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This paper presents the creep-fatigue interaction life consumption of industrial gas turbine blades using the LM2500+ engine operated at Pulrose Power station, Isle of Mann as a case study. The linear damage summation approach where creep damage and fatigue damage are combined was used for the creep-fatigue interaction life consumption of the target blades. The creep damage was modelled with the Larson-Miller parameter method while fatigue damage was assessed with the modified universal slopes method and the damage due to creep-fatigue interaction was obtained from the respective life fractions. Because of the difficulty in predicting the life of engine components accurately, relative life consumption analysis was carried out in the work using the concept of creep-fatigue interaction factor which is the ratio of the creep-fatigue interaction life obtained from any condition of engine operation to a reference creep-fatigue interaction life. The developed creep-fatigue interaction life consumption analysis procedure was applied to 8 most of real engine operation. It was observed that the contribution of creep to creep-fatigue interaction life consumption is greater than that of fatigue at all ambient temperatures. The fatigue contribution is greater at lower ambient temperatures as against higher ambient temperatures. For the case study, the overall equivalent creep-fatigue factor obtained was 1.5 which indicates safe engine operation compared to the reference condition. The developed life analysis algorithm could be applied to other engines and could serve as useful tool in engine life monitoring by engine operators.

Gas turbine blades, especially the compressor turbine blades of aero derivative turbines are exposed to high temperatures and can fail due to creep [

Aside the crack growth approach, there several other methods of investigating creep-fatigue interaction failure. Zhu et al. [

Creep-fatigue interactions can usually be modelled exploiting two widely used approaches. These are the isothermal method and the linear damage summation approach. Creep and other effects are not taken into account in the isothermal method. The linear damage summation rule takes the contributions of creep and fatigue and it is more or less combining the Palmgren-Miner rule for fatigue [

It is difficult to obtain very accurate results in life prediction of components. Also, fatigue life is stochastic in nature [

The linear damage accumulation method is applied in this work; hence the creep and the fatigue terms are considered separately and combined. The Larson-Miller parameter method is used for the creep while the modified universal slopes method is used for the fatigue life component. Blade thermal and stress models developed for the analysis together with creep-fatigue interaction model all incorporated in PYTHIA [

The creep life model employed is the Larson-Miller parameter method given by Equation (1),

t f = 10 ( LMP T − C ) (1)

where t f is the time to creep failure, T is the temperature of blade material in Kelvin (K), LMP is the Larson Miller parameter obtained from a Master curve and C is material constant usually of the order of 20. The creep life is estimated by developing blade stress and thermal models and each blade is divided to several sections. Details of how the creep life could be estimated could be found in [

D c , i = t i t f , i (2)

where t i is the time spent at a given stress-temperature combination, t f , i is creep fracture life at same stress-temperature combination and D c , i is a parameter which represents the fraction of life consumed.

The fatigue life model adopted in this work is the modified universal slopes method which is expressed in terms of nominal alternating stress amplitude, σ a as,

σ a = 0.623 σ u 0.832 E 0.168 ( 2 N f ) − 0.09 + 0.0196 ε f 0.155 σ u − 0.53 E 1.53 ( 2 N f ) − 0.56 (3)

where σ u is the ultimate tensile strength of the material, E is the Young’s Modulus of the material, ε f is the true fracture ductility, and N f is the number of stress cycles to failure. The fraction of life consumed due to fatigue at each stress-temperature combination is given as,

D f , i = N i N f , i (4)

where D f , i is a fatigue damage parameter, N i is the number of cycles accumulated at stress amplitude σ a , i and N f , i is the number of cycles to failure at stress amplitude σ a , i . Fatigue life is estimated using a developed stress model of the blades and details could be found in [

Using the linear accumulation model, the creep-fatigue interaction life at a given stress-temperature combination is expressed in terms of creep-Fatigue damage parameter D c + f given by Equation (5),

D c + f = t i t f , i + N i N f , i (5)

For a given period of engine operation, the creep-fatigue damage parameter will be the sum of the creep damage terms and the fatigue damage terms as in Equation (6),

D c + f = ∑ t i t f i + ∑ N i N f i (6)

The value of the creep-fatigue damage parameter ranges from zero to unity, and failure occurs at unity.

