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!"#$%&'(" *'+"#,'-% ./0,/""#,/0

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.12*34 8$B#%" K="#=,";

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.12*34 8$B#%" 6B&&$#+Recommended Texts:

1. R. C. Reed, The Superalloys: Fundamentals and Applications, CambridgeUniversity Press, Cambridge, UK, 2006.

2. Meherwan P. Boyce, Gas Turbine Engineering Handbook, Second Edition,

2002 by Butterworth-Heinemann, Boston, US.

3. I. J. Polmear, Light Alloys: From Traditional Alloys to Nanocrystals, Fourthedition, Butterworth-Heinemann, Elsevier Ltd. Oxford, UK, 2006.

4. F.C. Campbell, Manufacturing Technology for Aerospace Structural

Materials, 2006, Elsevier Ltd, Oxford, UK.

5. G. Ltjering, J. Williams, Titanium, Springer-Verlag Berlin, Germany, 2003

Course notes on blackboard for revision purposes background reading and

attendance at lectures necessary.

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Aerospace Materials Engineering

!

Section 1 Mechanical Design

! The Gas Turbine

!

Structural Integrity in Gas Turbines

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The Gas Turbine

!

There are many different forms of the gas turbine, but they allfollow the same basic work cycle:

!

This is achieved via:1. FAN intake of large volume of air

2. COMPRESSOR reduces the volume of air and raisesthe pressure

3. COMBUSTOR air + fuel mixed and ignited

4.

TURBINE exhaust gas expelled, turbine rotates fan andcompressor via shafts/gears

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Jet Engine

Compressor CombustionChamber

TurbineShaft

ExhaustNozzle

mVaircraft

mVjet

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*',/ ($E&$/"/+%G

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fan intake -60oC

IP compressor300oC

HP compressor 600oC

combustor 1200oC

TET >1200oC

Temperaturecapability is major governing factor controlling materials

selection. However, lowest density materials always preferred

Materials required all major or safety critical components under high

stress (centrifugal, flow and vibrational)E.g. fan blade CF(Centrifugal) load 100tonnes, HP blades CFload 20 tonnes

The Gas Turbine

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E.g., the fan system needs to be largeand lightto improveefficiency but without compromising strength.

At top speed the weight on each fan blade is equivalent toover 90,000kg!

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InletCase

Al Alloy

AccessorySectionAl or Fe Alloy

TurbineBlades

Ni Alloy

TurbineExhaust

CaseNi Alloy

Low PressureTurbineNi Alloy

High PressureTurbineNi Alloy

CombustionChamberNi Alloy

High PressureCompressor

Ti or Ni Alloy

Low PressureCompressor

Ti or Ni Alloy

FanTi Alloy

Fig. 6.1. Typical Material Distribution in Jet Engine

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Pressure and Temperature

Pressure(atmospheres)

0

40

Temperature(degrees C)

0

1500

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! Safety critical parts in gas turbines are those whose single failure

can jeopardize the operational capability of the aircraft! Critical components within the gas turbine are:

! Discs

! Shafts

! Fan and compressor blades

!Turbine blades

! Implications for a breach in structural integrity for any componentis assessed by a failure modes and effects analysis

! Allows service lives to be achieved at an acceptable level ofrisk

! Components such as casings, combustors and stators also have

structural integrity issues for the designer! Consequences of crack development are not as critical can

contain cracks and still operate safely

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!

Design stage balance stresses generated within a component

against material property data!

Key factors in design against maximum or static loads include:

! Modulus, Stiffness (Youngs Modulus), E

Specific stiffness = E/!

Maintain displacements and deflections within limits

! Proof Strength

Assessed in relation to 0.1% or 0.2% proof strength and ensuresstructure as a whole does not deform plastically and permanentlydistort.

! Ultimate Strength

Relates worst case operating condition to ultimate tensile strengthof the material (typical factor "1.5)

Ultimate strength #UTS/1.5

Specific strength = UTS/ !


