This wikipedia article has a list of countries that have done just that. See if you can detect any common themes between them. One thing to consider is just who it is you're defaulting on. This linked Forbes article indicates that, as of late last year, 65% of that debt is owned domestically. 16% is owned by social security. Think about that. Of our 18 trillion dollars of federal debt, almost 3 trillion is owed to social security. What happens if/when we default on that?