Steve Goddard

Composite Materials – Assignment 1Identifying Composite Materials

Contents

Introduction………………………………………………………………………..2

1.1 Significance of main technical terminology………………………………..3

1.2 Classification systems employed …………………………………………. 5

1.3 Requirements of matrix materials…………………………………………. 7

1.4 Examples of reinforcement materials, their properties,

Manufacturing methods & forms of supply…………………………………….8

1.5 Development of structural composites and their applications………….10

Bibliography……………………………………………………………………...12

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Steve Goddard

Introduction

Composites make up a huge variety of the materials we use in engineering today and basic composite materials have been around for thousands of years.

In this report I am going to provide a technical review on the current status of modern composite materials. This will include terminology used and the significance of this language, a description of the types of classification systems employed, requirements of matrix materials, examples of reinforcement materials with details on their properties, manufacturing methods and forms of supply and the development of structural composites and some of their possible applications.

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Steve Goddard

1.1 Significance of Main Technical Terminology

There are many terms used to describe the process and application of manufacturing and making composite components. This section of the report is going to list and explain the main terminology commonly used.

Additive An ingredient mixed into resin to improve properties (e.g., plasticizers, initiators, light stabilizers and flame retardants).

Adhesive Substance applied to mating surfaces to bond them together by surface attachment.

Amorphous Describes polymers with no crystalline component.

Anisotropic Not isotropic. Exhibiting different properties when tested along axes in different directions within the material.

Aramid Aromatic polyamide fibres. (Often referred to as Kevlar, DuPont's trademark.)

Autoclave Closed vessel for applying fluid pressure, with or without heat, to an enclosed object.

Bag moulding Moulding technique in which the composite structure is placed in a rigid mould and covered with a flexible impermeable layer of film whose edges are sealed, followed by consolidation and/or curing with pressure applied by vacuum, autoclave, press or inflation of the bag.

Catalyst Substance that promotes or controls curing of a compound without being consumed in the reaction.

Cure Irreversible alteration of the molecular structure and physical properties of a thermosetting resin by chemical reaction, typically stimulated by heat and/or the presence of catalysts, with or without applied pressure.

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Drape The ability of fabric or prepreg to conform to a contoured surface

Epoxy A thermosetting polymer containing one or more epoxide or oxirane groups, curable by reaction with amines or alcohols; used as a resin matrix in reinforced plastic products and as the primary component in certain structural adhesives. Cured epoxy resin is highly resistant to chemicals and water and its performance properties are relatively unaffected by extreme temperatures.

Filler A solid constituent, usually inert, added to a matrix to modify a composite's properties (e.g., increase viscosity, improve appearance or de-crease density) or to decrease part material cost.

Gel coat An unreinforced, clear or pigmented coating resin applied to the surface of a mould or part to provide a smooth, more impervious finish on the part exterior.

Hand layup A fabrication method in which reinforcement layers, preimpregnated or coated afterwards, are placed and arranged in a mould manually.

Honeycomb A lightweight cellular structure (typically hexagonal nested cells) used as core in composite sandwich structures. May be made from either metallic (e.g., aluminum) or nonmetallic (e.g., resin-impregnated paper or woven fabric) sheet materials. Rectangular sheets are adhesively bonded together in stacks, by means of parallel stripes of adhesive placed at regular intervals along one axis. Stacks are sliced across the transverse axis, and each sliced stack is expanded to form a honeycomb grid.

Impregnate To saturate the voids and interstices of a reinforcement with resin.

Isotropic Fibre directionality with uniform properties in all directions, independent of the direction of applied load.

Lay-up To place or the process of placing layers of reinforcing material into position in or on a mold; also used to refer to the reinforcing materials as placed in the mold ("the layup").

Mold An enclosed cavity or open form from which a composite component takes its shape, size and exterior surface appearance (also known as a tool).

Ply A single layer (or lamina) used to fabricate a laminate. Also, the number of single yarns twisted together to form a plied yarn.

Prepreg - Fibrous reinforcement (sheet, tape, tow, fabric or mat) preimpregnated with resin and capable of storage for later use. For thermosetting matrices the resin is usually partially cured or otherwise brought to a controlled viscosity, called B-stage. Additives (e.g., catalysts, inhibitors and flame retardants) are used to obtain specific end-use properties and/or improve processing, storage and handling characteristics.

