Composites

What are High Temperature Thermosets? (Service Temps up to 3000C)

Polyimides

What is a composite material?

Any material consisting of two or more components with different properties and distinct boundaries between the components.

What are medium temperature Thermosets? (Service Temps up to 2300C)

Bismaleimides (BMIs)

What is the purpose of Compression Test Rigs

The main consideration in compression testing is to avoid buckling of the thin laminate. This is achieved by having only a small length of the coupon unsupported.

The glass fibre diameter (5 to 20 m) is a function of.....

the size of the holes in the spinneret, the viscosity of the melt (which depends on glass composition and temperature), - the head of glass in the furnace and the rate of winding - The cooling rate experienced by the fibres is very high (greater than 10,0000 C per sec) - Hence the fibre structure is different from that of bulk glass, resulting in a higher tensile strength but lower elastic modulus and chemical resistance

What are the features of the ABD matrix?

- "ABD" matrix is the laminate stiffness matrix - The ABD matrix can be broken up into three 3x3 matrices - The 3x3 matrix [A] containing the Aij terms is known as the In-Plane stiffness matrix as it relates the in-planes forces (Nx, Ny and Nxy) to the in-plane strains (ex, ey and Ysy) - The 3x3 matrix [D] containing the Dij terms is known as the Bending stiffness matrix since it relates the bending moments (Mx, My and Mxy) to the curvatures (kx, ky and ksy) - The 3x3 matrix [B] containing the Bij terms is known as the Coupling stiffness matrix since it relates the bending moments (Mx, My and Mxy) to the in-plane strains (ex, ey and Ysy) and the in-planes forces (Nx, Ny and Nxy) to the curvatures (kx, ky and ksy) - The coefficients of the in-plane stiffness matrix [A] are independent of the stacking sequence of the laminate since they are essentially the sum of the ply stiffnesses multiplied by their thicknesses. - For a symmetric laminate, the coefficients Bij of the Coupling stiffness matrix are zero, hence there is no coupling between in-plane forces and out of plane deformations. In this case the in-plane and bending relations are decoupled, and they can be analysed independently. - In an unsymmetric laminate, the coefficients Bij are non-zero. This means that if an in-plane force is applied on an unsymmetric laminate, it will bend; conversely, if we apply a pure moment on an unsymmteric laminate, it will be not only bend but also stretch (the mid-plane will also be strained).

Manufacture of Pre-pregs**

- "Pre-pregs" are sheets of fibre reinforcement (unidirectional tapes or woven cloth) pre-impregnated with a resin already mixed with hardener - The partially cured (B stage) resin does not flow at room temperature but remains tacky (sticky to touch). B-staged epoxy pregregs are normally staged (partially cured) to about 15% of full cure for hand lay up and to about 25% of full cure for automated lay up (which is faster). - Uni-directional pre-preg consists of collimated fibre tows (about 10,000 fibres in a tow) in a uniform parallel sheet impregnated with B staged resin. Prepregs can also be made of cross ply or woven material - Prepreg rolls come in widths between 25 mm (tapes) and 3 to 4 metres - Carbon pre-pregs typically have 34% to 42% of resin content (resulting in about 70% fibre volume ratio after cure), and is about 0.125 mm thick. - Since the resin is partially cured pre-pregs have to be refrigerated (below -200C) and have limited shelf life and very limited bench life

General Laminating Procedure - Pre-preg Lay-up**

- "Pre-pregs" are sheets of fibre reinforcement (unidirectional tapes or woven cloth) pre-impregnated with partially cured (B stage) resin. - Pre-preg lay-up is virtually dry process, less messy, and with less safety issues. - Pre-preg lay-up offers better quality control of parts since the amount of resin is more or less predetermined - Pre-preg lay-up can achieve higher fibre volume ratios providing higher and more consistent mechanical properties - It is also less labour and time intensive. - However, raw material (pre-preg) is costly, requires refrigeration (more cost) and has limited shelf life.

What are the Lamina Engineering Properties?

- A single layer (lamina) or a unidirectional laminate is orthotropic along its principal material directions (axes along the fibre direction and perpendicular to it) - Longitudinal Modulus (Stiffness in fibre direction) E1 - Transverse Modulus (Stiffness perpendicular to fibres) E2 - Major Poisson's Ratio 12 (= -2/ 1 for applied 1) - Minor Poisson's Ratio 1 (= -1/ 2 for applied 2) - In Plane Shear Modulus G12 Note: Only 4 of the above five constants are independent. The Poisson's ratios are related to the Young's moduli through the relation:

What are the Two main methods of Ultrasonic Testing application?**

- A-Scan: Point Scan using hand held probes and CRT monitor; easy to use, but time consuming; good for field applications for localised inspections (Usually a coupling medium (gel, oil, water) is required between the probe head and part surface due to the poor Acoustic Impedance of Air) ) - C-Scan: Two-dimensional Scan over large areas using automatic traversing probes (can be pulse echo or through transmission), is used for inspecting large components (especially for quality assurance after manufacture) often using immersion tanks - Time of Flight Data is used to determine the depth at which defects (delaminations) occur

What are some Types of Fibre Metal Laminates?

- ARALL (Aramid Reinforced ALuminium Laminate): 0.2 mm to 0.4 mm thick sheets of Aluminium alloy 2024-T3 or T475 reinforced with aramid fibre reinforced epoxy (Kevlar) - GLARE (Glass Fibre Reinforced Laminate): 0.2 mm to 0.4 mm thick sheets of Aluminium alloy 2024-T3 or T475 reinforced with glass fibre reinforced epoxy nitrile adhesive (uni-directional or cross ply reinforcements available). - Weight savings of up to 20% can be achieved with GLARE in fatigue prone and impact sensitive areas - Proposed applications of GLARE include horizontal and vertical stabilizers, wing leading edges, Aircraft bulk cargo bay floors, landing gear doors, hardened unit load devices

The mechanical properties of the PMC are also affected by......(2)

- Alignment, flaws and strength variations of fibres - Temperature and Moisture in operating environment

Moisture absorption - Effects at Room Temperature

- All fibres to which resins will form a bond are preferentially wet by water. Presence of water in the fibre/resin interface may result in debonding of the fibres and hence loss of strength and stiffness - The presence of moisture (particularly a gradient across the thickness) can induce additional stresses due to the swelling of the laminate - Moisture trapped in voids and cracks can vaporise at high temperatures resulting in matrix cracking - Moisture trapped in voids and cracks may freeze at low temperatures; the resulting expansion can also initiate matrix cracking - Water acts as a plasticiser of the resin system and slightly degrades the mechanical properties at room temperature

What are some features of Intermetallics MMCs?

- Aluminides (FeAl, FeCrAlY) - chemically compatible with alumina - NiAl - excellent oxidation resistance and low density

What are the Test Standards for coupon testing?

- American Society for Testing and Materials (ASTM) - Suppliers of Advanced Composites Materials Association (SACMA) - Military Handbook 17 (MIL-HDBK-17): "Polymer Matrix Composites"

Impact Damage Resistance of PMCs**

- Among the common aerospace PMCs, GFRP has the highest impact resistance followed by aramid fibre composites (both better than that of Aluminium alloys) - The impact resistance of CFRP is lower than that of GFRP and Kevlar. The High Modulus CFRP has the least impact resistance. - Low velocity impact due to dropped tools, runway stones, etc can cause barely visible impact damage (BVID) through delaminations between the different layers - BVID can significantly affect the residual compressive strength and buckling resistance by causing significant reductions in the bending stiffness of the laminate

What are the features of Aramid Fibres?

- Aramid fibres (trade name Kevlar) were the first organic fibres suitable for high performance PMCs for aerospace applications - Specific properties of aramid fibres are much higher than those of glass fibres. - They have good tensile properties at temperatures up to 4000C. - But they have poor compressive strength which significantly limit their applications - Aramid fibres are able to absorb large amounts of energy during fracture due to (a) high strain-to-failure, (b) ability to undergo plastic deformation in compression, and (c) ability to defibrillate during fracture (yield strain is 0.3% in compression, compared to about 2% in tension) - Hence used for ballistic protection and engine containment rings. - Kevlar fibres are based on an aromatic polyamide (poly paraphenylene terphalamide; PPD-T). The aromatic rings contribute to high thermal stability - The strong covalent bonds in the polymer chain and weak Hydrogen bonding between chains result in anisotropic properties - The PPD-T dissolved in concentrated sulphuric acid is extruded through a spinneret at 1000C. The fibre precipitates after emerging from the spinneret and coagulates after passing through a cold water bath, which also removes the acid. - The fibre structure is essentially made up of bundles of fibrils (highly crystalline aligned polymer chains) that are weakly bonded together. - Aramid fibres fail by defibrillation process under tensile loading, which reduces sensitivity to flaws and increases energy absorption at failure - Strength reduces by about 20% at a temperature of 1800C, but declines rapidly thereafter - It is highly non-linear under compression due to the formation of kink bands resulting from in-phase compressive buckling of fibrils - Aramid fibres are hygroscopic (moisture absorption is around 4% at a relative humidity of 60% for Kevlar 49), but tensile strength is not affected much - Aramid fibres are prone to significant short term creep, but long term creep is negligible - They are susceptible to degradation in strength by exposure to UV, but is not affected in PMC due to protection by the resin matrix

What are the features of Filament Winding?**

- Automated manufacturing process that enables continuous reinforcement to be laid down at high speed and precision in predefined paths (generally geodesic or axisymmetric) - The process essentially involves impregnating continuous fibres with resin and winding them on to a stationary or rotating mandrel. The mandrel is removed after cure - By varying process parameters such as winding tension, winding angle and resin content, the desired part thickness, ply sequence and fibre volume fraction can be achieved.

