Materials and Processes Engineering

Polytechnique

Supplement to Case Study Course

Daniel Menard

19 October 2004

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Aeropace materials Aluminum, Steel, Titanium,

Stainless steel alloys

Hardware, castings, Al honeycomb, bearings

Kevlar, fiberglass, graphite, plastics

Adhesive films, adhesive pastes, Nomex, tapes

Sealants, fillers, paints, protective films, lacquers

Acids, bases, oxidants, reducers, wiping solvents, alkaline cleaners

Electrical components, flammability, documentation

Hydraulic fluids, cutting fluids, machining lubricants, jet fuel, engine oil

Aerospace Processes Machining, forming, bending,

drilling, riveting, fastening, metal bonding, heat treatment, shot peening, polishing, swaging, NDT techniques, corrosion resistance, fatigue, welding, plumbing, rolling, torquing,

Curing, bonding, assembling, painting, surface preparation, trimming, repairing, protecting, cutting, baking, cold working

Masking, anodizing, plating, conversion coatings, chemical milling, inspection, passivation

electrical bonding, material testing, design, identification, storage

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Introduction Objectives:

• Provide awareness of the main aerospace materials,their manufacturing processes and their subsequent conversion into useful parts

You will learn about:• Factors affecting selection of the correct material for the job

• Techniques used to alter shapes of metals

• Techniques to change material properties

• Techniques for joining materials

• Corrosion protection

• Composite construction

• Flammability compliance

• Short introduction to wing design (M&P point of view)

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Relation of Materials Selection to Design

Selection of the best material for a part involves the identification of the interrelationship between:

DESIGN

Service conditions

Function

Cost

MATERIALS

Properties

Availability

Cost

PROCESSING

Equipment selection

Influence on properties

Cost

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Engineering design is less than 5% of the cost of an airplane but influences more than 80% of the final cost

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Performance Characteristics of Materials

Physical properties• Viscosity, Density

Mechanical properties• Fatigue resistance, Strength

Chemical properties• Corrosion resistance,

Thermal properties• Thermal expansion, fire resistance

Electrical properties• Transmittivity, conductivity

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Performance Characteristics of Materials

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Standards and Specifications

Material & Process properties are usually formalized through standards and specifications• Standards intended to be used by as large a body as possible

- ASTM or ANSI

- SAE

• Aerospace manufacturers such as Bombardier write their own intended for more limited group

- Company specifications

- BAMS: Bombardier Aerospace Material Specification

- BAPS: Bombardier Aerospace Process Specification

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Metals and their processes

Major metal and alloy groups used in aircraft industry:

• Aluminum alloys

• Carbon and alloy steels

• Stainless steels

• Heat-resistant alloys

• Titanium alloys

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Aluminum alloys

Low density (~1/3 density of steel)

Low melting temperature ~1180 °F

Low hardness

Adequate strength, further strengthening achieved through:• Alloying

• Cold working

• Heat treatment

Very good fracture toughness

Very good corrosion resistance

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Aluminum alloys

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Carbon and alloy steels

High density

High melting temperature ~2760 °F

High hardness

High strength and fracture toughness• Strength -toughness combinations achieved through heat treatment

Excellent wear resistance

Poor corrosion resistance

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Performance Characteristics of Materials

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Stainless Steels

High density

High melting point ~2750 °F

High hardness

High strength and fracture toughness• Strength-toughness combinations achieved through heat treatment

Excellent wear resistance

Very good corrosion resistance

More expensive than steels

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Heat-resistant alloys

Reduced density

High melting point ~2500 °F

High static strength• Excellent high-temperature behavior

Very good corrosion resistance• Adequate oxidation resistance at elevated temperatures

• No special surface protection required

Expensive

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Performance Characteristics of Materials

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Titanium alloys

Low density (~1/2 density of steel)

High melting point ~3000 °F

Good strength• Further strengthening achieved through heat treatment

High fatigue strength

High fracture toughness

Very good high-temperature behavior

Excellent corrosion and oxidation resistance

Expensive

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Performance Characteristics of Materials

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Process Selection

The selection of a material must be closely coupled with the selection of a manufacturing process. • The goal in selecting a manufacturing process is to choose one that will

satisfy Engineering requirements and Production capabilities

• This will translate in: Adequate material and process properties, low cost and low cycle time manufacturing, acceptable quality; a good enough part

Main metal manufacturing processes broken down in a few broad classes:• Casting

• Deformation

• Material removal

• Heat treatment

• Joining

• Finishing

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Casting

What is casting?• Metal is molded into the required shape by pouring liquid metal into

an expendable pattern that is surrounded by a refractory slurry coating

Casting: • Can produce large or complex finished shape

- Avoids extensive machining and associated costs

• High dimensional accuracy

• Weight savings

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Deformation processes

Main deformation processing methods used in aircraft industry are:• Bending

• Hydro forming

• Stretch forming

• Forging

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What is bending?• The straining of flat sheet (or strip metal), by moving it around a

straight axis.

