1. Review of the Course STRENGH of AIRCRAFT I
AIRCRAFT MATERIALS
Review of the Course
STRENGH of AIRCRAFT I

2. Basic aircraft materials for airframe structures
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
Basic aircraft materials for airframe structures
Aluminium alloys
Magnesium alloys
Titanium alloys
Composite materials

3. Development of aircraft materials for airframe structures
other materials
Relative share of structural materials
composites
Ti alloys
Mg alloys
other Al alloys
pure AlZnMgCu
alloys
AlCuMg alloys
wood
pure AlCuMg
alloys
new Al
alloys
steel
Year

4. Structural materials on small transport aeroplane

5. Development of composite aerospace applications over the last 40 years

6. Composite share in military aircraft structures in USA and Europe
Structural materials on Eurofighter

7. Structural materials on Eurocopter

8. Comparison of mechanical performance of composite materials and light metals

9. Aluminium Alloys

10. Characteristics of aluminium alloys
Advantages
Low density 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

11. Reference aluminium alloys in airframe structure
Part
Control parametr
Reference alloys
Wing
Upper panels
Upper stringers
Lower panels
Lower stringers
Beams, ribs
compression
damage tolerance (DT)
tension + DT
static properties
7150-T6/T77
7050-T74
2024-T3, 2324-T39
2024-T3
7050-T74, 7010-T76
Fuselage
Stiffeners
Main frame
compression, DT, formability
tension/compression
complex
2024 clad-T3
7175-T73
T74
Other parts
All types
7010/7050/7075

12. Typical mechanical properties of alloy 2014 4. 4Cu-0. 8Si-0. 8Mn-0
Typical mechanical properties of alloy Cu-0.8Si-0.8Mn-0.5Mg , E = 72.4 GPa , ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil. cycles
Bare sheet 2014
186 97 18 90 T4
427
290 20
140 T6
483
414 13
125
Alclad sheet 2014
172 69 21 - T3
434
273

13. Typical mechanical properties of alloy 2024 4. 4Cu-1. 5Mg-0
Typical mechanical properties of alloy Cu-1.5Mg-0.6Mn, E = 72.4 GPa , ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil. cycles
Bare 2024
185 75 20 90 T3
485
345 18
140
T4, T351
470
325
Alclad 2024
180 -
450
310
440
290 19

14. Typical mechanical properties of alloy 2124 4. 4Cu-1. 5Mg-0
Typical mechanical properties of alloy Cu-1.5Mg-0.6Mn, E = 72.4 GPa , ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil. cycles
Plate L
T851
490
440 9 -
Plate LT
435
Plate ST
470
420 5
Better transvers properties, good strengths and creep resistance at higher temperatures - for application between 120 – 175 °C.

15. Typical mechanical properties of alloy 6061 1. 0Mg-0. 6Si-0. 3Cu-0
Typical mechanical properties of alloy Mg-0.6Si-0.3Cu-0.2Cr ; E = 68.9 GPa; ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil cycles
Bare 6061
124 55 25 62 T4
241
145 22 97 T6
310
276 12
Alclad 6061
117 48 -
228
131

16. Minimal mechanical properties of alloy 6056 1. 0Si-0. 9Mg-0. 8Cu-0
Minimal mechanical properties of alloy Si-0.9Mg-0.8Cu-0.7Mn-0.25Cr-0.2Ti+Zr; ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil cycles
Thin extrusions - L
T4511
355
245 16 -
T6511
380
360 10
T78511
335
Bare sheet - LT T4
265
135 18
T78
340
315 8

17. Mechanical properties of alloy 7050 6. 22Zn-2. 3Mg-2. 3Cu-0
Mechanical properties of alloy Zn-2.3Mg-2.3Cu-0.12Zr; E = 70.3 GPa; ρ = g/ccm
Direction
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 10 mil. cycles
Minimum properties - Die forgings T-736 (T-74), thickness up to 50 mm L
496
427 7 -
L-T
469
386 5
Minimum properties – Hand forgings T73652, thickness up to 50 mm
434
490
421
Typical properties – Plate T73651
510
455 11
Typical properties – Forgings T73652
524 15

