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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A metallographic study on transverse slices showed that the material structure of the nitrocarburized layer and the bulk material of the gear is satisfactory. At first, the regularities of the occurrence of fatigue cracks in the gear Z 4 are considered. The scatter of the operating time of the gear Z 4 under consideration at the moment of its fracture on the stand during the acceptance tests and in-service was in the range of 6-7285 hours. Providing that the rotational speed of the gear in the operating mode is 14,150 rpm, it was revealed that at the moment of the gear fracture, their in-service life was in the range of about 5∙10 6 -6∙10 9 cycles. It means that the gears with the minimum in-service life were fractured in the HCF region, while at the maximum in-service life, the fracture was realized in the VHCF region. As an example, one of the cases of gear failure from the zone with the least damage to the surface is considered. In 2015, a shutdown of the PS-90A engine of the first power plant of in-flight Tu-214 aircraft occurred. The crew of the aircraft was forced to do an emergency landing. The disassembly of the engine showed that in-flight engine shutdown is due to the fracture of the driven gear Z 4 (Fig. 2). The engine under consideration has been operating for 92 hours or 37 engine start and stop cycles (ESSC) since the new, i.e. the loading of the gear was realized by 14,150×60×92=7.8∙10 7 single loading cycles. The indicated lifetime corresponds to the transitional region from the VHCF to the HCF.

Fig. 2. Overview of the in-service failed gear Z 4 of the considered engine with lifetime in-service of 92 hours since the new (or 37 engine start and stop cycles). The arrow indicates the area of the fracture origin.

The fractographic analysis with using a scanning electron microscope EVO-40 (Carl Zeiss GmbH, Germany) showed that the main fracture has the HCF nature with origins of crack located on the surface of the bottom land from the side of the small module in the chamfer zone. The zone of initial crack propagation on the fracture surfaces is damaged. The nature of the crack propagation from the origin zone indicates that the crack initiated, as in most previous cases, from smoothed machining scores. The fracture surface in the whole area of fatigue crack development is characterized by the presence of meso-beach-marks, which result from a change in the loading operating modes of the gear during each flight cycle. As a result of counting the indicated regularly repeating meso beach-marks, it was found that the fracture of the gear was performed for at least 28 flight cycles. The high-cycle fatigue behavior of the gear fracture should not be associated with a high stress concentration due to the identified scores near the crack origin zone, but with the high stress state of the structural element in this zone. The relatively low lifetime of the gear (92 hours, 37 ESSC) compared to the rest of the gears operating without failure indicates the following. The realized in-service stress level is close to the fatigue limit of the material, σ–1. Therefore, the insignificant changes in the stress state level of the gear, which is associated, among other things, with the appearance of stress concentrators in the form of machining scores, can cause a scatter of durability by almost three orders of magnitude in the region of transition from VHCF to HCF.