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Understanding composites for large scale boat projects


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What is the issue?
If you are considering a large scale boat building project, or re-building / altering portions of a vessel, you will find there is a wide range of composites with specific applications that make this area complicated.

Why address this?
A successful large scale project will require a good working knowledge of composites in order that the appropriate selection, application of composite materials and processes are adhered to in order to fabricate your required structure.

How to address this?
This extensive overview will provide you with a complete understanding of composites enabling you to make appropriates decisions and construction strategies.

It looks at basic composite theory, properties of materials used and then the various processing techniques commonly found for the conversion of materials into finished structures.

Composite Theory

In its most basic form a composite material is one which is composed of at least two elements working together to produce material properties that are different to the properties of those elements on their own. In practice, most composites consist of a bulk material (the ‘matrix’), and a reinforcement of some kind, added primarily to increase the strength and stiffness of the matrix. This reinforcement is usually in fibre form. Today, the most common man-made composites can be divided into three main groups:

Polymer Matrix Composites (PMC’s) – These are the most common and will be discussed here. Also known as FRP - Fibre Reinforced Polymers (or Plastics) – these materials use a polymer-based resin as the matrix, and a variety of fibres such as glass, carbon and aramid as the reinforcement.

Metal Matrix Composites (MMC’s) - Increasingly found in the automotive industry, these materials use a metal such as aluminium as the matrix, and reinforce it with fibres such as silicon carbide.

Ceramic Matrix Composites (CMC’s) - Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibres, or whiskers such as those made from silicon carbide and boron nitride.


Polymer Matrix Composites

Resin systems such as epoxies and polyesters have limited use for the manufacture of structures on their own, since their mechanical properties are not very high when compared to, for example, most metals. However, they have desirable properties, most notably their ability to be easily formed into complex shapes.

Materials such as glass, aramid and boron have extremely high tensile and compressive strength but in ‘solid form’ these properties are not readily apparent. This is due to the fact that when stressed, random surface flaws will cause each material to crack and fail well below its theoretical ‘breaking point’. To overcome this problem, the material is produced in fibre form, so that, although the same number of random flaws will occur, they will be restricted to a small number of fibres with the remainder exhibiting the material’s theoretical strength. Therefore a bundle of fibres will reflect more accurately the optimum performance of the material. However, fibres alone can only exhibit tensile properties along the fibre’s length, in the same way as fibres in a rope.

It is when the resin systems are combined with reinforcing fibres such as glass, carbon and aramid, that exceptional properties can be obtained. The resin matrix spreads the load applied to the composite between each of the individual fibres and also protects the fibres from damage caused by abrasion and impact. High strengths and stiffnesses, ease of moulding complex shapes, high environmental resistance all coupled with low densities, make the resultant composite superior to metals for many applications.

Since PMC’s combine a resin system and reinforcing fibres, the properties of the resulting composite material will combine something of the properties of the resin on its own with that of the fibres on their own.


Overall, the properties of the composite are determined by:
(i) The properties of the fibre
(ii) The properties of the resin
(iii) The ratio of fibre to resin in the composite (Fibre Volume Fraction)
(iv) The geometry and orientation of the fibres in the composite

The first two will be dealt with in more detail later. The ratio of the fibre to resin derives largely from the manufacturing process used to combine resin with fibre, as will be described in the section on manufacturing processes. However, it is also influenced by the type of resin system used, and the form in which the fibres are incorporated. In general, since the mechanical properties of fibres are much higher than those of resins, the higher the fibre volume fraction the higher will be the mechanical properties of the resultant composite. In practice there are limits to this, since the fibres need to be fully coated in resin to be effective, and there will be an optimum packing of the generally circular cross-section fibres. In addition, the manufacturing process used to combine fibre with resin leads to varying amounts of imperfections and air inclusions. Typically, with a common hand lay-up process as widely used in the boat-building industry, a limit for FVF is approximately 30-40%. With the higher quality, more sophisticated and precise processes used in the aerospace industry, FVF’s approaching 70% can be successfully obtained.

The geometry of the fibres in a composite is also important since fibres have their highest mechanical properties along their lengths, rather than across their widths. This leads to the highly anisotropic properties of composites, where, unlike metals, the mechanical properties of the composite are likely to be very different when tested in different directions. This means that it is very important when considering the use of composites to understand at the design stage, both the magnitude and the direction of the applied loads. When correctly accounted for, these anisotropic properties can
be very advantageous since it is only necessary to put material where loads will be applied, and thus redundant material is avoided.

It is also important to note that with metals the properties of the materials are largely determined by the material supplier, and the person who fabricates the materials into a finished structure can do almost nothing to change those ‘in-built’ properties. However, a composite material is formed at the same time as the structure is itself being fabricated. This means that the person who is making the structure is creating the properties of the resultant composite material, and so the manufacturing processes they use have an unusually critical part to play in determining the performance of the resultant structure.


Loading

There are four main direct loads that any material in a structure has to withstand: tension, compression, shear and flexure.

