Interior crack initiation during the very-high-cycle fatigue of railway
wheel steel under axial loading and rolling contact loading
Abstract
In this paper, a comparative study of the very-high-cycle fatigue (VHCF)
behavior of railway wheel steel under axial loading and rolling contact
loading was conducted. Fatigue tests were performed with an ultrasonic
fatigue test machine under axial loading, and the fracture surfaces from
the fatigue tests and shattered rims taken from the failed railway
wheels were observed. The wheel steel under axial loading presents a
VHCF behavior with Mode I crack, and that under rolling contact loading
is a VHCF behavior with mix Mode II-III crack. For the VHCF behavior
with Mode I crack ,surface and interior crack initiation occurred with
equal probability at both low and high stress levels and produced a dual
linear S-N curve since the value of fatigue limits for the surface and
interior crack initiation are close. For the VHCF behavior with mix Mode
II-III crack, cracks were initiated from the interior Al
2O 3 inclusion and the fatigue life was
beyond 10 7 cycles. Fatigue bands were observed on the
fracture surface under rolling contact loading. The ferrite nanograins
formed due to the stress state of shear plastic strain with a large
compressive stress. The formed nanograins were softer than the matrix
caused by the redistribution of the carbon.