PSI - Issue 75

D. Jbily et al. / Procedia Structural Integrity 75 (2025) 158–175 Author name / Structural Integrity Procedia (2025)

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roughness. Their simulations demonstrated a clear impact of micropitting on gear dynamics and identified pinion acceleration and bearing forces as viable indicators for micropitting detection. The present work builds upon previous study (Jbily et al. (2024)), by providing a complementary analysis comparing shot peened and ground gears. Although shot peening is beneficial in theory, enhancing residual stress and improving surface roughness, it can paradoxically lead to increased micropitting damage under certain conditions. To explore the mechanisms behind this phenomenon, the study integrates microscopic examination, crystallographic analysis using electron backscatter diffraction (EBSD), and tribological testing in accordance with ASTM G133. The objective is to deepen the understanding of surface integrity and support the development of more effective strategies in gear surface engineering. 2. Background and previous results Contact fatigue tests are conducted using a back-to-back gear test rig (91.5 mm). An interrupted testing method is employed to monitor the initiation and progression of micropitting on the tooth flanks. Two variants of carburized spur gear specimens are evaluated: the reference specimen (abbreviated as REF), featuring case-hardened and ground tooth flanks typical of standard gear applications, and a second specimen with the same heat treatment and grinding, followed by an additional shot peening treatment (referred to as SP). Fatigue tests are carried out until the initiation of macropitting. The shot peening performed on the gear teeth led to higher residual stresses on the surface compared to the unpeened ground teeth. In the new condition, the metrological control shows that, compared to the REF teeth, the SP teeth exhibit: • Rsk and Rku values of similar magnitudes • Lower values of Ra, Rq, Pc, and Rz The gear contact fatigue tests conducted (Jbily et al. (2024)) showed: • That micropitting appears on both unpeened (REF) and shot peened (SP) teeth. • That for comparable test durations, the micropitting area on the shot peened flanks is larger than that on the ground flanks (see Fig. 1). • A difference in the evolution of micropitting between the REF teeth and the shot peened teeth.

REF Pinion

SP Pinion

Fig. 1. View of the flanks of the gear teeth after 1 hour (0.135*10 6 load cycle)

• Shot peening delayed Macropitting (the progression of micropitting to Macropitting); generalized macropitting appeared after about 60 hours of operation on the REF gears (Fig. 2), whereas it appeared after about 209 hours (28.2*10 6 cycles).