Composite Engineering Materials, Notas de estudo de Engenharia Elétrica

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SP Systems UK Ltd St Cross Business Park, Newport, Isle of Wight, U.K. PO30 5WU T +44 (0)1983 828000 F +44 (0) 1983 828100 E info@spsystems.com W www.spsystems.com SP Systems UK Ltd St Cross Business Park, Newport, Isle of Wight, U.K. PO30 5WU T +44 (0)1983 828000 F +44 (0) 1983 828100 E info@spsystems.com W www.spsystems.com SP Systems Guide to Composites SP Systems Guide to Composites Contents Introduction 1 Composite Theory 1 Polymer Matrix Composites 1 Loading 3 Comparison with Other Structural Materials 4 Resin Systems 8 Introduction 8 Resin Types 9 Polyester Resins 10 Vinylester Resins 13 Epoxy Resins 14 Gelation, Curing and Post-Curing 16 Comparison of Resin Properties 16 Adhesive Properties 16 Mechanical Properties 17 Micro-Cracking 18 Fatigue Resistance 19 Degradation from Water Ingress 19 Osmosis 20 Resin Comparison Summary 21 Other Resin Systems Used in Composites 21 Reinforcements 23 Properties of Reinforcing Fibres & Finishes 23 Basic Properties of Fibres 24 Laminate Mechanical Properties 25 Laminate Impact Strength 25 Comparative Fibre Cost 26 It is when the resin systems are combined with reinforcing fibres such as glass, car- bon 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 cou- pled 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 re- sulting composite material will combine something of the properties of the resin on its own with that of the fibres on their own. Fig. 1 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 res- ins, 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 gen- erally 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 Fibre FRP Composite Resin Strain Te ns ile S tre ss GTC-1-1098 - 2 industry, a limit for FVF is approximately 30-40%. With the higher quality, more sophis- ticated 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. How- ever, 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 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. Fig. 2 Compression Fig. 3 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. Fig. 3 GTC-1-1098 - 3 Shear Fig. 4 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 prop- erties 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 multi- layer composite (‘laminate’). Fig. 4 Flexure 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. Fig. 5 Comparison with Other Structural Materials Due to the factors described above, there is a very large range of mechanical prop- erties 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 compari- sons that follow therefore show a range of mechanical properties for the composite materials. The lowest properties for each material are associated with simple manu- facturing processes and material forms (e.g. spray lay-up glass fibre), and the higher properties are associated with higher technology manufacture (e.g. autoclave mould- ing 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. GTC-1-1098 - 4 Fig. 9 Fig. 10 Further comparisons between laminates made from the different fibre types are given later in this guide in the section on ‘Reinforcements’. Sp ec ifi c Te ns ile M od ul us Specific Tensile Modulus of Common Structural Materials 0 Woods Al. Alloys Titanium Steels E-Glass Composite S-Glass Composite Aramid Composite HS Carbon Composite IM Carbon Composite 10 20 30 40 50 60 110 70 80 100 90 120 GTC-1a-1098 - 7 Sp ec ifi c Te ns ile S tre ng th Specific Tensile Strength of Common Structural Materials 0 200 400 600 800 1000 1200 Woods Al. Alloys Titanium Steels E-Glass Composite S-Glass Composite Aramid Composite HS Carbon Composite IM Carbon Composite 2000 1400 1600 1800 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 gradi- ent) 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. Fig 11 It should also be noted that when a composite is loaded in tension, for the full me- chanical 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. 12 gives the strain to failure for E- glass, 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. Plastic Deformation Strain to FailureStrain (%) Te ns ile S tre ss Failure El as tic D ef or m at io n Ult. Tensile Strength GTC-1-1098 - 8 Fig. 12 Adhesive Properties of the Resin System High adhesion between resin and reinforcement fibres is necessary for any resin sys- tem. 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 com- posite 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 environ- ment. 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 com- posed of long chain-like molecules consisting of many simple repeating units. Man- made polymers are generally called ‘synthetic resins’ or simply ‘resins’. Polymers can 1 2 3 4 5 6 Strain (%) Epoxy Resin E-glass S-glass Aramid HS Carbon 3000 2000 1000Te ns ile S tre ss (M Pa ) GTC-1-1098 - 9 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 as follows, where ‘B’ indi- cates the reactive sites in the molecule. Schematic Representation of Polyester Resin (Uncured) Fig. 14 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 follows: Schematic Representation of Polyester Resin (Cured) Fig. 15 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 polym- erisation 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 GTC-1-1098 - 12 A B A B A B A A B A B A B A A B A B A B A S S S 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 in- cluding: ■ 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 addi- tion 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 con- tribute to increasing the fire-resistance of the laminate. Vinylester Resins Vinylester resins are similar in their molecular structure to polyesters, but differ prima- rily 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 polyes- ters. 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 counter- parts, and are frequently found in applications such as pipelines and chemical stor- age tanks. The figure 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. Idealised Chemical Structure of a Typical Epoxy Based Vinylester Fig. 16 The molecular chains of vinylester, represented below, can be compared to the sche- matic representation of polyester shown previously where the difference in the loca- tion of the reactive sites can be clearly seen: Schematic Representation of Vinylester Resin (Uncured) Fig. 17 GTC-1-1098 - 13 C=C–C–0–C–C–C–0– –C– 0 II C I 0 II * ** * Ester groups *denotes reactive sites n = 1 to 2 OH I –0–C–C–C–0–C–C=C I C OH I n B A A A A A B GTC-1-1098 - 14 B A A A A A B B A A A A A B B A A A A A B S S With the reduced number of ester groups in a vinylester when compared to a polyes- ter, the resin is less prone to damage by hydrolysis. The material is therefore some- times 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. Schematic Representation of Vinylester Resin (Cured) Fig. 18 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 res- ins 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 ide- alised chemical structure is shown in the figure below and is the most easily