Blade Turbines fatigue

Turbine rotating blades cannot be field-repaired if they are cracked. If one or two blades are damaged mechanically, the manufacturer may recommend field repair or replacement of the damaged blades. However, if several blades are fatigue cracked, it is recommended that the entire set be replaced, since the remaining blades have been exposed to the same operating conditions and, therefore, have little fatigue life left. 

Speidel M O, Denk J and Scarlin B 1991 Stress Corrosion Craoking and Corrosion Fatigue of Steam-Turbine Rotor and Blade Materials (Luxembourg Commission of the European Communities)... 

The operating schedule of a gas turbine produces low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the hfe of the hot sections (combustor, turbine nozzles, and blades). The life reduction effect of the number of starts on a combustor liner could be as high as 230 hours/start and on the turbine nozzles as high as 180 hours/start. The effect of full load trips can be nearly 2-3 times as great ... 

Because oxides are usually quite brittle at the temperatures encountered on a turbine blade surface, they can crack, especially when the temperature of the blade changes and differential thermal contraction and expansion stresses are set up between alloy and oxide. These can act as ideal nucleation centres for thermal fatigue cracks and, because oxide layers in nickel alloys are stuck well to the underlying alloy (they would be useless if they were not), the crack can spread into the alloy itself (Fig. 22.3). The properties of the oxide film are thus very important in affecting the fatigue properties of the whole component. 

What of the corrosion resistance of new turbine-blade alloys like DS eutectics Well, an alloy like NiaAl-NisNb loses 0.05 mm of metal from its surface in 48 hours at the anticipated operating temperature of 1155°C for such alloys. This is obviously not a good performance, and coatings will be required before these materials are suitable for application. At lower oxidation rates, a more insidious effect takes place - preferential attack of one of the phases, with penetration along interphase boundaries. Obviously this type of attack, occurring under a break in the coating, can easily lead to fatigue failure and raises another problem in the use of DS eutectics. 

The operating schedule of a gas turbine produces a low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the blade life. Table 11-1 shows fewer starts per operating time increases turbine life. 

Rust, T, E. and Swaminathan, V. P, Corrosion fatigue testing of steam turbine blading alloys , EPRI Workshop on Corrosion Fatigue of Steam Turbine Blade Materials, Palo Alto, California, September 1981, Pergamon Press, (1983)... 

ALTERNATIVE FATIGUE FORMULATIONS FOR VARIABLE AMPLITUDE LOADING OF FIBRE COMPOSITES FOR WIND TURBINE ROTOR BLADES... 

This paper looks at modem wind turbine rotor blades from the point of view of material fatigue. Characteristics of the rotor blades and loads are discussed and a simple commonly used lifetime prediction method is reviewed. Also, possible modifications to the various components of the fatigue calculations are discussed. 

However, there are indications that the commonly used lifetime prediction methods can lead to inaccurate results for the composite blade s lifetime under the strongly varying loads to which they are subjected. The present paper presents the preliminaries to an extensive study into the fatigue behaviour of wind turbine rotor blade materials under variable amplitude loading. This paper will discuss some of the problems and drawbacks of the currently prescribed lifetime prediction methods, and describes possible ways of tackling these. 

First, the most important loads on wind turbine rotor blades are described and a brief review of fatigue research in composites is presented to place the following approach in a broader perspective. The approach described in this paper concerns modification of the existing formulations. Possible modifications are illustrated with predictions for an existing data set, which was generated by subjecting composite specimens to different variable amplitude load sequences. 

Jian, C. Y., Hashida, T., Takahashi, H., and Saito, M., "Thermal Shock and Fatigue Resistance Evaluation of Functionally Graded Coatings for Gas Turbine Blades by Laser Heating Method," Composites Engineering, Vol. 5 (7), pp. 879-889,1995. 

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