They will default on their promises. That is why you either want to have your cash in your mattress, or in a very strong bank.
Or, most cynically, put your money in a very weak bank. If the banks start going down, you want yours to go down first when they're still paying on the 'insurance'.
Default will never ever happen. The USA government will never default on its citizens. It will print away all it needs, which is equivalent to defaulting, but it will never directly default. There is a reason why there has never been a deflationary collapse and the empire collapse are always hyper-inflationary.
The USA would only default on some foreign nation. The event would obviously trigger war, probably IIIWW.