COMPOSITE MATERIALS

UNIT1 INTRODUCTION TO COMPOSITES Fundamentals of composites, need for composites - Enhancement of properties. Classification of composites – Matrix-Polymer matrix composites (PMC), Metal matrix composites (MMC), Ceramic matrix composites (CMC). Reinforcement – Particle reinforced composites, Fibre reinforced composites. Applications of various types of composites

DEFINITION A composite material is defined as a material which is composed of two or more materials at a microscopic scale and has chemically distinct phases A composite material is heterogeneous at a microscopic scale but statistically homogeneous at macroscopic scale Composite is a structural material which consists of combining two or more constituents. The constituents are combined at a macroscopic level and are not soluble in each other. One constituent is called the reinforcing phase and the one in which its embedded is called the matrix

HISTORY OF COMPOSITES

Sinews were used to join and cover the horn and antler together. HISTORY OF COMPOSITES 3000 B.C The horn and antler were used to make the main body of the bow as it is very flexible and resilient. Sinews were used to join and cover the horn and antler together. Glue was prepared from the bladder of fish which is used to glue all the things in place. The string of the bow was made from sinew, horse hair and silk. The composite bow so prepared used to take almost a year for fabrication. The bows were so powerful that one could shoot the arrows almost 1.5 km away.

During World War II Military application Non-metallic shielding of Radomes (to house electronic radar equipments) Glass Fibre Reinforced Plastics (GFRP) The first application of wood - composite laminates in Havilland Mosquito Fighter/Bomber of British Royal Air- Force

Why we need these materials? 1.Strength 2. Stiffness 3. Toughness 4. High corrosion resistance 5. High wear resistance 6. High chemical resistance 7. High environmental degradation resistance 8. Reduced weight 9. High fatigue life 10. Thermal insulation or conductivity 11. Electrical insulation or conductivity 12. Acoustic insulation

EXAMPLES Naturally Found Composites Wood, where the lignin matrix is reinforced with cellulose fibers and bones in which the bone-salt plates made of calcium and phosphate ions reinforce soft collagen Advanced Composites Graphite/epoxy, Kevlar R/epoxy, and boron/aluminum composites

CLASSIFICATION OF COMPOSITES Composite materials are commonly classified at following two distinct levels: Matrix material Material structure

1. Holds the fibres together 2. Protects the fibres from environment MATRIX The matrix is the monolithic material into which the reinforcement is embedded and completely continuous and it is the primary phase FUNCTIONS 1. Holds the fibres together 2. Protects the fibres from environment 3. Protects the fibres from abrasion (with each other) 4. Helps to maintain the distribution of fibres 5. Distributes the loads evenly between fibres 6. Enhances some of the properties of the resulting material and structural component (that fibre alone is not able to impart). These properties are such as: transverse strength of a lamina Impact resistance 7. Provides better finish to final product

Based On Matrix Material

Polymer Matrix Materials(PMC) Polymers make ideal materials as they can be processed easily. It possess lightweight and desirable mechanical properties PMCs consists of a polymer (e.g., epoxy, polyester, urethane) reinforced by thin diameter fibers (e.g., graphite, aramids, boron). For example, graphite/ epoxy composites are approximately five times stronger than steel on a weight for-weight basis. DRAWBACKS: Low operating temperature, high coefficients of thermal and moisture expansion and low elastic properties in certain directions

Metal matrix composites(MMC) Metal matrix composites are mainly used to provide advantages over monolithic metals ADVANTAGES Higher elastic properties, higher service temperature, insensitivity to moisture , higher electric and thermal conductivities, better wear, fatigue and flaw resistances DISADVANTAGES Higher density

Ceramic Matrix Composites(CMC) CMC have a ceramic matrix such as alumina, calcium, alumino silicate reinforced by fibers such as carbon or silicon carbide ADVANTAGES High strength, hardness, high service temperature limits for ceramics, chemical inertness, and low density DISADVANTAGES Low fracture toughness. Under tensile or impact loading, they fail catastrophically.

Carbon–Carbon Composites These composites use carbon fibers in a carbon matrix. It is used in very high-temperature environments of up to 6000°F(3315°C), and are 20 times stronger and 30% lighter than graphite fibers. ADVANTAGES Withstand high temperatures, low creep at high temperatures, low density, good tensile and compressive strengths, high fatigue resistance, high thermal conductivity, and high coefficient of friction DISADVANTAGES High cost, low shear strength, and susceptibility to oxidations at high temperatures

Based On Reinforcing Material Structure Particulate Reinforced Composite (PRC) Fibre Reinforced Composite (FRC) Laminar Composite (LC)

Particulate Reinforced Composite Microstructural of metal and ceramics composites which show particles of one phase shown in the other are known as PRC Three dimensional reinforcement in composites offers isotropic properties because of the three systematical orthogonal planes Examples: Mica flakes reinforced with glass (Non metallic particles in non metallic matrix) Aluminium particles in polyurethane rubber (Metallic particles in non metallic matrix) Lead particles in copper alloys (Metallic particles in Metallic matrix)

Fibre Reinforced Composite The orientation of fibre in the matrix is an indication of the strength of the composite and the strength is greatest along the longitudinal direction of fibre Short length fibres incorporated by the open or close mould process are less efficient than non conventional shaped and hollow shaped fibres which shows better mechanical properties In hollow fibres, the transverse compressive strength is lower than that of solid fibre composites when the hollow portion is more than half the total fibre diameter. But, it is not easy to handle and fabricate

Laminar Composite These composites may be of several layers of two or more metal materials occurring alternatively or in a determined order more than once and in as many numbers as required for a specific purpose Clad and sandwich laminates

Reinforcements The role of the reinforcement in a composite material is fundamentally one of increasing the mechanical properties of the system. Typical reinforcements are asbestos, boron, carbon, metal glass and ceramic fibers, flock, graphite, jute, sisal and whiskers, as well as chopped paper, macerated fabrics, and synthetic fibers. The primary difference between reinforcement and filler is the reinforcement markedly improves tensile and flexural strength, whereas filler usually does not. Also to be effective, reinforcement must form a strong adhesive bond with the resin.

