Materials Aircraft

8/6/2019 Materials Aircraft

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[email protected] Page 1 of 14 July 2009

The Goldsmiths Company

Science for Society

Dept. Materials Science & Metallurgy

University of CambridgeLecture: Aircraft alloys

Rob Wallach

Materials for aircraft:why they dont fall down

University of Departmentof

Cambridge Materials Science

University of Cambridge

Goldsmiths Science for Society 2009

Safety: statistics

http://aviation-safety.net/statistics/period/stats.php?cat=A1

Aircraft weight

Boeing 747 ~400,000 kg or 400 tonnes

equivalent to ~ 5,000 people each 80 kg weight

Why dont aircraft fall down

www.grc.nasa.gov/WWW/K-12/airplane/

NASA Guides to Aeronautics:

AerodynamicsPropulsionHypersonicsCompressible AerodynamicsModel RocketsKites

orhow do they lift off ?

Airfoil behaviour

Lift forceis perpendicularto direction of motion.

Want this force large.

www.kidwind.org/ppresentations/Powerinthewind.ppt

= low

=midrange


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Aerofoil design: lift optimisation

www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Aerofoil design: lift optimisation

www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Weight distribution: commercial aircraft

1% saving in weight of empty aircraft increases available

revenue (payload) to 15% i.e. ~ 7% increase.

47%plane

7% carrier

4% spare

28% fuel

14% payload income

Weight reduced by: decreasing density of materials

increasing mechanical properties

Properties relevant to aircraft skin

Weight: mass per unit volume

Strength: load for shape change

Stiffness: resistance to bending

Fatigue life: resistance to cyclic load

Toughness: energy absorbed to break

Can heavier material with better property, give weight saving?

- need to consider mechanical loading on component,

- need to consider how it might fail or break.

Aim: to optimise those properties which are important.

density

yield stress y

modulus E

endurance limit

fracture toughness G

What are stiffness and strength?

StiffnessE

Permanentshape change

Strength[yield y]

unload

[SHAPE CHANGE]

[LOAD]

Application of a further load leading tonecking and failure

Stiffness and strength: summary

Stiffness

StrengthFailurestress

Ultimate tensilestress

Yieldstress


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Airbus A380

Stress distribution in an aircraft skin

[stress =forceor loadper unit area]

Stress analysis in aircraft skin

= Pr/t z= Pr/2tHoop stress Longitudinal stress

Optimise materials: merit index for lightness

Mass m of cylinder length L made frommaterial of density is:

m = volume (of cylinder and 2 end pieces) xdensity m = (2

r t L + 2

r2

t)

L

r

tassumer>>t

Aircraft skin will not fail by yielding (permanent shape change) if yield stress y> hence y > Pr/t

Onlyvariablethat isnotmaterial property or specified dimensioniswall thicknesst. Eliminate t between two equations: m= 2r (L+ r) Pr /

y

.

.

m = (2r t L + 2r2t)

Materials selection maps - usage

Definey/ as the merit index for specific strength

Higher values show materials with higher strengthsfor same mass.

In many design problems, there will be a value of specificstrength that has to be achieved to ensure safety.

To compare different materials, let y1/n

/ = k

and so log (y) = n log () + k'Graph oflog(y) versuslog() gives straight line of gradient n

where value of nis fixed by design problem & analysis used.

Materials on a given line are equivalent in terms of the ratio of

the two properties.

Materials selection maps - usage

Definey/ as the merit index for specific strength

Hence for lower mass, want higher strength or lower density

In many design problems, there is a value of specific strengththat has to be achieved to ensure safety.

To compare different materials, let y1/n/ = k and so log (

y) = n log () + k'

Graph oflog(y) versuslog() gives straight line of gradient n

m= 2r (L+ r) Pr /y

Materials on a given line are equivalent in terms of the ratio of

the two properties.

[Note: value of nis fixed by design problem & analysis used.]

or y

/= [2r (L+ r) Pr]/m

Ashby map for

materials selection:

yield strengthyversus density

Line of gradient n

Foams

Woods

Rubbers

Metals


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Strengths of wood and aluminium

[for a medium strength aluminium alloy]

Wood (pine) Aluminium alloy

Strength y 80 MN m-2 80 MN m-2Density 700 kg m-3 2800 kg m-3

For same strength, wood bar is ~ 4 times lighter than Al bar

Aircraft frame and skin materials

www.ultralightnews.com/plansbuyerguide/boredomfighter-aircraftplans.html

Boredom fighter: wood

Concorde: aluminium

. and stiffness?

