Characterization of Glass Fiber Reinforced Polymer Materials GF… · Fiber-reinforced polymers are...

Mechanical Engineering Department Materials Engineering 2 Laboratory

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Characterization of Glass Fiber Reinforced Polymer Materials 1

Content 1 Introduction ............................................................................................................................................. 3

2 Reinforcement Materials ......................................................................................................................... 4

2.1 Fiber types ........................................................................................................................................ 4

2.2 Fiber properties ................................................................................................................................ 5

2.3 Fiber forms ....................................................................................................................................... 6

2.4 Hybrids reinforcements .................................................................................................................... 7

3 Matrix Materials ....................................................................................................................................... 8

3.1 Polyesters ......................................................................................................................................... 8

3.2 Vinyl-esters ....................................................................................................................................... 8

3.3 Epoxies .............................................................................................................................................. 8

3.4 Phenolics ........................................................................................................................................... 9

3.5 Polyimides ........................................................................................................................................ 9

3.6 Special Forms .................................................................................................................................... 9

4 Materials Processes ................................................................................................................................ 10

4.1 Hand lay-up or contact molding ..................................................................................................... 10

4.2 Spray-up or spray molding ............................................................................................................. 11

4.3 Press molding ................................................................................................................................. 11

4.4 Vacuum-bag and autoclave molding .............................................................................................. 12

4.5 Resin transfer molding ................................................................................................................... 13

4.6 Injection molding ............................................................................................................................ 13

4.7 Filament winding ............................................................................................................................ 13

4.8 Pultrusion ....................................................................................................................................... 14

5 Mechanical Behavior of Fiber Reinforced Composite Materials ........................................................... 15

5.1 Young’s modulus ............................................................................................................................ 16

5.2 Tensile strength and the critical fiber length ................................................................................. 16

1 Adapted from: Edwards K.L.; An overview of the technology of fiber-reinforced plastics for design purposes, Materials and Design 19 (1998) 1-10; Ashby, M.F. & Jones, D:R:H., Engineering Materials 2: An Introduction to Microstructures, Processing and Design, Second Edition, Butterworth-Heinemann, 1998; and other sources.


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5.3 Compressive strength ..................................................................................................................... 18

5.4 Toughness ....................................................................................................................................... 18

5.5 Other considerations ...................................................................................................................... 19


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A composite material is a physical mixture of different components which interact in such a way that they alleviate each other’s shortcomings. It is this mutually enhancing support which gives composites their superior properties, as well as the superior property combinations2.

Composites materials based on fiber-reinforced polymers are recognized as possessing superior specific properties when compared to conventional engineering materials. However, the widespread use of these materials is still limited, with engineering designers choosing to stick with what they know best. This unfortunately tends to stifle innovation and leads to sub-optimal solutions. This article provides a simplified introduction to the technology of fiber-reinforced thermosetting plastics from the perspective of the information requirements of engineering designers. The use of detailed descriptions has been kept to a minimum and a concentration on the most regularly used material types, properties and manufacturing processes. The aim is to help engineering designers, who are not familiar with this technology, obtain a reasonable understanding and sufficient knowledge to pursue relevant topics further.

1 Introduction Traditionally, fiber-reinforced polymers were, and still are, used for decorative and lightly loaded structures. A typical general application might be a metal space frame clad with fiber-reinforced panels. Here the major loads are carried by the metal frame, the panels fill the interstices and provide a functional surface. Fiber-reinforced polymers are now regularly used in stress critical applications, as more and more designers are realizing the high specific strength and stiffness properties available from these materials. Unfortunately, this material substitution is not without drawbacks. In general the raw material costs are higher than for metals, but the processing costs are generally lower. Therefore, the material-processing package has to be taken into consideration when choosing these materials.

An obvious advantage of fiber-reinforced polymers over metals is the shape potential, particularly the ability to produce large double curvature geometry. Complex parts in metals often necessitate the use of multiple parts and processes including joining. Using fiber-reinforced plastics, the shape potential is virtually limitless and part consolidation reduces assembly time and costs. The most pronounced advantage of fiber-reinforced polymers over metals is the ability to tailor the material to a given application leading to efficient material utilization. However, the material is simultaneously processed at the component manufacturing stage. It is therefore important that the component is designed for the process. This is a major source of difficulty for designers new to these materials. A good working knowledge of the materials and processes is therefore essential to be an effective designer.

In addition to the considerations normally made when designing with conventional engineering materials, the designer using fiber-reinforced polymers has to also consider the selection of constituents, i.e. proportions, types, distribution and orientation, depending on the properties required and the selection of processes, the shape and production requirements.

This document provides sufficient guidelines and information for designers who are unfamiliar with these materials. A more detailed treatment of the various aspects of the subject can be found in reference material and by consulting material suppliers’ literature. It is recommended that the guidance of these suppliers and advice from experts in the field be sought as early as possible.

2 Prof. Ferd McGary, MIT, 1987.


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2 Reinforcement Materials There are a variety of reinforcing agents for fiber-reinforced polymers (FRP), such as long and short fibers, and whiskers. Fibers, especially long and continuous forms, provide the stiffest and strongest materials and it is for this reason that they are also the most common method of reinforcing FRPs3. The introduction of fibers into the matrix induces directionality or anisotropy in the material. The properties of the FRP are therefore highly dependent on the alignment of the fibers.

The variables that have a major influence on the properties of FRPs are:

fiber type;

alignment of the fiber;

distribution of the fiber;

fiber-matrix interface;

size and shape of the fiber; and

loading direction.

