Issue 38

H. Weil et alii, Frattura ed Integrità Strutturale, 38 (2016) 61-66; DOI: 10.3221/IGF-ESIS.38.08 63 Because the criterion of Dang Van uses Tresca, it is more restrictive than Crossland. Moreover, The Dang Van criterion takes into account the phase shift of the stress tensor components through Tresca stress by maximizing the difference of the principal stresses. M ODELLING Calculation of the fatigue lifetime he purpose is to determine the fatigue lifetime profile of a nitrided layer and for a given failure probability. The idea is to find the number of cycles N corresponding to E(N) = 1 at each depth through the nitridied layer. Despite the hydrostatic pressure and the applied stresses change along the depth, they are indepedent of the number of cycles N . However, the coeffients of criteria   N  and   N  must be calculated with a new number of cycles at each iteration [5]. Some assumptions are made for the calculations of the fatigue life: - The residual stress profiles are defined for a semi-infinite solid. The stress distribution as a function of the geometry is not taken into account. - The loading undergone by the material comes from a finite element simulation. However, the simulations are carried out on a non-nitrided material. The alteration of mechanical properties (yield strength…) of the nitrided layers is not considered in the calculation of the loading. - The coefficient   N  does not depend on the depth of the nitreded layer. Indeed, it is calculated by means of the Wöhler curves in the non-nitrided material. The Wöhler curves were obtained from rotating bending tests ( R 1    ) and tensile tests ( R 0.05   ) for a given failure probability. Simulation on simple specimens Simulations were made on simple geometry in order to test the calculation algorithm of the fatigue lifetime. The failure must be initiated in the superficial layer in order to test the nitriding quality. To meet these objectives, the rotating bending test and specimen geometry with notches were chosen (Fig. 1) [15]. Figure 1 : Geometry of notched cylindrical specimen for the rotating bending. Simulations were performed on Ansys to justify this choice. A loading corresponding to an alternating stress of ± 900 MPa is applied. The stress tensor is calculated on the diameter of the critical section of the test specimen. The results are implemented into the fatigue algorithm in order to find the fatigue lifetime profile. Fig 2. reports the equivalent stress of von Mises and the hydrostatic pressure in the critical section. The significant effect of notch is apparent. Indeed, the stresses are very high on the edges of the specimen. However the rotating bending test generates no stresses in the center. Therefore, such a sample is suitable for testing the nitrided surfaces rather than the core. T