Material Selection for
Aerospace Applications
Darren Pyfer, P.E.
Engineering Specialist Senior
October 16, 2001
Agenda
• Vought Aircraft Industries Corporate Overview
• Material Selection Criteria
• Material Types
• Material Forms
• Examples
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2
Vought Aircraft Industries
Corporate Overview
Vought Company Overview
•
Largest Single Supplier of Aerostructures to Boeing:
- Producing More of the
747 Structure Than Any
Other Commercial
Supplier for Boeing
- Producing More of the
C-17 Structure Than Any
Other Military Supplier
for Boeing
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4
Vought Company Overview (cont.)
•
Largest Single Supplier of Aerostructures to
Gulfstream Aerospace
– Designed and Build
Integrated Wing System
for the Gulfstream GV
As a Risk-sharing Team
Member
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5
Vought Company Overview (cont.)
•
Largest Single Supplier of Aerostructures to
Northrop on the B-2 Stealth Bomber Program
– Designed and Built the
Intermediate Wing
Section of the B-2
Bomber including the
Engine and Landing
Gear Bays
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Vought Commercial Products
737
747
GV
777
757
CFM56
CF6
GIV
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767
HAWKER 800
CF34
7
Vought Military Products
C-17
S-3
F/A-18E/F
F-14
E-8C/JSTARS
E-2C
EA-6B
V-22
Global Hawk
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P-3
T-38
Vought Product Line Summary
Empennage Fuselage Doors
737
747
757
767
777
GV
C-17
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Wings
Nacelle Control
Comp Surfaces
Material Selection Criteria
Static Strength
• Material Must Support Ultimate Loads Without
Failure. Material Must Support Limit Loads Without
Permanent Deformation.
– Initial Evaluation for Each Component
– Usually Aluminum Is the Initial Material Selection
– If Aluminum Cannot Support the Applied Load
Within the Size Limitation of the Component,
Higher Strength Materials Must Be Considered
(Titanium or Steel)
– If Aluminum Is Too Heavy to Meet the
Performance Requirements, Graphite/Epoxy or
Next Generation Materials Should Be
Considered
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Stiffness
• Deformation of Material at Limit Loads Must Not
Interfere With Safe Operation
– There Are Cases Where Meeting the Static
Strength Requirement Results in a Component
That Has Unacceptable Deflections
– If That Is the Case, The Component Is Said to Be a
‘Stiffness’ Design
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Fatigue (Crack Initiation)
• The Ability of a Material to Resist Cracking Under
Cyclical Loading
– Spectrum Dependant
– Stress Concentration Factors
– Component Is Limited to a Certain Stress Level
Based on the Required Life of the Airframe
– Further Processing May Improve Fatigue
Properties Such As Shot Peening or Cold Working
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Damage Tolerance (Crack Growth)
• The Ability of a Material to Resist Crack Propagation
Under Cyclical Loading
– Slow Crack Growth Design
– Use of Alloys With Increased Fracture Toughness
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Weight
• Low Weight Is Critical to Meeting Aircraft
Performance Goals
– Materials Are Tailored for Specific Requirements to
Minimize Weight
– Materials With Higher Strength to Weight Ratios
Typically Have Higher Acquisition Costs but Lower
Life Cycle Costs (i.e. Lower Fuel Consumption)
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Corrosion
• Surface Corrosion
– Galvanic Corrosion of Dissimilar Metals (see
Chart)
– Surface Treatments
– Proper Drainage
• Stress Corrosion Cracking
– Certain Alloys Are More Susceptible to Stress
Corrosion Cracking (see Chart)
– Especially Severe in the Short Transverse Grain
Direction
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Dissimilar Metal Chart
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Stress Corrosion Cracking (SCC) Chart
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Producibility
• Commercial Availability
• Lead Times
• Fabrication Alternatives
– Built Up
– Machined From Plate
– Machined From Forging
– Casting
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Cost
• Raw Material Cost Comparisons
– Aluminum Plate = $2 - $3 / lb.
– Steel Plate = $5 - $10 / lb.
– Titanium Plate = $15 - $25 / lb.
– Fiberglass/Epoxy Prepreg = $15 - $25 / lb.
– Graphite/Epoxy Prepreg = $50 - $100 / lb.
• Detail Fabrication Costs
• Assembly Costs
• Life Cycle Costs
– Cost of Weight (Loss of Payload, Increased Fuel
Consumption)
– Cost of Maintenance
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Specialized Requirements
• Temperature
• Lightning and Static Electricity Dissipation
• Erosion and Abrasion
• Marine Environment
• Impact Resistance
• Fire Zones
• Electrical Transparency
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Performance vs. Cost Dilemma
• Highest Performance For The Lowest Cost Is the
Goal of Every Airplane Material Selection.
