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CHAPTER 3

PROPERTIES OF GFRP MATERIALS

3.1 GENERAL

FRP composites are a state-of-the-art construction material, an

alternative to traditional materials such as concrete, steel and wood. Among

many applications of FRP in civil infrastructures, bridge decks have received

much attention because of their light weight, high strength-to-weight ratio,

and corrosion resistance. Other advantages of FRP bridge decks are the

reduction in bridge deck construction time and increase in service life. The

attractiveness of FRP composites as construction materials derives from a set

of advantages gleaned from the tailorability of this material class through the

synergistic combination of fibres in a polymeric resin matrix, wherein the

fibre reinforcements carry load in predesigned directions and the resin acts as

a medium to transfer stresses between adjoining fibres through adhesion and

also provides protection for the fibres. The selection of matrix and

reinforcement for fabrication of any composite product mainly depends on the

properties of matrix and reinforcement.

3.2 FIBRES

Fibres are the principal constituents in a fibre reinforced composite

material. They occupy the largest volume fraction in a composite laminate

and share the major portion of the load acting on a composite structure. The

effectiveness of fibre reinforcement depends on the type, length, volume

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fractions and orientation of fibres in the matrix. Proper selection of the fibre is

influenced by following characteristics.

Density

Tensile and Compressive strength

MOE

Fracture

Fatigue performance

Response to impact loads

Electrical and Thermal properties

Cost

Principal fibres in commercial use for production of civil

engineering applications are

i) Carbon

ii) Aramid

iii) Glass fibres

E-glass fibres have been employed in this study. A brief description

about its composition, advantages, and properties are presented below.

3.3 GLASS FIBRES

The most extensively used class of fibres in composites are those

manufactured from E-glass. E-glass is a low alkali borosilicate glass

originally developed for electrical insulation applications. It was first

produced commercially for composite manufacture in 1940’s, and its use now

approaches 2 MT per year worldwide. Many different countries manufacture

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E-glass and its exact composition varies according to the availability and

composition of the local raw materials. It is manufactured as continuous

filaments in bundles, or strands, each containing typically between 200 and

2000 individual filaments of 10-30 µm diameters. These strands may be

incorporated into larger bundles called roving and may be processed into a

wide variety of mats, clothes, and performs and cut into short-fibre formats.

Glass filaments have relatively low stiffness but very high tensile strength

(~3GPa). In spite of their initial very high strength, glass filaments are

relatively delicate and may become damaged by abrasion and by attack from

moist air. It is therefore always necessary to protect the newly drawn strands

with a coating or size (also referred to as a “finish”). This is usually applied as

a solution or emulsion containing a polymer that coats the fibres and binds the

fibres in the strand together (film former), a lubricant to reduce abrasion

damage and improve handling, additives to control static electric charges on

the filaments, and a coupling agent, usually a silane, that enhances the

adhesion of the filaments to the matrix resin and reduces property loss on

exposure to wet environments.

3.4 REINFORCEMENT FORMAT

The reinforcement fibres are generally available in the form of a

tow, or in a band. In some processing operations (e.g. filament winding),

tows, or rovings, of continuous fibres are converted directly into the

component. Following forms of GFRP are generally available:

CSM (Emulsion)

CSM (Powder)

WR

Spray - up Rovings

SMC Rovings

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Assembled Rovings

Direct Roving

Among these forms, the present study deals with CSM (Emulsion)

and WR.

3.4.1 Woven Rovings

Woven clothes and rovings are very widely used in the manufacture

of laminated structures. A simple plain weave WR allows a Vf of up to 0.6 to

be achieved in the laminate. In-plane strengths are much higher than for the

random materials. Stiffness, strength, and drape are also influenced by the

weave pattern. The plain weave leads to a high degree of crimp, which may

reduce stiffness by up to about 15% compared with a similar fraction of

straight fibres. Twill and satin weaves offer better drape, and the satin weaves

in particular have less crimp.

Five and eight-harness satin weaves are widely used in composite

laminates, especially in the lighter weights, which are more appropriate in

many highly stressed designs. The tighter fibre structure in cloths renders

them more difficult to infiltrate and consolidate than the random mats. WR

fabrics are specifically designed to meet most demanding performance,

processing and cost requirements. These fabrics deliver a unique combination

of properties. They offer one of the highest strength-to-weight ratios possible

for reinforced plastics and through careful selection and placement of fabrics,

designers can put the strength exactly where it is needed, making optimum

use of the fibre strength. WR fabrics provide the most economical solution for

raising glass content of laminates and increasing laminate stiffness and impact

resistance without adding thickness, weight or other non-reinforcing

materials. Figure 3.1 shows the typical WR mat.