• Metal flow takes place within the plastic range of the metal, so that the bent part retains a permanent set.

• Minimum bend radius is a function of metal type, condition, and thickness

• Metal cracks on tensile surface if bend radius smaller than a certain value

• Springback; dimensional change of the formed part after forming

Bending

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BENDING TERMSBENDING TERMSBENDING TERMSBENDING TERMS

SPRINGBACKSPRINGBACKSPRINGBACKSPRINGBACK

Bending

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Hydro Forming

What is hydro forming?• A sheet metal forming operation in which a pliable rubber pad

attached to a ram is forced by hydraulic pressure to become a mating die for a punch on a press bed.

• Developed in the aircraft industry for the limited production of a large number of diversified parts.

• Can readily produce contoured flanged parts

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Hydro Forming

TOOLING AND SETUP FOR HYDRO FORMINGTOOLING AND SETUP FOR HYDRO FORMING

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SINGLY CURVEDSINGLY CURVEDSINGLY CURVEDSINGLY CURVED

SHRINK FLANGESHRINK FLANGESHRINK FLANGESHRINK FLANGE

STRETCH FLANGESTRETCH FLANGESTRETCH FLANGESTRETCH FLANGE

CURVED SECTIONSCURVED SECTIONSCURVED SECTIONSCURVED SECTIONS

TYPICAL HYDRO-FORMED SHAPESTYPICAL HYDRO-FORMED SHAPESTYPICAL HYDRO-FORMED SHAPESTYPICAL HYDRO-FORMED SHAPES

Hydro Forming

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Stretch Forming

What is stretch forming?• The shaping of a metal sheet or part by first applying suitable tension

or stretch and then wrapping it around a die of the desired shape.

• Produces parts with of large radius of curvature

• Springback is reduced because stress gradient is relatively uniform

STRETCH FORMING TECHNIQUESTRETCH FORMING TECHNIQUE

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Forging

What is forging?• Metal is worked into the desired shape by impact via hammering.

• Outstanding grain structures

• Best combination of mechanical properties

29CLOSED-DIE FORGINGCLOSED-DIE FORGING

CLOSED-DIE FORGINGCLOSED-DIE FORGING

Relation of Materials Selection to Manufacturing-Forging

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Metal Removal techniques, Other techniques

Metal removal:• Machining

• Chemical Milling

Other Processes• Heat Treatment

• Joining

• Shot peening (grenaillage)

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Machining

Main mechanical metal removal method used in aircraft industry

What is machining?• Removing material from a metal part, usually using a cutting tool,

and usually using a power-driven machine.

Machining advantages include:• High dimensional tolerance

• Good surface finish

• Complex geometry capabilities

Precautions include:• Economics of machining

- Programming (NC)- Machining labor- Tooling

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Machining

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Chemical milling

Main chemical metal removal method used in aircraft industry

What is chemical milling?• Chemical milling is a metal removal method in which metal is

shaped into intricate shapes by masking certain portions and then etching away unwanted material.

Advantages include:• Allows reduced web thickness’ below practical limit of other

processes- Thickness of 0.010” as opposed to 0.040” for machining

• Can accommodate curved parts

Precautions include:• Reduction in service fatigue performance of parts

• Environment

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Heat Treating

Main heat treatment methods used in aircraft industry • Quenching and tempering of steels

• Age hardening of nonferrous alloys

What is heat treatment?• Heat treatment is defined as a controlled heating and cooling of a

solid metal or alloy by methods designed to obtain specific properties by changing the microstructure

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Heat Treating for steels

Annealing• Softens material and yields lowest mechanical properties

• Changes properties such as machinability by achieving desired microstructure

Normalizing• To refine grain structure subjected to high temperatures during hot work

operations that increase grain size

Quenching and tempering• Achieved by reheating a steel that has been previously hardened

• Used to manipulate properties such that optimum mechanicalproperties are achieved while providing high toughness

Surface hardening• Achieved through both heating and controlled cooling to put a hard,

wear-resistant surface layer on a part

• The area below the surface (core) is softer or tougher by comparison

• Commonly used surface hardening treatments include:- Carburizing, Nitriding, Carbonitriding

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Heat Treating for non-ferrous alloys

Annealing• Achieved by heating to a select temperature & holding for a period

of time. After heating, aluminum is cooled at a rapid rate through quenching

• Purpose is to soften metal to prepare it for a hardening treatment

Precipitation hardening (a.k.a. Aging)• Strengthening of metals by extremely small uniform particles that

precipitate from a supersaturated solid solution

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Joining

Mechanical• Bolting, Riveting

Metallurgical• Welding, Brazing

• Friction Stir Welding

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Mechanical Joining

What is mechanical joining?• Joint formed by inserting a fastener into two or more bodies to hold

them together.

