2. AIRCRAFT MATERIALS
1. Basic requirements
• High strength and stiffness
• Low density
=> high specific properties e.g. strength/density, yield strength/density,
E/density
• High corrossion resistance
• Fatigue resistance and damage tolerance
• Good technology properties (formability, machinability, weldability)
• Special aerospace standards and specifications
2. Basic aircraft materials for airframe structures
• Aluminium alloys
• Magnesium alloys
• Titanium alloys
• Composite materials
3. Development of aircraft materials for airframe structures
composites
Mg alloys
other Al alloys
pure AlZnMgCu
alloys
pure AlCuMg
alloys
new Al
alloys
steel
Year
AlCuMg alloys
wood
other materials
Relative share
of structural
materials Ti alloys
10. Characteristics of aluminium alloys
Advantages
• Low density 2.47- 2.89 g/cm³
• Good specific properties – Rm/ρ, E/ ρ
• Generally very good corrosion
resistance (exception alloys with
Cu)
• Mostly good weldability – mainly
using pressure methods
• Good machinability
• Good formability
• Great range of semifinished
products
(sheet, rods, tubes etc.)
• Long-lasting experience
• Acceptable price
Shortcomings
• Low hardness, susceptibility to
surface damage
• High strength alloys (containing Cu)
need additional anti-corrosion
protection:
– Cladding – surface protection using
a thin layer of pure aluminium or
alloy with the good corrosion
resistance
– Anodizing – forming of surface oxide
layer (Al2O3)
• It is difficult to weld high strength
alloys by fusion welding
• Danger of electrochemical corrosion
due to contact with metals:
– Al-Cu, Al-Ni alloys, Al-Mg alloys, Al-
steel
15. • Typical castings in aircraft structures
Al – front body of engine
32 kg - D=700 mm
Al- steering part - 1,1 kg
390 x 180 x 100 mm
Al – pedal - 0,4 kg
180 x 150 x 100 mm
Al – casing - 1,3 kg
470 x 190 x 170 mm
17. General characteristics of Mg alloys
• Pure magnesium
– Hexagonal crystal lattice
– ρ=1,74 g/cm³ , Rm=190 MPa, Rp0,2=95 MPa
– Used in metallurgy (alloying element in Al alloys, titanium metallurgy, ductile iron
metallurgy).
– Not used for structural purposes – magnesium alloys have better utility values
• Advantages of Mg alloys
– Low density (ρ = 1,76–1,99 g/cm³ ) → high specific strength (Rm/ ρ)
– Comparing Al alloys, lower rate of strength decrease in relation with temperature
– Lower notch sensitivity and higher specific strength at vibrating loads
– High damping capacity (influence of low modulus of elasticity ~47GPa)
– High specific bending stiffness (higher to 50 % comparing steel, to 20 % comparing Al)
→ high resistance against buckling
– High specific heat → minor temperature increasing at short time heating
– Very good machinability
– Applicability – most alloys up to 150 °C, some of them up to 350 °C.
18. • Shortcomings of Mg alloys
– High reactivity at increased temperatures
• Above 450 °C rapid oxidation, above 620 °C ignition (fine chips, powder)
• Melting and casting – protection against oxidation (chlorides, fluorides, oxides Mg,
powder sulfur, gases SO2, CO2).
– Lower corrosion resistance , generally difficult anti-corrosion protection
• Corrosion environment (air, sea water), impurities Fe, Cu, Ni forming intermetallic
compounds
• Electrochemical corrosion – in contact with the most of metals (Al alloys, Cu alloys, Ni
alloys, steel)
– Low formability at room temperature - most alloys cannot be formed without heating
– After forming – high strength anisotropy along and crosswise deformation –→
differences 20 to 30 %.