What are some examples of composite materials?

- Bamboo - Wood - Mud Bricks Reinforced with straw - Concrete - Reinforced concrete - Fibreglass - Kevlar Epoxy - Graphite Epoxy - Carbon/Carbon - SiC/SiC (Patriot Missile Radomes)

What are the features of Boron Fibres?

- Boron fibres are large monfilaments around 125-140 m in diameter (compared to 5-10 m for Carbon, 15- 20 m for Glass) - Boron is almost as hard as diamond, so Boron PMCs difficult to drill and machine. - Hence, in aerospace applications, Boron has largely been replaced by Carbon fibres, which are much cheaper, more machinable and formable - Still, several US aircraft (F-14, F15 and B-1) contain several Boron/epoxy components - Boron/epoxy is also employed extensively for composite patch repair of cracks on metallic airframe structures.

Manufacture of Boron Fibres

- Boron fibres are made by chemical vapour deposition (CVD) of boron onto a fine incandescent tungsten or pitch-based Carbon fibre core of about 10 m diameter (the Carbon fibre is coated with a thin layer of graphite to facilitate relative movement to relieve internal stresses) - Deposition is usually through hydrogen reduction of Boron trichloride (or bromide, iodide or fluoride) gas at a temperature over 10000C at atmospheric pressure in a glass reactor. - The Boron deposit grows at a slow rate of 3-4 m per minute; which makes it very expensive. - The tungsten core is also expensive, hence Boron fibres are made into large diameter filaments (~140 m) to save cost. - At temperatures over 12000C, some crystalline Boron is formed, which adversely affects mechanical properties.

Dry Fibre Forms - Others

- Braided Fabrics are more expensive than woven fabrics due to the complexity of the manufacturing process; however they offer greater strength per fabric weight. 2D braiding is used to make flat or tubular preforms (such as exhaust ducts), but are limited to components of small size in general - Non Crimp Fabrics: Multiaxial, multilayer warp-knit fabrics consisting of fibres held in plane by a stitched or knitted thermoplastic polymer fibre (nylon or polyester) or a flexible high performance fibre such as glass or aramid. Since the material is not crimped the properties are better than those of woven materials - Tapes: Woven or uni-directional fabrics less than 100 mm in width - 3D textile Preforms: Reinforcement is put together for a complete complex shaped component using weaving, braiding or knitting. The finished component requires only the addition of resin and curing.

What are the Adhesion and Interfacial Bond Strength of CFRPs?

- Carbon fibres are normally surface treated to develop adequate interfacial bonding. Too strong bonding is not desirable as the fibre should be capable of disbonding to alleviate local stress concentrations such as at microcracks - The composite strain to failure is dependent on the matrix properties rather than fibre strain to failure values - Matrix and interfacial bonding plays a very important role in compression by supporting the fibres against microbuckling which is the main mode of failure in compression - Matrix also plays a major role in interlaminar shear

What are the features of Carbon Fibres?

- Carbon fibres are widely used for airframes, engines and other aerospace applications as well as in other areas such as sporting goods where high specific strength and/or stiffness are important. - Three types of carbon fibres: High Modulus (HM, Type I), High Strength (HS, Type II), Intermediate Modulus (IM, Type III) - Carbon fibres are made from organic precursor materials by a process of carbonisation. - Carbon fibres for aerospace and other structural applications are mostly made from PAN (PolyAcryloNitrile) fibres (higher strength), other precursor commonly used is pitch. - Early generation of carbon fibres were made from rayon, however these have been phased out due to poor carbon yield and poor mechanical properties compared to PAN and pitch based carbon

What are some features of Carbon Carbon Composites?

- Carbon/Carbon has the best structural properties (specific strength, specific stiffness, creep resistance, at the highest operating temperatures (over 20000 C) of all materials - Further it has no significant chemical or thermal expansion compatibility problems - Its greatest disadvantage is susceptibility to oxidation at high temperatures - If protected against oxidation, or exposed only for short durations to oxidizing environments, it can withstand temperatures well over 20000 C. - Hence C/C is the main candidate for use in rocket nosecones, nozzles, and leading edges on hypersonic wings.

What are some features of Ceramic Matrix Composites?

- Ceramic matrices reinforced with continuous fibres, whiskers or particulates - High temperature applications in gas turbine engines and high temperature airframe/space structures - Ceramics are brittle and subject to microcracking at high temperatures - Hence reinforcing fibres with high strength and stiffness that have chemical stability and resistance to oxidation at high temperatures as well as matching CTEs are required - Hence it is common to use similar materials for both reinforcement and matrix such as SiC/SiC and alumina fibres in alumina matrix.

How do glass fibres compare with other with other PMCs?

- Cheaper for non or less load bearing structures - Density comparable to that of Boron/epoxy (B), higher than aramid/epoxy (A) and carbon/epoxy (C) - Lower specific stiffness than all three (A,B and C) Hence used mainly in building and marine industry and for construction of small aircraft - Not employed for weight critical load bearing structures, especially in large aircraft - Better dielectric properties than other PMCs

What are the Thermal Constants Required for an Orthotropic Lamina?

- Coefficient of thermal expansion in the fibre direction (alpha)1 - Coefficient of thermal expansion in transverse direction (alpha)2

What are the disadvantages of PMCs?

- Composite Materials are expensive - Less historical data and established design allowables - Corrosion problems can result from improper coupling with metals, especially when graphite is used (sealing is essential) - Degradation of structural properties under temperature extremes and wet conditions - Poor energy absorption and impact damage resistance - May require lightning protection - Expensive and complicated inspection methods - Reliable detection of substandard bonds is difficult - Internal nature of defects makes damage assessment difficult

Fatigue Resistance of PMCs - Compression Fatigue

- Compression fatigue resistance depends on the ability of the matrix to support the fibres to avoid microbuckling under compression which in turn depends on the interfacial bond strength and the presence of matrix cracks - In general compression fatigue can be as good as tension tension fatigue provided no disbonds and delamination damage have occurred. - AFRP has poorer compression fatigue performance due to the poor compression resistance of aramid fibres - Fatigue performance under compression is severely affected by hot/wet conditions, due to lowering of the Tg and softening of the matrix

General Laminating Procedure - Compression Die Moulding

- Compression or matched die moulding is two sided, employing two matching (male and female) dies that close to form a cavity with the required shape of the component - The dies, generally made of tool steel, can be internally heated, by electric elements, steam or hot oil pipes - The reinforcement fibres or fabrics are placed in the lower half and the two dies brought together to apply pressure for consolidation - Advantages; Excellent dimensional control, high quality surface finish on both sides, high production rates, good consolidation and high fibre volume ratio - Disadvantages: High Cost of tooling, consolidation pressure limited to capacity of hydraulic presses employed

What are the features of Phenolic Resins?

- Condensation polymerisation of phenol with formaldehyde under alkaline or strong acid conditions is used to produce a pre-polymer called "resol". - This is polymerised under heating or with acidic or basic catalysts to form densely cross-linked structure forming the matrix - Water and volatile by products are formed in the reaction, hence high pressures are required for curing - High void content

What are the features of Aluminium Metal Matrix?

- most common metal matrix, greatly improved properties when reinforced, light weight, easy to process - high temperature applications limited, do not offer much improved capability over PMCs based on BMI (bismaleimides) and polyimides - Better thermal and electrical conductivity over PMCs, but far more expensive

What are the features of Dry Fibre Forms?

- Continuous carbon, glass, aramid and other multifilament fibres are produced in various dry forms (no matrix) for subsequent processing using pre-impregnation (pre-preg), resin film or liquid resin injection techniques - Dry forms include continuous rovings, woven rovings, yarns and woven cloth. - Monofilament CVD fibres are available in dry form either as single fibre spool, or as a tape or cloth held together by polymeric fibres (sometimes attached by a light resin coating to a very fine glass fibre cloth). - Rovings consist of an untwisted bundle of strands (which contain a number 102, 204, etc, of filaments) of glass fibres (a term reserved mainly for glass fibres) - Tows are untwisted bundles of carbon fibres (1000 to 48,000) produced directly from the PAN precursor (a term reserved mainly for carbon fibres) - Note: Maintaining an even tension in rovings and tows is very important for subsequent processing - Yarns are twisted collection of strands or filaments ( about one turn per cm). The twist holds the fibres in place and maintains an even tension during subsequent processing such as weaving and filament winding.

