MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES -Composite materials are engineered materials made from two or more constituent materials that remain separate and distinct while forming a single component -Generally, one material forms a continuous matrix while the other provides the reinforcement - The two materials must be chemically inert with respect to each other so no interaction occurs upon heating until one of the components melts, an exception to this condition is a small degree of interdiffusion at the reinforcement-matrix interface to increase bonding
MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES Can you think of any examples of where composites are used?
MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES Composites can be found in: -Boat hulls -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
MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES 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) or sometimes referred to as organic matrix composites (OMC’s)
MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES MMC - Metal Matrix Composites -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. MMC use: -Two types of particulates ( dispersion strengthened alloys and regular particulate composites) -Or long fiber reinforcements
MECH 473 ~ LECTURE 15: COMPOSITES COMPOSITES PMC - Polymer Matrix Composites -The matrix is relatively soft and flexible -The reinforcement must have high strength and stiffness -Since the load must be transferred from matrix to reinforcement, the reinforcement-matrix bond must be strong CMC – Ceramic Matrix Composites -The matrix is relatively hard and brittle -The reinforcement must have high tensile strength to arrest crack growth -The reinforcement must be free to pull out as a crack extends, so the reinforcement-matrix bond must be relatively weak
MECH 473 ~ LECTURE 15: COMPOSITES 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.
MECH 473 ~ LECTURE 15: COMPOSITES SINTERED ALUMINUM POWDER (SAP) COMPOSITES -SAPs have an aluminum matrix with aluminum oxide (Al 2 O 3 ) particulate -The matrix can be strengthened by %14 SAPs are produced using different methods, two examples are as follows: -Al and Al 2 O 3 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 Al 2 O 3 on each particle, when the powder is compacted the Al 2 O 3 film fractures into tiny particles and becomes surrounded by the Al during sintering
MECH 473 ~ LECTURE 15: COMPOSITES SINTERED ALUMINUM POWDER (SAP) COMPOSITES Properties of SAP compared to 2024-T8, 7075-T6 and a boron fiber strengthened 1100 alloy.
MECH 473 ~ LECTURE 15: COMPOSITES THORIA-DISPERSED COMPOSTIES -An important group of dispersion-strengthened composites is thoria-dispersed (TD) metals -Thorium is an element on the periodic table (atomic number 90) -A common example is TD-nickel TD-nickel composites roduced by: -Powders of metallic Th and Ni are ball milled, compacted at high pressure and then sintered. -The compact is then heated in air and oxygen diffuses in to react with Th metal to form a fine dispersion of ThO2. -This method, internal oxidation is also used for fabricating the W-ThO2 composites. Electron micrograph of TD-Ni with 300 nm diameter ThO2 particles (X2000)
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES PARTICULATE MMC’S FOR ELECTRICAL CONTACTS -Electrical contacts used in switches, relays and motors must be quite wear resistant to stand up in service -Highly conductive metals such as Cu and Ag are relatively soft and thus show excessive wear when used as contacts resulting in arcing and poor electrical conduction. -The goal is to produce a contact that is both a good conductor and has excellent wear properties -This is done by using silver reinforced with tungsten particles, the Ag is a terrific conductor while the W provides good wear properties
MECH 473 ~ LECTURE 15: COMPOSITES PARTICULATE MMC’S FOR ELECTRICAL CONTACTS -The composite is made in two stages: -The composite is made in two stages: -First a low density compact with interconnected pores is produced by pressing and firing tungsten powders (figure a and b) -Liquid silver is then infiltrated into the connected voids under vacuum (figure c) bc -The final product has a continuous Ag and W structure which provides good electrical conductivity and wear resistance
MECH 473 ~ LECTURE 15: COMPOSITES CAST METAL PARTICULATE MMC’S -Al alloys for automotive connecting rods and pistons can be strengthened and hardened by the addition of SiC (silicon carbide) particles. -Al alloys for automotive connecting rods and pistons can be strengthened and hardened by the addition of SiC (silicon carbide) particles. -The SiC particles are introduced at a temperature at which the alloy is in the solid plus liquid state, ie., by “compocasting”.
