lecture65_2

Effect of design and material defects

on gas turbine blade failures

Harry BhanguConsultant Engineer

David FordSecretary-General European Investment Casters’ Federation

Design Engineering Aspects

byDr Harry Bhangu

Effect of Design and Material Defects on Gas Turbine Blade Failures


Compressor Turbine

Combustor

Fuel

Air

Hot Exhaust

Gases

Rotating shaft

Gas Turbine Process

INPUTS OUTPUTS


Applications

The main applications of the gas turbine are:

• Power generation

• Marine

• Surface vehicles

• Pumping

• Aero – Civil and defence


•Aerodynamics

•Blade vibration/HCF

•Manufacturability

•Repair

•Weight

•Disc/blade issues

•Service Life(Creep, LCF, oxidation

lives, etc.) •Cooling

•Blade material

•Cost

•Coatings

Turbine Blade Design Issues

Some of the above aspects can lead to failures in service if the design,

material and/or manufacture are defective


Design Drivers and Constraints

• ηthermal

•EMISSIONS

•LIFE

• Operational mode

• OPR

• TET

• Cost:

- Fuel

- Parts

•Fuel type:

- Gas

- Diesel

- Crude

• New Technologies

- Novel cooling techniques

- Materials

- Manufacturing methods

ηthermal = Thermal efficiency

Effect of design and material defects on gas turbine blade failures

Turbine Blade Design Issues…2

• Fuel costs account for ~50% of total operating costs

• Fuel burn characterised by ηthermal = power output/rate of fuel energy input

• In simple cycle application typical high technology gas turbine ηthermal ≈ 40%

(i.e. the thermal energy contained in the exhaust gases represents 60% of the

inlet fuel energy).

• If the ηthermal is improved by 1% assuming annual fuel cost of £1B the annual

saving is ≈ £10M

• Component life: Stage 1 turbine blade creep and oxidation life of 50,000+

hours. This is to be achieved by using the minimum amount of cooling air

which in turn requires

- CFD and FEA capabilities

- maximum metal temperature of 900º C

- novel cooling techniques

- modern superalloys

- ceramic surface coatings


FEA - First bending Von-Mises stresses

Turbine Blade Design Issues…3

CFD – Flow inside the blade

Flow separation bubbles


Gas Turbine Configurations

There are many configurations

some of which are:

•Single shaft

•Multi shaft

•Simple cycle

•Combined cycle

•Combined heat and power

•Closed loop

•Generating:

- 5 to 500+ MW electrical power

- 3600 or 3000 rpm

- 60 or 50 Hz respectively

POWER

TURBINE

GAS GENERATOR

HP/IP TURBINE

LP

COMPRESSOR

AIRFLOW


Types of Failures

• Integrity/safety/airworthiness

- Disc rupture, mainly on aero derivative machines (very remote)

- Vibration causing HCF

- forced response

- self excitation/flutter

- acoustic (combustion instabilities)

- Flame breakout fuel leak from the injector

- Oil fires

- Pressure vessel rupture

- Turbine blade failure rotor out-of-balance

compressor rubs and a possible fire depending on

the material combination of blades and rotor path

- FOD (foreign object damage)

• Failures resulting from casting and machining defects

• Life related

- Low cycle fatigue (LCF)

- creep

- oxidation/sulphidation corrosion due to burning/overheat


Blade Vibration – Mode Shapes

First Bending First Torsion

•Mode shapes highlight blade deflections

•If the mode shape and the frequency of the excitation source match

the mode shape and the natural frequency of the blade the resulting

resonance can create amplitudes large enough to fail the blade.

scale


Consequence of Stage 1 turbine blade failure


Food for thought…1

Specified life of stage 1 turbine blade is +50,000 hours ≈ 6 years!

For six years the blade has to rotate continuously at 3000 RPM in

a very harsh environment

•Local gas temperature of

~1527ºC is ~200 C higher than

alloy melting point of ~1350ºC

• Power output per blade is ~850

horsepower i.e.~7 x typical

family car


•Typical blade weight ≈ 9 Ibs

•Centrifugal force ≡ weight of a dumper truck or 4 London double-decker buses

• Rotate continuously at 3000 RPM for ~6 years while also vibrating!

Food for thought…2

Leave you with the thought what costs £1 to put right in the design

phase will cost £10 in development and £100 if it gets out into service.


Metallurgy of Turbine Aerofoils

Dr David Ford

DAF Associates


Turbine Blade Alloys

Alloys are made from nickel with many other metallic elements to create an alloy with these properties:

• Melting point 1400 oC• Maintain strength up to 1000 oC• Tensile strength of 50 tons/sq in at 1000 oC• Resistant to oxidation and corrosion• Resistant to impact damage.• Resistant to continual stress without deformation (creep )• Resistant to cyclic stresses (fatigue).

These alloys are called Superalloys and can only be formed into shape by casting.


