was revealed. • The microstructural evolutions of the samples with different surface rolling deformation severities during fatigue loading were revealed. • The residual stress evolutions during fatigue loading for the different surface rolling deformed samples were clarified. • A surface strengthening strategy was given to enhance the stress-controlled fatigue property of titanium alloy. This work investigates the effect of initial surface plastic deformation on the fatigue behavior of Ti-6Al-4V alloy based on the microstructure and residual stress evolution under fatigue loading. The high strain hardening ability, full play of compressive residual stress, and excellent surface integrity of samples with low plastic deformation severity resulted in relatively higher fatigue strength. Microstructural analysis revealed that the ultrasonic surface rolling process (USRP) applied to samples with low plastic deformation severity accommodated plastic strain by refining the grains, whereas the grains coarsened in the USRP samples with high plastic deformation severity for accommodating the plastic strain. [ABSTRACT FROM AUTHOR]

As a representative structure with an acute radial gradient in terms of residual stress (RS) and grains, the induction-hardened railway S38C axle exhibits excellent mechanical properties and fatigue resistance. To investigate the effect of gradient features on fatigue crack growth (FCG) behaviors, we designed three-point bending (TPB) tests with different gradient features, including the negative-gradient, positive-gradient, and matrix specimens. Finite element modeling was introduced to simulate the FCG of the TPB specimens in terms of the cyclic J -integral (Δ J) and equivalent plastic strain at the growing crack tip. The results reveal that the FCG rates in both negative- and positive-gradient structures decrease and a more pronounced tendency of decline is achieved in the positive-gradient ones. The substantial compressive residual stress (CRS) within the negative-gradient structure directly influences the crack tip area, while the CRS in the positive-gradient structure indirectly mitigates the crack driving force by impeding the overall structural deformation. Coarser grains aid in enhancing energy absorption along the zigzag crack path, thereby diminishing the crack driving force. Δ J and the equivalent plastic zone surrounding the crack tip indicate significant reductions in the presence of CRS. [ABSTRACT FROM AUTHOR]

The influence of plasma electrolytic oxidation (PEO) treatment and ultrasonic surface rolling process (USRP) pretreatment followed by PEO treatment on the fretting fatigue (FF) performance of Ti–6Al–4V alloy was investigated. PEO treatment yielded effective improvement in the FF life of the alloy at low stress levels, which was attributed to that the coating with high hardness suppressed crack initiation. The FF life of USRP-PEO-coated specimens was significantly improved and by at least a factor of 81 at a maximum alternating stress of 650 MPa compared to the uncoated ones. This resulted mainly from the fact that the high-hardness coating retarded the FF crack initiation, and the synergistic effect of the induced large deep-distributed compressive residual stress and gradient nanostructure retarded early FF crack propagation. The compressive residual stress played the dominant role in the FF life improvement. For the PEO-coated or USRP-PEO-coated specimens, the FF cracks were initiated at the interface of stick zone and slip zone in the fretting wear contact zone of the coating and then propagated through the coating and coating/substrate interface into the substrate. During fretting, the wear debris of the coating moved into the cracks and promoted early crack propagation as wedge effect. • PEO treatment improves the FF performance of titanium alloy. • USRP significantly improves the FF life of PEO-treated specimen. • The FF failure mechanism of PEO-treated/USRP-PEO-treated titanium alloy is studied. [ABSTRACT FROM AUTHOR]

