Multiaxial Fatigue

So far fatigue has been treated assuming that simple uniaxial loads were acting. The same S–N fatigue curves were obtained under uniaxial conditions, generally a push-and-pull tension or a fully reversed bending. Unfortunately, uniaxial loads are not that common as it would be expected. In most real cases stresses acting on a work piece or on a part of it are not uniaxial even dough the external load is uniaxial. It suffices to consider a component with a hole or a discontinuity where localized triaxial stress state develops. Besides the localized triaxiality, multiaxial loading represents the most general working condition for real components subjected to stresses acting simultaneously in different directions or, which is equivalent, to combined bending and torsion. The latter case is quite common. A torsion arm of a car rear axle or a helical spring or a gear shaft or a crankshaft are typical examples.