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1. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1118
Fatigue Life Estimation of Small Gas Turbine Blisk
Chethan R Chandran1, Dr. T. Rangaswamy2, Dr. Manjunath K3
1Student, Dept. of Mechanical Engineering, Govt. Engineering College, Hassan, Karnataka, India
2 Professor and Head Dept. of Mechanical Engineering, Govt. Engineering College, Hassan, Karnataka, India
3Asst. Professor, Dept. of Mechanical Engineering, Govt. Engineering College, Hassan, Karnataka, India
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Abstract - A Small Gas Turbine (SGT) blisk is used for the
power generation of a surveillance aircraft. So it is important
to predict the fatigue life of a SGT blisk for the scheduling the
maintenance and the safety of the aircraft. SGT blisk
undergoes mainly four types of loadings. They are centrifugal,
thermal, vibrational and aerodynamic loads. In the present
work, the fatigue life of a SGT blisk is simulated with the help
of an FEA package ANSYS. After defining an operating cycleof
2 hours; thermal, pressure and centrifugal loads are applied
on the blisk and stress values at different time steps are
extracted. From the analysis results it is observedthattheblisk
is operating in the elasto-plastic region. So a strain based
approach is suitable for the estimation of Low Cycle Fatigue
and the same is estimated using Smith Watson Topper mean
stress correction theory.
Key Words: Life Estimation, Fatigue, Structural analysis,
Finite element analysis, Small gas turbine, Blisk
1. INTRODUCTION
Fatigue life estimation of a gas turbine blisk is very
important in the operational point of view. Failure of a blisk
during operation will lead to the failure of the aircraft.
Conventional life estimation proceduresaretimeconsuming
and expensive. A simulation of the blisk under operating
conditions can save the time and cost of the life estimation
process. The objective of the present study is to estimate the
fatigue life of a small gas turbine blisk with the help of a FEA
package ANSYS.
A gas turbine, also called a combustion turbine, is a type of
internal combustion engine. It has an upstream rotating
compressor coupled to a downstream turbine, and a
combustion chamber or area, called a combustor, in
between. The main aim of the turbine is to extract maximum
energy from the working medium to convert into to a useful
work and by obtaining maximum reliability, less starting
time, minimum cost and less supervision. Freshatmospheric
air flows through a compressor that brings it to higher
pressure. Energy is then added by spraying fuel into the air
and igniting it so the combustion generates a high-
temperature flow. This high-temperature high-pressure gas
enters a turbine, where it expands down to the exhaust
pressure, producing a shaft work output in the process.
1.1 Turbine Rotor
A Blisk is an engine component in which rotordisc and blade
is manufactured a single component. The Blisk may be
manufactured as integrally cast or from a single solid piece
material or manufactured by welding between disc and
blades. The term Blisk is an acronym composed of the words
“blade” and“disk” whichmeans bladeddisc.Blisksarenamed
as IBR (Integrated bladed rotors) Meaning is that joint
between root dick and blade is not even required. Initially
blisks were used in small engines for helicopters. Blisk was
introduced in the yearsof 1980sformilitary aircraft engines
and later elaboratelyusedinturbofanandturbopropengines.
By eliminatingthe components for thedisk-bladeattachment
weight as well as the drag is reduced. Elimination of fir tree
or dovetail will reduce the chance of crack formation and
propagation. Blisks possess higher thermal efficiency.
Fig 1: Turbine rotor blisk
1.2 Super Alloys
A superalloy or high performance alloy exhibits excellent
mechanical strength, resistance to thermal creep
deformation, good surface stability, and resistance to
corrosion or oxidation. AdditionofelementssuchAluminium
and Chromium will provide oxidation or corrosion
resistance. Microstructure of the superalloysslow down the
crack propagation and thereby increased fatigue life. Nickel
(Ni) based superalloys have emerged as the material of
choice for these applications. The properties of these Ni
based superalloyscan be tailored to a certain extentthrough

2. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1119
the addition of many other elements, both common and
exotic, including not only metals, but also metalloids and
non- metals. INCONEL® alloy 718 is the alloy considered
here.
