PSI - Issue 30

S.P. Yakovleva et al. / Procedia Structural Integrity 30 (2020) 201–208

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Yakovleva S. P. et al. / Structural Integrity Procedia 00 (202 ) 0 0–00

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Fig. 3. Crack in the working blade #1 and the metal microstructure: a - view of a crack from the side of the root ridges (arrows show the initial cracks); b - the cross-section of a crack (arrow shows the direction of crack propagation); c - the central part of a crack (section 1 in Fig. 3, b ); d - the mouth of a crack (section 2 in Fig. 3, b) Blade #2 has undergone complete destruction along a feather that started from its entrance edge (Fig. 1, c ). The fracture is significantly damaged by high-temperature oxidation and impacts of fragments of the destructed blades. The structure of the least damaged section of the fracture at the output edge of a feather (Fig. 4, a ) reflects the crystallization macrostructure of the metal. The intercrystalline character of the fracture indicates the weakened macrograin boundaries and a sufficiently fast crack propagation rate. High-temperature operating conditions of the blade under thermocyclic and alternating loads accelerate diffusion and fatigue processes that lead to changes in the structure and active accumulation of thermal-fatigue damages as found by Kablov (2015), Getsov (2010), Logunov (2017). The leading edge of a feather refers to those areas of blades where cracks often take place as shown by Kostyuk and Frolova (2001). Fig. 4, b shows that the metal degradation has reached the level of multiple micropores formation including at the grain boundaries. In addition, as shown above, a dissolution of intermetallide in the nickel matrix. At large magnifications, numerous superstructural packaging defects are noticeable inside the intermetallide particles (Fig. 4, c ), the appearance of which means that the intermetallide loses the properties of its hardening phase as pointed by Viswanathan (1989), Shirzadi A. and Jackson (2014). Intercrystalline micro tears on a surface served as centers of origin of fatigue cracks (one of them is shown in Fig. 1, c ), the development of which was favored by porosity, depletion of the solid solution with intermetallide and softening of intermetallide particles. The rapid development of several cracks and their integration into main led to the final destruction of the working blade. Thus, the study of the destructed working blade revealed the accumulation of irreversible changes in the metal structure indicating its insufficient heat resistance and explaining the limited resource of the blade under the operating modes of GTU in the zone of climatically low temperatures.