When a country defaults...it's not like when a private citizen goes broke...where they declare bankruptcy and have their income garnished and assests seized.
When a country defaults....it says to the foreigners whom owes it money... "Sorry...I'm not paying you back!"
That's it!
The country keeps paying it's social security obligations, public servant's wages etc and the citizenry keep paying their taxes, taking their kids to school etc.
There are no international police or baliffs to throw a country in jail...no courts to garnish it's GDP...
If the foreign debtors don't like it ..they can try to recover their lost money by siezing sovereign assets by force with their armed forces. But there's a couple of problems with this.

The top 2 being..who's going to invade the US and take the Statue of Liberty, and most of the debt is owed to private businesses with no armed forces of their own.
The only real drawback to a country defaulting on it's debt is;
1) it may be seen as a risky place to lend to, making borrowing a little more expensive, and
2) facing trade embargoes from countries that have been defaulted on.
But once again whom would be stupid enough to become enemies with the U.S. and lock themselves out of the worlds richest economy.
Essentially, there's no downside to the US if it defaults!