lecture4_fibera

8/18/2019 Lecture4_fibera

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Reinforcing Agent- Fiber


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The four main factors that govern the fiber's

contribution are:1. The basic mechanical properties of the fiber

itself.

2. The surface interaction of fiber and resin (the'interface'). Depends on degree of bondingbetween fiber and matrix. Smaller fiber diameter,higher surface area, more in contact with resin.

3. The amount of fiber in the composite ('fiberVolume Fraction').4. The orientation of the fibers in the composite.

Properties of Reinforcing fibers


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The stiffness and strength of a laminate will increase in proportion to theamount of fiber present.However, above about 60-70% FVF although tensile stiffness may continueto increase, the laminate's strength will reach a peak and then begin todecrease due to the lack of sufficient resin to hold the fibers together

properly.

The orientation of the fibers creates highly 'direction-specific' propertiesin the composite. This 'anisotropic' feature of composites can be used to

good advantage in designs.


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Fiber Length:There is a critical length, lc, that the fibers must have to strengthen amaterial to their maximum potential. The critical length is given in

equation

The minimum fiber length for a continuous fiber composite

lc= critical lengthσf= tensile strength of the fiberd = diameter of the fibertc = shear strength of the bond between the matrix and the fiber

The fiber ends do not carry load; as the fiber length increases, therelatively ineffective portions of the fibers decrease.

The average stress in a fiber σf avg is given byσf avg = σf (1-lc /2l)

If the actually length of the fiber is at least 15 times greater than the lcvalue; the fiber is said to be "continuous"; Otherwise the composite is"discontinous."

Continuous fiber composites have superior mechanical properties.


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Glass Fiber

Contains SiO2 and other inorganic oxides

By variation of the “recipe”, different types of glass can beproduced.


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E-glass - Good tensile and compressive strength andstiffness, good electrical properties and relatively low cost,but impact resistance relatively poor. Depending on the typeof E glass the price ranges from about £1-2/kg. E-glass isthe most common form of reinforcing fiber used in polymermatrix composites.

C-glass - best resistance to chemical attack. Mainly used inthe form of outer layer of laminates used in chemical andwater pipes and tanks.

R, S or T-glass –have higher tensile strength and modulus

than E glass, with better wet strength retention. Developedfor aerospace and defence industries, and used in some hardballistic armour applications. Depending on the type of R orS glass the price ranges from about £12-20/kg.


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By blending quarry products (sand, kaolin, limestone,colemanite) at 1600C, liquid glass is formed. The liquid is

passed through micro-fine bushings and simultaneously cooledto produce glass fiber filaments from 5-24m in diameter. Thefilaments are drawn together into a strand (closely associated)or roving (loosely associated), and coated with a “size” toprovide filament cohesion and protect the glass from abrasion.

The fibers are cooled rapidly through Tg during drawing in order to

prevent crystallization.


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Other advantages

Resistance to high temperature- the softenng point is about 850°°°°C.

Transparency to visible light-takes the color of the matrix

Isotropy- thermal expansion is identical in axial and radial directions.

Disadvantage

Prone to surface damage

Dissolution of some of the oxides by moisture


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It is used extensively within the automotive and sport equipment sectors,

Glass fiber is also used to make pipes for drinking water, sewers,chemicals, and so on.

GRP is also widely used in the telecommunication industry for shroudingthe visual appearance of antennas, due to its RF permeability and lowsignal attenuation properties.

Applications


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Para-linked Aramid Fiber

TECHNORA : co-poly-(paraphenylene/3,4'-oxydiphenylene terephthalamide)

TWARON : poly-(paraphenylene terephthalamide) (PPTA)(Kevlar)

Meta-linked Aramid Fiber

TEIJINCONEX : poly-(metaphenylene isophthalamide) (MPIA)

Strength in aramid is enhanced by intermolecular hydrogen bonding.Because of their rod like structure, they have exceptionally high

modulus and high packing efficiency. They are more flexible thanglass fibers.

Aramid Fiber


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Aramid Fiber CharacteristicsNo melting pointLow flammabilityGood fabric integrity at elevated temperatures

Less abrasion

Disadvantage: impart yellow coloration, absorb moisture,difficult to cut.

Use

Reinforcement of rubber goods such as tires, ropes andcables.Aramid fabric reinforced composites for building of boatsand aircraft.


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Carbon Fiber from PAN

Carbon fiber is made by heating, oxidizing and carbonizing polyacrylonitrile(PAN) polymer fibers.

First, the PAN fiber is heated in air. The heat causes the cyano siteswithin the PAN polymer chain to form repeat cyclic units, oftetrahydropyridine.

Continuing the heating process in air, oxidation occurs. This process causesthe carbon atoms to kick off their hydrogen atoms, and the rings becomearomatic.