What are some Reinforcements for MMCs?

- Continuous fibres: Carbon and Boron, Alumina (Al2O3), Silicon Carbide and Nicalon (Si-C-O-N) - Whiskers: Ceramic whiskers such as Silicon Carbide, Alumina, Boron Carbide and Silicon Nitride, and Graphite - Particulates: SiC, Boron Carbide, Alumina, Titanium Carbide

Additives of Epoxy Resin

- Diluents are often added to reduce viscosity before cure - Flexibilizers are added to reduce elastic modulus and increase elongation to failure - Toughening agents increase fracture toughness and reduce crack propagation rates - Inert fillers, such as hollow glass microspheres are added to alter density, resin flow and effective modulus

What are the features of Thermoplastic Systems?

- Do not undergo chemical reaction while curing Short processing times (also dry and less messy), however higher pressures and temperatures are required - Much higher strains to failure, toughness, interlaminar shear strength and impact resistance than thermoplastics - Absorb much less moisture than thermosets, hence less deterioration in mechanical properties under wet conditions - Unlimited shelf life - Can be remelted and remoulded (healed) and welded - More expensive to process than thermosets - More susceptible to chemicals such as fuels, hydraulic oil,paint stripper than thermosets - More susceptible to creep than thermosets - Two types of thermoplastics: amorphous and semicrystalline

What are the different types of Glass Fibres?

- E Glass (calcium alumino-borosilicate) for electrical applications due to their higher electrical resistivity and low dielectric constant - S Glass (magnesium alumino-silicate) for structural applications due to their higher strength and low cost

What are the types of Fibres in Glass Fibre Composites (3)

- E-Glass: moderate strength and stiffness, low cost, non-structural and semi-structural applications - S-Glass: higher strength and higher stiffness than E-Glass, cost comparable to that of Carbon/epoxy, employed for some structural components - D-Glass: better dielectric properties than E-glass, protection against lightning strikes Note: Properties depend on type of reinforcement (chopped strand mat, woven rovings, unidirectional) and matrix type (epoxy, polyester)

What are some Examples of Applications of MMCs?

- Engine components such as fan and compressor blades, shafts, discs - Potential for use in Undercarriage parts - Carbon/Al and Carbon/Mg are used in satellite applications due to high specific strength and stiffness and high dimensional stability (due to high conductivity), but not useful in normal atmosphere at temperatures over 800 degs. C due to reactivity of carbon with the metals. - Boron/Al used in tubular structures in the Space Shuttle

Post Cured Properties of Epoxy Resin

- Epoxies cured with aromatic amines have higher Tg and better mechanical properties; hence are most commonly used for advanced aerospace composites - Epoxies cured with aliphatic polyamine curing agents are not suitable for use at temperatures above 500C. - Epoxy systems with aromatic amines and polyanhydrides cured at 1200C and 1800C, have service temperatures of use at temperatures of 1000C and 1500C. These can be post cured at 1500C-2200C to raise service temperatures to 1000C to 2500C range

Manufacturing of Epoxy Resins

- Epoxy resins are a class of organic compounds containing two or more epoxide groups (oxirane or glycidyl group of two carbons and an oxygen forming a triangle) - Epoxies are formed by reacting polyphenols or other active hydrogen compounds with epichlorohydrin under basic conditions. Most common phenol used is bisphenol A (Diphenylolpropane) - Epoxies are usually cured using curing agents ( hardeners). Hardeners with aliphatic amines give cold curing systems, while hardeners with aromatic amines and polyanhydrides give heat-curing systems.

What are the features of the Matrix Systems in CFRPs?

- Epoxy resins: service temperatures of up to 2000C, can be toughened by formulation - Bismaleimides (BMI): service temperatures of up to 2500C - Polyimides : service temperatures of up to 3200C - Thermoplastics (PEEK and PPS): offer higher toughness and lower moisture absorption comparedto thermosets

What are the features of Cure Monitoring?**

- Essential for the curing of thermosets which require rigid adherence to the programme of temperature, vacuum and pressure specified by the manufacturer - Autoclave and RTM cure processes can be optimised by real time monitoring of parameters such as pressure, resin viscosity and gel point - The sensor outputs may be employed in a feedback system to fine tune the application of pressure, vacuum and temperature - The measurement of temperature variations at various locations on the component using thermocouples is the simplest and most common method of cure monitoring - The measurement of electrical properties such as dielectric constant and dielectric loss tangent can be correlated to the degree of cure - Electrical conductivity measures the mobility of ions which is inversely proportional to the viscosity of the resin - The use of ultrasonic wave propagation and acoustic emission may be correlated to parameters such as degree of cure, porosity, viscosity, delaminations and fibre volume fractions - Optical Methods: Optical and infra red spectroscopy, fluorescence in the visible and UC spectrum, and refractive index measurements can be used in cure monitoring - Pressure and Compaction Sensing using pressure or displacement transducers are employed to monitor pressure

What are the features of Polyimide Resins?

- Extremely high Tg compared to other polymer matrices (200- 4000C). - Aromatic heterocyclic systems produced both by condensation and addition polymerizations - Condensation polyimides are thermoplastic and are used as matrices or toughening mechanisms in high temperature epoxy formulations (e.g., Avimid-N, Kapton from Dupont, LARC-TPI from NASA , Ultem from GE, Aurum from Mitsui) - Addition polyimides are thermosets. In general they release large quantities of volatiles unless high temperatures and pressures are employed, hence fabrication is complex and expensive

What are the limitations of Epoxy Resins?

- Fairly low toughness (GIC of 100-540 J/m2) (ductile metals: 50-200 kJ/m2 ) - Sensitive to Impact Damage - Limited Service temperature range (Epoxy systems cured at 1200C have a service temperature of 1000C to 1300C, while epoxies cured at 1800C have a service temperature of about 1500C). - Absorbs moisture, which reduces glass transition temperatures and hence significantly affects mechanical properties at elevated temperatures - Sensitive to UV exposure - Limited resistance to organic acids and phenols - Relatively high cost compared to polyesters, less convenient and longer cure than polyesters

What are the three damage categories that composites can be broadly classified into?

- Fibre breakage - Matrix cracks - Delaminations

Manufacturing of Polyester Resins

- First an intermediate low molecular weight (poly-) unsaturated polyester is produced from a mixture of dibasic acids and dihydric alcohols (glycols) or dihydric phenols. - These are diluted in a reactive solvent such as styrene which begins to polymerise when free radicals (initiator) and catalysts (accelerators) are added. The polymerised styrene reacts with the unsaturated polyester sites to form a 3D cross linked network - The curing of polyesters is by free radical polymerisation which requires only small quantities of initiators and can be finely controlled by the quantity of initiators used - The polymerisation is strongly exothermic.

What are the Assumptions of Classical Laminate Theory?

- For two-dimensional plane stress analysis, the strain is constant through the thickness - For bending, the strain varies linearly through the thickness - The laminate is thin compared with its in-plane dimensions - Each layer is quasi-homogeneous and orthotropic - Displacements are small compared with the thickness - The material behaviour is linear

What are some toughening mechanisms of Epoxy Resin?

- Formation of a solid solution with a more ductile polymer - Precipitation of elastomeric second phase - Development of interpenetrating polymer networks - The inclusion of elastomeric second phase can be achieved by adding an elastomer (rubber) to form a copolymer with the base resin which then precipitates out upon curing to form a dispersed second phase - Or by adding a very fine powder to form a dispersion - Or by both methods - Toughening can improve the fracture toughness of epoxies to between 450 and 550 J/m

What are the features of Glass Fibre Coatings**

- Friction between glass fibres is around unity - Mechanical damage due to abrasion with neighbouring fibres can cause significant reduction in strength - Within milliseconds of solidifying, fibres are coated with protective "size" to reduce friction as well as absorption of moisture - The size consists of a lubricant, binders such as starch and polyvinyl alcohol (to hold filaments together) and primers to improve adhesion between fibres and matrices - The primer (or finish) is a coupling agent in the form of an organosilane compound. The organic portion (eg epoxy for epoxy and phenolic matrices) interacts with the organic resins while the silane interacts with the inorganic fibre to improve bonding.

What are some Common Thermoset PMCs?

- Glass /Epoxy: low stiffness to weight ratio, low cost - Boron /Epoxy: high specific strength & stiffness, high cost - Graphite /Epoxy: high specific strength & stiffness, intermediate cost. - Kevlar /Epoxy: high impact damage resistance, intermediate cost, hygroscopic, low compressive strength

What are some examples of Ceramic Matrix Composites?

