Structural Fabrication - RCP Productions, Inc.

Vehicle Processing Readiness Course
Structural Fabrication
Instructed by
Steve Kane
Topics






Material Science
Shaping/Working Metals
Tools
Rivets/Bolts
Blueprint Reading and Layouts
Composites
Material Science

Solid – a substance held together by
electrical interactions between atoms
– Two basic forms – amorphous and crystalline
 Amorphous – formed by sudden cooling from liquid
to solid creating random orientations of atoms

Glass and plastics are examples of amorphous solids
 Crystalline – formed when atoms “link up” in specific
ways and create lattice structures

Metals are examples of crystalline materials
Elastic

When a load is placed on the material it will
deflect a certain amount
–
Materials exhibit a natural “recovery” mechanism
when stressed
 Stiffness plays a role
 Inherent with molecular bonds
–
As long as the molecular bond strength is not
exceeded, it will “snap back” or return to its
original position
 Deformation/return is known as “elastic”
Plastic flow

If molecular bond strength is exceeded
however, the material “gives”
–
Material has reached its “yield point”
 In metals, the crystal lattice shifts/slides until force is
relieved and molecular bonds re-establish
 This is known as “plastic flow”
80
70
60
Elastic region
Force
50
40
Plastic region
30
Yield point
20
10
Strain
0
10
20
Time
30
40
Brittle vs. Ductile

Amorphous and crystalline substances
exhibit one of two basic conditions:
–
Brittle – maintain load to yield point, then
fracture (crack)
 Glass, ceramics, some plastics fail by cracking first
 Little or no warning
–
Ductile – maintain load to yield point, then
“give”
 Silver, gold, aluminum, even steel “give” before total
failure
 Lose strength but maintain a certain amount of
functionality
Fracture Mechanisms

Factors that cause materials to break:
–
–
–
–
Stress – load on a given area
Strain – amount of deformation
Fatigue – cyclic strain (loading/unloading)
Splitting – forces “cleave” the inter-molecular
bonds

–
Applies to laminar materials (wood, composites)
Cracking – path of least resistance


Originates at a fatigue point
Follows grain boundaries
Stress

Internal resistance of a material to deform
when subjected to an external force
– In normal state, molecular bonds are in their
“happy” place
 Everything’s as it should be
 Molecular bonds “resist” being moved
– Can also be thought of as load on a given area
 How much force is applied
– Not to be confused with strength
 How much load a material can take before failure
Strain

Amount of deformation per unit length
which results from stress
–
Cyclic strain
(vibration,
pressurization
cycles, etc.) is a
major factor in
fatigue failure
Aloha Airlines April 1988 http://www.as.wm.edu/nondestructive.html
Fatigue

Occurs in metal and composites
– More prevalent in metals
– Occurs as a result of:
 Harmonic vibrations (propellers/rotors)
 Pressurization cycles (expansion/contraction)
 Corrosion (corrosion action leaves jagged “pit”)
– Breaking points:
 Fractures – generally occur at the weakest point
 Known as a “stress riser”


Corners of corrosion pits
At flaws such as scratches, gouges if metals allowed to
“oil can” (cold working)
Splitting

Forces exceed the strength of intermolecular bonds
– Generally applies to laminar materials
(wood, composites)
– Causes a “tearing away” or “cohesive”
failure
– Results from excessive transverse or
compressive loads
Force
Force
Cracking

Common in most materials
– More prevalent in metals
– Originates at a fatigue point




Fatigue/cold-working, i.e. cyclic loading/unloading
Nucleus is a flaw of some sort
Forces overcome molecular bonds
As bonds fail, neighboring bonds can no longer
support load and fail successively
 In metals, follows crystal lattice (grain) boundaries
 Reason cracks appear jagged
Fatigue flaw,
corrosion pit,
scratch, etc.
Crack
V
Grain
Boundaries
Crystals
(Grains)
Stiffness

Modulus of elasticity (Young’s Modulus)
– Stress/strain = E
– How much a material deforms from a given stress
Boron
Graphite
Kevlar
E-Glass
S-Glass
Steel
Aluminum
Titanium
0
25
50
75
100
Specific Stiffness (GPa)
125
150
175
Stiffness

Modulus of some common materials
Material
Rubber
Plastic soda bottle
Young’s Modulus (E)
1
200
Spruce wood
2,000
Concrete
2,500
Bamboo fibers
4,000
Bone
6,000
Beer glass
10,000
Diamond
170,000
Loads

Aerospace structures are designed to
compensate for the maximum anticipated
stress (load) and strain (deformation)
– Structures function to transfer (spread) loads
to adjacent areas
– Amount of load a structure can transfer is a
function of
 Material stiffness
 Material strength
Loads


Not all loads are created equal
Structural design must account for forces
from more than one direction
– Bending
 Tension load
– Shear
 Cuts like a knife
– Compression
 Crushing failure
– Buckling
 All the above
Aluminum Alloys
Modern aerospace structures are constructed from
many different metal alloys

– Aluminum
– Titanium
– Stainless steel
– Carbon steel
Military and aerospace use titanium and other highend metals

Most commercial aerospace applications favor
aluminum alloys

Aluminum Alloys
Aluminum alloys are formed when pure
aluminum is mixed with other substances