2.1 Fiber types

The most common type of fiber material used to reinforce FRPs is glass. The other regularly used fiber materials are aramid, carbon and in small quantities of boron.

Glass fiber

Glass fiber is a mixture of oxides. The principal oxide is silica with varying smaller amounts of calcium, aluminum, sodium, boron, and iron. There are several commercial grades of glass fiber of which the following are the most common:

E-glass, good electrical insulator and high strength.

C-glass, good chemical corrosion resistance.

S-glass, high silica content with high temperature performance, high strength and stiffness (military grade).

R-glass, civil version of S-glass.

A-glass, high alkali content for chemical resistance.

D-glass, low dielectric and low density.

L-glass, high lead content for radiation protection.

By far the most widely used grade is E-glass which accounts for almost 95% of total glass fiber production, but only a small amount of this fiber is used for electrical purposes.

Aramid fiber

This is the generic name for aromatic polyamide fibers, a class of synthetic organic fibers. A high modulus aromatic fiber is produced commercially by DuPont under the trade name Kevlar. The chemical structure of Kevlar fiber consists of oriented para-substituted aromatic units, a rigid rod-like structure, produced by melt-spinning from liquid polymer solutions. Kevlar has a crystalline structure with strong covalent bonding in the fiber direction and relatively weaker hydrogen bonding in the fiber transverse direction. The anisotropic nature of the fiber causes Kevlar to have poor compressive properties. Under compressive loads the fiber develops kink bands which eventually lead to ductile failure.

Kevlar fibers are available in three main types:

Kevlar: used for mechanical rubber goods such as tires.

Kevlar 29: used for cables, ropes, coated fabrics and ballistic protection.

3 Other reinforcement agents, such as flakes and particles are normally classified as ‘fillers’.


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Kevlar 49: used for reinforcement of rigid polymer matrices in aerospace, automotive and marine structures.

Carbon fiber

There are three main types of carbon fiber:

Type I; high modulus

Type II: high strength

Type III: general purpose

Carbon fiber is produced by the controlled pyrolysis of a precursor. The most commonly used precursors are pitch, rayon and poly-acrylo-nitrile (PAN). The successive stages of pyrolysis are as follows:

1) Oxidation: heating in an oxidizing atmosphere at 200~250⁰ C. 2) Carbonization: heating in a non-oxidizing atmosphere at 1,000⁰ C for high strength fiber. 3) Graphitization: heating in a non-oxidizing atmosphere at 2,500~3,000⁰ C for high modulus fiber.

Therefore by adjusting the process temperature it is possible to produce a range of carbon fibers with different mechanical properties, i.e. strength and stiffness. Furthermore, fibers can be chemically treated to modify the surface characteristics.

Boron fiber

Boron fiber is produced by high temperature reduction of boron trichloride vapor on a tungsten or carbon substrate. The strong, stiff boron fibers are much larger in diameter and denser than glass, aramid and carbon fibers.

Metal fiber

Occasionally metal or metal coated fibers of aluminum, zinc, stainless other steels are incorporated for strength or even for electromagnetic shielding.

2.2 Fiber properties

In general, a material drawn into fiber form becomes much stronger in the drawn direction than in its original form. In the case of glass, the resulting fiber, formed by melt drawing, is isotropic. However, for

Table 1: Typical properties of some popular reinforcing fibers. Fiber Type

Density (Mg/m3)

Strength (GPa)

Modulus (GPa) Strain to Failure (%)

Coef. of Th. Exp. (10-6/K) Long. Trans. Long. Trans.

Glass

E 2.60 2.4 73 73 3.8 5.0 5

R 2.53 3.5 86 86 4.1 4.0 4

Aramid

HM 1.45 3.0 130 5.4 2.1 -2.0 17

LM 1.44 2.8 65 4.3

Graphite

HM1 1.96 1.8 500 5.7 0.4 -1.5 30

HM2 1.80 3.0 300 1.0 -0.2

HT 1.78 3.6 240 15 1.5 -0.5 10

HST 1.75 5.0 240 2.1 -0.1

IM 1.77 4.7 295 1.6

Abbreviations: HM: high modulus; LM: low modulus; HM1, 2: high modulus; HT: high tensile strength; HST: high strain; IM: intermediate type.


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aramid, formed by wet spinning and drawing, and carbon fiber, formed by oxidation and carbonization, the resulting reinforcements are highly anisotropic. As a consequence, the fibers have different properties in the longitudinal (parallel to the fiber) and transverse (perpendicular to the fiber) directions. Also, all these fibers exhibit, in tension, elastic behavior up to the point of failure. There is no yield point and very low strain to failure. Table 1 shows a comparison of mechanical properties of reinforcing fibers used in fiber-reinforced polymers.

2.3 Fiber forms

Fibers can be supplied in a variety of configurations ranging from continuous untwisted fibers to chopped short fibers and weaved configurations.

Roving

Rovings are bundles of twisted yarn or untwisted strand of continuous filaments. The roving is available in a range of weights or ‘tex’; weighed in grams per kilometer.

Chopped strands

These are rovings which have been cut or chopped in to short lengths of between 3 and 50 mm. The fibers are normally mixed with resin and fillers for compression molding. Fiber lengths below 3 mm are available by ‘milling’.

Chopped strand mat (CSM)

This is a sheet of reinforcement material comprised of randomly dispersed chopped fibers usually 25~50 mm in length and held together with a resinous binder. CSM is produced in a variety of widths, lengths and weights.

Continuous fiber-reinforced mat (CFRM)

This is similar to CSM except the fiber is continuous and swirled in a random manner.