– Low shear strength and notch impact strength
– Low wear resistance
– Low diffusion rate during heat treatment → longtime processes , artificial aging is
necessary at precipitation hardening
– Relatively difficult joining – possible electrochemical corrosion, limited weldability
(hot cracking, weld porosity, possible welding techniques - inert gas welding, spot
welding)
20. Characteristics of titanium and titanium alloys
• Pure titanium - 2 modifications
– αTi – to 882 °C, hexagonal lattice
– βTi – 882 to 1668°C, cubic body centered lattice
– With alloying elements, titanium forms substitution solid solutions α and β
• Commercially pure titanium can be used as structural material in many applications,
but Ti alloys have better performance.
• Basic advantages of Ti
– Lower density comparing steel ( ρ = 4.55 g/cm³)
– High specific strength at temperatures 250 – 500 °C, when alloys Al, Mg already cannot
be used
– High strength also at temperatures deep below freezing point
– Good fatigue resistance (if the surface is smooth, without grooves or notches)
– Excellent corrosion resistance due to stabile layer of Ti oxide
– Good cold formability, some alloys show superplasticity
– Low thermal expansion => low thermal stresses
21. • Shortages of titanium
– High manufacturing costs => high prices (~8x higher comparing Al)
– Chemical reactivity above 500 °C – intensive reactions with O2, H2, N2, with refractory
materials of furnaces and foundry molds => brittle layers, which are removed with
difficulties
– Lower modulus of elasticity comparing steel ( E = 115 GPa against 210 GPa)
– Poor friction properties, tendency for seizing
– Poor machinability (low thermal conductivity → local overheating, adhering on tool,
above 1200 °C danger of chips and powder ignition.
– Welding problems (reactivity with atmospheric gases => welding in inert gas, diffusion
welding, laser beam welding, electron beam welding)
– Special manufacturing methods (vacuum melting and heat treating, manufacture of
castings in special molds – graphite molds and/or ceramic molds with a layer of carbon,
hot isostatic pressing - HIP)
• Preferred use of titanium alloys
– If strength and temperature requirements are too high for Al or Mg alloys
– At conditions, when high corrosion resistance is required
– At conditions, when high yield strength and lower density comparing steel are required
– Compressor discs, vanes and blades, beams, flanges, webs, landing gears, pressure
vessels, skin up to 3M, tubing…
– Increasing usage (Boeing 727 – 295 kg, Boeing 747 – 3400 kg)
23. Most composites consist of a bulk material (the ‘matrix’), and a
reinforcement, added primarily to increase the strength and stiffness of the
matrix. This reinforcement is usually in fibre form.
Today, the most common man-made composites can be divided into three main
groups:
Polymer Matrix Composites (PMC’s) – These are the most common and will
be discussed here. Also known as FRP - Fibre Reinforced Polymers (or Plastics)
– these materials use a polymer-based resin as the matrix, and a variety of fibres
such as glass, carbon and aramid as the reinforcement.
Metal Matrix Composites (MMC’s) - Increasingly found in the automotive industry,
these materials use a metal such as aluminium as the matrix, and reinforce it with fibres
such as silicon carbide (SiC).
Ceramic Matrix Composites (CMC’s) - Used in very high temperature
environments, these materials use a ceramic as the matrix and reinforce it with short fibres,
or whiskers such as those made from silicon carbide and boron nitride (BN).
24. Polymer fibre reinforced composites
Common fiber reinforced composites are composed of
fibers and a matrix.
Fibers are the reinforcement and the main source of strength
while the matrix 'glues' all the fibres together in shape
and transfers stresses between the reinforcing fibres.
Sometimes, fillers or modifiers might be added
to smooth manufacturing process, impart special properties,
and/or reduce product cost.
25. Polymer matrix composites
• The properties of the composite are determined by:
- The properties of the fibre
- The properties of the resin
- The ratio of fibre to resin in the composite (Fibre Volume Fraction)
- The geometry and orientation of the fibres in the composite
Properties of unidirectional
composite material
26. Main resin systems
• Epoxy Resins
The large family of epoxy resins represent some of the highest performance resins of those
available at this time. Epoxies generally out-perform most other resin types in terms of
mechanical properties and resistance to environmental degradation, which leads to their
almost exclusive use in aircraft components
• Phenolics
Primarily used where high fire-resistance is required, phenolics also retain their properties
well at elevated temperatures.
