Module 1

The Lecture Contains:

Definition of a Composite Material

History of Composites

The Constituents in a Typical Composite

References

Module 1: Introduction to Composites

  Lecture 1: Definition and Introduction

 Introduction

There is an unabated quest for new materials which will satisfy the specific requirements for various applications like structural, medical, house-hold, industrial, construction, transportation, electrical; electronics, etc. The metals are the most commonly used materials in these applications. In the yore of time, there have been specific requirements on the properties of these materials. It is impossible of any material to fulfill all these properties. Hence, newer materials are developed. In the course, we are going to learn more about composite materials. First, we will deal with primary understanding of these materials and then we will learn the mechanics of these materials.

In the following lectures, we will introduce the composite materials, their evolution; constituents; fabrication; application; properties; forms, advantages-disadvantages etc.  In the present lecture we will introduce the composite materials with a formal definition, need for these materials, their constituents and forms of constituents.

Definition of a Composite Material

A composite material is defined as a material which is composed of two or more materials at a microscopic scale and has chemically distinct phases.

Thus, a composite material is heterogeneous at a microscopic scale but statistically homogeneous at macroscopic scale. The materials which form the composite are also called as constituents or constituent materials. The constituent materials of a composite have significantly different properties. Further, it should be noted that the properties of the composite formed may not be obtained from these constituents.  However, a combination of two or more materials with significant properties will not suffice to be called as a composite material. In general, the following conditions must be satisfied to be called as a composite material:

1. The combination of materials should result in significant property changes. One can see significant changes when one of the constituent material is in platelet or fibrous

from. 2. The content of the constituents is generally more than 10% (by volume).

3. In general, property of one constituent is much greater   than the corresponding property of the other constituent.

The composite materials can be natural or artificially made materials. In the following section we will see the examples of these materials.

Why do we need these materials?

There is unabated thirst for new materials with improved desired properties. All the desired properties are difficult to find in a single material. For example, a material which needs high fatigue life may not be cost effective. The list of the desired properties depending upon the requirement of the application is given below.

1. Strength2. Stiffness3. Toughness4. High corrosion resistance5. High wear resistance6. High chemical resistance7. High environmental degradation resistance

History of Composites

The existence of composite is not new. The word “composite” has become very popular in recent four-five decades due to the use of modern composite materials in various applications. The composites have existed from 10000 BC. For example, one can see the article by Ashby [1].  The evolution of materials and their relative importance over the years have been depicted the Figure 1 of this article. The common composite was straw bricks, used as construction material.

Then the next composite material can be seen from Egypt around 4000 BC where fibrous composite materials were used for preparing the writing material. These were the laminated writing materials fabricated from the papyrus plant. Further, Egyptians made containers from coarse fibres that were drawn from heat softened glass.

One more important application of composites can be seen around 1200 BC from Mongols. Mongols invented the so called “modern” composite bow. The history shows that the early existing of composite bows dates back to 3000 BC as predicted by Angara Dating. The bow used various materials like wood, horn, sinew (tendon), leather, bamboo and antler. The horn and antler were used to make the main body of the bow as it is very flexible and resilient. Sinews were used to join and cover the horn and antler together. Glue was prepared from the bladder of fish which is used to glue all the things in place. The string of the bow was made from sinew, horse hair and silk. The composite bow so prepared used to take almost a year for fabrication. The bows were so powerful that one can throw the arrows almost 1.5 km away. Until the discovery of gun-powder the composite bow used to be a very lethal weapon as it was a short and handy weapon.

As said, “Need is the mother of all inventions”, the modern composites, that is, polymer composites came into existence during the Second World War. During the Second World War due to constraint impositions on various nations for crossing boundaries as well as importing and exporting the materials, there was scarcity of materials, especially in the military applications. During this period the fighter planes were the most advanced fighting means. The light weight yet strong materials were in high demand. Further, for application like housing of electronic radar equipments require non-metallic materials. Hence, the Glass Fibre Reinforced Plastics (GFRP) were first used in these applications. Phenolic resins were used as the matrix material. The first use of composite laminates can be seen in the Havilland Mosquito Bomber of the British Royal Air Force.

The composites exit in day to day life applications as well. The most common existence is in the form of concrete. The concrete is a composite made from gravel, sand and cement. Further, when it is used along with steel to form structural components in construction, it forms one further form of composite. The other material is wood which is a composite made from cellulose and lignin. The advanced forms of wood composites can be ply-woods. These can be particle bonded composites or mixture of wooden planks/blocks with some binding agent. Now a days, these are widely used in the furniture and construction materials.

An excellent example of natural composite is muscles of human body. The muscles are present in a layered system consisting of fibers at different orientations and in different concentrations.

These result in a very strong, efficient, versatile and adaptable structure. The muscles impart strength to bones and vice a versa. These two together form a structure that is unique. The bone itself is a composite structure. The bone contains mineral matrix material which binds the collagen fibres together. 

The other examples include: wings of a bird, fins of a fish, trees and grass. A leaf of a tree is also an excellent example of composite structure. The veins in the leaf not only transport the food and water but also impart the strength to the leaf so that the leaf remains stretched with maximum surface area. This helps the plant to extract more energy from sun during photo-synthesis.

What are the constituents in a typical composite?

In a composite, typically, there are two constituents. One of the constituent acts as a reinforcement and other acts as a matrix. Sometimes, the constituents are also referred as phases.

What are the types of reinforcements?

The reinforcements in a composite material come in various forms. These are depicted through Figure 1.1.           

1. Fibre: Fibre is an individual filament of the material. A filament with length to diameter ratio above 1000 is called as a fibre. The fibrous form of the reinforcement is widely used. The fibres can be in the following two forms:

a. Continuous fibres: If the fibres used in a composite are very long and unbroken or cut then it forms a continuous fibre composite. A composite, thus formed using continuous fibres is called as fibrous composite. The fibrous composite is the widely used form of composite.

b. Short/chopped fibres: The fibres are chopped into small pieces when used in fabricating a composite. A composite with short fibres as reinforcements is called as short fibre composite.

In the fibre reinforced composites, the fibre is the major load carrying constituent.

2. Particulate: The reinforcement is in the form of particles which are of the order of a few microns in the diameter. The particles are generally added to increase the modulus and decrease the ductility of the matrix materials. In this case, the load is shared by both particles and matrix materials. However, the load shared by the particles is much larger than the matrix material. For, example in an automobile type carbon black (as a particulate reinforcement) is added in rubber (as matrix material). The composite with reinforcement in particle form is called as particulate composite.

3. Flake: Flake is a small, flat, thin piece or layer (or a chip) that is broken from a larger piece. Since these are two dimensional in geometry, they impart almost equal strength in all directions of their planes. Thus, these are very effective reinforcement components. The flakes can be packed more densely when they are laid parallel, even denser than

unidirectional fibres and spheres. For example, aluminum flakes are used in paints. They align themselves parallel to the surface of the coating which imparts the good properties.

4. Whiskers: These are nearly perfect single crystal fibres. These are short, discontinuous and polygonal in cross-section.

The classification of composites based on the form of reinforcement is shown in Figure 1.2. The detailed classification further is given in Figure 1.3. The classification of particulate composites is depicted further in Figure 1.4. Some of the terms used in these classifications will be explained in the following paragraphs/lectures.

Figure 1.2: Classification of composites based on reinforcement type

Figure 1.3: Classification of fibre composite materials

Why is the reinforcement made in thin fibre form?

