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Hu, F. F.*; Qin, T. Y.*; Ao, N.*; Su, Y. H.; Zhou, L.*; 徐 平光; Parker, J. D.*; 篠原 武尚; Chen, J.*; Wu, S. C.*
Engineering Fracture Mechanics, 306, p.110267_1 - 110267_18, 2024/08
被引用回数：2 パーセンタイル：55.14(Mechanics)Non-destructive and quantitative mapping of gradient residual strain distribution in surface-hardened railway S38C axles could provide a positive reference for determining service lifetime and maintenance strategy. To tackle this concern, time-of-flight neutron Bragg-edge transmission imaging was employed by real axle samples with and without impacted crater. A novel and simple procedure to formulate the residual strain field was also developed in this work, with the transmission batch code in Appendix A. By mapping the global two- dimensional residual strains, it can be verified that the residual strains into the axle are uniformly distributed in the hoop direction. Subsequently, it was revealed that the axial and hoop residual strains, respectively in the cylinder and the long strip samples prepared from a real S38C hollow axle, indicated a gradient evolution distribution with a depth of 
8 mm, covering a range of -5500 1000 
for axial strains and -6500 1000 for hoop strains. More importantly, the maximum compressive lattice strain of the cylinder sample was increased by 15.61%, and 22.35% at the impacting speeds of 100, and 125 m/s, respectively; and that of the long strip sample increased by 29.17%, and 43.70%, respectively. It can thus be concluded that lattice strains have redistributed around the impact crater, demonstrating the local alteration of the residual strain field. These new findings suggest the localized variation in residual strains should be taken into account while evaluating the service damage evolution of railway axles, especially those affected by high-speed impacts during operation.
Hu, F. F.*; Qin, T. Y.*; Su, Y. H.; Ao, N.*; Zhou, L.*; 徐 平光; Parker, J. D.*; 篠原 武尚; Wu, S. C.*
no journal, ,
High-frequency induction hardened S38C axles are widely applied in Japan Shinkansen and China bulletin trains. Due to well-designed surface treatment, these axles produce a large gradient change in the material structure from the axle surface to the core, resulting in a large-layer depth of residual strain and microhardness. It is well known that the residual strain of a real component is of great importance for its long-cycle service assessment, mainly at the reduced non-destructive detect interval and low maintenance cost. Fortunately, the neutron beam has a high penetrating capacity, which provides the advantage of using Bragg-edge transmission imaging to characterize the residual strain and microstructure inside large engineering components. In our study, ring shaped specimens of the axle were prepared. The neutron transmission imaging experiments were conducted at RADEN in J-PARC MLF under proposal Nos. 2022A0298 and 2023A0069. The obtained results reveal that the 0-5 mm area of the axle surface is compressive residual strain, while the core region is the combination of the tensile residual strain and the compressive residual strain. In the presentation, we will also analyze and compare the changes in residual strain and microstructure in defective S38C axles at various fatigue stages. We are expecting to employ pulsed neutron imaging data to assess the fatigue resistance of the axle.
Liu, J. J.*; Qin, T. Y.*; Hu, F. F.*; 徐 平光; Su, Y. H.; Wu, S. C.*
no journal, ,
The railway axles are critical components for high-speed trains, subject to complex fatigue loads from wheels, rails, and the car bodies, directly affecting service safety. Medium carbon steel S38C axles after high-frequency induction hardening retain a significant gradient of triaxial residual stresses down to a depth of several millimeters, raising concerns about their fatigue integrity. This study measures the axial, hoop, and radial residual strains within the axles for the first time using advanced neutron diffraction. Utilizing the SIGINI Fortran subroutine, we reconstructed the global initial residual stress field from the measured data, revealing residual stresses of approximately -520 MPa (axial), -710 MPa (hoop), and -40 MPa (radial) beneath the surface. We also investigated the fatigue crack propagation behavior of S38C axles through numerical modeling in the context of fracture mechanics, finding that compressive residual stresses within 0-3 mm of the surface reduce the driving force for crack growth, preventing propagation as long as the crack depth remains below 3.7 mm for hollow S38C axles. These results suggest that the maximum defect size allowed in routine inspections is safe.
Hu, F. F.*; Qin, T. Y.*; Zhang, R.*; Ao, N.*; He, L. H.*; Su, Y. H.; 徐 平光; Wu, S. C.*
no journal, ,
The S38C railway axles demonstrate excellent fatigue resistance owing to the large-layer depth compressive residual stress in the hardened surface. However, during fatigue crack propagation, the residual stress may occur the stress relaxation problem, which results in a reduction of the damage tolerance capacity and the service lifetime. To tackle this concern, the time-of-flight neutron scatter methods, including the Bragg-edge transmission imaging and neutron diffraction, were employed to quasi-in-situ study the residual strain and RS relaxation during the fatigue crack advance with single-edge notch bending samples. The BET experimental results show that lattice parameters will change as the crack grows, which then leads to a decrease in residual strain. Moreover, it was discovered from the neutron diffraction test that all three RS components decrease as the crack propagation. The CRS in the axle surface layer is almost fully released when the crack propagates to the matrix material zone.