- Glass ceramic matrices can be toughened up to 50 times by the addition of suitable reinforcements - SiC (SCS-6) fibres in Silicon Nitride (Si3N4) matrix : has high specific strength and stiffness, toughness, notch insensitivity and thermal shock resistance up to 14000 C. Good creep resistance. - Silicon Carbide/Silicon Carbide (SiC/SiC): has high specific strength and stiffness, and toughness; retains structural strength well above 14000 C, good thermal conductivity, good complex shape capability.

What are some examples of Ceramic Matrices?

- Glass ceramics (e.g. Lithium aluminosilicate) for applications up to 5000 C) - Oxides (e.g. alumina and mullite) - Nitrides (e.g. Silicon Nitride) for applications up to 14000 C) - Carbides (e.g. Silicon Carbide) for applications over 14000 C

What are the features of Glass Fibres?

- Glass fibres are the most popular PMC reinforcement due to their high strength and low cost - Have been used in structural applications such as pressure vessels, rocket casings, boats, etc since 1960s - Primary airframe structural applications are generally limited to low speed aircraft such as gliders due to low stiffness compared to other PMCs. - GFRPs are employed in secondary structures where stiffness is not a major consideration such as in fairings, due to their lower cost compared to other PMCs - Due to their transparency to EM radiation, also employed in radomes and aerial covers. - Glass is an amorphous solid produced by cooling a viscous liquid at a very high rate to prevent formation of crystalline regions - manufactured mainly from silica (SiO2) melted with oxides of Al, B, Ca, Mg, Na, and K - Additives are used to lower melting point of silica for required viscosity at lower temperatures and for removal of gas bubbles - Additives significantly affect mechanical properties

What is the Effect of Flaws on Glass Fibres?

- Glass fibres, being essentially monolithic, linearly elastic brittle materials, are very sensitive to flaws in the form of submicroscopic inclusions and cracks - Commercial fibres are particularly prone to abrasion against other fibres, resulting in a reduction in strength of up to 20% compared to pristine fibres - Humid environments reduce the strength of glass fibres under sustained loading due to adsorption of moisture on to the surface causing "static fatigue" - Individual fibre strength is reduced by about 50% in a PMC, but the bundling of the fibres overcomes this effect

What is the Glass Fibre Manufacturing Process?**

- Glass is first melted at temps between 12500 and 14000 C - flows into an electrically heated platinum-rhodium alloy bushing or spinneret which contains a large number of holes at its base - The glass drops emerging from the holes are drawn into fibres at speeds of up to 50 m/s - They are cooled by a water spray and coated with a "size" by a contact with a rolling applicator - Finally the fibres are combined into a strand (of 52, 102 or 204 fibres) and wound on to a take-up spool

What are the reasons for Lower-than-Expected Usage of Composites?

- High Cost of raw materials and manufacture - High Cost of Certification - Low Resistance to Impact Damage and transverse cracking - Limited applicability at high temperatures - Advancement in aluminium alloys (improved toughness, fatigue resistance and corrosion resistance) - Development of new light weight alloys (Al-Li) - Low cost aerospace grade castings, mechanical alloying and super-plastic forming, diffusion bonding - Advanced joining techniques for metals (laser and friction welding, automated riveting)

What are the features of the fibres in CFRPs?

- High Strength, High Modulus, Intermediate Modulus, Ultra High Modulus, etc - Polyacrylonitrile (PAN) based fibres are the most popular since they have lower cost, better handling characteristics (higher strain to failure rates) than pitch based fibres and attractive overall mechanical properties - Pitch based fibres are more extensively used in satellite applications (despite higher cost) due to their superior stiffness, high electrical conductivity and low coefficient of thermal expansion

What are the features of Polyethylene Fibres?

- High modulus polyethylene fibres have specific gravity less than one - Commercial PE fibres currently available include Spectra (Allied Signal) and Dyneema (by DSM) - They are manufactured by drawing melt-crystallized PE to very high draw ratios - Other methods of manufacture include solution and gel spinning of very high molecular weight PEs. - Like Aramid fibres, PE fibres exhibit low compressive strength due to formation of kink bands - Specific strengths are comparable to Aramid fibres - PE fibres are susceptible to creep deformation and creep rupture under long term loading even at room temperatures. - PE fibres not suitable for prolonged static loading due to poor creep behaviour - Service temperature is limited to under 1000C. - However, PE has good chemical resistance and low moisture absorption characteristics (compare to UV sensitivity and hygroscopicity of Aramid fibres) - Since the material is non-polar in nature, difficult for bonding with resins; this is overcome by pre-bonding etching techniques , including corona discharge - The most attractive feature of PMCs based on PE fibres is their high toughness. Hence they are primarily promoted for ballistic protection and impact resistance.

What are the features of Poly-Phenylene Sulphide (PPS)?

- Highly Crystalline polymer with high thermal stability, chemical and fire resistance - Crystallizes rapidly above its Tg (900C); crystallinity is between 50 and 60%. - Due to crystallinity, the loss in strength above Tg is gradual, retaining good mechanical properties even at 2000C. - Inert to organic solvents, inorganic salts and bases - Not affected by aircraft fluids except paint stripper - Soluble in aromatic hydrocarbons and chlorinated aromatic compounds - Highly flame resistant - Can be used for aircraft interiors

What are the features of Pultrusion?

- Highly automated, continuous linear process for manufacturing of components with constant cross sectional profiles with fibre reinforced PMCs - The process essentially involves impregnating continuous fibres with a thermosetting resin and pulling them through a heated die to shape and cure the composite into a finished product - Usually the part is more or less cured by the time it emerges from the die - The pulling speed is related to the curing kinetics of the resin system and the die length. - Dies are typically one metre long and pulling speeds vary between 200 mm/min for epoxies and 3000 mm/min for commercial polyester products

What are some features of Hybrid Metal/PMC Composites?

- Hybrid metal/PMC composites (also known as Fibre Metal Laminates - FML) combine the advantages of metals (such as high resistance to low velocity impact damage) with the advantages of PMCs (such as high fatigue life). - They typically consist of thin sheets of metal (usually Aluminium) bonded together with a fibre reinforced adhesive. - Fibre metal laminates have much higher fatigue lives than monolithic alloys and greater impact resistance (due to the aluminium outer layers) than normal PMCs. - They have lower densities than aluminium, higher post yield strength and higher damping capacity - Disadvantages include higher sensitivity to blunt notches, lower elastic modulus than monolithic aluminium and possibility of earlier crack initiation, and high cost

General Laminating Procedure - Wet Lay-up**

- In Wet Lay-up layers of fibre or woven cloth are laid-up in the required configuration (laminating sequence) in an open mould and resin is added by brushing or spraying. Handling wet resins can be messy and has OH&S concerns. - Wet lay up also has problems of quality control due to lack of repeatability and difficulty in controlling the resin content, which can give rise to variations in weight, thickness and mechanical properties - It is also labour intensive - However it is cost effective, especially for small volume production (one off parts) and for small companies

General Laminating Procedure - Open Die Moulding - Consolidation

- In contact moulding pressure is applied by hand-rolling over a sheet of plastic film (used only for low stress applications) - In vacuum bagging, a flexible plastic membrane is laid over the lay-up to form a tight vacuum bag. - - The bag is evacuated, so the atmospheric pressure is used to consolidate the laminate. - Advantages: Inexpensive way of applying pressure, better consolidation, better control. - Disadvantages: Pressure limited to atmospheric pressure, more care needed to ensure uniform distribution of pressure, otherwise bubbles or voids may form

Fatigue Resistance of PMCs - Tension Fatigue of Cross Ply Laminates

- In the first phase matrix cracking in the 900 and other off axis plies accumulate leading to a characteristic damage state (CDS) with saturation of matrix cracks - In phase two, crack coupling and delaminations develop which slowly increase the stresses carried by the zero degree plies - In phase 3, the zero degree fibres begin to fail under the tensile load leading to final fracture - Note that the damage progression and loss of stiffness is high initially (matrix cracking), remains fairly steady during the second phase, and then again accelerates in the final phase of fibre fracture

What are the features of Vinyl-Ester Resins?

- Intermediate class of materials between epoxies and polyesters - The major ester ingredient is produced by reaction of a standard epoxy and methacrylic acid - The resins are cured by free radical reaction process like polyesters - Tougher and more chemically resistant than polyesters due to less cross linking - Large range with different mechanical properties, including rubber toughened resins

Fatigue Resistance of PMCs - Tension Tension Fatigue

- It is important to maintain high fibre to matrix stiffness ratio so that the matrix strains are kept low to avoid matrix cracking - Carbon fibres are generally fatigue resistant, so the S-N curve is generally flat at high strain rates but with a high scatter factor due to the breakage of fibres with large inherent flaws - At moderate strain levels fatigue resistance of CFRP is strain dependent with fibre bridged matrix cracking and disbonds the main damage characteristic - Fatigue of CFRP at low strain levels causes only a small amount of matrix cracking which can lead to environmental damage - Glass fibres exhibit degradation of strength under fatigue, possibly due to stress rupture due to accumulated time at peak stresses. Further the low modulus of glass fibres causes high matrix strains leading to matrix cracking, which exacerbates fatigue sensitivity by causing strain concentration as well as by exposing fibres to environment - The fatigue resistance of AFRP is in between that of CFRP and GFRP - In general, composites with moderately tough matrices and moderate fibre to matrix bond strength far better than those with high toughness and interfacial strength, since the former allows damaged fibres to be isolated from the good fibres by matrix cracking, disbonding and formation of delaminations

What are the features of Titanium and Titanium Alloys MMCs?