– Percentage of other substances will determine
material properties

Aluminum alloys come in two forms
– Wrought – grain structure is compressed (sheet,
rod, extrusions)
– Cast – coarse grain structure (wheels, engine
casings)
Aluminum Alloys
Aluminum alloy characteristics make them a good
choice :

– Light
– Easily formed
– Available in a multitude of product shapes
Are susceptible to corrosion when alloyed with other
materials however

– Can be protected:
 Cladding – adding a thin layer of pure aluminum to each side of the
alloy (pure aluminum is corrosion resistant)
 Conversion coatings and/or paint – anodizing, chem-films such as
Alodine, and catalyzed primers/paints (Koropon, enamels, epoxies)
Alloy Codes
Alloy Designation System
Major Alloying Element
Alloy Number
Pure Aluminum (99.00%)
Copper
Manganese
Silicon
Magnesium
Magnesium and Silicon
Zinc
10XX i.e. 1090
20XX i.e. 2024
30XX i.e. 3003
40XX i.e. 4047
50XX i.e. 5052
60XX i.e. 6061
70XX i.e. 7075
Aluminum Alloys

Wrought aluminum is divided into two categories:
– Non-heat treatable
– Heat treated
Non-heat treatable alloys are made with alloying
agents that do not respond to heat treatment

– Alloys like 1100 (pure aluminum), 3003 (aluminummanganese), 5052 (aluminum-magnesium) are non-heat
treatable
– Must be strain hardened
 Also known as cold-working
 Forces grains closer together (rolling, bending, forging)
Aluminum Alloys

Heat treated aluminum alloys are commonly
used as structural materials
– Four stages to heat treatment
 Heating – furnace or solution
 Quenching – rapid cooling
 Aging – holding at high temperature until hardened
 Cold working – rolling material to further compress grain structure
Note: If heat treatment is improperly performed, it can
cause Intergranular corrosion
Heat Treatment Paths
As Fabricated
Unstable
F Solution heat treated W
Stabilized
Naturally aged
Peak Strength
T4 Artificially aged T6
Overaged
Stabilized
T7
& cold water quenched
Cold worked
& naturally aged
Cold worked &
artificially aged
T3
T8
Annealed
O
Stabilized
Stabilized
Artificially aged
Peak Strength
Blueprints
and Drawings
• Working drawings, 3 types:
• Detail drawing – single part
• Assembly drawing – two or more parts assembled
together
• Installation drawing – all necessary information for a part
or an assembly to be installed
http://www.airweb.faa.gov
Blueprints
and Drawings
 Title
–
–
–
–
–
–
–
Block:
Drawing number
Name of the part
The scale
The date
Name of the firm
Name of the draftsman
Name of the person
who checked the
drawing
– Name of who submitted
the drawing
– Name of the person
who approved the
drawing
Blueprints
and Drawings
Orthographic Projection
 Shows
exact size, shape
 Six views
Blueprints
and Drawings
Tolerance
 When
a given dimension on a print allows
variation, the plus (+) figure indicates the
maximum and the minus (–) figure indicates the
minimum variation allowed.
 Called the “the extreme permissible dimensions”
of the hole or part.
 Example: Using 0.225” + 0.0025” - 0.0005”
 The hole could be as large as 0.2275” or as small
as 0.2250”
Basic Structures
Beam Theory:


Beams transfer loads between different locations
Make up a large proportion of the devices in
ordinary life
In aerospace
structures (such
as a wing), the
primary beams are
ribs and spars
Basic Structures

Structures incorporate designs to transfer
both length-wise and span-wise loads:
–
–
–
–
Spars – transfer longitudinal (length-wise) loads
Ribs – transfer shear, chord-wise (across width)
and compression (crushing) loads
Stringers – transfer secondary length-wise and
width-wise loads between ribs and spars
Skins – provide bracing (rigidity) between other
members, function to accommodate both
longitudinal and chord-wise loads
Basic Structures

Structural members are mechanically
fastened at seams, called “joints
–
Joints are “overlapped” to provide fastening area
 Known as “lap” joints
–
If members can not overlap they must be butted
together and joined with a third or “faying” edge
(3-way lap joint)
Lap joint
Fayed edge
joint
Basic Structures


Metal parts are sized by shearing
and the fastening areas for joints
are formed by bending
Material is first cut to size in a
shear
–

Sizes and squares material
Bends are then formed with a
metal “brake”
–
–
Most common is “box and pan”
brake which contains removable
segments
Forms the fastening area for lap
joints
Metal
Shear
Box and
Pan Brake
Basic Structures

Conical structures are formed with a slip roller
–
–

Three rollers connected by gears
Turned with a hand crank
Slip rollers can be individually adjusted
–
–
Used to make cylinders or ducts
Lap joints/toggle joints are formed at each end
Slip Roller
http://www.americanmachinetools.com/bending_rolls.htm
Basic Structures
 Files
are used to smooth
edges and remove burrs
– Double-cut, single cut
– Flat, mill, round (rattail),
triangular, half round, and
vixen
Most common are the flat,
mill, rattail, and vixen
Clean all files with a file card
File in forward direction only
(unless draw-filing)
Keep file well oiled
Double vs. single cut
File Card
Vixen File
Forming