identified characteristic of any more complex epoxy molecule. CH2 – CH – Idealised Chemical Structure of a Simple Epoxy (Ethylene Oxide) Fig. 19 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 chemi- cal resistance. Epoxies find uses as adhesives, caulking compounds, casting com- 0 tion are important. Polyester resins generally have the lowest adhesive properties of the three systems described here. Vinylester resin shows improved adhesive proper- ties over polyester but epoxy systems offer the best performance of all, and are there- fore 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 adhe- sive 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. Figs. 22 and 23 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 bi- products 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 Comparative Tensile Strength of Resins Fig. 22 Comparative Stiffness of Resins Fig. 23 GTC-1-1098 - 17 0 2 4 6 8 10 Polyester Vinlyester Epoxy Te ns ile S tre ng th (M Pa ) 7 days @ 20°C 5 hours @ 80°C 1 3 5 7 9 0 1 2 3 4 5 Polyester Vinlyester Epoxy Te ns ile M od ul us (G Pa ) 7 days @ 20°C 5 hours @ 80°C 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 com- pletely 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. Typical FRP Stress/Strain Graph Fig. 24 The strain that a laminate can reach before microcracking 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 se- verely 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-crack- ing 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. 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 GTC-1-1098 - 18 First Fibre/Resin Debonding Ultimate Tensile Strength Strain Strain to Failure Strain to First Fibre/Resin Micro-crack Te ns ile S tre ss the laminate. However, in an environment such as water or moist air, the micro-cracked laminate will absorb considerably more water than an uncracked 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 vari- ous resin systems is shown in Fig. 25. Typical Resin Stress/Strain Curves (Post-Cured for 5 hrs @ 80°C) Fig. 25 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 influ- enced 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, epoxy- based 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 air- craft 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, add- ing 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 long- term loss in mechanical properties. Both polyester and vinylester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecu- lar structures. As a result, a thin polyester laminate can be expected to retain only Epoxy 7%4.5%3% Strain St re ss Vinylester Polyester GTC-1-1098 - 19 Silicones Synthetic resin using silicon as the backbone rather than the carbon of organic poly- mers. 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. Typi- cal costs: >£50/kg. Polyimides Used where operation at higher temperatures than bismaleimides can stand is re- quired (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. Resin Systems Such as Silicones, BMI’s and Polyimides are Frequently Used for High Temperature Aircraft Parts. GTC-1-1098 - 22 Reinforcements The role of the reinforcement in a composite material is fundamentally one of increas- ing 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. GTC-1-1098 - 23 Basic Properties of Fibres and Other Engineering Materials Material Type Tensile Str. Tensile Modulus Typical Density Specific (MPa) (GPa) (g/cc) Modulus Carbon HS 3500 160 - 270 1.8 90 - 150 Carbon IM 5300 270 - 325 1.8 150 - 180 Carbon HM 3500 325 - 440 1.8 180 - 240 Carbon UHM 2000 440+ 2.0 200+ Aramid LM 3600 60 1.45 40 Aramid HM 3100 120 1.45 80 Aramid UHM 3400 180 1.47 120 Glass - E glass 2400 69 2.5 27 Glass - S2 glass 3450 86 2.5 34 Glass - quartz 3700 69 2.2 31 Aluminium Alloy (7020) 400 1069 2.7 26 Titanium 950 110 4.5 24 Mild Steel (55 Grade) 450 205 7.8 26 Stainless Steel (A5-80) 800 196 7.8 25 HS Steel (17/4 H900) 1241 197 7.8 25 The amount of fibre in the composite is largely governed by the manufacturing proc- ess 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 lami- nate’s strength will reach a peak and then begin to decrease due to the lack of suffi- cient 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’ prop- erties 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. GTC-1-1098 - 24 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 5-24µm 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. S- glass 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 vol- umes 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 4-13µm. 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 13-24µm. 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 re sultant 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 diam- GTC-1-1098 - 27 eters 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 fi- bres. 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 EXAMPLE: diameter (µ ) weight (tex) twist strands twist per metre E C 9 34 Z X2 S 150 E = Electrical C = Continuous Z = Clockwise S = High Strength S = Anti- clockwise Aramid Aramid fibre is a man-made organic polymer (an aromatic polyamide) produced by spinning a solid fibre from a liq- uid 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 applica- tions. 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 sup- plier offers several grades of aramid with various combinations of modulus and sur- face 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, car- bonisation and graphitisation of carbon-rich organic pre- cursors which are already in fibre form. The most com- mon 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 GTC-1-1098 - 28 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 in- creased 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 5-7µm. Carbon fibre has the highest specific stiffness of any commercially available fibre, very high strength in both ten- sion 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. Strength and Modulus Figures for Commercial PAN-based Carbon Fibres Grade Tensile Modulus Tensile Strength Country (GPa) (GPa) of Manufacture Standard Modulus (<265GPa) (also known as ‘High Strength’) T300 230 3.53 France/Japan T700 235 5.3 Japan HTA 238 3.95 Germany UTS 240 4.8 Japan 34-700 234 4.5 Japan/USA AS4 241 4.0 USA T650-35 241 4.55 USA Panex 33 228 3.6 USA/Hungary F3C 228 3.8 USA TR50S 235 4.83 Japan TR30S 234 4.41 Japan Intermediate Modulus (265-320GPa) T800 294 5.94 France/Japan M30S 294 5.49 France IMS 295 4.12/5.5 Japan MR40/MR50 289 4.4/5.1 Japan IM6/IM7 303 5.1/5.3 USA IM9 310 5.3 USA T650-42 290 4.82 USA T40 290 5.65 USA High Modulus (320-440GPa) M40 392 2.74 Japan M40J 377 4.41 France/Japan HMA 358 3.0 Japan UMS2526 395 4.56 Japan MS40 340 4.8 Japan HR40 381 4.8 Japan Ultra High Modulus (~440GPa) M46J 436 4.21 Japan UMS3536 435 4.5 Japan HS40 441 4.4 Japan UHMS 441 3.45 USA Information from manufacturer’s datasheets GTC-1-1098 - 29 Fibre Finishes Surface finishes are nearly always applied to fibres both to allow handling with mini- mum damage and to promote fibre/matrix interfacial