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

Last Time  Fantastic plastics!

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.

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

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.

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 increases by about 50x.  Surprisingly, carbon fiber has the same structure as (soft) graphite: But these sheets are long and thin in CF, whereas they are flat (and slippery) in graphite.

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

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

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.

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.

Nano-particle Solar Cells  Developed by Paul Alivisatos at Berkeley.  Nanometer (7x60) sized inorganic rods are oriented vertically and held in a polymer matrix.  Very simple (room temperature) process.  Potential for very low-cost, large area solar cells. 2 local companies work on this.

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 very low density.  But its not the best possible…

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 limited beam length!), and was by far the strongest structure for weight at the time.

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.

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

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 a 3 /12, (b is depth into the page). a

Honeycomb strength  In a honeycomb structure, the mass is concentrated in the top and bottom sheet.  The moment of inertia is I = b a h 2 / 4 (b is depth)  Much higher bending stiffness for a given weight (h >> a) h a/2

Cellular hierarchies  Honeycomb has some weakness. The cell faces can 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 number of plant materials.

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 foam has lower strength than theoretically possible.

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 uniform cells and better strength.

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…