Design Realization lecture 13

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Design Realization lecture 13

• John Canny/Dan Reznik
• 10/7/03

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Last Time

• Fantastic plastics!

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This time

• S-t-r-e-t-c-h-i-n-g material properties
  composites and cellular materials
• Chemistry takes us pretty far. But we can also
  customize material properties with geometry
• Composites distinct materials tightly bound
  together.
• Cellular materials customized fine structure for
  desired stiffness/strength.

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Composites Fiber-based

• Fiberglass is the classic composite
• Glass fibers (often woven)
• Two-part polyester or epoxy resin
• Epoxy strength 60MPa
• Glass fiber tensilestrength 500 MPa
• The composite can achieve a significant
  percentage of the fiber strength (300MPa
  typical), along the fiber direction.

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Composites Fiber-based

• Laminates to get strength in several directions,
  the fibers are either
• Laminated in sheets in different directions, or
• Made from a woven fabric with threads in several
  directions.
• Glasses are chosen for different attributes
• Tensile strength
• Stiffness
• Electrical insulation
• Glass and polymer do not react, but the polymer
  must adhere very well to the fiber for strength.

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Composites Carbon Kevlar

• Recall (lecture 10) that carbon fiber and kevlar
  fibers both have diamond-like tensile strength (
  4 GPa), or about 70x epoxy.
• Modulus also increasesby about 50x.
• Surprisingly, carbon fiberhas the same
  structureas (soft) graphiteBut these sheets
  are long and thin in CF, whereas they are flat
  (and slippery) in graphite.

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Workability

• Glass, carbon, kevlar sheets and two-part resins
  are easy to work with, and used for
• Boat making and repair.
• Custom surfboards, snowboards
• Motorcyle and auto racing.
• Furniture (e.g. chairs)
• Construction by mold-making,fiber laying, resin
  application.
• See http//www.fibreglast.com/

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Natural fiber composites

• Wood is a natural composite of cellulose fiber
  and a polymer called lignin.
• Bone is a hierarchical fiber composite
• Bone
• Osteons
• Lamella
• Collagen fibers
• Collagen fibrils

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Particle composites

• Fiber composites are ideal for improving tensile
  strength. Particle composites can
• Improve compressive stiffness.
• Decrease weight without sacrificing strength
  (hollow glass sphere polymer composite).
• Make the material magnetic (refrigerator
  magnets).
• Improve electrical or thermal characteristics
  (polymer metal composites).
• Traditional fiber and particle composites have
  fibers/particles of around micron size.

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Nano-particle composites

• Exciting area, has seen dramatic results lately.
• Much less exotic than it sounds.
• Many nano-particulate materials are commercially
  available at moderate cost.
• Advantages of nano-particles
• Allows small features (lt 1 micron) of composite,
  important for electronics or complex machines.
• Composite is more homogeneous, consistent
  physical behavior.
• Some material properties depend on dimension, and
  are tunable by particle size.

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Nano-particle Solar Cells

• Developed by Paul Alivisatos at Berkeley.
• Nanometer (7x60) sizedinorganic rods are
  orientedvertically and held in apolymer matrix.
  
• Very simple (room temperature) process.
• Potential for very low-cost, large area solar
  cells. 2 local companies work on this.

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Hierarchical materials

• Often we want large volume materials with low
  density e.g. for ships, packing and aircraft.
• How do you maximize strength?
• The classical triangular truss is a good design.
• Really 1-dimensional, so verylow density.
• But its not the best possible

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Hierarchical materials

• Long, straight members will buckle under high
  load.
• Strength can be increased using hierarchical
  structure(trusses made from trusses)
• The Eiffel tower used this structure (because of
  limitedbeam length!), and was by farthe
  strongest structure for weight at the time.

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Hierarchical material fabrication

• Its impossible to build small hierarchical
  trusses by conventional methods.
• But 3D printers are limited neither by complexity
  or by geometry (the many cavities which cant be
  created by casting or milling).
• Hierarchical structures are the natural way to
  build low-density, high-strength volumes with 3D
  printing.

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Cellular materials

• Honeycomb two flat sheets sandwiching a layer of
  honeycomb.
• Very strong resistanceto bending.
• Used for aircraft floors.
• Good vibration resistance.
• Soft honeycombs used for shock absorption.
  Sometimes visible in athletic shoes.

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Honeycomb strength

• Honeycomb is a very efficient structure for
  bending stiffness.
• In a normal Beam, the bending stiffness is EI,
  where E is Youngs modulus, I is the moment of
  inertia of the beam cross-section.
• I b a3 /12, (b is depth into the page).

a
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Honeycomb strength

• In a honeycomb structure, the mass is
  concentrated in the top and bottom sheet.
• The moment of inertia is
• I b a h2 / 4 (b is depth)
• Much higher bending stiffness for a given weight
  (h gtgt a)

a/2
h
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Cellular hierarchies

• Honeycomb has someweakness. The cell facescan
  collapse under pressure.
• By adding small cells to reinforce the large
  ones, we eliminate the weakness.
• This structure is used in animal bone, and a
  numberof plant materials.

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Plastic foams

• Plastic foams are usually thermoplastics.
• Traditional methods use volatile hydrocarbons
  mixed with the polymer.
• On heating, they create bubbles in the polymer.
• The voids are rather irregular, and the foamhas
  lower strength thantheoretically possible.

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Plastic foams

• Lately microcell foams have been developed.
• The foams use a gas (CO2 or Nitrogen) dissolved
  under pressure to create voids.
• Under sudden change in pressure/temperature,
  small voids form, and do not have time to join
  into larger voids.
• Result is more uniformcells and better strength.

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Plastic foams

• But the uniform cell foams are like single-scale
  trusses, and susceptible to failure across large
  faces. Greater strength would result from
  multi-scale cells.
• Still an open problem how to do this