Multiaxial fatigue limit for defective materials: mechanisms and experiments

To study the influence of defect geometry on the fatigue behaviour, experiments are conducted under tension and torsion on a C36 steel containing artificial defects of different sizes and shapes. Results show that the shape of the defect influences the fatigue limit at a given size. A study of fatigue mechanisms shows that cracks propagate in the plane perpendicular to the maximum principal stress direction because of the stress distribution around the defect. Nevertheless, it is shown that fatigue crack initiation in the maximum shear plane seems to be the governing mechanism at the tip of the defect for low stress concentration defect. Stage I of the fatigue process is not negligible for defective material. A multiaxial endurance criterion is used to analyse the experimental results: it appears necessary to calculate the local stress distribution around the defect to characterise the defect morphology in a multiaxial criterion.