Tension

Fig. 2 a shows a tensile load applied to a composite. The response of a composite to tensile loads is very dependent on the tensile stiffness and strength properties of the reinforcement fibres, since these are far higher than the resin system on its own.

Compression

Fig. 2 b shows a composite under a compressive load. Here, the adhesive and stiffness properties of the resin system are crucial, as it is the role of the resin to maintain the fibres as straight columns and to prevent them from buckling.

Shear

Fig. 2 c shows a composite experiencing a shear load. This load is trying to slide adjacent layers of fibres over each other. Under shear loads the resin plays the major role, transferring the stresses across the composite. For the composite to perform well under shear loads the resin element must not only exhibit good mechanical properties but must also have high adhesion to the reinforcement fibre. The interlaminar shear strength (ILSS) of a composite is often used to indicate this property in a multilayer composite (‘laminate’).

Flexure

Fig. 2 d shows flexural loads are really a combination of tensile, compression and shear loads. When loaded as shown, the upper face is put into compression, the lower face into tension and the central portion of the laminate experiences shear.


Comparison with Other Structural Materials

Due to the factors described above, there is a very large range of mechanical properties that can be achieved with composite materials. Even when considering one fibre type on its own, the composite properties can vary by a factor of 10 with the range of fibre contents and orientations that are commonly achieved. The comparisons that follow (figures 3- 7) therefore show a range of mechanical properties for the composite materials. The lowest properties for each material are associated with simple manufacturing processes and material forms (e.g. spray lay-up glass fibre), and the higher properties are associated with higher technology manufacture (e.g. autoclave moulding of unidirectional glass fibre prepreg), such as would be found in the aerospace industry.

For the other materials shown, a range of strength and stiffness (modulus) figures is also given to indicate the spread of properties associated with different alloys, for example.

Figures 3-7 clearly show the range of properties that different composite materials can display. These properties can best be summed up as high strengths and stiffnesses combined with low densities. It is these properties that give rise to the characteristic high strength and stiffness to weight ratios that make composite structures ideal for so many applications. This is particularly true of applications which involve movement, such as cars, trains and aircraft, since lighter structures in such applications play a significant part in making these applications more efficient. The strength and stiffness to weight ratio of composite materials can best be illustrated by the following graphs that plot ‘specific’ properties. These are simply the result of dividing the mechanical properties of a material by its density. Generally, the properties at the higher end of the ranges illustrated in the previous graphs are produced from the highest density variant of the material. The spread of specific properties shown in the following graphs takes this into account.


RESIN SYSTEMS


Introduction

Any resin system for use in a composite material will require the following properties:
1. Good mechanical properties
2. Good adhesive properties
3. Good toughness properties
4. Good resistance to environmental degradation


Mechanical Properties of the Resin System

The figure below shows the stress / strain curve for an ‘ideal’ resin system. The curve for this resin shows high ultimate strength, high stiffness (indicated by the initial gradient) and a high strain to failure. This means that the resin is initially stiff but at the same time will not suffer from brittle failure.

It should also be noted that when a composite is loaded in tension, for the full mechanical properties of the fibre component to be achieved, the resin must be able to deform to at least the same extent as the fibre. Fig. 9 gives the strain to failure for Eglass, S-glass, aramid and high-strength grade carbon fibres on their own (i.e. not in a composite form). Here it can be seen that, for example, the S-glass fibre, with an elongation to break of 5.3%, will require a resin with an elongation to break of at least this value to achieve maximum tensile properties.


Adhesive Properties of the Resin System

High adhesion between resin and reinforcement fibres is necessary for any resin system. This will ensure that the loads are transferred efficiently and will prevent cracking or fibre / resin debonding when stressed.


Toughness Properties of the Resin System

Toughness is a measure of a material’s resistance to crack propagation, but in a composite this can be hard to measure accurately. However, the stress / strain curve of the resin system on its own provides some indication of the material’s toughness. Generally the more deformation the resin will accept before failure the tougher and more crack-resistant the material will be. Conversely, a resin system with a low strain to failure will tend to create a brittle composite, which cracks easily. It is important to match this property to the elongation of the fibre reinforcement.


Environmental Properties of the Resin System

Good resistance to the environment, water and other aggressive substances, together with an ability to withstand constant stress cycling, are properties essential to any resin system. These properties are particularly important for use in a marine environment.


Resin Types

The resins that are used in fibre reinforced composites are sometimes referred to as ‘polymers’. All polymers exhibit an important common property in that they are composed of long chain-like molecules consisting of many simple repeating units. Manmade polymers are generally called ‘synthetic resins’ or simply ‘resins’. Polymers can be classified under two types, ‘thermoplastic’ and ‘thermosetting’, according to the effect of heat on their properties.

Thermoplastics, like metals, soften with heating and eventually melt, hardening again with cooling. This process of crossing the softening or melting point on the temperature scale can be repeated as often as desired without any appreciable effect on the material properties in either state. Typical thermoplastics include nylon, polypropylene and ABS, and these can be reinforced, although usually only with short, chopped fibres such as glass.

