Reinforcement and matrix A clear interface is present

Reinforcement and matrix A clear interface is present between the two phases in a composite: • Matrix: the continuous phase • Reinforcement: the dispersed and discontinuous phase But a key role is played by: • Reinforcement/matrix interface, usually improved thanks to proper coupling agents • Fillers: used to change the resin properties or very often lower the cost of the matrix

Reinforcement and matrix The role of fibres Increase the mechanical properties of the matrix (. . . but in specific directions) • The contribution of the fibres to the properties of a composite depends on: ü Fibre mechanical properties ü Volume fraction of fibres ü Orientation ü Fibre matrix adhesion (i. e. interface) The role of the matrix • Impregnate fibres taking them apart • Load transfer between fibres and with the matrix • Prevent the contact of fibres with the environment • Prevent mechanical damage of fibres • Provide toughness limiting the crack propagation • It is responsible of shear and compression properties and of interlaminar (out of plane) properties

Lamina (or ply) and laminates • Lamina : a single layer (ply) of fiber and matrix. • Laminate: Two or more laminas are stacked to provide the required properties (usually adopting different fiber orientation) • Continuous unidirectional – High longitudinal modulus and strength – Low transversal modulus and strength Laminate • Continuous bidirectional • Discontinuous unidirectional • Discontinuos random

Reinforcement and matrix Mechanical property Dominating composite constituent Fiber matrix Lamina 0° tension V 0° compression V V Shear V 90° tension V Laminate Tension V Compression V V In-plane shear V V Interlaminar shear V

Reinforcement and matrix Fibers are mainly responsible of the following properties: • Stiffness and strength (but in the direction in which they are oriented) • Density • Fatigue • Thermal and electrical conductivity • Coefficient of thermal expansion • Cost (. . . besides glass fibers. . ) The matrix is mainly responsible of the following properties: • Density • Shear strength • Interlaminar properties (shear and out of plane properties) • Compressive properties (microbuckling) • Load transfer to fibres (effectiveness of the reinforcement) • Environmental resistance • Maximum service temperature • Processing properties

Tensile test on fibres grips 10 -15 mm glue clamp fiber cuts 20 -30 mm 60 -90 mm clamp

Tensile properties of fibres • All fibres are linear elastic solid. • Break occurs without any plastic deformation • The strain to break can be quite high, usually higher than strain to yielding of steel and aluminium

Specific strength (106 m 2/s 2 Pa*m 3/Kg) Specific properties of fibers 2. 75 UHMWPE Kevlar 2. 25 HT (high strength) graphite S glass 1. 75 1. 25 Polymer matrix E glass 0. 75 HM (high modulus) graphite Steel 0. 25 Aluminum 25 50 75 100 125 150 175 Specific modulus (106 m 2/s 2 Pa*m 3/Kg)

EFFECT OF TEMPERATURE Graphite/polyimide 1400 Specific strength 1000 Aluminum metal matrix 600 400 titanium metal matrix Carbon-carbon 300 100 Al carbides, nitrates Ti Steel 0 500 1000 Super alloys Refractory materials 1500 2000 2500 3000 Temperature (°F) 3500 4000

EFFECT OF THE DIAMETER OF FIBRES 8 Torayca M 40 Torayca T 300 Hercules AS 4 7 Strength (GPa) 6 5 4 3 2 1 0 3 4 5 6 7 diameter ( m) 8 9 10

Commercial grades of fibre strands • Bundles of parallel fibers are collected at the production plant • The minimum number of fibers in a bundle is called Strand • Other commercial names of different combinations of strands are: ü Roving ü Yarn ü Fabric ü Mats • When dealing with carbon fibers the name Tow or End are often used in place of strand • The number of fibers in a tow of carbon fibers is indicated by a “k” (for kilo) • A bobin of carbon fibers labeled “ 12 k” is made of a a bundle of 12000 fibres

Linear mass density of fibers (tex) • The linear mass density of fibres is the ratio between the weight and a given length of a fibre bundle unit symbol definition equation tex Weigth in grams of 1000 m of a strand tex=(M/L)*1000 decitex dtex Weigth in grams of 10000 m of a strand dtex=(M/L)*10000 denier den Weigth in grams of 9000 m of a strand den=(M/L)*9000 • Conversion table unit tex den tex 1 tex*10 tex*9 dtex/10 1 dtex*0. 9 den*0. 111 den*1. 11111 1

tow (strand) Unidirectional (UD) 0° filament Woven +45° yarn discontinuous Woven 0/90 continuous

