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.

中文翻译：

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感应淬火碳钢 S38C 车轴的梯度残余应力和疲劳寿命预测：实验和模拟

中碳钢S38C车轴经过高频感应淬火后，仍保留了几毫米深度的三轴残余应力梯度分布，这已成为疲劳结构完整性的关键问题。为了解决这个问题，首次通过先进的中子衍射测量了车轴内部的轴向、环向和径向梯度残余应变。然后采用 SIGINI Fortran 子程序根据测量数据重建全局初始残余应力场。实验和模拟结果表明，轴表面以下保留了约-520 MPa（轴向）、-710 MPa（环向）和-40 MPa（径向）残余应力。随后，在断裂力学框架下对 S38C 车轴的疲劳裂纹扩展行为进行了数值研究。计算结果清楚地表明，距车轴表面0~3 mm深度处的残余压应力导致裂纹扩展驱动力较低，并且只要裂纹深度小于3.7 mm，疲劳裂纹就不会扩展。空心 S38C 车轴。这些结果进一步表明，从安全和经济的角度来看，例行检查中允许的最大缺陷尺寸是可以接受的。通过实验和模拟对全局梯度残余应力场进行精确测量和表征，可以为优化这些表面强化铁路车轴表面缺陷无损检测（NDT）里程间隔提供重要参考。