Thermosetting materials, or ‘thermosets’, are formed from a chemical reaction in situ, where the resin and hardener or resin and catalyst are mixed and then undergo a nonreversible chemical reaction to form a hard, infusible product. In some thermosets, such as phenolic resins, volatile substances are produced as by-products (a ‘condensation’ reaction). Other thermosetting resins such as polyester and epoxy cure by mechanisms that do not produce any volatile by products and thus are much easier to process (‘addition’ reactions). Once cured, thermosets will not become liquid again if heated, although above a certain temperature their mechanical properties will change significantly. This temperature is known as the Glass Transition Temperature (Tg), and varies widely according to the particular resin system used, its degree of cure and whether it was mixed correctly. Above the Tg, the molecular structure of the thermoset changes from that of a rigid crystalline polymer to a more flexible, amorphous polymer. This change is reversible on cooling back below the Tg. Above the Tg properties such as resin modulus (stiffness) drop sharply, and as a result the compressive and shear strength of the composite does too. Other properties such as water resistance and colour stability also reduce markedly above the resin’s Tg.

Although there are many different types of resin in use in the composite industry, the majority of structural parts are made with three main types, namely polyester, vinylester and epoxy.


Polyester Resins

Polyester resins are the most widely used resin systems, particularly in the marine industry. By far the majority of dinghies, yachts and work-boats built in composites make use of this resin system.

Polyester resins such as these are of the ‘unsaturated’ type. Unsaturated polyester resin is a thermoset, capable of being cured from a liquid or solid state when subject to the right conditions. An unsaturated polyester differs from a saturated polyester such as Terylene™ which cannot be cured in this way. It is usual, however, to refer to unsaturated polyester resins as ‘polyester resins’, or simply as ‘polyesters’.

In chemistry the reaction of a base with an acid produces a salt. Similarly, in organic chemistry the reaction of an alcohol with an organic acid produces an ester and water. By using special alcohols, such as a glycol, in a reaction with di-basic acids, a polyester and water will be produced. This reaction, together with the addition of compounds such as saturated di-basic acids and cross-linking monomers, forms the basic process of polyester manufacture. As a result there is a whole range of polyesters made from different acids, glycols and monomers, all having varying properties.

There are two principle types of polyester resin used as standard laminating systems in the composites industry. Orthophthalic polyester resin is the standard economic resin used by many people. Isophthalic polyester resin is now becoming the preferred material in industries such as marine where its superior water resistance is desirable.

Figure 10 a. shows the idealised chemical structure of a typical polyester. Note the positions of the ester groups (CO - O - C) and the reactive sites (C* = C*) within the molecular chain.

Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a monomer which is usually styrene. The addition of styrene in amounts of up to 50% helps to make the resin easier to handle by reducing its viscosity. The styrene also performs the vital function of enabling the resin to cure from a liquid to a solid by ‘cross-linking’ the molecular chains of the polyester, without the evolution of any by-products. These resins can therefore be moulded without the use of pressure and are called ‘contact’ or ‘low pressure’ resins. Polyester resins have a limited storage life as they will set or ‘gel’ on their own over a long period of time. Often small quantities of inhibitor are added during the resin manufacture to slow this gelling action. For use in moulding, a polyester resin requires the addition of several ancillary products.

These products are generally:
- Catalyst
- Accelerator
- Additives: Thixotropic, Pigment, Filler, Chemical/fire resistance

A manufacturer may supply the resin in its basic form or with any of the above additives already included. Resins can be formulated to the moulder’s requirements ready simply for the addition of the catalyst prior to moulding. As has been mentioned, given enough time an unsaturated polyester resin will set by itself. This rate of polymerisation is too slow for practical purposes and therefore catalysts and accelerators are used to achieve the polymerisation of the resin within a practical time period. Catalysts are added to the resin system shortly before use to initiate the polymerisation reaction. The catalyst does not take part in the chemical reaction but simply activates the process. An accelerator is added to the catalysed resin to enable the reaction to proceed at workshop temperature and/or at a greater rate. Since accelerators have little influence on the resin in the absence of a catalyst they are sometimes added to the resin by the polyester manufacturer to create a ‘pre-accelerated’ resin.

The molecular chains of the polyester can be represented in figure 10, where ‘B’ indicates the reactive sites in the molecule. With the addition of styrene ‘S ‘, and in the presence of a catalyst, the styrene cross-links the polymer chains at each of the reactive sites to form a highly complex three-dimensional network as presented in figure 11:

The polyester resin is then said to be ‘cured’. It is now a chemically resistant (and usually) hard solid. The cross-linking or curing process is called ‘polymerisation’. It is a non-reversible chemical reaction. The ‘side-by-side’ nature of this cross-linking of the molecular chains tends to means that polyester laminates suffer from brittleness when shock loadings are applied.

Great care is needed in the preparation of the resin mix prior to moulding. The resin and any additives must be carefully stirred to disperse all the components evenly before the catalyst is added. This stirring must be thorough and careful as any air introduced into the resin mix affects the quality of the final moulding. This is especially so when laminating with layers of reinforcing materials as air bubbles can be formed within the resultant laminate which can weaken the structure. It is also important to add the accelerator and catalyst in carefully measured amounts to control the polymerisation reaction to give the best material properties. Too much catalyst will cause too rapid a gelation time, whereas too little catalyst will result in under-cure.

