completely
finally
The stresses of her job became too great and she finally cracked.

up
He thought he'd never get through the ordeal without cracking up.

crack under the pressure, crack under the strain, show signs of cracking
He is under a lot of pressure but is showing no signs of cracking.

hard

on
to crack down hard on crime

Crack is used with these nouns as the subject:
egg, glass, ice, leather, mud, thunder, veneer, voice, whip, windscreen

Crack is used with these nouns as the object:
bone, case, code, crime, egg, gag, glass, grin, joke, knuckle, mirror, mystery, nut, password, puzzle, rib, safe, scheme, shot, skull, smile, whip, windscreen

Crack is used with these nouns:
cocaine, force, shot, team, troops