UCK 353E Aerospace Materials-Week6-2015
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Transcript of UCK 353E Aerospace Materials-Week6-2015
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Aerospace Materials
Week-6
Fiber-Polymer Composites
Fabrication and Mechanics
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Hand Lay-Up • Epoxy is applied manually onto the surface of the each fabric
• Fabric is laid-up by hand directly onto the tool
• Fabric plies are oriented with their fibres aligned in the main loading directions
• Vacuum bagging is applied to remove air from between the ply layers
• Most of the hand lay-up composites are cured inside an autoclave with high pressure and temperature
14.12.2015 UCK 353E-Aersopace Materials-Week6 2
• Most common pattern is [0/+45/-45/90]
( quasi-isotropic)
• Orientation of the plies is symmetric
around the mid-plane to ensure the
material is balanced
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Autoclave Autoclave used for consolidation and curing of composites
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Automated Tape Lay-up • Automated tape lay-up (ATL) is an automated process used to lay-up
prepreg tape in the fabrication of composite aircraft structures
• ATL is used instead of manual lay-up to reduce the time (by more than 70–
85%) and cost spent in the lay-up of prepreg tape on the tool
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The key component of
the ATL process is a
computer numerically
controlled tape-laying
head that deposits
prepreg onto the tool
at a fast rate
with great accuracy
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Automated Fibre Placement • Automated fibre placement (AFP) is the lay-up of individual prepreg tows
onto a mandrel using a numerically controlled fibre-placement machine
• The AFP process is used to fabricate large circumference and highly
contoured structures such as fuselage barrels, ducts, cowls, nozzle cones,
spars and pressure tanks
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• The placement head is
computer controlled via a
gantry system suspended
above the mandrel
• During fibre placement,
the mandrel is rotated so
the prepreg is wound into
the shape of the
component
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Resin Transfer Molding • RTM is a closed-mould process
• Fabric is placed inside the cavity between two matched moulds with their
inner surfaces having the shape of the final component
• Fabric plies are stacked to the required orientation and thickness inside the
mould, which is then sealed and clamped
• Liquid resin is injected into the mould by means of a pump
• The resin flows through the open spaces of the fabric until the mould is
completely filled. The resin viscosity must be low enough for easy flow
through the tiny gaps between the fibres and tows of the fabric
• After injection, the mould is heated in order to gel and cure the polymer
matrix to form a solid composite part
• After curing, the mould is opened and the part removed for edge trimming
and final finishing
• The RTM process can produce composites with high fibre volume content
(up to 65%), making them suitable for primary aircraft structures that require
high stiffness, strength and fatigue performance
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Resin Transfer Molding
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Vacuum-assisted Resin Transfer Molding
(VARTM) • Since it can be difficult to completely infuse some types of fabric with resin
which leaves dry spots or voids in the cured composite, a vacuum pump is
used to extract air from the cavity placing it in a state of low pressure
• With VARTM the resin is drawn into the mould under the pressure differential
created by the vacuum
• Resin percolates between the fibres and tows of the fabric until the mould is
filled
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Vacuum Bagging Resin Infusion • Vacuum bagging is an open mould process
• Using an open mould rather than the two-piece closed mould reduce the
tooling cost
• Fabric plies are stacked on the tool with the top layer being a resin
distribution fabric
• The fabric stack is enclosed and sealed within a flexible plastic bag that is
connected at one point to a liquid resin source and at another point to a
vacuum-pump system
• Air is removed by the vacuum pump which causes the bag to squeeze the
fabric layers to the shape of the mould surface
• Liquid resin flows into the bag under the pressure differential created by the
vacuum pump
• Resin is drawn through the tightly consolidated fabric as well as along the
top resin distribution fabric
• After the fabric is completely infused with resin, the material is cured at high
temperature within an oven
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Vacuum Bagging Resin Infusion
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Resin Film Infusion (RFI) • The process uses an open mould upon which layers of dry fabric and solid resin film
are stacked
• The film is a B-stage cured resin similar to the cure condition of the resin matrix in prepreg
• Film is placed at the bottom, top or between the layers of fabric
• The materials are sealed within a vacuum bag and then air is removed using a vacuum pump
• The entire assembly is placed into an autoclave and subjected to pressure and heat
• The temperature is increased to reduce the resin viscosity to a level when it is fluid enough to flow into the fabric layers under the applied pressure
• Once the infusion is complete the pressure and temperature are raised to consolidate and fully cure the component
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Filament Winding
• Filament winding is a
manufacturing process where
cylindrical components are
made by winding continuous
fibre tows over a rotating or
stationary mandrel
• Two types of filament winding:
- Wet winding
- Prepreg winding
• The filament-winding process is
used to produce cylindrical
composite components
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Pultrusion • Pultrusion is an automated,
continuous process used to manufacture composite components with constant cross-section profiles
• Continuous fibres (tows) are pulled off storage spools and drawn through a liquid resin bath.
