I'm feeling a bit confused about something.
As I understand it, the total kinetic energy absorbed by a thickness of material stressed to its breaking point is proportional to the area under the stress/strain graph describing the energy transfer. This area is roughly proportional to the product of the material's ultimate tensile strength and elongation to break.
Various references (matweb, materials engineering textbooks, etc) on polyethylene terephthalate give it an UTS of around 7000-8500 psi, and an ETB of around 110%-250%, depending on the type and source of PET, and on its method of manufacture, with 140% being most common. These same references give polycarbonate an UTS of around 9500-10500 psi, and an ETB of around 50%-120%, with 100% being most common. Just comparing UTS * ETB suggests PET should absorb as much or more kinetic energy per unit area than PC, but sources commonly give PC's Izod Impact rating
5x-6x as high as PET's.What am I missing here? Do the stress/strain graphs for these materials' Izod tests just look very, very strange? (Linear for PC, concave for PET, maybe?) Does Izod test something other than what I think it does? I can't find stress/strain graphs for PET or PC for the Izod or Charpy tests.
-- TTK