Aerospace Materials

1
Aerospace Materials

• Selection of materials for a given application
• Aerospace metal alloys
• Properties
• Relative costs
• Composite materials
• Material properties
• Analysis methods
• Manufacturing

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Material Selection
The first factor to be considered in selection of
a material for a given component is the
application.

• Operational features principal function of the
  component description of principal loads and
  environment
• Design Criteria most important design
  properties for satisfying the operational
  features
• Manufacturing Processes Material form and
  fabrication processes.

3
Properties for Screening/Rating Materials

• Static strength and stiffness properties
• Durability and damage tolerance properties
  (fracture toughness, fatigue and corrosion
  resistance, etc.)
• Physical properties (thermal and electrical
  conductivity, coefficient of thermal expansion)
• Producibility (cost, manufacturing
  considerations, etc.
• Weight
• Availability

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Metal Alloys for Aerospace Application
An alloy is a mixture or solid solution of two or
more metals.
The atoms of one replace the atoms of the other
or occupy interstitial positions between the
atoms.

• Aluminum alloys
• Titanium alloys
• Steels
• Magnesium alloys
• Nickel alloys
• Beryllium alloys

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Material Forms for Metals

• Sheet and plate a rolled, flat product
• Sheet thickness less than 0.250 in.
• Plate thickness 0.250 in. or greater
• Extrusion uniform cross section created by
  forcing metal through a series of dies
• Forging shape created by plastically deforming
  metal by compression, usually in closed dies.
  Forging creates high-strength, tough part with
  efficient use of material.
• Casting created by solidification of liquid
  material in a mold

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Applications for Material Forms

• Sheet and plate
• Sheets are used in skin of fuselage, wings,
  control surfaces, etc.
• Plates are machined to varying thickness create
  optimum shapes in high-cost parts
• Extrusion used for uniform cross section parts
  (e.g., stiffeners on spars, ribs) where higher
  strength is needed
• Forging nonuniform cross section parts where
  high-strength is needed.
• Casting lower cost parts in noncritical areas

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Aluminum Alloy Characteristics

• Aluminum alloys are the most widely used
  materials in aircraft structures.
• Al alloys are easily formed and machined.
• Al alloys are relatively inexpensive.
• Al alloys experience a significant reduction in
  strength at higher temperatures, limiting their
  application in supersonic aircraft.

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Aluminum Alloys
Aluminum alloys are identified by a four-digit
numbering system that signifies the primary
alloying element.

• 1000 99 elemental Al
• 2000 Copper
• 3000 Manganese
• 4000 Silicon
• 5000 Magnesium
• 6000 Magnesium and Silicon
• 7000 Zinc

Processing used to produce specific properties
(such as heat treatment) are designated by a
dashed suffix following the four-digit alloy,
e.g., 2024-T3.
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Aluminum Alloys for Airframe Structures

• Group 2000 Primarily in tension applications
  where fatigue and damage-tolerant design are
  critical
• Lower wing skins
• Pressurized fuselage skins
• Standard material has been 2024-T3
• Group 7000 Compression applications or where
  static strength is more important than fatigue or
  damage tolerance
• Upper wing surfaces
• Wing ribs
• Floor beams
• 7075-T6, especially in military jets

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Titanium Alloys

• Titanium alloys offer, with their higher
  strength, offer higher structural efficiencies
  than Al alloys.
• Ti alloys are offer selected due to high
  temperature endurance.
• Ti alloys are significantly more expensive than
  Al (high material cost, more difficult to form
  and machine).
• Galvanic corrosion resistance for fastening
  composite
• structures.
• The most common alloy is Ti-6Al-4V.

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Steel Alloys

• Steel contains iron with a small percentage of
  carbon (0.02 to 1.7). Other alloying elements
  are added to achieve specific properties such as
  strength, toughness, or corrosion resistance.
• The mechanical properties of steels can be varied
  significantly by heat treating.
• Some steels offer very high strength.
• Steel alloys are not widely used in airframe
  structures except where very high strength is
  needed.

