Upload
dyna3d
View
239
Download
1
Embed Size (px)
Citation preview
8/6/2019 Materials Aircraft
1/14
[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
8/6/2019 Materials Aircraft
2/14
[email protected] Page 2 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
3/14
[email protected] Page 3 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
4/14
[email protected] Page 4 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
5/14
[email protected] Page 5 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
6/14
[email protected] Page 6 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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.
8/6/2019 Materials Aircraft
7/14
[email protected] Page 7 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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).
8/6/2019 Materials Aircraft
8/14
[email protected] Page 8 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
9/14
[email protected] Page 9 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
10/14
[email protected] Page 10 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
11/14
[email protected] Page 11 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
12/14
[email protected] Page 12 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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]
8/6/2019 Materials Aircraft
13/14
[email protected] Page 13 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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
8/6/2019 Materials Aircraft
14/14
[email protected] Page 14 of 14 July 2009
The Goldsmiths Company
Science for Society
Dept. Materials Science & Metallurgy
University of CambridgeLecture: Aircraft alloys
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!