Colouring of the resin mix can be carried out with pigments. The choice of a suitable pigment material, even though only added at about 3% resin weight, must be carefully considered as it is easy to affect the curing reaction and degrade the final laminate by use of unsuitable pigments.


Filler materials are used extensively with polyester resins for a variety of reasons including:
- To reduce the cost of the moulding
- To facilitate the moulding process
- To impart specific properties to the moulding

Fillers are often added in quantities up to 50% of the resin weight although such addition levels will affect the flexural and tensile strength of the laminate. The use of fillers can be beneficial in the laminating or casting of thick components where otherwise considerable exothermic heating can occur. Addition of certain fillers can also contribute to increasing the fire-resistance of the laminate.


Vinylester Resins

Vinylester resins are similar in their molecular structure to polyesters, but differ primarily in the location of their reactive sites, these being positioned only at the ends of the molecular chains. As the whole length of the molecular chain is available to absorb shock loadings this makes vinylester resins tougher and more resilient than polyesters. The vinylester molecule also features fewer ester groups. These ester groups are susceptible to water degradation by hydrolysis which means that vinylesters exhibit better resistance to water and many other chemicals than their polyester counterparts, and are frequently found in applications such as pipelines and chemical storage tanks.

Figure 11 below shows the idealised chemical structure of a typical vinylester. Note the positions of the ester groups and the reactive sites (C* = C*) within the molecular chain.

The molecular chains of vinylester, represented below, can be compared to the schematic representation of polyester shown previously where the difference in the location of the reactive sites can be clearly seen in figure 12:

With the reduced number of ester groups in a vinylester when compared to a polyester, the resin is less prone to damage by hydrolysis. The material is therefore sometimes used as a barrier or ‘skin’ coat for a polyester laminate that is to be immersed in water, such as in a boat hull. The cured molecular structure of the vinylester also means that it tends to be tougher than a polyester, although to really achieve these properties the resin usually needs to have an elevated temperature postcure –see figure 12.


Epoxy Resins

The large family of epoxy resins represent some of the highest performance resins of those available at this time. Epoxies generally out-perform most other resin types in terms of mechanical properties and resistance to environmental degradation, which leads to their almost exclusive use in aircraft components. As a laminating resin their increased adhesive properties and resistance to water degradation make these resins ideal for use in applications such as boat building. Here epoxies are widely used as a primary construction material for high-performance boats or as a secondary application to sheath a hull or replace water-degraded polyester resins and gel coats.

The term ‘epoxy’ refers to a chemical group consisting of an oxygen atom bonded to two carbon atoms that are already bonded in some way. The simplest epoxy is a three-member ring structure known by the term ‘alpha-epoxy’ or ‘1,2-epoxy’. The idealised chemical structure is shown in the figure below and is the most easily identified characteristic of any more complex epoxy molecule –see figure 12.

Usually identifiable by their characteristic amber or brown colouring, epoxy resins have a number of useful properties. Both the liquid resin and the curing agents form low viscosity easily processed systems. Epoxy resins are easily and quickly cured at any temperature from 5C to 150C, depending on the choice of curing agent. One of the most advantageous properties of epoxies is their low shrinkage during cure which minimises fabric ‘print-through’ and internal stresses. High adhesive strength and high mechanical properties are also enhanced by high electrical insulation and good chemical resistance. Epoxies find uses as adhesives, caulking compounds, casting compounds, sealants, varnishes and paints, as well as laminating resins for a variety of industrial applications.

Epoxy resins are formed from a long chain molecular structure similar to vinylester with reactive sites at either end. In the epoxy resin, however, these reactive sites are formed by epoxy groups instead of ester groups. The absence of ester groups means that the epoxy resin has particularly good water resistance. The epoxy molecule also contains two ring groups at its centre which are able to absorb both mechanical and thermal stresses better than linear groups and therefore give the epoxy resin very good stiffness, toughness and heat resistant properties.

Figure 13 below shows the idealised chemical structure of a typical epoxy. Note the absence of the ester groups within the molecular chain.

Epoxies differ from polyester resins in that they are cured by a ‘hardener’ rather than a catalyst. The hardener, often an amine, is used to cure the epoxy by an ‘addition reaction’ where both materials take place in the chemical reaction. The chemistry of this reaction means that there are usually two epoxy sites binding to each amine site. This forms a complex three-dimensional molecular structure which is illustrated in Fig13.

Since the amine molecules ‘co-react’ with the epoxy molecules in a fixed ratio, it is essential that the correct mix ratio is obtained between resin and hardener to ensure that a complete reaction takes place. If amine and epoxy are not mixed in the correct ratios, unreacted resin or hardener will remain within the matrix which will affect the final properties after cure. To assist with the accurate mixing of the resin and hardener, manufacturers usually formulate the components to give a simple mix ratio which is easily achieved by measuring out by weight or volume.


