Fatigue behaviour and life prediction of lateral notched round bars under bending–torsion loading

Fatigue behaviour of lateral notched round bars made of DIN 34CrNiMo6 high strength steel under single bending, single torsion and combined bending–torsion was studied. Crack initiation and crack growth were monitored in situ using a high-resolution digital system. Fracture surfaces were examined by scanning electron microscopy and were replicated with a three-dimensional laser scanner. The notch effect was analysed using the equivalent strain energy density concept and the fatigue life predictions were carried out through the Coffin–Manson model. Criteria based on the principal stress field were proposed to predict the most likely initiation sites, surface crack paths and surface crack angles. Finally, very good correlations between experimental and predicted fatigue lives were observed, particularly for lives greater than 104 cycles.