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Figure 3.1 Woven rovings

3.4.2 Chopped Strand Mat (Emulsion)

Chopped strands are produced by cutting continuous strands into

short lengths. The ability of the individual filaments to hold together during or

after the chopping process depends largely on the type and amount of size

applied during the fibre manufacturing operation. Strands of high integrity are

called “hard” and those that separate more readily are called “soft”. Longer

strands are mixed with a resinous binder and spread in a two dimensional

random fashion to form CSMs. Thus a CSM is made up of random yet evenly

distributed strands chopped from continuous “E” Glass fibres into 50mm

length and bonded with “Emulsion binder”. It possesses excellent surface

bonding efficiency. These mats are suitable for hand lay - up mouldings and

provide nearly equal properties in all directions in the plane of the structure.

Figure 3.2 shows a typical CSM.

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Figure 3.2 CSM 450 E gsm MAT

3.5 FUNCTIONAL RELATIONSHIP OF POLYMER MATRIX

TO REINFORCING FIBRE

The matrix gives form and protection from the external

environment to the fibres. Chemical, thermal, and electrical performance can

be affected by the choice of matrix resin. But the matrix resin does much

more than this. It maintains the position of the fibres. Under loading, the

matrix resin deforms and distributes the stress to the higher modulus fibre

constituents. The matrix should have an elongation at break greater than that

of the fibre. It should not shrink excessively during curing to avoid placing

internal strains on the reinforcing fibres. If designers wish to have materials

with anisotropic properties, then they will use appropriate fibre orientation

and forms of uni-axial fibre placement. Deviations from this practice may be

required to accommodate variable cross section and can be made only within

narrow limits without resorting to the use of shorter axis fibres or by

alternative fibre re-alignment. Both of these design approaches inevitably

reduce the load-carrying capability of the molded part and will probably also

adversely affect its cost effectiveness. On the other hand, in the case of a

complex part, it may be necessary to resort to shorter fibres to reinforce the

molding effectively in three dimensions. In this way, quasi-isotropic

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properties can be achieved in the composite. Fibre orientation also influences

anisotropic behaviour.

3.6 MATRIX RESINS

There are mainly three different types of matrix materials- organic

polymers, ceramics and metals. Thermosetting polymer resins are the type of

matrix material commonly used for civil engineering applications. Polymers

are chain like molecules built up from a series of monomers. The molecular

size of the polymer helps to determine its mechanical properties. Polymeric

matrices have lowest density, hence, produce lightest composite materials. A

major consideration in the selection of matrices is the processing requirement

of the selected material. The most common thermosetting resins used in civil

engineering applications are polyesters, epoxies, and to a lesser degree,

phenolics. ISO and ER have been used in the study. Polyester resins are

relatively inexpensive, and provide adequate resistance to a variety of

environmental factors and chemicals. Epoxies are more expensive but also

have better properties than polyesters. Some of the advantages of epoxies over

polyesters are higher strength, slightly higher modulus, low shrinkage, good

resistance to chemicals, and good adhesion to most fibres.

The matrix resin must have significant levels of fibres within it at

all important load-bearing locations. In the absence of sufficient fibre

reinforcement, the resin matrix may shrink excessively, can crack, or may not

carry the load imposed upon it. Fillers, specifically those with a high aspect

ratio, can be added to the polymer matrix resin to obtain some measure of

reinforcement. However, it is difficult to selectively place fillers. Therefore,

use of fillers can reduce the volume fraction available for the load-bearing

fibres. Another controlling factor is the matrix polymer viscosity.

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3.6.1 Epoxy Resins

ERs are used in advanced applications including aircraft, aerospace,

and defense, as well as many of the first- generation composite reinforcing

concrete products currently available in the market. ERs are available in a

range of viscosities, and will work with a number of curing agents or

hardeners. The nature of epoxy allows it to be manipulated into a partially-

cured or advanced cure state commonly known as a “prepreg”. If the prepreg

also contains the reinforcing fibres the resulting tacky lamina can be

positioned on a mold (or wound if it is in the form of a tape) at room

temperature. ERs are more expensive than commercial polyesters and vinyl

esters.

Although some epoxies harden at temperatures as low as 80oF(30oC), all epoxies require some degree of heated post-cure to achieve

satisfactory high temperature performance. Large parts fabricated with ER exhibit good fidelity to the mold shape and dimensions of the molded part.

