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.     

Failure analysis of a set of compressor blades

This paper analyses the root causes of the failure of a set of blades belonging to the high pressure compressor of an aircraft engine. All these blades were manufactured using a 718 nickel base alloy. The performed study consisted in a fractographic analysis by scanning electron microscopy and a microstructural study using both scanning and optical microscopy. Phases which were present in the fracture surfaces were identified by means of X-ray energy dispersive spectrometry. As a result of this labour the failure was attributed to the impact of sand and stones; that is the so-called foreign object damage mechanism.     

Coating failure in compressor rotor blades of an aeroengine

This paper deals with coating failure in compressor rotor blades of an aeroengine. The failure modes and the causes were studied and prevention measures were suggested. Investigation revealed that blistering was responsible for the coating breakdown. The coating was a standard polymeric one (epoxy phenolic resin) and was being used successfully for this particular application for many years. Analysis showed that mechanical damages caused to the coating of the blades during overhaul and/or re-assembly of the engine were responsible for the failure.     

Fatigue strength and life of compressor blades for marine gas turbine engines

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 on the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on the fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied taking into account the effect of the medium (sea water), stress ratio in a cycle, and programmed mode of loading. The authors also consider fatigue strength of newly-manufactured blades and those in operation, as well as specific features of fatigue crack propagation in blades. They have substantiated a method for predicting the life of blades with cracks. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 28–40, January–February, 1999.     

Simulation of fatigue failure in composite axial compressor blades

Centrifugal forces are generated by a spinning impeller, of magnitudes that create large stresses. Aerodynamic forces are also imparted on an impeller blade, which varies with time and position. These two forces play different roles during compressor events. Damage accumulated from these events results in the fatigue failure of impeller material and structure. Therefore, it is important to design an impeller against dynamic and fatigue failure. The finite element method has been used in the study of impeller fracture mechanics and is regarded as an important tool in the design and analysis of material and structures.     

Corrosion resistance of Ti-6246 for high power aeroengine compressor discs and blades

Abstract

The corrosion resistance between 723 and 1123 K of Ti-6246 has been investigated. For temperatures between 723 and 823 K, the maximum weight gain in air was below the measurable range. For the temperature range 873–1123 K, there was substantial weight gain with reaction time with a maximum weight gain of 180×10^-4 g cm^-2 being obtained after 150 h at 1123 K. Weight gain for the vacuum tests remained constant over all temperatures and reaction times with a maximum observed increase of 3×10^-4 g cm^-2. There was evidence in favour of parabolic rather than linear increases in weight gain with time. The parabolic and linear rate constants increased with temperature and the activation energy associated with parabolic oxidation was estimated as 216 kJ mol^-1. The life of Ti-6246 in argon at 773 K was found at certain stresses to be almost twice that obtained in air. This difference could not be explained by the loss of load bearing cross-sectional area following oxidation.     

Springback and forward slip compensation in designing roller cavity surfaces for net-shape rolling compressor blades

Designing roller cavity surfaces for rolling blades is a nontrivial task. This paper presents a new method to design cavity surfaces for rolling compressor blades with considering the springback and forward slip compensation. Firstly, a springback compensation model was proposed to modify the shape of section curves, and a forward slip model was proposed to determine the location of section curves to compensate the springback and forward slip in rolling process. Secondly, an algorithm was proposed to transform the updated section curves to the cavity section curves around the roller axis. The cavities were reconstructed through the transformed section curves. Finally, the designed roller cavities for a variable cross-section compressor blade were validated by the finite element method and experiment. The results revealed that the designed cavities meet the net shape precision at pressure and suction surfaces of the blade. The compensated design of cavities for blade rolling can save both time and costs while keeping excellent design quality.     

Initiation of combustion of supersonic propane-air flow by magnetoplasma compressor discharge

Results are given of investigations of the initiation of ignition and combustion of an air-propane flow in a supersonic wind channel with preheating by a pulsed discharge of a low-power magnetoplasma compressor of the erosion type. The mode of explosive ignition of mixture with formation of a combustion wave propagating towards the flow is revealed. The effect of initial conditions (fuel excess coefficient, preheating of air) on the spatial position and intensity of ignition is investigated.     

Cement kiln dust induced corrosion fatigue damage of gas turbine compressor blades – A failure analysis

Compressor of one of the gas turbines installed in a power plant was stopped under emergency conditions. Primary investigation showed that almost all of the first stage blades and some of the next stages were severely damaged. In this study, one of the first stage broken blades was failure analyzed. The results showed that the corrosion pits were formed on the compressor blade surface due to the presence of Cl and S elements in the compressor inlet air. Since the power plant located in the vicinity of a cement company and also an oil refining company, the inlet air of compressor had large amounts of Cl and S containing compounds. The corrosion pits acted as stress concentration sites, and facilitated fatigue crack initiation and propagation, leading to final fracture of the blades.     

Failure analysis of Ti6Al4V gas turbine compressor blades

The failure mechanism of Ti6Al4V compressor blades of an industrial gas turbine was analysed by means of both experimental characterisations and numerical simulation techniques. Several premature failures were occurred in the high pressure section of the compressor due to the fracture of the blade roots. Metallurgical and mechanical properties of the blade alloy were evaluated. A 2D finite element model of the blade root was constructed and used to provide accurate estimates of stress field in the dovetail blade root and to determine the crack initiation in the dovetail.

The results showed no metallurgical and mechanical deviations for the blade materials from standards. SEM fractography showed different aspects of fretting fatigue including multiple crack initiation sites, fatigue beach marks, debris particles, and a high surface roughness in the edge of contact (EOC). The numerical model clearly showed the region of highest stress concentration at the front EOC of the blade root in the dovetail region, correlated closely with the experimentally characterised fatigue crack region. It was concluded that this failure has occurred due to the tight contact between the blade root and the disk in the dovetail region as well as low wear resistance of the blade root.     

Influence of plastic predeformation on the fatigue strength of compressor blades with defects

We study the influence of plastic predeformation by bending to create deep residual compressive stresses on the fatigue strength of 13Kh11N2V2MF steel specimens and compressor blades with cut and stamped V-shaped and semicircular defects, and also the use of design-based redistribution of the stresses in the blade (offsetting toward the edges) for the retardation of cracks in the stress concentrators. It is found that plastic predeformation increases the fatigue strength by about 20%, while offsetting with the creation of a compressive mean stress _m = –300 MPa in the defect region increases the fatigue strength by 2–2.5 times. We examine schemes for calculating the fatigue strength with account for the residual and mean stresses, and also examine the defect creation technique. Recommendations are made for increasing the strength of compressor blades with defects.Translated from Problemy Prochnosti, No. 10, pp. 76–88, October, 1994.