MECH 473 ~ LECTURE 15: COMPOSITES CAST METAL PARTICULATE MMC’S Compocasting of Al-SiC: -Partially solidified alloy is stirred to break up dendrites (fig. a) -Particles of SiC are added at this temperature (fig. b) -In a pressure die casting machine, the solid mixture becomes thixotropic to form a high density casting (fig. c)
MECH 473 ~ LECTURE 15: COMPOSITES CAST METAL PARTICULATE MMC’S Microstructure of cast Al Alloy reinforced with particles of SiC magnified X125
MECH 473 ~ LECTURE 15: COMPOSITES RULE OF MIXTURES For particulate composites, the rule of mixtures predicts the density of the composite as well as other properties (although other properties may vary depending on how the dispersed phase is arranged) Density, , is given as a fraction, f, as: Where the subscripts m and f refer to the matrix and fiber.
MECH 473 ~ LECTURE 15: COMPOSITES RULE OF MIXTURES For fiber reinforced composites, the rule of mixtures predicts the density of the composite as well as electrical and thermal conductivity along the direction of the fibers if they are continuous and unidirectional. Density, , is given as a fraction, f, as: Thermal and electrical energy can be transferred through the composite at a rate that is proportional to the volume fraction, f of the conductive material For thermal conductivity: For electrical conductivity:
MECH 473 ~ LECTURE 15: COMPOSITES RULE OF MIXTURES In a composite material with a metal matrix and ceramic fibers, the bulk of the energy would be transferred through the matrix. In a composite consisting of a polymer matrix containing metallic fibers, the energy would be transferred through the fibers. When the fibers are not continuous or unidirectional, the simple rule of mixtures may not apply. For example, in a metal fiber-polymer matrix composite, electrical conductivity would be low and would depend on the length of the fibers, the volume fraction of fibers and how often the fibers touch one another.
MECH 473 ~ LECTURE 15: COMPOSITES RULE OF MIXTURES The rule of mixtures can also be used to predict the modulus of elasticity when the fibers are continuous and unidirectional. Parallel to the fibers, the modulus of elasticity may be as high as: However, when the applied load is very large, the matrix begins to deform and the stress-strain curve is no longer linear. Since the matrix now contributes little to the stiffness, the modulus is approximated by:
MECH 473 ~ LECTURE 15: COMPOSITES RULE OF MIXTURES Perpendicular to the fibers, the modulus of elasticity may be as high as: There are many good examples provided in your text by Askland and Phule in the chapter, “Composites: Teamwork and Synergy in Materials”. Review this chapter and the examples provided.
MECH 473 ~ LECTURE 15: COMPOSITES FIBER REINFORCED COMPOSITES -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
MECH 473 ~ LECTURE 15: COMPOSITES 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.
MECH 473 ~ LECTURE 15: COMPOSITES 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 m to 150 m. 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.
MECH 473 ~ LECTURE 15: COMPOSITES 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)
MECH 473 ~ LECTURE 15: COMPOSITES FIBER ORIENTATION -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
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES FIBER ORIENTATION -A three dimensional weave is also possible -This could be found when fabrics are knitted or weaved together
MECH 473 ~ LECTURE 15: COMPOSITES FIBER PROPERTIES In most fiber-reinforced composites, the fibers are strong, stiff and lightweight. If the composite is to used at elevated temperatures, the fiber should also have a high melting temperature. The specific strength and specific modulus of fibers are important characteristics given by: Where TS is the tensile strength, E is the elastic modulus and is the density.
MECH 473 ~ LECTURE 15: COMPOSITES FIBER PROPERTIES -On the left is a graph showing specific strength vs. specific modulus for different types of fibers
MECH 473 ~ LECTURE 15: COMPOSITES 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 -Carbon fibers -Oxide ceramic and non-oxide ceramic fibers
MECH 473 ~ LECTURE 15: COMPOSITES 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 Designation:Property or Characteristic: E, electricallow electrical conductivity S, strengthhigh strength C, chemicalhigh chemical durability M, modulushigh stiffness A, alkalihigh alkali or soda lime glass D, dielectriclow dielectric constant -The most commonly used glass is E-glass, this is the most popular because of it’s cost
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Above Left: Roving Above Right: Filaments Right: Close up of a roving
MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Random mat and woven fabric (glass fibers)
MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Carbon fiber woven fabric
MECH 473 ~ LECTURE 15: COMPOSITES 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
MECH 473 ~ LECTURE 15: COMPOSITES 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, used in everything from boats to RVs to piping to Corvette bodies
MECH 473 ~ LECTURE 15: COMPOSITES 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.
