Can impact windows break?
Technically, yes virtually all glass is breakable at certain impact points. Impact windows are created with impact resistant (not impact proof) glass.
Impact windows are created by sandwiching layers of polyvinyl butyral (PVB) in between two sheets of glass. After heat treating, the result is a window that can withstand continuous wind speeds up to 200 mph and direct strikes by 2x4s and other heavy debris.
Perhaps more important than its ability to withstand breakage, is what happens when an impact resistant window actually does break. While the glass may shatter, the pane remains in place instead of falling from the window frame. This means your window still provides a barrier from the outside world until you can get it repaired.