PSI - Issue 48

Shanyavskiy A. et al. / Procedia Structural Integrity 48 (2023) 119–126 Shanyavskiy et al/ Structural Integrity Procedia 00 (2023) 000–000

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Contemporary gearboxes use gears that rotate at speeds of more than 10,000 rpm. This means that based on consideration of a single loading cycle of the gear per revolution, the durability of 10 9 loading cycles is achieved in just 2000 hours. It is obvious that for long-term operation of gears for more than 10,000 hours, it is necessary to provide a stress level that corresponds to the durability of at least 10 10 cycles for a smooth specimen, which corresponds to the VHCF region for material operation in a structure. One way or another, but for already designed and operating structure from materials which behavior in the VHCF region has not been studied and, therefore, the range of stress levels in which fracture occurs by the VHCF mechanism is unknown, early failure of parts with the formation of a subsurface fracture origin is possible. In the stress range corresponding to the transition from VHCF to HCF, the probability of crack initiation from both the surface and subsurface of the material is different. Therefore, in the indicated region of material fracture, it is necessary to use not one but two branches of the fatigue S-N curve in the analysis of its behavior, i.e. to take into account the bimodal distribution of fatigue life in the range of 10 7 -10 9 cycles (Shanyavskiy and Soldatenkov (2022)). With regard to the gears of reducers operating at high rotational speed, a bimodal distribution of fatigue life can be observed with a wide scatter of the operating time. So, for example, at the initial stage of operation of the PS-90A engine, the failures of the gears Z 4 of the central drive of the main reducer of the engine appeared (Fig. 1). Fig. 1(a) shows the scheme of gears positions in gearbox with indicating failed in-service gear Z 4 .

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Fig. 1. (a) Scheme of the considered engine reducer with failed gears under investigation, indicating the location of the gear Z 4 ; (b) view of the gear Z 4 fractured at an operating time of 7 hours 36 minutes or 39 engine start and stop cycles; z is the number of teeth.

According to the specifications, the gears are made of high-strength steel grade 20Kh3MVF, nitrocarburized and quenched to gain the Rockwell hardness numbers for the surface layer of ≥ 59HRC and for bulk material of 34 43.5HRC, then polished to 6 degree of roughness. The ultimate tensile strength of the bulk material is 900-1100 MPa as indicated by Shanyavskiy et al. (2017). Gear rotational speed is about 14,150 rpm. The fracture of gears initiated from the surface from the geometric stress concentrators. Therefore, it became necessary to establish the cause of in-service fracture, to summarize the results of the studies performed, and to give recommendations on how to eliminate the appearance of such cases. 3. Fatigue cracks in gears Z 4 of the analyzed engine reducer Initially, the gears under investigation were subjected to metal-physical analysis in order to determine the conformity of the material quality in terms of chemical composition, structure and mechanical characteristics, which are specified by the technical documentations for their manufacturing. The performed spectral analysis of the gear material showed that by the content of the main chemical elements (chromium, manganese, vanadium, molybdenum and tungsten), it belongs to steel grade 20Kh3MVF. Deviations in chemical composition from the grade of material specified by the technical conditions were not found in any of the investigated gears. The thickness of the nitrocarburized layer is in the range of 0.62-1.1 mm that meets the conditions given by the specifications for manufacturing of gears.