**3. Materials and Methods**

In the present paper, two different gear-samples geometries subjected to STBF loading have been modeled by means of Finite Element Model (FEM). In Table 1, the geometrical parameters of the gears are reported. The above-mentioned gears were studied experimentally in [38,40]. The authors have collected experimental images of cracks in several teeth

(related to the study conducted in [38,40]) in which failure by TBF occurred. Through these images, it has been possible to extrapolate the position of the nucleation point of the cracks in the teeth root region and the direction (angle) of early propagation (Section 3.1). FEM results have been elaborated through critical plane criteria to characterize the crack behavior in the *ρf P* (Section 3.2). The numerical and experimental results have been compared in Section 4.


**Table 1.** Geometrical parameters of the simulated gears—Gear A [38], Gear B [40].
