1.
If someone cracks up , they are under such a lot of emotional strain that they become mentally ill. ( INFORMAL )
She would have cracked up if she hadn’t allowed herself some fun.
PHRASAL VERB : V P
2.
If you crack up or if someone or something cracks you up , you laugh a lot. ( INFORMAL )
She told stories that cracked me up and I swore to write them down so you could enjoy them too...
We all just cracked up laughing.
PHRASAL VERB : V n P , V P