Section 5
Major Structural Materials for Aeronautical and Automotive Structures
An overview of the major materials used in aeronautical and automotive structures will be given in this section. The mechanical and physical properties of the materials will be highlighted, with an emphasis placed on the stiffness versus density and strength versus density of various materials.
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Contents• Structural Materials• Steels• Aluminium Alloys• Advanced Fibre-Reinforced Polymeric Composites• Summary of Major Mechanical Properties• Stiffness – Density Chart• Strength – Density Chart• Failure Modes of Structural Materials• Ductile Fracture• Brittle Fracture• Temperature Dependent Ductile-Brittle Fracture Transition• Allowable Stress and Factor of Safety• How to Obtain Data on Material Properties?• Credits & Notices
Structural Materials• Key factors to be considered for load bearing aeronautical or
automotive structures:– Strength-to-density ratio– Stiffness-to-density ratio– Fracture toughness and impact toughness– Suitability for manufacturing technique
• Aeronautical structural materials– Aluminium alloys– Advanced composites– Titanium alloys– Steels
• Automotive structural materials– Steels– Aluminium alloys– Magnesium alloys– Plastics– Advanced composites
Steels• Major steel alloying elements
– Carbon– Chromium– Nickel– Manganese– Molybdenum
• Steel categories– Carbon steels– Alloy steels– Stainless steels
• Carbon steels according to carbon content:
– Low-carbon steels (<0.3%) – inc. mild steel– Medium-carbon steels (0.3% - 0.7%)– High-carbon steels (0.7% - 1.4%)
• Maraging steels: alloy steels without carbon
• Mechanical properties of steels are controlled primarily by two means:
– Alloying– Heat treatment
Aluminium Alloys• Major aluminium alloying elements:
– Copper (Cu)– Magnesium (Mg)– Zinc (Zn)– Lithium (Li)
• Aluminium alloy catagories:– 2xxx (with Cu as major alloying element)– 5xxx (with Mg as major alloying element)– 7xxx (with Zn as major alloying element)
• Common grades:– 2024-T3– 5052– 7075-T3
• Al-Li alloys: higher strength, lower density and improved weldability, but are less tough
• Heat treatment condition code for aluminium alloys:
– F: as fabricated– H: strain hardened or cold worked– O: annealed (i.e. softened)– T: heat treated
Advanced Fibre-Reinforced Polymeric Composites
• One-dimensional fibrous reinforcement and three-dimensional matrix
• Typically long continuous fibre reinforcement (15 micron diameter) and matrix can be any material
• Typical fibres:– Carbon– Glass– Aramid (such as Kevlar)
• Typical matrix:– Epoxy– Polyester– Phenolic– Polyetheretherketone (PEEK)– Bismaleimide (BMI)– Polyimide
Advanced Fibre-Reinforced Polymeric Composites (continued)
• Most useful: polymer-based fibre-reinforced composites– Carbon Fibre-Reinforced Plastics (CFRP)– Glass Fibre-Reinforced Plastics (GRP)– Kevlar Fibre-Reinforced Plastics (KFRP)
• FRP’s are still considered brittle when compared with steel and aluminium alloys
• Mechanical properties: significantly direction-dependent and dependent on amount of fibre-reinforcement (volume fraction) and fibre-orientation
• In-plane mechanical properties better than properties in the through-the-thickness (for 2D layups)
http://en.wikipedia.org/wiki/File:Rutan.variEze.g-veze.arp.jpg
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Summary of Major Mechanical Properties
Density
(kg/m3)
E
(GPa)
y
MPa
ult
(MPa)
K1c
(MPa.m½)
(10-6 1/C)
Metals and Alloys
Aluminium Alloys
2024
7075
Cast Iron
Steels
mild steel
high strength
Nickel Alloy
Titanium Alloy
Magnesium Alloy
Polymers or Plastics
Epoxy and Polyester
PVC
Composites
CFRP*
GRP*
Ceramics
Alumina
Silicon Carbide
2800
2800
7200
7850
7850
8800
4500
1800
1250
1400
1750
1950
3750
3000
72
72
83-170
210
210
190
110
45
2-4
2-4
140
35
390
450
0.33
0.33
0.2-0.3
0.29
0.29
0.3
0.33
0.35
0.3-0.4
0.4
0.3
0.3
0.1-0.2
0.1-0.2
410
480
120-290
220
340-1000
200-1600
760-1000
80-280
30-100
46
630
300
5000
