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Materials & Material Properties
Ch. 4 - Material Properties, Airframe Stress Analysis and Sizing (Niu)
& Ch. 4 - Materials, Airframe Structural Design (Niu)

The materials bible: Military Handbook - Metallic Materials and Elements for Aerospace Vehicle Structures, MIL-HDBK-5G, 1999 (new editions are issued every couple of years - always use the latest).

Primary Materials Selection Critera• Static strength efficiency
• Fatigue strength
• Fracture toughness and crack growth rate
• Corrosion and embrittlement properties
• Compatibility with other materials
• Environmental suitability
• Availability and cost
• Fabrication characteristics
Despite all the materials and alloys available, aluminum alloys continue to find prominence in mostaircraft.

For pressurized fuselage cabins and lower wing skins -- two areas prone to fatigue through the long-continued application and relaxation of tension stresses -- the standard material is an aluminum alloy 2024-T3

For upper wing skins that have to withstand mainly compression stresses as the wing flexes upward during flight, 7075-T6 is most often used. 7075-T6 is also used extensively formilitary fighter aircraft structures, which generally have stiffer wings and -- except for the cockpit area -- an unpressurized fuselage. 7075-T6 is almost twice as "strong" as 2024-T3, and therefore the weight can be reduced correspondingly in suitable application.
Aluminum Alloy Groups

• Group 1000 contains 99% elemental aluminum
• Group 2000 Copper as the major alloying element• Group 3000 Manganese as the major alloying element
• Group 4000 Silicon as the major alloying element
• Group 5000 Magnesium as the major alloying element
• Group 6000 Magnesium and Silicon as the major alloying
• Group 7000 Zinc as the major alloying element

Basic Tempers used for aluminum alloys
• O Annealed
• F Asfabricated
• H Strain hardened
• T Heat treated (all aluminum alloys used in primary aircraft
applications are the heat treated tempers)

A typical heat treat designation for an extrusion is shown below:

T6 - type of heat treatment (6=solution heat treated and
artificially aged)
5 - means materials has beenstress relieved
1 - material was stretched to accomplish stress relief
(2 if compressive methods are used)
1 - indicated minor straightening was used to meet straightness
and flatness tolerances (0 if straightening is not allowed)
See MIL-HDBK-5 for complete description of symbols used.

Group 2000 - Primarily used in tension applications where fatigue and damage tolerant design iscritical, e.g., lower wing surfaces, pressurized fuselage skin, etc.
Group 7000 - Primarily used in compression applications where fatigue and damage tolerant design is not critical, e.g., upper wing surfaces, wing ribs, floor beams, etc.
Stress-Strain Curves

Uniaxial tensile and compression tests are generally performed to obtain the following basic mechanical properties:

[pic] Young'smodulus in tension
[pic] Young's modulus in compression
[pic] Poisson's ratio
[pic] yield stress in tension (defined by .2% offset)
[pic] ultimate stress in tension
[pic] yield stress in compression (defined by .2% offset)
[pic] ultimate stress in compression
[pic] (or [pic]) strain when ultimate stresses is reached
Some Symbols, Abbreviations, etc. used in MIL-HDBK-5

[pic] Modulus ofelasticity in tension
[pic] Modulus of elasticity in compression
[pic] Secant modulus of elasticity
[pic] Tangent modulus of elastcity
[pic] Elongation in percent, unit deformation or strain
[pic] Elastic strain
[pic] Plastic strain
F stress
[pic] Design tensile yield stress at which permanent strain is .002
[pic] Design tensile ultimate stress
G Modulus of rigidity, shear modulus

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