Fiber bundles definition • STRAND: fiber bundles obtained grouping a limited number of fibers in the production plant. • YARN: It is made of several twisted strands. Twisting is strictly necessary when short (natural) fibres are used in order to provide mechanical properties to the yarn (Fig. c). It is not suited as composite reinforcement as a consequence of the poor fibre wettability resulting from twisting, i. e. fibres are packed and resin cannot wet all fibres. Furthermore, the fibres are not perfectly aligned • ROVING: it is made of parallel and aligned multiple strands (Fig. d). The term roving is mainly used for glass fibres. Very large linear densities are available up to 4800 tex. • TOW is used for carbon fibres instead of “roving”. They are characterized by the number of fibres indicated as 3 k=3000 fibres. Small tows contain 12000=12 k fibres or less, large tows more than 12000 fibres yarn tow (strand)

Roving and strands Molten glass Glass filaments Glass strand Single end roving Roving process Multi-strand roving

Different name given to fiber bundles Continuous fibres strand (tow) rovings yarns fabrics unidirectional plain twill woven basket mat satin

Fabrics • Fabric: strands, roving or tows are placed in different directions in a plane. They can be obtained by a weaving process on loom • Fabrics are characterized by fibre orientation, fibre position and by their surface density expressed as the weight in grams for square meter, g/m 2. They can be: • Knitted fabric • Woven fabric • Unidirectional fabric (UD fabric) • Random mat

Woven fabrics (Tela) • Woven fabrics: they are obtained by weaving fibres in warp (ordito) and weft (trama) directions. Fibres are aligned in the warp (0°) direction and weaved with weft fibres (90°). Weaving imparts mechanical stability to the fabric. Building style provides: • Drape (ability of a fabric to coat complex surfaces with single and double curvatures) • Smoothness • Stability • Wettability • Porosity

Knitted fabrics • Knitted fabric: not suitable for composite fabrication as a consequence of poor fiber alignment.

Different types of woven fabrics Harness satin weave (5 harness) Plain weave Advantages: Fabric stability Resists distortion Disadvantages: Least drapable Lower properties Advantages: High drapability High density High mechanical properties Disadvantages: Difficult to handle Fiber orientation distortion Fill(weft) direction 90° Warp direction 0° https: //www. youtube. com/watch? v=G 3 G 1_Kgfbz. A&list=PLKb. TEzgdd. Xl 210 V 2 c. RXEXDOMG 48 q 6 N 1 L&index=1

Woven fabrics • • • Plain – Each warp fibre passes alternately under and over each weft fibre. The fabric is symmetrical, with good stability and reasonable porosity. However, it is the most difficult of the weaves to drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles. Twill – One or more warp fibres alternately weave over and under two or more weft fibres in a regular repeated manner. With reduced crimp, the fabric also has a smoother surface and slightly higher mechanical properties Satin – The ‘harness’ number used in the designation (typically 4, 5 and 8) is the total number of fibres crossed and passed under, before the fibre repeats the pattern. Satin weaves are very flat, have good wet out and a high degree of drape. The low crimp gives good mechanical properties. Satin weaves allow fibres to be woven in the closest proximity and can produce fabrics with a close ‘tight’ weave. However, the style’s low stability and asymmetry needs to be considered.

Woven fabrics • Basket – it is a plain weave where two or more warp strands are weaved with two or more weft strands • Leno – Leno weave improves the stability in ‘open’ fabrics which have a low fiber count, i. e. unidirectional fabrics

Most used woven fabrics • PLAIN: 1/1 warp and weft strands (threads) alternate. It is characterized by the highest crimp. – – – Advantages: High stability Quite easily wetted Disadvantages (Compared to a UD ): Limited drapability High crimp and related lower mech. prop. in comparison with UD • SATIN: 4 (harness number) or more warp strands are jumped by a weft strand. The fabric is less crimped – – – – Advantages (Compared to plain): Low crimp and hence higher mech. prop. High smoothness Easily wetted High drape Disadvantages (Compared to plain): Low stability and related risk of distortions in fiber orientation

Most used woven fabrics • TWILL: Warp stands are weaved with two or more weft strands – – – Advantages (Compared to plain): Better wetted and higher drape Higher smoothness Slightly higher mech. prop. Disadvantages (Compared to plain): Lower stability • BASKET: it is a plain weave where two or more warp strands are weaved with two or more weft strands – – Advantages (Compared to plain): Higher smoothness Disadvantages (Compared to plain): Lower stability