bond strength. With carbon and aramid fibres for use in composite applications, the surface finish or size applied usually performs both functions. The finish is applied to the fibre at the point of fibre manufacture and this finish remains on the fibre throughout the conversion process into fabric. With glass fibre there is a choice of approach in the surface finish that can be applied. Glass Fibre Finishes Glass fibre rovings that are to be used in direct fibre proc- esses such as prepregging, pultrusion and filament wind- ing, are treated with a ‘dual-function’ finish at the point of fibre manufacture. Glass fibre yarns, however, when used for weaving are treated in two stages. The first finish is applied at the point of fibre manufacture at quite a high level and is purely for protection of the fibre against damage during handling and the weaving process itself. This protective finish, which is often starch based, is cleaned off or ‘scoured’ after the weaving process either by heat or with chemicals. The scoured woven fabric is then separately treated with a different matrix-compatible finish spe- cifically designed to optimise fibre to resin interfacial characteristics such as bond strength, water resistance and optical clarity. Carbon Fibre Finishes Finishes, or sizes, for carbon fibres used in structural composites are generally epoxy based, with varying levels being used depending on the end use of the fibre. For weaving the size level is about 1-2% by weight whereas for tape prepregging or fila- ment winding (or similar single-fibre processes), the size level is about 0.5-1%. The chemistry and level of the size are important not only for protection and matrix com- patibility but also because they effect the degree of spread of the fibre. Fibres can also be supplied unsized but these will be prone to broken filaments caused by gen- eral handling. Most carbon fibre suppliers offer 3-4 levels of size for each grade of fibre. Aramid Fibre Finishes Aramid fibres are treated with a finish at the point of manufacture primarily for matrix compatibility. This is because aramid fibres require far less protection from damage caused by fibre handling. The main types of fibre treatment are composite finish, rubber compatible finish (belts and tyres) and waterproof finish (ballistic soft armour). Like the carbon fibre finishes, there are differing levels of composite application finish depending on the type of process in which the fibre will be used. GTC-1-1098 - 32 Fabric Types and Constructions In polymeric composite terms, a fabric is defined as a manufactured assembly of long fibres of carbon, aramid or glass, or a combination of these, to produce a flat sheet of one or more layers of fibres. These layers are held together either by mechnical interlocking of the fibres themselves or with a secondary material to bind these fibres together and hold them in place, giving the assembly sufficient integrity to be han- dled. Fabric types are categorised by the orientation of the fibres used, and by the various construction methods used to hold the fibres together. The four main fibre orientation categories are: Unidirectional, 0/90°, Multiaxial, and Other/random. These are described below. Further details of many aspects of the different materials are contained in the reinforcement section of the SP Systems Com- posite Materials Handbook. Unidirectional Fabrics A unidirectional (UD) fabric is one in which the majority of fibres run in one direction only. A small amount of fibre or other material may run in other directions with the main intention being to hold the primary fibres in position, although the other fibres may also offer some structural properties. While some weavers of 0/90° fab- rics term a fabric with only 75% of its weight in one direction as a unidirectional, at SP Systems the unidi- rectional designation only applies to those fabrics with more than 90% of the fibre weight in one direction. Unidirectionals usually have their primary fibres in the 0° direction (along the roll – a warp UD) but can also have them at 90° to the roll length (a weft UD). True unidirectional fabrics offer the ability to place fibre in the component exactly where it is required, and in the optimum quantity (no more or less than required). As well as this, UD fibres are straight and uncrimped. This results in the highest possible fibre properties from a fabric in composite component construction. For mechanical properties, unidirectional fabrics can only be improved on by prepreg unidirectional tape, where there is no secondary material at all holding the unidirectional fibres in place. In these prepreg products only the resin system holds the fibres in place. Unidirectional Construction There are various methods of maintaining the primary fibres in position in a unidirec- tional including weaving, stitching, and bonding. As with other fabrics, the surface quality of a unidirectional fabric is determined by two main factors: the combination of tex and thread count of the primary fibre and the amount and type of the secondary fibre. The drape, surface smoothness and stability of a fabric are controlled primarily by the construction style, while the area weight, porosity and (to a lesser degree) wet out are determined by selecting the appropriate combination of fibre tex and numbers of fibres per cm. GTC-1-1098 - 33 Warp or weft unidirectionals can be made by the stitching process (see information in the ‘Multiaxial’ section of this publication). However, in order to gain adequate stabil- ity, it is usually necessary to add a mat or tissue to the face of the fabric. Therefore, together with the stitching thread required to assemble the fibres, there is a relatively large amount of secondary, parasitic material in this type of UD fabric, which tends to reduce the laminate properties. Furthermore the high cost of set up of the 0° layer of a stitching line and the relatively slow speed of production means that these fabrics can be relatively expensive. 0/90° Fabrics For applications where more than one fibre orientation is required, a fabric combining 0° and 90° fibre orientations is useful. The majority of these are woven products. 