CHAPTER 3 PROPERTIES OF GFRP MATERIALS

Transcription

1 39 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

2 40 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) iii) Aramid 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

3 41 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 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

4 42 Assembled Rovings Direct Roving and WR. Among these forms, the present study deals with CSM (Emulsion) Woven Rovings Woven clothes and rovings are very widely used in the manufacture of laminated structures. A simple plain weave WR allows a V f 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.

5 43 Figure 3.1 Woven rovings 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.

6 44 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

7 45 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.

8 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 partiallycured 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 80 o F (30 o C), 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

9 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 Hardeners for Epoxy ERs can be cured at different temperatures ranging from room temperature to elevated temperatures as high as 347 o F (175 o C). 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 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

10 48 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 socalled 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.

11 49 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

12 50 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 ACCELERATOR MEKP and the other initiators cannot cure polyester (or) vinyl ester resins without promoters at ambient temperature because they decompose into

13 51 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 phthalate solution 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 Co 2+ to Co 3+ which occurs when the cobalt complex decompose the initiator 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 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 MOE, (in N/ mm²) Volume fraction, V % % % Poisson's ratio, ER

14 52 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 Micromechanics Transverse modulus, E T = (E f E m )/[(E m V f ) + (E f V m )] (3.1) Longitudinal modulus, E L = (E f V f ) + (E m V m ) (3.2) Longitudinal Poisson's ratio, LT = (V f f ) + (V m m ) (3.3) Transverse Poisson's ratio, TL = LT x (E T / E L ) (3.4) Shear modulus, G LT = G m {[(G f / G m ) (1 + V f ) + V m ] / [(G f / G m ) V m V f ]} (3.5) where G m = E m / [2 (1 + m)] G f = E f / [2 (1 + f)]

15 Simplified composite micromechanics Transverse modulus, E T = (E f E m )/[E f - V f (E f - E m )] (3.6) Longitudinal modulus, E L = (E f V f ) + (E m V m ) (3.7) Longitudinal Poisson's ratio, LT = (V f f ) + (V m m ) (3.8) Transverse Poisson's ratio, TL = LT x (E T / E L ) (3.9) Shear modulus, G LT = (G f G m )/[G f - V f (G f - G m )] (3.10) where, G m = E m / [2 (1 + m)] G f = E f / [2 (1 + f)] Carpet Plots Figure 3.3 Ratio plots for E x Figure 3.4 Ratio plots for E y

16 54 Figure 3.5 Ratio plots for G xy Figure 3.6 Ratio plots for xy Figure 3.7 Carpet plots for Figure 3.8 Carpet plots for laminate properties (E x ) laminate properties (E y ) Figure 3.9 Carpet plots for Figure 3.10 Carpet plots for laminate properties (G xy ) laminate properties ( xy )

17 55 Using Carpet plots given by Davalos et al (for WR + ISO resin) From Figure 3.3 (E x ) % / (E x ) 50% = 0.7 From Figure 3.4, (E y ) % / (E y ) 50% = 0.7 From Figure 3.5, (G xy ) % / (G xy ) 50% = 0.7 From Figure 3.6, ( xy ) % / ( xy ) 50% = 1.02 From carpet plot, Figure 3.7 (E x ) 50% = 5.55E+06 psi = GPa From carpet plot, Figure 3.8 (E y ) 50% = 1.60E+06 psi = GPa From carpet plot, Figure 3.9 (G xy ) 50% = 6.30E+05 psi = 4.35 GPa From carpet plot, Figure 3.10 ( xy ) 50% = 0.29 (E x ) % = 0.7 (E x ) 50% = psi = GPa (E y ) % = 0.7 (E y ) 50% = psi = 7.70 GPa (G xy ) % = 0.7 (G xy ) 50% = psi = 3.03 GPa xy) % = 1.02 ( xy ) 50% = Tsai Hahn s Equations Transverse modulus, E T = (V f + T1V m ) E f E m /[E m V f + T1V m E f ] T1 = for ER and E-Glass (3.11) Longitudinal modulus, E L = (E f V f ) + (E m V m ) (3.12) Longitudinal Poisson's ratio, LT = (V f f ) + (V m m ) (3.13) Transverse Poisson's ratio, TL = LT x (E T / E L ) (3.14) Shear modulus, G LT = (V f + T2V m ) G f G m / [G m V f + T2V m G f ] (3.15)

18 56 Where, G m = E m / [2 (1 + m)] G f = E f / [2 (1 + f)] T2 = 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 Micromechanics Simplified composite micromechanics Carpet plots (WR) Carpet plots (CSM) E x GPa) E y GPa) (in (in Gxy (in GPa) xy yx

19 57 Table 3.3 Material Properties of the E-Glass - Epoxy Composite Property Micromechanics Simplified composite Micromechanics E x (in GPa) E y (in GPa) Gxy (in GPa) Tsai Hahn s Equation xy yx 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.