Carapace is a scientific term for protective shell. Turtles and crabs have them, but humans don't — so we make do with bike helmets and elbow pads.
Otherwise known as outerwear for arthropods and turtles, carapace is a word you probably won't come across very often, unless you're a zoologist. But that doesn't mean you can't use it to describe someone who's a tough nut to crack. If a person has a gruff attitude meant to protect them emotionally, they've certainly enclosed themselves in a carapace of sorts.