Explanation:

They both are very similar in meaning. "To crush" kind of implies that the original state of the thing being crushed has been altered/distorted using force. I will try to show you what I mean through examples:

- He crushed his hand on a rock. (implies broken bones- sounds more gruesome than "smashed")

- He smashed his hand on a rock. (simply that he hit his hand really really hard on a rock)

- Will you crush graham crackers for the dish I'm making? (implies an altering of the physical state of the graham crackers)

- Will you smash the graham crackers? (sounds weird to say because it actually means "will you hit the graham crackers really hard?")

- The suv smashed right into the truck.

-The semi just crushed that other car. (implies that the car's shape was badly distorted by the semi. You can smash into another car, but in order to crush it you need some serious force to do so, that is why I chose to use a semi and a car in my example)

- You crushed my art project! -It was an accident, I'm sorry! (you can't really smash someone's art project by accident, but you can accidentally sit on it and crush it)