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES -The method of manufacturing composites is very important to the design and outcome of the product -With traditional materials one starts out with a blank piece of material ie: rod, ingot, sheet, etc and works it to produce the desired part. -However, this is not the case with polymer-matrix composites. -With these composites the material and the component are being produced at the same time, therefore we aim for the product to be a net or near net shape with little to no post processing
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Hand Lay-Up/Spray-Up -Oldest and most commonly used manufacturing method -Usually used to produce polyester or epoxy resin parts such as boat hulls, tanks and vessels, pick-up truck canopies -The method is quite simple, the resin and reinforcement is placed against the surface of an open (one sided) mold and allowed to cure or in the case of spray-up the resin/reinforcement is sprayed onto the mold with a spray gun -Often a gel coat is applied to the mold prior to produce a better surface quality and protect the composite from the elements -A gel coat is a resin usually 0.4 to 0.7 mm thick, commonly seen on the outer surface of smaller boats
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Hand Lay-Up/Spray-Up -The pros of this process include: low initial start up cost, easy to change mold/design, on-site production possible (ie portable process) -The cons include: labor intensive, the quality of parts depends on operator’s skill and therefore inconsistent, only one good side to the part
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Prepreg -A pregreg (short for preimpregnated) is a composite that comes with the resin already added to the reinforcement -This means that the only concern when working with prepreg is shaping the part -Since the resin is already mixed (resin and catalyst) there is a limited shelf life -For the same reason prepreg must be cured in an oven or autoclave
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Prepreg -Prepreg can be used in a few different ways -It can be placed against a mold similar to the hand lay-up method -Once placed in the mold the material must be compressed and cured according to a specific pressure/temperature cycle -This is often done by means of a vacuum bag where a thin plastic cover is secured overtop of the composite and the air is vacuumed out -This process can reduce manufacturing time and produce a stronger part (if a knitted preform is used) (if a knitted preform is used) -Another process used is ‘automated tape lay-up’ -This process uses a large automated roller similar to a packing tape roller -The roller applies the tape with pressure which eliminates the need for a vacuum bag - Automated tape lay-up is used to produce large parts, generally in aerospace applications and is also capable of 3-d parts
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Pultrusion -Similar to extrusion of metal parts -Pultrusion involves pulling resin-impregnated glass strands through a die -Standard extruded shapes can easily be produced such as pipes, channels, I-beams, etc.
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Filament Winding -A continuous reinforcement, either previously impregnated or impregnated during winding is wound around a rotating mandrel to form a composite part -Pros: fast lay-up speed, very accurate and repeatable product, possibility to use continuous fiber -Cons: expensive equipment, high cost for mandrel, poor surface finish, some shapes not possible -Examples: oxygen bottles for firemen, rocket motors, tennis rackets, shafts
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Resin Transfer Molding -Resin transfer molding is a manufacturing method that is quite similar to injection molding where plastic is injected into a closed mold -In the RTM process the preform (precut piece(s) of reinforcement) is placed in the mold, the mold is closed and the thermoset plastic matrix is injected into the mold, once the matrix is cured the part is ejected
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Resin Transfer Molding
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Resin Transfer Molding Pros: -Complex components can be produced -Components have two good surfaces -Component can be created within a fairly tight tolerance -High level of repeatability -Process can be automated and repeated -Process can be numerically modeled and analyzed Cons: -Molds often need to be designed using trial and error methods -‘race tracking’ may occur -‘wash out’ may occur -Air voids are easily formed with poor process parameters
MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING OF POLYMER- MATRIX COMPOSITES Resin Transfer Molding Researcher from Aerospace Manufacturing Technology Center in Montreal molding members for a helicopter
MECH 473 ~ LECTURE 15: COMPOSITES THE END ANY QUESTIONS?