0/ 90° fabrics can be produced by stitching rather than a weaving process and a de- scription of this stitching technology is given below under ‘Multiaxial Fabrics’. Woven Fabrics Woven fabrics are produced by the interlacing of warp (0°) fibres and weft (90°) fibres in a regular pattern or weave style. The fabric’s integrity is maintained by the mechani- cal interlocking of the fibres. Drape (the ability of a fabric to conform to a complex surface), surface smoothness and stability of a fabric are controlled primarily by the weave style. The area weight, porosity and (to a lesser degree) wet out are deter- mined by selecting the correct combination of fibre tex and the number of fibres/cm*. The following is a description of some of the more commonly found weave styles: Plain Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability and reasonable porosity. However, it is the most difficult of the weaves to drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles. With large fibres (high tex) this weave style gives excessive crimp and therefore it tends not to be used for very heavy fabrics. Twill One or more warp fibres alternately weave over and un- der two or more weft fibres in a regular repeated manner. This produces the visual effect of a straight or broken di- agonal ‘rib’ to the fabric. Superior wet out and drape is seen in the twill weave over the plain weave with only a small reduction in stability. With reduced crimp, the fabric also has a smoother surface and slightly higher mechani- cal properties. GTC-1-1098 - 34 Stitched 0/90o Fabrics 0/90° fabrics can also be made by a stitching process, which effectively combines two layers of unidirectional material into one fabric. Stitched 0/90° fabrics can offer mechanical performance increases of up to 20% in some properties over woven fabrics, due to the following factors: 1. Parallel non-crimp fibres bear the strain immediately upon being loaded. 2. Stress points found at the intersection of warp and weft fibres in woven fabrics are eliminated. 3. A higher density of fibre can be packed into a laminate compared with a woven. In this respect the fabric behaves more like layers of unidirectional. Other benefits compared with woven fabrics include: 1. Heavy fabrics can be easily produced with more than 1kg/sqm of fibre. 2. Increase packing of the fibre can reduce the quantity of resin required. Hybrid Fabrics The term hybrid refers to a fabric that has more than one type of structural fibre in its construction. In a multi-layer laminate if the properties of more than one type of fibre are required, then it would be possible to provide this with two fabrics, each contain- ing the fibre type needed. However, if low weight or extremely thin laminates are required, a hybrid fabric will allow the two fibres to be presented in just one layer of fabric instead of two. It would be possible in a woven hybrid to have one fibre running in the weft direction and the second fibre running in the warp direction, but it is more common to find alternating threads of each fibre in each warp/weft direction. Al- though hybrids are most commonly found in 0/90° woven fabrics, the principle is also used in 0/90° stitched, unidirectional and multiaxial fabrics. The most usual hybrid combinations are: Carbon / Aramid The high impact resistance and tensile strength of the aramid fibre combines with high the compressive and tensile strength of carbon. Both fibres have low density but relatively high cost. Aramid / Glass The low density, high impact resistance and tensile strength of aramid fibre combines with the good compressive and tensile strength of glass, coupled with its lower cost. Carbon / Glass Carbon fibre contributes high tensile compressive strength and stiffness and reduces the density, while glass reduces the cost. GTC-1-1098 - 37 Multiaxial Fabrics In recent years multiaxial fabrics have begun to find favour in the construction of composite components. These fabrics consist of one or more layers of long fibres held in place by a secondary non-structural stitching tread. The main fibres can be any of the structural fibres available in any combination. The stitching thread is usu- ally polyester due to its combination of appropriate fibre properties (for binding the fabric together) and cost. The stitching process allows a variety of fibre orientations, beyond the simple 0/90° of woven fabrics, to be combined into one fabric. Multiaxial fabrics have the following main characteristics: Advantages The two key improvements with stitched multiaxial fabrics over woven types are: (i) Better mechanical properties, primarily from the fact that the fibres are always straight and non-crimped, and that more orientations of fibre are available from the increased number of layers of fabric. (ii) Improved component build speed based on the fact that fabrics can be made thicker and with multiple fibre orientations so that fewer layers need to be included in the laminate sequence. Disadvantages Polyester fibre does not bond very well to some resin systems and so the stitching can be a starting point for wicking or other failure initiation. The fabric production process can also be slow and the cost of the machinery high. This, together with the fact that the more expensive, low tex fibres are required to get good surface coverage for the low weight fabrics, means the cost of good quality, stitched fabrics can be relatively high compared to wovens. Extremely heavy weight fabrics, while enabling large quan- tities of fibre to be incorporated rapidly into the component, can also be difficult to impregnate with resin without some automated process. Finally, the stitching proc- ess, unless carefully controlled as in the SP fabric styles, can bunch together the fibres, particularly in the 0° direction, creating resin-rich areas in the laminate. Fabric Construction The most common forms of this type of fabric are shown in the following diagrams: GTC-1-1098 - 38 SP Style Type X SP Style Type Y SP Style Type Z SP Style Type Q2 SP Style Type Q1 Roll Direction There are two basic ways of manufacturing multiaxial fabrics: Weave & Stitch With the ‘Weave & Stitch’ method the +45° and -45° layers can be made by weaving weft Unidirectionals and then skewing the fabric, on a special machine, to 45°. A warp unidirectional or a weft unidirectional can also be used unskewed to make a 0° and 90° layer If both 0° and 90° layers are present in a multi-layer stitched fabric then this can be provided by a conventional 0/90° woven fabric. Due to the fact that heavy rovings can be used to make each layer the weaving process is relatively fast, as is the subsequent stitching together of the layers via a simple stitching frame. To make a quadraxial (four-layer: +45°, 0°, 90°, -45°) fabric by this method, a weft unidirectional would be woven and skewed in one direction to make the +45° layer, and in the other to make the -45° layer. The 0° and 90° layers would appear as a single woven fabric. These three elements would then be stitched together on a stitching frame to produce the final four-axis fabric. Simultaneous Stitch Simultaneous stitch manufacture is carried out on special machines based on the knitting process, such as those made by Liba, Malimo, Mayer, etc. Each machine varies in the precision with which the fibres are laid down, particularly with reference to keeping the fibres parallel. These types of machine have a frame which simultane- ously draws in fibres for each axis/layer, until the required layers have been assem- bled, and then stitches them together, as shown in the diagram below. GTC-1-1098 - 39 Weft Unidirectional Layer 1 Layer 2 Skew to 45o Stitch the skewed layers together - 45o + 45o ± 45o fabric Courtesy Liba Maschinenfabrick GMBH GTC-1-1098 - 42 PVC Foam Closed-cell polyvinyl chloride (PVC) foams are one of the most commonly used core materials for the construction