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 dama ge formation and evolution within the active zone was evaluated from the hy steresis energy of unnotched and notched specimens. Due to head hardening o f 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. Fatigu e tests were performed on both unnotched and single edge notched (SEN) spec imens under tension-tension load control condition at 5 Hz. The maximum fat igue stress was 200 MPa, which is about 40% of the yield strength of the ma terial. The minimum to maximum stress ratio was 0.1. The crack length, numb er of cycles, and hysteresis loops were recorded during the tests from whic h the crack speed, the energy release rate, and the hysteresis energy for b oth notched and unnotched specimens were determined. The rate of energy dis sipation on damage formation was evaluated based on the difference between the hysteresis energy for the notched and the unnotched specimens. These da ta were used in the MCL theory to extract the specific energy of damage, ga mma'; a material parameter characteristic of the fatigue crack growth resis tance 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 exa mination of the active zone revealed damage species in the form of microcra cks, inter-granular separation, and plastic deformed material. It is these damages that have led to the crack deceleration in the second stage. The fr acture surface was also examined. The initiation region showed drawn-out la mellar pearlite. Ductile tearing and coarse ridges with intensive lamellar formation as well as microcracks were observed in the second region. The fo rmation of these damage species has also contributed to the crack decelerat ion in the second stage of fatigue crack growth kinetics. The unstable crac k growth region displayed cleavage facets initiated from the grain boundari es. (C) 2001 Kluwer Academic Publishers.