- Larger margin on temperature capability over PMCs - 100% increase in stiffness 50% increase in strength (when reinforced with SiC fibres) over conventional titanium alloys - Do not match PMCs in terms of mechanical properties at moderate temperatures and more expensive - Highly damage tolerant - High temperature applications include high speed transport, gas turbine engines.

What are the advantages of PMCs?

- Light weight - Resistance to corrosion - High resistance to fatigue damage - Reduced machining - Tapered sections and compound contours easily accomplished - Strength and stiffness can be tailored - Reduced number of assemblies and reduced fastener count when co-cured or co-consolidation is used - Absorb radar microwaves (stealth capability) - Near zero thermal expansion reduces thermal problems in outer space applications

What are the Engineering Constants for an Orthotropic Lamina (2D)?

- Longitudinal Modulus (Stiffness in fibre direction) E1 - Transverse Modulus (Stiffness perpendicular to fibres) E2 - Major Poisson's Ratio v12 (= -e2/ e1 for applied 1) - Minor Poisson's Ratio v21 (= -e1/ e2 for applied 2) - In Plane Shear Modulus G12 Note: Only 4 of the above five constants are independent (e.g. the minor Poisson's ratio can be obtained from the two "E" values and the major Poisson's ratio)

What are the features of Aramid Fibre Composites?

- Lowest density fibres, hence lower density than other PMCs - Specific strength and stiffness in between GFRP and CFRP - Good tensile properties, but highly non-linear behaviour in compression - Very poor compressive strength (worse than GFRP) due to fibre micro-buckling - Combination of non-linear compression and high tensile strength provides high impact resistance, especially for ballistic protection (e.g. helicopters) - Aramid fibre reinforced composites have favourable dielectric properties (hence used for reinforcement of radomes) - AFRP has elastic moduli in between that of GFRP and CFRP - Compressive and tensile moduli are similar but compressive strength is very low - Inter laminar shear strength is also low compared to other PMCs - Fatigue resistance is superior to aluminium alloys and GFRP but inferior to CFRP - Low creep rate (similar to glass) - Less susceptible to stress rupture than GFRP - AFRP is used to make hybrid composites with carbon and glass fibres to improve toughness and or strength in the presence of stress raisers - AFRP suffers from poor fibre matrix adhesion Fibres can be treated to improve interfacial bond strength by up to 20% - AFRP is difficult to cut and machine due to tendency of fibres to defibrillate under high compressive and shear stresses - Fibres are hygroscopic (absorb up to 6% by weight of moisture), hence properties degrade in the presence of moisture - Hence fibres have to be stored at low humidity - Absorption of moisture reduces strength and stiffness by about 5% at room temperature, but the effects are reversible - Effect of moisture absorption is more severe at high temperatures - High Capacity to absorb energy during impact and penetration - This is attributed to the combination of high strain to failure with low stiffness, complex failure modes involving kinking and defibrillation of fibres, interfacial disbonding and delamination - AFRP provides good ballistic performance for the above reasons - AFRP also has good vibration damping characteristics - AFRP particularly suited for pressure vessels due to their high tensile strength and resistance to mechanical damage. - Hence used as containment rings for jet engines

What are the features of Magnesium and Copper MMCs?

- Magnesium: light weight, good interface with reinforcements, poor corrosion resistance, useful in space applications - Copper: heavier than aluminium, higher shear strength than Aluminium at high temperatures, higher conductivity (used for cooling in NASP vehicle)

What are the features of Particulates?

- Mainly ceramic, to improve toughness of brittle matrices Carbon Nanotubes (stiffer and stronger, E > 1000 GPa, Su > 100 GPa)

What are Advanced (High Performance) Composite Materials?

- Materials made by embedding high strength/high modulus reinforcements within a matrix material, e.g. CFRP, SiC in Aluminium alloy matrix - Mainly used in applications that require high specific strength /stiffness (hence weight savings), such as aerospace industry

Dry Fibre Forms - Non Woven and Woven Fabrics

- Mats are non-woven fabrics which provide near-isotropic properties and are made of chopped (about 50mm long) or continuous strands randomly oriented and held together by a polymeric binder (mostly glass). - Woven Fabrics are made with conventional textile weaving looms with warp fibres that run in the loom direction and weft fibres perpendicular to them. Styles include plain weave (1x1) (good fabric stability), twill weave (e.g. 2x2 and 4x4) (note: diagonal lines on the design) , satin weave (5 harness and 8 harness - each weft passes over n-1 warp yarns) and basket weaves (two or more wefts interlacing with two or more warps). - Twill and satin weaves have less fibre crimping, better packing efficiency, better drapability, formability and in-plane properties - Woven Fabrics are available in hybrid forms (mixture of weaves) and with hybrid materials (e.g. carbon fibres in one direction and glass fibres in the other). - Woven fabrics also come in a "comingled" form wherein the reinforcing fibres are mixed with thermoplastic polymer fibres which melt during consolidation and flows to form the thermoplastic matrix - The periodic undulation of fibres (crimping) due to the interlacing of warps and wefts results in a significant loss of reinforcing efficiency due to the bending of the fibres (maximum efficiency is achieved when the fibres are absolutely straight and in plane) - Compressive strength is particularly affected due to crimping, however through thickness strength and delamination resistance improves.

What are some features of Metal Matrix Composites?

- Metal alloys reinforced with continuous fibres, whiskers or particulates - Higher temperature resistance than PMCs (up to 650 degs.C) , high melting point - High ductility and toughness - Higher dimensional stability (low CTEs) - Mostly heavier than PMCs - Costly, complex and limited fabrication techniques - Problems with thermal coefficient mismatch and poor interfacial bonding between reinforcements and matrices - Susceptible to corrosion

What are the Environmental Factors that affect the mechanical properties and performance of PMCs?

- Moisture Absorption - Elevated Temperatures - Other Fluids and Solvents - Ultra Violet Radiation - Lightning Strike

What are the features of Moisture Absorption?**

- Moisture is absorbed by matrix materials by the process of diffusion - Weight increases of the order of 1%-1.5% are encountered after exposure over long periods of time (months) - Water being a very polar molecule, the diffusion mechanism involves hydrogen bonding with polar sites in the polymer molecule - Among epoxies, polyesters and phenolics, epoxies are the most polar while phenolics have the least polarity. Hence water permeability is the highest for epoxy resins and the lowest for phenolic resins - Note: Aramid fibres can absorb up to 6% by weight of moisture - The total mass of moisture in a laminate M is obtained by the integration of c(z,t) across the thickness. The saturation level (M/M0, where M0 is saturation moisture mass) increases with time reaching an asymptotic value of 1. - For small values of time "t" it can be approximated as: - The greater the laminate thickness "h", the longer it takes for saturation condition to be achieved. The magnitude of the diffusion constant and hence the absorption rate increases rapidly as the temperature rises.

What are the features of Carbon Fibre Polymer Matrix Composites?

- Most extensively used in aerospace structural applications due to best specific strength and stiffness properties - Lowest density next to aramid fibre composites - CFRP properties can be tailored due to wide choice of fibre and matrix systems - Carbon fibre reinforcements are produced in an extensive range of forms including chopped fibre mats, a variety of woven fabric products, unidirectional tapes, multi-layer fabrics and 3D preforms

What are the features of Epoxy Resins?

- Most widely used in Aircraft Structures - Excellent chemical and mechanical properties at low temperatures - They have a low viscosity stage during curing, which allows good wetting and liquid resin forming techniques such as RTM (resin transfer moulding) - Low shrinkage, good adherence to most fibres - Wide viscosity range, hence large processing window and easy to process - Not prone to voiding, low volatile emissions - Good Chemical resistance - Large database for aerospace applications - Tg increases with increasing cure temperature - Good dimensional and thermal stability - Properties such as fracture toughness can be tailored to some extent by formulation (additives)

What are some of the Emerging NDI Techniques for Composites?