Resistance to bending but such that nopermanent shape change occurs.

For the same stiffness, need 4 times the area of wood

Stiffness of wood and aluminium

Wood (pine) Aluminium alloy

StiffnessE 20 GN m-2 80 GN m-2Density 700 kg m-3 2800 kg m-3

but wood is 4 times lighter so

stiffness of larger wood bar is same as aluminium bar

and both weigh the same

Ashby map formaterials selection:

stiffnessE

versus density

. if so equivalent, why not use wood more?

Choice of shape can improve stiffness considerably:

Tube of same area A

higher stiffness 2.5 S

Round solid bar: area A

bending stiffness S

Lighter tube of area A/4

same stiffness S

Can readily make metallic tubes but more difficult with wood


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LOAD

SHAPE CHANGE orINCREASE IN LENGTH

Yieldstrength

Stiffness

.

.

.

Recap of stiffness and strength

Workhardening, i.e.

strength is higher

Permanent

strain

Grains in a metallic alloy

Aluminium grain size and deformation

Before deformation After deformation at 325C

Strengthening from:- crystal or grain orientation- grain size (small better)- work-hardening

100 m

????

Metallic crystal structures

Metallic crystal structures

http://physchem.ox.ac.uk/~rkt/tutorials/surfaces/solids.html

face centredcubic fcc

body centredcubic bcc

How do metals change shape (plastically deform)?

concept of dislocations

but to break all bonds simultaneously, find

stress needed


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What are dislocations?

www.doitpoms.ac.uk/tlplib/dislocations/index.php

dislocation is a line fault, imperfect latticemetallic alloy contains typically 1010 m/m3

shear stress

What is work hardening and

what are dislocations?

edge dislocation screw dislocation

www.doitpoms.ac.uk/tlplib/dislocations/index.php

dislocation is a line fault, imperfect lattice

Deformed Cu - 0.7 wt.% Co

Deformed grain structure with high dislocation density.

(Humphreys & Martin 1967)

1 m

Aluminium cold rolled 95% and annealed 300C

1 m

How can we strengthen a metallic alloy more?

Range of strengths foraluminium alloys

(log scale)

Strongest alloy is ~ 10 times greaterthan pure Al

Strengthening mechanisms: crystalline materials

Ease of dislocation movement (yield stress) is affected by:

- melting point

- lattice type: available slip systems

- crystal or grain orientation

- grain size

- extent of work-hardening

- solid-solution alloying

- two-phase hardening

- precipitation hardening

www.aluminium.matter.org.uk

Strengthening relies on restricting dislocation movement soyield stress (start of permanent deformation) is higher.


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Strengthening precipitation hardening

Strengthening relies on restricting dislocation movement soyield stress (start of plastic deformation) is higher.

www.aluminium.matter.org.uk

Precipitation hardening:precipitates are small discrete particles in a metal withdifferent structure & chemical composition to matrix

Precipitate strengthening

GP2 or " zones ' - semi coherent - incoherent

Strengthening from precipitates impeding dislocation motion

Al 4 wt% Cu alloy

Why use different metal alloys in aircraft?

Titanium military aircraft

Aluminium civilian aircraft

Relative costs of metallic alloys

www.roymech.co.uk/Useful_Tables/Matter/Costs.html

Relative strengths of aluminium & titanium alloys

Ashby map formaterials selection:yield strengthyversus density

Aircraft skin temperatures (flying at 0.9 Mach)

www.simtec.gr/.../ cfd-intro/cfd-intro-8.htm

At T > 250C,precipitates in aluminium becomeless effective (coarsen & can even dissolve).


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What other properties need to be optimised?

de Havilland Comet

Comet disasters 1954

Comet had square windows

stress magnification at corners

fatigue

repeated variation in stress (magnitude < yield) failure

Shape of the Comet windows

under alternating stress (vibrations)

led to crack initiation and growth

Fatigue mechanism

www.key-to-steel.com/Articles/Art162.htm

In compression:sharpen, and ...

Metal containscracks

In tensioncracks open

andblunt

cracks grow

Repeated cycles result in crack growthand eventually fracture occurs

alternating stress

Fatigue lifetimes: S-N curve for Ti - 6 Al - 4 V

Higher applied stress (load) means shorter fatigue life.

Define fatigue strength as stress (load) to allow 107 cycles

MPa

fatiguestrength

Ashby map formaterials selection: fatigue strength versus


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What is fatigue?

March 1980: Alexander Keilland oil rig in North Sea Ekofisk field brokedue to fatigue fracture and capsized, killing 123 people.

