PSI - Issue 75

148 Luca Corsaro et al. / Procedia Structural Integrity 75 (2025) 140–149 Luca Corsaro , Francesca Curà, Raffaella Sesana / Structural Integrity Procedia (2025) 9 analysis of Table 2, it can be pointed out that optimal results were only achieved in case of 20MnCr5 gear. As a matter of fact, the σ FP-TCM , properly evaluated with the Thermographic Method, was in good agreement with both reference results, σ FP-ISO and σ FP-SC respectively. Moreover, the σ FP-TCM was comparable with the possible results provided by the Staircase variability. On the other hand, the σ FP-TCM estimated from the C45 induction hardened gear was different from the refence value calculated from ISO 6336-3 (2019) and ISO 6336-5 (2019) Standards. Table 2. σ FP results.

C45 induction hardened gear

20MnCr5 carburized gear

σ σ σ

934

650

FP-ISO

-

683 ± 50

FP-SC

626

640

FP-TCM

In order to provide a justification for the result obtained in case of C45 induction hardened gear, a metallographic analysis was carried out on one tooth with the objective of examining the hardening pattern that resulted from the surface treatment. The result indicated that the induction hardening surface treatment was not properly optimised for a bending resistance, as the hardening pattern was guaranteed up to the upper part of the tooth root, as shown in Fig. 7. A significant hardness reduction, down to 420 HV, was measured where the martensitic microstructure was not produced along the root (see letter B in Fig. 7). These measurements were lower than the corresponding values obtained in areas where the induction hardening process resulted in a complete microstructural transformation (see letter A in Fig. 7). In addition, the lowest hardness value measured at the tooth root is lower than the minimum value suggested by ISO 6336-5 (2019) in case of induction hardening surface treatment. From these considerations, it is clear that the reference value obtained from the computations was not representative of the real bending resistance of the tested gear, and a lower value should be considered.

Fig. 7. Metallographic analysis for the C45 induction hardened gear.

5. Conclusions The present study investigated the possibilities of the Thermographic Method for the fatigue limit estimation in case of a mechanical component such as gears. The following conclusions can be drawn. • The thermal emissions produced at the tooth root exhibited a non-stabilised temperature profile trend, as is typically observed in case of testing classical specimens. The thermal parameter analyses (integrated area) confirm the abrupt temperature increment produced from a certain load as a consequence of different intrinsic dissipations generated by micro plasticisation or dislocation movements during the bending fatigue tests. • The estimation of the F pn ∾ was carried out by analysing the thermal parameters evolution (integrated area) with the TCM. The value obtained in case of 20MnCr5 carburized gear was comparable to the reference ones. On the other hand, the C45 induction hardened gear exhibited a result that was not comparable with the reference value. This was due to the hardening pattern not adequately optimising for bending resistance.