There are various reasons because of which the reinforcement is made in thin fibre form. These reasons are given below.

a) An important experimental study by Leonardo da Vinci on the tensile strength of iron wires of various lengths (see references in [2, 3]) is well known to us. In this study it was revealed that the wires of same diameter with shorter length showed higher tensile strength than those longer lengths. The reason for this is the fact that the number of flaws in a shorter length of wire is small as compared longer length. Further, it is well known that the strength of a bulk material is very less than the strength of the same material in wire form.

The same fact has been explored in the composites with reinforcement in fibre form. As the fibres are made of thin diameter, the inherent flaws in the material decrease. Hence, the strength of the fibre increases as the fibre diameter decreases. This kind of experimental study has revealed the similar results [2, 3]. This has been shown in Figure 1.5 qualitatively.

Figure 1.5: Qualitative variation of fibre tensile strength with fibre diameter

b) The quality of load transfer between fibre and matrix depends upon the surface area between fibre and matrix. If the surface area between fibre and matrix is more, better is the load transfer. It can be shown that for given volume of fibres in a composite, the surface area between fibre and matrix increases if the fibre diameter decreases.

Let    be the average diameter of the fibres,   be the length of the fibres and  be the number of fibres for a given volume of fibres in a composite. Then the surface area available for load transfer is

(1.1)The volume of these fibres in a composite is

(1.2)

Now, let us replace the fibres with a smaller average diameter of  such that the volume of the fibres is unchanged. Then the number of fibres required to maintain the same fibre volume is

(1.3)

The new surface area between fibre and matrix is

(1.4)

Thus, for a given volume of fibres in a composite, the area between fibre and matrix is inversely proportional to the average diameter of the fibres.

c) The fibres should be flexible so that they can be bent easily without breaking. This property of the fibres is very important for woven composites. In woven composites the flexibility of fibres plays an important role. Ultra thin composites are used in deployable structures.

The flexibility is simply the inverse of the bending stiffness. From mechanics of solids study

the bending stiffness is  EI, where  is Young’s modulus of the material and  is the second moment of area of the cross section of the fibre. For a cylindrical fibre, the second moment of area is

(1.5)

Thus,

Flexibility (1.6)

Thus, from the above equation it is clear that if a fibre is thin, that is, small in diameter it is References:

o MF Ashby. Technology of 1990s: Advanced materials and predictive design. Phil. Trans. R. Soc. Lond. A. 1987; Vol. 322, pp. 393-407.

o JY Lund, JP Byrne. Leonardo Da Vinci's tensile strength tests: implications for the discovery of engineering mechanics Civil. Eng. and Env. Syst. 2001; Vol. 18, pp. 243-250.

o E de LaMotte, AJ Perry. Diameter and strain-rate dependence of the ultimate tensile strength and Young's modulus of carbon fibres. Fibre Science and Technology, 1970; Vol. 3, pp. 157-166.

o CT Herakovich. Mechanics of Fibrous Composites, John Wiley & Sons, Inc. New York, 1998.

o BD Agarwal, LJ Broutman, K Chandrashekhara. Analysis and Performance of Fibre Composites, 3rd Edition, John Wiley & Sons, Inc. New York, 2006.

o RM Jones. Mechanics of Composite Materials, Material Science and Engineering Series.2nd Edition, Taylor & Francis, 1999.

o AK Kaw. Mechanics of Composite Materials. 2nd Edition, CRC Press, New York, 2006.

o RM Christensen. Mechanics of Composite Materials. Dover Publications, 2005.o SW Tsai, HT Hahn. Introduction to Composite Materials, Technomic Publishing,

Lancaster, PA, 1980.o D Hull, TW Clyne. An Introduction to Composite Materials, 2nd ed., Cambridge

University, Press, New York, 1996. o IM Daniel, O Ishai. Engineering Mechanics of Composite Materials, Oxford

University Press, 1994.o Composite Handbook.o ASTM Standards.o SS Pendhari, T Kant, YM Desai. Application of polymer composites in civil

construction: A general review. Composite Structures, 2008; Vol. 84, pp. 114-124.

o CP Talley. J. Appl. Phys. 1959, Vol. 30, pp 1114.o http://composite.about.com/o http://www.netcomposites.com/o http://www.gurit.com/o http://www.hexcel.com/o http://www.toraycfa.com/o http://www.e-composites.com/o http://www.compositesone.com/basics.htmo http://www.wwcomposites.com/ (World Wide Search Engine for Composites) o http://jpsglass.com/o http://www.eirecomposites.com/

o http://www.advanced-composites.co.uk/o http://www.efunda.com/formulae/solid_mechanics/composites/comp_intro.cfm

Lecture 2: Reinforcement: Materials and Forms  

The Lecture Contains:

Types of Fibres

Boron Fiber

Carbon Fiber

Glass Fibre

References

Introduction

In the previous lecture we have introduced the composite. Then we have seen the constituents of a typical composite material. Further, based on the reinforcement, the classification of the composite was presented.

In the present lecture we will introduce natural fibres and some advanced fibres. We will see, in brief, the key features of these advanced fibres.

What are the functions of a reinforcing agent?

The functions of a reinforcing agent are:

1. These are the main load carrying constituents.

2. The reinforcing materials, in general, have significantly higher desired properties. Hence, they contribute the desired properties to the composite.

3. It transfers the strength and stiffness to the matrix material.

What are the functions of a matrix material?

The matrix performs various functions. These functions are listed below:

1. The matrix material holds the fibres together.

2. The matrix plays an important role to keep the fibres at desired positions. The desired distribution of the fibres is very important from micromechanical point of view.

3. The matrix keeps the fibres separate from each other so that the mechanical abrasion between them does not occur.

4. It transfers the load uniformly between fibers. Further, in case a fibre is broken or fibre discontinuity, then it helps to redistribute the load in the vicinity of the break site.

5. It provides protection to fibers from environmental effects.

6. It provides better finish to the final product.

7. The matrix material enhances some of the properties of the resulting material and structural component (that fibre alone is not able to impart). For example, such properties are: transverse strength of a lamina, impact resistance

What are the types of fibres?

The fibres that are used in the fabrication of a composite can be divided into two broad categories as follows:

A. Natural fibres and

B. Advanced fibres

A. Natural fibres

The natural fibres are divided into following three sub categories.

o Animal fibers: silk, wool, spider silk, sinew, camel hair, etc.o Plant/vegetable fibers: cotton (seed), jute (stem), hemp (stem), sisal (leaf), ramie,

bamboo, maze, sugarcane, banana, kapok, coir, abaca, kenaf, flax, raffia palm, etc.

o Mineral fibers: asbestos, basalt, mineral wool, glass wool.

B. Natural fibres Advanced fibers:

An advanced fibre is defined as a fibre which has a high specific stiffness (that is, ratio of

Young’s modulus to the density of the material, ) and a high specific strength (that is the

ratio of ultimate strength to the density of the material, ).

What are the advanced fibres?

The fibres made from following materials are the advanced fibres.

1. Carbon and/or Graphite2. Glass fibers 

Lecture 2: Reinforcement: Materials and Forms 

Figure 1.6: Periodic Table showing the materials used in advanced composites (blue blocks) and conventinal metals (yellow blocks)

Figure 1.6 shows the periodic table. The conventional metals are shown in yellow colour whereas the materials of the advanced fibres are shown in blue colour. It can be seen that the materials of the advanced fibres are lighter than the conventional metals. These materials occupy higher position as compared to metals in the periodic table. Thus, one can easily deduce that, in general, these materials have higher specific properties (property per unit weight) than that of metals.