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Release agent An specially formulated material placed between the mold and uncured resin/fiber (usually sprayed or painted on the mold surface) to prevent permanent bonding between the two during cure and facilitates demolding after cure.

Resin A solid or pseudo-solid polymeric material, often of high molecular weight, which exhibits a tendency to flow when subjected to stress, usually has a softening or melting range, and usually fractures conchoidally. As composite matrices, resins bind together reinforcement fibers and work with them to produce specified performance properties.

Vacuum-bag molding Molding technique wherein a part layed up on an open mold is cured under a layer of sealed film from which entrapped air has been removed by vacuum. The technique more effectively consolidates the laminate and reduces void content, compared to conventional open molding.

Voids A void is a small fault with the composite component; the most common are the micro voids. Larger voids in composites reduce strength and could propagate under service loads producing failures.

Viscosity Describes the tendency of a material to resist flow. Viscosity is measured in comparison with water, and computed in centipoise (cps). The higher the number, the greater the resistance to flow.

Weave To interlace fibers in a pattern, often based on a 0°/90° grid; the fabric pattern formed by interlacing yarns. Interlacing patterns vary. In plain weave, for instance, warp and fill fibers alternate to make both fabric faces identical. A satin weave pattern is produced by a warp tow over several fill tows and under one fill tow (e.g., eight-harness satin features one warp tow over seven fill tows and under the eighth).See below for some examples.

Images from Blue Road research.

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Steve Goddard

1.2 Classification Systems Employed

This diagram shows the structure of composite classification:

These are brief descriptions of the main categories:-

Particle Reinforced Materials

These are materials which have spheres of the reinforcing material dispersed throughout the matrix and can account for any desired volume of the composite. Cermets or cemented carbides are examples of particle reinforced composites in which hard ceramic particles are dispersed within a metal matrix. An example of this is cemented carbide cutting tools eg. Tungsten carbide in a cobalt mix. By varying the amount of cobalt and the size of the tungsten particles the properties, particularly the hardness and toughness of the resulting composite can be controlled.Many polymers incorporate particular fillers such as glass beads, silica flour and rubber particles. Polystyrene is toughened by incorporating particles in the matrix to produce high impact polystyrene. Foams are a particular composite in which gas bubbles are bound by the matrix material. The foam characteristics are governed by the density of the foam to that of the unfoamed matrix and the cellular structure of the foam. The foam can be open cell, closed cell or a mixture of the two. The gas bubbles in closed cell foams are not interconnected; Foams are used for furniture cushions, energy absorbent packaging, thermal insulation and the filler material for structural panels, these having skins covering the foam core.

Fiber Reinforced materials

The main functions of the fibers in a composite are to carry most of the applied loads to provide stiffness which means that they should have a high tensile strength and a high elastic modulus. The fibers used may be continuous, running the full length of the composite or discontinuous i.e. In short lengths. They can be aligned so that they are in the same direction or randomly orientated depending upon the directional properties required of the composite. Some commonly used fiber reinforcing materials are alumina, silicon carbide (Nicalon), boron, carbon, E-Glass, polyethylene (Spectra 1000) and polyamide (Aramid/Kevlar 49).

Structural

Structural composites such as laminates are composites in which materials are sandwiched together to give a stronger laminated structure, plywood being an example. High performance composite components consist of layers or laminae stacked in a predetermined arrangement. A unidirectional lamina is often referred to as a ply and a stack of laminate is called a laminate.

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Other ways of classifying composite materials

Another type of grouping for composite materials is:

Polymer Matrix Composites (PMC’s)

These are the most common form of composite. Also know as FRP as mentioned above Fibre Reinforced Polymers.

Metal Matrix Composites (MMC’s)

Increasingly found in the automotive industry, these materials use a metal such as aluminum as the matrix, and reinforce it with fibres, or particles, such as silicon carbide.

Ceramic Matrix Composites (CMC’s)

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.

1.3 Requirements of Matrix Materials

The matrix is the non-orientated material in which

the fibres of a composite are imbedded. The matrix of

fibrous composites may be metal, polymer or ceramic.

In general, metals and polymers are used as matrix

materials because some ductility is desirable. With

ceramic-matrix composites, the re-enforcing

component is added to improve fracture toughness.