Table 1: Material properties of Inconel 718
Particulars Units Inconel 718
Density Kg/m3 8193.3
Tensile Yield
Strength
MPa 965.27 (at 293.15 K)
690.00 (at 1080.0 K)
Tensile Ultimate
Strength
MPa 1241.0 (at 293.15 K)
724.00 (at 1080.0 K)
1.9857 x 105 (at
310.93 K)
Young’s Modulus MPa 1.9030 x 105 (at
477.59 K)
1.4686 x 105 (at
1080.0 K)
Poisson’s Ratio .30
Coefficient of
thermal Expansion
1.3158 x 10-5 (at
310.93 K)
C-1 1.3932 x 10-5 (at
477.59 K)
1.6038 x 10-5 (at
1080.0 K)
Thermal
Conductivity
W m-1
c-1
148.84 (at 473.15 K)
212.81 (at 1080.0 K)
2. METHODOLOGY
Super alloys have the capability to retain its shape at higher
stress values even at higher temperatures. Strain based
fatigue is applicable for both HCF and LCF. Here because of
the special material properties and the deformation in the
elastoplastic region, Strain based LCF is estimated.
Strain based approach for fatigue life estimation is
considered for structures undergoing plastic deformation.
The total strain amplitude can be resolved into elastic and
plastic strain components from the steady state hysteresis
loops. At large strains or short lives the plastic strain
component is predominant and at small strains or longer
lives the elastic strain component is predominant. Several
models dealing with mean stress effects or the strain life
fatigue behavior are available. One is Morrows’ parameter
and the other is Smith Watson and topper parameter. The
strain life approach is a comprehensive approach which can
be applied for the treatment of both low cycle and high cycle
fatigue.
Morrow Equation:
Smith Watson and Topper relationship:
In the above equations
= Strain Amplitude = Strength coefficient
= Mean Stress = Number of cycles to failure
= Strain coefficient b = Fatigue strength exponent
c = Fatigue ductility exponent
Both strain range andmaximumstressdeterminetheamount
of fatigue damage. Smith, Watson and Topper proposed a
theory in 1970 that include both the cyclic strain range and
maximum stressthis modelcommonlyreferredtoastheSWT
parameter. This method has been used for the present case.
3. ANALYSIS USING FINITE ELEMENT METHOD
Blade coordinates from CFD analysis for the turbine rotor
blisk for SGT is taken. The coordinates are imported to the
modelling software Solid worksand rotor bladeisgenerated.
Based on the engine configuration consideredfortheanalysis
the detailed modellingof the disk isdone insolidworks.Fora
proper mesh generation and to reduce the overall time for
analysis a periodic sector of the blisk is considered for the
analysis. The below figure shows the CAD models of the SGT
blisk and a periodic sector.
Fig 2: CAD model of SGT Blisk and Periodic Sector of Blisk

3. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
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For getting accurate results an optimized mesh is required.
To avoid the issue of stressconcentration that may arisedue
to poor meshing near blade fillet area is overcome by giving
a proper face sizing over the blade and the fillet. Maximum
face sizing in the disk region is set as 1.25mm and .4mm of
maximum face size is set in the fillet region. As a result a
mesh of 33367 elements with 59209 nodes is obtained with
an average Jacobian ratio of 1.0508.
Fig 3: Mesh generated on the blisk section
The pressure and temperature loadson the rotorbladefrom
a CFD simulation at design point are taken. The nodal
temperature values are saved ascsv file formatandthesame
is mapped over the blades. The maximumbladetemperature
is 1080 for the SGT blisk and the temperature at the bore is
assumed to be 500 K.
Thermo-structural analysis is carried out by considering
temperature dependent mechanical properties of the blisk
material. The pressure profile, which is also obtained from
ANSYS CFX solver is imported to the rotor blades. An
operational cycle of 10 time stepsis considered withvarying
RPM. The RPMs at different time steps are listed in the table
4.1 and the same are applied about the axis of rotation. Each
time step is divided into 4 sub steps.
Fig 4: Thermal Boundary Conditions, Imported thermal
loads over the blade
Fig 5: Imported pressure loads over the blade, Structural
boundary conditions
Table 2: Rotational velocity over time
Steps Time [s] X [rpm] Y [rpm] Z [rpm]
1 0 0 0 0
1 300 0 0 -50500
2 1440 0 0 -50500
3 2160 0 0 -55550
4 2880 0 0 -55550
5 3600 0 0 -45450
6 4320 0 0 -45450
7 5040 0 0 -55550
8 5760 0 0 -55550
9 6900 0 0 -50500
10 7200 0 0 0
4. FINITE ELEMENT ANALYSIS RESULTS
By finite element analysis ANSYS gives the temperature
distribution over the blade as a result of the simulation.
