Design Realization lecture 10

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Design Realization lecture 10

• John Canny
• 9/25/03

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Last Time

• Introduction to prototyping processes
• CNC machining
• PC board manufacture
• Laser cutters, plasma, water cutters
• 3D printing SLA, SLS, LOM, FDM
• Modular 3D printing
• Design review next Tuesday bring your prototypes!

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Materials Physical constants Length

• 1 m (meter) 39.37 inches
• 1 dm (decimeter) 0.1 m
• 1 cm (centimeter) 0.01m
• 1 mm 10-3 m 0.03937 inches
• 1 mil 10-3 inches 0.0254 mm
• Surface finish tolerances of this order
• Human hair diameter 1 to 4 mils
• 1 liter 1 cubic decimeter 0.001 cubic m

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Physical constants Length

• 1 ? (micron) 10-6 m 0.0394 mils
• Dust particles, smoke, yeast cell
• Particles 1 ? float in air, adhere to surfaces
• Infra-red light wavelength
• 1 nm (nano-meter) 10-9 m
• Visible light 400-700 nm
• Nano-particles (1-100s of nm)
• Large molecules
• 1 Å (Angstrom unit) 10-10 m 0.1 nm
• Most atom diameters are a few Å

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Mass, Force

• 1 kg (kilogram) mass of 1 liter of water (about
  2.2 lbs)
• 1 N (Newton) force required to accelerate 1 kg
  mass to 1 m s-2
• From Newtons law F ma
• Gravitational force on 1 kg 9.81 N
• Objects in free fall accelerate at 9.81 m s-2
• 1 amu (atomic mass unit) 1.66 x 10-27 kg
• Average mass of 1 neutron/proton
• Approximate mass of hydrogen atom

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Density of common materials

• Mass/volume

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Pressure

• Pressure force per unit area
• 1 Pa (Pascal) 1N per sq meter
• 1 psi (pound per sq. inch) 6,895 Pa
• 1 atmosphere 101,300 Pascals 14.7 psi
• Blood pressure is about 300 kPa
• Hydraulic pressure 10 1000 MPa

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Strength and Stiffness

• When pressure is applied to a material, it
  deforms in the direction of the pressure
• The pressure is called stress ?.
• The displacement ?L/L is strain ?. It is
  dimensionless.

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Stiffness

• Material stiffness is stress/strain and it is in
  units of pressure.
• aka Youngs modulus E ?/?
• Defined for stretching a cylindrical rod, it must
  always be gt 0.

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Stiffness and Compressibility

• When the rod stretches, its area normally
  decreases (to minimize volume change).
• Poissons ratio ? - axial strain/ linear strain
• It must lie between -1 and 0.5
• An incompressible material has ? 0.5.
• Most materials have ? between 0 and 0.5

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Shear modulus

• G is the ratio of shear strain to shear stress
• G is always positive and satisfies

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Strength and Stiffness

• Strength is the stress at which the material
  fails

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Stiffness of Common Materials

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Strength of Common Materials

• Yield to plastic region final breaking
  strength.

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Temperature

• Heat is kinetic (motion) energy of atoms.
• Temperature measures the energy per molecule in a
  gas, or energy per degree of freedom in a solid.
• E per molecule 3/2 kT, per dof ½ kT
• T is absolute temperature (?C 273) andk is
  Boltzmanns constant k 1.38 x 10-23 J/?

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Brownian motion

• At normal temperature (300? K), each particle has
  average energy 3/2 kT 6.3 x 10-21 J
• Particle energy is given by ½ mv2
• 0.1 mm particle, mass 10-9 kg, v is 3 x 10-7 m/s
• 10 micron particle, mass 10-12 kg, v is 1 x 10-5
  m/s
• 1 micron particle, mass 10-15 kg, v is 3 x 10-4
  m/s
• Molecule of atomic wt 100, v is 250 m/s

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Thermal conduction

• Thermal conductivity heat flow/temp. gradient

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Electrical conduction

• Resistivity, Electric field/(current per unit
  area)

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Metals

• Metals strong atomic bonds (high strength and
  melting point), but also high thermal and
  electrical conduction.
• Structure can be characterized as positive ions
  in a sea of electrons.
• Conductivity also implies strong reflection of
  light (shininess).