Needled mat

CFRM and CSM are sometimes stacked to achieve a desired aerial density, and then mechanically connected in the dry form (before applying the matrix resin) by a process called ‘needling’ or ‘needle-punching’. In this process, thousands of barbed needles penetrate the ply stack, consolidating it and providing a three-dimensional reinforcement. The resulting fiber entanglement imparts integrity to the composite mat.

Woven fabric

Fabric is produced from woven yarns. A ‘plain’ weave is the simplest form with warp and weft4 yarns alternatively passing under and over each other. A balanced weave is one in which the number and count of warp and weft yarns are equal. A satin weave has each warp and weft yarn going over one yarn and under a number of yarns. A five-harness or five-end satin weave, for example, has one warp yarn passing over four weft yarns before passing under the fifth yarn. A twill weave is one in which the warp and weft yarns which pass over each other are varied, recognizable by diagonal lines. A 2x2 twill weave has two over by two under. Compared with plain weave, both satin and twill weaves have better drape characteristics and are therefore more suitable for molding complex curvatures.

Woven fabrics are described by the weave pattern; the number of yarns in warp and weft and the yarn count. Woven fabrics are available in most reinforcement types and also in ‘hybrid’ or mixed fiber form making them very versatile.

4 Warp and weft are the two basic components used in weaving to turn thread or yarn into fabric. The lengthwise or

longitudinal warp yarns are held stationary in tension on a frame or loom while the transverse weft (sometimes woof) is drawn through and inserted over-and-under the warp.


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A range of filament diameter, number of strands and weave style are available. Woven roving is a coarse heavy weight woven fabric produced from strand rovings. This plain weave woven roving is the most common.

Tapes are also available, usually woven in a variety of weave styles and widths, normally less than 150 mm.

Non-crimp fabric

Non-crimp fabric is continuous reinforcement, usually roving, laid in planar form in straight lines usually at some specified angle to the weft direction and is held together with a light weight warp, or linking yarn (knit). The absence of reinforcement loops reduces ‘kink’ stresses and allows the fabric to shear easily and therefore drape into complex shapes. A variety of permutations are available but they can be categorized as unidirectional, biaxial, triaxial, etc.

Non-woven fabric

This is similar to non-crimp fabrics except the reinforcement is held together using an adhesive or binder. Generally, the fabric is unidirectional and is not easy to shape and is usually used as local reinforcement or stiffening and for components having flat or gentle curvature, e.g. panels.

Knitted fabrics

These are produced using conventional textile knitting processes. Using modern computer controlled knitting machines the process is capable of producing a large variety of patterns and shapes at high rates of production. Unfortunately, the reinforcement materials are easily damaged by the process and the high proportion of looping in the resulting fabrics leads to poor mechanical properties.

Braids

These are continuous tubes of material with a woven type structure formed by counter-rotating bobbins. These can subsequently be shaped into three-dimensional preforms. Most braiding has been limited to textile machinery normally used for making ropes and hoses, but with the recent adoption of computer control, increased flexibility and accuracy is provided.

Tissues, felts and veils

These are very light weight, random reinforcement, mats primarily used to improve the surface appearance of mouldings. They are normally placed either between the gelcoat (if used) and the heavier laminating reinforcement, or as the final lamination to block out the fiber pattern of the underlying reinforcement and provide a smooth surface finish. Metallized fiber tissues are sometimes incorporated for electromagentic shielding.

2.4 Hybrids reinforcements

A hybrid is a reinforcement, with two or more different types of fiber or fiber forms to achieve cost and/or performance benefits. There are two distinct categories of hybrid: combination laminates; and mixed fibers.

Combination laminates

In this category, mats of the form previously described are combined either chemically or mechanically to form a combination mat. A regularly used combination mat comprises woven roving and chopped strand mat. Also in this category are laminates with plies of different types of reinforcement.

Mixed fibers

Although this form of reinforcement can be considered a combination form, it is categorized separately because it combines different types of reinforcement material in a single ply. Mixed fibers are usually formed in to woven structures, but unidirectional forms are also available. There are hundreds of different combinations of weave style, reinforcement type and density to choose from. Also, different mixed fiber plies can be combined to form a combination laminate.


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Preforms

Preforms are dry fiber mats, usually CSM or CFRM, that have been performed into a desired shape prior to resin impregnation. The mats are a special performable grade coated with a thin thermoplastic binder which may be in emulsion, film or powder form. The preforming operation is normally carried out in a heated press which melts the thermoplastic binder present on this grade of mat and forms the desired shape. On removal of the heat and pressure the mat retains its formed shape. The now fixed shape preform is removed from the press and taken to another mound for the resin impregnation operation. The preforms have sufficient integrity to be stacked in quantity prior to subsequent molding.

3 Matrix Materials The role of the matrix in a composite material is to maintain the structural integrity by bonding the reinforcement together. The matrix also serves to provide environmental protection for the reinforcement. The choice of matrix material depends on the mechanical, thermal and chemical properties required as well as the processing methods available and permitted cost.

Polymer matrices are classified as either thermoplastic or thermoset, but thermosets are the most common. Thermosets are polymers which undergo a curing reaction or chemical crosslinking, where a resin of relatively low molecular weight is converted in to a material of high molecular weight. The process is irreversible and can take place at either room or elevated temperature. These materials do not re-melt when heated and eventually char and burn. The choice of thermoset matrix is considerable and by far the most regularly used in the industry. The most used generic groups are: polyester, epoxy, phenolic and polyimide resins. Some cured resin properties are shown in Table 2.

