Crack a smile指的是微笑或露齿一笑：

例句：

They were a tough class to teach, I told my best jokes and no one even cracked a smile.

他们真是很难教的一个班级。我讲了我最棒的笑话，居然没有一个人笑。

Crack a smile还可以用来表示在感觉不想微笑的时候挤出一丝笑容：

例句：

She told me a joke at the party and it just wasn't funny, I tried to crack a smile but just couldn't.

她在聚会上给我讲了一个笑话，但一点也不好笑，我试着挤出一丝笑容，但做不到。