Important to consider what is occurring as
metal is bent:
– Material on outside
of bend stretches
– Material on inside of
bend compresses
– Area unaffected by
either strain is “neutral
axis”
Compresses
Stretches
Setback
 Length
of the unbent
portion
– From edge of finished part
to position where brake
jaws secure material
– Factor in bend allowance
– Computed by adding bend
radius and material
thickness and applying the
“K-factor” (Setback = K *
(BR + MT))
 K-factor is computed by
dividing any angle by two and
finding the tangent equivalent
Bend Allowance

Amount the metal will stretch when formed
around the brake “nose”
– Must be subtracted from leg length
– Three variables:
 Radius of bend
 Thickness of metal
 No. of degrees of bend
http://www.sheetmetaldesign.com
Drilling
 Pneumatic
drill motors preferable
– More durable
– Variable speed
– Electrical extension cords may be
accidentally cut
 Battery-powered
–
–
–
–
offer convenience
Less expensive
Rechargeable
No pneumatic hoses to move around
Portable
Drilling
 When
drilling metals:
– Locate/position hole using center punch
– Drill speed is important
 Slow speed/high pressure/cutting fluid for hard
materials (high carbon, nickel, or stainless steels)
 Do not overheat tip!
– For precision holes (high strength applications):
 Drill pilot hole first (square to material) with one size
smaller drill bit (0.0003” to 0.007” undersize)
 Ream up to size
 Rotate reamer in cutting direction only!
 Never “back” reamer when removing
Drill Bits
 Twist
Drills
– Three main parts:
 Tip – looks like hourglass when viewed from point
 Body – composed of flutes and webbing
 Flutes – valleys between spiral webbing
 Aid in cooling the tip
 Provide path for cutting fluid to reach the tip
 Aid in chip removal
 Webbing – structure between flutes
 Shank – portion clamped in drill motor
– In aerospace, numbered drill sizes are used and
are slightly larger than the fasteners they
correspond to (#40, #30, #20, #11)
Note: Drill bits to ½ inch have straight shank, over
½ inch, tapered shank
Drill Bits
 Drill
bits to use for rivet installations
– See “Drill Sizes”
 #40
 #30
 #20
 #11
drill bit for 3/32” rivet
drill bit for 1/8” rivet
drill bit for 5/32” rivet
drill bit for 3/16” rivet
Example: #30 drill bit measures 0.1285” corresponds
to 1/8” rivet (0.125”)
Lightening Holes
 Used
to reduce weight in aerospace
fabrication
– Formed with hole saws or chassis punches
If hole saw is used, deburring and burnishing is
necessary
– Must be flanged to return strength and prevent oil
canning
Rib
Flanged
Lightening holes
Construction
 Deburring
Tool
– Removes disturbed edges
 Microstop
– Used when countersinking
– Precision depth adjustment
 Clecos
and Cleco Pliers
– Used as “clamps”
– Color coded for standard
drill/rivet sizes
Blind Holes
 Hole
Finder
– Used to ensure proper hole alignment when
drilling “blind” fastener holes
– Minimizes oversize or elongated holes
Dimpling
 Thin
skins (< 0.032”) cannot be machine
countersunk
– Must be dimpled
– Two methods
 Cold – less than .040”
 Hot – thicker sections
– Hand rivet squeezer
 Removable dies
 Rivet sets
 Dimpling anvils
Countersinking
 Hold
countersink tool
at right angle to work
– Do not tip
– Use of drill press may
be preferred
– Microstop allows
precision adjustment of
depth
Aerospace Hardware
 Rivets
– Manufactured head – produced during initial
fabrication
– Shop head – driven (bucked) at unformed end to
one-half (1½) times the shank diameter.
– Some rivets are ready for use as received and can
be driven immediately.
 Called “field” rivets, fairly corrosion resistant (aluminum
2117-T)
– Some must be refrigerated until use and driven
within an hour of removal from refrigeration.
 Known as “icebox” rivets, (Aluminum 2017-T; 2024-T)
Solid Shank Rivets
 Types
of solid shank rivets
– Universal (AN470 or MS20470)
– Round
– Flathead
– Countersunk (AN426 or MS20426)
– Brazier
Solid Shank Rivets

Head marked to identify specific material:
Rivet Measurement

Rivet Measurement:
– Diameter sized in 32nds of an inch
– Length sized in 16ths of an inch

Rivet length is different for universal (round head)
and countersunk rivets



Round head (MS20470) length is measured from underside
of head to tip of shank
Countersunk (MS20426) is measured from top of
countersunk head to tip of shank
For example:
– MS20426AD-4-8 countersunk rivet:
½”
 -4 is 4/32 (1/8) inch
 -8 is 8/16 (1/2) inch
MS20426
MS20470
Rivet Installation

Lay out rivet pattern:
– Edge distance is minimum of 2x rivet diameter
 Measured from edge of material to center of rivet or rivet center
to rivet center
– Spacing (pitch) is minimum 3x rivet diameter

Use a center punch to mark hole, then drill
– Use clecos to clamp material as each hole is completed
– Countersink if required
 Flat rivet set is used for installing countersunk rivets
 Rivet must be flush with material being riveted
– Deburr all holes and remove chips
– Length of un-driven rivet protruding through material is 1-½
shank diameter
– Driven (shop) end is 1-½ shank diameter when properly
installed
– Drive rivets in as few blows as possible to minimize coldworking
Rivet Removal
 Center
punch rivet to indent head
 Drill center of rivet with drill bit one size
smaller than rivet shank diameter
 Drill to the base of the rivet head only!
 Use a pin punch with a rolling motion to pop
off the rivet head
 Knock out the rivet shank with the pin punch
 Ensures
hole maintains same dimensions
 Do not drill into rivet shank!
Special Fasteners