Boron Fiber

This fibre was first introduced by Talley in 1959 [15]. In commercial production of boron fibres, the method of Chemical Vapour Deposition (CVD) is used. The CVD is a process in which one material is deposited onto a substrate to produce near theoretical density and

small grain size for the deposited material. In CVD the material is deposited on a thin filament. The material grows on this substrate and produces a thicker filament. The size of the final filament is such that it could not be produced by drawing or other conventional methods of producing fibres. It is the fine and dense structure of the deposited material which determines the strength and modulus of the fibre.

In the fabrication of boron fibre by CVD, the boron trichloride is mixed with hydrogen and boron is deposited according to the reaction

In the process, the passage takes place for couple of minutes. During this process, the atoms diffuse into tungsten core to produce the complete boridization and the production

of  and . In the beginning the tungsten fibre of 12  diameter is used, which

increases to 12  . This step induces significant residual stresses in the fibre. The core is

Figure 1.7: Schematic of reactors for silicon carbide fibres by Chemical Vapour Deposition

The key features of this fibre are listed below:

These are ceramic monofilament fiber. Fiber itself is a composite. Circular cross section.

Fiber diameter ranges between 33-400  and typical diameter is 140  . Boron is brittle hence large diameter results in lower flexibility. Thermal coefficient mismatch between boron and tungsten results in thermal

residual stresses during fabrication cool down to room temperature.

Boron fibres are usually coated with SiC or   so that it protects the surface  

Carbon Fiber:

The first carbon fibre for commercial use was fabricated by Thomas Edison.

Sixth lightest element and carbon- carbon covalent bond is the strongest in nature. Edison made carbon fiber from bamboo fibers. Bamboo fiber is made up of cellulose. Precursor fiber is carbonized rather than melting. Filaments are made by controlled pyrolysis (chemical deposition by heat) of a

precursor material in fiber form by heat treatment at temperature 1000-3000   The carbon content in carbon fibers is about 80-90 % and in Graphite fibers the

carbon content is in excess of 99%. Carbon fibre is produced at about 1300 while

the graphite fibre is produced in excess of 1900 . The carbon fibers become graphitized by heat treatment at temperature above 1800

. “Carbon fibers” term is used for both carbon fibers and graphite fibers. Different fibers have different morphology, origin, size and shape

The size of individual filament ranges from 3 to 147  .

Maximum use of temperature of the fibers ranges from 250 to 2000 .

Carbon Fiber:

The first carbon fibre for commercial use was fabricated by Thomas Edison.

Sixth lightest element and carbon- carbon covalent bond is the strongest in nature.

Edison made carbon fiber from bamboo fibers. Bamboo fiber is made up of cellulose. Precursor fiber is carbonized rather than melting. Filaments are made by controlled pyrolysis (chemical deposition by heat) of a precursor

material in fiber form by heat treatment at temperature 1000-3000   The carbon content in carbon fibers is about 80-90 % and in Graphite fibers the carbon

content is in excess of 99%. Carbon fibre is produced at about 1300 while the graphite

fibre is produced in excess of 1900 .

The carbon fibers become graphitized by heat treatment at temperature above 1800 . “Carbon fibers” term is used for both carbon fibers and graphite fibers. Different fibers have different morphology, origin, size and shape

The size of individual filament ranges from 3 to 147  .

Maximum use of temperature of the fibers ranges from 250 to 2000 . The use temperature of a composite is controlled by the use temperature of the matrix. Precursor materials: There are two types of precursor materials (i) Polyacrylonitrile

(PAN) and (ii) rayon pitch, that is, the residue of petroleum refining. Fiber properties vary with varying temperature.

Fiber diameter ranges from 4 to10  . A tow consists of about 3000 to 30000 filaments. Small diameter results in very flexible fiber and can actually be tied in to a knot without

breaking the fiber. Modulus and strength is controlled by the process. The procedure involves the thermal

decomposition of the organic precursor under well controlled conditions of temperature and stress.

Cross section of fiber is non-circular, in general, it is kidney bean shape. Heterogeneous microstructure consisting of numerous lamellar ribbons. Morphology is very dependent on the manufacturing process. PAN based carbon fibers typically have an onion skin appearance with the basal planes in

more or less circular arcs, whereas the morphology of pitch-based fiber in such that the basal planes lie along radial planes. Thus, carbon fibers are anisotropic.

Glass Fibre

Fibers of glass are produced by extruding molten glass, at a temperature around 1200   through holes in a spinneret with diameter of 1 or 2 mm and then drawing the filaments to

produce fibers having diameters usually between 5 to15 . The fibres have low modulus but significantly higher stiffness Individual filaments are small in diameters, isotropic and very flexible as the diameter is

small.

The glass fibres come in variety of forms based on silica   which is combined with other elements to create speciality glass.

What are the different types of glass fibres? What are their key features?

The types of glass fibres and their key features are as follows:

E glass - high strength and high resistivity S2 glass - high strength, modulus and stability under extreme temperature and corrosive

environment. R glass – enhanced mechanical properties C glass - resists corrosion in an acid environment D glass – good dielectric properties

References:

MF Ashby. Technology of 1990s: Advanced materials and predictive design. Phil. Trans. R. Soc. Lond. A. 1987; Vol. 322, pp. 393-407.

JY Lund, JP Byrne. Leonardo Da Vinci's tensile strength tests: implications for the discovery of engineering mechanics Civil. Eng. and Env. Syst. 2001; Vol. 18, pp. 243-250.

E de LaMotte, AJ Perry. Diameter and strain-rate dependence of the ultimate tensile strength and Young's modulus of carbon fibres. Fibre Science and Technology, 1970; Vol. 3, pp. 157-166.

CT Herakovich. Mechanics of Fibrous Composites, John Wiley & Sons, Inc. New York, 1998.

BD Agarwal, LJ Broutman, K Chandrashekhara. Analysis and Performance of Fibre Composites, 3rd Edition, John Wiley & Sons, Inc. New York, 2006.

RM Jones. Mechanics of Composite Materials, Material Science and Engineering Series.2nd Edition, Taylor & Francis, 1999.

AK Kaw. Mechanics of Composite Materials. 2nd Edition, CRC Press, New York, 2006. RM Christensen. Mechanics of Composite Materials. Dover Publications, 2005. SW Tsai, HT Hahn. Introduction to Composite Materials, Technomic Publishing,

Lancaster, PA, 1980. D Hull, TW Clyne. An Introduction to Composite Materials, 2nd ed., Cambridge

University, Press, New York, 1996. IM Daniel, O Ishai. Engineering Mechanics of Composite Materials, Oxford University

Press, 1994. Composite Handbook. ASTM Standards. SS Pendhari, T Kant, YM Desai. Application of polymer composites in civil construction:

A general review. Composite Structures, 2008; Vol. 84, pp. 114-124. CP Talley. J. Appl. Phys. 1959, Vol. 30, pp 1114. http://composite.about.com/ http://www.netcomposites.com/ http://www.gurit.com/ http://www.hexcel.com/ http://www.toraycfa.com/ http://www.e-composites.com/

http://www.compositesone.com/basics.htm http://www.wwcomposites.com/ (World Wide Search Engine for Composites) http://jpsglass.com/ http://www.eirecomposites.com/ http://www.advanced-composites.co.uk/ http://www.efunda.com/formulae/solid_mechanics/composites/comp_intro.cfm

Lecture 3: Reinforcement: Materials  

The Lecture Contains:

Alumina Fibre

Aramid Fibre

CVD on Tungsten or Carbon Core

NICALON TM by NIPPON Carbon Japan

References

Introduction

In this lecture we are going see some more advanced fibres. Further, we will see their key features, applications and fabrication processes.

