Fatigue behaviour of C40 specimens under combined traction and torsion loading, in phase and out of phase [Caratterizzazione a fatica di provini in acciaio C40 soggetti a trazione e torsione combinate, in fase e fuori fase]

The paper presents the preliminary results of a research program mainly oriented to the definition of methodologies for the design of mechanical components subjected to multiaxial fatigue in the presence of small values of the notch root radius. All results reported here were obtained from specimens in C40 carbon steel (normalised state). Firstly, the fatigue strength under fully reversed pure axial and pure torsion loading was investigated by testing plain and notched specimens; afterwards V-shaped specimens were tested under combined traction and torsion loading, in-phase and out-of-phase. For all V-notched specimens, the diameter was 20 mm, the notch depth and the opening angle were 4 mm and 90 degrees, respectively. Finally, the nominal value of the notch tip radius was 0.5 mm. In the presence of a such small radius the fatigue limit is no longer controlled by the theoretical stress concentration factor K t and no simple correlation exists between Kt and the fatigue strength reduction factor Kf mainly because the notch sensitivity index exhibits a large scatter, in particular under torsion. On the other hand the value of the notch root radius is too large for using an approach based on Notch Stress Intensity Factors (N-SIFs) for Mode I and III. These parameters from a theoretical point of view, are defined only for sharp V-shaped notches. Fatigue tests were carried out on a MTS 809 servo-hydraulic axial-torsional testing device with an axial cell of 100 KN and a torsion cell of 1100 Nm. V-notched specimens were tested under two nominal load ratios, R=-1 and R=0, while keeping constant and equal to unity the biaxiality ratio, λ=σa/τa. Therefore four Wöhler curves were obtained: two curves for the in-phase loading (Θ=0), two curves for the out-of-phase loading, with a phase angle equal to 90 degrees. During the tests both axial and torsional stiffness were monitored. After the fatigue tests, notch roots and fracture surfaces were analysed by means of optical and electronic microscopy, with the aim to allow a better interpretation of damage mechanics and fatigue crack nucleation and propagation. Finally, all data from notched specimens were summarised in two bands of limited scatter by using as parameters the range of the total strain energy density present at the notch tip or, alternatively, the range of the deviatoric strain energy density. A future extension of a local- energy-based approach should consider the values of the energy over a finite volume surrounding the notch tip and no longer the maximum local value of the strain energy density at the notch tip.