Multiaxial fatigue of V-notched steel specimens: a non-conventional application of the local energy method

The paper deals with the multi-axial fatigue strength of notched specimens made of 39NiCrMo3 hardened and tempered steel. Circumferentially V-notched specimens were subjected to combined tension and torsion loading, both in-phase and out-of-phase, under two nominal load ratios, R = -1 and R = 0, also taking into account the influence of the biaxiality ratio, lambda = tau(a)/sigma(a). The notch geometry of all axi-symmetric specimens was a notch tip radius of 0.1 mm, a notch depth of 4 mm, an included V-notch angle of 90 degrees and a net section diameter of 12 mm. The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading on plain and notched specimens. Furthermore the fracture surfaces are examined and the size of non-propagating cracks measured from some run-out specimens at 5 million cycles. Finally, all results are presented in terms of the local strain energy density averaged in a given control volume close to the V-notch tip. The control volume is found to be dependent on the loading mode.