of high performance sandwich structures. Although strictly they are a chemical hybrid of PVC and polyurethane, they tend to be referred to simply as ‘PVC foams’. PVC foams offer a balanced combination of static and dynamic properties and good resistance to water absorption. They also have a large operating temperature range of typically -240°C to +80°C (-400°F to +180°F), and are resistant to many chemicals. Although PVC foams are generally flammable, there are fire-retardant grades that can be used in many fire-critical applications, such as train components. When used as a core for sandwich construction with FRP skins, its reasonable resistance to styrene means that it can be used safely with polyester resins and it is therefore popular in many industries. It is normally supplied in sheet form, either plain, or grid-scored to allow easy forming to shape. There are two main types of PVC foam: crosslinked and uncrosslinked with the uncrosslinked foams sometimes being referred to as ‘linear’. The uncrosslinked foams (such as Airex R63.80) are tougher and more flexible, and are easier to heat-form around curves. However, they have some lower mechanical properties than an equiva- lent density of cross-linked PVC, and a lower resistance to elevated temperatures and styrene. Their cross-linked counterparts are harder but more brittle and will pro- duce a stiffer panel, less susceptible to softening or creeping in hot climates. Typical cross-linked PVC products include the Herex C-series of foams, Divinycell H and HT grades and Polimex Klegecell and Termanto products. A new generation of toughened PVC foams is now also becoming available which trade some of the basic mechanical properties of the cross-linked PVC foams for some of the improved toughness of the linear foams. Typical products include Divincell HD grade. Owing to the nature of the PVC/polyurethane chemistry in cross-linked PVC foams, these materials need to be thoroughly sealed with a resin coating before they can be safely used with low-temperature curing prepregs. Although special heat stabilisation treatments are available for these foams, these treatments are primarily designed to improve the dimensional stability of the foam, and reduce the amount of gassing that is given off during elevated temperature processing. Polystyrene Foams Although polystyrene foams are used extensively in sail and surf board manufacture, where their light weight (40kg/m3), low cost and easy to sand characteristics are of prime importance, they are rarely employed in high performance component con- struction because of their low mechanical properties. They cannot be used in con- junction with polyester resin systems because they will be dissolved by the styrene present in the resin. Polyurethane Foams Polyurethane foams exhibit only moderate mechanical properties and have a ten- dency for the foam surface at the resin/core interface to deteriorate with age, leading to skin delamination. Their structural applications are therefore normally limited to the GTC-1-1098 - 43 production of formers to create frames or stringers for stiffening components. How- ever, polyurethane foams can be used in lightly loaded sandwich panels, with these panels being widely used for thermal insulation. The foam also has reasonable el- evated service temperature properties (150°C/300°F), and good acoustic absorption. The foam can readily be cut and machined to required shapes or profiles. Polymethyl methacrylamide Foams For a given density, polymethyl methacrylamide (acrylic) foams such as Rohacell of- fer some of the highest overall strengths and stiffnesses of foam cores. Their high dimensional stability also makes them unique in that they can readily be used with conventional elevated temperature curing prepregs. However, they are expensive, which means that their use tends to be limited to aerospace composite parts such as helicopter rotor blades, and aircraft flaps. Styrene acrylonitrile (SAN) co-polymer Foams SAN foams behave in a similar way to toughened cross-linked PVC foams. They have most of the static properties of cross-linked PVC cores, yet have much higher elongations and toughness. They are therefore able to absorb impact levels that would fracture both conventional and even the toughened PVC foams. However, un- like the toughened PVC’s, which use plasticizers to toughen the polymer, the tough- ness properties of SAN are inherent in the polymer itself, and so do not change appre- ciably with age. SAN foams are replacing linear PVC foams in many applications since they have much of the linear PVC’s toughness and elongation, yet have a higher temperature performance and better static properties. However, they are still thermoformable, which helps in the manufacture of curved parts. Heat-stabilised grades of SAN foams can also be more simply used with low-temperature curing prepregs, since they do not have the interfering chemistry inherent in the PVC’s. Typical SAN products include ATC Core-Cell’s A-series foams. Other thermoplastics As new techniques develop for the blowing of foams from thermoplastics, the range of expanded materials of this type continues to increase. Typical is PEI foam, an ex- panded polyetherimide/polyether sulphone, which combines outstanding fire perform- ance with high service temperature. Although it is expensive, this foam can be used in structural, thermal and fire protection applications in the service temperature range -194°C (-320°F) to +180°C (+355°F). It is highly suitable for aircraft and train interiors, as it can meet some of the most stringent fire resistant specifications. Honeycombs Honeycomb cores are available in a variety of materials for sandwich structures. These range from paper and card for low strength and stiffness, low load applications (such as domestic internal doors) to high strength and stiffness, extremely lightweight components for aircraft structures. Honeycombs can be processed into both flat and curved composite structures, and can be made to conform to com- pound curves without excessive mechanical force or heat- ing. Thermoplastic honeycombs are usually produced by extrusion, followed by slicing to thickness. Other honeycombs (such as those made of paper and aluminium) are made by a multi-stage process. In these cases large thin sheets of the material (usu- ally 1.2x2.4m) are printed with alternating, parallel, thin stripes of adhesive and the sheets are then stacked in a heated press while the adhesive cures. In the case of aluminium honeycomb the stack of sheets is then sliced through its thickness. The slices (known as ‘block form’) are later gently stretched and expanded to form the sheet of continuous hexagonal cell shapes. In the case of paper honeycombs, the stack of bonded paper sheets is gently ex- panded to form a large block of honeycomb, several feet thick. Held in its expanded form, this fragile paper honeycomb block is then dipped in a tank of resin, drained and cured in an oven. Once this dipping resin has cured, the block has sufficient strength