DM308 Production Techniques 2 Lecture 8 - Composite Materials PDF

DM308 Production Techniques 2 Lecture 8 – Composite materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Properties of composites What is a composite? A composite material is a m...

DM308 Production Techniques 2 Lecture 8 – Composite materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Properties of composites What is a composite? A composite material is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure, differentiating composites from mixtures and solid solutions. reinforcement matrix composite Review of composite materials Metal matrix composites Ceramic matrix composites ` MMCs Matrix is ductile Matrix is strong in tension Typically reinforced with ceramics Electrically conducting Polymer matrix composites PMCs Matrix is very tough Matrix can have low density Typically reinforced with ceramics CMCs Matrix is hard and wear resistant Matrix has thermal resistance Reinforced with ceramics/metals Natural composites Nature is smart Typically PMCs Parts of plants and animals The original engineering materials Types of composites by matrix As discussed on the previous slide, composites are classified into families according to the matrix materials. MMCs ↓ CMCs ↓ PMCs ↓ Matrix → ---------------------Reinforcement ↓ Metal Ceramic Polymer Metal Powder metallurgy parts combining immiscible powders Cermets – ceramic+metal composites Brake pads. Ceramic Cermets, cemented carbides, fibre reinforced metals SiC reinforced alumina tool materials. CarbonSiC composites. Glass fibre/carbon fibre reinforced polymers Polymer Kevlar/aramid fibre reinforced polymers Types of composites by reinforcement The type of reinforcement can be classified according to their geometric shape and arrangement. This makes for a convenient secondary classification. Fibres used for composite materials Glass (PMCs) Aramid [Kevlar] (PMCs) Low cost High production rates Good property balance More elongation than carbon fibres Doe snot burn/oxidise Electrically insulating High strength and stiffness Low density Very high melting point for a polymer Weak by water absorption Breaks down under UV light Silicon carbide (MMCs/CMCs) Carbon (PMCs) Expensive High strength and stiffness Low density Electrically conducting Burns/oxidises Expensive High strength and stiffness High melting point capability Low thermal expansion Anisotropy in composites Materials for which the physical properties are the same in all directions are called isotropic. Particulate reinforced composites can generally be assumed to be isotropic. In continuous fibre composites alignment of the fibres can lead to non-isotropic properties. For example Young’s modulus in parallel (longitudinal) and perpendicularly (transverse) to the fibres will be different. Continuous fibres - longitudinal modulus Longitudinal (equal strain) When the load is parallel to the fibres, we have equal strain in both the matrix and the fibres: For a fibre fraction f, the overall stress s1 in the composite can be expressed in terms of the matrix stress s1mand fibre stress s1f : The longitudinal Young’s modulus is thus be expressed as: Which simplifies to the law of mixtures: Elastic moduli in fibre composites Transverse (equal stress) When the load is perpendicular to the fibres, we have equal stress both the matrix and the fibres: For a fibre fraction f, the overall strain e2 in the composite is: The transverse Young’s modulus is thus be expressed as: Substituting e2f and e2m definitions from Hooke’s law into above : What does this mean? Let us consider a composite containing 50% fibres of carbon fibres in an epoxy matrix: f = 50% = 0.5 Em = 4GPa Ef = 400GPa Parallel to the fibres the Young’s modulus is: Perpendicularly to the fibres the Young’s modulus is: This means that the composite stiff loaded parallel to the fibres but very compliant when loaded perpendicularly to the fibres. Overcoming anisotropy To overcome the anisotropic behaviour of unidirectional fibre composites (a) several solutions are possible. Firstly, we can use randomly oriented fibres as shown in (b), but it is difficult to a high density of fibre packing giving a lower maximum fibre fraction. This compromises strength. Arranging the fibres in bidirectional (c) or multidirectional (d) layers is a stronger solution, but is more challenging technologically. Laminates help control anisotropy Constructing a laminate from layered unidirectional ply arranged at different angles offers a high degree of property control. Laminates help control anisotropy The mathematics of laminate properties can be quiet complex. To help understand the behaviour, the DoITPoMS website below offers a convenient simulation: