Ductile:
* Examples are silly putty, lead solder, etc.
* When placed under tension, the material lengthens.
* At the center point of the opposing tensions, the cross-sectional area is reduced.
* A totally ductile solid will fracture at the center with a cross-sectional area of zero at the breakpoint.
Brittle:
* Examples are glasses, some steels.
* When placed under tension, the material does not laterally expand
* There is no real reduction of area at the center of the opposing stresses
* The smoother the surface of the fracture face, the more brittle the solid.
In addition to this, all solids eventually become brittle at some lowered temperature. We had this problem during World War II, when we would send our Norfolk and Long Beach made ships out in the North Atlantic. Our choice of steel was plenty ductile in the warm waters of the Gulf Stream and southern California, but when in the Northern Atlantic, suddenly they became brittle. Each ship split in half across its girth. This was because we had reached the critical temperature of the steel, causing it to become brittle, and the slightest little thing could then split the ship in half. Remember in Terminator 2 how Arnold froze the T-1000 in liquid nitrogen? That’s an extreme case, proving this property.
An Engineer in the World 