Microstructure-Based Model for Sharp Stress Raiser-Related Fatigue Stage Length Assessment

A model for evaluating the fatigue life of specimens/structure elements with sharp stress raisers/defects is presented. The model permits of computing the number of cycles to fatigue crack initiation and its growth from a sharp stress raiser to failure at a constant stress span with the only application of characteristics of static strength and microstructure of the initial material. The model can be used to assess the fatigue life of components that contain structural stress raisers and defects stemming from their manufacturing technique (surface roughness, surface cuts, scratches, and microcracks). The model reliability was verified with experimental results taken from the literature, calculations appeared to be in good agreement with experimental data. Fatigue curves to a grain-size crack initiation and to fracture of smooth specimens and those with a chemically-notched blunt raiser that simulates the casting defects in aircraft components were calculated. The two sets of specimens from a Ti–6Al–4V titanium alloy differing in the cross-section (rectangular and cylindrical) and in microstructure (different grain sizes). Smooth specimens exhibited the test surface roughness R_v = 10 and 19 μm (average dent depth), which was assumed to be a sharp raiser for calculations. The model need not long-term and labor-consuming high-cycle fatigue tests to construct the fatigue curve.