Gelation, Curing and Post-Curing

On addition of the catalyst or hardener a resin will begin to become more viscous until it reaches a state when it is no longer a liquid and has lost its ability to flow. This is the ‘gel point’. The resin will continue to harden after it has gelled, until, at some time later, it has obtained its full hardness and properties. This reaction itself is accompanied by the generation of exothermic heat, which, in turn, speeds the reaction. The whole process is known as the ‘curing’ of the resin. The speed of cure is controlled by the amount of accelerator in a polyester or vinylester resin and by varying the type, not the quantity, of hardener in an epoxy resin. Generally polyester resins produce a more severe exotherm and a faster development of initial mechanical properties than epoxies of a similar working time.

With both resin types, however, it is possible to accelerate the cure by the application of heat, so that the higher the temperature the faster the final hardening will occur. This can be most useful when the cure would otherwise take several hours or even days at room temperature. A quick rule of thumb for the accelerating effect of heat on a resin is that a 10C increase in temperature will roughly double the reaction rate. Therefore if a resin gels in a laminate in 25 minutes at 20C it will gel in about 12 minutes at 30C, providing no extra exotherm occurs. Curing at elevated temperatures has the added advantage that it actually increases the end mechanical properties of the material, and many resin systems will not reach their ultimate mechanical properties unless the resin is given this ‘postcure’. The postcure involves increasing the laminate temperature after the initial room temperature cure, which increases the amount of crosslinking of the molecules that can take place. To some degree this postcure will occur naturally at warm room temperatures, but higher properties and shorter postcure times will be obtained if elevated temperatures are used. This is particularly true of the material’s softening point or Glass Transition Temperature (Tg), which, up to a point, increases with increasing postcure temperature.


Comparison of Resin Properties

The choice of a resin system for use in any component depends on a number of its characteristics, with the following probably being the most important for most composite structures:

1 Adhesive Properties
2 Mechanical Properties
3 Micro-Cracking resistance
4 Fatigue Resistance
5 Degradation from water ingress


Adhesive Properties

It has already been discussed how the adhesive properties of the resin system are important in realising the full mechanical properties of a composite. The adhesion of the resin matrix to the fibre reinforcement or to a core material in a sandwich construction are important. Polyester resins generally have the lowest adhesive properties of the three systems described here. Vinylester resin shows improved adhesive properties over polyester but epoxy systems offer the best performance of all, and are therefore frequently found in many high-strength adhesives. This is due to their chemical composition and the presence of polar hydroxyl and ether groups. As epoxies cure with low shrinkage the various surface contacts set up between the liquid resin and the adherends are not disturbed during the cure. The adhesive properties of epoxy are especially useful in the construction of honeycomb-cored laminates where the small bonding surface area means that maximum adhesion is required. The strength of the bond between resin and fibre is not solely dependent on the adhesive properties of the resin system but is also affected by the surface coating on the reinforcement fibres. This ‘sizing’ is discussed later under ‘Reinforcements’.


Mechanical Properties

Two important mechanical properties of any resin system are its tensile strength and stiffness. Figure 14 show results for tests carried out on commercially available polyester, vinylester and epoxy resin systems cured at 20C and 80C. After a cure period of seven days at room temperature it can be seen that a typical epoxy will have higher properties than a typical polyester and vinylester for both strength and stiffness. The beneficial effect of a post cure at 80C for five hours can also be seen. Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Shrinkage is due to the resin molecules rearranging and re-orientating themselves in the liquid and semi-gelled phase. Polyester and vinylesters require considerable molecular rearrangement to reach their cured state and can show shrinkage of up to 8%. The different nature of the epoxy reaction, however, leads to very little rearrangement and with no volatile biproducts being evolved, typical shrinkage of an epoxy is reduced to around 2%. The absence of shrinkage is, in part, responsible for the improved mechanical properties of epoxies over polyester, as shrinkage is associated with built-in stresses that can weaken the material. Furthermore, shrinkage through the thickness of a laminate leads to ‘print-through’ of the pattern of the reinforcing fibres, a cosmetic defect that is difficult and expensive to eliminate.


Micro-Cracking

The strength of a laminate is usually thought of in terms of how much load it can withstand before it suffers complete failure. This ultimate or breaking strength is the point it which the resin exhibits catastrophic breakdown and the fibre reinforcements break.

However, before this ultimate strength is achieved, the laminate will reach a stress level where the resin will begin to crack away from those fibre reinforcements not aligned with the applied load, and these cracks will spread through the resin matrix. This is known as ‘transverse micro-cracking’ and, although the laminate has not completely failed at this point, the breakdown process has commenced. Consequently, engineers who want a long-lasting structure must ensure that their laminates do not exceed this point under regular service loads.

The strain that a laminate can reach before micro-cracking depends strongly on the toughness and adhesive properties of the resin system. For brittle resin systems, such as most polyesters, this point occurs a long way before laminate failure, and so severely limits the strains to which such laminates can be subjected. As an example, recent tests have shown that for a polyester/glass woven roving laminate, micro-cracking typically occurs at about 0.2% strain with ultimate failure not occurring until 2.0% strain. This equates to a usable strength of only 10% of the ultimate strength – see figure 15.

