THE ART OF MATERIAL SELECTION FOR THE DESIGN AND MANUFACTURE OF AEROSPACE VEHICLES

PERSONAL VIEW OF A SMALL AIRFRAMER‘S EMPLOYEEINTERNAL REFERENCE: MP-00-MI-10-061, ISSUE 1

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

2

ABOUT THE AUTHOR

Dr. Urs I. Thomann

• MSc. in materials science

• Graduate studies in corrosion resistant high strength steels

• Ph.D. in composites science

with Pilatus since 2003:

• Materials and processes specialist

• Project Manager, landing gear redesign

• Since 2006, Director Production ManagementTrainer Aircraft

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

3

Contents

• Driving forces for material selection

• Yesterday’s, today’s and tomorrow’s material mix in aeroplanes

• Some examples of material selections (or refusals)

• A spotlight on composites: benefits and challenges

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

4

MATERIAL SELECTION: DRIVING FORCES

1. Cost reduction

2. Cost reduction

3. Cost reduction

4. Weight reduction, linked with cost through operating cost reduction (increased payload/range)

5. Maintenance cost (life cycle cost reduction) Advanced technologies are „only“ the means to achieve all but

only financial goals in all phases of the product‘s life! Safety is always a built-in feature granted through compliance

with ever more stringent regulations as issued by (multi)national authorities (EASA, FAA,...)

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

5

COST REDUCTION THROUGH COMPOSITES

• Design integration fewer parts reduction of structural assembly labour cost reduction

• Low density/high strength reduction of empty weight increased payload/range increased operating profit

• Improved corrosion resistance lower life cycle cost

Potential estimated at 30 % weight reduction, 40 % cost reduction compared with standard metal leight weight design (1990‘s) BUT...

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

6

... THE ALUMINIUM FACTION DID NOT LAZE!

• Advanced joining technologies design integration fewer parts reduction of structural assembly labour cost reduction

• New alloys lower density/higher strength reduction of empty weight increased payload/range increased operating profit

Potential estimated at 20 % weight reduction, 20 % cost reduction compared with standard metal light weight design (1990‘s)

Friction Stir Welding

Laser Beam Welding

or extrusion

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

7

A380 – ALUMINIUM STRUCTURE BENCHMARK

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

8

AIRFRAME MATERIALS: PAST, PRESENT, FUTURE

Com

posi

te w

eigh

t pe

rcen

tage

Tendency:

• More composite materials

• Tailored matieral mix to improve over all systems performance

EXAMPLES OF MATERIAL SELECTIONS (OR REFUSALS)

INTERNAL REFERENCE: MP-00-MI-10-061, ISSUE 1

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

10

EFFICIENCY IMPROVEMENT THROUGH ADVANCED MATERIAL TECHNOLOGIES

• Higher combustion temperatures yield higher thermodynamic efficiency and thus lower fuel consumption

• Today‘s technology with single crystal nickel alloys and oxide dispersioned strengthened (ODS) super alloys with bleed air cooling cannot provide the required step change in fuel consumption

„New“ high temperature/high strength materials along with new design concepts required Ceramic matrix composites

max

ambientmax

T

TT

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

11

WEIGHT REDUCTION THROUGHHIGH STRENGTH MATERIALS

• Typical steel applications: Heavily stressed bolts, bushings and special fittings in the landing gear and engine pylon, moderately temperature stressed portions of engine shrouds,...

• Despite the tendency of decreasing steel weight fraction of the airframe there is still some weight saving potential by employing novel high strength, corrosion resistant steels

• However, such novel alloys like e.g. nitrogen alloyed pressure electro slag remelted austhenitic stainless steels are still not offered (nor demanded) in aerospace certificated grades

• Weight saving potential is probably not big enough to off-set certification cost

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

12

LESS OBVIOUS MATERIAL SELECTION CRITERIA:PC-21 FIREWALL

• Frame to separate cockpit from engine is manufactured from titanium

• Firewall has to withstand an engine fire for a defined duration without allowing the heat to penetrate into the front cockpit

• Titanium has much lower heat conductivity than steel or aluminium and retains reasonable strength at higher temperatures

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

13

ELASTOMERS

• Still the best material to cope with excessive wear experienced by the tires is natural rubber!

• O-ring seals and flexible hoses: make sure to select the right material depending on media to be sealed against or flowing through:

– Chloroprene withstands fuel but not ozone and UV light

– Isoprene is easy with ozone und UV light but not with fuel or hydraulic fluids

– Nitrile butadiene rubber (NBR) happily swims in hydraulic fluids but should not be exposed to ambient air with ozone and UV light

– Fluoropolymer rubbers are expensive but cope with almost every environment, even at somewhat elevated temperatures

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

14

POLYSULFIDE SEALANTS

• Sealants are the true cost savers throughout an aeroplane‘s life

• Making the pressurised fuselage air tight and the integral wing tank fuel tight is only the most obvious primary function of a true but modest champion

• Seals crevices to prevent corrosion due to moisture entrapment

• Releases chromates to prevent microbial attack in the integral tank

• Chromates also actively inhibit corrosion in general

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

15

COMPOSITES FOR PROTOTYPING

• Some composites manufacturing processes allow for quick prototyping at modest tooling and production cost