• The corrosion fatigue performances of EA4T axle steel fabricated via ultrasonic surface rolling process was investigated. • The enhancement mechanism of the strengthened specimens of the electrochemical behavior and corrosion fatigue resistance was clarified. • The dominant factor attributed to the superior corrosion fatigue property of the EA4T railway axle subjected to the USRP was revealed. Surface treatment has been validated to improve the fatigue crack resistance of engineering components, while the coupling effect of corrosive atmosphere and external loading is relatively less reported with detailed experimental studies. In this study, gradient structure (GS) axle EA4T alloy steel specimens were prefabricated by using the ultrasonic surface rolling process (USRP), and their corrosion fatigue resistances were investigated. First, the optimal USRP parameters were determined via orthogonal tests. After the USRP treatment, the microstructure, surface topography, surface roughness, residual stress, and electrochemical corrosion behavior were measured to characterize the surface under various conditions. The electrochemical tests results indicated that the compressive residual stress (CRS) is an important factor in improving the corrosion resistance of axle steel. Rotating bending fatigue tests in both air atmosphere and corrosion liquid showed that the fatigue and corrosion fatigue performances could be significantly improved with the strengthened layer obtained via the USRP. Factor separation analyses substantiated that the remarkable enhancement of fatigue and corrosion fatigue strengths can be mainly attributed to the CRS introduced by the surface strengthening treatment. Simultaneously, the GS layer and hardened layer exhibited a synergistic effect. [ABSTRACT FROM AUTHOR]

• The surface properties of EA4T axle steel treated by ultrasonic surface rolling process (USRP) are investigated. • Effect of USRP on the electrochemical corrosion resistance and corrosion fatigue performance is elucidated. • Residual stress relaxation mechanism of USRP-treated EA4T axle steel in air and corrosion is revealed. • Influence of residual stress relaxation on the corrosion fatigue resistance is clarified. The fatigue properties and residual stress relaxation behavior of EA4T axle steel treated by ultrasonic surface rolling process (USRP) are investigated in air and corrosive environments. The result reveals that the surface roughness (Ra) is decreased from 1.42 to 0.17 μm after the USRP. A surface layer with ultrafine grains, work hardening, and compressive residual stress (CRS) is achieved by the USRP. Compared with the as-machined specimens, the USRP specimens exhibit slightly lower electrochemical corrosion resistance but superior fatigue performance in air and corrosive environments. The dislocation density is significantly decreased in the corrosive environment during fatigue loads, accompanied by a rapid relaxation of surface CRS. The influence of USRP on the corrosion fatigue performance is elucidated. The residual stress relaxation mechanism and its influence on fatigue resistance are also analyzed. [ABSTRACT FROM AUTHOR]

• Effect of artificial notches is studied in terms of fatigue crack non-propagation behaviors. • Non-propagation crack rate firstly increases and then decreases with the increased notch depth. • Gradient residual stress and microstructure are responsible for the crack non-propagation. • Crack traces of post-mortem specimen in the hardened layer and the matrix are compared. Abnormal damages in railway axles can lead to a significant hazard to running safety and reliability. To this end, a surface treatment was selected to effectively inhibit fatigue crack initiation and growth. In this study, a single-edge notch bending fatigue test campaign with artificial notches was conducted to elucidate the fatigue crack non-propagation behavior in railway S38C axles subjected to an induction hardening process. The fatigue cracking behavior in the gradient structure was revealed by optical microscopy, electron backscatter diffraction, and fractography. The microhardness distribution was measured using a Vickers tester. The obtained results show that the microhardness of the strengthening layer is nearly triple that of the matrix. Owing to the gradient microstructures and hardness, as well as compressive residual stress, the fatigue long crack propagates faster once it passes through the hardened zone (approximately 2.0 mm in the radial depth). Thereafter, local retarding (including deflection, branching, and blunting) of the long crack occurs because of the relatively coarse ferrite and pearlite in the transition region and matrix. Totally, this fatigue cracking resistance is reasonably believed to be due to the gradient microstructure and residual stress. These findings help to tailor a suitable detection strategy for maximum defects or cracks in railway axles. [ABSTRACT FROM AUTHOR]

• Global residual stress of induction-hardened axles were measured by neutron diffraction. • 3D residual stress into the axle and its impact on fatigue crack propagation behavior was studied. • Fatigue damage tolerance was used to predict the critical crack and remaining life of S38C axles. Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about −520 MPa (axial), −710 MPa (hoop), and −40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0–3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles. [ABSTRACT FROM AUTHOR]