As the ultimate strength of a laminate in tension is governed by the strength of the fibres, these resin micro-cracks do not immediately reduce the ultimate properties of the laminate. However, in an environment such as water or moist air, the micro-cracked laminate will absorb considerably more water than an un-cracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fibre sizing agents, loss of stiffness and, with time, an eventual drop in ultimate properties.

Increased resin/fibre adhesion is generally derived from both the resin’s chemistry and its compatibility with the chemical surface treatments applied to fibres. Here the well-known adhesive properties of epoxy help laminates achieve higher microcracking strains. As has been mentioned previously, resin toughness can be hard to measure, but is broadly indicated by its ultimate strain to failure. A comparison between various resin systems is shown in Fig. 16.


Fatigue Resistance

Generally composites show excellent fatigue resistance when compared with most metals. However, since fatigue failure tends to result from the gradual accumulation of small amounts of damage, the fatigue behaviour of any composite will be influenced by the toughness of the resin, its resistance to microcracking, and the quantity of voids and other defects which occur during manufacture. As a result, epoxybased laminates tend to show very good fatigue resistance when compared with both polyester and vinylester, this being one of the main reasons for their use in aircraft structures.


Degradation from Water Ingress

An important property of any resin, particularly in a marine environment, is its ability to withstand degradation from water ingress. All resins will absorb some moisture, adding to a laminate’s weight, but what is more significant is how the absorbed water affects the resin and resin/fibre bond in a laminate, leading to a gradual and longterm loss in mechanical properties. Both polyester and vinylester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion in water for a period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

Fig. 17 demonstrates the effects of water on an epoxy and polyester woven glass laminate, which have been subjected to a water soak at 100C. This elevated temperature soaking gives accelerated degradation properties for the immersed laminate.

Osmosis

All laminates in a marine environment will permit very low quantities of water to pass through them in vapour form. As this water passes through, it reacts with any hydrolysable components inside the laminate to form tiny cells of concentrated solution. Under the osmotic cycle, more water is then drawn through the semi-permeable membrane of the laminate to attempt to dilute this solution. This water increases the fluid pressure in the cell to as much as 700 psi. Eventually the pressure distorts or bursts the laminate or gelcoat, and can lead to a characteristic ‘chicken-pox’ surface. Hydrolysable components in a laminate can include dirt and debris that have become trapped during fabrication, but can also include the ester linkages in a cured polyester, and to a lesser extent, vinylester.

Use of resin rich layers next to the gel coat are essential with polyester resins to minimise this type of degradation, but often the only cure once the process has started is the replacement of the affected material. To prevent the onset of osmosis from the start, it is necessary to use a resin which has both a low water transmission rate and a high resistance to attack by water. When used with reinforcements with similarly resistant surface treatment and laminated to a very high standard, blistering can then be virtually eliminated. A polymer chain having an epoxy backbone is substantially better than many other resin systems at resisting the effects of water. Such systems have been shown to confer excellent chemical and water resistance, low water transmission rate and very good mechanical properties to the polymer.


Resin Comparison Summary

The polyesters, vinylesters and epoxies discussed here probably account for some 90% of all thermosetting resin systems used in structural composites. In summary the main advantages and disadvantages of each of these types are:


Other Resin Systems used in Composites

Besides polyesters, vinylesters and epoxies there are a number of other specialised resin systems that are used where their unique properties are required:

Phenolics
Primarily used where high fire-resistance is required, phenolics also retain their properties well at elevated temperatures. For room-temperature curing materials, corrosive acids are used which leads to unpleasant handling. The condensation nature of their curing process tends to lead to the inclusion of many voids and surface defects, and the resins tend to be brittle and do not have high mechanical properties. Typical costs: £2-4/kg.

Cyanate Esters

Primarily used in the aerospace industry. The material’s excellent dielectric properties make it very suitable for use with low dielectric fibres such as quartz for the manufacture of radomes. The material also has temperature stability up to around 200C wet. Typical costs: £40/kg.

Silicones
Synthetic resin using silicon as the backbone rather than the carbon of organic polymers. Good fire-resistant properties, and able to withstand elevated temperatures. High temperature cures needed. Used in missile applications. Typical costs: >£15/ kg.

Polyurethanes
High toughness materials, sometimes hybridised with other resins, due to relatively low laminate mechanical properties in compression. Uses harmful isocyanates as curing agent. Typical costs: £2-8/kg

Bismaleimides (BMI)
Primarily used in aircraft composites where operation at higher temperatures (230C wet/250C dry) is required. e.g. engine inlets, high speed aircraft flight surfaces. Typical costs: >£50/kg.

Polyimides
Used where operation at higher temperatures than bismaleimides can stand is required (use up to 250C wet/300C dry). Typical applications include missile and aero-engine components. Extremely expensive resin (>£80/kg), which uses toxic raw materials in its manufacture. Polyimides also tend to be hard to process due to their condensation reaction emitting water during cure, and are relatively brittle when cured. PMR15 and LaRC160 are two of the most commonly used polyimides for composites.