Alumina Fibre

These are ceramics fabricated by spinning a slurry mix of alumina particles and additives to form a yarn which is then subjected to controlled heating.

Fibers retain strength at high temperature. It also shows good electrical insulation at high temperatures. It has good wear resistance and high hardness.

The upper continuous use temperature is about 1700  . Fibers of glass, carbon and alumina are supplied in the form of tows (also called ravings

or strands) consisting of many individual continuous fiber filaments. Du Pont has developed a commercial grade alumina fibre and known as Alumina FP

(polycrystalline alumina) fibre. Alumina FP fibres are compatible with both metal and

resin matrices. These fibres have a very high melting point of 2100 . They can

withstand temperatures up to 1000 without any loss of strength and stiffness properties at this elevated temperature. They exhibit high compressive strengths, when they are set in a matrix.

The Alumina whiskers are available and they exhibit excellent properties. Alumina whiskers can have the tensile strength of 20700 MPa and the tensile modulus of 427 GPa.

What are the applications of Alumina fibres?

The Alumina has a unique combination of low thermal expansion, high thermal conductivity and high compressive strength. The combination of these properties gives good thermal shock resistance. These properties make Alumina is suited for the applications in furnace use as crucibles, tubes and thermocouple sheaths.

The good wear resistance and high hardness properties are harnessed in making the components such as ball valves, piston pumps and deep drawing tools.

Aramid Fibre

These fibres are from Aromatic polyamide, that is, nylons family. Aramid is derived from “Ar” of Aromatric and “amid” of polyamide. Examples of fibres from nylon family: Polyamide 6, that is, nylon 6 and Polyamide 6.6,

that is, nylon 6.6 These are organic fibers. Melt-spun from a liquid solution Du Pont developed these fibers under the trade name Kevlar. From poly (p-phenylene

terephthalamide (PPTA) polymer. Morphology – radially arranged crystalline sheets resulting into anisotropic properties.

Filament diameter about 12  and partially flexible High tensile strength. Intermediate modulus Significantly lower strength in compression. 5 grades of Kevlar with varying engineering properties are available. Kevlar-29, Kevlar-

49, Kevlar-100, Kevlar-119 and Kevlar-129.

Silicon Carbide Fibre (SiC)

Silicon carbide fibres are ceramic fibers. These fibres are produced in similar fashion as boron fibres are produced. The fibres are produced by two methods as follows:

CVD on Tungsten or Carbon Core

NICALON™ by NIPPON Carbon Japan

CVD on Tungsten or Carbon Core:

This fiber is similar in size and microstructure to boron. The fibres are produced on both tungsten and carbon cores. These fibres are relativity stiff due to thicker diameter of the fibres. The diameter of

the fibres is about 140  .

The fibres have strength in the range of 3.4 – 4.0 GPa. Failure strain is in the range of 0.8 - 1%. The Young’s modulus is about 430 GPa. The fibres show high structural stability and strength retention even at temperatures

above 1000  .

The CVD with  as the reactant, SiC is deposited on the core as follows:

The SiC fibres produced on a tungsten core with a diameter about 12  . It shows a thin interfacial layer between the SiC mantle and the tungsten core. In case, when

carbon fibre is used the fibre diameter of the carbon fibres is about 33  . Both type of SiC fibre have smoother surfaces than a boron fibre. This is because there

is a deposition of small columnar grains as compared to conical nodules in boron fibres.

The SiC fibres produced with carbon core are used in light reinforced alloys. These fibres are produced with a surface coating. The composition of this coating varies from carbon rich from inner surface to silicon carbide at the outer surface.

The fibres that are used to reinforce the titanium have a protective layer which varies from a carbon rich to silicon rich and again to a composition which is rich in carbon at the surface. The outer surface acts as a protective surface and when it comes in contact with molten and highly reactive titanium. The fibres are made by Specialty Materials

NICALON™ by NIPPON Carbon Japan

The fibres are manufactured by the process of controlled pyrolysis (chemical deposition by heat) of a polymeric precursor.

The fiber is homogeneously composed of ultrafine beta-SiC crystallites and carbon. The filament is similar to carbon fiber in size.

The diameter of the fibre is about 14   The fibres more flexible due to small diameter. The fibres arranged in tows of 250 to 500 filaments per tow. These fibres come in two grades:

a. Ceramic Grade: provides good high temperature performance and mechanical properties

b. High Volume Resistivity Grade: It is a low dielectric fibre. It has good electrical and mechanical properties. These are used in dielectric structures.

Uses of the NICALON™ Fibres      These fibres are used to form fibrous products such as high temperature insulation, filters, etc. These fibres have high resistance to chemical attack. Hence, these can be used in harsh environments.

These are also used as a reinforcement in plastic, ceramic and metal matrix composites.

Cross Sectional Shapes of Fibres

The cross sectional shapes of fibre of various types we have studied above are different. The cross sectional shape of the fibres, although is assumed to be circular, is not circular in general. The various cross sectional shapes of the fibre are shown in Figure 1.10.  

Figure 1.10: Cross sectional shapes of fibres

Fiber Properties

The following are the important points regarding the fibre properties.

Density, axial modulus, axial Poisson’s ratio, axial tensile strength and coefficient of thermal expansion are some of the important properties.

Advanced fibers exhibit a broad range of properties. Properties of carbon fiber can vary significantly depending upon fabrication process. For the advanced fibres studied above one can attain either high modulus (> 700 GPa) or

high strength (> 5 GPa) but not both attainable simultaneously.  SCS-6, IM8, boron and sapphire fibers offer the best combination of stiffness and

strength but have large diameters and thus limited flexibility. However, IM8 fibers are exception for flexibility.

The specific stiffness of some of these fibres is almost 13 times of structural metals. Similarly, the specific strength of some of these fibres is almost 16 times of structural

metals. Weight saving, when the composites of these fibres are used, is tremendous due to high

specific stiffness and strength.

Actual properties of composite (fiber + matrix) are reduced. Specific properties are reduced even further when the loading is in a direction other than

the length direction of fibers. Tailorable properties. One can get the desired heat transfer or electrical conductivity with proper designing. The increased fatigue resistance is attainable with the use of these fibre composites. Aging effect can be significantly lowered.

Note: The fibres are classified based on their values of modulus as follows:

1. Ultra-high-modulus, type UHM (modulus > 450 GPa) 2. High-modulus, type HM (modulus between 350-450 GPa) 3. Intermediate-modulus, type IM (modulus between 200-350 GPa) 4. Low modulus and high-tensile, type HT (modulus < 100 GPa, tensile strength > 3.0 GPa)

References:

MF Ashby. Technology of 1990s: Advanced materials and predictive design. Phil. Trans. R. Soc. Lond. A. 1987; Vol. 322, pp. 393-407.