to be sliced into the final thicknesses required. In both cases, by varying the degree of pull in the expansion process, regular hexa- gon-shaped cells or over-expanded (elongated) cells can be produced, each with different mechanical and handling/drape properties. Due to this bonded method of construction, a honeycomb will have different mechanical properties in the 0° and 90° directions of the sheet. While skins are usually of FRP, they may be almost any sheet material with the appro- priate properties, including wood, thermoplastics (eg melamine) and sheet metals, such as aluminium or steel. The cells of the honeycomb structure can also be filled with a rigid foam. This provides a greater bond area for the skins, increases the mechanical properties of the core by stabilising the cell walls and increases thermal and acoustic insulation properties. Properties of honeycomb materials depend on the size (and therefore frequency) of the cells and the thickness and strength of the web material. Sheets can range from typically 3-50 mm in thickness and panel dimensions are typically 1200 x 2400mm, although it is possible to produce sheets up to 3m x 3m. Honeycomb cores can give stiff and very light laminates but due to their very small bonding area they are almost exclusively used with high-performance resin systems such as epoxies so that the necessary adhesion to the laminate skins can be achieved. Aluminium honeycomb Aluminium honeycomb produces one of the highest strength/weight ratios of any struc- tural material. There are various configurations of the adhesive-bonding of the alu- minium foil which can lead to a variety of geometric cell shapes (usually hexagonal). Properties can also be controlled by varying the foil thickness and cell size. The honeycomb is usually supplied in the unexpanded block form and is stretched out into a sheet on-site. Despite its good mechanical properties and relatively low price, aluminium honey- comb has to be used with caution in some applications, such as large marine struc- tures, because of the potential corrosion problems in a salt-water environment. In this situation care also has to be exercised to ensure that the honeycomb does not come into direct contact with carbon skins since the conductivity can aggravate galvanic corrosion. Aluminium honeycomb also has the problem that it has no ‘mechanical GTC-1-1098 - 44 GTC-1-1098 - 47 30 40 50 60 70 80 90 100 Density kg/m3 PVC Foam s Balsa Acry lic-F oam Ho ney com b ( Al. & No me x) Co m pr es si ve S tre ng th 30 40 50 60 70 80 90 100 Density kg/m3 PVC Foam s Balsa Acry lic-F oam Alu m. Ho ney com b No me x H on ey co mb Sh ea r S tre ng th much heavier than a foam or honeycomb core, is lower in density than the equivalent thickness of glass fibre laminate. Being so thin they can also conform easily to 2-D curvature, and so are quick and easy to use. Comparison of Core Mechanical Properties Figs. 34 and 35 give the shear strength and compressive strength of some of the core materials described, plotted against their densities. All the figures have been ob- tained from manufacturers’ data sheets. Compressive Strength v Core Density Shear Strength v Core Density Fig.34 Fig. 35 As might be expected, all the cores show an increase in properties with increasing density. However, other factors, besides density, also come into play when looking at the weight of a core in a sandwich structure. For example, low density foam materials, while contributing very little to the weight of a sandwich laminate, often have a very open surface cell structure which can mean that a large mass of resin is absorbed in their bondlines. The lower the density of the foam, the larger are the cells and the worse is the problem. Honeycombs, on the other hand, can be very good in this respect since a well formulated adhesive will form a small bonding fillet only around the cell walls (see Fig.36). Finally, consideration needs to be given to the form a core is used in to ensure that it fits the component well. The weight savings that cores can offer can quickly be used up if cores fit badly, leaving large gaps that require filling with adhesive. Scrim-backed foam or balsa, where little squares of the core are supported on a lightweight scrim cloth, can be used to help cores conform better to a curved surface. Contour-cut foam, where slots are cut part-way through the core from opposite sides achieves a similar effect. However, both these cores still tend to use quite large amounts of adhesive since the slots between each foam square need filling with resin to produce a good structure. In weight-critical components the use of foam cores which are thermoformable should be considered. These include the linear PVC’s and the SAN foams which can all be heated to above their softening points and pre-curved to fit a mould shape. For hon Glue fillets Honeycomb Foam Glue line Skin Skin eycombs, over-expanded forms are the most widely used when fitting the core to a compound curve, since with different expansion patterns a wide range of conform- ability can be achieved. Core/Laminate Bond for Foams and Honeycombs Fig. 36 GTC-1-1098 - 48 GTC-1-1098 - 49 Cores - Properties Corecell Linear PVC Cross-linked Cross-linked Copolymer PU rigid PEI/PES Aluminium Aluminium Aramid Aramid Property Test Unit foam foam PVC foam PVC foam foam foam foam honeycomb honeycomb honeycomb honeycomb low density high density high density closed-cell lengthways widthways lengthways widthways Apparent nominal ISO 945 kg/m3 50-200 50-80 40-80 100-200 200-400 60 80 density D 1622 lb/ft3 3.5-12.5 3.1-5.0 2.5-5.0 6.25-12.5 12.5-25 3.7 5 Compressive ISO 844 N/mm2 0.4-0.9 0.5-1.4 2.0-4.6 4.0-13.0 0.42 0.75 4.2 strength D 1621 psi 63-584 60-130 70-200 290-667 580-1885 61 110 620-73 125-1870 Tensile DIN 53455 N/mm2 1.2-1.8 0.5-1.9 2.6-6.0 strength C 297 psi 150-468 165-260 75-230 340-870 Flexural DIN 53455 N/mm2 1.9 strength D 790 psi 173-1024 276 Shear ISO 1922 N/mm2 0.5-1.2 0.4-1.2 1.6-3.5 3.0-8.0 0.41 0.9 2.38 1.48 strength C 273 psi 96-286 70-170 60-160 220-508 435-1160 59 130 85-480 45-395 E-modulus DIN 53457 N/mm2 37-56 26-75 110-223 155-350 20 45 compression D 1621 psi 2132-18408 5365-8120 3900-10850 15950-32346 22480-37600 2900 6530 148-16 6-90 E-modulus DIN53457 N/mm2 37-64 29-57 80-188 tensile D 1621 psi 5365-9280 4200-9700 12300-27270 E-modulus DIN53457 N/mm2 52 flexural D 790 psi 7458-42441 7540 Shear ASTM C 393 N/mm2 15-21 12-30 38-77 60-240 4.1 18 modulus psi 1699-6555 2175-3045 1750-4600 5450-11170 8700-34810 595 2610 63-14 31-7 3.7-17 2.0-9.0 Shear elongation ISO 1922 % 60-40 80 10-30 30-31 7-6 30 30 at break C273 Impact DIN 53453 kJ/m2 4.0-5.0 0.2-0.9 1.4-4.0 1.40-4.60 0.9 1.6 strength 1.9-2.4 0.007-0.29 0.33-1.01 0.71-2.34 0.4 0.4 Thermal DIN 52612 W/m K 0.033-0.035 0.029-0.033 0.038-0.042 0.048-0.055 0.030 0.035 conductivity C 177 0.229-0.243 0.19-0.23 0.333-0.382 0.208 Maximum operating DIN 53445 °C 55-60 65-75 80 80 150 190 temperature °F 130-140 149-167 176 200 300 375 Water absorption DIN 53428 Vol.