Abstract
The interior defect-induced fracture of surface-hardened metallic materials in the long life region has become a key issue on engineering design. In the present study, the axial loading test with fully reversed condition was performed to examine the fatigue property of a surface-carburized low alloy gear steel in the long life region. Results show that this steel represents the duplex S-N (stress-number of cycles) characteristics without conventional fatigue limit related to 107 cycles. Fatigue cracks are all originated from the interior inclusions in the matrix region due to the inhabitation effect of carburized layer. The inclusion induced fracture with fisheye occurs in the short life region below 5 × 105 cycles, whereas the inclusion induced fracture with fine granular area (FGA) and fisheye occurs in the long life region beyond 106 cycles. The stress intensity factor range at the front of FGA can be regarded as the threshold value controlling stable growth of interior long crack. The evaluated maximum inclusion size in the effective damage volume of specimen is about 27.29 μm. Considering the size relationships between fisheye and FGA, and inclusion, the developed life prediction method involving crack growth can be acceptable on the basis of the good agreement between the predicted and experimental results.
Highlights
Because of the protective effect against surface fatigue fracture, the surface-hardening technology has been extensively employed to enhance the fatigue strength or life of structural materials such as low carbon alloy steels [1]
Considering the size relationships between fisheye and fine granular area (FGA), and inclusion, the developed life prediction method involving crack growth can be acceptable on the basis of the good agreement between the predicted and experimental results
The reason is that there is a change of the fracture mode from the surface fatigue fracture at high stress level to the interior fatigue fracture at low stress level [3,4]
Summary
Because of the protective effect against surface fatigue fracture, the surface-hardening technology has been extensively employed to enhance the fatigue strength or life of structural materials such as low carbon alloy steels [1]. The reason is that there is a change of the fracture mode from the surface fatigue fracture at high stress level to the interior fatigue fracture at low stress level [3,4] It is because of the transition of fatigue fracture mode or crack nucleation site that some surface-hardened or high-strength structural materials can present peculiar S-N characteristics [5,6] and complicated fracture mechanisms [7,8,9,10]. Interior inclusion-induced crack nucleation and growth is the most common mode of interior fatigue fracture. It should be noted that a characteristic rough area with granular morphology
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