1 general:: Phrase(s): crack a smile to grin; to smile. • I was tellin’ my best jokes, but Jim never cracked a smile. • She looked surprised, and then cracked a big, beautiful smile.
McGrawhill's American Idioms And Phrasal Verbs
2 general:: Smile, as in Betty was a very serious person; she rarely cracked a smile. This colloquialism transfers crack in the sense of "break" to breaking a serious facial expression into a smile.
American Heritage Idioms