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