https://www.doitpoms.ac.uk/tlplib/fibre_composites/lamina_stiffness.php Laminate warping. Even when bonded in a laminate, the ply may end up distorting when loaded or heated. Thus, it is also important to layer the ply in a way that avoids such distortions. Computer simulations are sometimes needed to model the behaviour of geometrically complex components. What about strength and toughness? The strength of a composite will depend on three factors: 1. The strength of the reinforcement phase 2. The strength of the matrix phase 3. The strength of the reinforcement/matrix interface Fibre reinforced composites have the following failure modes: Tension Tension matrix cracking matrix+ fibre cracking Shear Compression matrix cracking kink band matrix+ fibre cracking Fabrics Using ready-made fabrics can save time during the lay-up process and help control the composite anisotropy. Below are the four types of fabric used for composite production: Unidirectional Woven Multiaxial Random Stress-strain curve of a fibre composite The stress strain curve of a tough but weak and compliant matrix containing brittle but strong and stiff fibres is a superposition of the individual phase stress-strain behaviour curves. Role of interfacial strength For tough matrices a high interfacial strength is desirable in order to favour matrix deformation. before after stress For brittle matrices a low interfacial strength is desirable so that interfacial de-bonding, crack deflection, fibre fracture, and fibre pull-out can occur. Sandwich materials Sandwich materials employ a lightweight core material covered by thinner skin sheet materials on either side. Even though the core material often has low strength, the resulting sandwich has high flexural strength (i.e. strong in bending and torsion). Sandwiching also allows the thickness of composite components to be increased where such geometry is a design requirement. E.g. for crash absorption or heat insulation purposes. Sandwich structures Closedcell foam core CFRP composite Foam core wood composite Honeycomb being “potted” with foam Aircraft wing section with aluminium honeycomb and closed cell foam. CFRP composite with fire-retardant aramid (Nomex) honeycomb CFRP panel with closed cell aluminium foam Manufacture of PMCs Polymer matrix materials A polymer matrix for composite applications must have the following properties: - Good mechanical properties - Able to deform to the same extent as the fibres - Good adhesive properties - Good toughness - Good resistance to environmental degradation Polymer matrix materials Thermoplastic polymers (ABS, Nylon, polypropylene) become soft and mouldable when heated. Because they are very viscous they can only be reinforced with short fibres or whiskers. Thermoset polymers (polyester, vinylester, epoxy) form as a result of a non-reversible chemical reaction from liquid resin and catalyst. They have a much lower viscosity during processing than thermoplastics making them good for continuous fibre composite manufacture. Thermoset resins Polyesters Advantages: - Easy to use - Lowest cost of resins available (£1-2/kg) Disadvantages: - Only moderate mechanical properties - High styrene emissions (toxic) in open moulds - High cure shrinkage - Limited range of working times - More difficult to mix than other resins Vinyl-esters Advantages: - Very high chemical and environmental resistance - Better mechanical properties than polyesters Disadvantages: - Post-cure generally required for best properties - High styrene content - Higher cost than polyesters (£2-4/kg) - High cure shrinkage Epoxies Advantages: - High mechanical and thermal properties - High water resistance - Long working times available - Temperature resistance can be up to 140°C wet / 220°C dry - Low cure shrinkage Disadvantages: - More expensive than vinyl-esters (£3-15/kg) - Critical mixing - Corrosive handling Hand/wet lay-up This is the oldest and most traditional method for manufacturing fibre-reinforced polymer composite products. It does not require technical skills or special machinery. The process is labour intensive and best suited to low volume manufacture of large parts. E.g. boat hulls, enclosures. A protective gel coat is first applied to the mould to provide a smooth finish. It is a layer of resin several mm thick which protects the fibre reinforced polymer and can incorporate dies. Layers of fibre mats are then laid down to form a laminate of required thickness and are coated in catalysed resin. The resin is then cured to make the finished product. Pros and cons of hand lay-up Pros: - No machinery needed - No special skills needed - Low set-up cost - Variety of shapes possible - Custom layering and fibre arrangement in laminate. Cons: - Labour