Advantages include:• No dimensional distortion

• No alterations in grain structure or heat treat properties

• Permits the joining of dissimilar materials

• Disassembly of joined components with relative ease

Precautions:• Interrupted stress flow; weaker structure

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Metallurgical Joining

What is metallurgical joining?• Joint formed by heat to produce the coalescence of metals.

Advantages include:• No interrupted stress flow; very strong structure

Precautions:• Inability to weld dissimilar metals

• Alterations in grain structure

• High risk of welding defects (due to melting)- Cracks, porosity, inclusions, shrinkage, segregation, etc.

• Cycle time, cost

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Metallurgical Joining (new technique)

Friction Stir Welding• Joint formed by using the heat created by a tool on the surface of

the two mating surfaces (not reaching the fusion transition).

Advantages include:• Ability to join dissimilar metals

• Allow for manufacturing savings- Smaller details

- Can be automated

Precautions:• Fairly new technology to be proven

• Alterations in grain structure to be determined

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Shot peening (grenaillage) What is shot peening?

• Shot peening is process in which the surface of a part is bombarded with small spherical media called shot.

• Objective is to induce a layer of compressively-stressed material that will improve service properties.

• Shot peen, under specific conditions can also be used for forming operations (such as wing planks)

Advantages include:• Substantial improvement in service performance of parts

- Stress corrosion cracking resistance- Fatigue resistance

Precautions:• Strict control required to avoid excessive work hardening

- Exhausts ductility of the surface material- Leads to micro-crack formation- Reduction in service performance

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Couverture partielleCouverture partielle Couverture complèteCouverture complète

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Corrosion

Aircrafts are designed for high intensity & high cycle life for service over many years

Appropriate corrosion protection is vital

Four elements are required to get corrosion:• Anode

• Cathode

• Contact

• Electrolyte

If in any way we can eliminate one of the elements, we can

prevent corrosion

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Corrosion Cell

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Corrosion Prevention Processes - Inorganic CoatingAnodizing of Aluminum

Electrolytic process which produces an oxide layer at the surface of a metal. It is a controlled corrosion process

Advantages include:• Improve corrosion resistance

• Increased abrasion resistance

• Increased paint adhesion

• Improved adhesive bonding

Precautions:• Anodizing may affect fatigue life of parts

• Expensive

ProcessCorrosion Protection

Coating Thickness

Chromic Acid Anodizing (CAA) Good 0.0001"Sulfuric Acid Anodizing (SAA) Better

0.0003 to 0.0005"

Hard Anodizing Excellent 0.002"

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Corrosion Prevention Processes - Inorganic CoatingChemical conversion coating of aluminum

Reaction of the metal surface with a solution which causes the formation of a protective molecular film• Also known as Alodine or Iridite (commercial products)

Advantages include:• Corrosion protection

• Slightly conductive, does not affect fatigue properties

• Excellent base for painting

• Can be done by immersiion or manual application

Precautions:• No abrasion resistance

• Limited corrosion protection vs Anodizing

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Corrosion Prevention Processes - Inorganic CoatingPlating of steel

The electro deposition of an adherent metallic coating upon an electrode for the purpose of obtaining a surface with properties or dimensions different from those of the basis metal.

Advantages• Improved corrosion resistance over steels

Precautions• Environment: use of cyanides, heavy metals

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Plating

Cadmium:• Applied on Steel to avoid dissimilar contact with aluminum

• 0.0005 inch thick

• used as a sacrificial coating due to it being anodic to steel

Chromium:• 0.002 to 0.004 inch thick

• Used where exceptional wear resistance and low friction properties are required

• Detrimental effect on fatigue properties

Nickel:• Used as an alternate to Cadmium plating, where resistance to over

400 degree F, or abrasion resistance are required.