• The resin-impregnated fibres exit the bath and are pulled through a series of wipers that remove excess polymer.
• After this, the fibre–resin bundles pass through a collimator before entering a heated die which has the shape of the final component
• As the material passes through the die it is formed to shape while the resin is cured
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Comparison of Fabrication Methods Method Reinforcement Matrix Advantage Disadvatage Application
Spray Lay Up
Chopped Fibers Polyester Low cost Heavy Light loaded structural panels
Hand Lay Up
Woven or knitted
All Easy Tooling Quality limited
Prouction boats
Vacuum Bagging
Variety Epoxy, phenolics Low void ct/ lay-up
Better quality
Cruising boats
Pultrusion Variety Variety Good structural properties
Cost may be high
Chemical Storage tanks
RTM Variety
epoxy, polyester, vinylester and phenolic
Fast, economic, controllable
Size limited Bridges, frameworks
VA-RTM Variety Variety Good quality
Expensive Airfraft components
Autoclave Woven clothes, prepregs
Epoxy, polyester, vinylester…etc.
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Machining of Composites
• The majority of processes used to manufacture composites
produce components to the near-net shape
• Most machining operations for composites simply involve
trimming to remove excess material from the edges and hole
drilling for fasteners. Trimming can be performed using high-
speed saws and routers, although care is required to avoid
edge splitting (delamination damage)
• Water jet cutting is a process involving the use of a high-
pressure stream of water containing hard, tiny particles that cut
through the material by erosion
• Hole drilling of composites requires the use of specialist drill
bits. Drilling must be performed using a sharp bit at the correct
force and feed-rate otherwise the material surrounding the hole
is damaged
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Mechanics of Continuous-Fibre Composites
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UCK 353E-Aersopace Materials-Week6
Properties are Determined by
Three Factors:
1.The materials used as component
phases in the composite
2.The geometric shapes of the
constituents and resulting structure of
the composite system
3.The manner in which the phases
interact with one another
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Two Approaches to Mechanical Properties
of Composites
Micromechanical Approach
• Micromechanics models
composites where matrix
and fibers modeled
seperately
– Average composite
properties achieved
Macromechanical Approach
• Deals with laminate
– Will put several plies to
evaluate stresses and
strains within plies
From micromechanics to Macromechanics 14.12.2015
UCK 353E-Aersopace Materials-Week6
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Courtesy of B.L. Wardle Class Notes, MIT 16.223 14.12.2015 UCK 353E-Aersopace Materials-Week6 19
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Hierarchy of micromechanics-based analysis for
composite structures
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Courtesy of B.L. Wardle Class Notes, MIT 16.223 14.12.2015 UCK 353E-Aersopace Materials-Week6 21
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Courtesy of B.L. Wardle Class Notes, MIT 16.223 14.12.2015 UCK 353E-Aersopace Materials-Week6 22
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Mechanical Properties of Various Materials Including Composites
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Assumptions
• Composite ply is:
– Macroscopically
homogeneous
– Orthotropic
– Linear elastic
– Free of stresses
• Fibers are:
– Macroscopically
homogeneous
– Orthotropic
– Linear elastic
– Regularly spaced and
aligned
• Matrix is: – Macroscopically
homogeneous
– Generally isotropic
– Linear elastic
• And fibers and matrix are perfectly bonded
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Determination of Mechanical Properties of
Composites • Overall, the properties of the
composite are determined by:
– The properties of the fibre
– The properties of the resin
– The ratio of fibre to resin in the
composite (Fibre Volume Fraction)
– The geometry and orientation of the
fibres in the composite
Rule of Mixtures (RoM)
Vf + Vm = 1