3.1 Polyesters

Polyesters are the most widely used thermosetting resins. Polyester is formed by a condensation reaction between a glycol and an unsaturated dibasic acid. The unsaturated resin dissolved in a monomer solvent, usually styrene, is cross-linked (radical copolymerization) by the addition of a catalyst and heat. At room temperature, an accelerator may also be added to speed up the reaction. The reactivity can be further controlled by the use of an inhibitor. The catalysts generally used are organic peroxides with accelerators based on cobalt soaps or tertiary amine. Some resin systems are supplied with the accelerator already included and are called ‘pre-accelerated’ resins. There are also many other ingredients added to impart various properties to the resin, such as thixotropic agents, flexiblizers and shrinkage control additives. The shrinkage of polyesters on curing is typically between 4 and 7% by volume.

3.2 Vinyl-esters

Vinyl-esters are similar to polyesters in that they cure by radical initiated polymerization. They are derived from the reaction of an epoxy resin with acrylic or methacrylic acid, and their properties can be varied by using different epoxy resins. They are generally tougher, have improved chemical resistance and are capable of higher operating temperatures than polyesters. They fall between polyesters and epoxies in terms of performance and cost.

3.3 Epoxies

Epoxies are formed by condensation of epichlorhydrin and polyhydroxy compounds. They range from low viscosity liquids to high melting point solids. Epoxies are normally supplied as a single constituent, the resin, with a second constituent, the hardener or cross-linking agent, has to be added. Typical hardeners are aliphatic amines, aromatic amines and anhydrides. A range of properties is therefore possible by combining different resins and hardeners. Epoxies have higher strength and adhesion to fibers than polyesters. The shrinkage of epoxies on curing is typically between 0.25 and 2% by volume.


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3.4 Phenolics

Phenolics are the oldest thermosetting resins and formed by condensation of phenol and aldehyde. The condensation reaction is usually promoted by heat but can also be initiated using a strong acid catalyst. Phenolic resins have good fire resistance with low smoke and toxic fume emission characteristics, but during cure produce high volatile contents, predominantly water and are unstable at room temperature. The pot life of the resin is therefore generally low and a high proportion of the mass of the hardened resin is made up of water. Also, the resin cannot be pigmented, its color being unstable and ranging from pink to brown. The shrinkage of phenolics on curing is typically between 8 and 10% by volume.

3.5 Polyimides

Polyimides are formed by condensation of an aromatic tetracarboxylic acid dianhydride and an aromatic amine. A subdivision of polyimides are maleimides of which the majority are dysfunctional orbismaleimides. The processing cycles for these materials are long and occur at high temperatures. A post-cure cycle is also necessary. They have the advantage of high operating temperatures with no volatiles generated during cure, but suffer from brittleness.

3.6 Special Forms

Gel coat

Gel coat is a dense, void-free layer of resin on the exterior of moldings to improve the surface finish. It is applied to the mold and cured, or partly cured, prior to applying the main resin. The gel coat can be either a normal resin or a modified resin. The latter are resins specially formulated to give tough, resilient films and are intended only to be used for gel coats. Most gel coats can be pigmented.

Pre-preg

Pre-preg is a thin sheet of partially cured, or ‘B-staged5’ resin containing reinforced fibers. The fibers can be woven or unidirectional. The pre-preg sheet usually comes with a plastic backing sheet or release film which is removed prior to laminating. The resin systems have until recently been thermosetting, predominantly epoxies, but now thermoplastic pre-preg sheeting has become available. Pre-pregs are normally supplied as rolls. The parameters that can typically be specified are: fiber type and grade and whether surface-treated, the resin type and content and the cured ply thickness. The material has a limited shelf life because of the partially cured state and therefore has to be stored at low temperatures in a freezer. The obvious advantage of pre-preg is its excellent quality control but at a much higher price than purchasing the constituent materials separately. Processing of pre-preg materials is normally by hand lay-up, vacuum bag and autoclave molding.

Sheet Molding Compound and Bulk Molding Compound

Sheet Molding Compound (SMC) is a form of prepreg. SMC comprises a mixture of chopped glass fibers, catalyzed polyester resin and mineral filler, e.g. calcium carbonate. Typical proportions might be: resin 35%, filler 40%, and glass fiber 25%. Processing is normally by compression molding in a matched mold.

Bulk Molding Compound (BMC) is similar except the material is presented in a dough like form. The fibers are usually much shorter than with SMC and processing is by compression molding or transfer molding. The properties obtained with BMC are inferior to SMC but the material is capable of being processed into more complicated shapes.

5 A “b-stage” resin is a blend solid resins in a solvent carrier, preparing a homogeneous liquid slurry that can be

coated on a surface. Most of the excess solvent is then evaporated at a temperature below the activation point of polymerization.


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Table 2: Typical cured properties of some popular thermoset resins.

Property Polyester Vinyl Ester Epoxy

Density (Mg/ m3) 1.10-1.46 1.15 1.11-1.40

Hardness (HRM) 70-115 80-110

Tensile Strength (MPa) 42-91 73 28-91

Tensile Modulus (GPa) 2.0-4.5 3.5 2.4

Flexural Strength (MPa) 103 80

Flexural Modulus (GPa) 4.2 3.5

Strain to Failure (%) 1.25 4-7

Coeff. of Thermal Exp. 10-6/K 9.9-18.0 53 8.1-11.7

Heat Distortion Temp. (⁰ C) 100-125 110-150

4 Materials Processes There are a wide variety of FRP processes. The choice of process is dependent on many factors, such as type of reinforcement and matrix materials, size, shape, quantity and cost. There are many specialized processes available, but only the most commonly used commercial processes are briefly described here.