• Nickel plating is not a sacrificial coating like Cadmium plating

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Corrosion Prevention Processes - Organic CoatingsPaints

Primers• Solvent or waterbased

• Epoxy base, Acrylic base

• Applied to treated surfaces, add extra corrosion film

• High resistance to chemicals, weathering, impact

Top Coat• Mainly Polyurethane

• Appearance, exteriors, cockpit, visible parts

• Extra corrosion barrier, sub floor line

• High resistance to chemicals, weathering, impact

• High Temperature Coatings

• Teflon Filled Coating

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Non metallic materials

Composites

Sealants

Paints (seen in corrosion protection)

Other materials

Flammability

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Composite materials

Definition: A combination of two or more materials, differing in form or composition on a macro-scale. The constituents retain their identity.

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Composite Materials

A composite, as used in the aerospace world, is a non-homogenous mixture of fibers and a matrix, designed to procure oriented strength to a part.

Advantages include:

• High strength and stiffness to weight ratio

• Good fatigue properties

• Fewer details, less assembly required

Precautions:

• Material cost

• Processing cost

• Repair Cost

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Composite Materials

Matrix

• Thermoset resins are used, such as Epoxy (mainly) and Phenolic.

• Cured at 250oF or 350oF. Also can be cured at room temperature for repair.

Fibers

• Graphite

• Kevlar

• Glass

Fiber’s style

• Woven (plain, 5HS, 8HS, etc.)

• Unidirectional Five Harness Satin (5HS)

Plain Weave (PW)

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Composite Materials

Graphite• Most widely used for primary structural applications

• Best balance of properties

Kevlar• Uses limited by low compressive strength

• Light weight

Glass• Lower stiffness, uses in secondary structures

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Fiberglass

Graphite (carbon)

Aramid (Kevlar)

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Composite Processing - Autoclave (Temperature, Pressure and Vacuum)

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Composite materials - Preparation for Curing

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Composite Typical Cure Cycle

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Autoclave Curing…Controlled process?

The Truth…..

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Lightning Protection on Composites

Because of their low electrical conductivity (compared to metals) outside composites parts need to be protected against lightning.

A metal foil, or a wire mesh, is included in the lay-up of the the part to conduct the electrical charge in case the part is hit by a lightning.

If not…..see next slide….

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Lightning Strike Test - Graphite Laminates, not protected with a metallic foil

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Composite Materials - RTM advancementsResin transfer Molding

Process of manufacturing composite parts using matched metal tools• Dry fabric is placed in a mold.

• The mold is closed tightly

• Resin is injected at high pressure in the mold

Advantages include:• Manufacturing cost (less details for same assembly)

• Complex parts can be manufactured rapidly

Precautions:• Initial set-up cost

• Properties vs metallic structures

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RTM Flap

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Fiber Metal Laminates (Glare)

New technology trying to use the best of both worlds

Fuselage panels using composite bonded with traditional aluminum

Used on Airbus 380

Advantages include:• Light weight

• Reasonable structural properties loss

Precautions:• Strength

• Cost (royalties)

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Composite Sandwich Panel Construction

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Types of Honeycomb Used at Bombardier

HEXAGONAL FLEX-CORE

OVEREXPANDED

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Bonded Structure Comparison

Relative stiffness

Relative strength

Relative weight

100

100

100

700

350

103

3700

925

105

t 2t 4t

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Sealants

The main purpose of sealants is to prevent contamination from entering or to prevent fluids from leaking

Polysulfide Sealants: • 95% of the sealants used

• Great fluid resistance

• Used for- Aerodynamic sealing

- Fuel tank sealing

- Pressure and environmental sealing

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Sealant types

Silicone Sealants• Used where a high temperature resistance is required (400 to 500

degrees F).

Firewall Sealants• Silicone sealant specifically formulated to meet the fireproof

requirements (2000 degrees F flame for 15 minutes)

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Sealing Philosophy - 3 levels of sealing

1st level - Environmental Sealing• Faying surface sealing

• Wet installation of fasteners

2nd level - Pressure Sealing• 1st level sealing

• Fillet sealing

3rd level - Fuel Tank Sealing (liquid tight sealing)• 2nd level sealing

• Brush Coat of Fasteners

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Fuel Tank Sealing

WET SIDE

DRY SIDE

OUTSIDE SURFACE

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Other Materials …

Adhesives• Epoxies

• Cyanoacrylates (Crazy Glues)

• Contact cements

• Silicones

Rubbers• Silicones

• Ethylene Propylene (Skydrol resistant)

Oils, Hydraulic fluids, Grease, Jet fuel, Plastics

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Flammability Regulations- United Airlines, McDonnell Douglas MD-90

Rotorblast – Engine disk hit on fuel line

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Flammability - Air France Airbus 340-211

Hydraulic pump fire

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Flammability - Air Canada McDonnell DouglasDC 9-32

Initial Lavatory fire

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Flammability Requirements

Interior flammability to FAR 25.853, FAR 25.855, FAR 25.856

All materials/assemblies within the aircraft passenger, cargo and crew areas must meet some of the following flammability requirements, depending on their location, size and function:

• Vertical burn

• Horizontal burn

• 45/60 degrees burn

• Smoke density

• Heat release

• Burn through

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Flammability - Testing

Cargo Liners-Oil Burner Test Seat Cushions-Oil Burner Test

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Information about Material Properties

A new material can’t be employed in a design unless the engineer has access to reliable material properties and costs.• The need for materials data evolve as a design proceeds from

conceptual to detail design.

