The Fatigue Behaviors of a Medium-Carbon Pearlitic Wheel-Steel with Elongated Sulfides in High-Cycle and Very-High-Cycle Regimes

Abstract

The effects of stress ratio (R), loading condition, and MnS inclusion on the fatigue behavior of a medium-carbon pearlitic wheel-steel were investigated by a combination of rotating (frequency of 52.5 Hz, 103–108) bending and ultrasonic (frequency of 20 kHz, 5 × 104–109) axial cycling tests in high-cycle and very-high-cycle regimes. All the S-N curves present horizontal asymptotic shapes and have clear fatigue limits. The fatigue limits (260–270 MPa) for R = −1 obtained by ultrasonic test are almost 140–150 MPa lower than that (400–410 MPa) obtained by rotating bending, and the limit values of R = 0.3 are almost in the range of 195–205 MPa. For rotating bending, the fatigue fractures were originated from the surface matrix of the specimen. Whereas for ultrasonic fatigue, both surface and interior crack initiation occurred, and cracks were all initiated from MnS inclusions regardless of stress ratios. The finite element method was employed to study the influence of MnS inclusions on crack initiation and propagation. The results show that high stress concentrates on the sides of the elliptical MnS inclusion rather than the tip of the inclusion.

Microstructure Study on Very-High-Cycle Fatigue of an Addi-tively Manufactured Aluminium Alloy via Advanced Charac-terization Methods

“…For grain structure, the results reported in this paper is not consistent with that in literature [4,5,9,17,29,[33][34][35][36]38]. There is no observed nanograins in the crack initiation region of VHCF at R = -1, but a few nanograins formed underneath a very local area with size about several micrometers on the fracture surface adjacent to the VHCF origin under R = 0.…”

“…Besides S-N data and the fractography, microstructure underneath the fracture surface is another important objective which can help us to comprehend the accumulative process of fatigue damage [1]. For high-strength steels, fine granular area (FGA) is a characteristic region of crack initiation in VHCF regime [25][26][27], which is identified as a thin layer with nanocrystalline and fine-grained structure [33][34][35][36] top on the both side of a pair of fracture surfaces [35]. For many titanium alloys, there are several rough area (RA) regions with crack initiation from high-cycle to very-high-cycle stage [5].…”

Microstructure Study on Very High Cycle Fatigue of an Additively Manufactured Aluminium Alloy via Advanced Characterization Methods

“…For the grain structure, the results reported in this paper are not consistent with those in the literature [4,5,9,17,29,[33][34][35][36]38,45]. There are no observed nanograins in the crack initiation region of VHCF at R = −1, but a few nanograins formed underneath a very local area with a size of about several micrometers on the fracture surface adjacent to the VHCF origin under R = 0.…”

“…Besides S-N data and the fractography, the microstructure underneath the fracture surface is another important objective which can help us to comprehend the accumulative process of fatigue damage [1]. For high-strength steels, fine granular area (FGA) is a characteristic region of crack initiation in VHCF regime [25][26][27], which is identified as a thin layer with nanocrystalline and fine-grained structure [33][34][35][36] top on the both side of a pair of fracture surfaces [35]. For many titanium alloys, there are several rough area (RA) regions with crack initiation from high-cycle to very high cycle stage [5].…”