Ideal for validation of concept studies specifically for full scale aerodynamic tests

Risk mitigation, development cost reduction

PC-21 UWT H-tail fin: 5 days from design to prototype

A SPOTLIGHT ON COMPOSITES:BENEFITS AND CHALLENGES

INTERNAL REFERENCE: MP-00-MI-10-061, ISSUE 1

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

17

ALUMINIUM VS. COMPOSITE TRUCTURE

Advan

tages

Aluminum

Challe

ng

es

Composite

• Long-term experience

• High automation level

• Advanced joining technologies

• Standardized material

• Standard Certification procedure

• Low density (weight reduction)

• High strength and stiffness

• Improved fatigue behavior

• Less corrosion

• Design freedom

• Reduced manufacturing costs

• Reduced Direct Operating Costs

• Fatigue

• Corrosion

• Subprocesses

• Design

• Impact sensitivity

• Environmental influences

• Material + manufacturing diversity

• Certification (not standardized mat.)

• High material cost

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

18

IMPROVED CORROSION RESISTANCE – ONLY HALF OF THE TRUTH!

• Yes, by and large carbon fibre composites are pretty much unaffected by corrosive environments, but...

• ... aluminium alloys are even more affected when in direct contact with carbon fibres due to extreme electrochemical potential difference between carbon and aluminium

Cadmium plated stainless steel/nickel fasteners needed: More expensive heavier than aluminium fasteners

More titanium in direct contact with carbon fibre composites employed: More expensive raw material and more complex production

processes than aluminium Similar specific strength/stiffness as aluminium

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

19

RAW MATERIAL DIVERSITYComposite

Reinforcement (Fibers)

Fiber

Carbon

Thermosets

Polymer

Matrix (Polymer)

Glass Aramid

Thermoplastics

Cyanesther

Epoxy

Bismaleimide

Phenolic

…

…

IMS

AS4

HTA

T800

UD fabric Woven fabric Mat

Natural

Filament

…

PPS

PEI

PEEK

HTS

T700

… … ……

…

…

…

…

…

…

…

…

…

…

…

Diversity due to user-defined raw material combination

Objective:

Material combination = Design and manufacturing requirements

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

20

MANUFACTURING PROCESS DIVERSITY

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

21

COMPOSITE – DESIGN, MANUFACTURING, MATERIAL

Interaction

Manufacturing

Design

Material

• Process limitations• Laminate quality:

- Fiber volume fraction- Internal and external defects- Dimensions

• Surface condition• Quantity• Quality control• Process qualification• Costs

• Design, e.g.:- Integral or differential- Monolithic and/or sandwich- Frame-Stringer or Spar-Rips, etc.

• Design philosophy- Safe life- Fail safe- damage tolerance

• Strength and stiffness requirements• Static and dynamic analysis• Further considerations:

- Inspection- Repair procedure - Lightning protection- Electrical grounding

• Material properties• Semi-finished products• Environmental influences:

- Temperature- Humidity

• Quality control• Availability• Price

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

22

CERTIFICATION

Composite Metal

Material Tests

• Generic specimens

• Determine material data

• Understand deformations and failure modes

Establish Design

Proof Tests

• Aircraft-specific specimens

• Demonstrate ultimate load or fatigue capability

• Include defects, damage, environmental effects

Validate Design

No tests due to standardized material and long-term experience

Very little tests in case of special design features

Same as composite

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

23

CERTIFICATION

E.g. coupons tests:• Mechanical properties, e.g.:

– Laminate: Strength and stiffness etc. in tension, compression and shear.– Engineering data: Strength in tension and compression with and without

holes; bearing strength; Compression After Impact strength• Physical properties, e.g.:

– Density, glass transition temperature Tg, volume fraction, cured ply thickness

• Environmental influences, e.g.:– From -55°C to +55°C OAT in dry and wet conditions– Contaminations (hydraulic fluid, jet fuel, solvents, paint stripper)

• Requirements for storage, handling, processing, machining etc. Data must be established by means of a qualification programme for each

specific composite material.

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

24

• Raw material testing

– Physical and chemical tests

– Mechanical coupons tests

• Manufacturing control

– Process control

• Component testing

– Visual inspection

– Dimension and weight control

– Ultrasonic inspection

– Mechanical test of coupons which accompanied the curing process

QUALITY CONTROL

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

25

SOME CRITICAL COMMENTS

• Use of composites in aerospace is about to degenerate to a marketing crusade

• Composites should not be used for the sake of composites usage but for their beneficial properties in some (but not all) applications

• There is still a lot of black metal design even in the most recently developed products, which by and large defeats most of the composite‘s advantages over standard materials

• The holy grail lies in design integration and eventually certification of advanced joining techniques

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

26

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

27

SUMMARY

• Deep knowledge of the present state of the art in each class of materials is essential

• There is no right or wrong material selection; it is rather a complex decision making process depending on

– OEM’s design and manufacturing skill and experience level

– Requirements

– Balance of value and cost

• Mastering the art of selecting the best performing material for any given purpose of application is really at the core of the successful design of an aerospace vehicle

MP-00-MI-10-061

U. Thomann

Materials for Aerospace Vehicles

21:3715

28

THANKS FOR YOUR ATTENTION