Fatigue fracture planes and expected principal stress directions under biaxial variable amplitude loading

Fatigue behaviour under multiaxial variable amplitude loading can be examined by applying the failure criteria based on the critical plane approach. Positions of the critical plane can be determined in relation to the principal stress or strain directions. In the present paper, the expected directions of the principal stresses under proportional and non-proportional loading have been obtained by averaging the instantaneous values of the Euler angles through special weight functions. The known weight functions based on stress parameters appear not to be efficient for each loading or material being analysed. Thus, the authors consider new weight functions based on energy parameters. The presented forms of weight functions are assessed by testing 18G2A steel specimens. It is concluded that the fatigue fracture plane position under multiaxial loading can efficiently be determined on the basis of an averaged direction of the maximum principal stress by employing a suitable weight function.