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Modulus, proof strength and ultimate tensile strength can be

obtained from a simple tensile test


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! Fracture Toughness, KIC

Values applied to ensure defects just below non-destructivetesting threshold limits do not cause catastrophic failure

! Creep

Major consideration is excessive growth of components thatcould lead to blade tip rubs. Typical design criterion is 0.1%plastic strain during the life of the component

The above design criteria are matched against minimum materialproperty levels

Safe life parts (e.g. Discs)Minimum level is where 99% of the population is expected to fall

with a confidence level of 95%

Damage tolerant partsMinimum level is where 90% of the population is expected to fallwith a confidence level of 95%


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!

A major consideration in component design is fatigue

!

Lifingmethod depends on structural sensitivity of thecomponent.

! Critical components such as discs, fan blades and to someextent shafts, are lifed on the basis of design curves

Design curves constructed from plain specimenfatigue

data

Specific stress concentration features in componentsuse notchedspecimens or similar

In components for which containment provision, or

consequences of a crack are not catastrophic, design curvesare less stringent.

SAFE LIFE APPROACH


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! Alternative approach to safe life is based on fracture mechanics

To predict an Equivalent Initial Flaw Size (EIFS)

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! Statistics provides the largest EIFS for the material to a probability level

and is used to establish a failure life for the component

!

Declared service life is 2/3rds of the calculated life for the largest EIFS! Fracture mechanics relations, for

example Paris expression to back track

from a known failure.

! Define an effective flaw that causedfailure in the observed number of cycles.


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What is Fracture toughness?

!

It is important for the engineer to accept that a structure/material

does contain defects. %

%

a

KIC

=Y! "a

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stresses up to a critical value of

stress intensity, KIC, beyond which

the crack propagates rapidly

%max

Design Criteria Static properties

*T'qE

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!1988 - a B737 operated by Aloha Airlines had the roof of the

first-class cabin tear away!The aircraft had stress-corrosion damage at a number of rivets

in the fuselage lap splices, and this permitted multiple small

cracks to link up to form a large crack

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Creep is the process of strain accumulation under

static loading

!Creep is most readily associated with hightemperature conditions, but it can occur at all

temperatures above absolute zero

! It is a time dependent or diffusion controlled

process and is therefore more prevalent at hightemperatures

!Temperature to allow diffusion amongst thecrystal lattice depends on the material and atomic

bonding ionic easier than covalent

!

For high temperature creep, metals > 0.3Tm,ceramics > 0.5Tm


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! Curve can be separated into three distinct stages according to

creep rate! Time to failure = rupture life decreases as temperature or

applied stress increases

time

primary

decreasing strain rate

secondary

constant strain rate

tertiaryincreasing strain rate

failure

time

Increasing temperature

orincreasing stress


h/,L'- "-'%L( %+#',/

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!

Cracks can develop in materials

even under apparently safe loads!

Fatigue failures account for 80-90%

of all service failures

!

Fatigue failures can occur under

cyclic stresses which are well below

the UTS, and often below the yield strength

of the material! It is therefore essential to know the fatigue characteristics of materials

the process of mechanical failure due to theinitiation and growth of cracks under cyclic

loading

!

We can characterise a materials or structures performance undercyclic loading by fatigue or endurance tests.

Design Criteria Fatigue

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Design Criteria Fatigue Test

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The fatigue response of a material is evaluated usually by

subjecting simple test piece geometries to load or strain controlcycles

2

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!

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max

min

!

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=R

Design Criteria fatigue

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There are significant differences between stress and strain control cycles. Acomparison is shown below for zero to maximum loading conditions

!"#$%&

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but accumulates strain with time! For strain control the max and min stress achieved changes

with time

Design Criteria fatigue

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! The S/N curve is the traditional way of representing fatigue data andis usually plotted as stress versus cycles to failure i.e. Fracture intoone or more pieces.

! If the applied stress is equal to the UTS of the material, it will failinstantly.