4.1 Hand lay-up or contact molding

The hand lay-up method, sometimes called contact molding, is simple and versatile and the most widely used FRP process. In its simplest form, the process consists of applying layers of reinforcing material against a single sided mold and working resin into the material with a brush and roller. After a suitable period of time to allow the resin to cure, the molding, with one smooth surface, is removed from the mold and trimmed to size. The process is slow and highly labor intensive. When the quantity of moldings is sufficiently large the resin and reinforcement material can be simultaneously applied by spraying on to the surface of the mold. After consolidating with a roller and curing, the molding can be removed and trimmed as before.

Hand lay-up, because of the high labor content and the dependence on the skill of the laminator has the inherent problem of variability in the finished moldings. However, the size and complexity of the moldings that can be produced by this method are endless.

Hand lay-up can be divided into two categories: wet laminating; and dry or pre-preg laminating.

Wet laminating

Using this method wet resin is applied to dry reinforcement by the methods described above. Even with high skill, it is difficult to accurately control the resin-to-fiber ratio. The process though is inexpensive and the materials have a long shelf-life, only being mixed prior to processing. As most systems are cured at ambient temperatures and pressures only basic tooling is required. Figure 16 shows a typical application of the wet laminating process.

6 Courtesy: FilaPly, http://www.filaply.com/wet-lay-up.html

Figure 1: Manual wet layup of GFRP5.


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Dry or pre-preg laminating

By this method the reinforcement is already impregnated with a resin which is partially cured (pre-preg). It is normal to consolidate the lay-up with external pressure by vacuum bagging and/or autoclave techniques described later. The advantage of pre-pregs is the consistency of the material. There is more accurate and repeatable control of the resin-to-fiber ratio, but at a much higher cost than wet-laminating. Most pre-preg systems require elevated temperature and pressure curing with molds capable of withstanding these conditions. The pre-preg systems also have to be

stored at low temperatures in freezers with a limited shelf life, usually 6~9 months. Figure 2 depicts a typical carbon-epoxy per-preg application7.

4.2 Spray-up or spray molding

In this semi-mechanized process, the operator manipulates a chopper/spray gun to dispense fibers and resin on to the mold. The gun simultaneously chops rovings and pumps resin producing a combined spray of GFRP. It is difficult to control the deposition of material accurately and this leads to local variations in fiber fraction and molding thickness. However, the material deposition rates are high compared to conventional hand lay-up, therefore, larger products can be produced consistently (see Figure 38).

4.3 Press molding

This process is operated at low (ambient) or high temperatures under pressure. There are a variety of different techniques used. The process provides moldings with two smooth surfaces as opposed to only one good surface by contact molding. The cycle times in the mold tend to be low, typically 10~100 min. There are several variants of which the following typify the range.

Leaky mold technique

Using this method a flat metal mold is manufactured to allow excess resin to escape. When the pressure is applied, the two mold halves come together to a pre-determined pressure or to depth stops to achieve the desired fiber volume fraction in the material. Both wet lay-up and pre-preg resin systems are used in this process.

7 Courtesy: Easy Composites. https://www.easycomposites.co.uk/#!/prepreg/tooling-prepregs/xpreg-xt135-carbon-

fibre-tooling-prepreg-backing.html 8 Courtesy: Boat Building: Basic Construction of Resin, Fiberglass, and Cores,

https://www.boats.com/resources/boat-building-construction-resin-fiberglass-cores

Figure 3: Wet spryup of GFRP7.

Figure 2: Manual pre-preg layup of CFRP6.


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Matched-die technique

This technique makes use of matched male and female molds to manufacture shaped components with minimal leakage of resin. Depending on the operating pressure and temperature and quantity of parts to be produced, the molds can be made from composite materials or metal. Both wet lay-up and pre-preg resin systems are used in this technique. The technique can be broadly divided into two categories: flash molding in which excess resin and air are squeezed out into a flash groove, and net or positive molding in which no resin escapes. In the latter, the quantity of materials must be accurately determined and a means of venting air and volatiles must be provided.

The materials used for compression molding range from wet impregnated mat or fabric to pre-impregnated materials. The latter is used for higher volumes of production with heated matched metal tools. There are basically two types of pre-impregnated material used: sheet molding compound (SMC) and bulk molding compound (BMC). SMC is a mixture of randomly oriented fibers, usually 25~50 mm in length, combined with resin and filler in sheet form. BMC is a mixture of short fibers, usually 6 mm in length, combined with resin and filler in dough form. A variety of agents are also added to each type to impart various properties.

A major problem with the process is the induced fiber orientation as the material flows under heat and pressure during mold closure (see Figure 49). This leads to shear flow and ‘weld’ lines where distinct boundaries are formed in the material and no fibers cross.

4.4 Vacuum-bag and autoclave molding

This technique is used on moldings which are large or of complex shape. The mold is either male or female in configuration. The technique is usually used with pre-preg resin systems although there are techniques for wet resin systems. Consolidation is achieved by covering the molding with an airtight membrane (see Figure 510) or ‘bag’ from which the air is subsequently removed with a vacuum pump providing up to one atmosphere of pressure. If pressures higher than 1 atm are required the whole assembly is placed inside an autoclave or heated pressure vessel. The cycle times tend to be long, often several hours and the labor content is high. The process is highly dependent on the skill of the laminator.

9 Courtesy: Lu, Eason, SMC mold trial, https://www.youtube.com/watch?v=nwWjUntMlEc 10 Courtesy: EpoxyCraft, https://www.epoxycraft.com/how-to-repair-laminates-using-epoxy-resins/

Figure 4: SMC part shape forces fiber orientation as the material flows under heat

and pressure during mold closure8.

