Crack Up

A few decades ago it might have been possible.

These days, to crack up/to break up usually has the meaning of being unable to keep from laughing out loud at something.

"To crack up" might be used in cases of a complete mental breakdown.

"To crack" suggests only the beginning of a rupture. Not a complete collapse.
Of course, sometimes it's implied that

So, these are wrong?

"After intense police interrogation, the suspect cracked."
"After intense police interrogation, the suspect cracked up."
"Some employees cracked under the increasing workload."
"Some employees cracked up under the increasing workload."

Sorry, I meant to indicate that "cracked up" is not generally used in these situations. "Cracked" is fine.

Sometimes we say, "He began to crack."

My earlier point was that figuratively speaking, a crack can be an early sign of a coming breakup/breakdown, etc.

Thanks Avangi.

As Avangi said:"These days, to crack up/to break up usually has the meaning of being unable to keep from laughing out loud at something."

In modern usage, there is a difference "crack" and "crack up".
If you say, "After intense police interrogation, the suspect cracked up", it will be taken to mean that the suspect broke out in laughter after being intensely questioned by the