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The heating process is continued in the absence of air. The heating processis now called carbonization, where the heat is raised to above 1300 oC.

Adjacent polymer chains are joined together to give a ribbon-like fusedring polymer.

The newly formed ribbons continue to condense together to form the

lamellar, basal plane structure of nearly pure carbon.

The polymer has nitrogen atoms along the edges of the basal planes andwhich are expelled as Nitrogen gas.


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Carbon fibers can also be made from pitch or cellulose.

Variation of the graphitisation process produces either high strength fibers (@

~2,600°C) or high modulus fibers (@ ~3,000°C) with other types in between.Once formed, the carbon fiber has a surface treatment applied to improvematrix bonding and chemical sizing which serves to protect it during handling.

Carbon fibers are grouped as high strength (HS), intermediate modulus (IM),high modulus (HM) and ultra high modulus (UHM). The filament diameter ofmost types is about 5-7mm.

Carbon fiber has the highest specific stiffness of any commercially availablefiber, very high strength in both tension and compression and a high resistanceto corrosion, creep and fatigue.

Their impact strength, however, is lower than either glass or aramid, withparticularly brittle characteristics being exhibited by HM and UHM fibers.

Carbon Fiber


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Other Advantages:

Chemical inertness-resistant to moisture and other commonchemicals

High electrical and thermal conductivity along the fiber axis

Dimensional stability-axial thermal expansion is extremely low


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Cost


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Fiber Finishes

Surface finishes are nearly always applied to fibers both toallow handling with minimum damage and to promotefiber/matrix interfacial bond strength.

Glass fiber are treated in two stages.

The first finish is applied at the point of fiber manufacture at quite ahigh level for protection of the fiber against damage during handling and

the weaving process itself. This protective finish, which is often starchbased, is cleaned off or 'scoured' after the weaving process either byheat or with chemicals.

The scoured woven fabric is then separately treated with a different

matrix-compatible finish specifically designed to optimise fiber to resininterfacial characteristics such as bond strength, water resistance andoptical clarity.

Glass fiber Finishes

Carbon fiber and aramid fiber need finish for matrix compatibility


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Fabric Types and Constructions

The four main fiber orientation categories are: Unidirectional, Woven,Multiaxial, Braids and Other/random.

UnidirectionalWoven

Multiaxial/chopped strand matBraids


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Other fibersPolyester

A low density, high tenacity fiber with good impact resistance but low modulus.It is useful where low weight, high impact or abrasion resistance, and low cost arerequired. It is mainly used as a surfacing material, as it can be very smooth, keepsweight down and works well with most resin types.

• QuartzA very high silica version of glass with much higher mechanical properties andexcellent resistance to high temperatures (1,000°C). However, the

manufacturing process and low volume production lead to a very high price(14mm - £74/kg, 9mm - £120/kg).

• BoronCarbon or metal fibers are coated with a layer of boron to improve the overallfiber properties. The extremely high cost of this fiber restricts it use to high

temperature aerospace applications and in specialised sporting equipment. Aboron/carbon hybrid, composed of carbon fibers interspersed among 80-100mmboron fibers, in an epoxy matrix, can achieve properties greater than eitherfiber alone, with flexural strength and stiffness twice that of HS carbon and1.4 times that of boron, and shear strength exceeding that of either fiber.

• CeramicsCeramic fibers, usually in the form of very short 'whiskers' are mainly used inareas requiring high temperature resistance. They are more frequentlyassociated with non-polymer matrices such as metal alloys.

• NaturalAt the other end of the scale it is possible to use fibrous plant materials suchas ute and sisal as reinforcements in 'low-tech' a lications.


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Jute compositeSisal epoxy

composite

Coconut coir

composite


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Particle and Fiber Reinforced Plastics

• A polymer matrix is strengthened/stiffened by a

particulate second phase.

• Particles restrict the mobility and deformation of thematrix by introducing a mechanical restraint, the degree of

restraint depending on the particulate spacing and on theproperties of the particle and matrix


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• Particulate fillers tend to increase the stiffness of amatrix resin, but may or may not increase the toughness ortensile strength.

• With short discontinuous fibers of high modulus material,

the mechanical load is and the shared between the matrixand the filler. While for continuous fibers, the fiberscarry most of the mechanical load, while the matrix servesto transfer stresses to the load-bearing fibers and to

protect them against damage


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Hybrid fibers:

Aramid and carbon fibers have similarcoefficients of expansion, they aresometimes used as hybrid fibrous additivesin composites. Aramid fibers contributetowards high impact strength and carbon

fibers towards high compressive strength.


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WhiskersAn extremely fine filamentary crystal withextraordinary shear strength and unusual electricalor surface properties.

Material Tensile Strength ( MPa *103 )Graphite 20Silicon carbide 20Silicon nitride 14Aluminum oxide 14-28

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