! For stresses less than this, failure will occur after a certain number ofcycles such a stress under static loading would not cause failure

Design Criteria Fatigue

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Other issue that must be built in at the design stage include

!

Blade impacts due to birds or smaller hard body objects which are

defined as Foreign Object Damage (FOD)

!

Release of a blade or part of an aerofoil

!

Damage that could cause growth to complete failure! Changes to vibration characteristics

!

Containment

! Large fan blades contain large amounts of kinetic and translational

energies and are potentially hazardous to the aircraft.

!

For the smaller blades further back in the engine, the containment issimpler


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Primary Threats

Foreign Object Damage (FOD)

Bird-strike fan and compressor

Hard Body FOD (runway debris)

Domestic Object Damage (DOD)

Fitters tools

Secondary Threats

Containment

Fan Blade (partial from bird-strike)Fan Blade (whole from primary fatigue failure in root)

Compressor casing (released blade)

Turbine Blades (released blade)


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!Engine Certification Tests


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Design Specific component issues (Disc)

Rotating Discs

Discs are required to support the compressor and

turbine blades and to ensure that the blades are

positioned correctly within the gas stream

Discs are integral links in the transmission oftorque between the turbines and the compressor

7$#"

D,E

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Frontof the engine where large

fan blades are situated, discs

comparatively squat (thick and

short) in geometry

- Small rim radius that is wide

in axial direction (large fan

blade root)

Discs become thinnerbut rim

radius larger as the blade size isreduces through the compressor

Largest discs are found in the HP

turbine

- Experience high temperature

gradients and consequently

thermal stresses in addition to

the mechanical loads arising

from rotation.

7$#"

D,E

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! Real discs are not plain sectioned but

have a bottle shape to maximize load

carrying capacity

! The resultant stress distribution is

calculated by finite element methods

! The radial and hoop stresses areusually approximately equal in the

diaphragm

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! HP disc components there are substantial variations in temperaturefrom bore to rim

!

Resultant thermal stress will again have hoop and radialcomponents

!

Thermal gradientincreases hoop stresses and reduces radial stressat the rim

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! During a flight cycle, the mechanical and thermal stressdistribution will react differently to applied operating conditions

!

Mechanical stresses are principally governed by the rotationalspeed squared

! Thermal stress depends on the temperature difference betweenthe bore metal and the rim which is a function of gas streamconditions and the applied cooling flow

! Under some operating conditions the centre cob region can be of

equivalent temperature or even hotter than the rim

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! The stress distribution in discs can be perturbed locally byadditional structural features

!

Include drive shafts, adjacent discs in a drum construction andblade geometry

! Stress concentration features include holes for cooling, boltedholes, radii at attachments and blade fixing features

! Important design features impact on integrity of disc

! Root fixings can

introduce complexstress fields

! Also contact stress

concentrations due to

blades bedding in

! Movement of blades

leads to frettingdamage and the

possibility of earlyfatigue crack initiation

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Disc failure is unacceptable and potentially catastrophic as the high energy

fragments cannot be contained within the engine.

Therefore, the Certification Regulations specify an overspeed test

requirement to cover the potential burst condition and fatigue tests on discs

that address design limiting features under cyclic loading.

The overspeed criterion allows for run-away situations in which the rotational

speed of the disc increases beyond the 100% r.p.m.typical of normaloperating conditions.

A disc with minimum properties made to the most adverse tolerances shall

run for 5 minutes at a speed equal to 125% of the maximum to be approved

with stabilised temperatures equal to the most adverse which could be

achieved in normal operation.

It is interesting to note that the 125% increase on speed can be related to the

150% or 1.5 factor applied to the UTS of a material in derivation of the

ultimate criterion.


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Design Specific component issues (Shaft)Shafts, like discs, are critical rotating components.

Main shafts experience a complex loading system.

Shaft: Transmit the torque generated in the turbine through to thecompressor.

They experience small axial loads which arise in the compressor

and turbine while also experiencing thin ring hoop stresses as aconsequence of rotation.