– Mutually Exclusive
– Compromise Is Required
– Define the Cost of Weight to the Aircraft
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Material Types
Aluminum
• Aluminum Accounts for ~80% of the Structural
Material of Most Commercial and Military Transport
Aircraft
• Inexpensive and Easy to Form and Machine
• Alloys Are Tailored to Specific Needs
• 2000 Series Alloys (Aluminum-copper-magnesium)
Are Medium to High Strength With Good Fatigue
Resistance but Low Stress Corrosion Cracking
Resistance.
– 2024-T3 Is the Yardstick for Fatigue Properties
• 5000 and 6000 Series Alloys Are Low to Medium
Strength but Easily Welded
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Aluminum (cont.)
• 7000 Series Alloys (Aluminum-zinc-magnesiumcopper) Are High Strength With Improved Stress
Corrosion Cracking Resistance but Most Have No
Better Fatigue Properties Than 2000 Series
– 7050 and 7075 Alloys Are Widely Used
– 7475 Alloy Provides Higher Fatigue Resistance
Similar to 2024-T3
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Aluminum Tempers
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Aluminum Tempers (cont.)
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Aluminum Tempers (cont.)
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Aluminum Comparison Chart
Material
2024-T3,
T351,
T42
Typical Application
High Strength Tension Applications. Best
Fracture Toughness/Slow Crack Growth Rate
and Good Fatigue life. Thick Forms Have Low
Short Transverse Properties including Stress
Corrosion Cracking.
2324-T3
8% Improvement In Strength Over 2024-T3 With
Increased Fatigue And Toughness Properties.
7075-T6,
High Strength Compression Applications.
T651, Higher Strength Than 2024-T3, But Lower
T7351 Fracture Toughness. T7351 has Excellent
Stress Corrosion Cracking Resistance and
Better Fracture Toughness Than T6.
7050-T7451 Better Properties Than 7075-T7351 In Thicker
Sections.
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Titanium
• Better Strength To Weight Ratio Than Aluminum or
Steel
• Typically Comprises ~5% By Weight in Commercial
Aircraft and Up To ~25% By Weight For High
Performance Military Aircraft
• Good Corrosion Resistance
• Good Temperature Resistance
• Good Fatigue And Damage Tolerance Properties In
The Annealed Form
• Typical Alloy Is Ti 6Al-4V Either Annealed or Solution
Treated and Aged
• High Cost For Metals
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Steel
• Steel May Be Selected When Tensile Strengths
Greater Than Titanium Are Necessary
• Steel Is Usually Limited to a Few Highly Loaded
Components Such As Landing Gear
• There Are Many Steel Alloys to Choose From (See
Chart); Select the One That Is Tailored for Your
Application.
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Steel (cont.)
Mil-Hdbk-5 List of Aerospace Steel Alloys:
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Composite
• The Embedding of Small Diameter High Strength High
Modulus Fibers in a Homogeneous Matrix Material
• Material Is Orthotropic (Much Stronger in the Fiber
Oriented Directions)
• Fibers
– Graphite (High Strength, Stiffness)
– Fiberglass (Fair Strength, Low Cost, Secondary
Structure)
– Kevlar (Damage Tolerant)
• Matrix
– Epoxy (Primary Matrix Material) to 250° F
– Bismaleimide (High Temp Applications) to 350° F
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Material Properties Comparison
Material
2024-T3 Aluminum
7075-T6 Aluminum
6Al-4V Titanium
Annealed
6Al-4V Titanium
Solution Treated and
Aged
15-5PH Stainless
Steel (H1025)
Fiberglass Epoxy
(Unidirectional)
Graphite Epoxy
(Unidirectional)
Ftu
Fty
(ksi) (ksi)
64
47
78
71
134
126
Fcy
(ksi)
39
70
132
E
Density
6
3
(10 psi) (lb/in )
10.5
.101
10.3
.101
16.0
.160
150
140
145
16.0
.160
154
145
152
28.5
.283
80
60
5
.065
170
140
22
.056
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Next Generation Materials
• Aluminum Lithium
• GLARE (Fiberglass Reinforced Aluminum)
• TiGr (Graphite Reinforced Titanium)
• Thermoplastics
• Resin Transfer Molding (RTM)
• Stitched Resin Fusion Injected (Stitched RFI)
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Mil-Hnbk-5 Overview
• Document Contains Design Information On The
Strength Properties of Metallic Materials and
Elements for Aerospace Vehicle Structures. All
Information and Data Contained in This Handbook
Have Been Coordinated With the Air Force, Army,
Navy, Federal Aviation Administration and Industry
Prior to Publication and Are Being Maintained As a
Joint Effort of the Department of Defense and the
Federal Aviation Administration.
