Transformation of defects into fatigue cracks; the role of Kt and defect scale on fatigue life of non-pristine components

Fatigue lives of 2 mm aluminium 2024-T351 sheet samples were measured. The samples contained introduced surface scratches between 50 μm and 180 μm deep, with root radii of 5, 25 and 50 μm. Fatigue cracks initiated from the scratches, final failure cracks being about 1 mm in length. Thus the entire crack growth process took place in the short crack regime. Fatigue crack growth rates in the sheet through thickness direction were measured using striation spacing measurements, allowing the calculation of crack growth lives as a fraction of total sample life. All scratches reduced the fatigue life compared with those of pristine samples. Whilst Kt was an important parameter determining extent of life reduction, notch root radius was equally important. Stress fields at the scratch root were studied using elastic-plastic finite element models. Life data and models were used to develop a unified approach for prediction of life of non-pristine components, incorporating effects of defect geometry.