Fatigue crack growth analysis of a premium rail steel

The fatigue crack growth behavior of a premium rail steel was studied using the Modified Crack Layer (MCL) theory. The rate of energy expended on damage formation and evolution within the active zone was evaluated from the hysteresis energy of unnotched and notched specimens. Due to head hardening of the rail, there is a vertical microstructure gradient inside the rail. In this work, the fatigue test specimens were sliced longitudinally from the head of a new rail near the web which represents the microstructure of the base material. The notch length to sample width ratio (a/w) was 0.1. Fatigue tests were performed on both unnotched and single edge notched (SEN) specimens under tension-tension load control condition at 5 Hz. The maximum fatigue stress was 200 MPa, which is about 40% of the yield strength of the material. The minimum to maximum stress ratio was 0.1. The crack length, number of cycles, and hysteresis loops were recorded during the tests from which the crack speed, the energy release rate, and the hysteresis energy for both notched and unnotched specimens were determined. The rate of energy dissipation on damage formation was evaluated based on the difference between the hysteresis energy for the notched and the unnotched specimens. These data were used in the MCL theory to extract the specific energy of damage, gamma'; a material parameter characteristic of the fatigue crack growth resistance of the rail steel. It was found that the value of gamma' is 1300 kJ/m(3). Three distinctive stages of crack growth kinetics were observed; crack initiation, stable crack growth and unstable crack growth. Microscopic examination of the active zone revealed damage species in the form of microcracks, inter-granular separation, and plastic deformed material. It is these damages that have led to the crack deceleration in the second stage. The fracture surface was also examined. The initiation region showed drawn-out lamellar pearlite. Ductile tearing and coarse ridges with intensive lamellar formation as well as microcracks were observed in the second region. The formation of these damage species has also contributed to the crack deceleration in the second stage of fatigue crack growth kinetics. The unstable crack growth region displayed cleavage facets initiated from the grain boundaries. (C) 2001 Kluwer Academic Publishers.