Determination of the critical plane and assessment of fatigue durability under various cyclic loading regimes

Analytical solutions are obtained to determine the critical plane orientation for multiaxial stress state. This critical plane is the plane of development of fatigue damage under cyclic loading. The cases of in-phase and antiphase cyclic loadings are considered for the classical fatigue range (low-cycle and high-cycle fatigue). Generalizations of the Findley fatigue criterion are proposed taking into account the orientation of the critical plane for modes of very-high-cycle fatigue under in-phase and antiphase cyclic multiaxial loadings. These generalizations are based on the similarity of the left and right branches of the bimodal fatigue curve. The procedure aimed at determining the parameters of the generalized criterion is described according to the data of two uniaxial fatigue tests for tension-compression at various coefficients of the cycle asymmetry. The stressed state of the compressor disk of the gas turbine engine are calculated by the finite element method for a low-cycle fatigue (take off-flight-landing cycles) and for very-high-cycle fatigue (vibrations of blades). For low-cycle fatigue the effect of aerodynamic, centrifugal and contact loads for the contact system of disk, blades, fixing pins and shroud are taken into account. For very-high-cycle fatigue the additional stresses due to high-frequency torsional vibrations of blades and shroud are calculated. In both cases, a stress concentration zone was defined in the vicinity of the contact between the disc and the blades, in which the fatigue damage originates. The distribution of stresses and the proposed generalizations of the fatigue fracture criteria were used to obtain estimates of disk durability for both low-cycle fatigue and very-high-cycle fatigue modes. Numerical analysis showed that real time durability for the considered fatigue modes can be very close taking into account the characteristic cycle period. Therefore, in predictions of a safe operation life both mechanisms of fatigue must be taken