Fracture diagnostics and operational workability evaluation of working turbine blades of aircraft engine

DOI: 10.34759/vst-2019-4-108-122

Аuthors

Kiselev F. D.

Air Force Central Research Institute of the Ministry of Defense of the Russian Federation, 2-5, Aerodromnaya str, Shchelkovo, Moscow region, 141103, Russia

e-mail: kiselev_fd@mail.ru

Abstract

The topmost constituent part of the study on determining the cause parts of destruction of the aircraft in operation is fracture diagnostics employing the methods of physics-of-metals analysis of the fracture structure, material structure and composition determining, defect detection control, mechanical properties characterization, parts strength and survivability analysis.

Diagnostics of aircraft turbine blades operational fractures was performed, factors contributing to destruction were revealed, and causes of blades destruction were established. The article considers operational damageability specifics, on frequent occasions differing from the test bench ones, the systematization results of loading types, fracture mechanism, and operational fractures of gas turbine engine blades.

Methodical aspects were developed and new techniques were elaborated for fracture diagnostics were developed. The article systematizes external, fractographic and metallographic signs of diagnostics characteristic to anomalous (abnormal) modes of the engine functioning and a blade fracture at normal aircraft engine functioning (operating parameters did not outrun the operational limitations). The suggested classification allows determining blades fractures while operative diagnostics with account for joint action of static, vibration and thermal stresses in the blade material. It helps identifying blades fractures by the operational fractures types and revealing thermo­loading factors, determining the fracture mechanism, outlining it from all set of mechanical and thermal loadings acting on the blade.

The article presents the results of experimental studies of cyclic crack resistance of the blade made of VZHL12U (equiaxial crystallization) and ZHS26, ZHS32 (directional crystallization and single-crystal version correspondingly) alloys. Characterization of the blades material resistance to fatigue destruction with kinetic diagrams plotting (dependence of the crack growth rate on the stress intensity factor) was performed at the temperature of 850°C with samples loading on the vibro-bench. Eigen oscillations frequencies of the samples were of 70-120 Hz. Pulsating stretching scheme with the frequency of 50 Hz was used as well. The values of the cycle asymmetry coefficient in both cases were 0.15 and 0.35.

According to the results of high-temperature test and fatigue crack growth rate measuring on the samples from the above said alloys, kinematic diagrams of fatigue destruction, i.e. dependence of fatigue crack growth rate on stresses intensity coefficient values were plotted.

Based on the conducted fractographic studies and their results comparison with experimentally obtained ones and schematic kinetic diagram of fatigue destruction the schemes are developed; fractographically illustrated stages of fatigue crack growth and various fracture micromechanisms at different sites of the kinetic diagram of fatigue fracture in the material of the samples and blades.

The results of the work can be applied for developing more advanced modifications of turbine blades of high reliability.

Keywords:

turbine blades of aircraft engines, destruction diagnostics, fatigue strength, stress concentrators, stress concentrators effect, growth rate of fatigue crack, stress intensity factor, threshold and limit of cyclic crack resistance

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