Reinforcements

The role of the reinforcement in a composite material is fundamentally one of increasing the mechanical properties of the neat resin system. All of the different fibres used in composites have different properties and so affect the properties of the composite in different ways. The properties and characteristics of common fibres are explained
below.

However, individual fibres or fibre bundles can only be used on their own in a few processes such as filament winding (described later). For most other applications, the fibres need to be arranged into some form of sheet, known as a fabric, to make handling possible. Different ways for assembling fibres into sheets and the variety of fibre orientations possible lead to there being many different types of fabrics, each of which has its own characteristics. These different fabric types and constructions are explained later.


Properties of Reinforcing Fibres & Finishes

The mechanical properties of most reinforcing fibres are considerably higher than those of un-reinforced resin systems. The mechanical properties of the fibre/resin composite are therefore dominated by the contribution of the fibre to the composite.

The four main factors that govern the fibre’s contribution are:
1. The basic mechanical properties of the fibre itself.
2. The surface interaction of fibre and resin (the ‘interface’).
3. The amount of fibre in the composite (‘Fibre Volume Fraction’).
4. The orientation of the fibres in the composite.

The basic mechanical properties of the most commonly used fibres are given in the following table. The surface interaction of fibre and resin is controlled by the degree of bonding that exists between the two. This is heavily influenced by the treatment given to the fibre surface, and a description of the different surface treatments and ‘finishes’ is also given here.

The amount of fibre in the composite is largely governed by the manufacturing process used. However, reinforcing fabrics with closely packed fibres will give higher Fibre Volume Fractions (FVF) in a laminate than will those fabrics which are made with coarser fibres, or which have large gaps between the fibre bundles. Fibre diameter is an important factor here with the more expensive smaller diameter fibres providing higher fibre surface areas, spreading the fibre/matrix interfacial loads. As a general rule, the stiffness and strength of a laminate will increase in proportion to the amount of fibre present. However, above about 60-70% FVF (depending on the way in which the fibres pack together) although tensile stiffness may continue to increase, the laminate’s strength will reach a peak and then begin to decrease due to the lack of sufficient resin to hold the fibres together properly.

Finally, since reinforcing fibres are designed to be loaded along their length, and not across their width, the orientation of the fibres creates highly ‘direction-specific’ properties in the composite. This ‘anisotropic’ feature of composites can be used to good advantage in designs, with the majority of fibres being placed along the orientation of the main load paths. This minimises the amount of parasitic material that is put in orientations where there is little or no load – see figure 19.


Laminate Mechanical Properties

The properties of the fibres given above only shows part of the picture. The properties of the composite will derive from those of the fibre, but also the way it interacts with the resin system used, the resin properties itself, the volume of fibre in the composite and its orientation. The following diagrams show a basic comparison of the main fibre types when used in a typical high-performance unidirectional epoxy prepreg, at the fibre volume fractions that are commonly achieved in aerospace components. The graphs in figure 20 show the strengths and maximum strains of the different composites at failure. The gradient of each graph also indicates the stiffness (modulus) of the composite; the steeper the gradient, the higher its stiffness. The graphs also show how some fibres, such as aramid, display very different properties when loaded in compression, compared with loading in tension.

Impact damage can pose particular problems when using high stiffness fibres in very thin laminates. In some structures, where cores are used, laminate skins can be less than 0.3mm thick. Although other factors such as weave style and fibre orientation can significantly affect impact resistance, in impact-critical applications, carbon is often found in combination with one of the other fibres. This can be in the form of a hybrid fabric where more than one fibre type is used in the fabric construction. These are described in more detail later. The data in figure 22 below are calculated on a typical price of a 300g woven fabric. Most fibre prices are considerably higher for the small bundle size (tex) used in such lightweight fabrics.


FIBRE TYPES

Glass

By blending quarry products (sand, kaolin, limestone, colemanite) at 1,600C, liquid glass is formed. The liquid is passed through micro-fine bushings and simultaneously cooled to produce glass fibre filaments from 5-24m in diameter. The filaments are drawn together into a strand (closely associated) or roving (loosely associated), and coated with a “size” to provide filament cohesion and protect the glass from abrasion.

By variation of the “recipe”, different types of glass can be produced. The types used for structural reinforcements are as follows:

a. E-glass (electrical) - lower alkali content and stronger than A glass (alkali). Good tensile and compressive strength and stiffness, good electrical properties and relatively low cost, but impact resistance relatively poor. Depending on the type of E glass the price ranges from about £1-2/kg. E-glass is the most common form of reinforcing fibre used in polymer matrix composites.

b. C-glass (chemical) - best resistance to chemical attack. Mainly used in the form of surface tissue in the outer layer of laminates used in chemical and water pipes and tanks.

c. R, S or T-glass – manufacturers trade names for equivalent fibres having higher tensile strength and modulus than E glass, with better wet strength retention. Higher ILSS and wet out properties are achieved through smaller filament diameter. Sglass is produced in the USA by OCF, R-glass in Europe by Vetrotex and T-glass by Nittobo in Japan. Developed for aerospace and defence industries, and used in some hard ballistic armour applications. This factor, and low production volumes mean relatively high price. Depending on the type of R or S glass the price ranges from about £12-20/kg.