Vf = Volume fraction fibre
Vm = Volume fraction matrix
EL = Longitidunal Modulus
ET = Transverse Modulus
EL=EfVf + EmVm
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Courtesy of B.L. Wardle Class Notes, MIT 16.223 14.12.2015 UCK 353E-Aersopace Materials-Week6 28
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Transverse Modulus, ET
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More on Series Model
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More on Series Model
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RoM equations for Unidirectional Composites
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Example-1
• Use the values of a given fiber from Hexcel company to
calculate Elc, and discuss the measured and predicted values of
this fiber through RoM
Courtesy of B.L. Wardle Class Notes, MIT 16.223
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Example-2
• Use the values of a given fiber from Hexcel company to
calculate ETc, and discuss the measured and predicted
values of this fiber through RoM
Courtesy of B.L. Wardle Class Notes, MIT 16.223
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How Well Does Mechanics of Materials,
Micromechanics Approach Work?
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Summary
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• Critical fiber length (lC) for effective stiffening & strengthening:
• Ex: For fiberglass, a fiber length > 15 mm is needed since this length
provides a “Continuous fiber” based on usual glass fiber properties
• Why? Longer fibers carry stress more efficiently!
Shorter, thicker fiber:
c
f d
t
s< 15length fiber
Longer, thinner fiber:
Poorer fiber efficiency
c
f d
t
s> 15length fiber
Better fiber efficiency
s (x) s (x)
Critical Points in Fiber Reinforcement
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Influence of Fiber Orientation
39
Schematic stress strain curves for brittle fiber and ductile matrix materials.
Fracture stresses and strains for both materials are noted.
Schematic stress–strain curve for an aligned fiber-reinforced composite
that is exposed to a uniaxial stress applied in the direction of alignment.
(a) continuous and aligned,
(b) discontinous and aligned,
and
(c) discontinuous and randomly
oriented fiber reinforced
composites.
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• Estimate of Ec and TS for discontinuous fibers:
-- valid when
-- Elastic modulus in fiber direction:
-- TS in fiber direction:
efficiency factor: -- aligned 1D: K = 1 (aligned )
-- aligned 1D: K = 0 (aligned )
-- random 2D: K = 3/8 (2D isotropy)
-- random 3D: K = 1/5 (3D isotropy)
(aligned 1D)
From: H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.
c
f d
t
s> 15length fiber
(TS)c = (TS)mVm + (TS)fVf
Ec = EmVm + KEfVf
Composite Strength
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Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, Third Edition, by Norman E. Dowling. ISBN 0-13-186312-6.
Fibrous Composites
41
Fracture surface showing broken fibers for a composite of
Nicalon-type SiC fibers in a calcium aluminosilicate(CAS) glass-ceramic matrix.
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Laminate - Lamina
• Basic building block of laminate is a lamina
• A laminate is a various directions bonded laminae
Unbonded various direction laminates
Major purpose of lamination: To tailor diretional dependence of strength and
stiffness of a composite material for structural element
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Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, Third Edition, by Norman E. Dowling. ISBN 0-13-186312-6.
Laminated Composites
43
Cross section of a ceramic-intermetallic composite having SiC fibers in a Ti3Al matrix.
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Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, Third Edition, by Norman E. Dowling. ISBN 0-13-186312-6.
Fabricating Laminates: Structural Composites
44
Sheets having various fiber directions Laminate w/ Al sheets bonded to
sheets of composite w/ Kevlar fibers in an
epoxy matrix
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From Micromechanics to Macromechanics
for Lamina/Ply Level Study in
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