Solid Shank Rivets are impractical beyond a
certain size
– Most made from aluminum
– Lose characteristics in larger diameters

Hi-Shear Rivets
– Built to withstand high shear loads
– Two styles
Conventional
Blind
Special Fasteners
 Conventional
high shear
– Two main parts
 Steel stud - same strength characteristics of steel bolts
 Crushable collar –usually 2117T4 aluminum
– Two head styles:
 Countersunk
 Flat
– Set with hand rivet squeezer or pneumatic rivet
gun with specially designed set
 Huck
 Cherry
Special Fasteners
 Cherry-lock
and Cherry-max rivets
– Blind rivet used in both high shear structural and
general use applications
– available in universal (protruding) and
countersunk head styles
Special Fasteners
 Hi-lok
Fasteners
– Two-piece assembly consisting of a threaded pin
and an aluminum collar (nut) with a break-away
wrenching element
 Torque limiting feature
Screws and Bolts
 Screws
–
–
–
–
Usually have lower material strength than bolts
Looser thread fit
Head shapes are formed to engage a driver
Shank may be threaded along its entire length
without a clearly defined grip
– Available as:
 Structural screws – fabricated from a material with a
high-tensile strength
 Machine screws - general purpose screws for nonstructural applications.

May contain drilled heads for installation of safety wire
when used in light mechanical applications
 Self-tapping screws - tap their own mating thread when
driven into untapped or punched holes slightly smaller
than the diameter of the screw
Sizing
 For
screws under ¼”; part numbers indicate
fastener diameter by a number designator
followed by a dash and the number of threads
per inch
– Sized in 0.013” increments beginning at 0.060” for a
#0 screw (i.e., #0, 2, 4, 6, 8, 10, etc.)
– For example, if a screw has a part number “10-32”
 “10” denotes the screw diameter as 0.190 inch
 “32” indicates the screw has 32 threads per inch
Sizing

For ¼” (0.250”) and over, screws are identified similar
to bolts, ie., a basic part number, which identifies
–
–
–
–
–
Screw type
Diameter
Head
Thread count
Length
Example: NAS604-6 =
.2500-28 UNJC-3A,
machine screw,
aircraft, pan head,
cruciform recess,
full-threaded, alloy
steel, .380 inch long
Screws and Bolts

Bolts
– Either general-purpose (AN) or close-tolerance (NAS or MS)
– Aerospace bolts are identified by code markings on the bolt
heads
 AN standard steel bolts are marked with either a raised cross
or asterisk, corrosion resistant steel is marked by a single
dash, and AN aluminum-alloy bolts are marked with two raised
dashes
+
*
AN Steel
AN Steel
_
Stainless Steel
- Aluminum
– Certain aerospace manufacturers produce bolts of different
dimensions or greater strength than standard types.
 Such bolts are made for a particular application, and it is of
extreme importance to use like bolts in replacement!
Sizing

Bolt construction normally consists of a hex-shaped
head, an unthreaded shank portion known as the
“grip”, and the shank portion containing threads
Sizing
 Bolt
dimensions are also obtained from the
fastener part number
– For example, if a bolt has a part number “AN3-5A”
 AN denotes the bolt as built to Air Force – Navy
specifications
 The number “3” indicates bolt diameter in 16ths” (3/16”)
 A “dash” indicates the bolt is a standard cadmium-plated
steel


“DD” would indicate an aluminum fastener (AN3DD5A)
“C” would indicate corrosion-resistant (stainless) steel
(AN3C5A)
 The number “5” indicates bolt length in 1/8ths, (5/8”)
from underside of head to shank tip
 The “A” denotes an undrilled shank


If there were no designator, (i.e., AN3-5) a drilled shank is
indicated
An “H” indicates a drilled head (AN3-5H)
Screws and Bolts
 Thread
protrusion
– Three threads engagement in a nut or tapped hole
is the accepted value for full bolt strength
 Three threads may or may not be enough and in general
practice, the engagement should at least equal the
diameter of the bolt
– In order to assure proper fastener strength, the
threads of all fasteners should be engaged as
required by the applicable specifications and
drawings
Heli-Coils
 Damaged
threads can be re-threaded with a
replacement thread insert called a heli-coil
– Heli-coils return threads to same dimensions as
original
 Hole with damaged threads is drilled over size
 A tap is used to install threads for heli-coil
 The heli-coil insert is installed with a special driving tool
until slightly below surface, then the drive tang is
“snapped” off
 When properly installed, the insert will remain slightly
below the surface
Torque

It is very important each member in a structure carry
no more or no less load than the others
– In order to distribute loads properly, torque is applied to bolts, screws
and nuts
– The formula for applying torque to fasteners is:
F * L = T Where F = Force applied
L = Length of lever (between centerline of drive and
centerline of applied force)
T = Torque applied
Example: Required torque is 100 in/lbs; lever length is 12”;
force necessary:
F * 12 = 100
F = 100/12 in.
F = 8.3333 lbs
Torque

If an extension is used to reach a fastener not
accessible to the drive centerline, the torque setting on
the wrench must be recalculated:
Tw = Te * A
B
Where A = Lever length (wrench)
B = Lever length + extension
Te = Required torque
Tw = Torque reading on wrench
Example: Required torque is 100 in/lbs; lever length is 12”;
extension length is 4”; torque wrench setting:
Tw = (100*12)/16 = 1200/16 = 75 in/lbs
Safety Wiring