• Bismaleimides (BMI)
Primarily used in aircraft composites where operation at higher temperatures (230 °C
wet/250 °C dry) is required. e.g. engine inlets, high speed aircraft flight surfaces.
• Polyimides
Used where operation at higher temperatures than bismaleimides can stand is required (use
up to 250 °C wet/300 °C dry). Typical applications include missile and aero-engine
components. Extremely expensive resin.
27. Fabric types and constructions
• Unidirectional fabrics
– The majority of fibres run in one direction only, a small amount of fibre may run in
other directions to hold the primary fibres in position
– Prepreg unidirectional tape- only the resin system holds the fibres in place
– The best mechanical properties in the direction of fibres
• Basic woven fabrics
– Plain -Each warp fibre passes alternately under
and over each weft fibre. The fabric is
symmetrical, with good stability. However,
it is the most difficult of the weaves to drape.
– Twill - One or more warp fibres alternately weave
over and under two or more weft fibres in a regular
repeated manner. Superior wet out and drape,
smoother surface and slightly higher mechanical
properties
28. Fabric types and constructions – cont.
– Basket -Basket weave is fundamentally the same
as plain weave except that two or more warp fibres
alternately interlace with two or more weft fibres.
An arrangement of two warps crossing two wefts
is designated 2x2 basket.It is possible to have 8x2,
5x4, etc. Basket weave is flatter, and, through
less crimp, stronger than a plain weave, but less stable.
• Hybrid fabric
– A hybrid fabric will allow the two fibres to be presented in just one layer of fabric.
– Carbon / Aramid - The high impact resistance and tensile strength of the aramid
fibre combines with high the compressive and tensile strength of carbon.
– Aramid / Glass - The low density, high impact resistance and tensile strength of
aramid fibre combines with the good compressive and tensile strength of glass,
coupled with its lower cost.
– Carbon / Glass - Carbon fibre contributes high tensile compressive strength and
stiffness and reduces the density, while glass reduces the cost.
29.
30. Fiber metal laminates
• Consist of
alternating thin
metal layers and
uniaxial or biaxial
glass, aramid or
carbon fiber
prepregs
31. Fibre metal laminates
• Developed types
- ARALL - Aramid Reinforced ALuminium Laminates (TU-DELFT)
- GLARE - GLAss REinforced (TU-DELFT)
- CARE - CArbon REinforced (TU-DELFT)
- Titanium CARE (TU-DELFT)
- HTCL - Hybrid Titanium Composite Laminates (NASA)
- CAREST – CArbon REinforced Steel (BUT - IAE)
- - T iGr – Titanium Graphite Hybrid Laminate (The Boeing Company)
• Advantages
Fibre metal laminates produce remarkable improvements in fatigue
resistance and damage tolerance characteristics due to bridging
influence of fibres. They also offer weight and cost reduction and
improved safety, e.g. flame resistance. They can be formed to limited
grade.
32. Fiber metal laminates - application
AIRBUS A 380
Panels of fuselage upper part – 470 m² , GLARE 4
Maximum panel dimensions 10.5 x 3.5 m
Weight saving - 620 kg
Adhesive bonded stringers from 7349 alloy
33. Sandwich materials
• Structure – consists of a lightweight core
material covered by face sheets on both
sides. Although these structures have a
low weight, they have high flexural
stiffness and high strength.
• Skin (face sheet)
– Metal (aluminium alloy)
– Composite material
• Core
– Honeycomb – metal or composite
(Nomex)
– Foam – polyurethan, phenolic,
cyanate resins, PVC
• Applications – aircraft flooring,
interiors, naccelles, winglets etc.
Sidewall panel for Airbus A320
