Effect of initial crack length on the transition of fatigue properties for precipitation hardened steel

<p>Two different types of precipitation hardened steels are chosen for the present study. The two steels differ in the nature of the precipitates they exhibit. The fatigue behavior of the two steels for initially grain and long crack is analyzed. The main focus point in this part is the crack propagation behavior. The two steels have soft copper precipitates and hard titanium precipitates. The study carried out incorporates the analytical, experimental and microstructural observation techniques. The two materials are tested for fatigue crack growth and crack growth is monitored with the help of replica technique. The outcome of the fatigue test reveals that the soft precipitate steel is having better fatigue properties in short crack regime and vice-versa. This points to a transition in the fatigue properties of two steels for a particular crack length. This concludes the further work on this theme. Though the steel having hard precipitate also exhibit finer grains, but instead of having superior properties in both regimes it does so only in long crack regime. Also the tensile strength of both type of steels is nearly same. As fatigue strength is a function of tensile strength but the observations here also violates this theory. The possible reasons for the observed phenomenon is explored with the help of DIC technique, FEM analysis technique and crack growth observations.</p>