Safety wire is installed to prevent bolts and screws
from moving in a loosening direction
– Safety wire counteracts any loosening tendency through a
tightening of the wire
– Safety wire can be installed either by hand or by use of
specially-designed safety wire pliers
– It is important the wire is not kinked or stretched during
installation
FAA Advisory Circular AC65-9A, 1976; Internet; faa.gov
Metal Repairs


When performing metal repairs, extent of
damage must be determined before initiating
Application is also important:

Low-speed applications or internal repairs – lapped
joints, surface patches, universal head rivets


High-speed applications (aerodynamic considerations)
– flush patches, countersunk rivets


Less complicated
More complicated
Bottom line:


Repair must be as small as possible
Necessary bearing and shear strength must be obtained
Metal Repairs

Formulas exist for determining the number of
fasteners to use:
–
To find number of rivets to use:
 NR = (L * RPI * %) * 2
NR = number of rivets
L = length of damage (break)
RPI = rivets per inch (found on a chart or table)
% is normally 75 (75 % of rivets required for a lap joint)
Access cover
Lap patch
X
X
X
X
Flush patch
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Inspection and Testing

How do we know a material is strong
enough?
–
–
–

We have intuition through life experiences
We “know” whether a material is strong through
its appearance
We wouldn’t cross a stream on a rotten log, we’d
look for something more “substantial
In general practice, material strength is
determined through testing
Inspection and Testing

Destructive vs. Non-destructive Testing
–
Destructive – part is destroyed during testing
to yield ultimate strength
 Information is applicable to all materials similarly
constructed
–
Non-destructive – material integrity is
maintained
 Information is derived using physical properties of
test/mediums to determine characteristics
Inspection and Testing

Destructive
–
Coupon testing
 Information derived from a representative sample
 Lap shears
 Tensile testing
–
Simulation
 Test fixtures, conditions re-created to simulate or
test environmental factors/conditions/material
configurations
 Material analysis
 Accident/incident reconstruction
Destructive Testing
Space Shuttle RCC Panel Testing, Southwest Research
Institute (SwRI), San Antonio, TX May, 2003
Before Simulated Foam Strike:
1.7 lb. Bipod Ramp
After Simulated Foam Strike:
Inspection and Testing

Non-destructive Testing
–
Three categories
 Visual – examination of a material through visual
observation
 Limited to accessible areas only

Visual and Remote Visual (mirror, borescope)
 Surface – examination of near-surface and
conditions open to the surface
 Limited to less than approx. ¼” depth

Penetrant, Eddy Current
 Volumetric – examination of conditions deep within a
material or volume
 Areas inaccessible to visual or surface methods

Ultrasonic, Radiographic
Nondestructive Testing


Six traditional (classic) methods:
–
–
–
–
–
Visual Testing – use of light as a probing medium
Penetrant Testing – use of a liquid/high contrast
Magnetic Particle Testing – use of magnetic lines of force
Eddy Current Testing – use of an induced current
Ultrasonic Testing – sound waves
–
Radiographic Testing – differential absorption of
penetrating radiation
Sub-methods (derivatives of classic methods)
–
–
–
Acoustic Emission – use of induced strain to elicit noise
Infrared/Thermography – use of heating/cooling
Shearography – use of strain and different wavelengths of
light
Composite
Characteristics
What Are Composites?
A combination of two or more materials
working together

– Each contributes its own structural properties
– Each retains its unique identity

Two major components (two-phase material):
– Plastic matrix - tough but flexible
– Ceramic or mineral fibers- strong but brittle
Two-Phase Material
 Fiber
reinforcement is strong component and
provides tensile properties
 Matrix is pliable and provides compressive
and shear strength properties
+
 Put
=
them together and the result is a failureresistant, tough material!!
Composites Forms
DEFINITION:
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Advanced Composites
• Advanced Composites
– Higher strength-to-weight ratios than ordinary
fiberglass
– More expensive than ordinary fiberglass
– Properties are usually precisely tailored to
achieve a specific objective
Fiber Types
• Three types of composite fibers comprise the
bulk of the composite industry :
– Fiberglass – used where economy is a
– consideration
– Carbon Fiber – used where strength and stiffness
– are primary design considerations
– Kevlar – used where impact resistance is an
– important consideration
– Hybrids – combinations of the three major types for
specific applications
Glass Fiber

Two main types:
– E-glass – used primarily for secondary structures
 500,000 psi tensile strength, 10,500 kpsi modulus of
elasticity
– S-glass – used in aviation/aerospace as primary
structure
 665,000 psi, tensile strength, 12,400 kpsi modulus of
elasticity
 Roughly 30% stronger than E-glass, triple the cost
– Variations:
 S2 glass – somewhat better properties
 Hollex – hollow, approx. same properties as E-glass, 30%
less weight
Carbon Fiber

Generally lighter than fiberglass
– At one time, prohibitively priced material
– Strength – 470,000 psi tensile strength, 34,000 kpsi
modulus of elasticity
 Higher compressive strength than glass
 Stiffer than glass with lower impact resistance
– Nasty handling characteristics:
 Breakage of structure exposes hundreds of needle ends
 Breathing dust from sawing or sanding must be avoided
– Three main types
 Type III – Moderate strength, used for tooling and carbon brakes
 Type II – High strength, used in various structures and
components
 Type I – High modulus, stiff, used in spacecraft, Space Shuttle
Aramid (Kevlar®)

