CHAPTER 1: INTRODUCTION TO COMPOSITE MATERIALS Prepared by: Tan Soo Jin

Composite Materials Definition A materials system composed of two or more physically distinct phases whose combination produces aggregate properties that are different from those of its constituents Generally, one material forms a continuous matrix while the other provides the reinforcement Examples: Concrete reinforced with steel Epoxy reinforced with graphite fibers. Plastic molding compounds containing fillers Rubber mixed with carbon black

Can you think of any other examples of where composites are used?

Composite Can Be Found In -The aerospace industry (structural components as well as engines and motors) -Automotive parts (panels, frames, dashboards, body repairs) -Sinks, bathtubs, hot tubs, swimming pools -Cement buildings, bridges -Surfboards, snowboards, skis -Golf clubs, fishing poles, hockey sticks -Trees are technically composite materials, plywood -Electrical boxes, circuit boards, contacts -Everywhere

From constituents to application

Classification

Components in a Composite Material Nearly all composite materials consist of two phases: Primary phase (matrix) - forms the matrix within which the secondary phase is imbedded Secondary phase (reinforcement) - imbedded phase sometimes referred to as a reinforcing agent, because it usually serves to strengthen the composite The reinforcing phase may be in the form of fibers, particles, or various other geometries

Functions of the Matrix Material Protect phases from environment Transfer stresses to phases Holds the imbedded phase in place, usually enclosing and often concealing it When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent

Natural Vs Synthetic Fibers Rayon Nylon Acetate Acrylic Spandex Polyester Natural Silk Cotton Wool Mohair Cashmere

Classification of Natural Fibers Natural fibers are classified according to their origin: Vegetable or cellulose Animal or protein Mineral

Cellulose Fibers Cotton—vegetable fiber; strong, tough, flexible, moisture- absorbent, not shape-retentive Rayon—chemically altered cellulose; soft, lustrous, versatile Cellulose acetate—cellulose that is chemically altered to create an entirely new compound not found in nature

Synthetic Fibers Made from derivatives of petroleum, coal, and natural gas. Nylon—most durable of man-made fibers; extremely lightweight Polyester—most widely used man- made fiber Acrylic—provides warmth from a lightweight, soft, and resilient fiber Spandex—extreme elastic properties

Materials for Fibers Fiber materials in fiber‑reinforced composites: Glass – most widely used filament Carbon – high elastic modulus Boron – very high elastic modulus Polymers - Kevlar Ceramics – SiC and Al2O3 Metals - steel The most important commercial use of fibers is in polymer composites

Continuous vs. Discontinuous Fibers Continuous fibers - very long; in theory, they offer a continuous path by which a load can be carried by the composite part Discontinuous fibers (chopped sections of continuous fibers) - short lengths (L/D = roughly 100) Important type of discontinuous fiber are whiskers ‑ hair-like single crystals with diameters down to about 0.001 mm (0.00004 in.) with very high strength

Particles and Flakes A second common shape of imbedded phase is particulate, ranging in size from microscopic to macroscopic Flakes are basically two‑dimensional particles ‑ small flat platelets The distribution of particles in the composite matrix is random, and therefore strength and other properties of the composite material are usually isotropic Strengthening mechanism depends on particle size

Classification of Composite Materials Traditional or natural composites – composite materials that occur in nature or have been produced by civilizations for many years Examples: wood, concrete, asphalt Synthetic composites - modern material systems normally associated with the manufacturing industries, in which the components are first produced separately and then combined in a controlled way to achieve the desired structure, properties, and part geometry

Examples of Natural Composites Wood Cellulose Fibers Lignin Matrix Bones Collagen Fibers Mineral Matrix

Phase of Composite Matrix Phase: Polymers, Metals, Ceramics, also, continuous phase, surrounds other phase Reinforcement Phase: Fibers, Particles, or Flakes also, dispersed phase, discontinuous phase Examples: – Jelly and mixed fruit – Wood (cellulose fibers in hemicellulose and lignin) – Bones (soft protein collagen and hard apatite minerals) – Pearlite (ferrite and cementite) → Interface between matrix and reinforcement