At any point of engine operation where the engine is operated for time t i , the time to creep-fatigue interaction failure t f , c + f is,

t f , c + f = t i D c + f , i = t i t i t f , i + N i N f , i (7)

Equation (7) could also be expressed in terms of cycles to failure N f , c + f as,

N f , c + f = N i D c + f , i = N i t i t f , i + N i N f , i (8)

The damage parameter could be extended to any period of engine operation; hence, the time to creep-fatigue failure as well as the number of cycles to creep-fatigue interaction failure could be estimated for any period of engine operation. This is termed equivalent creep-fatigue life (ECFL) as given by Equations (9),

ECFL = ∑ i = 1 m t i D c + f = ∑ i = 1 m t i ∑ i = 1 m t i t f , i + ∑ i = 1 m N i N f , i (9)

Equations (7) to (9) are obtained using that creep-fatigue interaction failure occurs when the creep-fatigue damage parameter is unity. No engine operator ever envisage to operate his engine to failure hence if creep-fatigue interaction failure of an engine component is tracked, parts are likely replaced when the value of D c + f obtained is close to unity. Also, it is difficult to predict engine life accurately; this necessitates comparing the life obtained to a reference life.

Creep-fatigue interaction factor is used to assess the severity of engine operation under creep-fatigue interaction life consumption; this is similar to the fatigue factor approach [

CFF = t f , c + f t f , c + f _ Re f (10)

where CFF is the creep-fatigue interaction factor, t f , c + f _ Re f is the time to creep-fatigue interaction failure at a reference point. For a complicated engine operation process involving different conditions and time frames of engine operation, the equivalent creep-fatigue interaction factor (ECFF) is used and this is given by Equation (11),

ECFF = ECFL t f , c + f _ Re f (11)

Equation (10) and Equation (11) could also be presented in terms of cycles to failure.

The creep-fatigue interaction analysis system developed in this work is shown in

Both creep-life consumption and fatigue life consumption are carried out in their respective life analysis systems in

The results of the creep-fatigue interaction life consumption could be in terms of time to failure or cycles to failure. But, they are presented in terms of relative engine

life consumption which is creep-fatigue interaction factor. The creep-fatigue interaction life consumption procedure developed in this work was applied to 8 different months of real engine operation. The aim is to track the engine life. The creep-fatigue interaction life of the hot section blades of LM2500+ engine operated in Isle of Mann was tracked in this work. The equivalent creep-fatigue factor for each day operation of the engine for the 8 different months of engine operation is shown in

During the months of January, February, March, and December where low ambient temperatures are recorded, the creep-fatigue interaction factors obtained are higher as in

The equivalent creep-fatigue interaction factor for each month is shown in

This indicates that the life consumption rate due to creep-fatigue interaction is 50% less compared to the reference engine operation condition. The engine operation in the entire period is thus favourable.

In order to show the dominance of creep life consumption in creep-fatigue interaction life consumption, the equivalent creep factors (ECF) and equivalent creep-fatigue factors for two different months (January with low ambient temperature and July with high ambient temperatures) of engine operation are compared as in

The creep-fatigue interaction life consumption in each month of engine operation is greater than the creep life consumed because of the presence of fatigue life consumption in the former. If creep life consumption is used as base life consumption, the additional life consumed in creep-fatigue interaction life consumption may be viewed as reduction in engine life.

The percentage reduction in life is evaluated taking creep life as the base life. As creep interacts with fatigue, the resultant life is less than the creep life and the decrease in life due to fatigue contribution decreases with an increase in fatigue factors. The reduction in life is highest in the months with low ambient temperatures where fatigue contributions are highest. At the conditions where creep dominance is more paramount, the results of creep life consumption is not much different from those of creep-fatigue interaction life consumption. Thus in such engine operation conditions, estimating only creep life consumption may

be sufficient. Generally, during high ambient temperature and low part load operation, creep life estimation alone may suffice.

Creep-fatigue interaction life analysis is considered in this work exploiting the linear creep and fatigue accumulation model. Creep fatigue analysis system and fatigue life analysis system developed in previous works are used to provide inputs to the creep-fatigue interaction life analysis system to track the creep-fatigue interaction life consumption of LM2500+ engine operated at Isle of Mann. In the 8 months of engine operation considered, using relative life analysis, the months with low ambient temperatures have high values of creep-fatigue interaction factor indicating safer engine operation. Creep dominates the creep-fatigue failure at the different conditions the engine was operated. This is expected as industrial gas turbines are operated in a bit stable power conditions and fatigue cycles accumulated over time are quite few. Also, the fatigue contribution to creep-fatigue interaction life is higher in the months with lower ambient temperature if engine is often shut down. As fatigue interacts with creep, about 10% reduction the life was obtained on the average for the average for the 8 months considered. As much as 14.5% life reduction was obtained in the months of March and December. The results obtained are in line with what is obtainable in real engine operation and the life tracking methodology developed could be applied to any engine. The effects of ambient temperature variation, shaft power level and engine degradation on creep-fatigue interaction life need to be investigated.

The authors declare no conflicts of interest regarding the publication of this paper.

Saturday, E.G. and Isaiah, T.-G. (2018) Creep-Fatigue Interaction Life Consumption of Industrial Gas Turbine Blades. Modern Mechanical Engineering, 8, 221-232. https://doi.org/10.4236/mme.2018.84015