- Non-Contact Ultrasonics - Real time Radiography - Pulse Thermography - Holographic Interferometry (PHITS) and digital Holography - Shearography and digital shearography - Acoustic Emission Techniques, including acousto-ultrasonics and acoustic impact - Vibration based techniques - On-line health monitoring systems using smart sensors such as optical fibres

General Laminating Procedure - Open Die Moulding

- Open die moulding is one sided - In open die moulding, layers of fibre or woven cloth are laid-up in the required configuration (laminating sequence) in an open mould and resin is added by brushing or spraying (wet lay up), or pre-pregs are laid up - Various methods are then employed to apply pressure to consolidate the lay-up - Curing is usually done at room temperature (using matrices that cure at RT), but heat may also be applied, using heater blankets, hot air blowers, ovens, or electric elements in the mould.

Manufacture of Carbon Fibres from PAN

- PAN (PolyAcryloNitrile) is an acrylic textile fibre produced by wet or dry spinning of the polymer or co-polymer. Dry spinning produced smooth fibres, while wet spinning (extrusion into a coagulating bath) produces a variety of cross sections, which may provide greater bonding surface area. - Stretching of fibres during spin reduces the diameter, aligns more molecular chains along the length, increasing stiffness - PAN fibre tows typically contain 10,000 fibres - Finished carbon fibres are between 5 and 10 m in diameter.

What are the features of Polyetherimide (PEI)?

- PEI is an amorphous high performance thermoplastic - Good dimensional stability, low shrinkage - Highly isotropic properties compared to crystalline polymers - Unreinforced PEI is one of the strongest engineering amorphous thermoplastics - Very tolerant to solvents (including halogenated solvents) - Main disadvantage is high viscosity in molten state which makes forming of components difficult

What are the features of Polymer Matrix Composites?

- PMCs are the most developed class of composite materials - Wide variety of applications, including aerospace, sports - PMCs are composed of light weight, high performance fibres in organic polymer matrices - PMCs can be fabricated into large complex shapes - Usually processed at low temperatures and pressures, reasonably low cost - Easier to machine, mould and fabricate (compared to other composites) - Limited high temperature applications - Susceptible to environmental degradation

Polyimide Resins - Examples of Polyimide Thermosets

- PMR-15: Have been used for engine casings and airframe of high speed military aircraft, but long term use over 1700C caused microcracking and embrittlement - PMR-II 5O, AFR 700B, etc: These are in general difficult to process and very expensive and have good mechanical properties only at elevated temperatures - Phenylethynyl terminated imide (PETI) from NASA: complex aromatic structures, high processing temperatures (~ 3700C), high Tg (> 2700C), long term thermo-oxidative stability, good mechanical properties, expensive.

What are the features of Ultrasonics NDI Technique?**

- Piezo-electric transducers are used to transmit ultrasonic pulses (using compression waves, shear waves, Lamb or Surface waves) - In pulse-echo technique the same transducer is used as a sensor to monitor the intensity of reflected signal - In through transmission technique, the intensity of the transmitted signal is measured using another transducer - Ultrasonic Testing works on the principle that the presence of a defect (void or delamination) causes a significant discontinuity in acoustic impedance (AI), causing most of the pulse energy to be reflected back (Values of AI of air and epoxy are respectively about 0.00043 and about 3.2 kg/(s mm2)) - Note that Ultrasonic Testing (UT) is mainly sensitive to planar defects perpendicular to the direction of the pulse (delaminations)

Manufacture of Carbon Fibres from Pitch

- Pitch is a relatively cheap pre-cursor, made up of a mixture of organic compounds - Carbon fibres produced at low cost from pitch have relatively poor mechanical properties - Ultra high stiff fibres can be produced from pitch, but involve elaborate procedures which are very expensive - Unlike with PAN, in the conversion of pitch to carbon fibres no tension is required to develop molecular orientation for high modulus and strength, due to the anisotropic liquid crystal nature of pitch. - Pitch based fibres exhibit greater tensile modulus than PAN based carbon fibres, due to the more highly graphetisable nature of the pitch precursor - However pitch based fibres have low compression and shear properties - Pitch based fibres are also more porous resulting in lower tensile strengths. - Pitch based fibres have much higher carbon content, around 80% compared to 50% for PAN precursors - In addition to stiffness, electrical and thermal conductivities of pitch based carbon fibres are very high. Hence they are widely used in space applications.

What types of materials are used in Liquid Resin Moulding Techniques?**

- Polyesters, epoxies, BMI, phenolics and cyanate esters - Resin viscosity has to be carefully selected depending on the complexity of the part - For low fibre volume mouldings (around 40%), resin viscosities up to 3500 centipoise are suitable - For aerospace applications (fibre volume ratios of 70%) viscosities less than 500 centipoise are used

What are the features of Poly-ether-ether-ketone (PEEK)?

- Polyketones are crystalline polymers with good high temperature resistance - Aromatic varieties include: Poly-ether-ketone (PEK) and Polyether-ether-ketone (PEEK) - PEEK has good mechanical properties, temperature tolerance and solvent resistance - Rapid cooling produces amorphous PEEK, then crystallinity is introduced by annealing; optimum level of crystallinity is 25-40% - PEEK is resistant to non oxidizing acids (HCl - hydrochloric acid), alkalies & solvents; dissolves only in concentrated sulphuric acid. - PEEK is expensive and mainly used in high performance and defence applications

What are the different type of Matrices?

- Polymer Matrices (Max Service Temp Tser < 3000C) (Thermosets & Thermoplastics) - Metal Matrices (Tser < 6500C) (magnesium, aluminium, titanium and titanium alloys, etc) - Ceramic Matrices (Tser < 14000C) (Silicon Carbide, alumina, glass, etc) - Carbon Matrix (Tser > 14000C)

What are the features of Amorphous Thermoplastics?

- Polymer chains in a random coil status with no order - Usually soluble in common solvents - Prepregs can be made with low viscosity solutions to lower processing temperatures, but composite is solvent susceptible - Amorphous thermoplastics are more susceptible to creep and fatigue damage than semi-crystalline ones - Good fire, smoke and toxicity characteristics - Mainly used on aircraft parts where high temperature performance or high toughness is required, and exposure to solvents is not an issue.

What are the features of Semi-crystalline Thermoplastics?

- Polymer chains with locally ordered regions (crystallites) where the chains are aligned in crystal formation - Crystallinity provides good solvent resistance - Crystallinity improves high temperature properties including creep resistance - Degree of crystallinity can be varied by process variations and cooling rates (rapid cooling results in low crystallinity, slow cooling or annealing produces higher crystallinity - Semi-crystalline polymers shrink more on solidification - Main disadvantage is lack of adhesion, fibres have to be coated with molten thermoplastic to form the composite

What are the features of Sulfone Derivatives?

- Polysulfone (PSU), Polyethersulfone (PES), Polyarylsulfone (PAS) are high performance amorphous thermoplastics - Good tolerance to high temperatures and fire - Good hydrolytic stability - Good retention of mechanical properties in hot/wet conditions - Self extinguishing, and produce little smoke, when they burn - Not resistant to all solvents

Fatigue Performance of GFRP

- Poor fatigue performance compared to other PMC's - Fatigue performance deteriorates more severely in hot/wet conditions due to moisture ingression through matrix cracks causing stress-corrosion damage to glass fibres - Residual strength and stiffness decrease initially with increasing fatigue cycles before becoming constant - GFRP are more susceptible to thermal fatigue, and heat damage under high frequency fatigue due to low thermal conductivity and low stiffness.

How do Polymer Matrix Composites (PMCs) differ from metallic alloys?

- Properties are not uniform in all directions - Strength and stiffness can be tailored to meet loads - Possess a greater variety of mechanical properties - Have poor through the thickness (short transverse) strength - Are usually laid up in two dimensional form, while metals may be used in billets, bars, forgings, castings, etc. - Have greater sensitivity to environmental heat and moisture - Possess greater resistance to fatigue damage - Damage propagates through delamination rather than through the thickness cracks

The mechanical properties of a unidirectional continuous composite depend on.........(4)

- Properties of Fibres - Properties of the Matrix - Volume Fraction of the Fibres (Matrix) - The fibre/matrix bond strength (interfacial strength)

What is the Function of Matrix in a Composite?

- Provides shape to the component - Holds the fibres together - Transfers load in and out of fibres - Separates the fibres to prevent failure from adjacent fibres - Protects fibres from environmental damage - Determines "matrix dominated" properties: Longitudinal compressive strength Transverse tensile strength Intra and interlaminar shear strength

What are the four stages of quality assurance?

- Quality Control of Raw Materials - Monitoring of Manufacturing Processes - Quality Assurance of Finished Product - Damage Monitoring of In-Service Part (using Non Destructive Inspection)

What are some Fabrication Techniques for MMCs?

- Rapid liquid metal processes such as squeeze casting - Powder metallurgy processes based on heating compacted metal powders to just below their melting points to consolidate them (Hot Isostatic Press) - Some conventional metal working techniques for particulate MMCs - Super plastic forming - Diffusion bonding (for Ti matrix systems) Note: Special diamond coated tools are required for MMCs due to their high wear resistance.