18. Typical mechanical properties of alloy 7075 5. 6Zn-2. 5Mg-1. 6Cu-0
Typical mechanical properties of alloy Zn-2.5Mg-1.6Cu-0.23Cr; E = 71.0 GPa; ρ = g/ccm
Temper
Tensile strength
MPa
Yield strength
Elongation %
Fatigue strength
At 500 mil. cycles
Bare 7075
228
103 17 -
T6, T651
572
503 11
159
T73
434
Alclad 7075
221 97
524
462

19. Use of aluminum-lithium alloys in commercial aircraft

20. Typical mechanical properties of aluminium- lithium alloys
Temper
Direction
Tensile strength
MPa
Yield strength
Elongation %
Alloy 2090: 2.7Cu-2.2Li-0.12Zr; E = 76 GPa, ρ = 2.59 g/ccm
T 83
(near peak aged) L
530
527 3 LT
505
503 6
45°
440
Alloy 8090: Li-1.3Cu-0.95Mg-0.12Zr; E = 77 GPa; ρ = 2.55 g/ccm
T8X
480
400
4.5
465
395
5.5
325
7.5

21. 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 – casing - 1,3 kg
470 x 190 x 170 mm
Al – pedal - 0,4 kg
180 x 150 x 100 mm

22. Magnesium Alloys

23. Basic wrought Mg alloys
Mg-Al-Zn (AZ)alloys
The most common alloys in aircraft industry, applicable up to 150 °C
Composition – 3 to 9 % Al, 0.2 to 1.5 % Zn, 0.15 to 0.5 % Mn
Increasing Al content → strength improvement , but growth of susceptibility to stress corrosion
Zn → ductility improvement
(Cd + Ag) as Zn replacement → high strength up to 430 MPa
Precipitation hardening → strength improvement + decrease of ductility
The most common alloy for sheet and plates – AZ31B (applicable to 100 °C)
Alloy
Composition
Semi-product
Rm, MPa
Rp0.2, MPa
Ductility,%
AZ31B-F
3.0Al-1.0Zn
bars, shapes
260
200 15
AZ61A-F
6.5Al-1.0Zn
310
230 16
AZ80A-T5
8.5Al-0.5Zn
380
240 7
AZ82A-T5
275
AZ31B-H24
sheet, plates
290
220

24. Mg-Zn-Zr alloys (ZK) Mg-Mn alloys (M) Zn → strength improvement
Zr → fine grain → improvement of strength, formability and corrosion resistance
Better plasticity after heat treatment
Alloying with RE a Cd → tensile strength up to 390 MPa
Application up to 150 °C
Mg-Mn alloys (M)
Good corrosion resistance, hot formability, weldability
Not hardenable → lower strength
Alloy
Composition
Semi-product
Rm, MPa
Rp0.2, MPa
Ductility, %
ZK60A-T5
5.5Zn-0.45Zr
bars, shapes
365
305 11
M1A-F
1.2Mn
255
180 12

25. Mg-Th-Zr (HK) Mg-Th-Mn (HM) Mg-Y-RE (WE) High temperature alloys
Example: alloy HK31A - service temperature 315 to 345 °C
Mg-Th-Mn (HM)
Medium strength
Creep resistance → service temperature up to 400 °C
Mg-Y-RE (WE)
Hardenability, formability, good weldability
Y → strength after hardening, Nd → heat resistance, Zr → grain refinement
Application to 250 °C
alloy
composition
semi-product
Rm, MPa
Rp0.2, MPa
ductility, %
HM21A-T8
2.0Th-0.6Mn
sheet, plates
235
130 11
HK31A-H24
3.0Th-0.6Zr
255
160 9
Mg-RE (WE)
8.4Y-0.5Mn-0.1Ce-0.35Cd
bars, shapes
410
360 4