10000
480
550
69-480
430
550-1200
400-2000
900-1200
140-340
30-120
-
-
-
-
-
45
23
6-20
100-200
50-154
-
55-115
-
0.4
-
32-45
20-60
4
3
23
23
10-12
12
14
12
10
27
55-90
80
-0.9
7
9
4
* Unidirectional (UD) with 60% fibre-volume fraction
Stiffness versus Density
Composites
Density (kg/m3)
100 300 1,000 3,000 10,000 30,0000.01
0.1
1
10
100
1,000Y
ou
ng
’s M
od
ulu
s (G
Pa)
Woods and Wood Products
Foams
Metals and Alloys
Polymers
Strength versus Density
Metals and Alloys
Composites
Woods and Wood Products
Polymers
Foams
Density (kg/m3)
100 300 1,000 3,000 10,000 30,0000.1
1
10
100
1,000
10,000
Str
eng
th (
MP
a)
Failure Modes of Structural Materials
• Investigation of failure of load-bearing structures is essential to design and understanding of service conditions
• Two characteristic modes of fracture– Ductile– Brittle
• Factors that affect how a structure will fail are:– Manufacturing process– Loading characteristics– Structural configuration– Environmental or service effects
• Fracture mode could change if some of these factors change
• Most mechanical failures involve both, with one being dominant
Ductile Fracture
• Ductile fracture is a gradual process of nucleation, growth and coalescence of micro-cracks in the structure + local stress concentration
• Extensive plastic deformation– Distortion– Considerable dissipation of energy (heat, sound)
• Fracture surface is rough, fibrous and dull– Characteristic ‘cup and cone’ shape
• Materials that often fail in this manner:– Ductile metals– Plastics at room and moderate temperatures
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Brittle Fracture• Brittle fracture defined by a sudden complete
disruption of the material with little or no plastic deformation
• Initiates from interior flaws or stress risers with little warning– Dissipates limited energy
• Fracture surface is clean, smooth and shiny– Characteristic chevron marks usually
perpendicular to loading direction• Materials that often fail in this manner:
– Hard metals (cast irons and other cast metals)– Ductile metals and plastics at below-zero
temperatures• Tensile strengths significantly lower than
compressive strengths• Flaw population has significant effect on
fracture behaviour– Brittle fracture is notch (surface finish) sensitive
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tensile_test.JPG available for download under a Creative Commons licence.
Temperature Dependent Ductile-Brittle Fracture Transition
FCC metals
Low-strength alloys (BCC metals, low carbon steel and thermoplastics)
High-strength alloys
Temperature
Ab
sorb
ed E
ner
gy
Allowable Stress and Factor of Safety
• Limit load = maximum in service load• Factor of safety is applied to the limit load (value decided by design
authority)– Ultimate load = 1.5 limit load– Proof load = 1.15 limit load (military aircraft)
• The structure must be able to withstand ultimate loads without failure
• The structure must be able to support limit loads without detrimental permanent deformation
• Procedure is to design a structure to zero Margin of Safety– Balance between safety and weight
01f
FMS
0.1f
FRF or 01
f
FMS
itlim
yield
xxxxxx
F = Allowable stress (from material properties data)
f = Applied ultimate stress (from calculations / analysis / test)
xxx denotes tension, compression, shear, buckling, bearing etc.
How to Obtain Data on Material Properties?
• You can access an excellent resource for both metallic and composites material properties from the web via any computer connected to the Loughborough network at www.knovel.com or www.matweb.com– MMPDS – Metallic Materials Properties
Development and Standardisation – A U.S. DoT publication
– MIL-HDBK-17 – Composite Materials Handbook (Vol. 2 contains material properties)
This resource was created by Loughborough University and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.
© 2010 Loughborough University.
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Credits
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