Fatigue Life Estimation of Notched Members

From an engineering view, it is convenient to separate the total fatigue life of notched members into two portions: crack initiation, which is spent nucleating and growing small cracks, and the crack propagation life, which is spent growing these cracks to final fracture. The difficulty in applying this concept has been in defining the size of an initiated crack in a smooth specimen and dealing with small crack growth in the plastic zone near the notch root.

These difficulties may be overcome by considering a simple model where the total fatigue life is the summation of the portion of life controlled by notch plasticity and the portion controlled by nominal stress and crack length. The local strain approach is used to compute the initiation life. Growth of small cracks in the notch plastic zone is assumed to be part of the initiation life. Fracture mechanics concepts are employed to estimate crack propagation lives assuming an initial crack size equal to the notch depth.

Twelve sets of fatigue data reported in the literature were analyzed to assess the validity of the model. These include variations in specimen type, notch size, notch acquity, and material properties. Good correlation between the analytical estimate and experimental data was observed.