Fracture mechanics approach to fatigue crack initiation at uniaxially loaded notches

A fracture mechanics model is developed to simulate cracking events that may have taken place at the root of a notch in a uniaxially loaded plate made of a two-phase alloy during the fatigue crack initiation life. The roughness of the notch surface resembles microcracks of different sizes and locations at the notch root along the plate thickness, where material grains of different phases, sizes, and strengths are distributed at random. Potential notch surface cracking activities are anticipated during loading cycles until the emergence of a through-thickness crack that propagates from the notch root on the plate surface along its width. Thus, the duration of the experimentally defined fatigue crack initiation at the notch root can be calculated. The model is applied to previously published experimental data on symmetrically U-shaped ferritic-pearlitic steel notched plates subjected to constant amplitude cyclic uniaxial stresses with different stress concentration factors. Plates with a random configuration were virtually tested. The comparison of experimental results with corresponding predictions validates the model. This work develops a fracture mechanics model for the FCI at uniaxially loaded notches.Potential notch root surface cracking activities during the FCI period are considered.The notch FCI life is spent propagating microcracks resembling its surface roughness.The model predicts the effect of some factors that control the notch FCI behavior.