Composite Materials

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  Composite Materials
  

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  Introduction
  A Composite material is a material system composed of two or more macro constituents that differ in shape and chemical composition and which are insoluble in each other. The history of composite materials dates back to early 20th century. In 1940, fiber glass was first used to reinforce epoxy.
  Applications:
  Aerospace industry
  Sporting Goods Industry
  Automotive Industry
  Home Appliance Industry
  
  

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  Advanced Aerospace Application:
  Lear Fan 2100 “all-composite” aircraft
  

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  Advanced Aerospace Application:
  Boeing 767 ,777, 787 airplanes w/ the latest, full wing box is composite):
  

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  Sporting Goods
  

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  Automotive
  

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  Various applications
  

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  • Composites:
  -- Multiphase material w/significant
  proportions of each phase.
  • Dispersed phase:
  -- Purpose: enhance matrix properties.
  MMC: increase sy, TS, creep resist.
  CMC: increase Kc
  PMC: increase E, sy, TS, creep resist.
  -- Classification: Particle, fiber, structural
  • Matrix:
  -- The continuous phase
  -- Purpose is to:
  - transfer stress to other phases
  - protect phases from environment
  -- Classification: MMC, CMC, PMC
  metal
  ceramic
  polymer
  Elyaf dokuma
  Terminology/Classification
  woven
  fibers
  cross
  section
  view
  0.5
  mm
  0.5
  mm
  

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  Composite Structural Organization: the design variations
  

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  Fig. 2 (a) Schematic diagram of an individual layer of honeycomb-like carbon called graphene and how this could be rolled in order to form a carbon nanotube; (b)–(d) HR-TEM images of single, double- and multi-walled carbon nanotubes (insets are their corresponding images).
  Fig. 1 SEM image of the smallest working gear (carbon nanotube/nylon composite); inset exhibits the fractured surface.
  

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  Composite Survey
  

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  • CMCs: Increased toughness
  Composite Benefits
  fiber-reinf
  un-reinf
  particle-reinf
  Force
  Bend displacement
  • PMCs: Increased E/r
  E(GPa)
  G=3E/8
  K=E
  Density, r [mg/m3]
  .1
  .3
  1
  3
  10
  30
  .01
  .1
  1
  10
  10
  2
  10
  3
  metal/
  metal alloys
  polymers
  PMCs
  ceramics
  Adapted from T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp. 139-146, 1984. Used with permission.
  • MMCs:
  Increased
  creep
  resistance
  20
  30
  50
  100
  200
  10
  -10
  10
  -8
  10
  -6
  10
  -4
  6061 Al
  6061 Al
  w/SiC
  whiskers
  s
  (MPa)
  e
  ss
  (s-1)
  

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  Composite Survey: Particle-I
  • Examples:
  - Spheroidite
  steel
  matrix:
  ferrite (a)
  (ductile)
  particles:
  cementite
  
  (
  Fe
  3
  C
  )
  (brittle)
  60 mm
  - WC/Co
  cemented
  carbide
  matrix:
  cobalt
  (ductile)
  particles:
  WC
  (brittle,
  hard)
  V
  m
  :
  
  5-12 vol%!
  600 mm
  - Automobile
  tires
  matrix:
  rubber
  (soft, ductile)
  particles:
  C
  (stiffer)
  0.75 mm
  Particle-reinforced
  Fiber-reinforced
  Structural
  

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  Composite Survey: Particle-II
  Concrete – gravel + sand + cement
  - Why sand and gravel? Sand packs into gravel voids
  Reinforced concrete - Reinforce with steel rebar or remesh
  - increases strength - even if cement matrix is cracked
  Prestressed concrete - remesh under tension during setting of concrete. Tension release puts concrete under compressive force
  - Concrete much stronger under compression.
  - Applied tension must exceed compressive force
  Particle-reinforced
  Fiber-reinforced
  Structural
  threaded
  rod
  nut
  Post tensioning – tighten nuts to put under rod under tension
  but concrete under compression
  

