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To assess the residual life of railway truck heavy couplers made of cast steel with initial defects, a testing campaign has been carried out to determine high cycle fatigue strength, fatigue crack growth rate, threshold, and fracture toughness. The numerical model of a damaged No. 17 heavy-haul coupler is established, by fully considering the assembly clearance and the complex nonlinear contact. The fracture mechanics based remaining life under the actual load spectrum is then found. The results show that the remaining life with an initial 4-mm-length surface crack inside the heavy coupler are about 704,300 km and 761,800 km when using the classical Paris law and the advanced NASGRO equation respectively. Based on the advanced NASGRO equation, approximately 12.8% of the life is retained when there is a 20-mm-length crack. The maximum load-carrying capacity of the cast steel couplers shows an exponential downward trend with increases in crack length. The critical size for a non-propagating crack can be assumed to be 20 mm after considering possible high impact loads in service. The predicted mileage is 664,100 km, i.e., the life is about 3.32 years, which is larger than the inspection interval currently used for a railway truck heavy coupler. [ABSTRACT FROM AUTHOR]

• Critical position of high-speed railway wheels is identified. • A stepwise fatigue assessment approach is conducted to predict the fatigue life of railway wheels. • Three fatigue crack growth rate models are compared to assess the residual life of the wheels with defects. The fatigue life assessment of railway wheels is a critical issue to establish an effective maintenance strategy. Presently, the traditional nominal stress method is commonly adopted, assuming the wheels are defect-free. However, the wheels are always subjected to railway ballast, corrosion, and more, rendering the traditional design method inadequate. To address this, our study introduces a stepwise fatigue assessment approach. Initially, the traditional nominal stress method is employed as the first level, followed by the application of advanced damage tolerance analysis as the second level, which takes into account the impact of defects. The safe-critical position of the wheels is identified using the finite element model. Three fatigue crack growth rate models are compared to assess the residual life of the wheels with defects. The numerical simulation results illustrate that the iLAPS model is proved to be effective, demonstrating the effectiveness of the developed assessment method. This work is expected to provide a scientific guidance for establishing and optimizing of maintenance strategy for railway wheels. [ABSTRACT FROM AUTHOR]

• Effect of corrosive medium on FCG behaviors in a structurally gradient steel with different pre-crack depths is clarified. • Relationship between pre-crack depth and FCG/CFCG rates is revealed. • Compared with corrosion, fatigue loading plays a major role in CFCG rate at a frequency of 5 Hz. • X-ray μCT results verified a retardation effect of anodic dissolution on CFCG rate. Considering the complex service environments that high speed railway axles are subjected to, the fatigue crack growth (FCG) behavior of a structurally gradient axle steel with different pre-crack depths both in air and a corrosive medium was investigated at a frequency of 5 Hz. The results indicated that in the high Δ K region, the FCG rate was dramatically accelerated by corrosion but the gap narrows as Δ K decreased. The accelerated corrosion FCG rate was a comprehensive result of the acceleration effect of the anodic dissolution, hydrogen-enhanced localized plasticity and the retardation effect of corrosion-induced crack-tip blunting. Despite the fact that the corrosion resistance gradually decreased as the pre-crack depth increased, the FCG rate in the corrosive medium gradually decreased. This was because fatigue loading played a more important role than corrosion in accelerating the corrosion FCG rate. [ABSTRACT FROM AUTHOR]

• Both the tensile strength and ductility of the HRAM-processed alloy are improved. • The HRAM-processed alloy is characterized by a high dislocation density and large α laths. • High dislocation density and large α laths lead to the improved strength and ductility, respectively. • The fatigue crack growth resistance of HRAM-processed alloy is isotropic. Hybrid in situ rolled wire + arc additive manufacturing (HRAM) is a novel printing technology capable of fabricating large-scale components by combining in situ micro-rolling with standard wire + arc additive manufacturing (WAAM), which enhances the ductility and tensile strength of the resulting parts. This study attempts to identify the aspects of the microstructure that are critical to the property improvement. The microstructure of the WAAM- and HRAM-processed Ti6Al4V alloys exhibited a basketweave morphology composed of α laths and β phases. The HRAM-processed alloy was characterized by a higher dislocation density and larger α laths compared to those of the WAAM-processed alloy. The relatively high dislocation density and large α laths are responsible for the improvement in the tensile strength and ductility, respectively. The spatial evolution of cracking for the HRAM-processed alloy was investigated using a specially designed uniaxial tension/compression rig mounted on the synchrotron X-ray computed microtomography platform. The failure originated from the nucleation of micro voids and surface cracks. [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 preferentially dissolved ferrite in pearlite and shedding of cementite lamellae contribute to the formation of corrosion pits. • Corrosion fatigue crack initiation and growth mechanism of U75V rail steel is clarified. • Corrosion fatigue crack initiation behavior is related to morphology ratio and local shape of corrosion pits. Understanding the corrosion fatigue mechanism of rail materials is of great importance to guide the assessment of the structural integrity of modern railway components. To this purpose, the corrosion fatigue crack (CFC) initiation and growth behaviors of heat-treated U75V rail steel are investigated under 3.5 wt% NaCl solution in terms of high-cycle fatigue, fatigue crack growth (FCG) rate, together with surface morphology and microstructure. Experimental results reveal that the U75V rail steel is susceptible to corrosion fatigue failure. Examinations on fractured samples clearly show that the electric potential difference between ferrite and cementite triggers the preferential anodic dissolution of ferrite, leading to corrosion fatigue crack initiation. The crack initiation is not only solely determined by the size of the corrosion pit, but also strongly influenced by its local shape. In comparison with the fatigue crack growth in air, the corrosion FCG tends to grow in a relatively straight path, because of hydrogen embrittlement. Additionally, the CFC behaviors indicate a tendency to transverse through soft-oriented grains and deflect upon encountering hard-oriented grains. [ABSTRACT FROM AUTHOR]

• Instrumented Charpy impact test was performed for a gradient steel structure of railway axles. • Gradient orientation in the S38C steel plays a crucial role in the impact behavior. • The gradient material with coarse grains to fine grains shows an excellent damage tolerance. • Hardening treatment at both inner and external surface are suggested to create a new structure. Induction hardened S38C axles exhibit the balance between good crack growth resistance at the body and wear resistance at the wheel seat, which can be physically attributed to the gradient structure with fine grains (FG) near the surface and coarse grains inside the base metal (BM). However, the impact toughness of such bulk gradient engineering structure is less reported particularlly when experiencing high-speed impacts. Therefore, the impact behaviors of the S38C steel are thoroughly examined by using the V-notch instrumented Charpy impact testing in this work. It is clearly found that the gradient order has a significant effect on the impact response. For the gradient structure termed as FG → BM, the absorbed energy is significantly lower than the BM structure but the higher strength and hardness, which indicates that the FG → BM structure is difficult to tailor the fatigue crack growth and the resulting fatigue life of railway axles. By contrast, the BM → FG structure exhibits more excellent impact resistance primarily owing to the micropore coalescence. To acquire the exceptional damage tolerance, the inner bore and external surface of the S38C axles can be sequentially strengthened to acquire an integrated gradient structure. [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]