intensive - Product quality depends on worker skill - Not suitable for mass production - Smooth finish only on one side unless male and female moulds are used. N.B. Hand lay-up is suitable for high-end aerospace components (below), but requires additional processing – prepreging and autoclave curing. Spray lay-up Spray lay-up helps speed up the traditional hand-lay up process. A special chopper gun is used to cut reinforcement fibres into short strands, mix them with resin and catalyst, and spray them onto the mould. Some hand finishing is usually still needed afterwards. The deposited material is then left to cure in air under normal atmospheric conditions. Pros and cons of spray lay-up Pros: - Widely used for many years - Low cost way of quickly depositing fibre and resin. - Low cost tooling Cons: - Laminates are resin rich and tend to be heavy. - Use of short fibres limits mechanical strength. - Resins must be low-viscosity to be sprayable reducing strength - Spray lay-up resins tend to be more harmful. - Only practical for glass fibres composite production. Vacuum bagging Vacuum bagging is a modification of the hand/wet lay-up process. The laid-up composite pre form is sealed in a flexible polymer bag or under a polymer film which is connected to a vacuum system. When the bag is evacuated, it applies pressure to the composite pre-form to help consolidate it. The vacuum pressure allows the use of denser weave fabrics and more viscous resins. Pros and cons of vacuum bagging Pros: - Higher fibre content laminates can be produced using standard wet lay-up techniques. - Lower porosity. - Better resin infiltration due to pressure – better strength. - Excess resin flows out into bagging materials. - Safer for workers as toxic volatiles are removed by vacuum. Cons: - The extra process adds cost. - A higher level of skill is required. - Product quality is still very dependent on worker skill. Filament winding Filament winding is used to produce hollow cylindrical products, such as pipes and tanks. Fibre tows are passed through a resin bath to coat them. The coated fibres are then wound onto a rotating mandrel. The automated fibre feeding mechanism moves back and forth along the mandrel laying down multiple fibre orientations. The rate of mandrel rotation also helps control fibre orientation. Pros and cons of filament winding Pros: - Fast and economic fibre impregnation and lay-down. - Nip rollers and dies can be used to control resin content. - No need to weave fibres into fabrics – reducing cost. - Ability to lay down complex patterns allows high strength. Cons: - Only convex components can be produced. - No easy way of arranging the fibres along the rotation axis. - Large mandrels are costly. - External surface is unmoulded and aesthetically unattractive. Pultrusion Pultrusion is a process for producing continuous fibre reinforced polymer composite “extrusions”. Multiple fibres are pulled through a resin bath to become coated. The coated fibres then pass through a heated die which accelerates the curing of the resin. The cured profiles are then automatically cut into sections of desired length. Applications: beams, girders, frameworks. Pros and cons of pultrusion Pros: - Continuous process: fast, and economic impregnation and curing of materials. - Resin content can be accurately controlled. - Fibres are straight, highly aligned and have a high volume fraction – good for mechanical properties. - Resin impregnation area can be enclosed to limit volatile emissions. Cons: - Fibres only unidirectional. - High set-up costs. - Limited geometric variety. Resin transfer moulding In resin transfer moulding the fibre fabrics are laid up inside a mould and are then covered using a second mould and clamped. The moulds are then sealed off and resin is then injected at high pressure. Vacuum suction can be used to assist resin infiltration between fibres. VARTM – vacuum assisted RTM The resin is then cured to make the final moulded product. A two-step process using a pre-forming stage can be used as shown on the right. Pros and cons of resin transfer moulding Pros: - High fibre volume laminates can be obtained with very low porosity. - Good health and safety + environmental control due to enclosure of resin. - Both sides of the component have a smooth moulded surface. - Geometrically complex mouldings are possible. Cons: - Matched moulds and tooling are expensive and heavy in order to withstand pressures. - Usually limited to smaller components. - Unimpregnated areas can sometimes occur resulting in very expensive scrap parts. Pre-preg process Prepreging is the process of pre-impregnating the reinforcement fibres or fabrics with a resin. The pre-preg can be laid up in the mould