JY Lund, JP Byrne. Leonardo Da Vinci's tensile strength tests: implications for the discovery of engineering mechanics Civil. Eng. and Env. Syst. 2001; Vol. 18, pp. 243-250.

E de LaMotte, AJ Perry. Diameter and strain-rate dependence of the ultimate tensile strength and Young's modulus of carbon fibres. Fibre Science and Technology, 1970; Vol. 3, pp. 157-166.

CT Herakovich. Mechanics of Fibrous Composites, John Wiley & Sons, Inc. New York, 1998.

BD Agarwal, LJ Broutman, K Chandrashekhara. Analysis and Performance of Fibre Composites, 3rd Edition, John Wiley & Sons, Inc. New York, 2006.

RM Jones. Mechanics of Composite Materials, Material Science and Engineering Series.2nd Edition, Taylor & Francis, 1999.

AK Kaw. Mechanics of Composite Materials. 2nd Edition, CRC Press, New York, 2006. RM Christensen. Mechanics of Composite Materials. Dover Publications, 2005. SW Tsai, HT Hahn. Introduction to Composite Materials, Technomic Publishing,

Lancaster, PA, 1980. D Hull, TW Clyne. An Introduction to Composite Materials, 2nd ed., Cambridge

University, Press, New York, 1996. IM Daniel, O Ishai. Engineering Mechanics of Composite Materials, Oxford University

Press, 1994. Composite Handbook. ASTM Standards. SS Pendhari, T Kant, YM Desai. Application of polymer composites in civil construction:

A general review. Composite Structures, 2008; Vol. 84, pp. 114-124. CP Talley. J. Appl. Phys. 1959, Vol. 30, pp 1114.

http://composite.about.com/ http://www.netcomposites.com/ http://www.gurit.com/ http://www.hexcel.com/ http://www.toraycfa.com/ http://www.e-composites.com/ http://www.compositesone.com/basics.htm

Lecture 4: Matrix Materials  

The Lecture Contains:

Matrix Materials used in Composites

Thermoplastic and Thermoset Matrix Materials

Comparison between Thermoplastics and Thermosets

The Different Forms of Composites

The Factors that Affect the Composite Properties

References

Introduction

In the previous lecture we have introduced various advanced fibres along with their fabrication processes, precursor materials and key features. In the present lecture we will introduce some matrix materials, their key features and applications.

What are the matrix materials used in composites? 

The matrix materials used in composites can be broadly categorized as: Polymers, Metals, Ceramics and Carbon and Graphite.

The polymeric matrix materials are further divided into:

1. Thermoplastic – which soften upon heating and can be reshaped with heat and pressure.

2. Thermoset – which become cross linked during fabrication and does not soften upon reheating.

The metal matrix materials are: Aluminum, Copper and Titanium.

The ceramic materials are: Carbon, Silicon carbide, Silicon nitride.

The classification of matrix materials is shown in Figure 1.11.

Figure 1.11: Matrix materials

What are the thermoplastic matrix materials? What are their key features?

The following are the thermoplastic materials:

1. polypropylene, 2. polyvinyl chloride, 3. nylon, 4. polyurethane, 5. poly-ether-ether ketone (PEEK), 6. polyphenylene sulfide (PPS), 7. polysulpone.

The key features of the thermoplastic matrix materials are:

1. higher toughness2. high volume 3. low cost processing4. The use temperature range is upto 225  .

What are the thermoset matrix materials? What are their key features?

The thermoset matrix materials are:

1. polyesters, 2. epoxies, 3. polyimides

The key features of these materials are given for individual material in the following.

Polyesters

1. Used extensively with glass fibers2. Inexpensive 3. Light weight 4. Temperature range upto 100 .5. Resistant to environmental exposures

 Epoxy

1. Expensive2. Better moisture resistance3. Lower shrinkage on curing4. Use temperature is about 175

Polyimide

1. Higher use temperature about 300 2. Difficult to fabricate

What are the problems with the use of polymer matrix materials?

1. Limited temperature range. 2. Susceptibility to environmental degradation due to moisture, radiation, atomic oxygen (in

space)3. Low transverse strength.4. High residual stress due to large mismatch in coefficients of thermal expansion between

fiber and matrix.5. Polymer matrix cannot be used near or above the glass transition temperature.6. Comparison between Thermoplastics and Thermosets:

The comparison between the thermoplastic and thermoset matrix materials is given in Table 1 below:

7. Table 1.1: Comparison between thermoplastics and thermosets.

Thermoplastics Thermosets

Soften upon heat and pressure Decompose upon heating

Hence, can be repaired Difficult to repair

High strains are required for failureLow strains are required for failure

Can be re-processed Can not be re-processed

Indefinite shelf life Limited shelf life

Short curing cycles Long curing cycles

Non tacky and easy to handleTacky and therefore, difficult to handle

Excellent resistance to solvents Fair resistance to solvents

Higher processing temperature is required. Hence, viscosities make the processing difficult.

Lower processing temperature is required.

What are the common metals used as matrix materials? What are their advantages and disadvantages?

The common metals used as matrix materials are aluminum, titanium and copper.

Advantages:

1. Higher transfer strength, 2. High toughness (in contrast with brittle behavior of polymers and ceramics)3. The absence of moisture and 4. High thermal conductivity (copper and aluminum).

Dis-advantages:

1. Heavier 2. More susceptible to interface degradation at the fiber/matrix interface and 3. Corrosion is a major problem for the metals

            The attractive feature of the metal matrix composites is the higher temperature use. The aluminum matrix composite can be used in the temperature range upward of 300ºC while the titanium matrix composites can be used above 800 .             What are the ceramic matrix materials? What are their advantages and disadvantages?

The carbon, silicon carbide and silicon nitride are ceramics and used as matrix materials.

Ceramic:

            The advantages of the ceramic matrix materials are:

1. The ceramic composites have very high temperature range of above 2000  .2. High elastic modulus 3. Low density

The disadvantages of the ceramic matrix materials are:

1. The ceramics are very brittle in nature.  2. Hence, they are susceptible to flows.

Carbon

The advantages of the carbon matrix materials are:

1. High temperature at 2200 2. Carbon/carbon bond is stronger at elevated temperature than room temperature.

The disadvantages of the carbon matrix materials are:

1. The fabrication is expensive2. The multistage processing results in complexity and higher additional cost.

It should be noted that a composite with carbon fibres as reinforcement as well as matrix material is known as carbon-carbon composite. The application of carbon-carbon composite is seen in leading edge of the space shuttle where the high temperature resistance is required. The carbon-carbon composites can resist the temperature upto 3000  .

The advantages of these composites are:

1. Very strong and light as compared to graphite fibre alone.2. Low density3. Excellent tensile and compressive strength4. Low thermal conductivity5. High fatigue resistance6. High coefficient of friction

The disadvantages include:

1. Susceptible to oxidation at elevated temperatures2. High material and production cost3. Low shear strength

Figure 1.12 depicts the range of use temperature for matrix material in composites. It should be noted that for the structural applications the maximum use temperature is a critical parameter. This maximum temperature depends upon the maximum use temperature of the matrix materials.

Figure 1.12: Range of use temperature for matrix materials in composites

What are the different forms of composites?

1. Unidirectional lamina:o It is basic form of continuous fiber composites. o A lamina is also called by ply or layer.o Fibers are in same direction.o Orthotropic in nature with different properties in principal material directions.o For sufficient number of filaments (or layers) in the thickness direction, the

effective properties in the transverse plane (perpendicular to the fibers) may be isotropic. Such a composite is called as transversely isotropic.

