Plausibility Check: Ceramic Armour

But if it fractures does that mean its useless against next shot?

No.

It is undoubtedly weakened, but not useless. The ceramic armour is encased in other armour, so it's still there held in place. Slightly weaker, but certainly not useless. It's rare (impossible?) to have two impacts at exactly the same point, anyway.
 
Top