An investigation of the propagation behavior of fretting fatigue cracks in titanium alloys

Computational modeling of crack propagation behavior is conducted because it is very difficult to track crack growth under fretting conditions. Modeling and analysis of fretting crack propagation behavior was performed using a fracture mechanics code FRANC2D (FRANC2D/L). To obtain better accuracy, the crack growth model was combined with the finite element sub-modeling technique to estimate the crack path and crack propagation life in fretting fatigue specimens. This involved the computation of crack trajectories and stress intensity factors along the crack path, using the code FRANC2D, using different values of effective stress for cylindrical and flat pad geometries. The computed crack propagation lives were used to estimate the crack initiation lives by employing the results of a previous experimental study. In addition, a parametric study was performed to investigate the effects of other loading parameters, such as the normal load P, friction coefficient f, and Q/fP ratio on the crack propagation behavior. The results show that as the effective stress increases, the crack propagation life decreases for both the cylindrical and flat pads. The effect of the effective stress on the cylindrical pad is more deleterious than that on the flat pad. Finally, the effective stress was found to have the dominant effect on the propagation life followed by the ratio Q/fP. The coefficient of friction f has the least effect of the crack propagation behavior.; The effects of shot-peening and its various intensities on the propagation behavior of fretting fatigue cracks were also investigated. The results show that shot-peening improved the propagation life of fretting fatigue crack by about 3.4 times that of the unpeened specimens. However, the shot-peening intensity has no significant effect on the propagation life of the fretting fatigue cracks. The shot-peening intensity has greater impact in the initiation phase of the fretting fatigue crack life. The normalized initiation life increases, while the normalized propagation decreases with the shot-peening intensity.