Lamination for Aircraft Grade Composites

- Requires high precision, quality control and high fibre to matrix ratios to ensure the best mechanical properties possible. - Usually involves large volume of parts and often components of large size - Often more complex in shape - More complex in cure process due to the use of high performance materials - Hence usually employs pre-pregs, automated lay-up procedures and autoclaves for curing under high pressure temperature, often with pre programmed cure cycles and sensors for precision monitoring of cure quality.

What are the main types of Liquid Resin Moulding Techniques?**

- Resin Transfer Moulding - Resin Film Infusion - Vacuum Assisted RTM

Silicon Carbide Fibres manufactured by CVD

- SiC fibres can be made by Chemical vapour deposition (CVD) or through a polymer precursor route. - SiC made by CVD is suitable for reinforcing Al and Ti alloys, as well as ceramics such as SiC, SiN or glass. - CVD fibres are produced by the reaction of Hydrogen with a mixture of chlorinated alkali silanes at the surface of a resistivity heated substrate fibre. - The resulting fibre is ploycrystalline -SiC, with a fairly coarse grain and low internal stresses, compared to microcrystalline Boron which has high internal stresses. - The deposition cost of SiC fibres is only a third of that of Boron - These fibres retain most of their strength up to 9000C. - CVD fibres are produced with a Carbon rich zone of several microns close to the surface to ensure stable interfacial bonding

Conversion from Pitch to Carbon Fibres (Stages)

- Stage 1 - Isotropic to Mesophase Pitch: Isotropic pitch is subjected to prolonged heating in an inert atmosphere at 400- 4500C to form a liquid crystal phase (mesophase) which is meltspun into fibre form - Stage 2 - Cross linking: To reduce relaxation, the pitch fibres are cross linked by heating for a short time at 3000C in atmosphere containing O2. - Stage 3 - Pre-Carbonisation : Heating at 10000C, to reduce the rate of gas evolution and creation of surface flaws - Stage 4 - Carbonisation - Stage 5 - Graphitisation

Conversion from PAN to Carbon Fibres (Stages)

- Stage 1 - Stabilisation: PAN is first stabilised in air at about 2500C by oxidation to form a thermally stable ladder polymer, with a high glass transition temperature (Tg). - Stage 2 - Carbonisation: Removal of N, O and H in an inert nitrogen atmosphere at 1200-16000C. Fibres develop full strength at 1500-16000C. Basal planes align along the fibre axis, but remain as extended 2D ribbons. At this stage Young's modulus is about 240 GPa, Ultimate Strain about 1.5%. - Stage 3 - Graphitisation: Final heat treatment at 1500-25000C to in an inert atmosphere (Argon), wherein basal layers grow and coalesce along the fibre direction, to provide higher E (up to 380 GPa) at the cost of lower strain capability (0.7%) and strength.

Types of Polyester in Polyester Resins

- Standard Polyesters contain saturated phthalic acid or glycol which are cheap - Variations of polyesters are produced by replacing these with specific alternative materials - The replacement of orthophthalic acid by isophthalic acid or the use of diphenylol instead of glycol improves strength and durability - The use of adipic acid improves flexibility and increases strain to failure - Halogenated anhydrides are used to produce fire retardant composites

Impact Strength of GFRP

- Superior ability to absorb impact energy compared to other PMC's - Impact resistance of S-Glass/epoxy is 4 to 7 times more than that of high strength CFRP, 35 times more than that of high modulus CFRP and about 10 times better than aircraft grade aluminium alloys

What are the Strength Values required for an Orthotropic Lamina (2D)?**

- Tensile strength in fibre direction u1t - Compressive strength in fibre direction u1c - Tensile strength in transverse direction u2t - Compressive strength in transverse direction u2c - Shear Strength u12

What are the Assumptions involved in determining elastic constants using Micromechanics?**

- The fibres are assumed to be homogeneous, linearly elastic, isotropic, regularly spaced, perfectly aligned and of uniform strength - The matrix is assumed to be homogeneous, linearly elastic and isotropic - The fibre matrix interface is assumed to be perfect with no voids or dis-bonds - The lamina is orthotropic along its principal directions - The composite is considered to be homogeneous (but anisotropic) at the macroscopic level, i.e., it behaves as one single material macroscopically

What are the features of Radiography NDI technique?**

- The level of absorption of X-rays passing through a medium depends on the physical density of the material ( density and the thickness through which it passes) - The presence of a defect (having a different density) will cause variations in the energy absorbed and hence in the intensity of X-rays transmitted - When recorded on a film, a region of lower density (or thickness) appears as a darker spot in comparison to a region of higher density (or thickness) - Radiography is rather insensitive to planar defects (thin areas of lower or higher density), but is very useful to pick up defects of larger depth (> 1 mm) - The highest energy levels of the X-rays produced depends on the Voltage applied between the anode and cathode - Transportable X-ray equipment have capacities of up to 400 KV which permits on-site inspection of up to 10 cm thick steel - For higher energy field applications Gamma sources are employed - Linear Accelerators can produce up to 300 MV - The main disadvantage is the hazardous nature of X-ray. Exposure to radiation is a health hazard and has to be monitored carefully

What are the features of Constituent Materials - Matrices?

- The matrix forms the shape of the component - Holds the fibres together - Transfers load in and out of fibres - Separates the fibres to prevent failure from adjacent fibres - Protects fibres from environment - Provides transverse, shear and compressive strengths

What are the similarities/differences between Carbon and Graphite?

- The name "graphite" is sometimes used interchangeably with carbon for carbon fibres, but strictly this is incorrect (Baker et al.) - "Graphite is a form of carbon in which strong covalently bonded hexagonal basal planes are aligned in a three dimensional lattice. The weak dispersive Van der Waal's bonding allows easy slip between basal planes, the basis for the lubricating properties of graphite". - "The atomic structure of carbon differs in that the basal planes have only a two dimensional order, which inhibits slip". - Hence graphite is carbon but all carbon is not graphite.

Silicon Carbide Fibres based on a Polymeric Precursor

- The polymer precursor route for manufacture of SiC fibres is based on a high molecular weight polymer containing Si and Carbon. - The polycarbon-silane polymer is melt spun to form a multifilament tow and then pyrolised in two temperature stages: at 5500C to stabilize the precursor to be infusible, and at 8500C to form an impure form of silicon carbide, containing SiC, SiO2 and free carbon. - Nicalon is the most well known of multifilament SiC from polymer precursor, which can be produced in a wide variety of textile forms allowing flexibility in manufacture of components. - Due to impurities, the elastic modulus of polymer precursor SiC is lower than that of bulk SiC (220 GPA compared to 450 GPa

What are the reasons for the very high strength of fibres?

- The probability of a flaw being present is inversely proportional to the volume of the materials for a given length (and fibres have very low cross sectional area) - Flaws can be minimised by proper manufacturing and coating procedures - Drawing and spinning impose high strains along the fibre axis producing a more favourable orientation of crystal structure - High cooling rate or rapid molecular deposition produces ultra fine grained structures with much higher properties. - Note: Some polymers (Aramid and high density Polyethylene also form very strong and stiff fibres)

What are the features of Constituent Materials - Reinforcements?

- The reinforcements provide strength and stiffness to the composite - They carry the load - Come in a wide variety of forms: Continuous fibres Whiskers Particulates Nanotubes

Manufacturing Processes for thermoplastics**

- Thermoplastic resins have very high viscosities - Liquid resin moulding techniques are not applicable However, conversion from intermediate to final product requires only application of heat and pressure - No chemical reaction or emission of volatiles involved (clean and dry process) - Manufacturing times are small (30 mins compared to 3 to 7 hours for thermosets) - Using high rate, automated manufacturing technologies, large through put can be achieved - Requires high temperatures (250 to 4500C) compared to thermosets (RT to 2000C) - Requires high consolidation pressures (up to 1500 kPa) compared to thermosets (1 to 6 atmospheres )

Determination of Strength Values for Material Characterisation**

- To determine strength values coupons have to be tested to failure, in tension, compression and shear - Unlike metals for which dog bone shaped coupons are tested in tension, for composites the coupons are rectangular, parallel-sided with end tabs (to prevent failure in the grips) - In general variability is much higher in composites, so it is necessary to test many more specimens (typically 15) to obtain statistically acceptable data. - Note that for acceptable strength values, it is necessary to ensure that failure occurs within the gauge region (well outside the grips)

Requirements of the Component Manufacturing Process**

- To orient the fibres in the appropriate directions and proportions to obtained the desired 2D or 3D properties - To produce the required shape of the component - To ensure that the required properties of the matrix and the matrix/fibre interface are obtained - To ensure that the fibres are not damaged or misoriented and distributed evenly as per design - To ensure that the composite is free from significant voiding and other defects

Fatigue Resistance of PMCs - BVID and Damage Growth

- Unlike GFRP and AFRP, in which fatigue strength of undamaged structures may be of concern, for CFRP fatigue is of concern only in the presence of barely visible impact damage (BVID) - The fatigue strength of damaged CFRP drops steeply to around 50% of that of undamaged structure initially but thereafter decreases only gradually with increasing damage size - The BVID delaminations grow marginally under fatigue loading, which leads to a small drop in fatigue strength or fatigue life. - Unlike metals, damage has a significant effect on static residual strength, but less on fatigue life in CFRP

Moisture absorption - Effects at Elevated Temperatures

- Water reduces the glass transition temperature (the temperature above which the polymer goes soft) of the matrix. A moisture content of about 1.2% can reduce the glass transition temperature of epoxy matrix by about 400-500C. - This degrades the high temperature performance of the laminate significantly. Properties that are strongly dependent on matrix performance are the most severely affected. In the case of fibre dominated laminates these are mainly compression and buckling properties. In matrix dominated laminates, tensile strength and stiffness are also affected. - Degradation in high temperature performance is of particular concern in high performance aircraft whose skin temperatures may vary from - 500C (during high altitude cruise) to over 1000C (due to aerodynamic heating at supersonic speeds) in a matter of seconds.