ERs can be formulated to achieve very high mechanical properties. However, certain hardeners (particularly amines), as well as the ERs themselves, can be

skin sensitizing, so appropriate personal protective procedures must always be followed. Some epoxies are also more sensitive to moisture and alkali. This

behaviour must be taken into account in determining long term durability and suitability for any given application. Curing time and increased temperature

required to complete cross-linking (polymerisation) depend on the type and amount of hardener used. Some hardeners will work at room temperature.

However, most hardeners require elevated temperatures. Additives called accelerators are sometimes added to the liquid ER to speed up reactions and decrease curing cycle times. The heat resistance of an epoxy is improved if it contains more aromatic rings in its basic molecular chain. If the curing

reaction of ERs is slowed by external means, (i.e., by lowering the reaction temperature) before all the molecules are cross-linked, the resin would be in

47

what is called a B-staged form. In this form, the resin has formed cross-links at widely spaced positions in the reactive mass, but is essentially uncured.

Hardness, tackiness, and the solvent reactivity of these B-staged resins depend on the degree of curing.

3.6.1.1 Hardeners for Epoxy

ERs can be cured at different temperatures ranging from room temperature to elevated temperatures as high as 347oF (175oC). Post curing is

usually done. Epoxy polymer matrix resins are approximately twice as expensive as polyester matrix materials. Compared to polyester resins, ERs

provide the following general performance characteristics:

A range of mechanical and physical properties can be obtained

due to the diversity of input materials

No volatile monomers are emitted during curing and processing

Low shrinkage during cure

Excellent resistance to chemicals and solvents

Good adhesion to a number of fillers, fibres, and sub-strates

3.6.2 Isophthalic Polyesters

Isophthalic polyesters, which use Isophthalic acid as a saturated

acid are premium resins. ISO is a medium viscosity, Unsaturated Polyester

Resin based on Isophthalic acid. It is specially designed for corrosion resistant

applications. It exhibits excellent mechanical properties along with good

chemical resistance. They cost about 20% more than orthos based on current

pricing but have improved corrosion resistance, superior mechanical

properties, and higher heat distortion temperatures. ISO rapidly wets the

surface of glass reinforcements resulting in fast curing and a tack free surface.

It is recommended for moderate chemical resistance applications. At

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moderate temperatures, the resin has good resistance to water, acids (dilute to

medium concentrations), weak bases and good resistance to petroleum

solvents. The FRP components manufactured using it exhibit excellent

hydrolytic stability and resistance to outdoor weather. There is no anhydrate

form of Isophthalic acid since the two acid groups are not on adjacent

carbons.

This requires that the isopolysters be made in two steps, by a so-

called double cook process, because the Isophthalic acid does not react as

quickly as the Maleic anhydride with the glycol. The double cook process has

two advantages that offset the higher lost: the oligomers are more consistent

batch-to-batch with the more uniform distribution of unsaturated

functionality, and they build higher molecular weight, are generally thought to

be responsible for the superior thermal, mechanical and chemical resistance of

isophthalic polyesters. The level of instaurations in the oligomer determines

the cross-link density of the cured resin, which in turn greatly affects the

properties of the resin. Decreasing the cross link density by increasing the

isophthalic acid: maleic anhydride ratio (IPA: MA) results in a reduction in

heat distortion temperature and Young’s modulus and an increase in failure

strain (elongation and break). Higher resin elongation enhances performance

in some application of polyester composites. A notable example is large

diameter pipe liner, which must resist cracking during installation to be

effective corrosion barriers. Also higher strain to failure resins are sometimes

used in structural application. Good co-relation exists between resin tensile

elongation and laminate mechanical properties in glass fibre reinforced

polyesters.

3.7 GEL COAT

Much is required of gel coats, and as a result, their formulation is

complicated. The basic problem is that a gel coat must cure in thin layers.

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This is made difficult by the low resin mass and high mould mass, both of

which minimize exothermic temperature, a situation aggravated by the effect

of air inhibition of the free radical cure mechanism in gel coat. In addition gel

coats must be durable, i.e. must be resistant to cracking and crazing, must not

blister, and must retain colour and gloss after long exposure to UV light

clearly. All these criteria cannot be met indefinitely but it should be realized

that gel coats are remarkable for how well they perform.