Properties of Composite Materials In selecting a composite material, an optimum combination of properties is usually sought, rather than one particular property Example: fuselage and wings of an aircraft must be lightweight and be strong, stiff, and tough -Several fiber‑reinforced polymers possess this combination of properties Example: natural rubber alone is relatively weak -Adding significant amounts of carbon black to NR increases its strength dramatically

Properties are Determined by Three Factors: The materials used as component phases in the composite The geometric shapes of the constituents and resulting structure of the composite system The manner in which the phases interact with one another

Fibers Illustrate Importance of Geometric Shape Most materials have tensile strengths several times greater as fibers than in bulk By imbedding the fibers in a polymer matrix, a composite material is obtained that avoids the problems of fibers but utilizes their strengths The matrix provides the bulk shape to protect the fiber surfaces and resist buckling When a load is applied, the low‑strength matrix deforms and distributes the stress to the high‑strength fibers

Other Composite Structures Laminar composite structure – conventional Sandwich structure Honeycomb sandwich structure

Other Laminar Composite Structures Automotive tires - consists of multiple layers bonded together FRPs - multi‑layered fiber‑reinforced plastic panels for aircraft, automobile body panels, boat hulls Printed circuit boards - layers of reinforced plastic and copper for electrical conductivity and insulation Snow skis - composite structures consisting of layers of metals, particle board, and phenolic plastic Windshield glass - two layers of glass on either side of a sheet of tough plastic

Composites can be classified by their matrix material which include: -Metal matrix composites (MMC’s) -Ceramic matrix composites (CMC’s) -Polymer matrix composites (PMC’s)

Metal Matrix Composites (MMC’s) A metal matrix reinforced by a second phase The matrix is relatively soft and flexible. The reinforcement must have high strength and stiffness Since the load must be transferred from the matrix to the reinforcement, the reinforcement-matrix bond must be strong. Reinforcing phases: Particles of ceramic (these MMCs are commonly called cermets) Fibers of various materials: other metals, ceramics, carbon, and boron

Common Metal Matrices: Metal Matrix Material Common Metal Matrices: -Metal martices include aluminum, magnesium, copper, nickel, and intermetallic compound alloys -MMCs are better at higher temperatures than PMCs although production is much more difficult and expensive -MMCs can have applications such as fan blades in engines, clutch and brake linings, engine cylinder liners, etc.

Dispersion Strengthened MMC’s -Dispersion strengthened alloys can be considered as composites because there is little or no interaction between the two components and the reinforcement is not soluble in the metal matrix. -The dispersoids are usually 10-250 nm diameter oxide particles and are introduced by physical means rather than chemical precipitation. -They are located within the grains and at grain boundaries but are not coherent with the matrix as in precipitation hardening -The dispersed particles are sufficiently small in size to impede dislocation movement and thus improve yield strength as well as stiffness. -Dispersion strengthened alloys are somewhat weaker than precipitation hardened alloys at room temperature but since overaging, tempering, grain growth or particle coarsening do not occur on heating, they are stronger and more creep resistant at high temperatures.

Sintered Aluminum Powder (SAP) Composites -SAPs have an aluminum matrix with aluminum oxide (Al2O3) particulate -The matrix can be strengthened by 14% SAPs are produced using different methods, two examples are as follows: -Al and Al2O3 powders are blended then compacted at high pressure then sintered like a ceramic. -Al powder is heated in air to form a thick film of Al2O3 on each particle, when the powder is compacted the Al2O3 film fractures into tiny particles and becomes surrounded by the Al during sintering.

Cemented Carbides (CERMETS) -Cemented carbides are an example of regular particulate MMC’s (as opposed to dispersion strengthened MMC’s) -Carbides such as WC (tungsten-carbide) are used for cutting tool inserts but this hard ceramic is very brittle so it cracks or chips under impact loads, to remedy this cobalt is used as a matrix -Co-WC (cobalt tungsten-carbide) cermets are produced by pressing Co and W powders into compacts, which are heated above the melting point of Co -On cooling the carbide particles become embedded in the solidified Co, which act as a tough matrix for the WC particles -In addition to its strength and toughness, Co is also selected because it wets the carbide particles to give a strong bond

Cemented Carbides (CERMETS) -Cemented carbides are commonly used as inserts for cutting tools -I’m sure you’ve seen these in the machine shop Figure (from left to right): Cutting tool inserts, a milling tool and a lathe tool

Cemented Carbide Photomicrograph (about 1500X) of cemented carbide with 85% WC and 15% Co

Applications of Cemented Carbides Tungsten carbide cermets (Co binder) - cutting tools are most common; other: wire drawing dies, rock drilling bits and other mining tools, dies for powder metallurgy, indenters for hardness testers Titanium carbide cermets (Ni binder) - high temperature applications such as gas‑turbine nozzle vanes, valve seats, thermocouple protection tubes, torch tips, cutting tools for steels Chromium carbides cermets (Ni binder) - gage blocks, valve liners, spray nozzles, bearing seal rings

Ceramic Matrix Composites (CMC) A ceramic primary phase imbedded with a secondary phase, which usually consists of fibers Attractive properties of ceramics: high stiffness, hardness, hot hardness, and compressive strength; and relatively low density Weaknesses of ceramics: low toughness and bulk tensile strength, susceptibility to thermal cracking CMCs represent an attempt to retain the desirable properties of ceramics while compensating for their weaknesses

Polymer Matrix Composites (PMC) A polymer primary phase in which a secondary phase is imbedded as fibers, particles, or flakes Examples: most plastic molding compounds, rubber reinforced with carbon black, and fiber‑reinforced polymers (FRPs) FRPs are most closely identified with the term composite

Polymer Matrix Material -There are two basic categories of polymer matrices: -Thermoplastics -Thermoset plastics -Roughly 95% of the composite market uses thermosetting plastics -Thermoseting plastics are polymerized in two ways: -By adding a catalyst to the resin causing the resin to ‘cure’, basically one must measure and mix two parts of the resin and apply it before the resin cures -By heating the resin to it’s cure temperature

Polymer Matrix Material Common thermosetting plastics: -Phenolics: good electrical properties, often used in circuit board applications -Epoxies: low solvent emission (fumes) upon curing, low shrink rate upon polymerization which produces a relatively residual stress-free bond with the reinforcement, it is the matrix material that produces the highest strength and stiffness, often used in aerospace applications -Polyester: most commonly used resin, slightly weaker than epoxy but about half the price, produces emission when curing.

Fiber‑Reinforced Polymers -Fiber reinforced composites provide improved strength, fatigue resistance, Young’s modulus and strength to weight ratio over the constituent materials. -This is achieved by incorporating strong, stiff, yet brittle fibers into a more ductile matrix. -Generally speaking the fiber supplies the strength and stiffness while the matrix binds the fibers together and provides a means of transferring the load between fibers -The matrix also provides protection for the fibers

Common Fiber Reinforced Polymer Structure Most widely used form of FRP is a laminar structure, made by stacking and bonding thin layers of fiber and polymer until desired thickness is obtained By varying fiber orientation among layers, a specified level of anisotropy (direction dependence of the physical properties) in properties can be achieved in the laminate Applications: parts of thin cross‑section, such as aircraft wing and fuselage sections, automobile and truck body panels, and boat hulls

Characteristics of Fiber Reinforced Composites -Many factors must be considered when designing a fiber- reinforced composite including the length, diameter, orientation, amount and properties of the constituents, and the bonding between them. -The method used to produce the final product is also very important as it dictates the type of properties just mentioned as well as the quality of the product.

Characteristics of Fiber Reinforced Composites Fiber length and diameter: Fiber dimensions are characterized by their aspect ratio l/d where l is the fiber length and d is the diameter. The strength improves when the aspect ratio is large. Typical fiber diameters are from 10 mm to 150 mm. Fibers often fracture because of surface imperfections. Making the diameter small reduces its surface area, which has fewer flaws. Long fibers are preferred because the ends of the fiber carry less of the load. Thus the longer the fiber, the fewer the ends and the higher the load carrying capacity of the fibers.