E Glass Fibre Types

E Glass fibre is available in the following forms:
a. strand - a compactly associated bundle of filaments. Strands are rarely seen commercially and are usually twisted together to give yarns.
b. yarns - a closely associated bundle of twisted filaments or strands. Each filament diameter in a yarn is the same, and is usually between 4-13m. Yarns have varying weights described by their ‘tex’ (the weight in grammes of 1000 linear metres) or denier (the weight in lbs of 10,000 yards), with the typical tex range usually being between 5 and 400.
c. rovings - a loosely associated bundle of untwisted filaments or strands. Each filament diameter in a roving is the same, and is usually between 13-24m. Rovings also have varying weights and the tex range is usually between 300 and 4800. Where filaments are gathered together directly after the melting process, the resultant fibre bundle is known as a direct roving. Several strands can also be brought together separately after manufacture of the glass, to give what is known as an assembled roving. Assembled rovings usually have smaller filament diameters than direct rovings, giving better wet-out and mechanical properties, but they can suffer from catenary problems (unequal strand tension), and are usually higher in cost because of the more involved manufacturing processes.

It is also possible to obtain long fibres of glass from short fibres by spinning them. These spun yarn fibres have higher surface areas and are more able to absorb resin, but they have lower structural properties than the equivalent continuously drawn fibres.

Glass Fibre Designation

Glass fibres are designated by the following internationally recognised terminology: glass type yarn type filament strand single strand no. of multi strand no. turns

Aramid
Aramid fibre is a man-made organic polymer (an aromatic polyamide) produced by spinning a solid fibre from a liquid chemical blend. The bright golden yellow filaments produced can have a range of properties, but all have high strength and low density giving very high specific strength. All grades have good resistance to impact, and lower modulus grades are used extensively in ballistic applications. Compressive strength, however, is only similar to that of E glass.

Although most commonly known under its Dupont trade name ‘Kevlar’, there are now a number of suppliers of the fibre, most notably Akzo Nobel with ‘Twaron’. Each supplier offers several grades of aramid with various combinations of modulus and surface finish to suit various applications. As well as the high strength properties, the fibres also offer good resistance to abrasion, and chemical and thermal degradation. However, the fibre can degrade slowly when exposed to ultraviolet light.

Aramid fibres are usually available in the form of rovings, with texes ranging from about 20 to 800. Typically the price of the high modulus type ranges from £15-to £25 per kg.

Carbon
Carbon fibre is produced by the controlled oxidation, carbonisation and graphitisation of carbon-rich organic precursors which are already in fibre form. The most common precursor is polyacrylonitrile (PAN), because it gives the best carbon fibre properties, but fibres can also be made from pitch or cellulose. Variation of the graphitisation process produces either high strength fibres (@ ~2,600C) or high modulus fibres (@ ~3,000C) with other types in between. Once formed, the carbon fibre has a surface treatment applied to improve matrix bonding and chemical sizing which serves to protect it during handling.

When carbon fibre was first produced in the late sixties the price for the basic high strength grade was about £200/kg. By 1996 the annual worldwide capacity had increased to about 7,000 tonnes and the price for the equivalent (high strength) grade was £15-40/kg. Carbon fibres are usually grouped according to the modulus band in which their properties fall. These bands are commonly referred to as: high strength (HS), intermediate modulus (IM), high modulus (HM) and ultra high modulus (UHM). The filament diameter of most types is about 5-7m. Carbon fibre has the highest specific stiffness of any commercially available fibre, very high strength in both tension and compression and a high resistance to corrosion, creep and fatigue. Their impact strength, however, is lower than either glass or aramid, with particularly brittle characteristics being exhibited by HM and UHM fibres.


Fibre Type Comparisons

Comparing the properties of all of the fibre types with each other, shows that they all have distinct advantages and disadvantages. This makes different fibre types more suitable for some applications than others. Figure 23 provides a table with basic comparison between the main desirable features of generic fibre types. ‘A’ indicates a feature where the fibre scores well, and ‘C’ indicates a feature where the fibre is not so good.


Other Fibres
There are a variety of other fibres which can be used in advanced composite structures but their use is not widespread. These include:

Polyester
A low density, high tenacity fibre with good impact resistance but low modulus. Its lack of stiffness usually precludes it from inclusion in a composite component, but it is useful where low weight, high impact or abrasion resistance, and low cost are required. It is mainly used as a surfacing material, as it can be very smooth, keeps weight down and works well with most resin types.

Polyethylene
In random orientation, ultra-high molecular weight polyethylene molecules give very low mechanical properties. However, if dissolved and drawn from solution into a filament by a process called gel-spinning, the molecules become disentangled and aligned in the direction of the filament. The molecular alignment promotes very high tensile strength to the filament and the resulting fibre. Coupled with their low S.G. (

With thanks to:
With thanks to Martin Armstrong, Structural Polymer Systems Ltd. e-mail: info@spsystems.com web: http://www.spsystems.com





















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