Design Realization lecture 10

1. Design Realization lecture 10 John Canny 9/25/03

2. 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!

3. 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

4. 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

5. 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 Newton’s 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

6. Density of common materials • Mass/volume

7. 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

8. L P L 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.

9. Stiffness • Material stiffness is stress/strain and it is in units of pressure. • aka Young’s modulus E = / • Defined for stretching a cylindrical rod, it must always be > 0.

10. Stiffness and Compressibility • When the rod stretches, its area normally decreases (to minimize volume change). • Poisson’s 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

11. Shear modulus • G is the ratio of shear strain to shear stress: • G is always positive and satisfies:

12. Strength and Stiffness • Strength is the stress at which the material fails:

13. Stiffness of Common Materials

14. Strength of Common Materials • Yield to plastic region & final breaking strength.

15. 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 Boltzmann’s constant k = 1.38 x 10-23 J/

16. 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

17. Thermal conduction • Thermal conductivity = heat flow/temp. gradient

18. Electrical conduction • Resistivity, Electric field/(current per unit area)

19. 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).

20. Ferro-Metal Chemistry • Metal properties can be enhanced by mixing in other materials. • Steel is an alloy of iron and carbon (< 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 metal’s surface, protecting it from corrosion.

21. 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.

22. 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.

23. 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.

24. Shape-Memory Alloy • Two main metal phases are shown below:

25. 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).

26. 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.

27. Shape-Memory Alloy • Nitinol has the following attributes:

28. 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.

29. 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.

30. 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.

31. 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

32. 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!).