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  • Elastic modulus, Ec, of composites:
  -- two approaches.
  • Application to other properties:
  -- Electrical conductivity, se: Replace E in the above equations
  with se.
  -- Thermal conductivity, k: Replace E in above equations with k.
  Composite Survey: Particle-III
  lower limit:
  1
  E
  c
  =
  V
  m
  E
  m
  +
  V
  p
  E
  p
  c
  m
  m
  upper
  limit:
  E
  =
  V
  E
  +
  V
  p
  E
  p
  “rule of mixtures”
  Particle-reinforced
  Fiber-reinforced
  Structural
  Data:
  Cu matrix
  w/tungsten
  particles
  0
  20
  4
  0
  6
  0
  8
  0
  10
  0
  150
  20
  0
  250
  30
  0
  350
  vol% tungsten
  E(GPa)
  (Cu)
  (
  W)
  

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  Composite Survey: Fiber
  Fibers themselves are very strong
  Provide significant strength improvement to material
  Ex: fiber-glass
  Continuous glass filaments in a polymer matrix
  Strength due to fibers
  Polymer simply holds them in place and environmentally protects them
  Particle-reinforced
  Fiber-reinforced
  Structural
  

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  Fiber Loading Effect under Stress:
  

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  • Critical fiber length (lC) for effective stiffening & strengthening:
  • Ex: For fiberglass, a fiber length > 15 mm is needed since this length
  provides a “Continuous fiber” based on usual glass fiber properties
  Composite Survey: Fiber
  Particle-reinforced
  Fiber-reinforced
  Structural
  fiber diameter
  shear strength of
  fiber-matrix interface
  fiber strength in tension
  • Why? Longer fibers carry stress more efficiently!
  Shorter, thicker fiber:
  Longer, thinner fiber:
  Poorer fiber efficiency
  Adapted from Fig. 16.7, Callister 7e.
  Better fiber efficiency
  s
  (x)
  s
  (x)
  

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  Fiber Load Behavior under Stress:
  

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  Composite Survey: Fiber
  Fiber Materials
  Whiskers - Thin single crystals - large length to diameter ratio
  graphite, SiN, SiC
  high crystal perfection – extremely strong, strongest known
  very expensive
  
  Particle-reinforced
  Fiber-reinforced
  Structural
  Fibers
  polycrystalline or amorphous
  generally polymers or ceramics
  Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE
  Wires
  Metal – steel, Mo, W
  

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  Fiber Alignment
  aligned
  continuous
  aligned random
  discontinuous
  Adapted from Fig. 16.8, Callister 7e.
  

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  Behavior under load for Fibers & Matrix
  

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  Composite Strength: Longitudinal Loading
  Continuous fibers - Estimate fiber-reinforced composite strength for long continuous fibers in a matrix
  
  Longitudinal deformation
  c = mVm + fVf but c = m = f
  volume fraction isostrain
  Ece = Em Vm + EfVf longitudinal (extensional)
  modulus
  f = fiber
  m = matrix
  Remembering: E = / and note, this model corresponds to the “upper bound” for particulate composites
  

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  Composite Strength: Transverse Loading
  In transverse loading the fibers carry less of the load and are in a state of ‘isostress’
  
  c = m = f =  c= mVm + fVf
  
  
  transverse modulus
  
  Remembering: E = /
  and note, this model corresponds to the “lower bound” for particulate composites
  

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  An Example:
  Note: (for ease of conversion)
  6870 N/m2 per psi!
  UTS, SIModulus, SI
  57.9 MPa3.8 GPa
  2.4 GPa399.9 GPa
  (241.5 GPa)
  (9.34 GPa)
  

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  • Estimate of Ec and TS for discontinuous fibers:
  
  -- valid when
  
  -- Elastic modulus in fiber direction:
  
  
  
  
  
  
  