Repeated variation in stress (magnitude < yield) failure

Aloha Airlines Flight 243

Boeing 737-200, Hawaii, 28 April 1988

www.aloha.net/~icarus/

Review: properties relevant to aircraft skin

Weight: mass per unit volume

Strength: load for shape change

Stiffness: resistance to bending

Fatigue life: resistance to cyclic load

Toughness: energy absorbed to break

density

yield strength y

modulus E

endurance limit

fracture toughness G

What is toughness?

toughness is energy absorbed in breaking [fracture] shown by area under stress versus strain curve

Loador stress

Shape changeor strain

Improving toughness

Toughness is energy absorbed when breaking- blunting or diverting crack uses energy- increase area of f ractured surfaces uses energy.

Desirable material properties for strength and toughness:

- high yield stress with good subsequent ductility- small grain size- ease of dislocation motion ahead of crack to blunt it- phase transformation ahead of crack (specialised)also delamination in composites

Learning from nature: shells and trees


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micro fibrils in cell walls

cross-section

Tree branch cross-section Timber cell structure - tracheids

Composites: energy absorption

Burti & Schumacher, Hockenheim track, 29 July 2001

Carbon fibre reinforced composites

200 250 seats; range 7,000 - 8,500 nautical miles; Mach 0.85Graphite epoxy resin composite body & wings

Boeing 787 Dreamliner Ashby map formaterials selection:

KIc versus


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wood

fabric

Aircraft structural materials

aluminium

titanium

composites

steel

acrylic

rubber

www.machinedesign.com/ASP/strArticleID/56410/strSite/MDSite/viewSelectedArticle.asp

Wood: joint design (dovetail)

screws

glue

Fabric: sewn

glue

zipper

Joining approaches

Aluminium: riveting

Titanium: welding fusion and stir friction

Composites: joint design

screws

adhesives

Joining approaches: Duxford What gets us up? aircraft jet engines

Jet engines

T1902

Jet engine

1. SuckSquash court per sec.

2. Squeeze40x atmospheric pressure

3. BangCombust at 2000C

4. BlowForced through turbine


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Titanium

Nickel

Steel

Aluminium

Composites

Materials in gas-turbine aeroengines Rolls Royce 535 engine

Pressure(atmospheres)

Temperature(C)

0

40

0

1500

Pressure and temperature distributions Properties relevant to aircraft engines

Weight: density

Stiffness: modulus E

Strength: yield strength y

Toughness: fracture toughness G

Fatigue life: endurance limit

Creep life: hours under given load appliedat specified temperature

What is creep?

Creep : time-dependent permanent deformation

hence plastic and notelastic deformation

under action of applied stress where applied < yield

Creep ratedepends on:

material

stress

temperature: creep is negligible unless:

T > 0.3 - 0.4 Tm for metals

T > 0.5 - 0.6 Tm for ceramics

T > Tg for polymers (Tg is glass transition temp.)

Stages of creep

strain

time t

transient[shake down]

steady state

tertiary[run away]


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Steady state creep

Creep strain rate and its temperature dependence are shown by:

where A, A' and A are constants,

nis power law creep (or stress) exponent (typically ~ 3 - 8)

Dis the diffusion coefficient at a given temperature T.

Dislocation move

plus

atoms diffuse

Turbine blade: requirements

Weight: density

Stiffness: parent alloy selection

Strength y : small grain size

alloying (solid solution, precipitates)

Toughness: small grain sizeno internal defects

Fatigue life: small grain size

no stress raisers

Creep life: large grains single crystal

Turbine blades in Ni superalloys

Conventionally cast Directional solidificationSingle crystal

matrixsolid solution

disordered fcc

65%intermetallic

Ni3(Al,Ti)

ordered cubic-P

Cube-cube orientation with coherent interfaces

dark field dark field

Nickel-base superalloy (nimonic)

thermal barriercoating

multi-pass cooling

Cooling air

single pass cooling

Controlling turbine blade temperatures

Wrought

DirectionalsolidificationCast

Maximum turbine-entry temperatures


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Flight: past, now and the future?

T2545

Wood

Aluminium and titanium

Composites: carbon fibre

Nickel alloys

Steels

Rubber

Aircraft tyres

ConcordeConcordeJuly 2000

Properties relevant to aircraft

Weight: density

Stiffness: modulus E

Strength: yield strength y

Toughness: fracture toughness G

Fatigue life: endurance limit

Creep life: hours under given load applied

at specified temperature

Aircraft sustainability?

Materials for aircraft: why they dont fall down

. even if we might!

Materials Aircraft

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