PSI - Issue 80

Francesco Manni et al. / Procedia Structural Integrity 80 (2026) 177–186 Francesco Manni/ Structural Integrity Procedia 00 (2019) 000–000

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conditions are often required. To enhance the performance of these components, treatments are often used to improve the mechanical properties of materials. To achieve this, various methods exist, some exploit the plastic deformation of the metal to induce favourable residual stresses (Barbieri et al., 2022), while others, such as surface treatments, may also interact with the composition of the material itself, affecting its crystalline matrix (Jegou et al., 2018). Among these, nitriding is particularly effective in increasing the hardness of the surface layers, significantly improving fatigue and wear resistance (Conrado et al., 2017; Pye, 2003). The increase in hardness is due to the absorption of nitrogen into the crystalline matrix, which leads to the formation of particularly hard secondary phases, such as nitrides, as well as the expansion and distortion of the matrix itself (Hernandez et al., 2008; Torchane et al., 1996) . This process also generates a residual compressive stress state in the surface of the material, which has beneficial effects on fatigue resistance. However, the presence of such stresses also induces a tensile residual stress state in the internal zones, which negatively impacts the strength of the component. The nitriding treatment allows nitrogen to diffuse into the material in a nitrogen-rich atmosphere. It is known that diffusion follows a quadratic trend (Cavaliere et al., 2009), as does the increase in hardness, whereas residual stresses do not exhibit a monotonic behaviour (Podgornik et al., 2011), moving towards the inner regions of the material and reaching a maximum below the surface. This complex stress state makes it challenging to predict the behaviour of the component in operation and to identify the critical zones where damage nucleation may occur. In particular, while the treated surface layer exhibits high resistance, the residual tensile stresses in the internal regions can promote damage formation beneath the surface (Boiadjiev et al., 2014), making its detection more difficult. In the present study, a hybrid system for automotive use has been analysed. In fact, in an architecture featuring different power sources, an intermediate element between the two is necessary such as a transmission chain (Mangeruga et al., 2023) or, as adopted in this configuration, a gear train. These systems, due to the high torques and speeds involved, have often required the use of high-performance components to ensure effective power transmission between the two propulsion units. Furthermore, the large number of transients they encounter throughout their service life necessitates the management of highly variable load histories over time, sometimes characterized by stresses exceeding design limits, which could generate unforeseen behaviours within the material of the components. Unfortunately, the usage of nitrided gears leads to significant cost increases. Therefore, it becomes essential to have the ability to determine whether a component subjected to temporary overloads can still be used. In fact, any plastic deformation caused by excessive overloading could significantly alter the residual stresses, thus making a fatigue analysis necessary (Barbieri et al., 2019) on the damaged component to predict the behaviour of the system. This study has a twofold objective related to the considered test case. On one hand, a numerical calculation method for modelling nitrided gears and their meshing is developed. On the other hand, it is determined whether a gear that has undergone an overload still retains a fatigue strength sufficiently comparable to the original component. The paper is structured as follows. First, the motivations behind the study are presented. Secondly, the discretization of the model and the applied boundary conditions are exposed. Subsequently, a methodology is conceived to impose the calculated stress state within a Finite Element (FE) model. Then, various different meshing configurations are simulated via static analyses to identify the most damaging ones. Finally, the effect of a complete overload cycle is analysed to observe the full evolution of the stress state. To conclude the study, fatigue analyses have been carried out to evaluate the variation in strength of a wheel subjected to overload. Finally, some conclusions end the paper. 2. Background and Motivation Fig. 1 shows a typical shape of the fracture that occurs in the tooth as a result of the breakage, while point A indicates the crack initiation zone. Considering the test case, it has been observed that only the gears previously subjected to overload have been later prone to failure after being put back into operation. This suggested that the failure has been related to an alteration of the stresses developed on the surface through nitriding, and therefore, more generally, to a decrease in the fatigue strength of the material. Consequently, it has been necessary to develop a model capable of, first, simulating the conditions imposed by the surface treatment and, then, investigating how potential plasticization caused by an overload could jeopardize the residual stresses.