Figure 5: Vacuum bag molding process9.


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4.5 Resin transfer molding

In this process, dry reinforcement is placed between two matched mold halves. In its simplest form, resin is injected into the cavity and through the reinforcement which is trapped or pinched at the edges of the

mold. The effect of the pinching is to provide a path for air to escape but block the passage of resin. There are variations to this which lead to more accurate moldings with less material wastage. Removal of air is achieved by the use of a vacuum or by vapor purge. The resin is injected into the mold by gravity, pressure-pot or by positive displacement pump. Both cold and hot resin systems are used, the latter providing more rapid curing and control of viscosity. The process has the advantage of accurate fiber management with reproducible moldings. With automation, the process is capable of reproducible quality and high volume production with cycle times down to minutes using reactive resin

systems (see Figure 611).

4.6 Injection molding

This process is similar to injection molding of thermoplastics. The process uses a matched metal tool which is closed prior to injecting the material, usually BMC. The heated matched tools are held in a press and the material is fed by a screw. The screw itself is fed by a stuffing box. Degradation of the material is a major drawback of the process. A variation of the process is ‘transfer molding’ in which a single charge of material is introduced in to the tool cavity by a ram. The flow of the material in both processes leads to fiber orientation in the moldings resulting in similar problems to compression molding.

4.7 Filament winding

Filament winding is a mechanized process. In its simplest form, the winding machine is similar to a lathe, the payout eye replacing the cutting tool and a cylindrical the workpiece. As the mandrel rotates, resin impregnated filaments are pulled on to its surface via the payout eye which is mechanically traversed parallel to the mandrel axis producing a helical pattern. On completion of the winding process, the resin is cured and the mandrel removed. Modern winding machines are multi-axis and computer controlled, being capable of winding non-axisymmetric and branched shapes. Also, because of the complexity of generating data for winding patterns quickly and accurately they are also being interfaced to sophisticated CAD/CAE/CAM systems. The process produces accurate and

11 Courtesy: Orenco Composites, http://orencocomposites.com/processes/rtm/

Figure 6: Resin transfer molding process10.

Figure 7: Wet filament winding process10.


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reproducible parts but is relatively slow; cycle times for machine utilization are fairly low (see Figure 712). Specialized machinery, however, can produce continuous cross-sectional pipes at high rates of production.

Braiding is a special form of filament winding in which several counter-rotating bobbins simultaneously orbit a stationary mandrel Figure 813 shows such a process which produces much faster products with significantly enhanced properties. The resulting lay-up is a dry ‘basket-weave’ construction which has to be followed by a resin impregnation process such as RTM. The process produces similar shapes to filament winding but at higher rates of production.

Mandrel wrapping involves the use of a mandrel on to which a pre-preg material is wrapped, then

cured and consolidated by heat and pressure. The process is limited to simple tubular shaped components, but at a much lower cost than filament winding. Special purpose-built equipment is available for semi-automated production. The ‘rolling-table’ is most commonly used, consisting of two platens, one of which moves horizontally relative to the other. The pre-preg is cut to shape, depending on the tube profile and lay-up and laid on the bottom platen. The pre-preg leading edge is attached lightly to the mandrel and rolled between the plates under controlled pressure to give optimum consolidation without disturbing the fiber orientation. Final consolidation is achieved by wrapping with a shrink film, which is best applied automatically to ensure an even tension. The film shrinks during the oven cure cycle, flowing the resin and compacting the rolled pre-preg layers. When the cure cycle is complete, the shrink film is removed and the mandrel extracted leaving a FRP tube, with molded interior and exterior surfaces.

4.8 Pultrusion

Pultrusion is another mechanized process for producing continuous long sections. The process consists of pulling impregnated filaments together with mat or fabric through a heated die. The timing of the process is arranged such that as the material leaves the die it is sufficiently cured to enable it to be gripped by a ‘haul-off’ mechanism. In theory the process could run indefinitely provided sufficient raw materials can be supplied. In fact, a small diameter rod is coiled several kilometers long directly on to drums. Normal sections are automatically cut to length. Both solid and hollow sections can be produced with a quite complex cross-sectional shape. The process is capable of producing consistent quality at high rates of production (see Figure 914).

There are a couple of special variants of the basic pultrusion process that are worth noting. Pulforming extends the capability of pultrusion, allowing curved shapes to be formed with variable cross-section, provided the cross-sectional area remains constant. Pulwinding is a combination of pultrusion and filament-winding, allowing helical and hoop filaments to be incorporated in the pultruded section.

12 Source: Don Boonking, https://www.youtube.com/watch?v=HPBmbkyi9Tw 13 Braley, M. & Dingeldein, M., Advancements in Braided Materials Technology, A&P Technology, Inc., Cincinnati,

Ohio, xxxx. 14 Courtesy: Transmission Innovations Inc, http://blog.transmissioninnovations.com/how-frp-cross-arms-are-made-

the-pultrusion-process/

Figure 8: Braided filament wilding process11.


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Figure 9: Pultrusion line and typical product14.

5 Mechanical Behavior of Fiber Reinforced Composite Materials In the context of FRPs, the material consists of high modulus reinforcing fibers embedded in a low modulus polymeric matrix. The combination of the two distinct phases allows load to be transferred between fibers due to elasticity in the matrix. The matrix also serves to separate and protect the fibers. The stiffness and strength of a FRP depends on several characteristics and combinations of these. Issues to consider include the following:

Proportions of fiber and resin;

distribution and orientation of fiber;

type of fiber;

type of resin;

length of fiber for discontinuous fiber ; and

void content.