From the figure (5.1), it is noticed that maximum
temperature is at the middle region of the blade and it
gradually decreases towards the center of the disk. Same
thermal loads are considered further to carry out structural
analysis.
The structural analysis is carried out at the steady state
condition with varying RPMsat 10 time steps.Themaximum
equivalent stress (Von-Mises stress), maximum principal
stress, equivalent elastic strain, equivalent plastic strain,
maximum principal elastic strain are obtained as shown in
the following figures

4. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1121
Fig 6: Temperature distribution over the blisk, Equivalent
stress
Fig 7: Maximum principal stress, Equivalent plastic strain
Fig 8: Equivalent elastic strain, Maximum principal elastic
strain
4.2 Fatigue Analysis Results
Considering the value of infinite life as 1e9, the life is
evaluated in ANSYS Fatigue Tool with SWT strain life
approach. At the maximum loading conditionthatisat2160s
SWT theory predicted a life of 759 cycles. Maximum damage
value observed is 8.7806e5. The area where the maximum
damage is occurred is where the least life is predicted. The
value of Damage more than 1 indicates that the design will
fail before the infinite life.
Fig 9: Damage occurred, minimum number of cycles to
failure (Life)
From the Biaxiality indication results which is ranging from
.40273 to .99971 it is evident that the structure is
undergoing bi-axial stresses and the maximum principal
stress component has to be taken for predicting the
minimum life. As the infinite life cycle value is set at 1e9 and
he Design fails at 759 cycles, the Factor of Safety calculated
by fatigue tool is .16824
Fig 10: Biaxiality indication, Factor of Safety
4.4 Theoretical Calculation Results
Number of Cycles to Failure using SWT Equation
The SWT equation is given below
In the current simulation
= 771MPa (Maximum principal Stressdevelopedduring
the cycle) = 3.275 x 10-3
= 2355 = Number of cycles to failure
= .0128 b = -0.103
c = -0.973 E = 1.903 * 105

5. International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1122
By substituting the above values, we get an equationasfollows
After solving the numerical equation for Nf, Nf=71733.7
Fig 11: Representation of loading cycle. Green line
connects the maximum and minimum principal stress
values at each time step.
Here, there are 40 loading cycles in the operating cycle of 2
hours.
So the number of operating cycles = 1793.3
Table 3: Results comparison
Calculation method Number of cycles
to failure
FE Analysis by ANSYS workbench 759
Theoretical calculation 1739
By considering the loading history for 40 time steps (4 sub-
steps in each time step), the average of maximum and
minimum stress values are taken for the calculation.
Theoretical calculations predicted a life of 1793 cycles.
Which is almost the double of estimated life by ANSYS
fatigue tool (759 cycles). ANSYS Fatigue tool predicts the
least life by considering the maximum principalstress value
which is attained at 2160s. So the average of maximum and
minimum Principal stress values considered is higher than
the theoretical calculations, which will lead to a reduced life
compared to theoretical calculations.
5. CONCLUSION
The detailed thermal and structural analysis showed that
during the operating cycle at design condition and 110% of
the design condition the blisk maintains its natural shape.
Plastic strain of only 0.0016525 is developed. But the
presence of plastic strain indicates that strain life approach
is suitable for the operating conditions.
By considering the loading history for 40 time steps (4 sub-
steps in each time step), theoretical calculations predicted a
life of 1793 cycles. Ansys fatigue tool estimated a life by
considering the maximum principal stress value of 771 MPa
at 2160 seconds, which must predict a life lesser than the
theoretical calculated value and the desired result (759
cycles) is obtained.
ACKNOWLEDGEMENT
We would also liketo thank Mr. Prathapanayaka R, Principal
Scientist and in charge of Versatile Turbine Test Rig,
Propulsion Division for guiding and inspiringmetocarryout
this project work with interest. His constant inputs,
feedbacks and encouragement were invaluable.
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