without any further addition of resin. Resin is deposited onto a spooled out release paper at a carefully controlled thickness using a doctor blade. A carrier paper is also spooled out and fed between the heated rollers alongside the coated release paper. The fibres or fabric are fed between the two rolls of paper as they enter heated rollers. The roll pressure acts to infiltrate the resin between the fibres. Pros and cons of prepregs Pros: - Maximum strength properties. Prepregs avoid excess epoxy ~35% vs. ~50% for conventional lay-up. This minimises brittleness. - Part uniformity and repeatability, vs. human lamination techniques. - Reduced mess and waste. - Reduced curing time. -Better cosmetics. Prepregs virtually eliminate air bubbles and a smooth, glossy surface is more easily attainable. Cons: - Prepregs are expensive. - Limited shelf life. Heat cures the resin so freezing is needed to prolong the prepreg shelf life. - Prepregs require heat and vacuum bagging to be cured which increases costs. Automated prepreg tape lay-up Prepreg fibre tape can be laid down using automated multi-axis apparatus or robotic arms. The fibre tape is supplied from a reel on a backing tape which peels away. Tape is laid down in strips/segments which are sectioned off by built in cutters. The tape segments can be laid down onto a variety of moulds using a variety of patterns. The lay-up apparatus can incorporate built in heaters to facilitate curing. Pros and cons of automated tape lay-up Pros: - Precise control of laminate structure through layering. i.e. accurate localised control of mechanical strength. - In situ curing is possible with built-in infra red heaters. - Faster than hand lay-up. - Complex moulds can be used. Cons: - Expensive to set up. - Shape complexity limited by number of robotic motion axes. - Requires complex programming. Autoclave curing Autoclaves are computerised “pressure cookers” that help improve the curing of thermoset composites after lay-up. Subjecting the composite pre-forms to high temperatures and pressures accelerates the curing process. It also allows high fibre-to-resin ratios and greatly reduced porosity which gives the highest strength composites. In autoclaves, the pressure acting on the composite is more uniform than that provided by a vacuum bag alone. Thus, composite products with more complex shapes can be produced reliably. Pros and cons of autoclave curing Pros: - Precise control curing process which is faster than conventional wet lay-up methods. - Superior mechanical properties. - Uniform pressure distribution allows more complex 3D shapes. Cons: - Expensive to set up. - Additional safety protocols must be in place. Manufacture of MMCs Squeeze infiltration Squeeze casting is a method suitable for manufacturing fibrereinforced MMCs. Molten alloy is poured over a fibrous pre-form and pressure is applied using either a hydraulic ram or pressurised inert gas. The liquid metal infiltrates the spaces between the fibres and solidifies to form the final MMC. Stir-casting For manufacturing particulate and short fibre reinforced MMCs stir-casting is one possible cost-effective method. The reinforcement phase is simply added to a molten alloy which is stirred either mechanically or using electromagnetic induction. Stirring the melt allows a more-uniform distribution of the reinforcement phase in the melt that would not be achievable with squeeze casting. Stirring must be done carefully to overcome or avoid the sinking floating of the reinforcement or scattering due to Archimedes’/centrifugal forces. Diffusion bonding of foils For high-performance applications, precision lay-ups can be manufactured by layering continuous ceramic fibres between metal alloy foils. The fibres are initially held together in a binder which is burned off prior to consolidation. The MMCs are consolidated by applying heat and pressure which diffusion-bonds the the fibres and the foils. To apply the heat and pressure a hot-isostatic press or a closed-die hot forge. Summary Composites are materials that combine two or more materials with different physical properties to obtain new unique properties not obtainable in the separate materials. Composite = matrix phase + reinforcement phase Composite families by matrix: PMCs, MMCs and CMCs Reinforcement types: particulate, short fibre, continuous fibre Continuous fibre composites can offer superior properties, but the fibre direction is important as it gives rice to anisotropy. The majority of composites fail by cracking so a strong and tough composite will be able to resist crack growth. The interface between the reinforcement and matrix phases is very important. It can help deflect cracks and inhibit their propagation.