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Originally Posted by Celestialsailor  I am surprised someone would say that Acrylic is the same strength as PolyC. I use to make bullet resistant windshield for the police. The other claim I found odd was that the "scratches in Lexan(polyC) can precipitate failure". |
Polycarbonate gets what many confuse as being "strength" from its elasticity and flexibility, not from ultimate tensile strength. It is resistant to shock loads, almost to the extent of being unbreakable, due to those characteristics giving it resistance to high mechanical energies - but that not due to its tensile strength in which it does not have any unique advantage over, for example, acrylic.
In fact it suffers the disadvantage in many applications that by the time one reaches its ultimate tensile strength it will have elongated (ie permanently deformed) somewhere between 70-100%. So it behaves like strong gum. It is the characteristics of flexibility and elongation under stress that are reasons why it is not a good material for hatches and windows (nor especially for companionway washboards). It is an excellent material for many other things, especially where resistance to shock is required.
Another poster refers to aircraft transparencies and mentions that those are acrylic, and that is typically so, wheras one would assume that polycarbonate would be the better choice if it really were better for windows. On large aircraft the cockpit transparencies may be special glasses often laminated with a plastic such as PCB but as far as I am aware acrylic is the material used for smaller aircraft. It may be that polycarbonate is used in some smaller transparencies, but if so it would be in limited manner where its disadvantages are offset by its advantages and be used in conjunction with another material such as acrylic (passenger cabin transparencies are normally acrylic but it may be in some instances that polycarbonate is used as a secondary material, I don't know - as you've probably noticed these transparencies are made up of multiple panes).
Scratches on polycarbonate, as on most (all?) other materials create stress raisers which can precipitate failure. Given its flexibility and elasticity I suspect, but have not investigated, that its mode of failure would be different to that of a more brittle material such as acrylic, high carbon steel, etc but have a mode of failure it surely will - probably exaggerated elongation at the point of the scratch.
EDIT: Meant to mention that as far as I am aware, if polycarbonate is used in bullet resistant windows in vehicles it is used in a laminate with glass. The glass ofsetting the structural deficiencies of the polycarbonate and the polycarbonate giving the resistance to high mechanical energies.