Estimation of fatigue life under multiaxial loading by varying the critical plane orientation

Fatigue life of different metallic materials under multiaxial loading is evaluated by employing the critical plane-based criterion proposed by some of the present authors. According to such a criterion, the multiaxial fatigue strength is assessed through an equivalent stress expressed by a linear combination of the normal stress amplitude and the shear stress amplitude acting on the critical plane. The critical plane orientation is determined by a novel expression, which is a non-linear function of the ratio between the fully reversed stress fatigue limits (τaf,−1/σaf,−1), and such an expression can be employed for any value of this ratio. Some experimental fatigue life data related to smooth specimens under combined normal loading (tension or bending) and torsion loading are compared with the theoretical estimations deduced through the above criterion.