Influence of heat-resistant coatings on the state of thermal stress of model of gas turbine engine blades in nonsteady heat exchange

The finite-element method was applied to calculate the thermal state and the state of stress and strain of a wedge-shaped model with a heat-resistant coating (heat shield) subjected to heat cycling by a gas stream. We analyzed the influence of the method of discretization of the section of the model and of the method of specifying the characteristics of the materials of the base and of the coating on the accuracy of the solution of the problem of thermoelasticity. We obtained data on the influence of the technological factors and of the thickness of the heat shield on the kinetics of the state of thermal stress of the material of the models. It was established that a ceramic layer changes the asymmetry of the cycle to the side of tensile stresses and shifts the maximum of the stresses into the range of lower temperatures.Translated from Problemy Prochnosti, No. 6, pp. 24–30, June, 1994.     

Thermal fatigue resistance of protective coatings for gas turbine blades

Results of the investigation of thermocyclic strength of flat corset-type specimens (hour-glass-shape) (Getsov, L. B. et al., Industr. Lab., 1982, 7, 44)* from nine heat resistant alloys on a nickel base with protective coatings: electron-beam, i.e. single-layer system M—Cr—Al—Y, multilayer, dual-layer, diffusion, slurry and plasma spray coatings are represented. It is shown that the service life of electron-beam coatings is considerably higher than that of the pack cementation and slurry alumosilicide coatings. Dual-layer overlay coatings with an external ceramic layer have the optimum characteristics. Regularities are established for microcrack initiation and propagation in coatings and high-temperature alloys, depending upon the technology of the coating's deposition and test conditions.     

Effect of overloads on the growth of fatigue cracks in materials for gas turbine blades

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 4, pp. 47–52, April, 1988.     

Vibration based damage detection of rotor blades in a gas turbine engine

This paper describes the problems concerning turbine rotor blade vibration that seriously impact the structural integrity of a developmental aero gas turbine. Experimental determination of vibration characteristics of rotor blades in an engine is very important from fatigue failure considerations. The blades under investigation are fabricated from nickel base super alloy through directionally solidified investment casting process. The blade surfaces are coated with platinum aluminide for oxidation protection. A three dimensional finite element modal analysis on a bladed disk was performed to know the likely blade resonances for a particular design in the speed range of operation. Experiments were conducted to assess vibration characteristics of bladed disk rotor during engine tests. Rotor blade vibrations were measured using non-intrusive stress measurement system, an indirect method of blade vibration measurement utilizing blade tip timing technique. Abnormalities observed in the vibration characteristics of the blade tip timing data measured during engine tests were used to detect the blade damage. Upon disassembly of the engine and subsequent fluorescent penetrant inspection, it was observed that three blades of the rotor assembly were identified to have damaged. These are the blades that exhibited vibration abnormalities as a result of large resonant vibration response while engine tests. Further, fractographic analysis performed on the blades revealed the mechanism of blade failures as fatigue related. The root cause of blade failure is established to be high cycle fatigue from the engine run data history although the blades were put into service for just 6 h of engine operation.     

Thermophysical measurements during thermal cycling of gas turbine engine blades with ceramic coatings

A design-experiment method is proposed for high-frequency induction heating of turbine blades and models of combustion chamber flame tubes of gas turbine engines with heat-resistant ceramic coatings. The proposed method has been developed taking into account the electrophysical and thermophysical properties exhibited by the materials in thermal cycling tests. The results of thermophysical measurements, design-experiment studies of the nonstationary thermal state of the parts with coatings with the use of a thermal vision system, and thermal cycling tests of rotating blades and models of flame tubes with heat-resistant ceramic coatings are presented.     

Assessment of the crack resistance of multiple-seated lock joints of gas turbine blades

We present results of assessing the crack resistance of three-seat lock joint of gas turbine blade under low-cycle fatigue and creep conditions of material in the presence of a hypothetical crack. The stress intensity factor is calculated by the finite element method. __________ Translated from Problemy Prochnosti, No. 5, pp. 89–95, September–October, 2008.     

Fatigue strength of gas turbine compressor blades

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed into the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied considering the effect of the medium (sea water) and stress ratio in a cycle, as well as fatigue strength of newly-manufactured blades and those after being in operation. Specific features of fatigue crack propagation in blades have been considered and a method for predicting the life of blades with cracks has been justified.     

Investigation of Intergranular Corrosion in 2nd stage gas turbine blades of an aircraft engine

Intergranular corrosion is one of the decisive tools for the life of turbine blade for its continuous use. The study was to ascertain the occurrence of Intergranular Corrosion (IGC) in the turbine blades of an aircraft. For this purpose 2nd stage turbine blades made up of Nickel Based Superalloy Udimet 500 were used to find out attack of IGC in the top & middle portion of blades as the same was detected during Eddy Current Testing. The turbine blades having different flying hours (700, 1700 & 2700) were selected for specimen while simulating actual conditions which the turbine blades encounter during flight & maintenance/overhaul. High temperature exposure (at 900 °C for 1min & 950 °C for 1min) and alkaline media exposure (at normal and aggressive condition) were introduced to find out the damaging results. It was found that Cr carbide is precipitated at GBs on exposure to 900 °C and an excessive Cr carbide precipitation at GBs on exposure to 950 °C due to Intergranular Corrosion. On exposure to alkaline media, pitting was observed within grains and at grain boundaries on the specimen of blades.