3.8 PARTICULATE FILLERS

Particulate fillers are not reinforcements in the sense that stiffness

and strength of the resin are greatly enhanced, but they are widely used in

composite formulations. Typical fillers are the various forms of chalk

(calcium carbonate), silica aerogels, glass ballotini, glass and polymer micro

balloons, and carbon black. Their main function is to modify the matrix resin

and especially to improve the surface finish. Since resins are very expensive,

it will not be cost effective to fill up the voids in a composite matrix purely

with resins. Fillers are added to the resin matrix for controlling material cost

and improving its mechanical and chemical properties. Fillers are added to a

polymer matrix for one or more of the following reasons:

Reduce cost (Since most filler are much less expensive than the

matrix resin)

Increase modulus

Reduce mould shrinkage

Control viscosity

Produce smoother surface

Particulate fillers are not reinforcements in the sense that stiffness

and strength of the resin are greatly enhanced, but they are widely used in

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composite formulations. The three major types of fillers used in the composite

industry are the calcium carbonate (Chalk), kaolin, and alumina trihydrate.

Other common fillers include mica, feldspar, wollastonite, silica, talc, and

glasses. When one or more fillers are added to a properly formulated

composite system, the improved performance includes fire and chemical

resistance, high mechanical strength, and low shrinkage. Other improvements

include toughness as well as high fatigue and creep resistance. Some fillers

cause composites to have lower thermal expansion and exotherm coefficients.

Wollastonite filler improves the composites' toughness for resistance to

impact loading. Aluminum trihydrate improves the fire resistance or

flammability ratings. Some high strength formulations may not contain any

filler because it increases the viscosity of the resin paste. High viscosity resins

may have a problem wetting out completely for composite with heavy fibre

reinforcement.

3.9 CATALYST

There are numerous initiators that can be used to cure polyesters and when considered in combination with various amounts of promoters and

co-promoters, should be realized that cure behaviour can be adjusted over a wide range. Resins can be catalyzed to gel in few minutes or few hours at

room temperature or at elevated temperature. Inhibitors are chemicals whose main function are to increase storage life of resins, and as such are added by

the manufacturer. They apparently work by consuming free radicals, so cure can only proceed after all the inhibitor is depleted. Methyl Ethyl Ketone

Peroxide (MEKP) 50% solutions in pithalate plasticizer selected as catalyst.

3.10 ACCELERATOR

MEKP and the other initiators cannot cure polyester (or) vinyl ester resins without promoters at ambient temperature because they decompose into

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free radicals too slowly. The function of the promoter usually cobalt napthenate (CoNaP) is to decompose the initiator rapidly at room

temperature. The promoter is true catalyst that is, it is not consumed in the curing reactions, and so only a small amount of cobalt salt is needed to

decompose the initiator. It is usually added to the resin as a dioctyl phthalatesolution that is 6% by weight of cobalt. It imparts a slight purple hue to the resin, which turns to brown when the transition state of the cobalt change from Co2+ to Co3+ which occurs when the cobalt complex decompose the

initiator.

3.11 CHALK

It is used as filler in many systems, particularly sheet and bulk

moulding compounds. Its function is to replace part of the resin matrix, reducing thermal and cure shrinkage and thus improving surface finish. These

fillers also reduce the cost as they are cheaper than either the resin or the (glass fibre) reinforcement.

3.12 CALCULATION OF PROPERTIES OF THE COMPOSITE

Table 3.1 presents the various properties obtained for E - Glass

fibre, ER and ISO from the manufacturer. The properties include MOE,

Volume fraction and Poisson's ratio.

Table 3.1 Properties of E-Glass Fibre, ISO and ER

Properties E - Glass Fibre ISO ER

MOE, (in N/ mm²) 72400 3450 5000Volume fraction, V 33.33 % 66.67 % 66.67 % Poisson's ratio, 0.22 0.33 0.30

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The properties of GFRP composites depend on the properties of

material constituents (i.e., reinforcing fibre, matrix) and the corresponding

volume fractions. The following methods are available for the calculation of

material properties of the composite based on the properties of its

constituents.

i) Micromechanics

ii) Simplified composite micromechanics equations (Chamis)

iii) Carpet Plots

iv) Equations given by Tsai - Hahn

The methods (i), (ii) and (iii) can be adopted for E-Glass - ISO

composites and methods (i), (ii) and (iv) are suitable for E-Glass - ER

composites

3.12.1 Micromechanics

Transverse modulus, ET = (Ef Em)/[(Em Vf) + (Ef Vm)] (3.1)

Longitudinal modulus, EL = (Ef Vf) + (Em Vm) (3.2)

Longitudinal Poisson's ratio, LT = (Vf f) + (Vm m) (3.3)

Transverse Poisson's ratio, TL = LT x (ET / EL) (3.4)

Shear modulus, GLT = Gm {[(Gf / Gm) (1 + Vf) + Vm] /

[(Gf / Gm) Vm+ 1 + Vf]} (3.5)

where Gm = Em / [2 (1 + m)]

