TURBINE BLADE SUBJECTED TO CREEP AND FATIGUE LOADING

The main cause of turbine blade failure is high cycle fatigue. Fatigue failure is related to repeat cycling of the load on a structural member. The fatigue life of a structural member i.e. the number of load cycles it can survive is in general determined by the magnitude of the stress cycles. The exact relation between the magnitude of the stress and the fatigue life depends on the material properties of the structural member. In general higher stresses lead to a shorter fatigue life. For some materials fatigue only occurs if stresses exceed a certain minimum level for other materials there is no minimal stress level. If the stresses that are present on the turbine blade during operation and the material properties of the turbine blade are known then an estimation of the fatigue life of the turbine blade can be made.

Generally fatigue failure occurs as follows. After a number of load cycles a crack is initiated. This usually occurs at a point of relatively high stress concentration i.e. points with sharp geometrical discontinuities or points with relatively rough or soft surfaces. Once the crack is initiated it advances incrementally through the material with each stress cycle. In general this advance is very slow up to a certain point where it accelerates. The final failure occurs very rapidly. High cycle fatigue corresponds to failure after a relatively large number of load cycles. High cycle fatigue occurs at stress levels well below the yield strength of the material where deformation is elastic. The failure of a structural member is not caused by excessive loading but by the repeated cycling of the load.

Another cause of static stress failure of blades is blade overheating related to the departures from normal operating conditions. Such failures are detected by metallographic methods based on metal structural variations throughout the entire blade section or in its separate regions as well as on the formation of thick de-alloyed surface layers. Thus, cracks on ZhS6K-alloy guide blade edges were discovered after GT adjustment testing for 103 h. with 57 start-ups.

Coating cracking is induced by a local corrosion failure of blade base metal under the coating. A method for testing small-size coating specimens has been developed. The method makes it possible to observe strain relief characteristics during testing and to study the mechanisms of crack initiation and propagation in a coating up to specimen failure. A series of thermal fatigue tests was performed using different super alloy specimens with different coatings. The mechanisms of micro-crack initiation and suppression in multi-layer coatings have been determined.

LIFE PREDICTION UNDER CREEP & FATIGUE LOAD

The life prediction, more generally the evaluation of the behaviour of a rupture mechanism, or several mechanisms at the same time, is a significant task in order to ensure the reliability of the system. The cycle is non reversible to deal with the mean stress; equivalent reversible stress is calculated with the

Goodman rule .

Life prediction for fatigue (N_f):

Nf is defined in this work by the Manson Coffin formulation. Manson Coffin established that the alternative strain is related to the number of stress cycles to rupture, by the equation:

E is the elasticity modulus, σ_f material strength, and b and c for the majority of materials are equal to -0.12 and - 0.6.

Life prediction for creep (N_c):

Creep for metal is described by the Larson Miller parameter (LMP). T is the temperature in K degrees; N_c is the time to creep rupture in hours. The LMP C coefficient is taken equal to 20.

Model for fatigue-creep interaction:

Many approaches have been developed these last years to predict the safe life of materials subjected to high temperatures. To consider the interaction between fatigue and creep damage, several rules of damage accumulation could be used. This accumulation can be considered as linear or non linear. In this work two damage accumulation models are used: the linear Miner rule eq. and the non linear Chaboche rule.

It is considered that during a cycle, the damage of creep passes from the damage D0 to D1 and that the fatigue damage increases at the end of the cycle from D1 to D2. Two equations and give respectively the creep and fatigue interactive damage for one cycle:

The coefficients α, β and k are the material data, defined experimentally. In the two cases, the damage is complete when damage accumulation reaches the unity. Then the rupture becomes certain. We can then predict the number of cycles to failure when: