If it's a crack with no "spider web" pattern and no apparent impact point on the glass or case, then you stand a good chance of getting a replacement under warranty.
Otherwise, Apple's only remedy for any cracked screen is to pay their complete out of warranty replacement cost. Apple doesn't do repairs for customer devices on any iPhone. All out of warranty replacements cost either battery replacement fee (only if everything is working) or the standard replacement fee. They send exchanged iPads to a place where they may be recycled or refurbished.