Abstract
Fatigue crack initiation behaviour of notched 34CrNiMo6 high-strength steel bars with transverse blind holes and lateral U-shaped notches subjected to proportional bending-torsion loading is studied. Crack initiation at the geometric discontinuities is examined in situ using a high-magnification digital camera. Crack initiation mechanisms and failures modes are investigated after the fatigue testing program via scanning electron microscopy. Fatigue crack initiation lifetime is predicted using an averaged Total Strain Energy Density approach in conjunction with the Theory of Critical Distances. Crack initiation sites, crack surface angles and surface crack paths at the geometric discontinuities are successfully determined a priori from the maximum value of the first principal stress, the first principal direction, and the first principal stress field at the notch surface, respectively. Overall, fatigue lifetime predictions are very well correlated with the experimental observations.
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