Highest strength-to-weight ratio in tension of
any commercially available fiber
– 430,000 psi tensile strength, 19,000 kpsi modulus
of elasticity
– 43% lighter than glass; 20% lighter than carbon
– Outstanding in tension, not good in compression
– Three formulations:
 Kevlar 49 – used for structures
 Kevlar 29 – used as protective lamination for
protection against puncture
 Kevlar 129 – used for body armor
Polymer Fibers

Polyethylene (SPECTRA®)
– High tensile strength
– Lightweight
– Used underwater, in ballistics, and body armor

Advantages:
– Low moisture absorption
– Excellent chemical resistance

Disadvantages:
– Limited temperature range (< 220º F)
Mineral Fibers

Boron
– Very stiff and strong
– Used primarily in aerospace, sporting goods
– Seeing some application in metal repairs

Advantages:
– No galvanic corrosion potential exists
– Thermal expansion similar to steels

Disadvantages:
– Cannot be woven, filaments snap when bent
– Matrix does not bond well with mineral fibers
– Only available in prepreg unidirectional tape
– Single source: Textron Specialty Products, Lowell,
MA
Mineral Fibers

Quartz
– High temperature applications
– Low dielectric properties
– Similar to fiberglass
– Used in Space Shuttle thermal insulation

Ceramic
– Ultra high temperature applications
– Used in Space Shuttle thermal insulation
Fiber Design

All filaments in a strand are taken from the
same batch at the same time
– A single bundle of parallel untwisted strands is
called a “roving”
Also known as a “tow”
–Rovings are grouped together and twisted with
other strands to make yarn
 Delivered to weavers who manufacture cloth
– Rovings are used to manufacture stitch-bonded
fabrics
Tows normally used in unidirectional products
– Roving size is expressed in yield (linear yards per
pound)
Textile
Terminology
 Warp
Direction
– Parallel to the “Long” direction of the roll
– Also defined as parallel to the “selvage” edge

Fill Direction
– 90 degrees to the Warp direction
– Selvage edge to selvage edge
– Sometimes called “Weft” direction

Selvage Edge
– Tightly woven edge to prevent edge raveling
– Parallel to warp threads

Bias
– A 45 degree angle to the warp threads
– Fabric can be stretched along the bias but seldom along warp
Textile Terminology
Weave Terminology

Warp clock - Fiber orientation figure
– Very important to consider fiber
direction
– A component in all composite drawings
 Counterclockwise – plies viewed from
manufacturing standpoint (inside looking
out)
 Clockwise – plies viewed from repair
standpoint (outside looking in)
-45º
90º
0º
+45º
90º
Fiber Orientation

Advanced Composites are by design
– Orientation of fibers is proportional to properties
 The more fibers in a given direction, the stronger and
stiffer
Control of fiber angles to +/- 2 degrees
 Unidirectional (0) degrees for tension, compression, or
bending
 Woven fabrics eliminate crimps (stacked)
 Biax (+/- 45’s) degrees for shear
Triax (0, +45, -45) for tensile strength and shear
Fabric Weaves

Common weave styles:
– Plain – yarns interlaced in alternating fashion
– Basket (twill) – double strands interlaced two by two;
traditional composite “look”
– Crowfoot (4HS) – over three yarns, under one for pliability
– Five-Harness Satin (5HS) – over four yarns, under one
– Eight-Harness Satin (8HS) – over seven yarns, under one

Symmetrical vs. unsymmetrical
– Symmetrical weave patterns have an equal number of yarns
in both warp and fill directions
» Equal strength in both directions
– Unsymmetrical weaves have different numbers of yarns in
warp and fill directions, different sizes or different materials
» Strength not equal in both directions
Weave Patterns
Weave Patterns
Matrix Systems
Matrix
The “glue” that holds structural fibers
together

– Transfers loads between fibers
– Structural properties:
 Compression
 Shear
 Resistance to delamination
• Two categories:
– Thermosetting – becomes rigid once cured
– Thermoplastic – becomes pliable with the
application or reapplication of heat
Matrix
Types
of thermosetting resins:
– Epoxy – Elevated temperature curing
– Polyester – Low cost, room temperature curing
– Vinyl ester – Low-cost, low viscosity, room
temperature curing
Generally, curing process generates heat
“exothermic reaction” which may cause
carbonizing (burning) or shrink-cracking if
left uncontrolled
– Mixing quantities in containers insufficient to

dissipate heat generated
Matrix

Thermosetting resins are two-part systems
– Resin
– Hardener
Proper curing depends on the proper ratio of
resin to hardener:

– For epoxy matrix systems (most popular), 5 parts
resin to 1 part hardener is optimum



Too little resin – enhances exothermic reaction;
brittleness; insufficient strength
Too much resin – excess weight; insufficient strength
Optimum ratio: 60% fiber; 40% resin to achieve
proper (design) strength
Matrix
Methods for obtaining proper matrix resin-tohardener ratio:

– Scale
 Zero scale
 Weigh mixing container
 Re-zero scale
 Add one part hardener to five parts resin by weight
– Volume
 Use graduated mixing container
 Add one part hardener to five parts resin by volume
– Ratio pump
 Simplest method
 Pump chambers designed to deliver proper amounts
 One full pump resin, one full pump hardener
Matrix