They are also subjected to bending moments due to inertiaeffects and aerodynamic factors.

A major consideration with regard to bending moments is ablade-off incident. This can be particularly severe in the case of

large fan blades.

From a fatiguepoint of view there are also major and minor cyclic components. The

major torque component, for instance, can have minor vibratory oscillations. Theobjective is to define maximum principal stresses and maximum shear stresses inorder to establish a viable safe life for the shaft. The process is complicated by the

fact that shafts also contain hot spot features such as fillet radii, machined features, oilholesand most particularly spline teeth.

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Design Specific component issues (Blades)

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In blade geometries that contain stressconcentrating features additional factors

must be taken into consideration:

Consider the cooled turbine blades

The cooling holes leading edge, trailing

edge and film are stress raisers andpotential sites for crack development.

Temperature gradients between internalcooling channels and external hot

surfaces can produce significant thermalstress leading to cyclic variations of strain

and temperature or thermal fatigue.

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High temperature creep will cause blade

extension which must be controlled in order to

avoid excessive blade tip rubbing. Also give

rise to significant internal stress redistribution

and potentially could be a source of damage

development.

One further set of design criteria that address

the structural integrity of blades are related to

the consequences of impact of objects onto

the aerofoils. The objects can be soft bodies

and relatively large in size such as birdsor

they can be hardand relatively small such as

stonesingested from runways. In either case

the aerofoil is likely to incur damage and its

structural integrity may be impairedparticularly if the blade also experiences a

HCF (High Cycle Fatigue) vibration mode.


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Structural Integrity of fan and compressor blades

The critical issues with regard to the largefan blades:

! LCF: arising from the rotational forces and gas

pressures during a normal operating cycle.!

HCFcaused by high frequency excitations due to

influences such as flutter and resonances.

!

Consequences of soft and hard body impacts.

!

Containment

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Turbine blades

! Creep(cooled turbine blades)

These blades operate in a gas stream that is hotter than the

blade metal melting temperature. They survive because of the

cooling air that is tapped from the HP compressor. The cooling

air, however, adds to the complexity of the stress analysis for the

components. It gives rise to temperature gradients across theaerofoil section. Creep then acts to redistribute stress between

the hotter and cooler parts of this section.

! Thermal fatigue

The cyclic variations occur in which the strain at each position of

the aerofoil section varies in or out of phase with temperature

Design against thermal fatigue is a major consideration


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Coatings

Turbine blades are a system incorporating the base alloy and the coating technology.Without the coating materials modern blades would not survive for long, even with

internal cooling, in the aggressive gas stream. Design issues relate to the integrity ofthe coatings and the implications of damage to the coating material which could lead

to ingress of oxygen and corrosive species.

Aeroplanes, aerodynamics and wings

Aeroplanes fly because the wings push air downwards and the air reacts by pushingthe wing up Newtons third law. The resultant lift or upward force cause the plane to

rise. A consequence of the process is that air pressure is reduced above the wing faster moving fluid and increased below the wing (Bernoullis principle). The angle the

wings make with the horizontal is called the angle of attack. The angle causes the

downward push on the air. A steeper angle of attach increases the downward

movement. Lift is one of four primary forces acting on the wing/plane. The others are:Weight offsets liftDrag friction imposed by the air

Thrust propelling force

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Threats and Material Regimes

Blades

Untwist, frequency

LCF, HCF

Notch fatigue

Bird strike / FOD

Trailing blade

Discs

UST

LCF

Cold dwell

FCGR

Creep

Statics

Fatigue

Containment

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Materials technology drivers

Environmental Impact

Cost

Safety

Performance

Strength

Temperature

capability

Density

Cost

Time

Predictive

capability

Materialdevelopment

Process development

Improved methodology

Modelling

Process control

Customer

requirement

Design

requirement

Materials

technology

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Specific Strength

Nickel Alloy

Steel

Aluminium Alloy

Titanium Alloy

Temperature

Date post:	02-Jun-2018
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