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Today’s expression “crack up” is a separable phrasal verb which is used to talk about something or someone which makes a person laugh. For example:

Peter told such a funny joke this morning. It really cracked me up.

Nancy is one of the funniest people I know. She can always crack me up.

We can also use “crack up” as an intransitive phrasal verb. For example:

I cracked up during my class because my friend was making funny faces at me.

My friend often cracks up when he gets nervous. I hope he doesn’t do that for his presentation today.

So, as you can see, we use “crack up” as a separable phrasal verb when we focus on another person making us laugh, but when we focus on the person who laughs, we use it as an intransitive phrasal verb.