% 2.3 7 day Data from Reinforced Plastics Handbook, 1st edition. Reprinted by permission of the publishers. GTC-1-1098 - 52 Materials Options: Resins: Any, e.g. epoxy, polyester, vinylester, phenolic. Fibres: Any, although heavy aramid fabrics can be hard to wet-out by hand. Cores: Any. Main Advantages: i) Widely used for many years. ii) Simple principles to teach. iii) Low cost tooling, if room-temperature cure resins are used. iv) Wide choice of suppliers and material types. v) Higher fibre contents, and longer fibres than with spray lay-up. Main Disadvantages: i) Resin mixing, laminate resin contents, and laminate quality are very dependent on the skills of laminators. Low resin content laminates cannot usually be achieved without the incorporation of excessive quantities of voids. ii) Health and safety considerations of resins. The lower molecular weights of hand lay-up resins generally means that they have the potential to be more harmful than higher molecular weight products. The lower viscosity of the resins also means that they have an increased tendency to penetrate clothing etc. iii) Limiting airborne styrene concentrations to legislated levels from polyesters and vinylesters is becoming increasingly hard without expensive extraction systems. iv) Resins need to be low in viscosity to be workable by hand. This generally compromises their mechanical/thermal properties due to the need for high diluent/styrene levels. Typical Applications: Standard wind-turbine blades, production boats, architectural mouldings. GTC-1-1098 - 53 Vacuum Bagging Description This is basically an extension of the wet lay-up process described above where pressure is applied to the laminate once laid-up in order to improve its consolidation. This is achieved by sealing a plastic film over the wet laid-up laminate and onto the tool. The air under the bag is extracted by a vacuum pump and thus up to one atmosphere of pressure can be applied to the laminate to consolidate it. Materials Options: Resins: Primarily epoxy and phenolic. Polyesters and vinylesters may have problems due to excessive extraction of styrene from the resin by the vacuum pump. Fibres: The consolidation pressures mean that a variety of heavy fabrics can be wet-out. Cores: Any. Main Advantages: i) Higher fibre content laminates can usually be achieved than with standard wet lay-up techniques. ii) Lower void contents are achieved than with wet lay-up. iii) Better fibre wet-out due to pressure and resin flow throughout structural fibres, with excess into bagging materials. iv) Health and safety: The vacuum bag reduces the amount of volatiles emitted during cure. Main Disadvantages: i) The extra process adds cost both in labour and in disposable bagging materials ii) A higher level of skill is required by the operators iii) Mixing and control of resin content still largely determined by operator skill Typical Applications: Large, one-off cruising boats, racecar components, core-bonding in production boats. Sealant Tape Vacuum Bagging Film Release Film (Perforated) Release Coated Mould Laminate Peel Ply Breather/Bleeder Fabric To Vacuum Pump To Vacuum Gauge GTC-1-1098 - 54 Filament Winding Description This process is primarily used for hollow, generally circular or oval sectioned components, such as pipes and tanks. Fibre tows are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fibre feeding mechanism, and rate of rotation of the mandrel. Materials Options: Resins: Any, e.g. epoxy, polyester, vinylester, phenolic. Fibres: Any. The fibres are used straight from a creel and not woven or stitched into a fabric form. Cores: Any, although components are usually single skin. Main Advantages: i) This can be a very fast and therefore economic method of laying material down. ii) Resin content can be controlled by metering the resin onto each fibre tow through nips or dies. iii) Fibre cost is minimised since there is no secondary process to convert fibre into fabric prior to use. iv) Structural properties of laminates can be very good since straight fibres can be laid in a complex pattern to match the applied loads. Main Disadvantages: i) The process is limited to convex shaped components. ii) Fibre cannot easily be laid exactly along the length of a component. iii) Mandrel costs for large components can be high. iv) The external surface of the component is unmoulded, and therefore cosmetically unattractive. Angle of fibre warp controlled by ratio of carriage speed to rotaional speed Moving Carriage Fibres Resin Bath Nip RollersRotating Mandrel To Creel GTC-1a-1098 - 57 iii) Possible labour reductions. iv) Both sides of the component have a moulded surface. Main Disadvantages: i) Matched tooling is expensive, and heavy in order to withstand pressures. ii) Generally limited to smaller components. iii) Unimpregnated areas can occur resulting in very expensive scrap parts. Typical Applications: Small complex aircraft and automotive components, train seats. Other Infusion Processes - SCRIMP, RIFT, VARTM etc. Description Fabrics are laid up as a dry stack of materials as in RTM. The fibre stack is then covered with peel ply and a knitted type of non-structural fabric. The whole dry stack is then vacuum bagged, and once bag leaks have been eliminated, resin is allowed to flow into the laminate. The resin distribution over the whole laminate is aided by resin flowing easily through the non-structural fabric, and wetting the fabric out from above. Materials Options: Resins: Generally epoxy, polyester and vinylester. Fibres: Any conventional fabrics. Stitched materials work well in this process since the gaps allow rapid resin transport. Cores: Any except honeycombs. Main Advantages: i) As RTM above, except only one side of the component has a moulded finish. ii) Much lower tooling cost due to one half of the tool being a vacuum bag, and less strength being required in the main tool. iii) Large components can be fabricated. Mould Tool Vacuum Bag Reinforcement Stack To Vacuum Pump Resin drawn across and through reinforcements by vacuum Peel Ply and/or Resin Distribution Fabric Resin Sealant Tape GTC-1-1098 - 58 iv) Standard wet lay-up tools may be able to be modified for this process. v) Cored structures can be produced in one operation. Main Disadvantages: i) Relatively complex process to perform well. ii) Resins must be very low in viscosity, thus comprising mechanical properties. iii) Unimpregnated areas can occur resulting in very expensive scrap parts. iv) Some elements of this process are covered by patents (SCRIMP). Typical Applications: Semi-production small yachts, train and truck body panels. Prepregs Autoclave Description Fabrics and fibres are pre-impregnated by the materials manufacturer, under heat and pressure or with solvent, with a pre-catalysed resin. The catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months, of useful life when defrosted. However to prolong storage life the materials are stored frozen. The resin is usually a near-solid at ambient temperatures, and so the pre-impregnated materials (prepregs) have a