Epoxy Resins:
– Two-part system – polymerized during manufacture
 Resin and hardener cross-link or form chemical bonds
when mixed together
– Advantages:
 Good environmental resistance
 Wide variety of formulations available for many
applications (wet resin, prepreg, adhesive)
 Matrix of choice for aerospace applications
– Disadvantages:
 More expensive than polyester or vinyl ester
 Emits toxic smoke when burned
Matrix

Polyester
– Polymerization prevented during manufacture by
addition of “inhibitors”
 Cured by addition of a catalyst
 Promotors and accelerators can also be added to speed
cure
– Advantages:
 Good environmental resistance
 Inexpensive
– Disadvantages:
 High styrene emissions
 High shrinkage during cure
 Emits toxic smoke when burned
Matrix

Vinyl Ester
– Hybrid of epoxy and polyester
 Polymerization also inhibited during manufacture
 Cured by addition of a catalyst
 Promotors and accelerators can also be added to speed
cure
– Advantages:
 Better environmental resistance than polyester
 More strength and fiber adhesion than polyester
 Less shrinkage during cure than polyester
– Disadvantages:
 High styrene emissions
 Dangerous to use - Risk of explosion if improperly mixed
 Emits toxic smoke when burned
Matrix

Exotic resins:
– BMI – Bismaleimide
 Used in high performance, high temperature applications
 Expensive
– Cyanate ester
 High temperature, corrosion resistance
 Expensive
– Polyimide
 Used in thermoplastics
Sandwich Structures
Sandwich Structures
 Combination
of strong, thin skins, relatively
light “core” material
– Produces very efficient structures with high
stiffness to weight ratios

Core chief purpose:
– Passes shear forces between the skin surfaces
– Allows substantially improved structural
properties in thicker sections with only slight
increase in weight
Sandwich Structures
 Disadvantages
of sandwich construction:
– Sandwich structures have thin skins that can be
easily damaged
 Susceptible
to moisture intrusion:
– Can cause unintentional weight gain
– Freezing if subjected to lower temperatures of
higher altitudes may cause disbonds
– If core becomes contaminated with oil, fuel or
hydraulic fluid, it is almost impossible to remove
completely
Sandwich Structures
 Core
Materials:
– Hex core
 Common hexagonal shape
 Suitable for flat panels
 Difficult to bend
– Common variations:
 O-X core – over-expanded
 Facilitates curving
 Flex-core
 Compound curvature
 Exceptional formability
Sandwich Structures
 Core
Materials (cont):
– Foam
 Higher densities than honeycombs
 Greater crush resistance
 Do not bond as well to skins
– Common varieties:
 Polyurethane
Moderate mechanical properties
Structural applications limited to formers to
create frames or stringers for stiffening
components
Widely used for thermal insulation
Sandwich Structures
 Core Materials (cont):
– Polystyrene
 Used extensively in sail and surfboard manufacture
 Light weight (40kg/m3 ), low cost, easy to sand
characteristics
 Low mechanical properties
 Cannot be used with polyester resin systems,
dissolved by the styrene present in the resin
– Polyvinyl chloride (PVC)
 Most common core material for high performance
sandwich structures
 Referred to simply as „PVC foams‟
Sandwich Structures
 Core Materials (cont):
– Wood
 Described as „nature‟s honeycomb‟
 On microscopic scale, similar to the cellular hexagonal
structure of synthetic honeycomb
– Types
 Plywood
Not widely used - heavy
 Balsa
Excellent material for sandwich structures
Popular for marine structures
» Can have moisture problems
Used less frequently in aviation due to flammability
Sandwich Structures
 Core
Materials (cont):
– Wood (cont)
 Wood cores are susceptible to moisture attack
 Will rot if not well surrounded by laminate or resin
– Syntactic core
 Composite in a sense
 Microspheres mixed with thermosetting resin
 Strength and density can be controlled
 Can be made heat resistant by application of heat
resistant resins
 Can be easily formed into complex shapes
Shaping Cores

Hot Wire Cutting
– Easiest method for closed-cell foams
– Care must be taken with smoke

Band saw
– Can be used to rough shape
– Applicable to all types of cores (honeycomb, foam,
wood)
– Miter/tilt table can be used for angled cuts

Razor knife
– Can be used with straight edge for thin materials
(foam board, wood, nomex honeycomb)
– Trim work
Shaping Cores
 Sanding:
– Some foam core materials like polyurethane
and wood are easy to sand and shape
 Can be sanded with belt sanders, hand sanders
 Polyurethane foams can even be sanded with
another piece of like foam
Safety Considerations

When working with composites, Personal Protective
Equipment (PPE) must be worn:
– Airborne particulates are generated whenever
composite materials are cut or sanded
 Sanding residue is an irritant and uncured matrix
systems are suspected carcinogens
 Use of shop coats, gloves, dust masks and eye
protection is mandatory!
 Imperative to keep particulate matter out of airways, eyes,
pores of skin
– Uncured matrix materials cannot be mixed/diluted
with water!
 Must be kept out of drains, sewers
 Let stand until cured, then can be discarded as normal
trash
Manufacturing Processes