26. Typical properties of several cast magnesium alloys
composition
product Rm
MPa
Rp0.2
ductility %
AM60A-F
6.0Al-0.13Mn
pressure die casting
205
115 6
AZ91A-F
9.0Al-0.13Mn-0.7Zn
230
150 3
AZ63A-T6
6.0Al-3.0Zn-0.15Mn
sand casting
275
130 5
AZ91C-T6
8.7Al-0.13Mn-07Zn
145
AZ92A-T6
9Al-2Zn-0.1Mn
AM100A-T61
10.0Al-0.1Mn 1
QE22A-T6
2.5Ag-2.1RE-0.7Zr
260
195
WE43A-T6
4.0Y-3.4RE-0.7Zr
250
165 2
ZK61A-T6
6.0Zn-0.7Zr
310 10
EZ33A-T5
3.3RE-2.7Zn-0.6Zr
160
110

27. Titanium Alloys

28. 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

29. Properties of important wrought titanium alloys
Temper
Rm, MPa
Rp0.2, MPa
Elongation, %
E, GPa
α and pseudo α
Ti-5Al-2,5Sn
annealed 16
110
Ti-5,6Al
875
750 8 -
Ti-11Sn-1Mo-2,2Al-5Zr-0,2Si 15
114
α + β
Ti-3Al-2,5V 20
107
Ti-6Al-4V
hardened
1170
1100 10 14
Ti-6Al-2Sn-2Zr-2Cr-2Mo-0,25Si
1275
1140 11
122
pseudo β and β
Ti-10V-2Fe-3Al
112
Ti-15V-3Cr-3Al-3Sn
6 - 12

30. stress relief annealing
Cast titanium alloys
Comparison with wrought alloys
Similar chemical composition
Higher content of impurities, specific casting structure and defects (e.g. porosity)
Lower ductility and fatigue life
Often better fracture toughness
Manufacture of shape castings
Good casting properties (fluidity, mold filling)
Hydrogen absorption, porosity
Vacuum melting, special molds, hot izostatic pressing of castings (HIP)
HIP – heating close to solidus + pressure of inert gas (elimination and welding of voids due to plastic deformation) – conditions 910 to 965 °C/100 MPa/2 h.
Examples of cast alloys
Alloy
Heat Treatment
Rm, MPa
Rp0.2, MPa
A5 , %
Ti-6Al-4V
stress relief annealing
880
815 5
Ti-6Al-2Sn-4Zr-2Mo
970°C/2h + 590°C/8h
860
760 4
Ti-15V-3Cr-3Al-Sn
955°C/1h + 525°C/12h
1120
1050 6

31. Composite Materials

32. 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).

33. 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.

34. 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

35. 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.

38. Properties of composites
UD laminate
Properties directionally dependent
Quasi-isotropic laminate
Properties nearly equal in all directions
Tensile strength, MPa
Angle between fibers and stress, °

39. Properties of epoxy UD prepreg laminates Fibre fracture volume typical for aircraft structures
Fabrics and fibres are pre-impregnated by the materials manufacturer with a pre-catalysed resin. The catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months, of useful life. To prolong storage life the materials are stored frozen (e.g. -20°C). High fibre contents can be achieved, resulting in high mechanical properties.

40. Fiber metal laminates
Consist of alternating thin metal layers and uniaxial or biaxial glass, aramid or carbon fiber prepregs

41. Fibre metal laminates Advantages
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.

42. Standard FML configurations
Type
Configuration
Metal alloy
Prepreg constituents
Prepreg orientation
ARALL 2
2/1 – 6/5
2024-T3
Aramid-epoxy
unidirectional
ARALL 3
7475-T76
GLARE 1
Glass-epoxy
GLARE 2
GLARE 3
Cross-ply
GLARE 4
Cross-ply /unidirectional

43. Mechanical properties of FML
Laminate
Metal thickness mm
Prepreg
thickness mm
Tensile
strength
MPa
Yield E
GPa
Density
g/ccm
ARALL 1
0.3
0.22
897
535
67.5
2.16
ARALL 2
849
411
68.3
GLARE 1
0.25
1494
530
62.2
2.42
GLARE 2
0.2
1670
416
60.9
2.34
1449
406
63.0
0.4
1295
399
64.5
2.47
GLARE 3
382
51.3

44. Fatigue resistance of FML comparing to 2024 alloy

45. 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 kg Adhesive bonded stringers from 7349 alloy

46. 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

47. Effectivness of sandwich materials