Metal and polymer matrixes are more common so I will

focus on them.

The matrix has several purposes; it binds the fibres together and acts as the medium by

which externally applied stress is transmitted and distributed to the fibres. The matrix should

therefore be ductile. Another important requirement for a composite material is that the fibre

should have a higher elastic modulus than the matrix. The second function of the matrix is to

protect the individual fibres from surface damage as a result of mechanical abrasion or

chemical reactions with the environment. Such interactions may introduce surface flaws which

can lead to the formation of cracks that can lead to failure at low tensile stress levels. Finally

the matrix separates the fibres and prevents the propagation of brittle cracks from fibre to

fibre (due to its relative softness and plasticity) which can cause catastrophic failure. Basically

the matrix acts as a barrier to crack propagation, even though some of the individual fibres

fail, total composite fracture will not occur until large number of adjacent fibres, once having

failed, form a cluster of critical size.

It is essential that adhesive bonding between fibre and matrix be high to minimize fibre pull-

out. Bonding strength is an important factor in the selection of a matrix-fibre combination. The

ultimate strength of the composite depends, to a large degree, on the magnitude of this bond;

adequate bonding is essential to maximize the stress transmittance from the weak matrix to

the strong fibres.

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Briefly the main functions and requirements of the matrix of a composite material are to:-

Protect the fibres in the structure before and during fabrication

Distribute and transfer loads

Keeps the fibres in position In the structure

Carry interlamina shear

Control the thermal and chemical properties of the composite

1.4 Examples of reinforcement materials, their properties, manufacturing methods and forms of supply

Glass Fibre

Glass fibres can be expected to have the following properties: -

High strength-to-weight ratio (good strength as the weight increases) Good dimensional stability (dimensions remain stabile when process is complete) Good resistance to heat (ability to resist increases/decreases in temperature) Good resistance to moisture (ability to resist moisture attack) Good resistance to corrosion (ability to resist environmental attack) Good electrical insulation properties (ability to not conduct electricity)

Glass Fibre Reinforcements are manufactured by drawing monofilaments of glass from furnace. Within the Furnace is molten glass, drawing this will produce strands of glass fibre, these are collected to forms yarns or rovings. These rovings are either produced in continuous or woven forms, thus fashioning woven rovings. The continuous strands or chopped strands are also made in to reinforcing mats held together by a resinous binder.

Carbon Fibre

Carbon Fibres can be expected to have the following properties: -

Very High strength Lightweight High stiffness (high modulus of elasticity)

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Picture 1 – S-Glass fibre woven cloth, ready for wet lay up.Source: http://www.chinesemol.com/

Steve Goddard

Carbon Fibre is produced using a 3 stage process. And involves the use of heat, including the stretching and oxidisation of the fibres resulting in high modulus, high strength carbon fibres. Each carbon filament thread is a bundle of many thousand carbon filaments. A common method for making carbon filaments is the oxidation and thermal pyrolysis of polyacrylonitrile (PAN). The PAN is heated to 300 degrees C in air which breaks many of the hydrogen bonds and oxidises the material, it is then placed into a furnace and heated to approximately 2000 degrees c which induces graphitization of the material which changes it molecular bond structure.

Carbon FibreSource: https://www.ragingspeed.co.uk/

Kevlar

Kevlar Fibres can be expected to have the following properties: -

Very High strength Lightweight High stiffness (high modulus of elasticity) Damage resistance Resistance to fatigue and stress rupture

Aramid or Kevlar (as it is also known as) is used in the defence industry. Kevlar is available in many different ways for example pre-preg and tape form it is also available for use in wet and dry lay ups. It is made using a solvent spinning process.Kevlar 49 is supported in a matrix and used for aircraft structures, boat hulls and bicycles.Kevlar 29 is unsupported and used in brake linings and armour etc.Kevlar is used in tires and rubber goods

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Chemical formula of Carbon Fibres.Source: http://www.elmhurst.edu

Steve Goddard

1.5 Development of structural composites and their applications

Composite materials had been known in various forms throughout the history of mankind, the history of modern composites probably began in 1937 when salesmen from theOwens Corning Fiberglass Company began to sell fiberglass to interested parties around theUnited States. Fiberglass had been made, almost by accident in 1930, when an engineer became intrigued by a fiber that was formed during the process of applying lettering to a glass milk bottle.The initial products for this finely drawn molten glass were as insulation (glass wool) but structural products soon followed.It was then realized that the aircraft industry would be a likely customer for this new type of material because the many small and vigorous aircraft companies seemed to be creating new aircraft designs and innovative concepts in manufacturing almost daily with many of these innovations requiring new materials.