• Corrosion medium leads to the considerable decrease in fatigue strength of high-speed railway EA4T axle. • The fatigue limits of damaged specimens obtained by the experiment were higher than those evaluated by the Murakami's model. • Multiple sites of crack initiation were observed in notched specimens subjected to a corrosive environment. • A three-parameter K-T diagram was established to correlate the stress range and defect size with corrosion fatigue life. The combined effects of aggressive media and artificial surface indentation on the fatigue behaviours were investigated to evaluate the defect tolerance of high-speed hollow railway EA4T steel axles. Corrosive rotating-bending fatigue tests were performed on both smooth and notched specimens to assess their fatigue strengths and failure mechanisms. Based on the corrosion testing campaign, which mainly included examinations of the microstructure, residual stress, and fatigue crack resistance, material deformation underneath the bottom of indentations triggered corrosion fatigue cracks. The results also showed that the corrosive media was responsible for the decrease in fatigue strength, and notched specimens were more sensitive to 3.5 wt% NaCl solution than to air. The fracture surface indicated that cracks were mainly initiated from the notch of specimens in air, whereas cracks were formed from multiple sources in specimens in the corrosive medium. Finally, a three-parameter Kitagawa-Takahashi diagram was established to correlate the threshold stress range (Δ σ th) and critical defect size (a) with the corrosion fatigue life (N) in terms of the El-Haddad model, thus providing a scientific reference for the service safe region identification of high-speed railway axles subjected to corrosive environments. [ABSTRACT FROM AUTHOR]

• FF strength is effectively improved by USRP with a GNS surface layer. • Compressive residual stress yields more contribution than GNS in raising FF strength. • FF cracks of USRP sample yield a serrated path due to compressive residual stress. • Mechanical surface polishing treatment reduces the FF property. A gradient nanostructured (GNS) surface layer is synthesized on the surface of a Ti-6Al-4V alloy by means of the ultrasonic surface rolling process (USRP). The mean grain sizes on the topmost surface and at depth of 10 μm from the treated surface are approximately 45.8 and 87.5 nm, respectively. Fretting fatigue (FF) testing shows a 113.6% improvement in the fatigue strength after USRP as compared with the untreated condition. The mechanism through which the USRP improves the FF properties can be attributed to the comprehensive effect of the compressive residual stress, GNS surface layer, surface work hardening, and surface roughening. Among these, the compressive residual stress is the main factor in determining the FF behavior, due to the beneficial effect of compressive residual stress on the suppression of FF crack initiation and premature growth. [ABSTRACT FROM AUTHOR]

• Externally impacted process was simulated using the Johnson-Cook material model. • Impacting speeds of 100 m/s and 138 m/s were selected for introducing the foreign damage. • Johnson-Cook material model can effectively rebuild the damage evolution. • The external impacting inside railway axles shows a velocity dependence. Modern railway axles are prone to external impacts under in-service, leading to fatigue crack initiation and failure accidents. In this work, the impacting responses of railway axles induced by foreign object damage (FOD) were investigated by using finite element modelling with the Johnson-Cook (J-C) constitutive law and failure model. J-C material model parameters of railway axle EA4T alloy steel were determined by the quasi-static compression test, Split-Hopkinson pressure bar test and high-speed tensile tests at room temperature and high temperatures. For validation purpose, typical FOD craters were generated using the air gun device at two impacting velocities of 100 m/s and 138 m/s. Based on statistical size and morphology as well as residual stress distribution around the crater, it can be concluded that the derived J-C material model can effectively depict the impacting behavior of EA4T alloy steel. It is also observed that the dimensions of the defect and the residual stress are positively correlated with the assumed running speed of railway vehicles. [ABSTRACT FROM AUTHOR]