Meaning: Have a nervous breakdown
Example: He CRACKED UP after his son died and had to take a couple of months off work.
Meaning: Have bad reception on a mobile phone
Example: You'll have to talk louder- you're CRACKING UP.
Meaning: Burst out laughing
Example: Everybody CRACKED UP when he told the joke.
Meaning: Damage a car badly
Example: He CRACKED his car UP last night when he came off the road.