PSI - Issue 75

Luca Corsaro et al. / Procedia Structural Integrity 75 (2025) 140–149 Luca Corsaro , Francesca Curà, Raffaella Sesana / Structural Integrity Procedia (2025)

143

4

Fig. 1. (a) Thermal profile; (b) Fatigue limit estimation. The difference in the evolution of the thermal parameters was indicative of the intrinsic dissipation generated within the material. As a matter of fact, when the loading levels were below the fatigue limit, the intrinsic dissipations were mainly caused by anelastic phenomena, while micro plasticisation or dislocation movements were responsible for the intrinsic dissipations produced when the loading levels were above the fatigue limit. This enables the identification of a transition zone where the fatigue limit can be estimated. In the specific application of tooth root bending fatigue strength estimation, as presented in this work, the TCM methodology was adapted on the basis of the nominal pulsating force, rather than the traditional amplitude stress (F pn , as reported in Fig. 1 (b)). Regarding the thermal parameter (TP* as shown in Fig. 1 (b)), the area obtained from an integration of the temperature profile was considered for each specific loading condition. This approach enables the estimation of F pn ∾ , providing a rapid and non-destructive procedure as an alternative to that estimated by the Staircase method. 2.2. ISO 6336 Standards calculations The ISO 6336 Standards (see ISO 6336-3 (2019) and ISO 6336-5 (2019)) were adopted for the calculation of load capacity of the investigated spur gears. More in detail, the Method B was chosen for the computations of both the tooth root stress ( σ F ) and the permissible bending stress ( σ FP ). The σ F is determined as the product between the maximum local principal stress produced at the tooth root and specific stress correction factors. In case of spur gears, and considering STBF test conditions, the σ F is calculated as follows: = (1) where, in this work, the F t is evaluated considering the tangential contribution of the applied force during the bending fatigue test (F pn ). On the other hand, an estimation of the limit value of the tooth root stress ( σ FP ) can be performed correcting the nominal stress number (relevant to the influences of the material, the heat treatment and the surface roughness of the test gear root fillets) with coefficients that take into account the mechanical and geometrical parameters of the gear under investigation (see ISO 6336 -3 (2019) and ISO 6336-5 (2019)), obtaining: = (2) where σ Flim is determined according to the hardness of the gear under investigation (see ISO 6336 -5 (2019)). In this study, the fatigue limits in terms of bending fatigue stress were obtained by using both equation 1 and equation 2. In case of experimental activities (Thermographic and Staircase methodologies), it is evident that the