The pace of composite development was accelerated during World War II. Not only were even more aircraft being developed and, therefore, composites more widely used intooling, but the use of composites for structural and semi-structural parts was being explored and then adopted. For example, during the war the last parts on an aircraft to be designed were the ducts. Since all the other systems were already fixed, the ducts were requiredto go around the other systems, often resulting in ducts that were twisting, turning and placed in the most difficult locations. Metal ducts just couldn’t easily be made in these complicated shapes. Composites were the answer. The composites were hand layed up on plaster mandrels which were made in the required shape. Then, after the resin had cured, the plaster mandrels were broken out of the composite parts.Non-aircraft applications included cotton-phenolic ship bearings, asbestos4, phenolic switchgears, cotton/asbestos-phenolic brake linings, cotton-acetate bayonet scabbards,and thousands of others.

Many other composite improvements were developed during WWII including some innovative manufacturing methods such as filament winding and spray-up. Sandwich structures using a cellular core, fire resistant composites, and prepreg materials were also developed during this time of development opportunity.

After the war focus was changed to anything that could be designed with composites, manufacturers had to find new markets to apply their materials now that there was not so much demand for aircraft parts.The demand for automobiles seemed like the logical application for composites and by 1947 a fully composite body automobile had been made and tested. This car was reasonably successful and led to the development of the Corvette in 1953.One special use for composite materials was that of Convair Aircraft Company. The company had thought that WWII pilots may want to continue their flying and also include families on holidays. Convair developed a set of detachable wings which could be attached to a special composite car; this allowed the driver to rent a wing assembly at one airport, fly to the vacation site, turn in the wing assembly, and drive away. Prototypes were made and successfully demonstrated.Some of the products made during the post-war era have now emerged as major markets for composite materials. These include tubs and shower assemblies, non-corrosive pipes, appliance parts, trays, storage containers, and furniture.Several innovative manufacturing methods were also developed in the late 1940's and early 1950's including pultrusion (by Goldsworthy), vacuum bag molding, and large-scale filament winding.

In the 1950’s aerospace applications really pushed the advancement of composites, Richard Young of the W. M. Kellogg Company began using filament winding for making small rocket motors. This technology was purchased by Hercules and was the basis for the large-scale rocket motor business which was at the heart of the space race.In 1961 a patent was issued to A. Shindo for experimentally producing the first carbon (graphite) fiber.

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New fibers were also introduced with boron filaments becoming available in 1965 and Aramid fibers (Kevlar®) offered commercially by DuPont in 1971. Fibers made from ultra high molecular weight polyethylene were made in the early 1970's. These advanced performancefibers, along with fiberglass and carbon fibers, have led to tremendous developments in aerospace, armour (structural and personal), sports equipment, medical devices, and many other high performance applications. The development of new and improved resins has also contributed to the expansion of the composites market, especially into higher temperature applications and applications where high corrosion resistance is needed.Today, the composites marketplace is widespread. As reported recently by the SPI Composites Institute, the largest market is still in transportation (31%), but construction (19.7%), marine (12.4%), electrical/electronic equipment (9.9%), consumer (5.8%), and appliance/business equipment are also large markets. The aircraft/aerospace market represents only 0.8% which is surprising seeing its importance in the origins of composites.

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Glossary

GFRP – Glass Fiber Reinforced PlasticCFRP – Carbon Fiber Reinforced PlasticCMC – Ceramic Matrix CompositeFRC – Fibre Reinforced CompositeMMC – Metal Matrix CompositePMC – Polymer Matrix CompositePAN – Polyacrylonitrile

Bibliography

www.efunda.com

Lecture Notes

www.gurit.com - Gurit Guide to Composites

http://www.eurocomposites.com

A History of Composite Materials - A. Brent Strong/Brigham Young University

www.pipexstructuralcomposites.co.uk

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