Design Realization lecture 13

1. Design Realization lecture 13 John Canny/Dan Reznik 10/7/03

2. Last Time • Fantastic plastics!

3. 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.

4. 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.

5. 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.

6. 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) graphite:But these sheets are long and thin in CF, whereas they are flat (and slippery) in graphite.

7. 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/

8. 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

9. 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.

10. 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 (< 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.

11. 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.

12. 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…

13. 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.

14. 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.

15. 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.

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

17. 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 >> a) a/2 h

18. 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.

19. 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.

20. 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.

21. 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…