FRP materials are non-homogeneous and anisotropic, therefore the properties will vary throughout the structure. In a similar way to the reinforcing fibers, the behavior of a FRP under load is elastic up to the point of failure with no yield point or plastic behavior. The strain to failure is also low with a correspondingly small amount of work done. This lack of yielding makes the material very ‘notch’ or stress concentration sensitive. Therefore, localized stresses have to be critically analyzed.

The mechanical properties of a FRP depends on the properties of the constituent materials; the reinforcement and matrix, in particular the quantity and orientation of the fiber. The fiber orientation ranges from random as in chopped strand mat (CSM), through bi-directional as in woven fabric, to unidirectional. For random orientation and it is assumed ‘in-plane’ only, a FRP has equal properties in all directions (pseudo isotropic), for bi-directional orientation, a FRP has equal properties in the two directions and for unidirectional orientation, the properties are greatest parallel to the fiber. If the same amount of fiber is used in each case, the unidirectional FRP will have the highest mechanical properties and the random FRP the lowest. However, the properties of a unidirectional FRP are only superior in the direction parallel to the fibers; just a small angle away from the fiber-axis and the mechanical properties ‘drop-off’ considerably to very low values. The transverse properties of the FRP, perpendicular to the fiber direction, are close to that of the base resin.

Fiber volume fraction is another very important FRP property controlling factor. Typical fiber volume fractions obtainable are the following:

unidirectional, 50~70%;

bi-directional, 30~55%; and

random, 15~35%.


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The properties of the composite can be predicted approximately using the available constituent properties by a rule-of-mixtures. This simple mathematical model can be used to predict the tensile modulus, density and Poisson’s ratio. However, the model is a poor predictor of tensile strength, but does provide a maximum value. The reason for this is the matrix normally behaves in a non-linear manner and fails at a different level of stress to the fiber.

5.1 Young’s modulus

Assuming there is a perfect bond between the fiber and the matrix, then by the rule-of-mixtures, the tensile modulus parallel to the fiber direction, for example, is

1f f m fE E v A E v (1)

where,

E==composite modulus in the direction parallel to the fibers;

Ef=fiber modulus;

Em=matrix modulus;

vf=fiber volume fraction; f

A=fiber efficiency factor.

=1 for unidirectional,

=1/2 for bi-directional,

=3/8 for random.

1

1f f

f m

v vE A

E E

(2)

where,

E=composite modulus in the direction perpendicular to the fibers.

The moduli parallel and perpendicular moduli for a composite with, say, 50% of fibers, differ greatly: a uniaxial composite (one in which all the fibers are aligned in one direction) is exceedingly anisotropic. By using a cross-weave of fibers the moduli in the 0 and 90° directions can be made equal, but those at 45° are still very low. Approximate isotropy can be restored by laminating sheets, rotated through 45°, to give a plywood-like fiber laminate.

5.2 Tensile strength and the critical fiber length

Many fibrous composites are made of strong, brittle fibers in a more ductile polymeric matrix. Then the strain-stress curve looks like the heavy line in Figure 10. The strain-stress curve is linear, with slope E= (eq. 1) until the matrix yields. From there on, most of the extra load is carried by the fibers which continue to stretch elastically until they fracture. When they do, the stress drops to the yield strength of the matrix (though not as sharply as the figure shows because the fibers do not all break at once). When the matrix fractures, the composite fails completely.

In any structural application it is the peak stress which matters. At the peak, the fibers are just on the point of breaking and the matrix has yielded, so the tensile strength

Figure 10: The strain-stress curve of a continuous fiber composite.

Strain (ε)

Stre

ss (

σ)

Fibers Fracture

Matrix FractureMatrix

Yielding


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(σTS) is given by the yield strength of the matrix, σmy, and the fracture strength of the fibres, σf

f, can be combined using a rule of mixtures in the following way:

1f m

TS f f f yv v (3)

This is shown as the line rising to the right in Figure 11. Once the fibers have fractured, the strength rises to a second maximum determined by the fracture strength of the matrix, as:

* 1 m

TS f fv (4)

Where σ*TS is the remnant tensile strength; and

σmf is the failure strength of the matrix.

Figure 11: The variation of peak strength Figure 12: Model of the load transfer with volume fraction of fibers. from the matrix to the fiber in a unidirectional chopped reinforcement material.

Figure 11 shows that adding too few fibers does more harm than good: a critical volume fraction vfcritical of fibers must be exceeded to give an increase in tensile strength. If there are too few, they fracture before the peak is reached and the ultimate strength of the material is reduced.

For many applications (e.g. body pressings), it is inconvenient to use continuous fibers. It is a remarkable feature of these materials that chopped fiber composites (convenient for molding operations) are nearly as strong as those with continuous fibers, provided the fiber length exceeds a critical value.