2. Woven fabrics:o Examples of woven fabric are clothes, baskets, hats, etc.o Flexible fibers such as glass, carbon, aramid can be woven in to cloth fabric, can

be impregnated with a matrix material.o Different patterns of weaving are shown in Figure 1.13.

Typical weaving patterns are shown in Figure 1.13.

Figure 1.13: Types of weave

3. Laminate:

1. Stacking of unidirectional or woven fabric layers at different fiber orientations.2. Effective properties vary with:

1. orientation2. thickness3. stacking sequence

A symmetric laminate is shown in Figure 1.14.

Figure 1.14: A symmetric laminate

Hybrid composites:

The hybrid composite are composites in which two or more types of fibres are used. Collectively, these are called as hybrids. The use of two or more fibres allows the combination of desired properties from the fibres. For example, combination of aramid and carbon fibres gives excellent tensile properties of aramid and compressive properties of carbon fibers. Further, the aramid fibres are less expensive as compared to carbon fibres.

What are the factors that affect the composite properties?

There are various factors upon which the properties of the composite depend. Following are the various factors:

1. Properties of the constituent materials. Apart from this, the properties of other phases present, like additives, fillers and other reaction phases also affect the properties of the composite.

2. Length of the fibre.3. Orientation of the fibres (with respect to the loading direction).4. Cross sectional shape of the fibre.5. Distribution and arrangement of the fibres in the matrix material.6. Proportions of the fibre and matrix material, that is, volume fractions of the constituent

materials.

Notation for Composite Designation:            The composites are designated by the combination of the fibre and matrix system. The fibre and

matrix materials are separated by a slash  , that is, fibre material/matrix material. Further, one needs to specify the volume fractions of the constituents. In general, the fibre volume fraction is

specified. For example: AS4/PEEK, , that is, a carbon composite with AS4 fibres and PEEK as the matrix material with fibre volume fraction of 45%. Other examples are: T300/5208, T700/M21, Kevlar/Epoxy, Boron/Al, SCS-6/Ti-15-3, S2 Glass/Epoxy.

Lecture 5: Terminologies  

The Lecture Contains:

Terminologies Used in Fibrous Composites

The Advantages of Composite Materials

References

Introduction

In this lecture we are going to discuss some of the terms and their definitions that are used in the composites. These terms will be frequently used in our course. We will conclude this lecture with advantages and disadvantages of the composite materials.

Terminologies Used in Fibrous Composites

The following are the useful terminologies used in the composite related studies.

1.  Filament: individual element2.  Strand: Bundles of 204 filaments or multiple of these.3.  Rowing: Combination of strands to form thicker parallel bundles.4.  Yarns: strands are twisted to form yarns.5.  Aspect ratio: The ratio of length to diameter of a fiber.6.  Bicomponent fibers: A fiber made by spinning two compositions concurrently in each

capillary of the spinneret.7.  Blend: A mix of natural staple fiber such as cotton or wool and synthetic staple fibers such as

nylon, polyester. Blends are made to take advantages of the natural and synthetic fibers.8.  Braiding: Two or more yarns are intertwined to form an elongated structure. The long

direction is called the bias direction or machine direction.9.  Carding: Process of making fibers parallel by using rollers covered with needles.10.  Chopped strands: Fibers are chopped to various lengths, 3 to 50 mm, for mixing with resins.11.  Continuous fibers: Continuous strands of fibers, generally, available as wound fiber spools.12.  Cord: A relatively thick fibrous product made by twisting together two or more plies of yarn.13.  Covering power: The ability of fiber to occupy space. Noncircular fibers have greater

covering power then circular fibers.14.  Crimp: Waviness along the fiber length. Some natural fibers e.g. wool, have a natural crimp.

In synthetic polymeric fibers crimp can be introduced by passing the filament between rollers

having teeth. Crimp can also be introduced by chemical means. This is done by controlling the coagulation of the filament to produce an asymmetrical cross-section.

15.  Denier: A unit of linear density. It is the weight in grams of 9000m long yarn. This unit is commonly used in the US textile industry.

16.  Fabric: A kind of planar fibrous assembly. It allows the high degree of anisotropy characteristic of yarn to be minimized, although not completely eliminated.

17.  Felt: Homogeneous fibrous structure made by interlocking fibers via application of heat, moisture and pressure.

18.  Filament: Continuous fiber, i.e. fiber with aspect ratio approaching infinity.19.  Fill: see Weft.20.  Handle: Also known as softness of handle. It is a function of denier (or tex), compliance,

cross-section, crimp, moisture absorption, and surface roughness of the fiber.21.  Knitted fabric: One set of yarn is looped and interlocking to form a planar structure.22.  Knitting: This involves drawing loops of yarns over previous loops, also called interlooping.23.  Mat: Randomly dispersed chopped fibers or continuous fiber strands, held together with a

binder. The binder can be resin compatible, if the mat is to be used to make a polymeric composite.

24.  Microfibers: Also known as microdenier fibers. These are fibers having less than 1denier per filament (or less than 0.11 tex per filament). Fabrics made of such microfibers have superior silk-like handle and dense construction. They find applications in stretch fabrics, lingerie, rain wear, etc.

25.  Monofilament: A large diameter continuous fiber, generally, with a diameter greater than 100 m.

26.  Nonwovens: Randomly arranged fibers without making fiber yarns. Nonwovens can be formed by spunbonding, resinbonding, or needle punching. A planar sheet-like fabric is produced from fibers without going through the yarns spinning step. Chemical bonding and/or mechanical interlocking is achieved. Fibers (continuous or staple) are dispersed in a fluid (i.e. a liquid or air) and laid in a sheet-like planar form on a support and then chemically bonded or mechanically interlocked. Paper is perhaps the best example of a wet laid nonwoven fabric where we generally use wood or cellulosic fibers. In spunbonded nonwovens, continuous fibers are extruded and collected in random planar network and bonded.

27.  Particle: Extreme case of a fibrous form: it has a more or less equiaxial form, i.e. the aspect ratio is about 1.

28.  Plaiting: see Braiding.29.  Rayon: Term use to designate any of the regenerated fibers made by the viscose,

cuprammonium, or acetate processes. They are considered to be natural fibers because they are made from regenerated, natural cellulose.

30.  Retting: A biological process of degrading pectin and lignin associated with vegetable fibers, loosening the stem and fibers, followed by their separation.

31.  Ribbon: Fiber of rectangular cross-section with width to thickness ratio greater than 4.32.  Rope: Linear flexible structure with a minimum diameter of 4mm. it generally has three

strands twisted together in a helix. The rope characteristics are defined by two parameters, unit mass and break length. Unit mass is simply g/m or ktex, while breaking length is the length of rope that will break under the force of its own weight when freely suspended. Thus, break length equals mass at break/unit mass.

33.  Roving:  A bundle of yarns or tows of continuous filaments (twisted or untwisted).34.  Spinneret: A vessel with numerous shaped holes at the bottom through which a material in

molten state is forced out in the form of fine filaments or threads.35.  Spunbonding: Process of producing a bond between nonwoven fibers by heating the fibers to

near their melting point.36.  Staple fiber: Fibers having short, discrete lengths (10-400 mm long) that can be spun into a

yarn are called staple fibers. This spinning quality can be improved if the fiber is imparted a waviness or crimp. Staple fibers are excellent for providing bulkiness for filling, filtration, etc. Frequently, staple natural fibers, e.g. cotton or wool, are blended with staple synthetic fibers, e.g. nylon or polyester, to obtain the best of both types.

