If you have a cracked screen Dell will most likely say that is accidental damage, which is not covered by your standard 3 year warranty on the monitor.   Customer damage like cracked screens just isn't covered on the monitor's warranty.   There's stuff like accidental damage insurance you can probably buy for that separately from third parties like Geek Squad protection for example for Best Buy purchases, not sure if they have monitor plans. I suspect you'd probably have a hard time if you told Dell it was a defect that caused the crack.  
If my AW2518H LCD is broken and I have a 3 year advanced exchange warranty, I think with the basic warranty would they fix it?
AW2518H, LCD broken, warranty