Unformatted text preview: Metals Moderately high stiffness (E), Moderately high strength (σys), Can be enhanced by alloying Can be enhanced by processing Mechanical work, Heat treatment, High ductility - easy to deform, High toughness (KIC ) - resistant to fracture , Low electrical resistivity , High thermal conductivity (κ), High optical reflectivity – shiny surface, High reactivity – poor corrosion resistance Ceramics (Crystalline and Amorphous) Non-metallic, inorganic solids, High stiffness , High strength, High hardness, High wear/abrasion resistance, High temperature stability (crystalline)– Low reactivity – resistant to corrosion/oxidation– Resistant to creep/stress relaxation, High electrical resistivity, Moderate to high thermal conductivity, Low ductility – difficult to form, Low fracture toughness - Low tolerance for stress concentrations/defects/flaws -Low tolerance for high contact stresses Polymers and Elastomers Organic solids based on long chain molecules of C or Si Low density, Low stiffness (E), especially for elastomers Moderate strength (high strength/weight ratio), Moderate fracture toughness , Poor high temperature stability,- Few have useful strength above 150C, High electrical resistivity, Low thermal conductivity (κ), High ductility,– Easiest to form, especially to mold into complex shapes, Low cost (raw material and manufacturing Deep, steep, narrow wells correspond to: Higher bond energy, Higher melting and vaporization point, Higher elastic (Young’s) modulus, Lower coefficient of thermal expansion Van der waals This bonding type is common in polymers and gases Tin Pest – Transformation at low temperatures from one crystalline form to a less dense crystalline form Polymer Crystallinity • Polymers can range from fully amorphous (0% crystalline) to up to 95% crystalline, Crystallites are 1-10 um across and scatter light making semi-crystalline polymers translucent to opaque. Polymers are never 100% crystalline like, like metals or ceramics. There is always some amorphous connecting material. The concept of crystallinity is not valid for crosslinked or networked polymers which form a 3-D amorphous structure. In linear or branched polymers, crystallinity increases with less chain branching and smaller branches (HDPE v. LDPE), branches interfere with close packing, Smaller side groups (Polyethylene v. Polystyrene) Bulky side groups interfere with close packing. More polar side groups (Polyvinyl Chloride vs. Polyethylene) Polar side groups strengthen hydrogen and vanderWaals bonding promoting packing. Regular tacticity, Isotacticity, and to a lesser extent syndiotacticity, promote close packing– Less complex repeat units Longer and more complex repeat units interfere with close packing Thermoplastics Polymers that soften when heated and harden when cooled These polymers have a fixed melting point. These polymers are linear or branched polymers . Intra-chain bonding is covalent....
View Full Document
- Spring '11
- Materials Science, Strength of materials