37.  Tenacity: A measure of fiber strength that is commonly used in the textile industry. Commonly, the units are gram-force per denier, gram-force per tex, or Newton per tex. It is a specific strength unit, i.e. there is a factor of density involved. Thus, although the tensile strength of glass fiber is more than double that of nylon fiber, both glass and nylon fiber have a tenacity of about 6g/den. This is because the density of glass is about twice that of nylon.

38.  Tex: A unit of linear density. It is the weight in grams of 1000m of yarn. Tex is commonly used in Europe.

39.  Tow: Bundle of twisted or untwisted continuous fibers. A tow may contain tens or hundreds of thousands of individual filaments.

What are the advantages of the composite materials?

The following are the advantages of composites:

1. Specific stiffness and specific strength:

The composite materials have high specific stiffness and strengths. Thus, these material offer better properties at lesser weight as compared to conventional materials. Due to this, one gets improved performance at reduced energy consumption.

2. Tailorable design:

A large set of design parameters are available to choose from. Thus, making the design procedure more versatile. The available design parameters are:

1. Choice of materials (fiber/matrix), volume fraction of fiber and matrix, fabrication method, layer orientation, no. of layer/laminae in a given direction, thickness of individual layers, type of layers (fabric/unidirectional) stacking sequence.

2. A component can be designed to have desired properties in specific directions.

3. Fatigue Life:

   The composites can with stand more number of fatigue cycles than that of aluminum. The critical structural components in aircraft require high fatigue life. The use of composites in fabrication of such structural components is thus justified.

4. Dimensional Stability:

Strain due to temperature can change shape, size, increase friction, wear and thermal stresses. The dimensional stability is very important in application like space antenna. For composites, with proper design it is possible to achieve almost zero coefficient of thermal expansion.

5. Corrosion Resistance:

Polymer and ceramic matrix material used to make composites have high resistance to corrosion from moisture, chemicals.

6. Cost Effective Fabrication:

The components fabricated from composite are cost effective with automated methods like filament winding, pultrusion and tape laying. There is a lesser wastage of the raw materials as the product is fabricated to the final product size unlike in metals.

7. Conductivity:

The conductivity of the composites can be achieved to make it a insulator or a highly conducting material. For example, Glass/polyesters are non conducting materials. These materials can be used in space ladders, booms etc. where one needs higher dimensional stability, whereas copper matrix material gives a high thermal conductivity.

The list of advantages of composite is quite long. One can find more on advantages of composite in reference books and open literature.

What are the disadvantages of Composites?

1. Some fabrics are very hard on tooling.2. Hidden defects are difficult to locate.3. Inspection may require special tools and processes.4. Filament-wound parts may not be repairable. Repairing may introduce new problems.5. High cost of raw materials.6. High initial cost of tooling, production set-up, etc.7. Labour intensive.8. Health and safety concerns.9. Training of the labour is essential.10. Environmental issues like disposal and waste management.11. Reuse of the materials is difficult.12. Storage of frozen pre-pregs demands for additional equipments and adds to the cost of

production.13. Extreme cleanliness required.14. The composites, in general, are brittle in nature and hence easily damageable.15. The matrix material is weak and hence the composite has low toughness.16. The transverse properties of lamina or laminate are, in general, weak.17. The analysis of the composites is difficult due to heterogeneity and orthotropy.

Lecture 6: Applications  

The Lecture Contains:

Applications of Composite Materials

References

Lecture 6: Applications 

Infrastructure Structures:

Corrosion is a major design consideration such as in the chemical and on off- shore oil plate forms

Skeletal Structures Walls and Panels Doors, Windows, Ladders, Staircases Chemical and Water Tanks Cooling Towers Bridge Decks Antenna Dishes Bridge enclosures Aerodynamic fairings

Industrial:

Drive, conveyer belts, hoses, tear and puncture resistant fabrics, rotor vanes, mandrels,

ropes, cables.

Medical:

Wheelchairs, Crutches, Hip joints, Heat valves, Dentistry, Surgical equipments

Electronic:

Chips in electronic computing devices are laminated hybrid systems composed of a number of layers (materials) which serve different functions.

Chip must have good heat transfer properties and must be able to withstand induced thermal stresses without delaminating.

The composite finds a vast usage in electronic packaging materials. The Styrofoam, particle bonded materials formed from paper pulp, air-bubble cushioned plastic sheets, etc. are some of the popular materials used in the packing.

Military:

Helmets, bullet proof vests, impact resistant vehicles, lighter and less detectable ships, portable bridges.

Marine:

          The Glass reinforced fibre plastics are used in:

Ship and Boat Hulls Masts Instrument Panels Hydrofoils Hovercrafts Propellers Propulsion shafts Rudders Heat exchangers Flywheel Piping Ventilation ducts

Engine and equipment foundations

Wind Power Engineering:

Rotor blades including blade shell, integral webs, spars or box structure. Mast Generator housing

Lecture 7: Fabrication Processes  

The Lecture Contains:

Wet/Hand Lay-Up

Spray Lay-Up

Autoclave Curing

Filament Winding

Pultrusion

References

 Introduction:

In this lecture we will see some of the important fabrication processes of composites. Further, we will see their advantages, disadvantages and applications.

What are the fabrication processes of composites materials? Describe the processes in brief along with the materials used in the process, their advantages, disadvantages and applications.

The fabrication processes are described in the following along with their advantages, disadvantages and applications in the following.

A. Wet/Hand Lay-Up:

The fibres are first put in place in the mould. The fibres can be in the form of woven, knitted, stitched or bonded fabrics. Then the resin is impregnated. The impregnation of resin is done by using rollers, brushes or a nip-roller type impregnator. The impregnation helps in forcing the resin inside the fabric. The laminates fabricated by this process are then cured under standard atmospheric conditions. The wet/hand lay-up process is depicted in Figure 1.15.

The materials that can be used have, in general, no restrictions. One can use combination of resins like epoxy, polyester, vinylester, phenolic and any fibre material.

Advantages:

o The process results in low cost tooling with the use of room-temperature cure resins.o The process is simple to use. o Any combination of fibres and matrix materials are used.o Higher fibre contents and longer fibres as compared to other processes.

Disadvantages:

o Since the process is worked by hands, there are safety and hazard considerations.o The resin needs to be less viscous so that it can be easily worked by hands.o The quality of the final product is highly skill dependent of the labours. o Uniform distribution of resin inside the fabric is not possible. It leads to voids in the

laminates.o Possibility of diluting the contents.

Applications:B. Spray Lay-Up:

Fibre is chopped in a hand-held gun and fed into a spray of catalyzed resin directed at the mould. The deposited materials are left to cure under standard atmospheric conditions.

The fabrication method is depicted in Figure 1.16.

The polyester resins can be used with glass rovings is best suited for this process.

Figure 1.16: Wet or hand lay-up fabrication

Advantages:

The spray-up process offers the following advantages:

o It is suitable for small to medium-volume parts.o It is a very economical process for making small to large parts.o It utilizes low-cost tooling as well as low-cost material systems.