Conceptual design• Materials Selector

- Database for range of metals

- ASM Metals Reference Book

Detail design• At this stage, very precise data are required. Data are best found

in:- Data sheets issued by materials producers

- Actual tests performed on the material from which the part will be made

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Costs vs material selection

Prices for materials are complex:• Nature

• Transformation

• Competition

• Transportation

Many extra costs may be added:• Grades and tolerances

• Inspection & testing

• Size of order; cost of inventory vs just in time

• Packing, marking, storage conditions

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CRJ Wing

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Wing Planks

Integrally machined parts

• Integrated stringers design drove material selection

• Al 2000 series discarded because of poor damage tolerance

• Al 7075 T6 discarded because of poor stress corrosion cracking

• Upper planks 7150-T7751 for better Static and Compression

• Lower planks 7475-T7351 for better damage tolerance

Surface treatment• Shot peen (Forming and Saturation)

• Clean after shot peen, organic and iron residues (Acid or alkaline etch)

• Chromic Acid Anodizing

• Epoxy primer and Polyurethance topcoat (Exterior)

• Fuel Tank Coating (Interior)

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Wing Planks

Shot peen forming

• Wing planks are formed to achieve right curvatures

• Avoid machining of thick plates (material and process savings)

• Span, Twist, Chord forming to desired shape

• Manufacturing issues: Dimples, paint surface appearance

Span

Ch

ord

Twist

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Fuel access doors

Composite door (internal)

Aluminum door (external)

Conductivity with rest of structure critical

Avoid dissimilar material (corrosion)

Beware of in-flight flex leading to fretting leading to corrosion

Use of aluminized seal (flexible, conductive, compatible)

Interior: Fuel

Exterior: Aerodynamic and conductive surface

Ext. Door

Int. Door

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Leading edges

Al 6013

Formability, Corrosion resistance – No Clad

Formed

Machines pockets (Chemical milling)

No surface treatment

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Inspection and Testing

To ensure material behavior and condition per specifications, inspection and testing must be performed

Testing can be divided into two categories:• Nondestructive

• Destructive

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Destructive Testing

What is destructive testing?• A test used for determining the constitutive properties of structural

parts in which the test subject is at least partially destroyed

Chemical analysis Tensile testing Failure analysis Metallurgical testing

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Destructive Testing - SEM and Microscopic Surface Examination

Microscopic examination is the study of the surface of metals and alloys by scanning electron microscopy (SEM)

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Failure Analysis

Importance of failure analysis• Associated with economic losses

• May also be associated with human losses

Failure analysis requires:• Understanding of analysis methods

• Knowledge of aircraft components and systems

• Knowledge of failure modes

A successful investigation may result in improvements in:• Design

• Manufacturing

• Inspection procedures

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Modes of Failure

Typical Modes of Failure

• Failure associated with overload

• Failure associated with fatigue

• Failure associated with high temperature

• Environmentally assisted fractures

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Modes of Failure

Failure associated with fatigue

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Modes of Failure

Environmentally assisted fractures

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Destructive Testing - Tensile Testing

Main purpose to determine mechanical properties of material

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Destructive Testing - Light Microscope & Metallographic Examination

Metallographic examination is the study of the structure of metals and alloys by light microscopy using prepared surfaces

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Non Destructive Testing

A test used for determining the quality/characteristics of a material, part, or assembly, without permanently altering the subject or its properties.

VISUAL EXAMINATION PENETRANT TESTING RADIOGRAPHIC TESTING

ULTRASONIC EXAMINATION

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Conclusions

The material and process selection methodology goes through a complex fight between multiple characteristics

The Design engineer must understand very well what are the true objectives and requirements in order to overweigh some of these characteristics

There are never perfect scenarios, at the end, the engineer must compromise between pros and cons

The objective is to have the best design possible with the functionality, acceptable quality, and a short manufacturing cycle time, all at the right cost.

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Thank you - Merci

Questions???