Gf = Ef / [2 (1 + f)]

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3.12.2 Simplified composite micromechanics

Transverse modulus, ET = (Ef Em)/[Ef - Vf(Ef - Em)] (3.6)

Longitudinal modulus, EL = (Ef Vf) + (Em Vm) (3.7)

Longitudinal Poisson's ratio, LT = (Vf f) + (Vm m) (3.8)

Transverse Poisson's ratio, TL = LT x (ET / EL) (3.9)

Shear modulus, GLT = (Gf Gm)/[Gf - Vf(Gf - Gm)] (3.10)

where, Gm = Em / [2 (1 + m)]

Gf = Ef / [2 (1 + f)]

3.12.3 Carpet Plots

Figure 3.3 Ratio plots for Ex Figure 3.4 Ratio plots for Ey

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Figure 3.5 Ratio plots for Gxy

Figure 3.6 Ratio plots for xy

Figure 3.7 Carpet plots for Figure 3.8 Carpet plots for laminate properties (Ex) laminate properties (Ey)

Figure 3.9 Carpet plots for Figure 3.10 Carpet plots for

laminate properties (Gxy

) laminate properties (xy

)

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Using Carpet plots given by Davalos et al (for WR + ISO resin)

From Figure 3.3 (Ex)33.33 % / (Ex) 50% = 0.7

From Figure 3.4, (Ey)33.33 % / (Ey) 50% = 0.7

From Figure 3.5, (Gxy)33.33 % / (Gxy) 50% = 0.7

From Figure 3.6, ( xy)33.33 % / ( xy) 50% = 1.02

From carpet plot, Figure 3.7 (Ex) 50% = 5.55E+06 psi

= 38.28 GPa

From carpet plot, Figure 3.8 (Ey) 50% = 1.60E+06 psi

= 11.03 GPa

From carpet plot, Figure 3.9 (Gxy) 50% = 6.30E+05 psi

= 4.35 GPa

From carpet plot, Figure 3.10 ( xy) 50% = 0.29

(Ex)33.33 % = 0.7 × (Ex) 50% = 3885000 psi = 26.71 GPa

(Ey)33.33 % = 0.7 × (Ey) 50% = 1120000 psi = 7.70 GPa

(Gxy)33.33 % = 0.7 × (Gxy) 50% = 441000 psi = 3.03 GPa

xy)33.33 % = 1.02 × ( xy) 50% = 0.296

3.12.4 Tsai Hahn’s Equations

Transverse modulus, ET = (Vf + T1Vm) Ef Em/[Em Vf + T1Vm Ef ]

T1 = 0.516 for ER and E-Glass (3.11)

Longitudinal modulus, EL = (Ef Vf) + (Em Vm) (3.12)

Longitudinal Poisson's ratio, LT = (Vf f) + (Vm m) (3.13)

Transverse Poisson's ratio, TL = LT x (ET / EL) (3.14)

Shear modulus, GLT = (Vf + T2Vm) Gf Gm/

[Gm Vf + T2Vm Gf ] (3.15)

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Where, Gm = Em / [2 (1 + m)]

Gf = Ef / [2 (1 + f)]

T2 = 0.316 for ER and E-Glass

Summary of the properties of the composite calculated by various

methods is given in Tables 3.2 and 3.3.

Table 3.2 Material Properties of the E-Glass - Isophthalic Polyester Composite

Property MicromechanicsSimplified composite

micromechanics

Carpet plots (WR)

Carpet plots

(CSM)

Ex (in GPa) 26.433 26.433 26.71 14.92

Ey (in GPa) 5.055 7.664 7.70 14.82

Gxy (in GPa)

2.44 2.90 3.03 5.29

xy 0.293 0.293 0.296 0.41

yx 0.056 0.085 0.085 0.41

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Table 3.3 Material Properties of the E-Glass - Epoxy Composite

Property Micromechanics Simplified composite Micromechanics

Tsai Hahn’s Equation

Ex (in GPa)

27.467 27.467 27.467

Ey (in GPa)

7.250 10.810 6.49

Gxy (in GPa)

3.44 4.09 4.40

xy 0.293 0.293 0.293

yx 0.077 0.115 0.050

3.13 CONCLUDING REMARKS

ER and ISO are chosen as resin WR and CSM are chosen as matrix

for the present study. The appropriate properties have been obtained by using

four popular methods, namely (i) Micromechanics, (ii) Simplified composite

micromechanics, (iii) Carpet plots and (iv) Equations given by Tsai - Hahn.

The properties are tabulated in 3.1, 3.2 and 3.3 are used for analytical

evaluation.