Consider the peak stress that can be carried by a chopped-fiber composite for which the yield strength in shear of the matrix can be expressed as:

1

2

m m

s y (5)

Furthermore, consider a cylindrical fiber of finite length immersed in such a matrix, as can be seen in Figure 12. The peak force transferred between fiber and matrix could be expressed as:

m

sF d x (6)

The force on the fiber thus increases from zero at its end to a given value at a distance x from the end:

Volume Fraction of Fibers (vf)

Stre

ngt

h (

σ)

100%0% vf critical

Distance along Fiber (x)

Axi

al F

orc

e o

n F

iber

(F)

½ 1

Fiber

Matrix

d

0


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0

xm m

s sF d dx F d x (7)

The force which will just break the fiber is:

2

4

f

c f

dF

(8)

Equating these two forces, we find that the fiber will break at a distance from its end as follows:

4

f

f

c m

s

dx

(9)

If the fiber length is less than 2xc, the fibers do not break – but nor do they carry as much load as they could. If they are much longer than 2xc, then nothing is gained by the extra length. The optimum strength (and the most effective use of the fibers) is obtained by chopping them to the length 2xc in the first place. The average stress carried by a fiber is then simply σf

f/2 and the peak strength (by the argument developed earlier) reduces to:

12

f

f f m

TS f y

vv

(10)

This is more than one-half of the strength of the continuous-fiber material (eq. 3), or it is if all the fibers are aligned along the loading direction. That, of course, will not be true in a chopped-fiber composite. In a car body, for instance, the fibers are randomly oriented in the plane of the panel. Then only a fraction of them – about ¼ – are aligned so that much tensile force is transferred to them, and the contributions of the fibers to the stiffness and strength are correspondingly reduced.

5.3 Compressive strength

The compressive strength of composites is less than that in tension. This is because the fibers buckle or, more precisely, they kink – a sort of co-operative buckling, shown in Figure 13. So while brittle ceramics are best in compression, composites are best in tension.

5.4 Toughness

The toughness Gc of a composite (like that of any other material) is a measure of the energy absorbed per unit crack area. If the crack simply propagated straight through the matrix (toughness Gm

c) and fibers (toughness Gf

c), we might expect a simple rule-of-mixtures as follows:

1f m

c f c f cG v G v G (11)

But it does not usually do this. We have already seen that, if the length of the fibers is less than 2xc, they will not fracture. And if they do not fracture they must instead be pulled out as the crack opens (Figure 14). This gives a major new contribution to the toughness. If the matrix shear strength is σm

s (as before), then the work done in pulling a fiber out of the fracture surface is given approximately by:

2

2 2

0 0 8

l lm m

s s

lFdx d xdx d (12)

Tension Compression

Figure 13: Composites fail in compression by kinking, at a load which is lower than

that for failure in tension.


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The number of fibers per unit crack area is 4vf/π d2 (because the volume fraction is the same as the area fraction on a plane perpendicular to the fibers). So the total work done per unit crack area is:

2

2

2

4

8 2

f fm m

c s s

v vlG d l

d d

(13)

This assumes that l is less than the critical length 2xc. If l is greater than 2xc the fibers will not pull out, but will break instead. Thus optimum toughness is given by setting l = 2xc in eqn. 13 to give the following:

22

2 2 2

2 2 4 8

ffff f f fm m

c s c s m m

s s

v v v ddG l x l

d d

(14)

The equation says that, to get a high toughness, you should use strong fibers in a weak matrix (though of course a weak matrix gives a low strength). This mechanism gives CFRP and GFRP a toughness (50 kJ m−2) far higher than that of either the matrix (5 kJ m−2) or the fibers (0.1 kJ m−2); without it neither would be useful as an engineering material.

5.5 Other considerations

The prediction of transverse and shear properties can also be treated mathematically using similar methods.

The selection of a suitable composite material involves comparing the properties. A direct comparison of the properties of composite materials on a weight fraction basis is not easy because the densities of the constituents need to be considered. It is therefore much easier to compare the properties of composite materials on a volume fraction basis. However, in production it is more convenient to work in terms of weight. Therefore, conversion between the two is a regular activity. The conversion between weight fraction and volume fraction is given by the following relationship:

1

1

f f

f

f f m f

m f

d vw

d v d v

w w

(15)

where,

wf=fiber weight fraction;

wm= matrix weight fraction;

df= fiber density; and

dm=matrix density.

A major disadvantage of using glass FRPs in light weight structures is the low stiffness resulting from the low modulus of elasticity, even when in unidirectional form. In terms of specific properties, however, the

l/2

Fiber

Matrix

x

d

σ

σ

l

Figure 14: Material toughened by fiber pull out of the fracture surface.


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stiffness is comparable with most metals and with higher strength. Table 3 shows a comparison of mechanical properties of composites and commonly used engineering metals. This table demonstrates that FRPs have much higher specific properties than commonly used engineering metals. The tensile properties of aramid and carbon FRPs are better than glass FRPs, but at a significant cost penalty.

However, if flexural and compressive properties are considered a different result is obtained. In particular, the compressive strength, bending strength and shear strength of aramid FRPs are very poor and much lower than for carbon and glass FRPs. The major contributors to this are the tendency for the fiber to buckle easily and the poor bonding obtained between fibers and resins. The viscoelastic behavior of aramid fiber composites lead to higher work of fracture and the high ductility leads to better damping characteristics.

Table 3: Comparison of mechanical properties of popular composites and metals.

Material Density (Mg/m3)

Tensile Strength

GPa

Tensile Modulus

GPa

Specific

Strength Stiffness

Composites a

E glass 2.1 1.1 45 0.5 21

Aramid 1.4 1.4 75 1.0 54

Type I Carbon 1.5 1.1 220 0.7 147

Type II Carbon 2.0 1.5 140 0.8 70

Metals

Steel 7.8 1.3 200 0.2 26

Aluminum 2.8 0.3 73 0.1 26

Titanium 4 0.4 100 0.1 25

a) Sixty percent fiber volume fraction unidirectional reinforcement.

For long-term properties (static and dynamic loading), carbon FRPs are generally far superior to glass and aramid FRPs. The behavior of FRPs under fatigue or creep conditions are however, complicated subjects. They are also important in most design work but it is essential that a reasonable level of understanding of the underlying theory is achieved before the properties can be sensibly applied.

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