PSI - Issue 80

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

183 7

possible. In this case, a significantly larger plastic deformation zone can be observed. Fig. 9 depicts two distinct areas of the gear tooth that undergo plastic deformation. The first is located at the root of the tooth, on the flank opposite to the one engaged in meshing. This is the result of the stresses imposed by the bending moment acting on the tooth, compressive on this flank, as the stresses induced by nitriding, which results in the two contributions adding up. On the root on the meshing flank, however, the opposite behaviour occurs, here the stresses generated due to tooth bending are tensile, and thus subtract their contribution from those imposed by the surface treatment, leading to a less critical condition for the component. The second area is instead located beneath the contact surface, with a peak that occurs consistent with what is observed in the literature at the interface between the nitrided and non-nitrided zones. This occurs because nitriding induces an equi-biaxial compressive stress state in the outermost layers, parallel to the treated surface, whereas the residual stresses are negligible in the direction perpendicular to it. When a pressure such as that imposed by tooth contact is applied, an additional compressive stress is generated, perpendicular to the induced ones, resulting in a stress state closer to hydrostatic hence, less damaging. At the interface between the treated and untreated zones, however, the residual stresses are tensile. The application of a third compressive stress component leads to a highly deviatoric stress state, which is therefore more critical for the component. Plastic deformation in this second region is particularly critical, as it alters the stress field on the meshing flank, shifting the favourable compressive surface stresses toward zero or even tensile values. Moreover, contrary to what has been hypothesized by observing the fractures, the plastic deformation zone is not located where crack initiation occurs, but rather just below the contact.

Fig. 8. Contact status of the second meshing configuration .

Interface between nitrided and non-nitrided zone

[max]

Contact point

Tooth root of the non-meshing flank

Tooth root of the meshing flank

[min]

Fig. 9. Plastic strain after the application of the overload, detail of a tooth section.