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Basis of Properties
• Material Property Selection Is Dependant on the
Criticality of the Structural Component
– Critical Single Load Path Structure
– A Basis (99% Probability of Exceeding)
– S Basis (Agency Assured Minimum Value)
– Other Primary Structure With Redundant Load
Paths
– B Basis (90% Probability of Exceeding)
– Without a Test, A or S Basis May Be Required
– Secondary Structure
– B Basis (90% Probability of Exceeding)
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Grain Direction
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Material Properties (Mil-Hdbk-5) Example
• Type
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Material Forms
Sheet
• Rolled Flat Metal Thickness Less Than .25”
– Fuselage Skin
– Fuselage Frames
– Rib and Spar Webs
– Control Surfaces
– Pressure Domes
• Good Grain Orientation
• Many Parts and Fasteners
• Fit Problems
– Straighten Operations
– Shims
– Warpage
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Plate
• Rolled Flat Metal Thickness Greater Than .25”
– Wing and Tail Skins
– Monolithic Spars and Ribs
– Fittings
• Unitized Structure; Fewer Fasteners
• Grain Orientation Can Be a Problem
• High Speed Machining Has Lowered Fab Costs
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Extrusion
• Produced By Forcing Metal Through a Forming Die At
Elevated Temperature To Achieve The Desired Shape
– Stringers
– Rib and Spar Caps
– Stiffeners
• Grain Is Aligned in The Lengthwise Direction
• Additional Forming and Machining Required
• Used In Conjunction With Sheet Metal Webs
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Forging
• Produced by Impacting or Pressing The Material Into
The Desired Shape
– Large Fittings
– Large Frames/Ribs
– Odd Shapes
• Control Grain
Orientation
• Residual Stresses
Can Cause
Warpage
• Tooling Can Be
Difficult
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Casting
• Produced By Pouring Molten Metal Into A Die To
Achieve The Desired Shape
– Nacelle/Engine Components
– Complex Geometry
• Dramatically Lowers Part and Fastener Counts
• Poor Fatigue And Damage Tolerance Properties
• High Tooling Costs
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Composite
• Produced By Laying Fabric, Laying Tape, Winding,
Tow Placement and 3D Weaving or Stitching
– Skins
– Trailing Edge Surfaces
– Interiors and Floors
• Properties Can be
Oriented To Load Direction
• Excellent Strength To
Weight Ratio
• High Cost Of Material and
Processes
• Poor Bearing Strength
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Examples
Upper Wing Cover
• Skin - 7075-T651 Aluminum
Plate
• Stringers - 7075-T6511
Aluminum Extrusion
• After Machining; Age Creep
Formed To -T7351/-T73511
• Compression Dominated
• Reduces Compressive Yield
Strength
• Greatly Increases Stress
Corrosion Resistance
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Lower Wing Cover
• Skin - 2024-T351 Aluminum
Plate
• Tension Dominated
• Good Ultimate Tensile Strength
• Very Good Fatigue and
Damage Tolerance Properties
• Stringers - 7075-T73511
Aluminum Extrusion
• High Ultimate Tensile Strength
• Good Damage Tolerance
Properties
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Spars
• 7050-T7451 Aluminum Plate
• High Tensile and
Compressive Strength in
Thick Sections
• Good Stress Corrosion
Resistance
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Fixed Trailing Edge Surface
• Graphite/Epoxy Fabric
• Aramid/Phenolic
Honeycomb
• Fiberglass/Epoxy Fabric
Corrosion Barrier
• Secondary Structure
• Stiffness Design
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Leading Edge
• 2024-0 Clad Aluminum
• Heat Treated to -T62 After Stretch Forming to Shape
• Clad For Corrosion Resistance
• Polished For Appearance
• De-icing by Hot Air/Bird Strike Resistance
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Landing Gear Support Beam
• Titanium 6Al-4V Annealed
Forging
• High Strength and Stiffness
• Critical Lug Design
• Height is Limited By Wing
Contours
• Annealed Form
Is Good For
Fatigue And
Damage
Tolerance
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Wing to Body Attachments
• PH13-8Mo Cres Steel Bar
• Critical Lug Design
• High Strength
Requirement
• Good Corrosion
Resistance
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Flap Tracks
• PH13-8Mo Cres Steel Bar
• Geometry Is Very Limited
By Requirement To Be
Internal To The Wing
• Results In Very High
Stress Levels
• High Stiffness Is Required
To Meet Flutter and Flap
Geometry Criteria
• Good Corrosion
Resistance
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