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Steel Alloys
Steel alloys are identified by a four-digit
numbering system.
The first two digits identify the primary
alloying elements, while the last two signify the
carbon content.

• 4130 Cr-Mo with 0.3 C
• 4340 Ni-Cr-Mo with 0.4 C

Standard heat treats are identified by the
ultimate tensile strength, e.g., 180 ksi.
AISI 4340 is used for thicker parts than 4130
because it can be heat treated to a greater depth.
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Mechanical Properties
Most common metallic materials used for aerospace
design are Aluminum, Steel, and Titanium. The
properties for these materials are contained in
MIL-HDBK-5. Typical data includes Tension
Compression stu Ultimate Stress scu
Ultimate Stress sty Yield Stress sty
Yield Stress E Modulus of Elasticity Ec
Modulus of Elasticity e Elongation Shear
Bearing ssu Ultimate Stress sbru
Ultimate Stress G Modulus of Rigidity sbry
Yield Stress
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MIL-HDBK-5 Terminology
Strength values are reported using symbol F. For
example Fty sty The values reported are
minimum guaranteed values based on testing
multiple specimens. The statistical confidence
in the values are given using the following
bases A Basis At least 99 percent of all
mechanical property values are expected to fall
above the specified property values with a
confidence of 95 percent. B Basis At least 90
percent of all mechanical property values are
expected to fall above the specified property
values with a confidence of 95 percent. S Basis
Minimum mechanical property values specified by
various agencies.
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MIL-HDBK-5
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Example Material Selection for Minimum Weight
Design
A commonly used criterion in selecting materials
for aerospace structures design is minimum
weight. This involves selecting the proper
combination of material and design dimensions
that result in the part with minimum
weight. Consider the three loading conditions
below. It is assumed that the only design
dimension to selected is thickness t.
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Example Material Selection for Minimum Weight
Design (Continued)
The expressions relating the applied external
loads to the induced stresses are
Buckling
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Example Material Selection for Minimum Weight
Design (Continued)
The weight of the member can be expressed in
terms of the material density ? and geometric
dimensions
W L b t ? Solving for t from the
expressions relating loads to stress and
substituting above
(buckling)
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Example Material Selection for Minimum Weight
Design (Continued)
Weight comparison of different materials may be
conducted using the expressions previously
derived.
Axial Load
Buckling
Bending
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Example Material Selection for Minimum Weight
Design (Continued)
1.02
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Material Cost
The material cost data provided in the handout
are normalized based on the cost of 2024 Al
sheet, which is has been widely used in the
structures of existing aircraft.
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Fatigue Failure

• Nearly every component in an aircraft structure
  is subjected to fluctuating loads, including the
  loads of pressurization, takeoff, and landing.
• Discontinuities such as windows, doors, and
  rivets cause stress concentrations which mean
  these areas are of particular concern.
• Because materials subject to fluctuating loads
  fail at stresses much lower than the stresses
  that cause failure under static loads, fatigue
  behavior must be considered in selection of
  materials.
• Fatigue behavior is expressed as a graph of
  failure stress as a function of cycles to cause
  failure (S-N curve).

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Typical S-N Curves
For many steels the S-N curve levels is
asymptotic to a minimum value, as shown for 4130,
HT to 125 ksi.
Aluminum alloys do not exhibit this asymptotic
behavior, as shown for Al 2024-T4.
Palmgren-Minor theory
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Sandwich Structure

• The sandwich consists of a core material,
  typically honeycomb, between two higher strength
  face sheets.
• The face sheets and core may be made from an
  aluminum alloy or a nonmetallic composite.
• The face sheets possess high in-plane strength,
  and the core separates them to increase bending
  resistance.
• Design of the sandwich component must consider a
  variety of failure modes, including shear failure
  of the bond between face sheets and core.

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Example of Typical Transport Structure Boeing
737 horizontal stabilizer assembly
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Typical Transport Structural ComponentBoeing 737
horizontal stabilizer rib (sandwich stiffened)
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Sandwich-Stiffened Structural ComponentBoeing
767 outboard aileron