Chapter 16: Composites

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*ISSUES TO ADDRESS...• What are the classes and types of composites?• What are the advantages of using composite materials?• How do we predict the stiffness and strength of the various types of composites?Chapter 16: Composites


*CompositeCombination of two or more individual materials Design goal: obtain a more desirable combination of properties (principle of combined action) e.g., low density and high strength


*• Composite: -- Multiphase material that is artificially made.• Phase types: -- Matrix - is continuous -- Dispersed - is discontinuous and surrounded by matrix Terminology/ClassificationAdapted from Fig. 16.1(a), Callister & Rethwisch 8e.


*• Dispersed phase: -- Purpose: MMC: increase sy, TS, creep resist. CMC: increase KIc PMC: increase E, sy, TS, creep resist. -- Types: particle, fiber, structuralTerminology/ClassificationReprinted with permission from D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47.woven fiberscross section view


*Classification of CompositesAdapted from Fig. 16.2, Callister & Rethwisch 8e.


*Classification: Particle-Reinforced (i)


*Classification: Particle-Reinforced (ii)Concrete – gravel + sand + cement + water - Why sand and gravel? Sand fills voids between gravel particlesReinforced concrete – Reinforce with steel rebar or remesh - increases strength - even if cement matrix is crackedPrestressed concrete - Rebar/remesh placed under tension during setting of concrete - Release of tension after setting places concrete in a state of compression - To fracture concrete, applied tensile stress must exceed this compressive stress


*• Elastic modulus, Ec, of composites: -- two “rule of mixture” extremes:• Application to other properties: -- Electrical conductivity, se: Replace E’s in equations with se’s. -- Thermal conductivity, k: Replace E’s in equations with k’s.Adapted from Fig. 16.3, Callister & Rethwisch 8e. (Fig. 16.3 is from R.H. Krock, ASTM Proc, Vol. 63, 1963.)Classification: Particle-Reinforced (iii)


*Classification: Fiber-Reinforced (i)Fibers very strong in tension Provide significant strength improvement to the composite Ex: fiber-glass - continuous glass filaments in a polymer matrix Glass fibers strength and stiffness Polymer matrix holds fibers in place protects fiber surfaces transfers load to fibers


*Classification: Fiber-Reinforced (ii)Fiber Types Whiskers - thin single crystals - large length to diameter ratios graphite, silicon nitride, silicon carbide high crystal perfection – extremely strong, strongest known very expensive and difficult to disperse Fibers polycrystalline or amorphous generally polymers or ceramics Ex: alumina, aramid, E-glass, boron, UHMWPE Wires metals – steel, molybdenum, tungsten


*Fiber Alignmentaligned continuousaligned random discontinuousAdapted from Fig. 16.8, Callister & Rethwisch 8e. Transverse directionLongitudinal direction


*• Aligned Continuous fibers• Examples:From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp. 987-998, 1988. Used with permission.-- Metal: g'(Ni3Al)-a(Mo) by eutectic solidification.Classification: Fiber-Reinforced (iii)matrix: a (Mo) (ductile) fibers:g’ (Ni3Al) (brittle)2 mm


*• Discontinuous fibers, random in 2 dimensions• Example: Carbon-Carbon -- fabrication process: - carbon fibers embedded in polymer resin matrix, - polymer resin pyrolyzed at up to 2500ºC. -- uses: disk brakes, gas turbine exhaust flaps, missile nose cones.• Other possibilities: -- Discontinuous, random 3D -- Discontinuous, alignedAdapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p. 151. (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL.Classification: Fiber-Reinforced (iv)500 m


*• Critical fiber length for effective stiffening & strengthening:• Ex: For fiberglass, common fiber length > 15 mm neededClassification: Fiber-Reinforced (v)fiber diametershear strength of fiber-matrix interfacefiber ultimate tensile strength• For longer fibers, stress transference from matrix is more efficient


*Composite Stiffness: Longitudinal LoadingContinuous fibers - Estimate fiber-reinforced composite modulus of elasticity for continuous fibers Longitudinal deformation c = mVm + fVf and c = m = f volume fraction isostrain Ecl = EmVm + Ef Vf Ecl = longitudinal modulus c = composite f = fiber m = matrix


*Composite Stiffness: Transverse LoadingIn transverse loading the fibers carry less of the load c= mVm + fVf and c = m = f =  Ect = transverse modulusc = composite f = fiber m = matrixisostress


*• Estimate of Ecd for discontinuous fibers: -- valid when fiber length < -- Elastic modulus in fiber direction:efficiency factor: -- aligned: K = 1 (aligned parallel) -- aligned: K = 0 (aligned perpendicular) -- random 2D: K = 3/8 (2D isotropy) -- random 3D: K = 1/5 (3D isotropy)Values from Table 16.3, Callister & Rethwisch 8e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.)Composite Stiffness


*Composite Strength• Estimate of for discontinuous fibers:


Composite Production Methods (i)Pultrusion Continuous fibers pulled through resin tank to impregnate fibers with thermosetting resin Impregnated fibers pass through steel die that preforms to the desired shape Preformed stock passes through a curing die that is precision machined to impart final shape heated to initiate curing of the resin matrix Fig. 16.13, Callister & Rethwisch 8e.


Composite Production Methods (ii)Filament Winding Continuous reinforcing fibers are accurately positioned in a predetermined pattern to form a hollow (usually cylindrical) shape Fibers are fed through a resin bath to impregnate with thermosetting resin Impregnated fibers are continuously wound (typically automatically) onto a mandrel After appropriate number of layers added, curing is carried out either in an oven or at room temperature The mandrel is removed to give the final productAdapted from Fig. 16.15, Callister & Rethwisch 8e. [Fig. 16.15 is from N. L. Hancox, (Editor), Fibre Composite Hybrid Materials, The Macmillan Company, New York, 1981.]


*• Laminates - -- stacked and bonded fiber-reinforced sheets - stacking sequence: e.g., 0º/90º - benefit: balanced in-plane stiffnessAdapted from Fig. 16.16, Callister & Rethwisch 8e. Classification: Structural


*Composite Benefits


*• Composites types are designated by: -- the matrix material (CMC, MMC, PMC) -- the reinforcement (particles, fibers, structural) • Composite property benefits: -- MMC: enhanced E, s, creep performance -- CMC: enhanced KIc -- PMC: enhanced E/, sy, TS/ • Particulate-reinforced: -- Types: large-particle and dispersion-strengthened -- Properties are isotropic • Fiber-reinforced: -- Types: continuous (aligned) discontinuous (aligned or random) -- Properties can be isotropic or anisotropic • Structural: -- Laminates and sandwich panelsSummary


*Core Problems:Self-help Problems:ANNOUNCEMENTSReading:

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Last Updated: 8th March 2018

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