Strong enough for what?
Most screen protectors protect the screen from scratches and rub marks. None of them will protect the screen if you drop the phone face down onto a hard pebble - whether the screen protector alone breaks or the protector and the screen break depends on how scratch-proof the screen is. The more scratch-proof something is, the less break-proof it is.
But is the screen protector strong enough to keep the screen from breaking if you keep dropping the phone screen-side down? No. Not until they invent transparent steel. (And that would scratch like crazy.)