adv composites

8/10/2019 Adv Composites

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Advanced Materials for Design and TechnologyAdvanced Materials for Design and Technology

Advanced Composite MaterialsAdvanced Composite Materials

Content:Content:

Introduction;

Linear Elastic Stress-strainCharacteristics;

Stress-strain Relations for PlaneStress;

Plane-stress Stress-strain Relations ina Global Coordinate System;

Failure theories;

Materials Degradation;

Manufacturing Process andApplications.


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Assessment criteriaAssessment criteria

Course works

Group discussions 10 %

In-class assignments 10 %

Home works 10 %

Group design projects 30 %

Written examination 40%

Total 100 %


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IntroductionIntroduction

Definition of Composite Materials

In its most basic form a composite material is one, which is

composed of at least two elements working together to produce

material properties that are different to the properties of thoseelements on their own. In practice, most composites consist of a bulk

material (the matrix), and a reinforcement of some kind, added

primarily to increase the strength and stiffness of the matrix. This

reinforcement is usually in fibre form. Today, the most commonman-made composites can be divided into three main groups:


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Polymer Matrix Composites (PMCs) These are the most

common and will be discussed here. Also known as FRP -

Fibre Reinforced Polymers (or Plastics) these materials use

a polymer-based resin as the matrix, and a variety of fibres

such as glass, carbon and aramid as the reinforcement.

Metal Matrix Composites (MMCs) - Increasingly found in

the automotive industry, these materials use a metal such as

aluminium as the matrix, and reinforce it with fibres such as

silicon carbide.

Ceramic Matrix Composites (CMCs) - Used in very high

temperature environments, these materials use a ceramic as

the matrix and reinforce it with short fibres, or whiskers such

as those made from silicon carbide and boron nitride.


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Most commonly, composite materials have a bulk phase, which is

continuous, called the matrix, and one dispersed, non-continuous,

phase called the reinforcement, which is usually harder and stronger.

The concept of composite materials is ancient: to combine different

materials to produce a new material with performance unattainable

by the individual constituents. The essence of the concept of

composites is this: the bulk phase accepts the load over a large

surface area, and transfers it to the reinforcement, which being

stiffer, increases the strength of the composite. The significance

here lies in that there are numerous matrix materials and as many

fiber types, which can be combined in countless ways to produce

just the desired properties.

Polymer Matrix CompositesPolymer Matrix Composites


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Illustrating the combined effect on Modulus of the addition of fibres to

a resin matrix.

FRP Fibre-reinforcedpolymer/plastic


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Overall, the properties of the composite aredetermined by:

The properties of the fibre The properties of the resin

The ratio of fibre to resin in the composite (Fibre Volume

Fraction (FVF), Vf)

The geometry and orientation of the fibres in the composite


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Components of Composite Materials

Bulk phase: matrix materialsBulk phase: matrix materials

Polymers

Metals

Ceramics

Reinforcement: fibers andReinforcement: fibers andparticulateparticulate

Glass

Carbon

Organic Boron

Ceramic

Metallic

InterfaceInterface


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The reinforcement system in a composite material strongly

determines the strengthening mechanism in a composite. It is thusconvenient to classify composites according to the characteristics of

the reinforcement, such as length, orientation etc.

Classification of Composite Materials


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A fibre reinforced plastic (FRP) has been recognised as one of

the most innovative materials in the applications ranging from

the aerospace industry to prevalent sport goods and facilities for

more than 30 years. It has been demonstrated that the FRP

could be successfully used to replace conventional materials for

most primary structural elements in modern aircraft with safe

and durability.

Fiber types: Carbon, E-glass and

Kevlar (Aramid)

Resin types: Polyester, Vinyl ester,

Phenolic and Epoxy


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Fabric types:


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Weft-knitted glass

fabrics


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FibresFibres

Reinforcements are not necessarily in the form of long fibers. They

can be particles, whiskers, discontinuous fibers, sheets etc. A greatmajority of materials is stronger and stiffer in the fibrous form thanin any other form. This explains the emphasis on using fibers incomposite materials design.

Fibers used in advanced composites have very high strength andstiffness but low density. They also should be very flexible (toallow a variety of methods for processing) and have high aspectratio (length/diameter), that allows a large fraction of the applied to

be transferred via the matrix to the fiber.

Fibers are added to a ductile matrix (like polymers and metals)usually to make it stiffer, while fibers are added to a brittle matrix

(like ceramics) to increase toughness.


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MatrixMatrix

Polymers used as matrix materials are commonly referred to as

resins. The matrix resin generally account for 30 to 40%, byvolume, of a composite material. In addition to maintaining the

shape of the composite structure, aligning the reinforcements

(fibres), and acting as a stress transfer medium, the matrix protects

the fibre from abrasion and corrosion. More importantly, thelimitation of a composite may well be a function of matrix

properties, such as thermal stability, chemical inertness, load

transferability, moisture absorbability, mouldability and curing

temperature.


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ThermosetsThermosets

Thermosetting resins are the most common type of matrix system

for composite materials. They have become popular for a numberof reasons, including low melt viscosity, good fibre impregnation,

and fairly low processing temperatures. They are also lower in cost

compared to thermoplastic resins.

Polyester

Vinyl ester

Epoxy Polyimide

50 110

100 150

50 250280 320

40 90

127

100 20070 120

1.2 4.5

3 4

2 63.1 4.9

Resin type Tg Tensile strength (MPa) E (GPa)


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Resin SystemsResin Systems

Any resin system for use in a composite material will require

the following properties:

1. Good mechanical properties

2. Good adhesive properties

3. Good toughness properties

4. Good resistance to environmental degradation


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The figure below shows the stress / strain curve for an 'ideal'

resin system. The curve for this resin shows high ultimate

strength, high stiffness (indicated by the initial gradient) and a

high strain to failure. This means that the resin is initially stiffbut at the same time will not suffer from brittle failure.


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Advanced Materials for Design and TechnologyAdvanced Materials for Design and TechnologyIt should also be noted that when a composite is loaded intension, for the full mechanical properties of the fibre

component to be achieved, the resin must be able to deform to

at least the same extent as the fibre. The figure below gives

the strain to failure for E-glass, S-glass, aramid and high-

strength grade carbon fibres on their own (i.e. not in a

composite form). Here it can be seen that, for example, the S-

glass fibre, with an elongation to break of 5.3%, will require a

resin with an elongation to break of at least this value to

achieve maximum tensile properties.


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InterfaceInterface

1. More often than not, the interface between fiber and matrix is rather

rough, instead of ideal planar.2. To obtain desirable properties in a composite, the applied load should

be effectively transferred from the matrix to the fibers via the

interface. This means that the interface must be large and exhibit

strong adhesion between fibers and matrix. Failure at the interface(called debonding) may or may not be desirable. This will be

explained later in fracture propagation modes.

3. Interfacial strength is measured by simple tests that induce adhesive

failure between the fibers and the matrix. The most common is the

Three-point bend test or ILSS (interlaminar shear stress test)


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Typical Internlaminar (Short Beam) Shear Test Set-up


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A good bonding interface Debond at the interface


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Mechanical PropertiesMechanical Properties


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Mechanical PropertiesMechanical Properties


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