The US can never default. FRN's are not money. They are a promise to pay money. No one ever even attempts to collect so how can there ever be a default until people redeem their FRN's for lawful money?
U.S. v. Thomas 319 F.3d 640 Paper currency, in the form of the Federal Reserve Note, is defined as an "obligation[] of the United States" that may be "redeemed in lawful money on demand." 12 U.S.C. § 411 (2002). These bills are not "money" per se but promissory notes supported by the monetary reserves of the United States.