TYPES OF REINFORCEMENTS

FIBRES Fibers are the important class of reinforcements, as they satisfy the desired conditions and transfer strength to the matrix constituent influencing and enhancing their properties as desired. Different types of fibers are glass fibers, silicon carbide fibers, high silica and quartz fibers, aluminina fibers, metal fibers and wires, graphite fibers, boron fibers, aramid fibers and multiphase bers fibers Glass fibers: Most commonly used, inexpensive, easy to manufacture and possess high strength and stiffness. Major disadvantage is that it is prone to break when subjected to high tensile stress for a long time Metals fibers It is easily produced using several fabrication processes and are more ductile and it possess high strengths and temperature resistance. Their weight and the tendency to react each other through alloying mechanisms are major disadvantages.

Filled composites On adding filler materials to plastic matrix it change or enhance the properties of the composites. Filler particle may have irregular structure or have geometrical shapes like polyhedrons, short fibres or spheres Filler materials occasionally impart color to the composites It increases stiffness, thermal resistance, stability, strength and abrasion resistance, porosity and coefficient of thermal expansion However the methods of fabrication are very limited. They also shorten the life span and weaken the composites

Microspheres The key properties of the microsphere are specific gravity, stable particle size, strength and controlled density Solid glass microspheres manufactured from glass are most suitable for plastics. It is coated with a binding agent which bonds itself as well as the sphere’s surface to the resin. This improves the bonding strength and removes absorption of liquid into the separations around the spheres Solid glass microspheres have relatively low density so it influences the commercial value and weight of the finished product. Studies have indicated that their inherent strength is carried over to the finished moulded part of which they form a constituent. Hollow microspheres are essentially silicate based made at controlled specific gravity. They are larger than solid glass spheres used in polymers and commercially supplied in a wider range of particle sizes. Due to the modification, the microspheres are rendered less sensitive to moisture, thus reducing attraction between particles. This is very vital in highly filled liquid polymer composites where viscosity enhancement constraints the quantum of filler loading.

Whiskers Single crystals grown with nearly zero defects are termed whiskers. They are usually discontinuous and short fibers of different cross sections made from several materials like graphite, silicon carbide, copper, iron etc. Typical lengths are in 3 to 55 nm ranges. Whiskers differ from particles in that, whiskers have a definite length to width ratio greater than one. Whiskers can have extraordinary strengths upto 7000 MPa Flakes It is often used in place of fibers as can be densely packed. Metal flakes that are in close contact with each other in polymer matrices can conduct electricity and heat, while mica flakes can resist both. Flakes are not expensive to produce and usually cost less than fibers Parallel flakes filled composites provides uniform mechanical properties in the same planes as the flakes. Flake composites have a higher theoretical modulus of elasticity than fibre reinforced composites. They are easy to handle and relatively cheap to produce

Particulate Reinforced Composites Microstructures of metal and ceramic composites which show particles of one phase strewn in the other are known as particulate reinforced composites In particulate composites, the particles strengthen the system by the hydrostatic correction of fillers in the matrices and by their hardness relative to the matrix Solidification of Composites Directional solidification of alloys is adopted to produce in-situ fibers. They are really a part of the alloy being precipitated from the melt, while the alloy is solidifying. This comprises eutectic alloys wherein the molten material degenerates to form many phases at a steady temperature. When the reaction is carried out after ensuring the solidifying phases, directionally solidified eutectics result.

Parameters affecting the properties of fibrous composites 1. Length of the fibre 2. Orientation of the fibre (with respect to the loading direction) 3. Shape of the fibre 4. Distribution of the fibres in matrix material 5. Properties of the fibres 6. Properties of the matrix material 7. Proportion of fibre and matrix material

Advantages of composite material over conventional material High strength to weight ratio High specific strength High stiffness Design Flexibility High fatigue properties High corrosion resistance Low coefficient of thermal expansion High internal damping Ease of Manufacturing Reduced part count, near net shape Low scrap and machining etc

Disadvantages High cost of raw materials and fabrication. Composites are more brittle than wrought metals and thus are more easily damaged. Transverse properties may be weak. Matrix is weak, therefore, low toughness. Reuse and disposal may be difficult. Difficult to attach. Repair introduces new problems, for the following reasons: Materials require refrigerated transport and storage and have limited shelf life. Hot curing is necessary in many cases requiring special tooling. Hot or cold curing takes time. Analysis is difficult.

Applications Aerospace/Military Civil Electronic Energy Automobile/Transportation Sports Medical Marine

Aerospace

Helicopter Blade

SPORTS

Wind Energy

Medical

Civil/Infrastructure

Marine

Automobile/Transportation