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 http://www.fibreglast.com/
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…