Woven fabrics Weave styles- comparison of properties Plain Twill Stability good acceptable Poor drape Poor Good excellent acceptable good excellent Poor Smoothness Poor acceptable excellent Poor Balance Good Poor Good excellent acceptable Very Poor acceptable excellent Poor Density Symmetrical Low crimp Satin basket

An example of a technical data sheet

Unidirectional fabrics • Unidirectional fabrics: Most of fibers (75 -95% by weight) are placed in one direction and only a few fibers are weaved in the orthogonal direction. These last (often of different chemical type) are just used to keep aligned fibers. • This fabric is used to fabricate unidirectional laminas: – almost all the reinforcement is placed in a single direction – unidirectional laminas can be stacked with different orientations to produce more complex laminates

An example of a technical data sheet

Multiaxial Non-Woven Fabrics These fabrics are made of layers of parallel unidirectional fibers with different orientation (for instance 0/ 90°/45°, ecc. ) held in place by stitches, often made with polyester or glass threads. 0° Advantages • • +45° Higher mechanical properties as a consequence of the almost perfect alignment of fibres, i. e. no crimp 90° Quasi isotropic laminates can be obtained using textile technologies. This can reduce processing time, since predefined stacks of several laminas can be bought -45° Good wettability of the fibres Stitches increase the interlaminar (out of plane) properties and resistance to delamination Disadvantages: • Stitches can alter fibre alignment and could be not well impregnated, being often used of twisted yarns of polyester fibres • Higher costs

Multiaxial Non-Woven Fabrics • Weave & Stitch – With the ‘Weave & Stitch’ method the +45 and -45 layers can be made by weaving weft unidirectionals and then skewing the fabric, on a special machine, to 45. Open structure with +45° / 90° / -45° / 0°

Multiaxial Non-Woven Fabrics Guiding rolls creel fibers collecting rolls Stitched multiaxial Fiber mat bobins Stitching machine Deposition combs Needle feed chain

An example of a technical data sheet E=glass, C=carbon, A=aramide

Tow spreading to reduce the fabric thickness

Tow spreading to reduce the fabric thickness

Tow spreading to reduce the fabric thickness: mechanical properties of laminates obtained with decreasing ply thicknesses AE=Acoustic Emission

Braiding Braids are produced by interlacing fibres in a spiral nature to form a tubular fabric. – The diameter of the tube is controlled by the number of fibres in the tube’s circumference, the angle of the fibres in the spiral, the number of intersections of fibre per unit length of the tube and the size (tex) of the fibres in the assembly. The interlacing can vary in style (plain, twill, etc. ) as with 0/90 woven fabrics. – The process allows the fibres to move between angles of about 25 and 75, depending on the number and tex of the fibres. – Braids can be found in such composite components as masts, antennae, drive shafts and other tubular structures that require torsional strength. Fi b th res is m di ov re e cti in on –

Fi b th res is di mo re ve ct in io n Braiding https: //www. youtube. com/watch? v=V 49 Jvz 5 X_lc https: //www. youtube. com/watch? v=MC 00 z. Ekf. BRM

Overbraiding • During overbraiding the preform is fabricated on a core, which has the inner geometry of the desired preform • By reciprocating the core through the braiding point a preselected number of layers can be braided • The core is usually removed

Hybrid fabrics • Fabrics containing different fibers woven according to different geometries • Often they are used for their appealing aspect, i. e. aesthetic purposes Other properties • Carbon/Aramide: both are low density fibers. Carbon fibers provide stiffness and aramide fiber provide impact resistance • Aramide/ Glass: Good impact resistance. Glass fibers are responsible of reduced stiffness and higher density but also of lower costs • Carbon/Glass: Glass fibers reduce all mechanical properties in comparison to a full carbon fabric. However cost is significantly reduced. hybrid carbon 45° and aramide 0°

An example of a technical data sheet E=glass, C=carbon, A=aramide

Chopped fibers Strands can be cut to produce chopped fibres. Chopped fibres can be in the range of mm or cm. In the first case are used as reinforcement of thermoplastic polymers. In the second they are used in composite technologies for low technology applications (for instance spray-up) Chopped fibres of 30 -60 mm are used to make chopped strand mats. A proper binder is used to keep in place fibres in the mat. The binder should be dissolved by thermosetting matrix when the mat is impregnated. Veils are very thin mats used at composite surface for aesthetic purposes

Chopped strand mat production Needling or chemical bonding Rovings

Volume and weight fraction of reinforcement •

Volume and weight fraction of reinforcement •

Fiber content in composite laminates obtained from fabrics •

Void content •

Void content •

AGATE (Advanced General Aviation Transport Experiments ) materials qualification procedures