Characteristics of Fiber Reinforced Composites -As can be seen from this plot, the strength of the composite increases as the fiber length increases (this is a chopped E-glass- epoxy composite)

-The effect of fiber orientation and strength can be seen in the plot -Maximum strength is obtained when long fibers are oriented parallel to the applied load -The effect of fiber orientation and strength can be seen in the plot

Fiber Orientation -The properties of fiber composites can be tailored to meet different loading requirements -By using combinations of different fiber orientation quasi-isotropic materials may be produced Figure (a) shows a unidirectional arrangement Figure (b) shows a quasi-isotropic arrangement

Types of Fibers Some commonly used fibers for polymer matrix composites: -Glass fibers -Carbon fibers -Aramid fibers Some commonly used fibers for metal matrix composites: -Boron fibers -Oxide ceramic and non-oxide ceramic fibers

Glass Fibers -Due to the relatively inexpensive cost glass fibers are the most commonly used reinforcement -There are a variety of types of glass, they are all compounds of silica with a variety of metallic oxides -The most commonly used glass is E-glass, this is the most popular because of it’s cost Designation: Property or Characteristic: E, electrical low electrical conductivity S, strength high strength C, chemical high chemical durability M, modulus high stiffness A, alkali high alkali or soda lime glass D, dielectric low dielectric constant

Carbon Fibers -Carbon fibers have gained a lot of popularity in the last two decades due to the price reduction “Carbon fiber composites are five times stronger than 1020 steel yet five times lighter. In comparison to 6061 aluminum, carbon fiber composites are seven times stronger and two times stiffer yet still 1.5 times lighter” -Initially used exclusively by the aerospace industry they are becoming more and more common in fields such as automotive, civil infrastructure, and paper production

Aramid Fibers -Aramid fibers are also becoming more and more common -They have the highest level of specific strength of all the common fibers -They are commonly used when a degree of impact resistance is required such as in ballistic armour -The most common type of aramid is Kevlar

Comparative Cost of Fiber Reinforcement

Commercially Available Forms of Reinforcement Filament: a single thread like fiber -Roving: a bundle of filaments wound to form a large strand -Chopped strand mat: assembled from chopped filaments bound with a binder -Continuous filament random mat: assembled from continuous filaments bound with a binder -Many varieties of woven fabrics: woven from rovings

Commercially Available Forms of Reinforcement Above Left: Roving Above Right: Filaments Right: Close up of a roving

Commercially Available Forms of Reinforcement Random mat and woven fabric (glass fibers)

Commercially Available Forms of Reinforcement Carbon fiber woven fabric

Fiber Reinforced Polymer Properties High strength‑to‑weight and modulus‑to‑weight ratios Low specific gravity - a typical FRP weighs only about 1/5 as much as steel; yet, strength and modulus are comparable in fiber direction Good fatigue strength Good corrosion resistance, although polymers are soluble in various chemicals Low thermal expansion - for many FRPs, leading to good dimensional stability Significant anisotropy in properties

Fiber Reinforced Polymer Applications Aerospace – much of the structural weight of todays airplanes and helicopters consist of advanced FRPs Automotive – somebody panels for cars and truck cabs Continued use of low-carbon sheet steel in cars is evidence of its low cost and ease of processing Sports and recreation Fiberglass reinforced plastic has been used for boat hulls since the 1940s Fishing rods, tennis rackets, golf club shafts, helmets, skis, bows and arrows

Other Polymer Matrix Composites In addition to FRPs, other PMCs contain particles, flakes, and short fibers as the secondary phase Called fillers when used in molding compounds Two categories: Reinforcing fillers – used to strengthen or otherwise improve mechanical properties Examples: wood flour in phenolic and amino resins; and carbon black in rubber Extenders – used to increase bulk and reduce cost per unit weight, but little or no effect on mechanical properties

Guide to Processing Composite Materials The two phases are typically produced separately before being combined into the composite part Processing techniques to fabricate MMC and CMC components are similar to those used for powdered metals and ceramics Molding processes are commonly used for PMCs with particles and chopped fibers Specialized processes have been developed for FRPs

Advantages of Composite Materials Composites can be very strong and stiff, yet very light in weight, so ratios of strength‑to‑weight and stiffness‑to‑weight are several times greater than steel or aluminum Fatigue properties are generally better than for common engineering metals Toughness is often greater too Composites can be designed that do not corrode like steel Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone

Disadvantages and Limitations of Composite Materials Properties of many important composites are anisotropic ‑ the properties differ depending on the direction in which they are measured – this may be an advantage or a disadvantage Many of the polymer‑based composites are subject to attack by chemicals or solvents, just as the polymers themselves are susceptible to attack Composite materials are generally expensive Manufacturing methods for shaping composite materials are often slow and costly