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Ferro-Metal Chemistry

• Metal properties can be enhanced by mixing in
  other materials.
• Steel is an alloy of iron and carbon (lt 2).
  First producing in China around 300 BC.
• High-carbon steels are stiffer, stronger, more
  brittle.
• Stainless steel adds chromium, which forms a
  tightly packed oxide layer on the metals
  surface, protecting it from corrosion.

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Ferro-magnetism

• Iron is an important material for its magnetic
  properties, which depend on crystal structure
• Ferritic and Martensitic steels are magnetic
• Austenitic steels are not
• The boundaries are not clear non-magnetic
  (including most common stainless) steels can be
  worked into a magnetic state.

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Flavors of Magnets

• The current killer magnet material is NIB
  (Neodymium-Iron-Boron), which is about 4x
  stronger than the strongest ferrite.
• Actually NIB is Nd2Fe14B, so its mostly iron
• Very stiff and brittle (safety glasses!),
  flammable!
• Refrigerator magnets use ferrite particles (e.g.
  Strontium Ferrite SrFe12O19) in an elastomer
  (flexible plastic).
• The magnetic field is actually periodic.

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Liquid Magnets

• There are magnetic liquids ferro-fluids, which
  contain simple ferrite (Fe3O4) with fatty acid
  molecules attached to them.
• The fatty acid chains are attracted to an oil
  medium and help the magnetic particles
  dissolve in the oil.
• A magnet will also holdthe liquid in an
  invertedcontainer.

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Shape-Memory Alloy

• Two main metal phases are shown below

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Shape-Memory Alloy

• In steel, the martensite/austenite transition is
  influenced by alloying, cold-working etc.
• In shape memory allow, the transition is caused
  by a small change in temperature.
• The best-known shape memory allow is Nitinol NiTi
  (Nickel Titanium).

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Shape-Memory Alloy

• The austenite is stiffer and has lower volume.
• Heating SMA wire causes it to contract with some
  force. Strains of 3-5 are typical.

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Shape-Memory Alloy

• Nitinol has the following attributes

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Aluminum and Alloys

• Aluminum is a versatile metal that is light, has
  very good thermal and electrical conduction.
• Easy to machine (mill or drill).
• Tricky to weld (need to remove oxygen).
• Strength is not high, but can be improved by
  alloying with many other metals.
• Titanium-aluminum alloys offer excellent
  strength/weight, and dominate the aircraft
  industry.

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Brass

• Brass is an alloy of Copper and Zinc.
• It has good corrosion resistance, electrical
  conduction, and is easy to machine.
• A close relative is bronze, which includes some
  other metal like tin or phosphor.
• It offers a range of attractive shades and is
  polishes well.

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Surface treatments

• Plain metals are often susceptible to corrosion
  in water or air. Treatments include
• Galvanizing coating ferrous metal with zinc, or
  zinc-based paint.
• Electroplating deposit a variety of metals on
  another metal surface.
• Anodizing for Aluminum, creates a thicker oxide
  layer on the surface,possibly with other
  metals.

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Metals limitations

• Material properties are not programmable.
• Very high melting point
• Structure-dependent properties
• Complex manufacturing processes
• Small vocabulary of basic materials (periodic
  table!), and compatible combinations

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Metals summary

• Metals are essential for strength, cost and
  electrical, magnetic and thermal properties.
• Aluminum is a very easy material to work with,
  and has good finishing properties.
• Customization cost is moderate, e.g. custom
  extrusions.
• Steel workhorse for maximum strength.
• Needs heavier tooling (or outsource your CAD
  model!).