Limitations:

The following are some of the limitations of the spray-up process:

o It is not suitable for making parts that have high structural requirements.o It is difficult to control the fiber volume fraction as well as the thickness. These

parameters highly depend on operator skill.o Because of its open mold nature, styrene emission is a concern.o The process offers a good surface finish on one side and a rough surface finish on

the other side.o The process is not suitable for parts where dimensional accuracy and process

repeatability are prime concerns. The spray-up process does not provide a good surface finish or dimensional control on both or all the sides of the product.

o Cores, when needed, have to be inserted manually.o Only short fibres can be used in this process.o Since, pressurized resin is used the laminates tend to be very resin-rich.

o Similar to wet/hand lay-up process, the resins need to be of low viscosity so that it can be sprayed.

Applications:Simple enclosures, lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubs, shower trays, some small dinghies.

C. Autoclave Curing:

The key features of this process are as follows:

o An autoclave is a closed vessel for controlling temperature and pressure is used for curing polymeric matrix composites.

o Composites to be cured is prepared either through hand lay up or machine placement of individual laminae in the form of fibers tape which has been impregnated with resin.

o Components is then placed in an autoclave and subjected to a controlled cycle of temperature and pressure.

o After curing, the composite is “solidified”.o One can use the fibres like carbon, glass, aramid etc. along with any resin.

Advantages:

o Large components can be fabricated.o Since, the curing of matrix material is carried out under controlled environment

the resin distribution is better as compared to hand or spay lay-up processes.o Less possibility of dilution with foreign particles.o Better surface finish.

Disadvantages:

o Initial cost of tooling is high.o Running and maintenance cost is high.o Not suitable for small products.

Applications:The process is suitable for aerospace, automobile parts like wing box, chassis, bumpers, etc.

Lecture 7: Fabrication Processes 

D. Filament Winding:

This process is an automated process. This process is used in the fabrication of components or structures made with flexible fibers. This process is primarily used for

hollow, generally circular or oval sectioned components. Fibre tows are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fibre feeding mechanism, and rate of rotation of the mandrel. The wound component is then cured in an oven or autoclave.            One can use resins like epoxy, polyester, vinylester and phenolic along with any fibre. The fibre can be directly from creel, non-woven or stitched into a fabric form.            The filament winding process is shown in Figure 1.17.

Advantages:

o Resin content is controlled by nips or dies. o The process can be very fast.o The process is economic.o Complex fibre patterns can be attained for better load bearing of the structure.

 Disadvantages:

o Resins with low viscosity are needed.o The process is limited to convex shaped components. o Fibre cannot easily be laid exactly along the length of a component.

E. Pultrusion:

It is a continuous process in which composites in the form of fibers and fabrics are pulled through a bath of liquid resin. Then the fibres wetted with resin are pulled through a heated die. The die plays important roles like completing the impregnation and controlling the resin. Further, the material is cured to its final shape. The die shape used in this process is nothing the replica of the final product. Finally, the finished product is cut to length.

In this process, the fabrics may also be introduced into the die. The fabrics provide a fibre direction other  0°. Further, a variant of this method to produce a profile with some variation in the cross-section is available. This is known as pulforming.

The resins like epoxy, polyester, vinylester and phenolic can be used with any fibre.

The pultrusion process is shown in Figure 1.18.

Advantages:

o The process is suitable for mass production.o The process is fast and economic.o Resin content can be accurately controlled.o Fibre cost is minimized as it can be taken directly from a creel.o The surface finish of the product is good.o Structural properties of product can be very good as the profiles have very straight

fibres.

 Disadvantages:

o Limited to constant or near constant cross-section components.o Heated die costs can be high.o Products with small cross-sections alone can be fabricated.

Applications:

Beams and girders used in roof structures, bridges, ladders, frameworks

Figure 1.18: PultrusionLecture 8: Fabrication Processes

The Lecture Contains:

Braiding

Vacuum Bagging

Resin Transfer Molding - RTM

Centrifugal Casting

References

Lecture 8: Fabrication Processes

B. Vacuum Bagging:

This is basically an extension of the wet lay-up process described above where pressure is applied to the laminate once laid-up in order to improve its consolidation. This is achieved by sealing a plastic film over the wet laid-up laminate and onto the tool. The air under the bag is extracted by a vacuum pump and thus up to one atmosphere of pressure can be applied to the laminate to consolidate it.

Materials Options:

o Resins: Primarily epoxy and phenolic. Polyesters and vinylesters may have problems due to excessive extraction of styrene from the resin by the vacuum pump.

o Fibres: The consolidation pressures mean that a variety of heavy fabrics can be wet-out.

o Cores: Any.

Advantages:

o Higher fibre content laminates can usually be achieved than with standard wet lay-up techniques.

o Lower void contents are achieved than with wet lay-up. o Better fibre wet-out due to pressure and resin flow throughout structural fibres,

with excess into bagging materials.

o Health and safety: The vacuum bag reduces the amount of volatiles emitted during cure.

Disadvantages:

o The extra process adds cost both in labour and in disposable bagging materials.o A higher level of skill is required by the operators. o Mixing and control of resin content still largely determined by operator skill.

Applications:

Large one-off cruising boats, race car components, core-bonding in production boats.

Figure 1.19: Vacuum Bagging Resin Transfer Molding - RTM

The process consists of arranging the fibres or cloth fabrics in the desired configuration in a preform. These fabrics are sometimes pre-pressed to the mould shape, and held together by a binder. A second matching mould tool is then clamped over the first. Then pressurized resin is injected into the cavity. Vacuum can also be applied to the mould cavity to assist resin in being drawn into the fabrics. This is known as Vacuum Assisted Resin Transfer Moulding (VARTM) or Vacuum Assisted Resin Injection (VARI). The laminate is then cured. Both injection and cure can take place at either ambient or elevated temperature.

In this process, the resins like epoxy, polyester, vinylester and phenolic can be used. Further, one use the high temperature resins such as bismaleimides can be used at elevated process temperatures. The fibres of any type can be used. The stitched materials work well in this process since the gaps allow rapid resin transport. Some specially developed fabrics can assist with resin flow.

Advantages:

The process is very efficient. Suitable for complex shapes.

High fibre volume laminates can be obtained with very low void contents. Good health and safety, and environmental control due to enclosure of resin. Possible labour reductions. Both sides of the component have a moulded surface. Hence, the final product gets a

superior surface finish Better reproducibility. Relatively low clamping pressure and ability to induce inserts.

Disadvantages:

o Matched tooling is expensive and heavy in order to withstand pressures. o Generally limited to smaller components. o Unimpregnated areas can occur resulting in very expensive scrap parts.

Applications:

The applications include the hollow cylindrical parts like motor casing, engine covers, etc.

Figure 1.21: Centrifugal casting

D. Centrifugal Casting:

In this process the chopped fibres and the resin is sent under pressure to the cylindrical moulding. The moulding is rotating. Due to centrifugal action, the mixture of resin and chopped fibres get deposited on wall of the moulding. Thus, the mixture gets the final form of the product.

Advantages:

1. Suitable for small hollow cylindrical products.2. Economic for small production.

Disadvantages:

3. Complex shape can not be made.4. Resin with low viscosity is needed.5. The finish of the inner side of the product is not good.6. The structural properties may not be good as the chopped fibres are used.

Figure 1.21: Centrifugal casting

Applications:The applications include the hollow cylindrical parts like motor casing, engine covers,

etc.