uck 353e aerospace materials-week6-2015

Aerospace Materials

Week-6

Fiber-Polymer Composites

Fabrication and Mechanics

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

Autoclave Autoclave used for consolidation and curing of composites


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


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

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


• 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

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


Resin Transfer Molding


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


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


Vacuum Bagging Resin Infusion


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


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


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


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.


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


Mechanics of Continuous-Fibre Composites


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

14.12.2015 17

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

18

Courtesy of B.L. Wardle Class Notes, MIT 16.223 14.12.2015 UCK 353E-Aersopace Materials-Week6 19

Hierarchy of micromechanics-based analysis for

composite structures


Mechanical Properties of Various Materials Including Composites


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


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


Transverse Modulus, ET


More on Series Model


RoM equations for Unidirectional Composites


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


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


How Well Does Mechanics of Materials,

Micromechanics Approach Work?


Summary


• 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

38 14.12.2015 UCK 353E-Aersopace Materials-Week6

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.

14.12.2015 UCK 353E-Aersopace Materials-Week6

• 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

40 14.12.2015 UCK 353E-Aersopace Materials-Week6

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.


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



Laminated Composites

43

Cross section of a ceramic-intermetallic composite having SiC fibers in a Ti3Al matrix.



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


From Micromechanics to Macromechanics

for Lamina/Ply Level Study in


uck 353e aerospace materials-week6-2015

Documents