Method for Predicting the Fatigue Life of Geometrically Discontinuous Structures Under Combined Bending and Torsion

The fatigue damage model based on theory of damage mechanics is capable of predicting the fatigue life under multiaxial loading. Meanwhile, the application of critical plane method in the prediction of multiaxial fatigue life has made certain progress. According to the law of thermodynamics, a new damage evolution equation is developed in the present study to predict the fatigue life of geometrically discontinuous structure under tension-torsion loading based on damage mechanics and the critical plane method. The essence of this approach is that the strain parameter of the uniaxial nonlinear fatigue damage model is replaced with the equivalent strain, which consists of the relevant parameters of the critical plane. However, it is difficult to calculate the stress–strain status and the critical plane position of geometrically discontinuous structure by theoretical methods because of the existence of stress concentration and the multiaxial nonproportional characteristics. Therefore, a new numerical simulation method is proposed to determine the critical plane of geometrically discontinuous structure under multiaxial loading by means of the finite element method and MATLAB software. The fatigue life of notched specimens subjected to combined bending and torsion is predicted using the proposed method, and the result is compared with those from the experiments and the Manson–Coffin law. The comparisons show that the proposed method is superior to the Manson–Coffin law and is capable of reproducing the experimental results reasonably when the geometry of the structure is complex. It completely meets the needs of engineering practice.