  -- TS in fiber direction:
  efficiency factor:
  -- aligned 1D: K = 1 (aligned )
  -- aligned 1D: K = 0 (aligned )
  -- random 2D: K = 3/8 (2D isotropy)
  -- random 3D: K = 1/5 (3D isotropy)
  (aligned 1D)
  Values from Table 16.3, Callister 7e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.)
  Composite Strength
  Particle-reinforced
  Fiber-reinforced
  Structural
  (TS)c = (TS)mVm + (TS)fVf
  Ec = EmVm + KEfVf
  

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  • 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.
  Composite Survey: Fiber
  Particle-reinforced
  Fiber-reinforced
  Structural
  matrix:
  a
  (Mo) (ductile)
  fibers:
  g
  ’ (Ni3Al) (brittle)
  2 mm
  -- Ceramic: Glass w/SiC fibers
  formed by glass slurry
  Eglass = 76 GPa; ESiC = 400 GPa.
  (a)
  (b)
  fracture
  surface
  From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig. 11.20, p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Used with permission of CRC
  Press, Boca Raton, FL.
  

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  • Discontinuous, random 2D fibers
  • Example: Carbon-Carbon
  -- process: fiber/pitch, then
  burn out at up to 2500ºC.
  -- uses: disk brakes, gas
  turbine exhaust flaps, nose
  cones.
  • Other variations:
  -- Discontinuous, random 3D
  -- Discontinuous, 1D
  Composite Survey: Fiber
  Particle-reinforced
  Fiber-reinforced
  Structural
  (b)
  fibers lie
  in plane
  view onto plane
  C fibers:
  very stiff
  very
  strong
  C matrix:
  less stiff
  less strong
  (a)
  efficiency factor:
  -- random 2D: K = 3/8 (2D isotropy)
  -- random 3D: K = 1/5 (3D isotropy)
  Ec = EmVm + KEfVf
  

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  Looking at strength:
  

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  • Stacked and bonded fiber-reinforced sheets
  -- stacking sequence: e.g., 0º/90º or 0/45/90º
  -- benefit: balanced, in-plane stiffness
  Adapted from Fig. 16.16, Callister 7e.
  Composite Survey: Structural
  Particle-reinforced
  Fiber-reinforced
  Structural
  • Sandwich panels
  -- low density, honeycomb core
  -- benefit: light weight, large bending stiffness
  honeycomb
  adhesive layer
  face sheet
  Adapted from Fig. 16.18,
  Callister 7e. (Fig. 16.18 is
  from Engineered Materials
  Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.)
  

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  Composite Manufacturing Processes
  Particulate Methods: Sintering
  Fiber reinforced: Several
  Structural: Usually Hand lay-up and atmospheric curing or vacuum curing
  

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  Open Mold Processes
  Only one mold (male or female) is needed and may be made of any material such as wood, reinforced plastic or , for longer runs, sheet metal or electroformed nickel. The final part is usually very smooth.
  
  Shaping. Steps that may be taken for high quality
  1. Mold release agent (silicone, polyvinyl alcohol, fluorocarbon, or sometimes, plastic film) is first applied.
  2. Unreinforced surface layer (gel coat) may be deposited for best surface quality.
  

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  Hand Lay-Up: The resin and fiber (or pieces cut from prepreg) are placed manually, air is expelled with squeegees and if necessary, multiple layers are built up.
  Hardening is at room temperature but may be improved by heating.
  Void volume is typically 1%.
  Foam cores may be incorporated (and left in the part) for greater shape complexity. Thus essentially all shapes can be produced.
  Process is slow (deposition rate around 1 kg/h) and labor-intensive
  Quality is highly dependent on operator skill.
  Extensively used for products such as airframe components, boats, truck bodies, tanks, swimming pools, and ducts.
  

