LECTURE 15: COMPOSITE MATERIALS By: Casey Keulen

1. MECH 473 ~ LECTURE 15: COMPOSITES LECTURE 15:COMPOSITE MATERIALSBy: Casey Keulen

2. MECH 473 ~ LECTURE 15: 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 COMPOSITES

3. MECH 473 ~ LECTURE 15: COMPOSITES Can you think of any examples of where composites are used? COMPOSITES

4. MECH 473 ~ LECTURE 15: 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 COMPOSITES

5. MECH 473 ~ LECTURE 15: 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) COMPOSITES

6. MECH 473 ~ LECTURE 15: 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 COMPOSITES

7. MECH 473 ~ LECTURE 15: 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 COMPOSITES

8. MECH 473 ~ LECTURE 15: COMPOSITES -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. DISPERSION STRENGTHENED MMC’S

9. MECH 473 ~ LECTURE 15: 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 SINTERED ALUMINUM POWDER (SAP) COMPOSITES

10. MECH 473 ~ LECTURE 15: COMPOSITES Properties of SAP compared to 2024-T8, 7075-T6 and a boron fiber strengthened 1100 alloy. SINTERED ALUMINUM POWDER (SAP) COMPOSITES

11. MECH 473 ~ LECTURE 15: COMPOSITES -An important group of dispersion-strengthened composites is thoria-dispersed (TD) metals -Thorium is an element on the periodic table (atomic number 90) THORIA-DISPERSED COMPOSTIES -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)

12. MECH 473 ~ LECTURE 15: COMPOSITES -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)

13. MECH 473 ~ LECTURE 15: COMPOSITES -Cemented carbides are commonly used as inserts for cutting tools -I’m sure you’ve seen these in the machine shop CEMENTED CARBIDES(CERMETS) Figure (from left to right): Cutting tool inserts, a milling tool and a lathe tool

14. MECH 473 ~ LECTURE 15: COMPOSITES -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 PARTICULATE MMC’S FOR ELECTRICAL CONTACTS

15. MECH 473 ~ LECTURE 15: COMPOSITES -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) PARTICULATE MMC’S FOR ELECTRICAL CONTACTS -The final product has a continuous Ag and W structure which provides good electrical conductivity and wear resistance b c

16. MECH 473 ~ LECTURE 15: COMPOSITES -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”. CAST METALPARTICULATE MMC’S

17. MECH 473 ~ LECTURE 15: COMPOSITES 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) CAST METALPARTICULATE MMC’S

18. MECH 473 ~ LECTURE 15: COMPOSITES Microstructure of cast Al Alloy reinforced with particles of SiC magnified X125 CAST METALPARTICULATE MMC’S

19. MECH 473 ~ LECTURE 15: COMPOSITES 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, r, is given as a fraction, f, as: RULE OF MIXTURES Where the subscripts m and f refer to the matrix and fiber.

20. MECH 473 ~ LECTURE 15: COMPOSITES 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, r, is given as a fraction, f, as: RULE OF MIXTURES For thermal conductivity: For electrical conductivity: 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

21. MECH 473 ~ LECTURE 15: COMPOSITES 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. RULE OF MIXTURES

22. MECH 473 ~ LECTURE 15: COMPOSITES 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: RULE OF MIXTURES 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:

23. MECH 473 ~ LECTURE 15: COMPOSITES Perpendicular to the fibers, the modulus of elasticity may be as high as: RULE OF MIXTURES 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.

24. MECH 473 ~ LECTURE 15: 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 FIBER REINFORCED COMPOSITES

25. MECH 473 ~ LECTURE 15: 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

26. MECH 473 ~ LECTURE 15: 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

27. MECH 473 ~ LECTURE 15: 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) CHARACTERISTICS OF FIBER REINFORCED COMPOSITES

28. MECH 473 ~ LECTURE 15: COMPOSITES -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

29. MECH 473 ~ LECTURE 15: COMPOSITES -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 FIBER ORIENTATION Figure (a) shows a unidirectional arrangement Figure (b) shows a quasi-isotropic arrangement

30. MECH 473 ~ LECTURE 15: COMPOSITES -A three dimensional weave is also possible -This could be found when fabrics are knitted or weaved together FIBER ORIENTATION

31. MECH 473 ~ LECTURE 15: COMPOSITES 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: FIBER PROPERTIES Where TS is the tensile strength, E is the elastic modulus and r is the density.

32. MECH 473 ~ LECTURE 15: COMPOSITES -On the left is a graph showing specific strength vs. specific modulus for different types of fibers FIBER PROPERTIES

33. MECH 473 ~ LECTURE 15: COMPOSITES 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 TYPES OF FIBERS

34. MECH 473 ~ LECTURE 15: COMPOSITES -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 GLASS FIBERS -The most commonly used glass is E-glass, this is the most popular because of it’s cost

35. MECH 473 ~ LECTURE 15: COMPOSITES -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 CARBON FIBERS

36. MECH 473 ~ LECTURE 15: COMPOSITES -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 ARAMID FIBERS

37. MECH 473 ~ LECTURE 15: COMPOSITES COMPARATIVE COST OF FIBER REINFORCEMENT

38. MECH 473 ~ LECTURE 15: COMPOSITES -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

39. MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Above Left: Roving Above Right: Filaments Right: Close up of a roving

40. MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Random mat and woven fabric (glass fibers)

41. MECH 473 ~ LECTURE 15: COMPOSITES COMMERCIALLY AVAILABLE FORMS OF REINFORCEMENT Carbon fiber woven fabric

42. MECH 473 ~ LECTURE 15: COMPOSITES MATRIX MATERIALS

43. MECH 473 ~ LECTURE 15: COMPOSITES -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

44. MECH 473 ~ LECTURE 15: COMPOSITES 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 POLYMER MATRIX MATERIAL

45. MECH 473 ~ LECTURE 15: COMPOSITES 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. METAL MATRIX MATERIAL

46. MECH 473 ~ LECTURE 15: COMPOSITES MANUFACTURING WITH POLYMER MATRIX MATERIALS

47. MECH 473 ~ LECTURE 15: 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 MANUFACTURING OF POLYMER-MATRIX COMPOSITES

48. MECH 473 ~ LECTURE 15: 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 MANUFACTURING OF POLYMER-MATRIX COMPOSITES -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

49. MECH 473 ~ LECTURE 15: 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 MANUFACTURING OF POLYMER-MATRIX COMPOSITES

50. MECH 473 ~ LECTURE 15: 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 MANUFACTURING OF POLYMER-MATRIX COMPOSITES