Conventionally cast (equiax) turbine blade

Individual grains (crystals) formed on solidification


Alloy C Cr Ti Al Co Mo W Nb Zr B Ta Hf Ni

IN713 0.05 12.0 0.6 5.9 4.5 2.0 0.1 0.01 Rem

IN738 0.11 16.0 3.4 3.4 8.5 1.75 2.6 0.9 0.06 0.01 1.75 Rem

MM 247 0.16 8.5 1.0 5.6 10.0 0.65 10.0 0.04 0.015 3.0 1.4 Rem

Composition of conventional turbine superalloys

Typical Microstructure of a superalloy

x200


Alloy structure at high magnification


Hard particles of precipitate (Ni3 (Al Ti)) strengthen the Ni base alloy

and remain stable at high temperatures.


Metallurgical Failures in Turbine Blades


Metallurgical failures in Turbine Blades

1. creep


Blade showing creep

extension due to

centrifugal stresses during

service


Showing material deformation during creep.


Metallurgical structure of blade showing creep during engine service

Possible to predict engine temperature and life in a failed turbine blade by examining the microstructure.


Metallurgical failures in Turbine Blades

2. Fatigue


Metal fatigue occurs when the material is subjected to a high number of cyclic stresses.

There are two types of fatigue common in gas turbines:

1. High cycle fatigue which occurs when the component is subjected to very fast and a high number of vibration stresses, e.g. 108 cycles at a stress ½ the tensile strength

2. Low cycle fatigue when the component is subject to cyclic thermal stresses. The stresses are usually higher than stresses which cause high cycle fatigue, e.g. 104 cycles at ¾ the tensile strength.


Fatigue cracks have a characteristic form which makes

a fatigue crack easy to identify.


Typical laboratory fatigue test piece showing beach marks


Striations between beach

marks

A beach mark is formed at the

end of an operating cycle e.g.

when the engine stops and

restarts. It is possible to

determine the number

operations from the start of a

fatigue crack before the part

failed.

Striations are formed at each

stress cycle and it is possible to

determine if the crack is high or

low cycle fatigue from the

number and size of striations.


Beach marks in aerofoil with a high cycle fatigue crack


Material advances:

1. Directional solidification

• Increases creep life by a factor of 2

• Increased fatigue life by a factor of 10


Advanced materialsDirectionally solidified material

Columnar crystals

Increased strength and a 10x increase in fatigue life


Material advances:

2. Single crystal

• Increases creep life by a factor of 10

• Increased fatigue life by a factor of 20

Superalloys crystallise on

solidification as a face centred

cubic (FCC) structure. The

atoms are positioned as cubes

with one in each corner and

one on each face of the cube.

This crystal structure gives the

material specific properties.


111

110

100

111

110

Φ

θ


Stress directions within the crystal structure have different material properties

Controlled solidification selects and orientates a single crystal in the most

beneficial orientation in the turbine blade

Single crystal superalloys

001 011

111

POOR

POOR

INTERMEDIATE

EXTREMELY

POORINTERMEDIATE

GOOD

VERY GOOD

GOOD

VERY GOOD

BEST

012

Reference Direction,

RD

Reference Plane, RP

001

Creep ResistanceExploits crystal anisotropy



Single crystal alloys

Projections for Rene N4, using Hermann expressions

0

50

100

150

200

250

300

350

0 20 40 60 80 100

Theta, °

Modulu

s, G

Pa

E, phi=0

G, phi=0

E, phi=10°

G, phi = 10°

E, phi =45°

G, phi=45°

low modulus =

good fatigue

resistance

Orientation of

crystal controlled

to a low modulus

direction.

Good News

Crystal direction with the lowest modulus is also

the crystal direction with the highest strength

so

Buy one get one free.



Single crystal casting


Alloy Cr Co Mo W Ta Al Re Ti Hf Ni

CMSX 4 6.5 9.0 0.6 6.0 6.5 5.6 3.0 1.0 0.1 Rem

PWA 1484 5.0 10.0 2.0 6.0 9.0 5.6 3.0 - 0.1 Rem

Rene N5* 7.0 8.0 2.0 5.0 7.0 6.2 3.0 - 0.2 Rem

Single crystal alloys

Rhenium scarce element with no natural ore – price £ 4-8 K/ kg


Single crystal defects

Second crystal


Single crystal defects

Recrystalisation


Quality assurance -1

Process control:

Ensures the manufacturing process is consistent

Each stage of manufacture subject to a controlled procedure

Material control:

Each alloy master heat is chemically overchecked on receipt

Each casting is chemically checked to ensure no contamination

Heat treatment validated by mechanical testing and microstructure


Quality assurance -2

Quality control:

Each component is subjected to extensive inspection:

Dimensional inspection internal and external dimensions

X-ray which may also include microfocus and CT scan for

internal defects

Fluorescent penetrant inspection

Ultrasonic inspection for wall section size

Grain size and form

For single crystals:

Crystal orientation

Crystal defects

Microstructure

cracks


Identified defects during manufacture


To conclude:

Modern gas turbines blades are made from the most highly

developed alloys to date and cost typically > £100/kg

They operate at temperatures well in excess of their melting

point but are designed for lives well in excess of 10000 hours.

They are sensitive to manufacturing defects but are subjected to

rigorous inspection processes to ensure that they meet the

highest manufacturing standards.

Failures are rare but if uncontained can cause extensive

damage.


Thank you for your attention and success to your business

lecture65_2

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