What are the drivers for improved materials for aerospace applications?

- Weight Reduction - Improved Performance - Reduced Acquisition Cost - Reduced Through-Life Support Cost

General Laminating Procedure - Wrapping

- Wrapping is alternative to filament winding, using tapes, fabrics or pre-pregs to wrap around a removable mandrel to form the lay-up - The laminate is cured under pressure and the mandrel is removed - Special machines are avaliable for wrapping so the process can be automated - Pressure can be applied by use of shrink film, vacuum bag or autoclave, or by using silicon rubber bladders between the mandrel and the lay-up, which are pressurized after wrapping - Advantages: High through put, good quality control - Disadvantages: High initial investment, not suitable for all parts, limited in shape and size - Employed for small parts like fishing rods, golf clubs and tennis racquets

Steps involved in manufacture of laminates in aircraft industry

- cutting and kitting in a "clean room" environment under controlled humidity and temperature (Usually by hand requiring skilled and dedicated labour, but has been automated to a great extent) - Lay-up or ply-stacking in the required configuration and shape using appropriate tools - Bagging - Preparing for cure with accessories and vacuum sealing - Autoclave Curing / Co-curing - Debagging, Finishing and Painting - Trimming and Drilling

What are the features of Fibres?

- made from light elements (Carbon, Boron), -silicon compounds (silica and silica based glasses, SiC, SiN) or organic materials based on long chain molecules of C, H and N - produced by pyrolysis (e.g. Carbon) or - Chemical Vapour Deposition (CVD) (e.g. Boron) - Single filaments or tows of over 25000 filaments

What are the advantages/disadvantages of Polyimide Thermosets?

Advantage - Stability at high temperatures (up to 3000C) - Resistance to most chemicals - Can be formulated to give better mechanical properties Disadvantages - High Cost - Difficulty in processing

What are the advantages/disadvantages of Vinyl-Ester Resins?

Advantages - Combines chemical resistance of epoxies with easy processing of polyesters - Lower cross linking, hence better mechanical properties than polyesters - Improved interfacial bond strength with fibres Disadvantages - High cost compared to polyesters - Higher shrinkage levels than epoxies

What are the advantages/disadvantages of Phenolic Resins?

Advantages - Excellent resistance to high temperatures, especially under oxidizing conditions - Phenolics have good ablation properties, forming char readily, yielding a superficial layer of carbon, which burns away protecting the underlying composite - Hence good as fire retardant materials Disadvantages - High pressures needed for polymerization, hence difficult to fabricate - Significantly lower mechanical properties due to high void content

What are the advantages/disadvantages of GFRP?

Advantages - Low cost - Superior Impact Resistance - Better dielectric Properties Disadvantages - Lower specific stiffness than all other PMCs - Poorer fatigue performance - Susceptible to Stress rupture - Degradation of Properties due to moisture and other aggressive environments

Advantages and Disadvantages of Polyester

Advantages of Polyester - Initial low viscosity that permits good wetting of the fibres - Low cost (inexpensive and readily available raw materials, easy long term storage) - Cure conditions can be modified easily without too much expertise - Easy manufacture of modifications for a variety of specific applications - Excellent environmental durability Disadvantages of Polyester - High exotherm and high shrinkage on cure (this leads to residual stresses and poor interfacial bonding with fibres) - Lower values for mechanical properties than epoxies - Systems with adequate shear strength tend to be brittle (low toughness) - Toughening mechanisms are relatively ineffective - Poor chemical resistance to even dilute alkali

What are the advantages/disadvantages of Liquid Resin Moulding Techniques?**

Advantages: - Can be used for manufacturing complex shapes and parts - Materials can be selected to obtain the desired properties - Process can be tailored to each individual type of component to maximise efficiency Disadvantages: - Tooling is very costly - Initial set up time is time consuming and expensive - Process is time consuming and laborious - High operator skill and experience required - Very low through put

What are the advantages/disadvantages of Pultrusion?

Advantages: - Continuous process - Fully automated - High Throughput - Can use inexpensive forms of reinforcement - Can be employed to make very long components - No lamination or autoclave curing is required - No requirement to refrigerate raw materials Disadvantages: - Tooling can be costly - Initial set up time is time consuming and expensive - Process can only be applied to parts with constant cross section

What are the advantages/disadvantages of Filament Winding?**

Advantages: - High speed and high precision process - Fully automated - High fibre volume ratios can be achieved - Can be employed to make very large components - No lamination or autoclave curing is required - No requirement to refrigerate raw materials Disadvantages: - Tooling is very costly - Initial set up time is time consuming and expensive

What are some features of Thermoplastic PMCs?

Can be melted and remoulded, high toughness, high impact resistance, intermediate cost

What is coupon testing?

Coupons are tested to determine material properties (elastic constants, strengths, fracture properties, fatigue and environmental degradation)

What is Fatigue Testing?

Due to the relative insensitivity of undamaged coupons to fatigue, fatigue testing is usually conducted on detailed, sub-element and full scale levels under accelerated spectrum loading with and without damage

Fatigue Resistance of PMCs - Effect of Notches and Stress Concentrations &Fatigue Frequency

Effect of Notches and Stress Concentrations - Contrary to metals, in which stress concentrations have a greater effect on fatigue life than static strength, in composites the effect of stress concentrations on fatigue is minimal due to formation of microcracking and delaminations Effect of Fatigue Frequency - In general variations in frequency of loading does not have any significant effect on fatigue life of CFRP, whereas in the case of GFRP, frequencies over 5 Hz can reduce fatigue performance due to accumulation of heat damage due to the poor thermal conductivity of glass fibres. However, the accumulation of heat damage depends on matrix properies and the thickness of the laminate.

What is environmental testing?

Effect of exposure to Humidity is conducted using environmental chambers, often using accelerated moisture conditioning using elevated temperatures.

What are low temperature Thermosets? (Service Temps 1000- 1500C)

Epoxies Polyesters Vinyl Ester resins Phenolic resins

What is the meaning of Heterogeneous?

Has different material properties at all points

What is the meaning of Anisotropic?

Has different material properties in different directions with no planes of symmetry

What is the meaning of Homogeneous?

Has same material properties at all points

What is the meaning of Isotropic?

Has the same material properties in all directions (infinite number of planes of symmetry)

What is the meaning of Orthotropic?

Has three planes of symmetry for its material properties Note: An orthotropic material has 9 independent constants in 3D and 4 independent constants in 2D

What is Combined Loading and Environmental Testing?

Involves accelerated testing combining mission profiles, cyclic loads, environment and typical temperature excursions (eg. ENSTAFF)

What is Laminate Theory?

Laminate Theory establishes the relationships between loads (in plane forces and moments) and deformations (strains and curvatures) in a multilayer laminate.

What is a laminate?

Laminate is a product made by bonding together two or more thin layers (plies or laminae) of materials. Most common composites employed in aircraft industry are polymer matrix laminates.

What is the meaning of Transversely Isotropic?

Material with one plane (at every point) in which the properties are the same in all directions

What is Micromechanics?**

Mechanical properties of a single lamina (unidirectional laminate) can be computed from the known properties of the fibres and the matrix using : - Micromechanics - Mechanics of Materials approach (using simplified models of the composite) - Theory of elasticity approach (produces upper and lower bound exact analytical or numerical solutions) - Finite element modelling

Stress Rupture & Environmental Degradation of GFRP

Stress Rupture - GFRP is susceptible to stress rupture (static fatigue) Environmental Degradation - Fibre/matrix interfacial strength degrades significantly in aggressive environments (moisture and other chemicals)

What are the features of Wiskers?

Ultra strong, stiff, short fibres made up of single crystals (dia 1 to 10 µm, length < 1mm)

What are Reinforcements for Ceramic Matrices?

Usually similar ceramic particulates, such as SiC, SiN, Al2O3)