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  A spray gun supplying resin in two converging streams into which roving is chopped
  Automation with robots results in highly reproducible production
  Labor costs are lower
  SPRAY-UP MOLDING
  

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  Cut and lay the ply or prepreg under computer control and without tension; may allow reentrant shapes to be made.
  Cost is about half of hand lay-up
  Extensively used for products such as airframe components, boats, truck bodies, tanks, swimming pools, and ducts.
  Tape-Laying Machines
  (Automated Lay-Up)
  

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  Filament Winding
  Ex: pressure tanks
  Continuous filaments wound onto mandrel
  Adapted from Fig. 16.15, Callister 7e. [Fig. 16.15 is from N. L. Hancox, (Editor), Fibre Composite Hybrid Materials, The Macmillan Company, New York, 1981.]
  
  

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  Filament Winding Characteristics
  Because of the tension, reentrant shapes cannot be produced.
  CNC winding machines with several degrees of freedom (sometimes 7) are frequently employed.
  The filament (or tape, tow, or band) is either precoated with the polymer or is drawn through a polymer bath so that it picks up polymer on its way to the winder.
  Void volume can be higher (3%)
  The cost is about half that of tape laying
  Productivity is high (50 kg/h).
  Applications include: fabrication of composite pipes, tanks, and pressure vessels. Carbon fiber reinforced rocket motor cases used for Space Shuttle and other rockets are made this way.
  

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  Pultrusion
  Fibers are impregnate with a prepolymer, exactly positioned with guides, preheated, and pulled through a heated, tapering die where curing takes place.
  Emerging product is cooled and pulled by oscillating clamps
  Small diameter products are wound up
  Two dimensional shapes including solid rods, profiles, or hollow tubes, similar to those produced by extrusion, are made, hence its name ‘pultrusion’
  

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  Composite Production Methods
  Pultrusion
  Continuous fibers pulled through resin tank, then preforming die & oven to cure
  Adapted from Fig. 16.13, Callister 7e.
  Production rates around 1 m/min.
  Applications are to sporting goods (golf club shafts), vehicle drive shafts (because of the high damping capacity), nonconductive ladder rails for electrical service, and structural members for vehicle and aerospace applications.
  

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  PREPREG PRODUCTION PROCESSES
  Prepreg is the composite industry’s term for continuous fiber reinforcement pre-impregnated with a polymer resin that is only partially cured.
  Prepreg is delivered in tape form to the manufacturer who then molds and fully cures the product without having to add any resin.
  This is the composite form most widely used for structural applications
  

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  Manufacturing begins by collimating a series of spool-wound continuous fiber tows.
  Tows are then sandwiched and pressed between sheets of release and carrier paper using heated rollers (calendering).
  The release paper sheet has been coated with a thin film of heated resin solution to provide for its thorough impregnation of the fibers.
  PrePreg Process
  

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  The final prepreg product is a thin tape consisting of continuous and aligned fibers embedded in a partially cured resin
  Prepared for packaging by winding onto a cardboard core.
  Typical tape thicknesses range between 0.08 and 0.25 mm
  Tape widths range between 25 and 1525 mm.
  Resin content lies between about 35 and 45 vol%
  PrePreg Process
  

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  The prepreg is stored at 0C (32 F) or lower because thermoset matrix undergoes curing reactions at room temperature. Also the time in use at room temperature must be minimized. Life time is about 6 months if properly handled.
  Both thermoplastic and thermosetting resins are utilized: carbon, glass, and aramid fibers are the common reinforcements.
  Actual fabrication begins with the lay-up. Normally a number of plies are laid up to provide the desired thickness.
  The lay-up can be by hand or automated.
  PrePreg Process
  

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  • Composites are classified according to:
  -- the matrix material (CMC, MMC, PMC)
  -- the reinforcement geometry (particles, fibers, layers).
  • Composites enhance matrix properties:
  -- MMC: enhance sy, TS, creep performance
  -- CMC: enhance Kc
  -- PMC: enhance E, sy, TS, creep performance
  • Particulate-reinforced:
  -- Elastic modulus can be estimated.
  -- Properties are isotropic.
  • Fiber-reinforced:
  -- Elastic modulus and TS can be estimated along fiber dir.
  -- Properties can be isotropic or anisotropic.
  • Structural:
  -- Based on build-up of sandwiches in layered form.
  Summary