light sticky feel to them, such as that of adhesive tape. Unidirectional materials take fibre direct from a creel, and are held together by the resin alone. The prepregs are laid up by hand or machine onto a mould surface, vacuum bagged and then heated to typically 120-180°C. This allows the resin to initially reflow and eventually to cure. Additional pressure for the moulding is usually provided by an autoclave (effectively a pressurised oven) which can apply up to 5 atmospheres to the laminate. Materials Options: Resins: Generally epoxy, polyester, phenolic and high temperature resins such as polyimides, cyanate esters and bismaleimides. Fibres: Any. Used either direct from a creel or as any type of fabric. To Vacuum Pump Cores: Any, although special types of foam need to be used due to the elevated temperatures involved in the process. Main Advantages: i) Resin/catalyst levels and the resin content in the fibre are accurately set by the materials manufacturer. High fibre contents can be safely achieved. ii) The materials have excellent health and safety characteristics and are clean to work with. iii) Fibre cost is minimised in unidirectional tapes since there is no secondary proc- ess to convert fibre into fabric prior to use. iv) Resin chemistry can be optimised for mechanical and thermal performance, with the high viscosity resins being impregnable due to the manufacturing process. v) The extended working times (of up to several months at room temperatures) means that structurally optimised, complex lay-ups can be readily achieved. vi) Potential for automation and labour saving. Main Disadvantages: i) Materials cost is higher for preimpregnated fabrics. ii) Autoclaves are usually required to cure the component. These are expensive, slow to operate and limited in size. iii) Tooling needs to be able to withstand the process temperatures involved iv) Core materials need to be able to withstand the process temperatures and pressures. Typical Applications: Aircraft structural components (e.g. wings and tail sections), F1 racing cars, sporting goods such as tennis racquets and skis. GTC-1-1098 - 59 Estimating Quantities of SP Formulated Products Laminating Resins Resin/ Hardener Mix Required (kg) = A x n x WF x R.C x 1.5*(1 - R.C.) Where: A = Area of Laminate (sq.m) n = Number of plies WF = Fibre weight of each ply (g/sq.m) R.C. = Resin content by weight Typical R.C.’s for hand layup manufacturing are: Glass - 0.46 Carbon - 0.55 Aramid - 0.61 Gelcoats and Coatings Solvent Free Resin/ Hardener Mix Required (kg) = A x t x ρm x 1.5* 1000 Solvent Based Resin/ Hardener Mix Required (kg) = A x t x ρm x 1.5* 10 x S.C. Where A = Area to be coated (sq.m) t = Total finished thickness required (µm) ρm = Density of cured resin/hardener matrix (g/cm 3) S.C. = Solids content of mix (%) *Assuming 50% wastage, for resin residue left in mixing pots and on tools. This wastage figure is based on SP Systems’ experience of a wide variety of workshops, but should be adjusted to match local working practices. Laminate Formulae Fibre Volume Fraction From Densities FVF = (ρC - ρm) (assuming zero void content) (ρF - ρm) Fibre Volume Fraction from Fibre Weight Fraction ρF ρm 1 FWF -11 + FVF = 1 Fibre Weight Fraction from Fibre Volume Fraction Cured Ply Thickness from Ply Weight CPT (mm) = WF ρF x FVF x 1000 Where FVF = Fibre Volume Fraction FWF = Fibre Weight Fraction ρc = Density of Composite (g/cm 3) ρm = Density of Cured Resin/ Hardener Matrix (g/cm 3) ρF = Density of Fibres ( g/cm 3) WF = Fibre Area Weight of each Ply (g/sqm) (ρF - ρm) x FVFρm + FWF = ρF x FVF Imperial/Metric Conversion Tables For SP Products The bold figures in the central columns can be read as either the metric or the British measure. Thus 1 inch = 25.4 millimetres: or 1 millimetre = 0.039 inches. °C °F -18 0 0 32 5 41 10 50 15 59 20 68 25 77 30 86 35 95 40 104 45 113 50 122 55 131 60 140 65 149 70 158 75 167 80 176 85 185 90 194 95 203 100 212 105 221 110 230 Mil (thou) Microns (µM) 0.039 1 25.40 0.079 2 50.80 0.118 3 76.20 0.157 4 101.60 0.197 5 127.00 0.236 6 152.40 0.276 7 177.80 0.315 8 203.20 0.354 9 228.60 Inches mm 0.039 1 25.4 0.079 2 50.8 0.118 3 76.2 0.157 4 101.6 0.197 5 127.0 0.236 6 152.4 0.276 7 177.8 0.315 8 203.2 0.354 9 228.6 Pints Litres 1.760 1 0.568 3.520 2 1.137 5.279 3 1.705 7.039 4 2.273 8.799 5 2.841 10.559 6 3.410 12.318 7 3.978 14.078 8 4.546 15.838 9 5.114 Feet Metres 3.281 1 0.305 6.562 2 0.610 9.843 3 0.914 13.123 4 1.219 16.404 5 1.524 19.685 6 1.829 22.966 7 2.134 26.247 8 2.438 29.528 9 2.743 US Quarts Litres 1.057 1 0.946 2.114 2 1.892 3.171 3 2.838 4.228 4 3.784 5.285 5 4.73 6.342 6 5.676 7.400 7 6.622 8.457 8 7.568 9.514 9 8.514 Yards Metres 1.094 1 0.914 2.187 2 1.829 3.281 3 2.743 4.374 4 3.658 5.468 5 4.572 6.562 6 5.486 7.655 7 6.401 8.749 8 7.315 9.843 9 8.230 US Gal. Litres 0.264 1 3.785 0.528 2 7.570 0.792 3 11.355 1.056 4 15.140 1.32 5 18.925 1.584 6 22.710 1.848 7 26.495 2.112 8 30.280 2.376 9 34.065 Ounces Grams 0.035 1 28.350 0.071 2 56.699 0.106 3 85.048 0.141 4 113.398 0.176 5 141.748 0.212 6 170.097 0.247 7 198.446 0.282 8 226.796 0.317 9 255.146 Imp.Gal. Litres 0.220 1 4.546 0.440 2 9.092 0.660 3 13.638 0.880 4 18.184 1.100 5 22.730 1.320 6 27.277 1.540 7 31.823 1.760 8 36.369 1.980 9 40.915 Sq. yds Sq. metres 1.196 1 0.836 2.392 2 1.672 3.588 3 2.508 4.784 4 3.345 5.980 5 4.181 7.176 6 5.017 8.372 7 5.853 9.568 8 6.689 10.764 9 7.525 Miles Kilomtrs. 0.621 1 1.609 1.243 2 3.219 1.864 3 4.828 2.485 4 6.437 3.107 5 8.047 3.728 6 9.656 4.350 7 11.265 4.971 8 12.875 5.592 9 14.484 oz/sq.yd g/sq.m 0.029 1 33.9 0.059 2 67.9 0.088 3 101.8 0.118 4 135.8 0.147 5 169.7 0.177 6 203.6 0.206 7 237.6 0.236 8 271.5 0.265 9 305.5 lb/cu.ft kg/cu.m 0.062 1 16.0 0.125 2 32.1 0.187 3 48.1 0.250 4 64.1 0.312 5 80.2 0.374 6 96.2 0.437 7 112.2 0.499 8 128.2 0.561 9 144.3 Pounds Kilograms 2.205 1 0.454 4.409 2 0.907 6.614 3 1.361 8.818 4 1.814 11.023 5 2.268 13.228 6 2.722 15.432 7 3.175 17.637 8 3.629 19.842 9 4.082 US Gal. Imp. Gal. 1.200 1 0.833 2.401 2 1.666 3.601 3 2.499 4.802 4 3.332 6.002 5 4.165 7.203 6 4.998 8.403 7 5.831 9.604 8 6.664 10.804 9 7.497 ksi N/mm2 (MPa) 0.145 1 6.9 0.290 2 13.8 0.435 3 20.7 0.580 4 27.6 0.725 5 34.5 0.870 6 41.4 1.015 7 48.3 1.160 8 55.2 1.305 9 62.1 Msi Gpa 0.145 1 6.895 0.290 2 13.790 0.435 3 20.685 0.580 4 27.580 0.725 5 34.475 0.870 6 41.370 1.015 7 48.265 1.160 8 55.160 1.305 9 62.055 Cu. Feet Cu. Metres 35.315 1 0.028 70.629 2 0.057 105.944 3 0.085 141.259 4 0.113 176.573 5 0.142 211.888 6 0.170 247.203 7 0.198 282.517 8 0.227 317.832 9 0.255 lb/cu.in g/cu.cm 0.036 1 27.7 0.029 0.8 22.1 0.031 0.85 23.5 0.033 0.9 24.9 0.034 0.95 26.3 0.038 1.05 29.1 0.040 1.1 30.4 0.042 1.15 31.8 0.043 1.2 33.2 °C °F 115 239 120 248 125 257 130 266 135 275 140 284 145 293 150 302 155 311 160 320 165 329 170 338 175 347 180 356 185 365 190 374 195 383 200 392 205 401 210 410 215 419 220 428 225 437 230 446 Fluid Oz. Litres 35.21 1 0.028 70.42 2 0.057 105.63 3 0.085 140.84 4 0.114 176.06 5 0.142 211.27 6 0.170 246.48 7 0.199 281.69 8 0.227 316.90 9 0.256