Manual Wet Layup:
– Oldest, simplest composite technique
 Usually requires use of a “tool” or mold
 Requires release agent
 Surface can be wet out/sprayed with resin or a “gel” coat
 Gel coat allowed to partially cure before addition of fabric
 Fabric can be applied dry and saturated with resin:
 Excess resin is removed by hand-working
 Sprayed with chop gun (chops and mixes continuous
fibers with pre-metered resin)
Manufacturing Processes

Imperfections in surface can be addressed
with fillers made from chopped materials
mixed with matrix
– Two forms:
Structural
Cellulose fiber (flox) – strength comparable to foam
 Milled glass – stronger than flox, heavier; nasty handling
characteristics
Non-structural
Micro (glass) balloons – very lightweight filler, easily sanded
– Imperfections can be located with a “smudge”
stick
 Made from chalk and a straight edge
 Rubbed across surface to identify any high or low spots
Manufacturing Processes

Manual Wet Layup:
Source: http://www.rockymountaincomposites.com
Manufacturing Processes
 Vacuum
Bagging
– Purpose:
 Apply down force for
ply consolidation and
force laminate to
contour to tool
surface
 Also extracts excess
resin, air and volatiles
 Down force equals
29.92 in. Hg or 14.7
psi at sea level
 Loses one half pound
per 1,000 ft elevation
Courtesy : Gary Eisenberg; Antelope Valley Community College;
Lancaster , CA
Manufacturing Processes
 Vacuum Bagging Materials and
Equipment
– Bagging film
 Can be any plastic film
 Preferably thermoplastic (Nylon,
polyethylene, etc.)
– Release film
 Can be bagging film
 Teflon coated fabric
 Superior durability, can be reused
 “Perf” - release film with holes to allow
air/resin to flow out
 Gives bondable finish to part
 Minimizes resin bridging, wrinkles
removed after cure leaving matte surface
ready for paint or bond
Courtesy : Gary Eisenberg;
Antelope Valley Community
College; Lancaster , CA
Manufacturing Processes

Vacuum Bagging Materials and Equipment, cont.)
– Breather
 Spun polyester mat or batting
 Allows even air/resin removal
– Bleeder
 Absorbs excess resin
 Avoids clogging breather
 Any cheap woven goods
Can even be peel ply
– Peel Ply
 Usually Dacron (polyester cloth)
 Can be unfinished glass cloth
 Absorbs excess resin,
contaminates
Courtesy : Gary Eisenberg; Antelope Valley
Community College; Lancaster , CA
Cures
Curing
 Room
Temperature Cure
– Most common
– Used with wet-layup techniques
 Materials are assembled, fabric is wet-out with resin,
allowed to dry in ambient conditions
 Typically 4-6 hours to initial gel for epoxies
 Times can be adjusted for polyesters/vinyl esters
 24-48 hours for final cure
– Cannot be used with prepregs
Curing
 Typical
heated press
– Pressure, elevated temperature cure
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Curing
 Oven with vacuum hook up
– Elevated temperature cure
– Vacuum source for down force
– Can be used for prepregs
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Curing
 Typical
Autoclave
– Elevated temperature cure
– Can provide several atmospheres of down force
to compact thick sections
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Prepregs
Prepregs
 Disadvantages
with wet lay-up can be
overcome with Prepregs
 Prepreg fiber is pre-impregnated with resin
(hence the name)
–
–
–
–
–
Any type fiber can be used
Amount of resin can be controlled
Must be frozen below zero
Usually only good for a year
Oven-cure between 250 to 350 degrees F.
Prepregs
 Advantages:
–
–
–
–
Little mess or smell
Lower exposure to health risks
Resin/fiber ratio controlled
Many high performance matrix systems
available
– Easier to cut to shape than dry materials
Prepregs
 Disadvantages:
–
–
–
–
–
–
–
–
More expensive
Short shelf life (12 months)
Frozen storage required
Proper thawing critical to quality of finished
part
Requires vacuum bagging
Thicker laminates require autoclave cure
Sensitive to dirt and oil contamination
Sensitive to moisture contamination
Prepregs
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Prepregs
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Prepregs
• Prepreg Machine
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Prepregs
 Prepreg
forms:
Unidirectional
Fabric
Courtesy : Gary Eisenberg; Antelope Valley Community College; Lancaster , CA
Curing Prepregs
 Elevated
Temperature Cure
– Virtually all prepregs require elevated
temperatures to allow the matrix to cure
– Most common ways to raise temperature:
 Oven
Virtually any enclosure that can be heated
 Heated Press
Good for high production rates on flat components,
common in production settings
 Autoclave
Pressurized oven, expensive, used for thicker
sections (more than eight layers) to compress
material
Acknowledgements:
• Maria Clinton and Gary Eisenberg, Instructors, Aerospace Technology; Antelope
Valley Community College, Lancaster, CA
• Aircraft Sheet Metal; Nick Bonacci, Jeppson Sanderson Training Products, 1987
• Understanding Aircraft Composite Construction; Basics of Materials and
Techniques for the Non-Engineer; Zeke Smith; Aeronaut Press; Napa, CA; 1996
• Advisory Circular AC43.13-1b, Chg 1; Acceptable Methods, Techniques, and
Practices - Aircraft Inspection and Repair; Federal Aviation Administration; 9-27-2001
• Advisory Circular AC65-9a, Airframe and Powerplant Mechanics General Handbook;
Federal Aviation Administration; 1-1-1978